Optical and Electronic Studies of Photostability and Charge Dynamics

By

Yi Zheng Tan

A dissertation submitted in partial fulfillment of

the requirements for the degree of

Doctor of Philosophy

(Chemistry)

at the

UNIVERSITY OF WISCONSIN-MADISON

2013

Date of final oral examination: 06/20/13

The dissertation is approved by the following members of the Final Oral Committee: Robert J. Hamers, Professor, Chemistry John C. Wright, Professor, Chemistry Song Jin, Associate Professor, Chemistry Randall H. Goldsmith, Assistant Professor, Chemistry Joel A. Pedersen, Professor, Soil Science

i

Optical and Electronic Studies of Photostability and Charge Dynamics

Yi Zheng Tan

Under the supervision of Professor Robert J. Hamers

University of Wisconsin-Madison

Abstract

This thesis encompasses three areas of research:

(1) Ligand chemistry for enhanced photostability of CdSe quantum dots (QDs): Chalcogenide quantum

dots such as CdSe and PbSe have potential as absorbers for QD-sensitized solar cells, but their

practical utility is limited by fast degradation when exposed to ambient environments. Our work

explored how the molecular structure of small thiol ligands affects the photostability of CdSe QDs.

We found that electron donating conjugated ligands enhanced the photostability by effectively

trapping oxidative holes from the QD. To further this chemistry, we developed an in-situ

functionalization method of conjugated dithiocarbamates on CdSe/TiO2, and explored the para-

substituent effect of these conjugated ligands on the photostability of CdSe.

(2) Spectroelectrochemistry of the iodide-triiodide redox couple: Alternatives to the standard Pt

counter electrode to the dye-sensitized solar cells are conducting polymers and carbon

nanomaterials. To try to understand differences in their mechanisms, we discuss our efforts with a

home-built spectroelectrochemical cell to enable spectral measurement of changes in the iodine

species with a voltage sweep.

(3) Surface photovoltage (SPV) measurement for charge dynamics of materials: SPV measurement is a

contactless technique to characterize semiconductor surfaces. We developed a transient-SPV

measurement to directly measure fast charge transfer processes that uses ns-pulsed excitations

onto a capacitive coupled sample. To resolve dynamics faster dynamics down to the fs-timescale, we

ii developed an SPV measurement technique using delayed ultrafast laser pulses. The non-linear

behaviour of SPV allows time-averaged electronic measurements. With this technique, we

successfully measured nanosecond to picosecond relaxation dynamics of a few highly doped

semiconductors. We further tracked changes in carrier dynamics of natural pyrite after various

surface treatments.

iii

Acknowledgements

It has been a long 6 years, and I am grateful to so many people for their help and support.

First and foremost, I would like to thank my advisor, Bob Hamers for his guidance and support

throughout my years here. Bob, your enthusiasm and optimism for science has been a constant source

of motivation and inspiration, especially when things were not going as well as I had hoped. I am very

grateful to the Hamers group, past and present, for the mentorship, the friendship, and overall for being

a supportive and nurturing academic family. I would like to especially thank my officemates and

labmates, Jeremy, Drew, Bo, Mike, Michelle, Courtney, Lee, Caroline, Marco, Linghong, and Arielle for

making the workplace not only a productive but also a fun and exciting environment. Mike, you are

much missed. Marco, thanks to you, I will eternally be the ‘Chef’ of the ‘Hamers Group Muppets’. I am

also grateful to group members who started with me at the same time, Kacie, Xin, and Rose. Your

company has made my graduate school life less stressful.

I have been very fortunate to be involved in collaborations outside my group. I am grateful to

Song Jin and John Wright for helpful discussions and suggestions to improve my research. A large part of

my research has been highly collaborative between the Jin and Wright groups, so I tremendously

appreciate my collaborators from these two groups, especially Skye, Andrei, Miguel, Dan, Blaise, Qi, and

Kyle for your contributions to my projects. I am happy to have helped your research in some way too,

however minor.

Finally, I am grateful to my family and friends. I would like to thank my old college friends,

especially John. Even though we have parted ways since our undergraduate days, you have kept our

friendship going strong. Jon, without you, I do not think I would have made it this far in graduate school.

Thank you for all your love and support.

iv

Table of Contents

Abstract .......................................................................................................................................................... i

Acknowledgments ........................................................................................................................................ iii

Table of Contents ......................................................................................................................................... iv

Chapter 1 Introduction and Background ..................................................................................................... 1

1.1 The Advent of Dye Sensitized Solar Cells ............................................................................................ 1

1.2 The Iodide-Triiodide Redox Couple and Counter Electrode ............................................................... 2

1.3 Inorganic Nanocrystals as Potential Absorbers for Sensitized Solar Cells .......................................... 4

1.4 The Issue of Quantum Dot Stability .................................................................................................... 7

1.5 Surface Photovoltage Spectroscopy for Measurements of Charge Transfer and Charge Dynamics

................................................................................................................................................................. 10

1.6 Scope of Thesis .................................................................................................................................. 13

1.7 References......................................................................................................................................... 14

Chapter 2 Influence of Hole-Sequestering Ligands on the Photostability of CdSe Quantum Dots ........... 19

2.1 Introduction ...................................................................................................................................... 19

2.2 Experimental ..................................................................................................................................... 21

2.2.1 Chemicals .............................................................................................................................. 21

2.2.2 Preparation of Nanocrystalline TiO2 Films ............................................................................ 21

2.2.3 Synthesis of CdSe QDs .......................................................................................................... 22

2.2.4 CdSe-TiO2 Adduct Formation and Subsequent Ligand Exchange ......................................... 22

2.2.5 Fourier Transform Infrared (FTIR) Spectroscopy .................................................................. 23

2.2.6 X-ray Photoelectron Spectroscopy (XPS) .............................................................................. 23

2.2.7 Photodegradation Studies .................................................................................................... 23

v 2.2.8 Photoluminescence ............................................................................................................... 24

2.2.9 Density Functional Theory (DFT) Calculations ...................................................................... 24

2.2.10 Fabrication of CdSe Sensitized TiO2 Solar Cells ................................................................... 25

2.3 Results ............................................................................................................................................... 25

2.3.1 FTIR Characterization of Functionalized CdSe/TiO2 Surfaces ............................................... 25

2.3.2 Photostability in Water ......................................................................................................... 27

2.3.3 Comparison of Molecular Coverages .................................................................................... 31

2.3.4 Photostability in Air............................................................................................................... 31

2.3.5 Photoluminescence ............................................................................................................... 33

2.3.6 DFT Calculations .................................................................................................................... 35

2.4 Discussion .......................................................................................................................................... 36

2.5 Effect of DMATP in a Liquid Junction Solar Cell ................................................................................ 40

2.6 Extending the DMATP Passivation Method to PbS and PbSe QDs ................................................... 42

2.7 Conclusions ....................................................................................................................................... 42

2.8 References......................................................................................................................................... 43

Chapter 3 Photostability of CdSe Quantum Dots Functionalized with Small Conjugated Dithiocarbamates

(DTCs) .......................................................................................................................................................... 50

3.1 Introduction ...................................................................................................................................... 50

3.2 Experimental ..................................................................................................................................... 51

3.2.1 Chemicals .............................................................................................................................. 51

3.2.2 Preparation of Nanocrystalline TiO2 Films ............................................................................ 51

3.2.3 Synthesis of CdSe QDs .......................................................................................................... 52

3.2.4 CdSe-TiO2 Preparation and Ligand Modification .................................................................. 52

3.2.5 Fourier Transform Infrared (FTIR) Spectroscopy .................................................................. 53

vi 3.2.6 X-ray Photoelectron Spectroscopy (XPS) .............................................................................. 53

3.2.7 Water Photostability Studies ................................................................................................ 53

3.2.8 Photoluminescence (PL) ....................................................................................................... 54

3.3 Results ............................................................................................................................................... 54

2.3.1 FTIR and XPS Characterization .............................................................................................. 54

2.3.2 Water Photostability of R-Ph-DTC Functionalized CdSe-TiO2 ............................................... 60

2.3.3 Photoluminescence ............................................................................................................... 62

2.3.4 Photostability of DTC vs. Thiol Bound CdSe-TiO2 Surfaces ................................................... 64

3.4 Discussion .......................................................................................................................................... 66

3.5 Conclusions ....................................................................................................................................... 68

3.6 References......................................................................................................................................... 68

Chapter 4 Spectroelectrochemistry of the Iodide-Triiodide Redox Couple ............................................... 73

4.1 Introduction ...................................................................................................................................... 73

4.2 Mechanism of Electrochemical Reactions of the Iodide-Triiodide Redox Couple on Platinum ....... 74

4.3 Experimental ..................................................................................................................................... 75

4.3.1 Materials ............................................................................................................................... 75

4.3.2 Electrochemistry ................................................................................................................... 75

4.3.3 Spectroelectrochemistry ....................................................................................................... 76

4.4 Results and Discussion ...................................................................................................................... 76

4.4.1 Absorption Spectra of Iodine Species ................................................................................... 76

4.4.2 Electrochemistry of the Iodide-Triiodide Couple on Pt and PEDOT-PSS .............................. 78

4.4.3 Spectroelectrochemistry ....................................................................................................... 79

4.5 Conclusions ....................................................................................................................................... 83

4.6 References......................................................................................................................................... 83

vii Chapter 5 Surface Photovoltage Techniques for Measurements of Charge Transfer and Charge Dynamics

.................................................................................................................................................................... 86

5.1 Introduction ...................................................................................................................................... 86

5.2 SPV Cell Setup ................................................................................................................................... 87

5.3 Steady-State SPV Experiments .......................................................................................................... 89

5.4 Transient SPV Experiments ............................................................................................................... 91

5.5 Ultrafast SPV Experiments ................................................................................................................ 99

5.6 Conclusions ..................................................................................................................................... 103

5.7 References....................................................................................................................................... 105

Chapter 6 Conclusions and Outlook.......................................................................................................... 107

Appendix ................................................................................................................................................... 109

A1 Molecular Coverage Calculations from XPS Data for Ligands on Nanoparticles ............................. 109

A2 Technical Design and Drawing of the Spectroelectrochemistry Cell ............................................... 115

A3 Technical Design and Drawing of the SPV Cell ................................................................................ 118

A4 Additional Data on Surface Photovoltage (SPV) Measurement ...................................................... 119

A4.1 Steady-State SPV Measurement .......................................................................................... 119

A4.1.1 CdSe Quantum Dot (QD) Functionalized on Doped Single Crystal Rutile (SCR) TiO2

(110) ................................................................................................................................ 119

A4.2 Transient SPV Measurement ............................................................................................... 121

A4.2.1 Bulk Diamond ...................................................................................................... 121

A4.2.2 N719 Dye on Porous Nanocrystalline TiO2 Films ................................................. 125

A4.2.3 TiO2 (Single Crystal Rutile) ................................................................................... 126

A4.2.4 ZnO (Single Crystal Sample and Porous Nanocrystalline Film) ............................ 128

A4.2.5 Iron Pyrite (FeS2) .................................................................................................. 130

viii A4.2.6 Layered Chalcogenide Nanostructures of MoS2, WS2, and WSe2 ....................... 132

A4.2.7 Sensitized SnO2 ................................................................................................... 133

A4.2.8 Hematite (α-Fe2O3) Nanowires on FTO ............................................................... 134

A4.3 Ultrafast SPV ........................................................................................................................ 135

A4.3.1 Ultrafast SPV of n-GaAs ....................................................................................... 135

A4.3.2 Ultrafast SPV of Synthetic Single Crystal Iron Pyrite ........................................... 137

A4.4 References ........................................................................................................................... 138

1

Chapter 1

Introduction and Background

1.1 The Advent of Dye Sensitized Solar Cells

Energy from the sun is the one of the most viable clean, renewable resources to meet our

increasing world energy demand. We can convert sunlight directly into electricity with photovoltaic

cells. Current cells that are widely used are Si wafer-based p/n junctions. For these cells, defects in the Si

wafer crystals increase the recombination rates of electrons and holes, so high efficiency relies on

having highly crystalline materials, which are expensive to manufacture. To lower cost, there has been

some success with the so-called ‘second generation solar cells’ in which thin films are used instead of

wafers, with materials such as cadmium telluride, copper indium gallium selenide (CIGS), and gallium

arsenide.1 However, they still suffer from cost issues (exotic materials such as indium are rare and

expensive), and also potential toxicity from the use of cadmium.

In 1991, O’Regan and Grätzel demonstrated the success of a dye-sensitized solar cell (DSSC) as

another low-cost alternative in the family of second generation solar cells.2 Sensitization is a way to

enhance the generation of photocurrent obtained from a semiconductor electrode by introducing an

absorber (traditionally a metal-complex dye) that can absorb light at longer wavelengths and transfer

photo-excited charges into the semiconductor.3 Before the work of O’Regan and Grätzel, sensitized

systems have been studied for potential light harvesting applications but suffered from very low

efficiencies. These studies focused on flat surfaces, and therefore had the problem of low absorption by

the thin monolayer of sensitizer. O’Regan and Grätzel revolutionized the concept of this sensitized

system by introducing a porous network of interconnected nano-sized crystals of anatase TiO2 as the

electron acceptor, thus greatly increasing the surface area of the electrode. The high surface coverage of

the sensitizing dye makes the electrode highly absorptive, and subsequently can potentially generate

2 increased photocurrents. TiO2 is a stable, low cost semiconductor with a lot of potential in

photoelectrochemical applications, but its large bandgap (3.2 eV for anatase) does not ideally match the

solar spectrum. The dye, informally named N3, is a Ru-bipyridyl complex with two carboxylic acid groups

on each bipyridine ligand and has an absorption onset in the near IR region. The carboxylic acids serve as

anchoring groups; they readily bind to the surface of TiO2. Figure 1.1(a) shows the general schematic of

a sensitized solar cell, and Figure 1.1(b) shows the relevant energy levels of a dye sensitized-TiO2 system.

Since the lowest unoccupied molecular orbital (LUMO) level of the Ru-dye is more reducing than the

conduction band edge of the TiO2, there is a driving force for the photo-excited electrons to transfer

from the dye into the TiO2. These electrons then travel through the nanocrystalline film to a transparent

conducting fluorine-doped tin oxide (FTO) coated glass and into an external circuit. The electron from

the oxidized dye is then replenished with an iodide-triiodide redox electrolyte. This electrolyte has a

higher reduction potential than the highest occupied molecular orbital (HOMO) of the dye, so it can

reduce the dye back to its ground state. A platinized transparent conducting glass serves as the counter

electrode to complete the cell.4

The work of O’Regan and Grätzel has spurred a large amount of research, both on the

mechanism of the cell and alternative materials for each cell component to improve cost and

performance. To date (7/11/2013), a search for “sensitized solar cells” on Google Scholar yields ~36,700

papers, and the original Nature paper2 has been cited 14,970 times.

1.2 The Iodide-Triiodide Redox Couple and Counter Electrode

After the dye injects the electron into the TiO2 semiconductor in a dye-sensitized solar cell

(DSSC), a redox electrolyte has to replenish the electron back to the dye. The electrolyte should have a

redox potential that is slightly more negative in reducing potential than the HOMO level of the dye to

enable successful dye regeneration (see Figure 1.1). The very first cell used an iodide-triiodide redox

couple, and since then there have been alternatives explored, most promisingly the Co2+/3+ complexes.5-9

3

Sensitizer-TiO2

Redox couple e-

Counter Pt/FTO

FTO

e-

e-e-

hν

e-

hν

CB

VB

S

S+

Eredox

TiO2

E

(a)

(b)

Figure 1.1: (a) General scheme of a sensitized solar cell (b)

Relevant energy levels involved in electron transfer of a cell

4 Yet the majority of successful cells are the ones that utilized the iodide-triiodide couple. The

effectiveness is understood to be due to the nature of the oxidation-reduction kinetics of this redox

couple. Oxidation of the iodide to triiodide by the dye is kinetically much faster than its back reduction.9-

13 Thus, once the electrons are injected into the TiO2, recombination of these electrons back to the

triiodide is inhibited.

The counter electrode in a DSSC functions to complete the cell by regenerating the oxidized

electrolyte triiodide species back to iodide. On this side of the cell, a fast reduction of triiodide to iodide

is desirable. The conventional counter electrode used is a platinized electrode, since Pt catalyzes

triiodide reduction. However, for reasons of cost and availability, this material is not ideal if we want to

make DSSCs economical for practical commercialization. Alternatives have been widely explored with

moderate success, including organic conducting polymers,14-16 carbon nanomaterials,17,18 various metal

oxides,19-21 sulfides,22,23 nitrides,21,24 and carbides.21,25 This shows the potential of further lowering the

cost of DSSCs, making their use more attractive.

1.3 Inorganic Nanocrystals as Potential Absorbers for Sensitized Solar Cells

In the first demonstration of the DSSC by O’Regan and Grätzel, they used a Ru-bipyridyl dye (N3)

as the absorber.2 Since then, there has been a great deal of effort in finding alternative sensitizers either

to extend the dye absorption deeper into the near IR and/or to lower the cost. Although only a thin layer

of dye is needed per photovoltaic cell, Ru metal is expensive and its cost could be prohibitive for

practical commercialization.26 A large research area that has branched out of the DSSC is the use of

inorganic semiconducting nanocrystals as the potential sensitizer.27

Nanocrystals (crystalline particles with length scales of 1 – 100 nm) have unique physical and

electronic properties that differ from their macro-sized bulk counterparts due to their small sizes.

Among them are quantum dots (QDs), semiconducting nanocrystals with sizes comparable or smaller

than their exciton Bohr radii, leading to quantum confinement in all directions. The Bohr radius depends

5 on the nature of the crystal lattice structure, so this quantity differs for each semiconducting material.

The physical confinement results in electronic properties that are in between small isolated molecules

and large macro-sized crystals, as illustrated in Figure 1.2(a). In an isolated molecule, electronic energy

levels are discrete; we commonly describe these levels with molecular orbitals, HOMOs and LUMOs. On

the other end, bulk crystalline semiconductors have continuous energy levels that form bands; the filled

band is termed the valence band (VB), while the empty band is the conduction band (CB), and the

minimum energy to excite an electron from the VB into the CB is the band gap (Eg). As we decrease the

size of semiconductor crystals to below the exciton radius, the energy of its charge carriers increases

due to the increased spatial confinement, and the electronic band structure becomes discretized,

resembling more like an isolated molecule. The bandgap in a QD becomes larger than that of the bulk.

Due to the spatial nature of the quantum confinement, the resulting electronic structure is highly shape

and size dependent. Smaller QDs have stronger confinement effects, and therefore larger bandgaps.28

Since we can tune the electronic properties of these nanocrystals with size and shape, one of the

advantages in using them as sensitizing absorbers is the opportunity to optimize the bandgap of the

nanocrystal to ideally match the solar spectrum.

Various nanocrystal materials have been studied as sensitizers, more popularly the

chalcogenides like CdS, CdSe, PbS, and PbSe. They have been paired with a number of wide band gap

semiconductors such as TiO2, SnO2, and ZnO to act as electron acceptors for sensitized solar cells.27 Since

the physical sizes and the electronic energies of semiconducting nanocrystals are intimately connected,

there is some bandgap engineering that needs to be considered. Figure 1.2(b), adapted from Tvrdy et.

al.,29 shows the energy alignments of a series of CdSe QD sizes to the bulk band energies of a few

electron acceptor materials. CdSe (Bohr radius = 5.4 nm),30 due to the position of its CB edge in the bulk,

is able to inject electrons into the CB of all of the electron acceptors shown in Figure 1.2(b) at all QD

sizes. The variation in QD size and electron acceptor material, however, changes the thermodynamic

6

(a)

(b)

(c)

nanocrystal

diameter (nm)

Figure 1.2: (a) Energy diagram showing the intermediate electronic property

of a semiconductor nanocrystal, from ref. 28 (b) Energy level alignments of

various CdSe QD sizes with a few metal oxide electron acceptors, from ref.

29 (c) Energy level alignment of PbSe QDs to TiO2, from ref. 31

7 driving force for charge injection and affects the rate of electron transfer. In contrast, as shown in Figure

1.2(c),31 PbSe QDs, despite having a large exciton Bohr radius (46 nm),32 can only inject electrons from

their lowest excited state into the TiO2 CB when the QD diameter is less than 5 nm.

Another unique opportunity in the area of nanocrystal sensitized solar cells is the observation of

multiple exciton generation (MEG) and longer lived hot carriers in nanocrystals. These processes would

allow us to extract more energy from high energy electrons without wasting it in the form of heat from

thermalization of the carriers to the CB minimum. Recently, there have been some successful reports of

extracting hot carriers and multiple excitons in sensitized solar cells.33 The ability to tune the electrical

properties with size and shape ,along with the higher absorption coefficient (as opposed to a molecular

dye)26 and other unique properties (long lived hot carriers and MEG) make semiconducting nanocrystals

attractive absorbers for sensitized solar cells. However, this is a relatively new area (as opposed to the Si

photovoltaic technology), and there is still much to learn if it is to compete with conventional

photovoltaic cells.

1.4 The Issue of Quantum Dot Stability

Stability can be described in different ways when discussing nanomaterials. One is colloidal

stability, simply defined as the ability of the material to remain suspended in a particular solvent

indefinitely. A stable nanomaterial suspension in this respect will not aggregate and settle out of the

solution. This stability is highly dependent on the nature of the organic ligands that coat the materials.

Nanomaterials coated with long alkyl hydrocarbon chains will be colloidally stable in non-polar solvents,

while nanomaterials with hydrophilic ligands (e.g. carboxylic acid, ethylene glycol) will be stably

suspended in polar solvents. Weakly bound ligands, or ligands that are kinetically labile could also cause

colloidal instability. The other description of stability is chemical stability, in which the nanomaterials are

resistant to chemical change or decomposition. Often times, colloidal stability and chemical stability are

dependent on one another, especially when dealing with the interaction of ligand chemistry and

8 nanomaterials. A colloidally unstable suspension may lead to chemical instability, and a chemically

unstable ligand may lead to an unstable colloidal suspension. My work focuses on chemical stability,

specifically photostability, so this is what I will mean when I use the term stability.

As discussed in detail in the previous section, quantum dot (QD) sensitized solar cells have

emerged to be exciting alternatives to their dye-based counterparts. These cells utilize inorganic QDs to

absorb light from the sun, and transfer the electrons to a wide band gap semiconductor, thus creating

separation of holes and electrons. Most widely studied of these QDs are the chalcogenides such as CdSe,

PbS, and PbSe. However, these chalcogenide QDs suffer from photostability problems when exposed to

ambient conditions, which could limit their practical long-term utilization in solar cells. The

photostability problem becomes worse in a sensitized cell. Since charges are spatially separated, the

electrons or holes could react with oxygen or water to produce reactive species that will in in turn

degrade the QDs.

Chalcogenide QDs like CdSe, PbS, and PbSe can oxidize when exposed to air and water. X-ray

photoelectron studies of CdSe QD thin films exposed to air showed that the Se2- quickly oxidizes to SeO2.

However, the surface Cd2+ is somewhat more intact, due to the ligands that are bound to the Cd sites of

the QDs. When the ligands were desorbed from the surface, only then were CdOx species detected.34

SeO2 species were also detected by X-ray absorption near-edge structure analysis.35 CdSe QDs exposed

to water (and light) behaved differently: the CdSe degraded into insoluble elemental Se and soluble Cd-

hydroxide ions.36 In colloidally suspended CdSe QDs, photochemical instability can depend on the nature

of the ligand,37 or the solvent environment.38 Aldana et al. studied water photostability of CdSe QDs

functionalized with thiol ligands, and found that upon photoexcitation of the CdSe QD, the holes

generated, in combination with oxygen, oxidizes the thiol into a disulfide that then becomes unbound to

the QD. By varying the nature of the thiol ligands (length of alkyl chain, conjugated vs. alkyl, monothiol

9

Figure 1.3: (a) CdSe QD coated with organic

dendron ligands, from ref. 39 (b) CdSe QD with

a ZnS shell, along with energy levels showing

the potential well created

ZnS

CdSe

CB

VB

(a)

(b)

10 vs. dithiol), they found that the most photostable CdSe QDs are the ones that are coated with long

hydrocarbon chains to prevent diffusion of oxygen into the QD core.37

There have been multiple methods developed to prevent the photo-oxidation of chalcogenide

QDs. Following the initial work of Aldana et al., as shown in Figure 1.3(a), ligands with large branched

hydrocarbons or dendrons were synthesized and found to stabilize QDs.39,40 Another route in using

organic ligand passivation is the encapsulation of the QD with polymers that contains multiple binding

groups on the polymer chain.41,42 QDs can also be protected from photodegradation with a wide-band

gap inorganic shell. The most common shell material for CdSe QDs is ZnS. As shown in Figure 1.3(b), the

energy levels of the ZnS shell are such that they trap the CdSe energy levels into a potential well, forcing

the photogenerated charges away from the surface. The inorganic shell also passivates the QD core

from surface and defect states.28 This passivation method is important for applications that require

stable, bright luminescence from the core, such as fluorescent tagging of biological materials.43

Insulating shell overcoats, such as ZnS,44 TiO2,45 and Al2O3

46,47 have also been used to protect QD-

sensitized systems either to passivate the absorbing QD from photodegradation or to reduce

recombination losses. In this case, since electrons need to transfer out of the QD into the electron

acceptor, the control of the shell thickness is important to not severely impede electron transfer.48,49

1.5 Surface Photovoltage Spectroscopy for Measurement of Charge Transfer and Charge Dynamics

As semiconductor technology becomes increasingly important in applications like computers

and optoelectronic devices, there is a continuing need to deepen our understanding of optical and

electronic properties of charge carriers in various semiconductor materials. Various spectroscopic

methods have been developed and utilized to characterize charge carriers in semiconductors; the most

common ones are pump-probe (either in transmission or reflection mode), photoluminescence,

photoemission, and surface photovoltage spectroscopies.50 Surface photovoltage (SPV) spectroscopy

has emerged to be very attractive and useful due to the advantages it has over the other techniques. It

11 is a non-contact, non-destructive technique does not require the sample to be either transmissive or

reflective (unlike pump-probe), have strong radiative emission bands (unlike photoluminescence), or be

performed under ultrahigh vacuum conditions (unlike photoemission).

SPV measurements rely on the spatial separation of charge, either due to a semiconductor

space charge layer, or charge transfer from a dye/semiconductor heterojunction. The space charge

forms due to the fact that the termination of lattice periodicity at the surface of a semiconductor

introduces dangling bonds, surface atom rearrangements, or surface defects. This creates surface states

on the semiconductor that are usually located in the middle of the bandgap. At equilibrium, the charge

carriers redistribute between surface and bulk states, and subsequently the near-surface region of the

semiconductor develops an internal electric field. This deviation in electronic energies from the bulk is

represented and described by a band bending of the conduction and valence energy bands. Figure 1.4(a)

illustrates the band bending typical of an n-type semiconductor. As the semiconductor is photoexcited,

electrons generated feel this electric field, so charges separate spatially, changing the voltage at the

surface. SPV is the result of this photoinduced change in voltage. For an n-type semiconductor, electrons

will be driven deeper into the bulk, while the holes are left at the surface. In a dye/nanocrystal-

sensitized heterojunction system, spatial separation of charges is achieved by a charge transfer from the

sensitizer into an appropriate semiconductor acceptor. Specifically, for the dye-sensitized or the CdSe-

sensitized TiO2 systems discussed previously, electrons are transferred into the conduction band of TiO2,

while the holes are left on the sensitizing material (see Figure 1.1).

There are two main experimental configurations for the detection and measurement of SPV. The

first is the Kelvin probe method that relies on the direct relationship between surface work function and

surface photovoltage. I will not expand further on the Kelvin probe method (please see literature cited if

interested), as my SPV work focusses on the second approach, the metal-insulator-semiconductor (MIS)

setup. The MIS structure creates a capacitor-like arrangement, as shown in Figure 1.4(b). Upon

12

Figure 1.4: (a) Energy diagram showing band bending of a typical n-

type semiconductor (b) Schematic of the MIS capacitive setup for SPV

measurement. The setup illustrates the SPV signal resulting from the

spatial separation of charges in a sensitized system

-

+ hν

Ec

Ev

EF

(a)

hν

sense electrode

sample

+ + + +

- - - -

Insulating space

(b)

Sensitizer

13 illumination of the photoactive material, charges of one type accumulate on the surface due to the SPV

mechanisms discussed above. The ‘sense’ electrode, held close to the sample, builds up the charge of

the opposite sign and the corresponding voltage change can be externally measured. To achieve high

signal-to-noise, this is typically performed by illuminating with chopped light and measuring with a lock-

in amplifier.51

SPV spectroscopy has traditionally been used to characterize Si wafers and other simple single

crystal semiconductors (e.g. surface defect sites, effect of surface treatments), but more recently has

been extended to a few nanostructured semiconductors.52-54 The non-contact setup that is independent

of light reflection or scattering makes it very convenient for opaque or rough surfaces that are common

for nanostructured materials. This technique has also been used to characterize charge transfer across

dye and inorganic nanocrystal sensitized systems.55-58 Transient SPV techniques, on the other hand, are

less well-developed, and would be extremely beneficial for characterization of fast dynamics of charge

transfer (as in a sensitized solar cell), and exploration of earth-abundant semiconductors such as

hematite and iron pyrite that have short carrier lifetimes (< 1 ns) for solar photo-electrical/-

electrochemical applications.

1.6 Scope of Thesis

Chapters 2 and 3 are about the potential of using small organic conjugated ligands to enhance

the photostability of CdSe QD sensitized TiO2. Unlike long insulating hydrocarbons or insulating wide

bandgap shell, these ligands could provide an alternative pathway to passivation of air-sensitive

nanocrystals. Chapter 2 explores the influence of hole-trapping thiols on the photostability of CdSe

quantum dots (QDs). We show that by having conjugated ligands with electron-donating substituents,

we can stabilize the QD by a hole transfer, and subsequently the delocalization of the hole in the ligand.

Chapter 3 is further photostability studies of CdSe QDs, in which we explored the use of conjugated

bidentate ligands, specifically dithiocarbarmates (R-NCS2-). We were able to functionalize

14 dithiocarbamate (DTC) ligands on CdSe-TiO2 surfaces in-situ, and further found that the

dithiocarbarmates enhanced the photostability of CdSe QDs more than the monodentate thiols. This

enhancement is likely due to an increased electronic coupling to the QD. Chapter 4 is a brief work on the

possibility of using spectroelectrochemistry to elucidate the mechanism of triiodide reduction on Pt and

poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), an alternative DSSC counter electrode

material. Chapter 5 describes our efforts in the development of surface photovoltage (SPV) methods for

measurement of charge transfer and charge dynamics, in particular time-resolved methods, both in the

nanosecond and in the picosecond timescales. Finally, in chapter 6, I discuss my outlook and possible

future directions of my thesis work.

1.7 References

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2. O'Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar-Cell Based on Dye-Sensitized Colloidial TiO2 Films, Nature 1991, 353, 737-740.

3. Memming, R. Semiconductor Electrochemistry; Wiley-VCH: Weinheim, Germany, 2001.

4. Grätzel, M. Dye-Sensitized Solar Cells, J. Photochem. Photobiol. C-Photochem. Rev. 2003, 4, 145-153.

5. Klahr, B. M.; Hamann, T. W. Performance Enhancement and Limitations of Cobalt Bipyridyl Redox Shuttles in Dye-Sensitized Solar Cells, J. Phys. Chem. C 2009, 113, 14040-14045.

6. Nusbaumer, H.; Moser, J. E.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. Co-II(dbbiP)(2)(2+) Complex Rivals Tri-iodide/Iodide Redox Mediator in Dye-Sensitized Photovoltaic Cells, J. Phys. Chem. B 2001, 105, 10461-10464.

7. Nusbaumer, H.; Zakeeruddin, S. M.; Moser, J. E.; Grätzel, M. An Alternative Efficient Redox Couple for the Dye-Sensitized Solar Cell System, Chem.–Eur. J. 2003, 9, 3756-3763.

8. Sapp, S. A.; Elliott, C. M.; Contado, C.; Caramori, S.; Bignozzi, C. A. Substituted Polypyridine Complexes of Cobalt(II/III) as Efficient Electron-Transfer Mediators in Dye-Sensitized Solar Cells, J. Am. Chem. Soc. 2002, 124, 11215-11222.

9. Hamann, T. W.; Ondersma, J. W. Dye-Sensitized Solar Cell Redox Shuttles, Energy Environ. Sci. 2011, 4, 370-381.

15 10. Anderson, A. Y.; Barnes, P. R. F.; Durrant, J. R.; O'Regan, B. C. Simultaneous Transient Absorption

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11. Boschloo, G.; Hagfeldt, A. Characteristics of the Iodide/Triiodide Redox Mediator in Dye-Sensitized Solar Cells, Acc. Chem. Res. 2009, 42, 1819-1826.

12. Clifford, J. N.; Palomares, E.; Nazeeruddin, M. K.; Grätzel, M.; Durrant, J. R. Dye Dependent Regeneration Dynamics in Dye Sensitized Nanocrystalline Solar Cells: Evidence for the Formation of a Ruthenium Bipyridyl Cation/Iodide Intermediate, J. Phys. Chem. C 2007, 111, 6561-6567.

13. Rowley, J.; Meyer, G. J. Reduction of I-2/I-3(-) by Titanium Dioxide, J. Phys. Chem. C 2009, 113, 18444-18447.

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15. Kanciurzewska, A.; Dobruchowska, E.; Baranzahi, A.; Carlegrim, E.; Fahlman, M.; Girtu, M. A. Study on Poly(3,4-ethylene dioxythiophene)-Poly(styrenesulfonate) as a Plastic Counter Electrode in Dye Sensitized Solar Cells, J. Optoelectron. Adv. Mater. 2007, 9, 1052-1059.

16. Li, Q.; Wu, J.; Tang, Q.; Lan, Z.; Li, P.; Lin, J.; Fan, L. Application of Microporous Polyaniline Counter Electrode for Dye-Sensitized Solar Cells, Electrochem. Commun. 2008, 10, 1299-1302.

17. Murakami, T. N.; Graetzel, M. Counter Electrodes for DSC: Application of Functional Materials as Catalysts, Inorg. Chim. Acta 2008, 361, 572-580.

18. Trancik, J. E.; Barton, S. C.; Hone, J. Transparent and Catalytic Carbon Nanotube Films, Nano Lett. 2008, 8, 982-987.

19. Hou, Y.; Wang, D.; Yang, X. H.; Fang, W. Q.; Zhang, B.; Wang, H. F.; Lu, G. Z.; Hu, P.; Zhao, H. J.; Yang, H. G. Rational Screening Low-Cost Counter Electrodes for Dye-Sensitized Solar Cells, Nat. Commun. 2013, 4, 1583-1583.

20. Wu, M.; Lin, X.; Hagfeldt, A.; Ma, T. A Novel Catalyst of WO2 Nanorod for the Counter Electrode of Dye-Sensitized Solar Cells, Chem. Comm. 2011, 47, 4535-4537.

21. Wu, M.; Lin, X.; Wang, Y.; Wang, L.; Guo, W.; Qu, D.; Peng, X.; Hagfeldt, A.; Graetzel, M.; Ma, T. Economical Pt-Free Catalysts for Counter Electrodes of Dye-Sensitized Solar Cells, J. Am. Chem. Soc. 2012, 134, 3419-3428.

22. Wang, M.; Anghel, A. M.; Marsan, B.; Ha, N.-L. C.; Pootrakulchote, N.; Zakeeruddin, S. M.; Graetzel, M. CoS Supersedes Pt as Efficient Electrocatalyst for Triiodide Reduction in Dye-Sensitized Solar Cells, J. Am. Chem. Soc. 2009, 131, 15976-15977.

16 23. Sun, H.; Qin, D.; Huang, S.; Guo, X.; Li, D.; Luo, Y.; Meng, Q. Dye-Sensitized Solar Cells with NiS

Counter Electrodes Electrodeposited by a Potential Reversal Technique, Energy Environ. Sci. 2011, 4, 2630-2637.

24. Li, G.-R.; Wang, F.; Jiang, Q.-W.; Gao, X.-P.; Shen, P.-W. Carbon Nanotubes with Titanium Nitride as a Low-Cost Counter-Electrode Material for Dye-Sensitized Solar Cells, Angew. Chem. Int. Ed. 2010, 49, 3653-3656.

25. Wu, M.; Lin, X.; Hagfeldt, A.; Ma, T. Low-Cost Molybdenum Carbide and Tungsten Carbide Counter Electrodes for Dye-Sensitized Solar Cells, Angew. Chem. Int. Ed. 2011, 50, 3520-3524.

26. Peter, L. M. The Grätzel Cell: Where Next?, J. Phys. Chem. Lett. 2011, 2, 1861-1867.

27. Kamat, P. V. Quantum Dot Solar Cells. The Next Big Thing in Photovoltaics, J. Phys. Chem. Lett. 2013, 4, 908-918.

28. Smith, A. M.; Nie, S. Semiconductor Nanocrystals: Structure, Properties, and Band Gap Engineering, Acc. Chem. Res. 2010, 43, 190-200.

29. Tvrdy, K.; Frantsuzov, P. A.; Kamat, P. V. Photoinduced Electron Transfer from Semiconductor Quantum Dots to Metal Oxide Nanoparticles, Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 29-34.

30. Albe, V.; Jouanin, C.; Bertho, D. Confinement and Shape Effects on the Optical Spectra of Small CdSe Nanocrystals, Phys. Rev. B 1998, 58, 4713-4720.

31. Acharya, K. P.; Alabi, T. R.; Schmall, N.; Hewa-Kasakarage, N. N.; Kirsanova, M.; Nemchinov, A.; Khon, E.; Zamkov, M. Linker-Free Modification of TiO2 Nanorods with PbSe Nanocrystals, J. Phys. Chem. C 2009, 113, 19531-19535.

32. Kang, I.; Wise, F. W. Electronic Structure and Optical Properties of PbS and PbSe Quantum Dots, J. Opt. Soc. Am. B: Opt. Phys. 1997, 14, 1632-1646.

33. Nozik, A. J. Nanoscience and Nanostructures for Photovoltaics and Solar Fuels, Nano Lett. 2010, 10, 2735-2741.

34. Katari, J. E. B.; Colvin, V. L.; Alivisatos, A. P. X-ray Photoelectron Spectroscopy of CdSe Nanocrystals with Applications to Studies of the Nanocrystal Surface, J. Phys. Chem. 1994, 98, 4109-4117.

35. Hines, D. A.; Becker, M. A.; Kamat, P. V. Photoinduced Surface Oxidation and Its Effect on the Exciton Dynamics of CdSe Quantum Dots, J. Phys. Chem. C 2012, 116, 13452-13457.

36. Xi, L.; Lek, J. Y.; Liang, Y. N.; Boothroyd, C.; Zhou, W.; Yan, Q.; Hu, X.; Chiang, F. B. Y.; Lam, Y. M. Stability studies of CdSe Nanocrystals in an Aqueous Environment, Nanotechnology 2011, 22.

37. Aldana, J.; Wang, Y. A.; Peng, X. G. Photochemical Instability of CdSe Nanocrystals Coated by Hydrophilic Thiols, J. Am. Chem. Soc. 2001, 123, 8844-8850.

17 38. Manner, V. W.; Koposov, A. Y.; Szymanski, P.; Klimov, V. I.; Sykora, M. Role of Solvent-Oxygen

Ion Pairs in Photooxidation of CdSe Nanocrystal Quantum Dots, ACS Nano 2012, 6, 2371-2377.

39. Wang, Y. A.; Li, J. J.; Chen, H. Y.; Peng, X. G. Stabilization of Inorganic Nanocrystals by Organic Dendrons, J. Am. Chem. Soc. 2002, 124, 2293-2298.

40. Guo, W. H.; Li, J. J.; Wang, Y. A.; Peng, X. G. Luminescent CdSe/CdS Core/Shell Nanocrystals in Dendron Boxes: Superior Chemical, Photochemical and Thermal Stability, J. Am. Chem. Soc. 2003, 125, 3901-3909.

41. Potapova, I.; Mruk, R.; Prehl, S.; Zentel, R.; Basche, T.; Mews, A. Semiconductor Nanocrystals with Multifunctional Polymer Ligands, J. Am. Chem. Soc. 2003, 125, 320-321.

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19

Chapter 2

Influence of Hole-Sequestering Ligands on the Photostability of CdSe Quantum

Dots

A part of this work is published in J. Phys. Chem. C, 117, 313 – 320

2.1 Introduction

Inorganic semiconductor quantum dots (QDs) are promising alternatives to organic dyes as

visible-light absorbers in sensitized solar cells.1-3 Inorganic QDs are advantageous because their optical

and electronic properties are size dependent and therefore can be tuned to optimize solar absorption as

well as the energy alignment between QD and metal oxide acceptor for favourable electron transfer.4-6

In addition, recent studies have demonstrated that some QDs have the ability to support hot electrons7,8

or multiple excitons per incident photon.9-12 Various semiconductor QDs such as CdSe,4,13 PbSe,5,6,8,14 and

PbS11,15,16 have been studied as sensitizers for quantum dot sensitized solar cells.

While QDs have many outstanding properties, the practical utilization of chalcogenide QDs is

hindered by their propensity to undergo oxidation,17-21 requiring strictly air- and water-free conditions to

remain stable. Protective inorganic shells such as ZnS13 and Al2O322 have been used to passivate the

surface and control their photooxidation, but these wide-bandgap shells may introduce potential

barriers to charge transfer if their thickness is not properly controlled.23,24

Compounds containing aromatic amino groups are widely used as hole conductors, but in

general these are larger molecules or polymers with more extensive conjugation.25,26 These groups are

also widely used in donor-π-acceptor type structures for dye-sensitized solar cells, with the arylamine

acting as an electron donor.27-29 A similar approach can be used to remove oxidizing holes from a QD, by

functionalizing the QD with hole-accepting ligands. This chapter is about our work on the influence of

20

Oleic Acid (OA)

1-Dodecanethiol (DT)

4-Dimethylaminothiophenol (DMATP)

Thiophenol(TP)

Figure 2.1: Ligand molecules used to cap CdSe QDs

21 hole-accepting ligands on the stability of CdSe QDs and on CdSe-sensitized TiO2 (referred to as

CdSe/TiO2). Figure 2.1 shows the ligands investigated here. Our results show that small conjugated

ligands slow photocorrosion in comparison with long alkyl ligands. In particular, an electron donating

amino group in the conjugated ligand, such as in 4-dimethylaminothiophenol (DMATP, see Figure 2.1),

provides remarkable stabilization of CdSe QDs. We combine photocorrosion, photoluminescence, and

density functional calculations to understand how molecular structure of the ligand affects QD stability.

Our results show that removing localized charge from the sulfur with an electron-donating amino group

stabilizes the ligand and minimizes photocorrosion of the QD. Finally, we discuss our attempts in utilizing

DMATP-passivated CdSe in a liquid junction QD sensitized solar cell, and our efforts in extending this

DMATP ligand passivation idea to PbS and PbSe QDs.

2.2 Experimental

2.2.1 Chemicals

Trioctylphosphine oxide (TOPO) 99+%, CdO 99.99% metal basis, Oleic Acid (OA) 90%,

Trioctylphosphine (TOP) 90%, Selenium 99.99% metal basis, 3-Mercaptopropionic Acid (MPA) 99+%, 1-

Dodecanethiol (DT) 98+%, Thiophenol (TP) ≥99% were purchased from Sigma-Aldrich. 4-

Dimethylaminothiophenol (DMATP) was purchased from Oakwood Products.

2.2.2 Preparation of Nanocrystalline TiO2 Films

Fluorine-doped tin oxide (FTO) coated glass (Hartford Glass) was pre-cleaned with detergent,

acetone and ethanol. Anatase TiO2 nanoparticles (average diameter = 20 nm) in a form of a paste (Ti-

Nanoxide, T20/SP, Solartonix) was screen-printed onto the FTO glass to give ~ 2 μm thick films. The

screen-printing process produces multiple films with the same thickness and mesoporous structure.

Each experimental set described here was performed using films prepared from the same batch of films.

These films of 0.5 cm in diameter were then sintered and annealed in air following a procedure adapted

from literature30 at 325 C for 5 min, at 375 C for 5 min, and at 450 C for 15 min, and finally, at 500 C

22

for 30 min. Before use, the films were given an additional annealing step at 500 C for 15 min to remove

any adsorbed water and organic contaminants.

2.2.3 Synthesis of CdSe QDs

The synthesis was adapted from Peng and Peng.31 Briefly, 3.1 g TOPO, 0.23 g CdO, and 1.8 g OA

are combined into a 3-necked flask. The mixture was then heated under Ar flow until it turned optically

clear (~290 C). The mixture was then allowed to cool to 250 C. A solution of TOPSe made by combining

0.04 g Se and 1.2 mL TOP was injected to the flask. The reaction was cooled to ~80 C, and was then

quenched with toluene. The CdSe QD solution was purified four times by precipitation and

centrifugation with methanol. The size and concentration of the CdSe QDs were determined from the

wavelength and absorbance of the first exciton peak, using empirical relationships established by Peng

and co-workers.32 This analysis yielded particle diameters that were typically 3.0 - 3.2 nm. The QDs were

kept in toluene and in the dark until further use. These as-synthesized QDs will be referred to as OA-

CdSe.

2.2.4 CdSe-TiO2 Adduct Formation and Subsequent Ligand Exchange

After synthesis, the CdSe nanoparticles were linked to TiO2 by one of two methods: (1) using a

bifunctional ligand to provide a covalent CdSe-TiO2 linkage, or (2) by direct physical adsorption. Ligand

exchange was performed after the CdSe nanoparticles were attached to TiO2 to investigate the effect of

the ligands on physical properties of the CdSe nanoparticles. Attachment of CdSe to TiO2 via covalent

linkage was performed using 3-mercaptopropionic acid (MPA). This approach provides obtain good

control over CdSe coverage.33-35 The TiO2 films were immersed in 0.1 M MPA in anhydrous acetonitrile

(ACN) for 6 – 8 h in the dark. They were rinsed with anhydrous ACN and toluene. Samples were then

immersed in 20 – 50 μM CdSe QDs in toluene (QD molarities in this work are defined by number of

moles of QDs per liter of solution) for 16 – 18 h in the dark, and rinsed with toluene. Functionalization

with ligands depicted in Figure 2.1 was then performed by soaking the samples in a solution of the

23 respective ligand (0.1 M in toluene, or just pure toluene for the OA sample) for ~24 h in the dark. They

were rinsed with toluene, then heptane, and dried with N2. Experiments were also conducted using

direct absorption of CdSe QDs to TiO2 with no linker; in this case the CdSe QDs (also 20 – 50 μM) were

precipitated with methanol, centrifuged, and resuspended in dichloromethane (DCM). The TiO2 films

were then immersed in the QD/DCM solution for 24 h,35 rinsed with DCM, and dried with N2.

2.2.5 Fourier Transform Infrared (FTIR) Spectroscopy

All measurements were taken with a Vertex 70 (Bruker) spectrometer at a resolution of 4 cm-1

and constant dry-air purging. The L-CdSe/TiO2 surfaces were measured in single-bounce reflection

absorption mode (VeeMax II accessory, Pike Technologies) with p-polarized light at an incident angle of

50 from the sample normal. The spectra were referenced against a clean TiO2 film prepared using

identical procedures. To minimize effects of atmospheric water and CO2 each sample and the clean TiO2

reference sample were measured as closely together in time as possible. This method of referencing

produced the most reproducible results.

2.2.6 X-ray Photoelectron Spectroscopy (XPS)

Measurements were done on a custom-built XPS system (Physical Electronics) with an Al-Kα

source (Model 10-610, 1486.6 eV photon energy), torroidal monochromator (Model 10-420), and

hemispherical analyzer with a 16 channel detector array (Model 10-360). We used an electron takeoff

angle of 45 and measured at a resolution of 0.05 or 0.1 eV. Peak areas were obtained by fitting the

spectra to a Voigt function. Shirley baseline corrections were used as needed.

2.2.7 Photodegradation Studies

For studies in H2O, the samples were sandwiched into a cell with another piece of FTO glass and

a 127 μm spacer; the open region was filled with 18 MΩ·cm H2O (Barnstead Nanopure). The cell has

windows to allow light penetration and time-dependent absorption studies without disassembling the

cell. The sample was illuminated through the plain FTO piece and water onto the sample. Light from a

24 solar simulator (Newport 91160, equipped with AM1.5G filters and set to 100 mW/cm2 as measured by

a Scientech calorimeter) was passed through a filter transmitting only light with wavelengths longer

than 475 nm. This filter was used to ensure that light is only absorbed by the CdSe QD, and not by the

TiO2. With the filter, the irradiance was measured to be 81 mW/cm2. This optical setup was used for all

of our degradation studies. Transmission UV-visible absorbance spectra (Shimadzu, UV-2401PC) were

obtained up to 10 min of exposure time. For studies in air, samples were left under the light under

ambient conditions without assembly into a cell. Absorption spectra were taken at exposure times up to

15 min.

2.2.8 Photoluminescence (PL)

The QDs were precipitated with methanol, centrifuged and resuspended with chloroform to give

~2 μM concentration. In all of the experiments, they were excited with 450 nm light, close to the second

absorption peak that corresponds to the second excitonic transition. Ligands, at a concentration of ~70

μM, were added and mixed immediately before experiments. Steady state experiments were performed

using an ISS K2 fluorometer. For transient PL measurements, 3 ns pulses from a laser (Ekspla NT340,

Nd:YAG with an optical parametric oscillator 250 μJ/pulse, 20 pulses/sec) were used to excite the QDs.

The transient fluorescence was collected with a photomultiplier tube (Hamamatsu R6357, rise time 1.4

ns) and recorded using an oscilloscope (Agilent, DSO5054A, 500MHz).

2.2.9 Density Functional Theory (DFT) Calculations

In order to better understand the nature of charge separation in these compounds, we

performed DFT calculations of the relevant molecules, using a Cd6Se6 cluster to model the CdSe surface.

Calculations were performed using the Gaussian09 program using the B3LYP hybrid density functional

and the LANL2DZ basis set for all atoms.36 Calculations on the free molecules were also performed using

the Dunning-Hay D95 basis set;37 since these results were nearly identical to those using the LANL2DZ

basis set all results reported here used the latter. The CdSe cluster was constrained to maintain the

25 property symmetry while leaving all bond distances unconstrained. A Natural Bond Orbital (NBO)

analysis38 was used to determine the charges on the individual atoms.

2.2.10 Fabrication of CdSe Sensitized TiO2 Solar Cells

TiO2 electrodes on FTO glass were made following a procedure adapted from literature.30 The

TiO2 films were first subjected to a TiCl4 pre-treatment to obtain a dense 1 nm thick TiO2 film. Three

screen-print passes were done with a drying step at 125 C for 5 min in between passes to yield 3 μm

thick films. We did not deposit a scattering layer because we found that the presence of the diffuse film

blocked the absorption of QDs into the pores of the TiO2 film. The films were subsequently annealed

using the procedure described above. After annealing, the films were subjected to another TiCl4

treatment step. The films were cleaned by heating at 500 C for 15 min prior to use. CdSe QDs were

functionalized to the TiO2 by direct physical absorption (no linker used) as described above. The redox

electrolyte was 0.22 M Co(bpy)3(PF6)2, 0.033 M Co(bpy)3(PF6)3, 0.2 M tert-butylpyridine, and 0.1 M

LiClO4.39 The counter electrode was a platinized FTO glass, made by coating the FTO piece with a drop of

0.1 M H2PtCl6 solution in ethanol and heating the piece at 450 C for 15 min.30

2.3 Results

2.3.1 FTIR Characterization of Functionalized CdSe/TiO2 Surfaces

Figure 2.2(a) shows FTIR spectra of CdSe/TiO2 functionalized with different ligands used in this

work (labeled as L-CdSe/TiO2 where L = OA, DT, TP, or DMATP). Spectra of the neat ligands are shown in

Figure 2.2(b). The CdSe QDs, as functionalized on TiO2 exhibit C-H peaks at 2856 and 2928 cm-1, close to

the 2854 and 2924 cm-1 of neat OA and a C-H peak at 3005 cm-1 from the C=CH group of oleic acid. These

nanoparticles show two peaks at 1551 and 1404 cm-1 that are characteristic of the carboxylate group,

while neat OA shows a single large peak at 1710 cm-1 in agreement with previous studies.40,41 The peaks

at 1551 and 1404 have previously been attributed to the asymmetric and symmetric stretching vibration

modes (respectively) of carboxylate groups bonded to CdS surfaces while 1710 cm-1 is typical

26

0.05

Inte

nsi

ty

OA

DT

TP

DMATP

Wavenumbers (cm-1)

-C-H=C-H

3000 2600 1600 1200

(Ar)C-H

carboxylate

(Ar)C-H

1800 1600 1400 1200 10003200 3000 2800 2600

0.05Oleic Acid

1-Dodecanethiol

Thiophenol

4-Dimethylaminothiophenol

Inte

nsi

ty

Wavenumbers (cm-1)

(a)

(b)

Figure 2.2: (a) IR spectra of L-CdSe/TiO2 indicating binding of ligands to the

surface. The 3160-2980 cm-1 region is enlarged 8x and overlaid directly

above each spectra. Ar = Aromatic (b) IR spectra of neat ligands

27 of the C=O stretch of a free carboxylic acid.40 Both OA on CdSe and MPA bound to TiO2 contribute to

these carboxylate peaks. Therefore, they are still present even after the displacement of OA molecules

with other ligands. The DT functionalized sample, shows similar features but is marked by the absence

of any significant intensity near 3005 cm-1, thereby indicating removal of the oleic acid groups. The TP-

modified samples show a C-H feature at 3061 cm-1, slightly lower that the ~3070 cm-1 observed for the

parent compound, while and DMATP-CdSe/TiO2 samples show a peak at 3076 cm-1. These features are

nearly identical to those observed previously for thiophenol on gold and attributed to the aromatic C-H

stretching modes.42 The DMATP samples show a more complex spectrum in the 2700-3000 cm-1 region

where the C-H modes of the -N(CH3)2 groups would be expected; this region is similar to that of the pure

parent compound and of N,N-dimethylaniline.43 Our FTIR data establish that ligand exchange from the

initially functionalized samples is successful, although some small amounts of OA may remain. The

asymmetric CH2 stretch of OA-CdSe/TiO2 and DT-CdSe/TiO2 occur at 2927 and 2926 cm-1 respectively,

slightly larger than the value of 2924 cm-1 for neat OA and DT. In contrast, prior studies have found that

the asymmetric mode decreases by ~ 6 – 8 cm-1 when forming a crystalline monolayer.44 Thus, our FTIR

data indicate that the OA and DT layers formed on CdSe are in a very fluidic local environment.

2.3.2 Photostability in Water

Figure 2.3(a) shows visible absorption spectra of OA-CdSe QDs that were linked to TiO2 films and

were then exposed to water and light. After illumination, the CdSe exciton peak broadened, amplitude

decreased, and the peak position slightly shifted. The dark control shown in Figure 2.3(b) on the other

hand, shows very little change in the excitonic peak, indicating that light is required to induce

degradation. A shift and broadening would indicate that the size and distribution of the QD have

changed as a result of photo-corrosion. While loss of the exciton features could also be attributed to

desorption of whole QDs from the TiO2 film, control experiments performed under dark conditions show

that the nanoparticles are stable in the dark; this is a photodegradation process of the QD/TiO2

28

0.4

0.6

0.8

1

A/A

0

1050exposure time (min)

OAOA,

dark controlDTTPDMATP

0.3

0.2

0.1

0.0

600500

peak only

raw data(OA, 0 min)

baseline

Wavelength (nm)

0.05

Ab

s

Wavelength (nm)

(a)

(c)

(e)

(d)

Ab

s

0 min 3

5 10

600550500

0 min310

600550500

OA,

dark control

OA

600560520

OAA

bs A0

A

0.02

(b)

Figure 2.3: (a) Visible absorption of OA-CdSe/TiO2 exposed to light and H2O at 0, 3, 10

min (b) the dark control for L = OA. Spectra stacked for clarity (c) Baseline subtraction

procedure used to obtain the peak. (d) Resulting peaks after baseline subtraction for L=

OA. (e) Fraction of original peak amplitude, A/A0 versus exposure time for all ligands.

Error bars are standard deviations obtained from four separate samples for each ligand

29 structure. After light exposure, significant changes were observed in the first exciton peak, but this peak

was riding on a large rising background. For subsequent analyses, we isolated the exciton peak by

subtracting the rising background as depicted in Figure 2.3(c) to quantify this degradation. The

background was fitted to a line in the region around the peak. Figure 2.3(d) shows the results of the

baseline subtraction. To systematically track the degradation, we calculated the fraction of original peak

amplitude, A/A0. A0 is the amplitude at the peak of the baselined curve at time = 0 min, while A is the

amplitude of time > 0 min, obtained at same wavelength as A0. This ratio was calculated for each sample

first, then averaged over multiple samples (prepared identically) for each ligand to evaluate the

statistical variation between samples.

We compared the photodegradation of CdSe/TiO2 adducts before and after substitution of the

native CdSe ligands with three different ligands (L) depicted in Figure 2.1: 4-dimethylaminothiophenol

(DMATP), thiophenol (TP), and 1-dodecanethiol (DT). Figure 2.3(e) shows the calculated ratio A/A0

plotted versus exposure time of the L-CdSe/TiO2 adducts after illumination in water. These data show

that thiol-substituted ligands on CdSe are more stable than those of the starting OA-CdSe/TiO2 adducts.

A comparison of all four ligands reveals that the order of stability is DMATP > TP ≈ DT > OA.

When comparing the thiol ligands used here, somewhat surprisingly, our results show that the

shorter, phenyl-terminated ligands provide comparable (in the case of TP) or better (in the case of

DMATP) stability than the long-chain dodecanethiol. In contrast, Aldana et al. reported that for thiols

terminated with carboxylic acids, the aromatic thiol-coated CdSe QDs were less stable than aliphatic

thiol-coated ones.45 Conjugation in the molecule can provide protection against degradation, with

DMATP outperforming the rest, as measured by the noticeably smaller change in the A/A0 values.

Notably, the dimethylamino group at the distal end of the DMATP molecule provides better protection

than the similar molecule lacking this group (i.e. thiophenol).

30

(a) S(2s) (b)

300

Co

un

ts

Binding Energy (eV)

No linker

240 235 230 225

DMATP

TP

DT

OA

3x1014

2

1

0

Co

vera

ge(m

ole

cule

s /

cm2)

Figure 2.4: (a) XPS spectra of the S(2s) region of L-CdSe/TiO2. Spectrum of CdSe

functionalized on TiO2 without MPA linker is also shown in the plot to confirm

that the higher binding energy peak is from Se(3s). (b) Molecular coverage for

each ligand calculated from the Cd(3d5/2) and C(1s) intensities

31 2.3.3 Comparison of Molecular Coverages

To investigate whether there were significant differences in packing of ligand molecules on the

CdSe QDs, XPS measurements were performed on freshly prepared samples to obtain their relative

coverages on CdSe QDs. Quantitative analysis of the S and Se regions is complicated by the fact that the

S(2s) and S(2p) peaks have significant overlap with the Se(3s) and Se(3p) peaks. Figure 2.4 shows the

sulfur 2s region; here, the peak at 230 eV was assigned to Se(3s), while the peak at 227 eV is assigned to

S(2s). This assignment was verified by fact that only the peak at 230 eV was observed when CdSe was

functionalized directly on TiO2 without using MPA linker. Quantitative analysis of molecular packing

densities on nanoparticulate samples must take into account the geometric shape of the nanoparticles

and inelastic scattering taking place within the nanoparticle core and the surface ligands.20 To properly

account for electron scattering effects, we used direct numerical integration to determine the ratio of C

to Cd signal expected from QDs of 1.6 nm radius surrounded by an organic layer, including full scattering

corrections. Details of the numerical integration and the parameters used are described in the Appendix

A1 section of this thesis. Figure 2.4(b) shows the resulting molecular packing densities determined from

the XPS data. The data show that DT, TP, DMATP molecules have similar packing densities; that of OA is

somewhat smaller, likely due to the labile nature of the carboxylic acid ligands, and the unsaturated

nature of the ligand, which disrupts packing crystallinity.46 These data show that the enhanced

photostability of DMATP cannot be explained simply on the basis of molecular packing densities.

2.3.4 Photostability in Air

The enhanced stability of DMATP-functionalized CdSe/TiO2 films is also evident in air. Figure 2.5

shows A/A0 values of L-CdSe/TiO2 plotted versus exposure time. The degradation in air is slightly slower

than that in water; while the OA sample had ~60% of the original peak amplitude after 10 min exposure

in air, the same OA ligand had only ~40% of the original amplitude after the same amount of exposure

time in water. In this case, desorption of whole QDs should not occur, as samples were not immersed in

32

0.6

0.8

1

A/A

0

151050Exposure time (min)

OA DMATP TP DT

Figure 2.5: Photostability under light and ambient air. Fraction of original peak

amplitude, A/A0 plotted versus exposure time for all ligands. Error bars are standard

deviations obtained from four to five separate samples for each ligand. Inset:

photograph of samples after about two weeks being left out on the bench top.

33 liquid, therefore the photodegradation measured was from corrosion of the QD itself. The photo

degradation effects can also be seen visually in a photograph of the CdSe/TiO2 films that have been

functionalized with the ligands in this work exposed to ambient lighting for about two weeks (Figure 2.5

inset). After several days, only the DMATP-capped CdSe/TiO2 has retained its dark orange colour, the

films capped with other ligands became lighter in colour, turning from orange-red to pale-orange. The

enhanced stability of QDs that are capped with DMATP compared to other ligands is readily visible.

2.3.5 Photoluminescence

To understand the origins of this stability trend, we performed photoluminescence (PL)

experiments on the CdSe QDs in solution functionalized with the different capping ligands. Figure 2.6(a)

shows the steady-state emission of the CdSe QDs, while Figure 2.6(b) shows the transient luminescence.

Figure 2.6(a) shows that the thiol-capped CdSe QDs has much smaller fluorescence (~100-fold reduction

in intensity) compared with the OA-terminated QDs (note also the scale change, as the signal from the

OA-capped QDs has been reduced 10-fold). Thiol-capped CdSe QDs quenches the PL, as seen and

investigated by many others,47-51 and is thought to be due to hole transfer to the thiol end group.48,52

Trapping of holes in the ligand is a non-radiative pathway, therefore as the propensity of hole transfer

increases, PL quenching increases. The extent of quenching by capping ligand also agrees the

photostability trend; ligands that trap holes more efficiently lead to increased CdSe stability. When the

holes are pulled away from the CdSe into the ligand, oxidation to the CdSe itself is prevented. With the

DMATP ligand, we observed a > 3000-fold reduction in PL signal as compared to the OA-capped CdSe

QDs.

To better understand the PL dynamics, we also performed time-resolved PL on the ligand

functionalized CdSe QDs in solution using 450 nm excitation from a pulsed laser (~3 ns pulses, 20 Hz).

The PL decay from the CdSe QDs was observed to be multi-exponential, consistent with prior studies.53,54

Our data were fit best to a biexponential function, consistent with prior reports of the nanosecond

34

Ligand A1 τ1 (ns) A2 τ2 (ns) <τ> (ns)

OA 0.580 ± 0.005 9.4 ± 0.1 0.462 ± 0.006 41.3 ± 0.4 34.2 ± 4.5

DT 0.661 ± 0.004 7.7 ± 0.1 0.392 ± 0.004 48.8 ± 0.5 40.2 ± 5.6

TP 0.756 ± 0.007 6.5 ± 0.1 0.290 ± 0.007 39.8 ± 0.1 29.9 ± 2.2

DMATP 0.977 ± 0.005 3.9 ± 0.1 0.098 ± 0.004 32.2 ± 1.5 16.7 ± 4.5

Figure 2.6: (a) Steady-state and (b) transient PL of L-CdSe in chloroform.

Ligand concentration was 35x of QD (c) Bi-exponential fits from TR-PL

Inte

nsi

ty

700650600550Emission

wavelength (nm)

OA (x0.1)DTTPDMATP

2x106

(a) (b)

0.1

1

No

rmal

ized

sig

nal

40200

Time (ns)

(c)

35 dynamics being controlled by two primary populations of trap states.51,53,54 For the OA-capped QDs, we

found a short time constant of ~8 ns and a longer time constant of ~40 ns. Results of the fits are shown

in Figure 2.5(c). Both the amplitude and the time constant of the PL transients changed after ligand

modification. The short time constant decreased slightly with DT, more with TP, and then DMATP with

the shortest time constant of ~4 ns. We interpret these changes as reflecting the dynamics of hole

transfer from the QD to the ligands. Fast hole transfer from CdSe QDs has also been observed with

similar nitrogen and sulfur containing aromatic molecules.55,56 Burda et al.55 found that hole transfer

from CdSe QDs to 4-aminothiophenol occurs in 3 ps, while Huang et al.56 used phenothiazine and

measured hole transfer of 300 ps to 40 ns depending the ligand concentration. Due to the resolution

limit of our instrument, we cannot precisely determine the rate of transfer in our DMATP-CdSe system

except that it must be faster than 3 – 4 ns.

2.3.6 DFT Calculations

The above results demonstrate that all thiol groups strongly quench the luminescence from the

CdSe QDs, suggesting that hole transfer from CdSe to the thiol group of the molecule is facile for all

three thiols investigated. However, DMATP is unique in its ability to reduce photodegradation. To help

understand this phenomenon, we used density functional calculations to help characterize the system.

Using DFT calculations on the free ligands and using Koopmans' Theorem57,58 to relate the ionization

potential to the energy of the highest-occupied molecular orbital, we estimate ionization potentials of

6.3 eV for butanethiol, 6.1 eV for thiophenol, and 5.3 eV for DMATP; this shows that DMATP has the

largest driving force for injection of electrons into the excited CdSe QD. Because prior work indicated

that the resulting holes trapped on the interfacial S atoms induces disulfide formation and subsequent

desorption of the ligands,45 we also performed calculations on a Cd6Se6 cluster with the thiol ligands

attached. In these calculations the Cd and Se atoms were terminated with H atoms except for one

exposed Cd-Se pair whose local geometry mimicked that of the non-polar CdSe( 0211 ) surface, the

36 lowest-energy face of bulk CdSe.59 Energies were calculated for the molecule-surface cluster in neutral

form, the cation in the neutral-optimized geometry, and for the fully relaxed cation. While these clusters

are too small to adequately represent the electronic structure of the CdSe QDs, previous studies have

shown that clusters of similar size adequately represent trends in ligand binding energies.60 Using the

Natural Bond Orbital (NBO) analysis we determined the natural charges associated with the individual

atoms, which allows us to determine how much of the charge was localized on the molecule. Figure

2.7(a) shows the optimized molecular structure for DMATP molecule on the Cd6Se6 cluster. It is notable

that the geometry around the N atom is locally planar instead of pyramidal, in agreement with previous

studies.61,62

We calculated the charge distribution on the molecule-cluster adduct in the neutral state and

then for the cation, in both the sudden limit (i.e. using the neutral geometry) and in the adiabatic limit

(after full relaxation of the cation) for butanethiol, thiophenol, and DMATP. Calculations in the adiabatic

and sudden limit yielded similar values. Using the NBO analysis on the neutral and cation for each

molecule-Cd6Se6 complex, we determined the charge on the molecule (including the S atom linker) and

also the amount of charge on the S atom alone. Figure 2.7(b) summarizes these calculations. For

butanethiol (a mimic for dodecanethiol), only ~0.22 of the total +1 charge is on the molecule. More

importantly, however, is that most of that charge is localized on the S atom of the thiol linker. In

contrast, TP and especially DMATP have both a larger fraction of the charge localized on the molecule,

and yet have a smaller total charge on the S atom. Thus, in addition to being a more effective electron

donor to the CdSe, the conjugated linker DMATP is also more effective at removing the charge away

from the oxidation-sensitive thiol group.

2.4 Discussion

While surface-bound ligands are widely known to play an important role in QD photostability,

the links between ligand structure, photocorrosion, and optical properties are complex and not yet fully

37

Figure 2.7: (a) Energy-minimized structure of DMATP on Cd6Se6 cluster. (b) Results of NBO

analysis of charge distribution on the molecule and on the linking S atom.

38 understood.45,63-65 When CdSe is optically excited, the holes can oxidize surface atoms from Se2- to

elemental Se0 in the presence of water,66 or SeO2 in the presence of air.20 While photooxidation can be

problematic for QDs in applications such as fluorescence imaging, the problems are particularly acute in

structures such as QD-sensitized solar cells1-3 because in solar cells the excited electron is transferred

from the QD into an electron acceptor, leaving the QD overall positive charged and therefore

particularly susceptible to oxidation.

Previous studies have shown that densely packed ligands can passively stabilize QDs against

photooxidation by preventing diffusion of oxygen and/or water to the nanoparticle surface, thereby

helping to prevent formation of higher oxidation products.45,65 Ligands can also play an active role by

donating electrons to fill holes in the QD valence band, shutting off the pathway for radiative bandgap

photoluminescence47 and eliminating the driving force for photocorrosion of the QD core. However,

even in this case oxidation of the thiol group or other components of the ligand may lead to their loss

from the surface. Aldana et al.45 reported that photooxidation of ligand-modified QDs is initiated by

diffusion of oxygen through the molecular layer to the QD core, where they oxidize the thiol head

groups to disulfides that are then released from the surface. Based on this and other studies, it can be

inferred that stabilization of the CdSe QDs requires four criteria: (1) stable binding of the ligand to the

QD, (2) tight packing of the molecular chains to prevent diffusion of oxygen and water to the QD core,

(3) the ability to inject electrons from the ligand into the QD, and (4) the ability to stabilize positive

charge on the ligand in a manner that does not lead to subsequent oxidation of the ligands.

Our experiments demonstrate that the first criterion is met by using thiols. This conclusion is in

agreement with previous investigations of molecular layers bound to CdSe via amines,48,59,60,67 carboxylic

acids,59,60 phosphine oxides,59,60,67 phosphonic acids,59,68,69 dithiocarbamates70 and thiolates.45,51,67 These

prior studies have generally found amines and carboxylic acids are among the weakest, while

phosphonic acids and thiols are the strongest ligands to CdSe QDs.

39 The most significant result of our work lies in our demonstration that short aromatic thiols such

as DMATP can be highly effective at reducing photodegradation of CdSe. This result is surprising in light

of studies by Aldana et al.,45 who concluded that CdSe QDs capped with aromatic thiols were less stable

than those capped with aliphatic thiols. Our studies show that the nature of the substituent groups on

the aromatic ring play a very important role, evidenced by the fact that dimethylaminothiophenol

(DMATP) is significantly more effective than the parent thiophenol molecule. The apparent discrepancy

between our results and those of Aldana et al. can be resolved by noting that in Aldana's studies,45 4-

mercaptobenzoic acid (MBA) was the only aromatic thiol investigated. However, MBA contains an

electron-withdrawing carboxylic acid group in the para position, which makes MBA a poorer electron

donor than thiophenol.71 Thus, while MBA may be poorer than alkyl thiols at reducing photocorrosion,

replacing the electron-withdrawing carboxylate group with an electron-donating group such as the

dimethylamino group of DMATP greatly enhances the photostability and achieves nearly complete

quenching of the luminescence. Our data show that while thiophenol reduces the CdSe fluorescence

intensity by a factor of 100, DMATP reduces the fluorescence intensity by a factor of >3000, to below

our detection limit. Thus, we conclude that DMATP is a significantly more effective electron donor than

thiophenol.

In addition to the electron-donating ability of the ligand, the location of the residual positive

charge can also play an important role in the resulting photostability because oxidation of S atoms leads

to formation of disulfide linkages and desorption of the molecular layers, thereby leaving the

nanoparticle core exposed to subsequent degradation.45 Our computational results in Figure 2.7 show

that in addition to DMATP being a highly effective electron donor, only a small fraction of the resulting

positive charge remains localized on the oxidation-sensitive S atom. The ability to delocalize the

resulting positive charge after electron donation is likely an important factor affecting the photostability

of ligand-modified CdSe QDs. The relative importance of the electron donation vs. electron

40 delocalization can also be qualitatively estimated using Hammett constants σp and σp

+, the former

reflecting substituent-induced changes in electron donation in the absence (σp) and presence (σp+) of

resonance stabilization. For -N(CH3)2 they are quite different (σp 0.83, σp+ 1.70).71,72 The large

negative value of σp+ for -N(CH3)2 shows that it is a strong electron donor, while the large difference

between σp and σp+ shows that -N(CH3)2 substantially enhances resonance stabilization of the DMATP

cation.

The ability to achieve corrosion protection using short, conjugated ligands is important because

long alkyl chains are expected to prevent facile electron transfer and are therefore not likely to be

suitable for applications such as nanocrystal- or QD-based thin film optoelectronic devices73,74 that

require electronic communication between QDs. Short, conductive ligands are needed to increase

coupling from QDs or nanocrystals.75-79

2.5 Effect of DMATP in a Liquid Junction Solar Cell

To test whether DMATP functionalized QDs would still enhance the photostability of CdSe in a

more practical system, we fabricated liquid junction CdSe sensitized TiO2 solar cells with a Co(II/III)

bipyridyl redox mediator and a platinized FTO counter electrode. We compared the DMATP modified to

the as prepared OA-CdSe/TiO2 sample. The DMATP modified solar cell was observed to have a lower

photovoltaic performance than the OA-CdSe/TiO2 cell. To compare their performance stabilities, we

measured the short circuit photocurrents as a function of time under chopped light illumination from a

solar simulator (100 mW/cm2, AM 1.5G). The DMATP cell started out at low currents that gradually

increased, while the OA cell had a steady current that eventually decreased. We discovered, by

disassembling the cell and performing FTIR experiments, that the DMATP ligands had desorbed from the

surface of the cell after the photocurrent-time scans. The observation of ligand desorption from CdSe

QDs indicated that a thiol ligand may not bind strongly enough in a liquid-junction type cell. Perhaps this

41 ligand passivation idea would be more useful in a solid-state solar cell such as a CdSe-polymer hybrid

cell.

2.6 Extending the DMATP Passivation Method to PbS and PbSe QDs

PbSe (QD diameter = 5.2 nm) and PbS (QD diameter = 6.1 nm) were synthesized following

procedures published by others.80-82 Size and concentration of these QDs were determined from the

wavelength and the absorbance of the first exciton peak.83-85 For photostability experiments, we note

that FTO glass does not transmit in the near infrared (NIR) region, so TiO2 nanocrystalline films were

prepared on clean pieces microscope glass to enable transmission absorption experiments. We show in

the FTIR spectra in Figure 2.8(a) that DMATP can also bind to PbSe QDs. Figures 2.8(b) and 2.8(c) show

NIR absorption spectra of samples exposed to light and air at various time points of OA- and DMATP-

PbSe/TiO2 respectively. Figure 2.8(d) shows a plot of fractional change in the peak absorbance (of the

first exciton) as a function of exposure time. Surprisingly, in contrast to the CdSe case, the DMATP

capped PbSe was observed to have worse photostability than the OA capped. Similar experiments were

performed on PbS QDs; we also found no enhanced stabilization of DMATP ligands functionalized on PbS

QDs (as compared to the OA functionalized ones). The contrast in the DMATP-QD stability of the PbSe

and PbS cases vs. the CdSe case can be explained when we consider the valence band (VB) positions of

the QDs. The highest occupied molecular orbital (HOMO) level of DMATP was calculated to be -5.3 eV

vs. vacuum (see section 2.3.6 above). The VB position of a 3-nm small CdSe QD is ~ -5.5 eV. Therefore

the DMATP ligand is able to accept holes from the CdSe QDs. However, the VB position of PbSe at a

diameter of 5.2 nm is ~ -5 eV, and VB position of PbS at a diameter of 6.1 nm is ~ - 4.9 eV.86 In this case,

DMATP ligands do not have enough energy to accept the holes generated by the PbSe and PbS QDs. We

show that ligand passivation via hole transfer is also very dependent on the band position of the QD,

and a different ligand design is needed to better match the ligand HOMO level to the VB position of the

QD.

42

Figure 2.8: (a) IR spectra of OA- and DMATP-PbSe/TiO2 (b) – (c) Photostability of OA-

and DMATP-PbSe/TiO2. Transmission absorption spectra after 0, 10, 20, and 30 min

exposure to air and light (d) Fractional change in peak absorbance of OA, and

DMATP-PbSe/TiO2 as a function of time

0.1

Inte

nsi

ty

3200 2800Wavenumbers (cm-1)

1600 1200

OA

DMATP

PbSe-TiO2(FTO)

0.01

Ab

s

200016001200

Wavelength (nm)

OA 0 min102030

0.02

200016001200

DMATP

-0.8

-0.6

-0.4

-0.2

0.0Fr

ac. c

han

ge in

p

eak

abs

3020100Time (min)

OADMATP

Wavelength (nm)

(a)

(b) (c) (d)

43 2.7 Conclusions

Our results show that aromatic ligands bearing electron-donating substituents can provide

excellent protection of CdSe QDs against water and air oxidation during illumination when

functionalized on a QD sensitized TiO2. By stabilizing the sulfur end of the molecule with electron

donating groups, these small molecules can shuttle holes quickly and away from the thiol and the QD

surface, thus inhibiting oxidation of the QD and also providing protection for the interfacial thiol linker.

However, we found that DMATP ligands will desorb from the CdSe surface after a short cycling of a

liquid-junction solar cell. Lastly, in utilizing this ligand passivation idea, the positions of the ligand HOMO

level and QD VB must be such that the ligand is energetically able to accept the holes from the QD.

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50

Chapter 3

Photostability of CdSe Quantum Dots Functionalized with Conjugated

Dithiocarbamates

3.1 Introduction

Chalcogenide quantum dots (QDs) have the potential as the photoactive material in sensitized

solar cells1 and other optical devices2-4 but are prone to photodegradation from water and air.5-8 Various

conventional methods have been utilized to passivate the surface, including large-bandgap shells,9,10

long hydrophobic hydrocarbon chains11 and organic polymer encapsulation.12,13 When using organic

ligands to passivate QDs, short conductive ligands are more desirable for QD applications that require

electrical transport such as in photodetectors, photovoltaic devices, and QD-LEDs.14,15 Conductive

organic ligands significantly affect the QD surface electronic structure16-18 and could play a significant

role in controlling the QD photostability.19

Of the conventional ligands for synthesis and manipulation of QDs, thiols are widely used, as

thiols thermodynamically bind more strongly than other common ligands such as carboxylic acids,

phosphonic acids, and amines.20 However, thiols still have inherent problems with photooxidation21 and

labile surface chemistry.22 Bidentate ligands with two chelating sulfur groups are promising functional

groups enhance the binding to QDs,23,24 one of which is the dithiocarbamate (DTC) group (R-NHCS2-)

Although bidentate ligands are expected to bind more strongly to the QD, there is some evidence that

bidentate binding may not necessarily lead to more stable QDs;21,25 more studies are needed to

determine whether there is a relationship between chelating ligand attachment and QD photostability.

Current methods to attach DTC ligands to CdSe QD surfaces involve the separate synthesis of

the ligand molecule or a purification step of the QDs.17,26,27 DTC molecules have been functionalized in-

situ on gold surfaces, under mildly basic conditions.28,29 In this chapter, we discuss the successful in-situ

51 functionalization of DTC molecules on CdSe-TiO2 films, and the utilization of this chemistry to explore

the effect of this bidentate DTC group attachment on QD photostability. With a series of para-

substituted conjugated DTC ligands, we show the increase in QD photostability trended with the

increase in electron-donating ability of the substituent. Finally, we compare the effect of the bidendate

DTC vs. a monodentate thiol on QD photostability.

3.2 Experimental

3.2.1 Chemicals

Trioctylphosphine oxide (TOPO, 99+%), CdO (99.99+% metal basis), oleic acid (OA, 90%),

trioctylphosphine (TOP, 90%), selenium, (99.99% metal basis), 4-(trifluoromethoxy)aniline( 98%), aniline

(≥99%), p-anisidine (≥99%), N,N-dimethyl-p-phenylenediamine (97%), carbon disulfide (anhydrous,

≥99%), triethylamine (TEA, ≥99.5%),and 4-methoxythiophenol (97%) were purchased from Sigma-

Aldrich and used without further purification.

3.2.2 Preparation of TiO2 Nanocrystalline Films

Fluorine-doped tin oxide (FTO) coated glass (Hartford glass) was cleaned with detergent,

acetone, and ethanol. Porous anatase nanocrystalline TiO2 films were then screen-printed onto the FTO

glass from a commercially available paste (Solaronix T/SP). Each pass of the screen-printing process

produced layers of approximately 1 μm thick. The screen printing process also produced multiple

samples of the same thickness and mesoporous structure, and each experiment in this work was

conducted with the same batch of films. After the screen-printing step, the TiO2 films were then

annealed in air using a recipe adapted from literature:30 at 325 C for 5 min, at 375 C for 5 min, at 450

C for 15 min, and finally at 500 C for 30 min. Before use, the films were cleaned by heating them at

500 C for 15 min in air to remove any additional organic contaminants. For XPS and FTIR studies, thin

films from one screen-print pass were used. Thicker films resulted in interference effects in the FTIR

spectra. For photostability studies with UV-visible spectroscopy, films from two passes were used; the

52 higher optical densities obtained resulting from increased CdSe QD loading gave better comparisons

among ligands.

3.2.3 Synthesis of CdSe QDs

The QDs were synthesized using a procedure adapted from Peng and Peng.31 First, 3.1 g TOPO,

0.24 g CdO, and 1.9 g OA were combined in a three-necked flask and heated in an Ar atmosphere until

the mixture turned optically clear (~290 C). The mixture was allowed to cool until ~250 C and a

solution of TOPSe (made from 1.2 mL TOP and 0.045 g Se) was quickly injected. The temperature

dropped to 220-230 C after injection and the formed QDs were allowed to cool to ~80 C. The QDs

were quenched with toluene and purified four times by precipitation using methanol and centrifugation.

The size and concentration of the QDs were determined by UV-visible spectroscopy from the

wavelength and absorbance of the first exciton peak, using empirical relationships established by Yu et

al.32 Typically, the QDs from this synthesis procedure were 3 – 3.2 nm in diameter.

3.2.4 CdSe-TiO2 Preparation and Ligand Modification

The CdSe QDs were attached to the TiO2 by directly immersing a clean TiO2 film into a solution

of QDs for 24 h. Although using a linker may give more control over QD coverage, studies have shown

reduced electron transfer rate from the linked heterostructures.33-35 Furthermore, commonly used

linkers like mercapto-alkanoic acids contain sulfur and the lack of these sulfur containing linkers on our

CdSe-TiO2 samples made FTIR and XPS characterization of the DTC functionalization more convenient.

To prepare the CdSe-TiO2 surfaces the TiO2 films were immersed in 15 – 25 μM (defined as moles of

CdSe QD per liter) solution in dichloromethane (DCM)33 for 24 h, then rinsed with DCM and dried with

N2. To further functionalize these films with DTC molecules, the films were immersed in a stirred, Ar-

degassed solution of 50 mM R-aniline, 50 mM CS2, and 50 mM TEA in methanol for 4 h in the dark. They

were then rinsed thoroughly with methanol and dried. To functionalize the CdSe-TiO2 with 4-

53 methoxythiophenol (MeO-Ph-SH), the films were similarly immersed in an Ar-degassed 50 mM solution

of MeO-Ph-SH in methanol for 4 h, then rinsed with methanol and dried.

3.2.5 Fourier-Transform Infrared Spectroscopy (FTIR)

FTIR characterization of the functionalized CdSe-TiO2 films was performed in single bounce

reflection mode with a Vertex 70 (Bruker) with p-polarized light at an incident angle of 50o from sample

normal. An identically prepared bare TiO2 film served as the reference spectra for each experiment, and

the functionalized samples were measured immediately after the reference. This method minimized the

effect of atmospheric water and CO2. Reference spectra of the parent aniline were measured in

transmission mode on ZnSe plates. All measurements were done at a resolution of 4 cm-1.

3.2.6 X-ray Photoelectron Spectroscopy (XPS)

We performed XPS measurements with a custom-built XPS system (Physical Electronics) with an

Al-Kα source (Model 10-610, 1486.6 eV photon energy), toroidal monochromator (Model 10-420), and

hemispherical analyzer with a 16 channel detector array (Model 10-360). Measurements were done at

an electron take-off angle of 45 with a resolution of 0.1 eV. The resulting XPS peaks were fit to a Voigt

function to obtain peak areas.

3.2.7 Water Photostability Studies

Samples were sandwiched into a custom-made cell with a Teflon spacer (127 μm thick) and

another piece of FTO. The open region as defined by the spacer was filled with 18 MΩ H2O (Barnstead

Nanopure). The cell has holes to allow for light penetration and absorption measurement without

disassembly. Samples were illuminated through the FTO piece and water with light from a solar

simulator after passing through a filter that only allows transmission of wavelengths longer than 475

nm. The solar simulator (Newport 91160) was equipped with AM1.5G filters and set to 100 mW/cm2 (as

measured by a Scientech calorimeter). The light after the filter was measured to be 81 mW/cm2. We

used the filter to ensure that the light could only be absorbed by the CdSe QDs, and not the TiO2.

54 Transmission measurements using a UV-visible spectrometer (Shimadzu UV-2401PC) were taken at

various time points of light exposure.

3.2.8 Photoluminescence (PL)

We prepared solution-based QDs functionalized with DTC ligands for the PL experiments. The

CdSe QDs were precipitated, centrifuged and resuspended in chloroform. R-aniline and CS2 were added

into the solution at a concentration of 50 mM each and the entire mixture was stirred in the dark for 4

h. To get rid of excess starting reagents, the QD mixture was precipitated with methanol, and

resuspended with chloroform. PL experiments were done at a concentration of 1.5 μM with a

flourometer (ISS K2) with 450 nm as the excitation wavelength.

3.3 Results

3.3.1 FTIR and XPS Characterization

As a start, we functionalized the CdSe-TiO2 surfaces using an aniline with a fluorine containing

substituent: OCF3-aniline. The CF3 group serves as a molecular tag since it has relatively unique

absorption features in the IR at ~1200-1300 cm-1 and the F(1s) region in the XPS also does not overlap

with any other regions present on the spectra. Figure 3.1 shows IR and XPS spectra of the resulting

functionalization of DTC after a 4 h reaction along with the proper control experiments. The unmodified

sample shows carboxylate stretches at 1535 and 1412 cm-1 resulting from oleate capped QDs.36 When

the CdSe-TiO2 sample was exposed to both OCF3-aniline and CS2 the carboxylate peaks disappeared and

we observed new peaks at 1510, 1271, and 1227 cm-1. As a reference, the IR spectrum of neat OCF3-

aniline is included. Prominent peaks of this neat liquid occur at 1350 – 1150 cm-1 and 1510 cm-1,

characteristic of the –CF3 and the aromatic C=C stretches respectively. The peaks of the reacted samples

correspond to those of the parent compound, with a slight change of the peak shape in the CF3 region.

Leaving out the CS2 showed that the aniline compound itself does not bind onto the CdSe surface, as

shown in the IR of ‘No CS2’ sample. Exposure of reactants to a bare TiO2 surface also resulted in no

55

F(1s) Se(3s) andS(2s)

1800 1600 1400 1200 1000

Neat OCF3-aniline

Anil+TEA+CS2

No TEA

No CS2

unreacted

Wavenumbers (cm-1)

0.05

Inte

nsi

ty

(a)

Co

un

ts

695690685

Anil+CS2+TEA

No TEA

No CS2

unreacted

500

200

235 230 225

Binding Energy (eV)

200

404 402 400 398 396

N(1s)(b) (c)

(d)

Figure 3.1: (a) FTIR of CdSe-TiO2 samples functionalized with OCF3-aniline, CS2, and

TEA along with the relevant controls. A spectrum of the parent aniline is included as

reference (b) – (d) XPS scans of the F(1s), Se(3s)/S(2s), and N(1s) regions of the

unreacted, and the OCF3-aniline, CS2, and TEA functionalization with the controls of

no CS2, and no TEA added

56 binding (not shown), indicating that the DTCs were bound only to the CdSe surface. Surprisingly, we

found that the TEA base was not needed for functionalization. This result, in which the TEA base was left

out in the reaction mixture, is shown in Figure 1(a) as the ‘No TEA’ sample. In fact, leaving out the base

gave more intense peaks, indicating more ligands attached to the surface.

XPS results gave further confirmation of successful reaction. Figures 3.1(b), (c), and (d) shows

the F(1s), Se(3s)/S(2s), and N(1s) spectra from the functionalization. F(1s) was only present when both

OCF3-aniline and CS2 are present. S(2s) and Se(3s) signals occur in the same region and gave overlapping

peaks. The unmodified sample (which contained no sulfur) yielded the binding energy (BE) of the Se(3s)

was at 229.5 eV. The other peak at 226.8 eV was therefore assigned to the S(2s) as this peak only

occurred when samples were exposed CS2. The N(1s) region was riding on the tail edge of the large

Cd(3d5/2) peak, resulting in a rising background. However, there was a clear nitrogen peak in each of the

‘No TEA’ and the ‘Anil+CS2+TEA’ peaks. Peak area ratios of the F(1s), S(2s), and N(1s) could indicate the

structure of the molecule at the surface after correcting the values to their respective atomic sensitivity

factors. The elemental ratios of the dithiocarbamate molecule derived from OCF3-aniline (OCF3-Ph-DTC)

are F : N 3 : 1 and F : S 1.5 : 1. For the ‘No TEA’ sample, peak area ratios were F : N 3.2 : 1, and F:S

1.3 : 1, indicating good agreement with the OCF3-Ph-DTC.

The sample that was exposed to the TEA base (‘Anil+CS2+TEA’) gave less absolute fluorine signal

than when the base was excluded, similar to what was observed in the FTIR results. The F/Cd peak area

ratio was 0.4 for the sample without TEA exposure but was only 0.07 when TEA was included as a

reagent. The calculated F to N ratios for the ‘Anil+CS2+TEA’ sample yielded 1.2 : 1. These elemental

ratios indicate excess nitrogen on the surface. The observation of excess nitrogen could mean that some

TEA base is also binding and consequently is blocking functionalization of the DTC. This functionalization

condition differs from the typical organic synthesis of DTC molecules, in which basic conditions is

required.24,37

57

Figure 3.2: (a) FTIR of grafting kinetics at 15 min, 4, and 17 h. A spectrum of the parent aniline is

included as reference (b) Transmission absorption spectra at these reaction times (c) The CF3

peak at 1270 cm-1 was fit to a Voigt function after a linear baseline subtraction. (d) Transmission

absorption spectrum of the functionalized sample was fit to a Gaussian peak after a cubic

baseline subtraction. (e) Plot of FTIR peak area as a function of reaction time. Also plotted is the

fraction A/A0 vs. reaction time. A is the Gaussian peak amplitude derived from the transmission

absorption spectrum after reaction, while A0 is the peak amplitude from the same sample before

reaction

15 min

4 h

17 h

ref

0.05

1600 1400 1200 1000

Inte

nsi

ty0.025

600550500

Ab

s

0.02

1350 1250 1150

Raw dataFit

Voigtpeak

Linear baseline

0.02

600550500

Wavelength (nm)

Gaussian peak

Cubicbaseline

Raw dataFit

Ab

s

Inte

nsi

ty

Wavenumbers (cm-1)

Wavenumbers (cm-1) Wavelength (nm)

4

3

2

1Pea

k A

rea

(arb

. un

its)

151050Reaction time (h)

1.0

0.9

0.8

A/A

0

IR peak area

A/A0

(a) (b)

(c) (d)

(e)

A

58 Because of the small diameter of the spherical nanoparticle and because the electron escape

depths are comparable to the nanoparticle diameters, quantitative analysis of the XPS data requires

accounting for the shape and size of the nanoparticles. We previously used numerical integration for an

organic ligand shell surrounding a spherical particle with radius of 1.6 nm to properly account for

electron scattering of this geometry.19 In this work, we have adapted it to include a sulfur head group as

an interface between the organic ligand and CdSe core (see Appendix section A1 of this thesis for more

details). Molecular coverage of the ‘No TEA’ sample calculated using the S/Cd ratio was 1.7 x 1014

molecule/cm2. This sample, in which the TEA was left out of the functionalization procedure, was the

one that gave the correct stoichiometric ratios corresponding to the formed DTC molecule.

For grafting kinetics, similar functionalization conditions were used (OCF3-aniline and CS2, 50mM

each in methanol) but were allowed to react for times ranging from 15 min to 17 h before rinsing the

sample with methanol and dried. FTIR and UV-visible spectroscopy measurements were used to track

the extent of reaction. Figures 3.2(a) shows FTIR spectra of functionalized films at selected times. A trace

of the parent OCF3-aniline compound is also included as reference. Figure 3.2(b) shows the visible

absorption spectra of the same samples. For quantification, we used one of the CF3 IR stretches at ~1270

cm-1 and plotted the peak area as a function of reaction time. As shown in Figure 3.2(c), the peak area

was fitted with a Voigt function with a linear baseline. Surprisingly, instead of the saturation behaviour

as observed when all the possible sites on the surface are filled up, we found an increase first, then a

decrease. The decrease indicated that some etching of the DTC molecule at long reaction times, which

was also observed from the absorption spectra of the functionalized samples. We quantified the visible

absorption feature of the QD by extracting the peak height of the first exciton peak. The raw absorption

spectrum was fitted to a Gaussian with a baseline as shown in Figure 3.2(d). A cubic function was used

to fit the baseline as done by others.38 We then took the fraction of the amplitude derived from the

Gaussian fit, A, from the value (of the same sample) before reaction, A0, to obtain the fraction remaining

59

Figure 3.3: Functionalization of R-Ph-DTCs, R = H, MeO, and NMe2.

The parent aniline compound is included in grey as reference

Inte

nsi

ty

1800 1600 1400 1200 1000Wavenumbers (cm-1)

0.01

0.02

Inte

nsi

ty

1800 1600 1400 1200 1000

Wavenumbers (cm-1)

0.02

Inte

nsi

ty

1800 1600 1400 1200 1000Wavenumbers (cm-1)

60 or A/A0. The peak height, A was normalized to the peak height of the same sample before reaction, A0.

We plotted the A/A0 values as a function of time along with the FTIR peak area data in Figure 3.3(e). The

A/A0 values were observed to follow the same trend as the IR peak areas. At time < 4 h, the A/A0

increased as the peak evolved into the red-shifted DTC-QD. At optimal functionalization time (time=4 h),

the A/A0 became ~1. At longer times, the peak height decreased which might mean that some etching of

the QDs had occurred.

We further extended this surface functionalization to other para-substituted phenyl groups, R-

Ph-DTC, where R is either H, MeO, or NMe2. FTIR spectra of these samples are shown in Figure 3.3. All

reactions were done at the same concentrations (50 mM each of R-aniline and CS2 in methanol) without

the addition of TEA base, and at reaction time of 4 h. As observed from conducting DTC grafting kinetics

with R = OCF3 (discussed above), the DTC functionalization was optimal at this reaction time.

3.3.2 Water Photostability of R-Ph-DTCs Functionalized CdSe-TiO2

With the series of para-substituted R-Ph-DTC ligands on CdSe-TiO2, we investigated the

photostability of the QDs in water. Figure 3.4(a) shows the evolution of the transmission absorption of

the OCF3-Ph-DTC functionalized sample for up to a total of 10 min of light and water exposure. At 0 min,

we observed a peak at 559 nm corresponding to the first exciton peak. After exposure to water and

light, the peak shifted, broadened and its amplitude decreased, indicating photodegradation to the QDs.

Exposure to light was needed to induce this degradation; no change was observed in a control sample

kept in the dark, as observed previously.19 To compare ligand effects in QD photostability, we extracted

the amplitude of the first exciton peak to quantify the rate of degradation using the same procedure

described above. The A0 used was the value at time = 0 min. Figure 3.4(c) shows the plot of A/A0 values

over exposure time for each set of ligand functionalized CdSe. The error bars are standard deviations

from four equivalently prepared samples to account for sample scatter. Included in this plot is a

comparison to OA and an (OA) dark control. OA sample exhibited the worst photostability in water of all

61

Figure 3.4: Water photostability data of CdSe-TiO2 functionalized with R-

Ph-DTCs. (a) Raw absorption spectra of the R = OCF3 samples after

exposure to light and water for 0, 2, 5, and 10 min. (b) Fitting procedure

of the absorption spectrum to obtain the Gaussian amplitude (c) A/A0

plotted as a function of exposure time of the OA, and R= OCF3, H, MeO,

and NMe2. Also plotted is a dark control (with the OA ligand

functionalized sample)

0.4

0.3

0.2

Ab

s

600550

00 min020510

1.0

0.8

0.6

0.4

A/A

0

1050Time (min)

(a) (b)

(c)

NMe2

Raw data (0 min)Fit

0.4

0.2

0.0

600550500Wavelength (nm)

Gaussian peak

Baseline (cubic)

62 of the ligands studied here. We found significant differences in QD photostability when we vary the

para-substituents of the R-Ph-DTC functionalized CdSe QD. Of these samples, the stability trend of the

samples of functionalized with R-Ph-DTCs is as follows: R = NMe2 > MeO > H ≈ OCF3. The increase in

electron donating ability of the R group was observed to increase the photostability of CdSe QDs. In the

time scale of this 10 min long photostability experiment, the photostability of the R = NMe2, most

electron donating substituent was remarkably within the error bars of the dark control.

3.3.3 Photoluminescence

We further investigated the mechanism of this increased stability. One explanation for this

stability is the hole transfers to the ligand molecule upon photoexcitation of the CdSe QD. Efficiency of

hole transfer can be measured with photoluminescence (PL) quenching experiments. This quenching has

been observed with CdSe QDs functionalized with thiols39-42 and dithiocarbamates;26,43 as photoexcited

holes are trapped in the ligand, the radiative recombination pathways of the QD decrease.44 To see

whether the photostability enhancement with electron-donating DTC ligands resulted from increased

hole transfer efficiency, we performed PL quenching experiments of CdSe QD suspended in chloroform

functionalized with R-Ph-DTCs, R = OCF3, H, MeO. For the PL experiments, we did not include the NMe2

substituent since the aniline starting reagent is visibly coloured and interfered with the PL signal from

the CdSe QDs. These experiments were done at an excitation wavelength of 450 nm. Results of these

experiments are shown in Figure 3.5. The PL spectrum of an OA-capped QD sample is also shown for

comparison. Overall, the DTC ligands quenched the PL as compared to the as-made (OA-capped) sample.

PL quenching efficiency trended as follows: R = OCF3 (amplitude of the PL peak is 63% of that of the OA)

< H (41% of OA) < MeO (1.4% of OA). To eliminate the effects of possible weakly bound anilines, we also

performed control experiments with exposure of QD suspensions to only the corresponding anilines (no

CS2 added), and they did not result in these significant changes in the PL signal. The PL trend suggests

that increasing the electron-donating ability of the DTC ligand results in the increased hole transfer

63

2x105

Inte

nsi

ty

700650600550500Emission wavelength (nm)

OA

OCF3

HMeO

R-Ph-DTC, R =

Figure 3.5: Steady-state photoluminescence

spectra of solution-based CdSe QDs functionalized

with R-Ph-DTCs

64 efficiency from the QD into the ligand. The effective hole transfer into the ligand could explain the

photostability trend, in which holes are pulled away from the CdSe, thus protecting the QD core.

3.3.4 Photostability of DTC vs. Thiol Bound CdSe-TiO2 Surfaces

Increased stability also requires a strong and stable binding group. The most common binding

mode of the DTC group to transition metal ions is the bidentate configuration.37 Since the two sulfur

groups can bind to the CdSe surface in a bidentate fashion, DTCs can potentially bind more strongly than

thiols, the more commonly used head group used in QD ligand chemistry. Although the NMe2

substituent gave the best stability, this ligand also absorbs in the same region as the CdSe first exciton

peak and could interfere with comparative experiments with its corresponding thiol. Therefore, to make

the best comparison, we chose the thiol and DTC ligands with the MeO substituent. Both molecules lack

absorption features in the 475 – 600 nm region. Similar water photostability experiments were

performed as depicted and described in Figure 3.4, with exposures up to 20 min. Figure 3.6(a) shows the

A/A0 values over exposure time. The DTC functionalized CdSe was observed to be more stable than the

thiol. To investigate whether this effect simply resulted from molecular packing differences in the thiol

and DTC, XPS was performed. From XPS, we obtained 1.9 DTC molecules and 2.3 thiol molecules per nm2

of each ligand-QD surface. This shows that there were no significant differences in the molecular

packing; in fact we obtained more thiol than DTC molecules on the QD surface. We also investigated

rate of ligand loss over time with FTIR to determine whether the increased stability came from the

bidentate binding of the DTC head group. Experiments in the dark showed no significant changes in

ligand coverage when exposed to water for either ligand. Figure 3.6(b) shows the FTIR spectra of DTC or

thiol bound CdSe at 0 and 20 min of exposure to water in the dark. The characteristic features of the

surface-bound ligands remained the same, indicating no significant desorption of ligands in the

timescale of the experiment. Thus, the increased stability of DTC is not due to increased packing density

65

Figure 3.6: Photostability CdSe-TiO2 bound with

dithiocarbamate vs. with thiol of the MeO substituent

(a) A/A0 plotted as a function of time of exposure to

light and water (b) FTIR of the samples before and after

20 min of exposure to water in the dark

1.0

0.8

0.6

0.4

A/A

0

20151050Time (min)

MeO-Ph-DTC

MeO-Ph-SH

Wavenumbers (cm-1)

(a)

(b)MeO-Ph-DTC MeO-Ph-SH

0 min

20 min

0 min

20 min

0.02

1600 1400 1200

0.05

1600 1400 1200

Inte

nsi

ty

66 (which would restrict access to water and oxygen to the NP-ligand interface) or due to a stronger

binding to CdSe.

3.4 Discussion

Our experimental results show a few important pieces of information. First, we show that we

can functionalize DTCs on CdSe-TiO2 surfaces in-situ. However, in contrast to previous methods on gold,

in which a base was necessary to form the DTC, we found it preferable to omit the base for CdSe.

Second, the para-substituents on the DTC molecule have a large influence on the water photostability of

CdSe QDs, with electron-donating substituents providing higher stability. Finally, a comparison of thiol

and DTC binding groups yielded higher stability with the DTC group. Surprisingly however, this enhanced

stability is not due to differences in packing density or in the binding strengths of the functional groups.

We first address the role of the amine base on DTC functionalization to CdSe surfaces.

Although the synthesis of DTC molecules is traditionally done in the presence of base as the proton

acceptor,24,45 we found that the functionalization proceeded without it, and its presence even inhibited

the functionalization on CdSe. Amines can bind onto the surface of CdSe-TiO2, and consequently there

were some competitive binding of the TEA base onto our surfaces. Our work shows that while in-situ

functionalization of surfaces is convenient, we must also make sure that none of the individual starting

reagent readily binds to the surface and interfere with the grafting chemistry.

Next, we address the CdSe photostability functionalized with DTC vs. thiols. There have been

previous studies comparing stabilities of a bidentate sulfur ligand and the monodentate, with Au

surfaces (DTC vs. thiol head group) and solution-based CdSe QDs (carbodithiodate vs. thiol head group)

that found better stability with the bidentate ligands.23,46 However, the Au work is a thermal desorption

study in an ultrahigh vacuum XPS chamber.46 The carbodithiodate vs. thiol functionalized CdSe QDs work

was performed in non-aqueous solvents23 which may not accurately represent ligand-QD behaviour in

ambient air or water. In contrast, our studies were done in the presence of water and with a TiO2

67 electron acceptor. The aqueous instability of bidentate ligands was similarly found by Aldana et al.;21 in

fact, they found worse photostablity in the dithiol bound CdSe QDs than in the monothiol bound case,

due to ligand oxidation processes that takes place in aqueous conditions. Our work show that utilizing a

ligand head group that can chelate strongly to the QD may provide a stable ligand-QD adduct, but does

not necessarily give a more water- and photo-stable QD-TiO2 system, since the ligand themselves are

also susceptible to degradation from photogenerated charges in water and/or air. It is important to not

only have ligands that bind strongly, but also have hole-trapping electronic stabilization effects, as we

will discuss next.

Although we found no significant difference in ligand desorption of light exposed CdSe-DTC vs.

the CdSe-thiol QDs, the CdSe-DTC QDs were still more photostable. Studies on molecular junctions of

DTCs on Au46,47 and CdSe43 have found increased conductivity due to the resonant stabilization nature of

the NCS2 group resulting in a better π-conjugated system (as compared to a thiol). Similarly in our

system, the increased photostability in DTC functionalization could have also resulted from an enhanced

electronic effect from the DTC binding group. We further found that the CdSe-DTC photostability

depends on the para-substituent of phenyl DTCs. Previously, we showed enhanced QDs photostability

with electron-donating aromatic thiol ligands is due to the delocalization of the photogenerated holes

into the ligand molecule.19 Here we also observed this delocalization effect from the electron-donating

DTC groups. Our work suggests the importance of ligand electronic effects that could come from both

the ligand head group as well as the ligand molecular structure on the photostability of QDs.

Lastly, we note that the interpretation of the para substituent effects in our photostability

results is inconsistent with the work of Frederick et al.48 They investigated the para-substituent effect R-

Ph-DTCs on the electronic structure of CdSe QDs and counter-intuitively found that electron-

withdrawing ligands delocalized the holes more than the electron-donating. Ligand-CdSe behaviour is

known to be highly dependent on synthesis conditions and ligand passivation.49-52 Frederick et al.’s

68 experiments and DFT calculations were performed with amine passivated CdSe QDs. We did not use

amines in our system. Furthermore, our work focused on the water photostability of CdSe (with a TiO2

electron acceptor) and its relationship to the ligand molecular structure.

3.5 Conclusions

We have successfully functionalized conjugated dithiocarbamates on CdSe-TiO2 surfaces in-situ

to enable studies in factors that control ligand-QD photostability. When we compared the stability of

these conjugated DTCs to the corresponding thiols, we showed that the DTCs are more stable, but the

stability is not from the stronger binding of the DTC head group to the CdSe. The increased stability is

likely from an enhanced electronic effect of the DTC group. Furthermore, we found that the increased

electron donating ability of the substituent led to more stable QDs with a series of para-substituted

phenyl DTC ligands, and that this effect is from the favourable transfer and delocalization of the

photoexcited holes from the QD into the ligand.

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38. Norris, D. J.; Bawendi, M. G. Measurement and Assignment of the Size-Dependent Optical Spectrum in CdSe Quantum Dots, Phys. Rev. B 1996, 53, 16338-16346.

39. Koole, R.; Schapotschnikow, P.; Donega, C. d. M.; Vlugt, T. J. H.; Meijerink, A. Time-Dependent Photoluminescence Spectroscopy as a Tool to Measure the Ligand Exchange Kinetics on a Quantum Dot Surface, ACS Nano 2008, 2, 1703-1714.

40. Liu, I. S.; Lo, H.-H.; Chien, C.-T.; Lin, Y.-Y.; Chen, C.-W.; Chen, Y.-F.; Su, W.-F.; Liou, S.-C. Enhancing Photoluminescence Quenching and Photoelectric Properties of CdSe Quantum Dots with Hole Accepting Ligands, J. Mater. Chem. 2008, 18, 675-682.

41. Munro, A. M.; Jen-La Plante, I.; Ng, M. S.; Ginger, D. S. Quantitative Study of the Effects of Surface Ligand Concentration on CdSe Nanocrystal Photoluminescence, J. Phys. Chem. C 2007, 111, 6220-6227.

42. Wuister, S. F.; Donega, C. D.; Meijerink, A. Influence of Thiol Capping on the Exciton Luminescence and Decay Kinetics of CdTe and CdSe Quantum Dots, J. Phys. Chem. B 2004, 108, 17393-17397.

43. Zotti, G.; Vercelli, B.; Berlin, A.; Virgili, T. Multi layers of CdSe Nanocrystals and Bis(dithiocarbamate) Linkers Displaying Record Photoconduction, J. Phys. Chem. C 2012, 116, 25689-25693.

44. Ning, Z.; Molnar, M.; Chen, Y.; Friberg, P.; Gan, L.; Agren, H.; Fu, Y. Role of Surface Ligands in Optical Properties of Colloidal CdSe/CdS Quantum Dots, Phys. Chem. Chem. Phys. 2011, 13, 5848-5854.

45. Ewing, S. P.; Lockshon, D.; Jencks, W. P. Mechanism of Cleavage of Carbamate Anions, J. Am. Chem. Soc. 1980, 102, 3072-3084.

46. von Wrochem, F.; Gao, D.; Scholz, F.; Nothofer, H.-G.; Nelles, G.; Wessels, J. M. Efficient Electronic Coupling and Improved Stability with Dithiocarbamate-Based Molecular Junctions, Nature Nanotechnol. 2010, 5, 618-624.

47. Wessels, J. M.; Nothofer, H. G.; Ford, W. E.; von Wrochem, F.; Scholz, F.; Vossmeyer, T.; Schroedter, A.; Weller, H.; Yasuda, A. Optical and Electrical Properties of Three-Dimensional Interlinked Gold Nanoparticle Assemblies, J. Am. Chem. Soc. 2004, 126, 3349-3356.

48. Frederick, M. T.; Amin, V. A.; Swenson, N. K.; Ho, A. Y.; Weiss, E. A. Control of Exciton Confinement in Quantum Dot-Organic Complexes through Energetic Alignment of Interfacial Orbitals, Nano Lett. 2013, 13, 287-292.

72 49. Bullen, C.; Mulvaney, P. The Effects of Chemisorption on the Luminescence of CdSe Quantum

Dots, Langmuir 2006, 22, 3007-3013.

50. Green, M. The nature of quantum dot capping ligands, J. Mater. Chem. 2010, 20, 5797-5809.

51. Guyot-Sionnest, P.; Wehrenberg, B.; Yu, D. Intraband relaxation in CdSe nanocrystals and the strong influence of the surface ligands, J. Chem. Phys. 2005, 123.

52. Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Highly luminescent monodisperse CdSe and CdSe/ZnS nanocrystals synthesized in a hexadecylamine-trioctylphosphine oxide-trioctylphospine mixture, Nano Lett. 2001, 1, 207-211.

73

Chapter 4

Spectroelectrochemistry of the Iodide-Triiodide Redox Couple

4.1 Introduction

Dye sensitized solar cells (DSSCs) offer a low cost alternative to the more expensive

conventional single crystalline silicon p-n junction cells.1 In this liquid junction solar cell, a dye absorbs

sunlight and transfers electrons into a mesoporous TiO2 nanocrystalline film. A redox shuttle then

replenishes the electrons and a counter electrode (CE) completes the cell. The standard electrolyte

material for the redox shuttle is the iodide-triiodide couple and the standard CE material is platinized

transparent conducting glass. There have been efforts to replace the iodine based electrolytes, most

promisingly with cobalt complexes,2,3 but most do not perform as well as the iodine-based ones. The

iodide-triiodide redox couple yielded the best performing cells because the large difference in oxidation-

reduction kinetics reduces electron recombination rates. On the dye-TiO2 side, oxidation kinetics from

the iodide to triiodide is much faster than the reverse reaction, thus reducing electron recombination

from the dye or the TiO2 back to the redox electrolyte.4

On the CE side however, a fast reduction of triiodide is desirable to regenerate the iodide

species. Pt has been the classical CE material due to its high stability in the iodine electrolyte and

catalytic properties of triiodide reduction.5 Alternatives to the rare and expensive Pt have been

explored, among them conductive organic polymers,6-8 carbon materials,9,10 inorganic materials such as

metal sulfides,11,12 metal oxides,13,14 metal carbides,15 and metal nitrides.15,16 While the mechanism of

triiodide reduction is well-studied on Pt,5,17 the mechanism has not been fully understood for these Pt-

free CEs. One can speculate a similar mechanism as on the Pt; a rational screening method has been

done recently through density functional theory calculations. This work explored a large series of metals

and inorganic semiconductors.13 Studies of carbon materials and conducting polymers have largely

74 focused on optimizing the electrode physical micro-structure, not on the fundamental mechanism of

catalysis.7,9 A better understanding of the iodine redox chemistry on these organic materials is important

for the design of inexpensive, effective CE materials.

Spectroelectrochemistry is a useful tool that allows us to monitor changes in the concentrations

of the redox species if there are signature differences in their absorption behaviour. Since the iodine

species used in DSSCs have differentiable UV-visible absorption spectra, we could use this technique to

compare electrochemical behaviour of different electrodes. This chapter discusses the use of in-situ UV-

visible spectroscopy to identify electrogenerated iodine species and monitor their concentrations

changes as function of electric potential. We will first explore changes at a platinum electrode, then

extend it to one of the conducting polymers used as an alternative CE in DSSC, poly(3,4-

ethylenedioxythiophene) poly(styrenesulfonate) or PEDOT-PSS.

4.2 Mechanism of Electrochemical Reactions of the Iodide-Triiodide Redox Couple on Platinum

The electrochemistry of iodine involves multiple species. Starting from iodide (I-), it can be

oxidized to triiodide (I3-), and then be further oxidized to iodine (I2):

(A) 3I- ↔ I3- + 2e-

(B) 2I3- ↔ 3I2 + 2e-

Furthermore, the concentrations of these three polyiiodide species are in equilibrium with each other, I-

+ I2 I3-, with the equilibrium highly shifted toward the formation of triiodide (Keq = 107 at 25 C).5 On

Pt, the reaction mechanism is as follows:

(1) I3- (sol) I2(sol) + I- (sol)

(2) I2 (sol) + e- + S I- + I(ads)

(3) I (ads) + e- I- (sol)

75 S represents an active site on the electrode surface. The (sol) indicates the species is in solution while

(ads) indicates the species is adsorbed on the surface. The rate determining step of this reduction is

reaction (2), in which the iodine adsorbs to an active site of the Pt and produces an iodide ion.5,17

4.3 Experimental

4.3.1 Materials

Lithium iodide (anhydrous, 99%, Acros Organics), Iodine (≥99.8%, Columbus Chemical Industries,

Inc.), Lithium perchlorate (99.99%, Aldrich), and Poly(3,4-ethylenedioxythiophene)-

poly(styrenesulfonate), 1.3 wt% dispersion in H2O (Aldrich) were used as received. Fluorine-doped tin

oxide (FTO) coated glass (Hartford Glass) was cleaned with detergent, then rinsed with acetone and

ethanol before use. Platinum foil (0.127 mm thick, 99.9% metal basis, Alfa Aesar) and platinum wire

(0.25 mm diameter, 99.99%, Aldrich) were cleaned by soaking in concentrated H2SO4 overnight. They

were rinsed thoroughly with deionized H2O, and dried immediately prior to use. For the PEDOT-PSS

electrodes, the polymer solution was spin-coated on FTO glass at 5000 rpm for 1 min, and dried at 110

C for 5 min. The thickness of the polymer film was 70 nm as measured by ellipsometry.

4.3.2 Electrochemistry

Cyclic voltammetry was performed with a Gamry potentiostat (Series G 300) with a Pt wire as

the counter electrode and another Pt wire as the reference electrode. The Ag/AgCl and the Ag/Ag+ are

commonly used reference electrodes, but we had long term problems from the formation of AgI

precipitating out and clogging the Vycor frit. Therefore, we resorted to using Pt wire as a pseudo-

reference electrode. All cyclic voltammograms shown here were performed with 3 mM LiI, 0.5 M LiClO4

in acetonitrile (ACN) at a scan rate of 25 mV/s, starting at the open circuit potential. LiClO4 was added as

a supporting electrolyte and did not have any redox activity in the potential window explored here.

76 4.3.3 Spectroelectrochemistry

Experiments were performed using a home-built cell in specular reflection mode. A detailed

technical design and drawing of the cell is shown in the Appendix A2 section of this thesis. A fused silica

window was fitted onto the top of the cell to allow light penetration. The cell was made leak-proof by

sealing the multiple components with properly positioned Kalrez O-rings. Contact from the electrodes to

the potentiostat clips was made through Cu tape. Electrolyte solution was flowed through the cell at a

constant rate of 0.6 mL/min with a syringe pump. A fiber optic cable bundle, angled normal to the

working electrode surface, served to deliver and collect the reflected light. UV-visible light was sourced

from a Mickropack (DH-2000) unit, equipped with deuterium (UV) and tungsten (visible-NIR) lamps, and

the subsequent reflected light was fed into an Ocean Optics spectrometer (USB-2000). Raw intensity

scans were taken every 2 s, at voltage steps of 50 mV. A reference scan (I0) was taken at open circuit

potential (0 mV) and the absorbance was then calculated using A = -log(I/I0).

4.4 Results and Discussion

4.4.1 Absorption Spectra of Iodine Species

The absorption spectra of the three different iodine species in acetonitrile (ACN) are shown in

Figure 4.1(a), taken in specular reflection mode with Pt as the substrate. To make the triiodide ions, we

mixed equal concentrations of LiI and I2. The equilibrium constant of I- + I2 I3- is markedly shifted to

the right,5 so almost all of the iodine species should exist as triiodide ions. Iodide ions exhibit absorption

only in the deeper UV region (220 – 275 nm), while the triiodide ions have three distinct peaks at 243,

291, and 362 nm, with the last peak extending into the visible. The absorption of molecular iodine in

ACN is mainly in the visible, with a peak at 460 nm, and two smaller peaks at 291 and 362 nm. These

spectra were not taken at the same concentrations; we had to dilute the triiodide solution > 25x to put

all of them at the same scale because the triiodide ions absorbs much more strongly than the iodine.

77

Figure 4.1: (a) Absorption spectra of the different

iodine species (b) Cyclic voltammograms (CVs) of

iodide-triiodide redox couple on Pt and PEDOT-PSS

2.0

1.5

1.0

0.5

0.0

Ab

s

600500400300Wavelength (nm)

I-

I2

I3-

(a)

(b)

0.05

0.00

-0.05

Cu

rren

t (m

A)

-0.5 0.5Voltage vs. Pt

PEDOT -PSS

Pt

A

A’

B’

B

78 The absorption coefficients (in units of L/mol-cm) are 24091 at 363 nm and 44881 at 293 nm for I3

-, but

only 837 at 463nm for I2.18

4.4.2 Electrochemistry of the Iodide-Triiodide Couple on Pt and PEDOT-PSS

Figure 4.1(b) shows cyclic voltammograms of 3 mM LiI, 0.5 M LiClO4 in ACN on Pt or PEDOT-PSS

electrodes at a scan rate of 25 mV/s. Starting at the open circuit potential (0 mV vs. Pt), the cell was

cycled toward more oxidizing potential first, and then reversed. For both Pt and PEDOT-PSS samples, we

observed two oxidization and two reduction peaks. As discussed previously, these peaks corresponded

to the oxidation of iodide to triiodide (reaction A), subsequently another oxidation from triiodide to

iodine (reaction B), and their respective reduction reactions when the potentials were reversed. For Pt,

the peak to peak splittings for the respective processes are:

(A) ∆EA = Ep,a(A) – Ep,c(A’) 149 – 13 = 136 mV

(B) ∆EB = Ep,a(B) – Ep,c(B’) 465 – 393 = 72 mV

These values indicate that reaction A is more sluggish than reaction B, as observed by others, and the

slow reaction of process A was attributed to a non-electroactive layer of iodine absorbed onto the

surface of Pt, impeding the reduction of triiodide.18,19

The iodide-triiodide oxidation-reduction behaviour was more sluggish on PEDOT-PSS than on Pt,

as observed by larger overpotentials and peak to peak splittings. For this electrode, the ∆EA = 572 mV

and ∆EB = 93 mV. Another striking difference on this electrode is the very broad reduction peaks for both

processes. Peak widths could indicate the reversibility of a reaction, and a broader peak indicates a less

reversible reaction.20 The large discrepancy of oxidation and reduction kinetics on this polymer could be

explained by the adsorption of non-electroactive iodine species in the film, blocking electron transfer.

Molecular iodine is known to form charge transfer complexes with conjugated organic molecules,21 and

a few previous studies have shown that these complexes form with thiophene22 and PEDOT.23 Once

79 iodine is formed, it can then be trapped in the microstructure of the polymer film through the formation

of these complexes, as shown by X-ray photoelectron spectroscopy studies.24

4.4.3 Spectroelectrochemistry

Figure 4.2 shows the results from spectroelectrochemistry experiments on Pt electrode. The

cyclic voltammogram is shown on Figure 4.2(a). To show changes in the spectral behaviour, the

voltammogram is divided into three regions. Representative spectra are shown in Figures (b), (c), and (d)

corresponding to regions 1, 2, and 3 respectively. Region 1 is the oxidation of iodide to triiodide and

subsequently from triiodide to iodine. In the absorbance spectra of this region, we detected an increase

in the concentration of triiodide as expected at potentials of the first oxidation peak (reaction A).

However, we did not detect the presence of molecular iodine as a separate peak at potentials of the

second oxidation peak (reaction B). Iodine was probably produced, but triiodide also absorbs in the

same region and both species have similar absorption coefficients at 462 nm (ε 837 L/mol-cm for

iodine, ε 1053 for triiodide).18 Furthermore, since iodide is in excess in the bulk solution, any iodine

that was produced would be quickly converted into triiodide. Sweeping back reduces the iodine to

triiodide (reaction B’, region 2). In this region, we observed the further increase of triiodide

concentration. At the reducing potential of region 3, the triiodide ions are reduced back to iodide ions.

The absorption spectra in this region confirmed this reaction, and showed a decreasing trend in the

triiodide concentration. We were able to map out the trends in triiodide concentrations with potential

cycling that corresponded to the oxidation-reduction reactions in the cell.

Figure 4.3 shows results from a similar spectroelectrochemistry experiment performed on the

PEDOT-PSS electrode. The cyclic voltammogram is shown in Figure 4.3(a). In Figure 4.3(b), we also

included the reference spectra of the three iodine species, taken from specular reflectance of the

PEDOT-PSS film on FTO glass. Due to differences in substrates, these spectra were observed to be

slightly different from those observed in the Pt case in Figure 4.1(a). However, general features were

80

Figure 4.2: (a) CV of iodide-triiodide redox couple on Pt (b)-(d)

Absorption spectra corresponding to regions (b) 1, (c) 2, and (d)

3 from the CV scan

0.08

0.06

0.04

0.02

0.00

Ab

s

500400300Wavelength (nm)

800 mV600400

0.2

0.1

0.0

-0.1

Cu

rren

t (m

A)

-0.2 0.2 0.6

Voltage vs. Pt

on Pt, 25mV/s

1

2

3

0.04

0.02

0.00A

bs

500400300

Wavelength (nm)

100 mV300500700

0.06

0.04

0.02

0.00

Ab

s

500400300Wavelength (nm)

100 mV-100-300

1

2 3

(a) (b)

(c) (d)

81 qualitatively similar. With this electrode, the spectroelectrochemistry experiment had an extra

complication from the electrochromic behaviour of PEDOT-PSS. The polymer film as spincoated

exhibited a light blue colour. At oxidizing potentials, the polymer becomes more doped and less

coloured in the visible. At reducing potentials, dedoping of the polymer occurs, and it becomes more

coloured.25 Absorption spectra from two marked regions are shown in subfigures 4.3(c) and 4.3(d). To

spectrally differentiate the iodide electrochemistry from the film electrochromism, we also performed a

background experiment with an electrolyte solution without any LiI. We included representative spectra

from this experiment in figures 4.3(c) and 4.3(d).

The changes in the electrolyte species on the PEDOT-PSS film were observed to be similar to the

Pt. The triiodide concentration increased at oxidizing potentials as shown in Figure 4.3(c), and decreased

at reducing potentials as in Figure 4.3(d) corresponding to the peaks from the cyclic voltammogram.

Again, we did not detect any iodine formation at the proper potentials. Comparing the spectral

experiments with and without the iodide ions revealed changes in the polymer film when exposed to

the iodide electrochemistry. Absorbance changes are more striking in the iodide solution especially in

the 500 – 750 nm region; the film lost more colour when oxidized and became more coloured when

reduced as compared to the background. This region did not correspond to native absorption signatures

of any iodine species and could be attributed to physical changes in film. The original counterion for the

film is PSS, but it could be swapped out after electrochemical cycling to other negatively charged ions in

the solution (iodide, triiodide, or perchlorate). Changes in polymer counterion are known to affect the

spectroelectrochemical behaviour of the film.26 Another explanation could be formation of charge

transfer complexes of PEDOT chains to iodine species. The charge transfer complex between iodine and

thiophene was found to have absorption peaks at 299 and 368 nm.22 If PEDOT-iodine complex has

similar absorption bands, it would be difficult to differentiate due to overlaps with the triiodide

absorption. However, it is more likely that the absorption bands are shifted, since PEDOT is different

82

Figure 4.3: (a) CV of iodide-triiodide redox couple on PEDOT-PSS. (b)

Absorption spectra (reflection mode) of the different iodine species

on PEDOT-PSS electrode (c)-(d) Absorption spectra corresponding to

regions (c) 1 and (d) 2 from the CV scan

1.0

0.5

0.0

Ab

s

600500400300

Wavelength (nm)

I-

I2

I3-

0.08

0.04

0.00

Ab

s

800600400

Wavelength (nm)

500mV1000

background1000mV

0.15

0.10

0.05

0.00

Ab

s

800600400

Wavelength (nm)

-100mV-500

background-500mV

0.04

0.02

0.00

-0.02

Cu

rren

t (m

A)

-0.5 0.5Voltage vs. Pt

PEDOT-PSS25mV/s 1

2

1 2

(a) (b)

(c) (d)

83 from the parent monomer thiophene with its delocalization of charges throughout the polymer chain.

More studies are needed to understand the changes in the PEDOT-PSS after electrochemical cycling to

explain these spectral differences in the films.

4.5 Conclusions

Understanding the behaviour of iodide-triiodide electrochemistry on Pt-free counter electrodes

for DSSCs is important for the design of better performing cheaper catalyst materials for triiodide

reduction. Spectroelectrochemistry is a potentially useful tool for this study, since there are spectral

differences in the various polyiodide species. We show that we can successfully monitor spectral

changes in the triiodide concentrations with cyclic voltammetry for both Pt and PEDOT-PSS (an

alternative CE material) electrodes. On PEDOT-PSS, the spectral changes in the visible region were found

to be due to the electrochromic behaviour of the polymer film; this can mask the detection of any iodine

species in that absorb in the visible. Complementary experiments are needed to monitor physical

changes in the PEDOT-PSS films with electrochemical cycling of polyiodine electrolytes.

4.6 References

1. Oregan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar-Cell Based on Dye-Sensitized Colloidial TiO2 Films, Nature 1991, 353, 737-740.

2. Feldt, S. M.; Gibson, E. A.; Gabrielsson, E.; Sun, L.; Boschloo, G.; Hagfeldt, A. Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells, J. Am. Chem. Soc. 2010, 132, 16714-16724.

3. Yella, A.; Lee, H. W.; Tsao, H. N.; Yi, C. Y.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W. G.; Yeh, C. Y.; Zakeeruddin, S. M.; Gratzel, M. Porphyrin-Sensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12 Percent Efficiency, Science 2011, 334, 629-634.

4. Gregg, B. A.; Pichot, F.; Ferrere, S.; Fields, C. L. Interfacial Recombination Processes in Dye-Sensitized Solar Cells and Methods to Passivate the Interfaces, J. Phys. Chem. B 2001, 105, 1422-1429.

5. Macagno, V. A.; Giordano, M. C.; Arvia, A. J. Kinetics and Mechanisms of Electrochemical Reaction on Platinum with Solutions of Iodine-Sodium Iodide in Acetonitrile, Electrochim. Acta 1969, 14, 335-357.

84 6. Li, Q.; Wu, J.; Tang, Q.; Lan, Z.; Li, P.; Lin, J.; Fan, L. Application of Microporous Polyaniline

Counter Electrode for Dye-Sensitized Solar Cells, Electrochem. Commun. 2008, 10, 1299-1302.

7. Saito, Y.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. I-/I-3(-) Redox Reaction Behavior on Poly(3,4-ethylenedioxythiophene) Counter Electrode in Dye-Sensitized Solar Cells, J. Photochem. Photobiol. A 2004, 164, 153-157.

8. Ahmad, S.; Yum, J.-H.; Zhang, X.; Grätzel, M.; Butt, H.-J.; Nazeeruddin, M. K. Dye-Sensitized Solar Cells Based on Poly (3,4-ethylenedioxythiophene) Counter Electrode Derived from Ionic Liquids, J. Mater. Chem. 2010, 20, 1654-1658.

9. Murakami, T. N.; Graetzel, M. Counter Electrodes for DSC: Application of Functional Materials as Catalysts, Inorg. Chim. Acta 2008, 361, 572-580.

10. Trancik, J. E.; Barton, S. C.; Hone, J. Transparent and Catalytic Carbon Nanotube Films, Nano Lett. 2008, 8, 982-987.

11. Wang, M.; Anghel, A. M.; Marsan, B.; Ha, N.-L. C.; Pootrakulchote, N.; Zakeeruddin, S. M.; Graetzel, M. CoS Supersedes Pt as Efficient Electrocatalyst for Triiodide Reduction in Dye-Sensitized Solar Cells, J. Am. Chem. Soc. 2009, 131, 15976-15977.

12. Sun, H.; Qin, D.; Huang, S.; Guo, X.; Li, D.; Luo, Y.; Meng, Q. Dye-Sensitized Solar Cells with NiS Counter Electrodes Electrodeposited by a Potential Reversal Technique, Energy Environ. Sci. 2011, 4, 2630-2637.

13. Hou, Y.; Wang, D.; Yang, X. H.; Fang, W. Q.; Zhang, B.; Wang, H. F.; Lu, G. Z.; Hu, P.; Zhao, H. J.; Yang, H. G. Rational Screening Low-Cost Counter Electrodes for Dye-Sensitized Solar Cells, Nat. Commun. 2013, 4, 1583-1583.

14. Wu, M.; Lin, X.; Hagfeldt, A.; Ma, T. A Novel Catalyst of WO2 Nanorod for the Counter Electrode of Dye-Sensitized Solar Cells, Chem. Comm. 2011, 47, 4535-4537.

15. Wu, M.; Lin, X.; Wang, Y.; Wang, L.; Guo, W.; Qu, D.; Peng, X.; Hagfeldt, A.; Graetzel, M.; Ma, T. Economical Pt-Free Catalysts for Counter Electrodes of Dye-Sensitized Solar Cells, J. Am. Chem. Soc. 2012, 134, 3419-3428.

16. Li, G.-R.; Wang, F.; Jiang, Q.-W.; Gao, X.-P.; Shen, P.-W. Carbon Nanotubes with Titanium Nitride as a Low-Cost Counter-Electrode Material for Dye-Sensitized Solar Cells, Angew. Chem. Int. Ed. 2010, 49, 3653-3656.

17. Dane, L. M.; Janssen, L. J. J.; Hoogland, J. G. The Iodine/ Iodide Redox Couple at a Platinum Electrode, Electrochim. Acta 1968, 13, 507-518.

18. Hanson, K. J.; Matlosz, M. J.; Tobias, C. W.; Newman, J. Electrochemistry of Iodide in Propylene Carbonate .2. Theoretical-Model, J. Electrochem. Soc. 1987, 134, 2210-2215.

19. Hubbard, A. T.; Osteryou.Ra; Anson, F. C. Further Studies of Iodide-Iodine Couple at Platinum Electrodes by Thin Layer Electrochemistry, Anal. Chem. 1966, 38, 692-697.

85 20. Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.;

Wiley, 2000.

21. Benesi, H. A.; Hildebrand, J. H. A Spectrometric Investigation of the Interaction of Iodine with Aromatic Hydrocarbons J. Am. Chem. Soc. 1949, 71, 2703-2707.

22. Ulagendran, V.; Kumar, R.; Jayakumar, S.; Kannappan, V. Ultrasonic and Spectroscopic Investigations of Charge-Transfer Complexes in Ternary Liquid Mixtures, J. Mol. Liq. 2009, 148, 67-72.

23. Biallozor, S.; Kupniewska, A. Study on Poly(3,4-ethylenedioxythiophene) Behaviour in I-/I-2 Solution, Electrochemistry Commun. 2000, 2, 480-486.

24. Kanciurzewska, A.; Dobruchowska, E.; Baranzahi, A.; Carlegrim, E.; Fahlman, M.; Girtu, M. A. Study on Poly(3,4-ethylene dioxythiophene)-Poly(styrenesulfonate) as a Plastic Counter Electrode in Dye Sensitized Solar Cells, J. Optoelectron. Adv. Mater. 2007, 9, 1052-1059.

25. Groenendaal, L. B.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. Poly(3,4-ethylenedioxythiophene) and Its Derivatives: Past, Present, and Future, Adv. Mater. 2000, 12, 481-494.

26. Domagala, W.; Palutkiewicz, D.; Cortizo-Lacalle, D.; Kanibolotsky, A. L.; Skabara, P. J. Redox Doping Behaviour of Poly(3,4-ethylenedithiothiophene) - The Counterion Effect, Opt. Mater. 2011, 33, 1405-1409.

86

Chapter 5

Surface Photovoltage Techniques for Measurements of Charge Transfer and

Charge Dynamics

5.1 Introduction

The surface photovoltage (SPV) effect is the illumination-induced change in the surface potential

of a material, resulting from spatial redistribution of electrons and holes. SPV measurement can be non-

contact, and non-destructive, making methods that take advantage of the SPV attractive for

characterization of electronic properties of semiconductor surfaces. SPV has conventionally been used

to study steady-state properties of semiconductors, or slow relaxation rates with timescales from

microseconds to minutes.1 New opportunities in nanotechnology for potential application in

photoelectrochemical (PEC) devices involve fast electron transfer dynamics in the range of nanosecond

to femtoseconds.2 Furthermore, low-cost, environmentally friendly materials such as iron oxide3 and

iron pyrite4 are candidates for newer generation PEC devices, but still suffer from problems of low

efficiencies resulting from very short carrier lifetimes in that same range. SPV methods can potentially

be utilized to study these materials, but faster measurement methods need to be developed to study

processes below the microsecond recombination regime.

There are two standard methods for SPV measurement: (1) contact potential difference with a

Kelvin probe, and (2) the metal-insulator-semiconductor (MIS) setup. These methods rely on capacitive

coupling to the material of interest with a sense electrode.5 Here, we use the MIS capacitive setup to

perform SPV measurements. Since the MIS capacitor approach does not require a vibrating electrode

system, the MIS arrangement is simpler, and therefore more versatile for incorporation into the fast

transient experiments that we describe in this chapter.

87 This chapter discusses work on the more conventional steady-state SPV measurement methods,

and also the less explored transient methods. To measure fast relaxation rates, we have developed an

ultrafast SPV method that can overcome the limitations of direct transient measurement with an

oscilloscope. With these measurement techniques, we were able to determine the optical bandgaps of

several semiconductors, the presence of charge transfer across a heterostructured system, the sign of

charge localized on the surface, and finally measure fast carrier recombination times. The data

presented in this chapter are the few experiments chosen to illustrate the different SPV measurement

techniques; more data can be found in the Appendix section of this thesis.

5.2 SPV Cell Setup

Samples were sandwiched into a custom made cell with a sense electrode separated by an

insulating spacer (Surlyn, Kapton, or Teflon, with thicknesses of 25, 75, or 127 μm). Figure 5.1(a) shows

the schematic of the cell construction, while Figure 5.1(b) is a photograph of an actual cell. A technical

drawing of the cell design is in the Appendix section of this thesis. In this configuration, light penetrates

through the sense electrode into the sample. A range of sense electrode materials were used: (1)

transparent conducting oxide (fluorine-doped tin oxide, FTO, or tin-doped indium oxide, ITO) coated

glass for visible light, (2) Pt mesh for the ultraviolet (UV) or near infrared (NIR), and (3) semitransparent

Cr electrode (~5 nm Cr evaporated on a piece of microscope glass slide) for the NIR. The spacer has a

hole punched out, so the open space between the two electrodes is air. Experiments have also been

performed by filling the open region with dodecane, a hydrophobic organic hydrocarbon, to help

prevent photo-oxidation of CdSe quantum dots (QDs). However, replacing ambient air with dodecane

only delayed the problem, and did not completely prevent photooxidation. We note though, that since

dodecane has a larger dielectric constant than air (approximately 2x), larger signals were observed (~2x),

due to an increased capacitance of the cell (SPV relies on measurement through a capacitively coupled

arrangement, and capacitance is proportional to the dielectric constant of the medium between the

88

Figure 5.1: (a) Schematic of the components involved in an

SPV cell (b) Photograph of the cell

Kapton spacer

Assembled

FTO glass

Top, bottom, screws

1 cm

(b)

(a)

89 capacitor electrodes). Exposure of the samples to air was successfully prevented by either (1)

performing the experiment in a nitrogen-purged flow cell, or (2) pre-assembling the cell in an Ar-filled

glovebox. Screwing the entire cell together made a tight seal around the open space, preserving the

inert environment between the electrodes. The second approach would not work if one were to use a

metal mesh as the sense electrode (e.g. for experiments involving UV or NIR light).

5.3 Steady-state SPV Experiments

In the case of steady-state measurements, samples were irradiated with light sourced from a

250W quartz-tungsten-halogen lamp (Newport Model 66884) that was focused into a scanning

monochromator (Acton SpectroPro-275, grating blazed at 500 nm). The light beam was mechanically

chopped (Thorlabs MC1000-A) to enable lock-in detection. An additional filter was added to remove the

UV light (< 400 nm) generated from second order diffraction processes of the monochromator. The SPV

signal was measured with a Stanford Research Systems lock-in amplifier (SRS830 DSP). Figure 5.2(a)

shows a schematic of the setup. A pre-amplifier (Femto DLPCA-200) was sometimes used to increase

signal sensitivity. To normalize the signal from the lamp intensity profile across the wavelength range, a

pyroelectric detector (Spectrum SPH-21) was used.

This steady-state SPV technique can be used to obtain bandgap characteristics of

semiconductors. Figure 5.2(b) shows the SPV spectrum of an n-type GaP(100) single crystal doped with

sulfur (spacer used: 25 μm thick Surlyn sheet, sense electrode: FTO glass, chopping frequency = 100 Hz).

GaP is a semiconductor that has an indirect bandgap of 2.22 eV (558 nm). Accordingly, we observed an

onset of SPV signal at about 550 nm. The spectrum has two distinct regions, a shallow tail (460 – 550

nm) and a steep increase (wavelengths below 460 nm), which correspond to the indirect and the direct

(at 2.78 eV or 446 nm) gaps of the GaP crystal respectively.6

In the case of sensitized systems or heterostructures, SPV measurements can be used to detect

charge transfer across the interface. Figure 5.2(c) shows the SPV spectrum of CdSe quantum dots (QDs)

90

Sample/probeChopper

Monochromator

Lock-in amplifier

Lamp

Filter(a)

S-doped GaP(100)

650550450

SPV

(ar

b. u

nit

s)

Wavelength (nm)

(b)CdSe QD-TiO2

(c)SP

V (

arb

. un

its)

600500Wavelength (nm)

1.2

1.0

0.8

Ab

s

SPVAbs

Figure 5.2: (a) A schematic of the steady-state SPV measurement

setup. (b) Plot of SPV signal vs. wavelength for S-doped GaP(100)

crystal. (c) Spectrally resolved SPV scan of CdSe QD sensitized TiO2.

Plotted in the same window is a transmission absorption spectrum of

the same sample

91 attached to a TiO2 nanocrystalline film on FTO with a bifunctional mercaptopropionic acid linker

(assembled in an Ar-filled glovebox, spacer used: 25 μm thick Kapton sheet, dodecane gap, sense

electrode: FTO glass, chopping frequency = 590 Hz, Femto amplifier at x105 V/A gain, low speed).

Preparation of the CdSe-TiO2 sample is described elsewhere in this thesis (see Chapter 2). Also included

in the figure is the visible absorption spectrum of the sample. The CdSe QD starts to absorb at 600 nm

and has peak at ~560 nm, corresponding to the first exciton transition. The SPV signal matches the

absorption spectrum of the CdSe well. This shows that optical excitation of the CdSe QDs results in a

spatial separation of charge on the CdSe-TiO2 sample. It is not possible to generate SPV signals

separately from CdSe QDs (there is no band bending effects due to their small sizes) or from TiO2 (its

bandgap is in the UV). The energy level of the conduction band of the CdSe QDs is such that it can

transfer electrons into the conduction band of the TiO2.7 Therefore, the measured SPV signal comes

from the charge transfer and charge separation at the interface of CdSe and TiO2. This SPV technique is a

convenient way to verify presence of charge transfer in sensitized systems and heterostructures.

5.4 Transient SPV Experiments

Instead of using a continuous light source, SPV measurements can also be performed with

pulses of light. If the timescale for the dynamics of charge separation is longer than the pulse duration, it

is possible to extract time-resolved information of the SPV. This section focuses on transient SPV

experiments with nanosecond pulses.

The schematic for the transient SPV setup is shown in Figure 5.3(a). Pulses from a commercially

purchased tunable laser (Ekspla NT342B-SH, third harmonic (355 nm) generation from Nd:YAG with an

optical parametric oscillator, tunable from 210 to 2300 nm, ~3 ns pulse duration) served as our light

source. The intensity of the laser pulses was controlled by the angle of a polarizer and was measured

with a pyroelectric detector (Coherent EnergyMax sensor). The SPV transients were amplified first (Fast

ComTec model TA2000B-3, a unit in AC mode giving 40x amplification, or model TA2000B-1, a unit in DC

92 mode giving 10x amplification, both models have 50 Ω input and output impedances), and then were

recorded with a digital oscilloscope (Agilent Model DSO5054A). A Faraday cage enclosed the sample cell

and amplifier setup; it was necessary to minimize the external electromagnetic interference, especially

when dealing with small SPV signals. It is important to note how the two electrodes are connected to

the amplifier/oscilloscope, since this affects the sign of the transient signal. All data in this section is

presented with the sample connected to ground, as illustrated in Figure 5.3(a). If the electrodes were

flipped (i.e. sense electrode connected to ground), then the sign of the transients we observe would be

the opposite of what is shown here.

The sign of the transient signals is indicative of the sign of charge that is accumulated on the

surface; this could be valuable information especially when dealing with less well-studied materials of

unknown doping type. Figure 5.3(b) illustrates this sign dependence with highly doped p-type and n-

type Si(111) samples (boron doped for p-type, phosphorus doped for n-type, both have resistivity values

of 0.001 - 0.004 Ω·cm, and were purchased from Addison Engineering). The Si piece was contacted

through the back with either Pt foil (for the p-type sample) or Zr foil (for the n-type sample). The spacer

used was 127 μm thick Teflon sheet, sense electrode was FTO glass, and the laser was at 700 nm with

0.1 mJ/cm2 per pulse. For both samples, we observed an initial rise of the signal, and then a recovery

with an opposite sign. This shape of the transient is interpreted as the capacitive charging (the rise) and

discharging (the recovery) of the charges generated in the sample-sense electrode system. This

displacement current from the capacitive charging and discharging was observed due to the relatively

small input impedance (50 Ω) of the oscilloscope. The timescales of the transient signal depend on the

different equivalent RC time constants (established by contact resistances, sample internal space charge

layer, and external capacitive-like arrangement of the system). The analysis of the relationship between

the RC time constants, and the timescales of the signal observed will be discussed in more detail later.

The n-type Si sample initially exhibited a positive signal, corresponding to the accumulation of positive

93 charges at the surface due to the nature of the band banding at the surface (see chapter 1). Conversely,

the p-type sample gave an initial negative signal, due to negative charge accumulation at the surface.

As with steady-state SPV spectroscopy discussed in the previous section, the transient SPV

technique can be used to obtain spectral information with the use of a tunable laser as the light source.

The increased intensity from the laser (as opposed to a tungsten lamp coupled to a monochromator) can

give larger SPV signals in weakly absorbing materials. However, the incident intensity of light should not

be too intense of course – we wouldn’t want to destroy the sample! We show in Figures 5.3(c) – (e) that

this spectrally resolved transient SPV technique can characterize charge transfer across heterostructures

as well as determine the doping type and optical band gap of novel nanostructures. Transient signals

were collected at intervals of wavelengths, then the peak heights or peak areas from each trace were

obtained and plotted against wavelength. Using peak areas is more accurate, as this capacitive

measurement produces a photocurrent, and the peak area is then proportional to the total charge

separated upon excitation.

Figure 5.3(c) shows transient signals from a sample of CdSe-TiO2 heterostructure on FTO glass

(see above section for sample preparation details) at three different wavelengths, while Figure 5.3(d)

shows the peak height (from each transient) plotted versus wavelength. Also included in this Figure (d) is

the transmission absorption spectrum of the sample. In this spectrally resolved figure, the points for λ >

600 nm were artificially set to zero; although the transient data was collected, there was no detected

SPV signal. This CdSe-TiO2 transient SPV data matched the absorption spectrum, indicating charge

transfer (and spatial charge separation) across the heterostructure. The information obtained from this

experiment is similar to the steady state SPV experiment. However, here we can additionally verify the

type of charge that is localized on the surface. (We note that the determination of charge type can be

achieved with steady state SPV experiments via the CPD/Kelvin Probe method, but not via MIS

94

Figure 5.3: (a) The schematic of the transient SPV measurement setup. (b) Transient

SPV signals of n-type and p-type silicon (c) Transient signal of CdSe QD sensitized

TiO2 sample at 700, 600, and 560 nm (d) Spectrally resolved SPV scan of CdSe QD

sensitized TiO2, obtained from peak heights of each transient curve. Plotted in the

same window is the transmission absorption spectrum of the same sample. (e) Peak

areas obtained from SPV transients of pyrite NWs plotted against wavelength. The

inset shows the raw transient signal taken at 1000 nm. Adapted from ref 9

oscilloscopeamp

sample

sense

Am

plit

ud

e(a

rb. u

nit

s)

80400Time (ns)

700 nm600560

SPV

am

plit

ud

e(a

rb. u

nit

s)

650550450Wavelength (nm)

0.6

0.5

0.4

0.3

Ab

s

SPVAbs

n-type Si

p-type Si

(a)

(b)

(c) (d)

(e)

Sign

al (

V) 0.02

0.00

3020100-10

-0.005

0.000

3020100-10Time (ns) Time (ns)

λ=1000 nm

20100-10

-1.0

-0.5

0.0

Time (ns)

Sign

al

95 measurement method). The observation of the positive transient signal indicates that holes are trapped

on the surface (i.e. the CdSe QDs), in agreement with the band alignments of CdSe QDs and TiO2.7

We have also successfully used this technique to characterize novel semiconductor

nanostructures of hematite (α-Fe2O3)8 and iron pyrite (FeS2).

9 Iron pyrite is an earth abundant material

that has a lot of potential in photovoltaic applications due to its high absorption coefficient and the well-

matched bandgap (0.95 eV) to the output from the sun. However, there are still challenges to making an

efficient photovoltaic material. Bringing materials down to the nanoscale has additional advantages,

primarily the increased surface area and smaller distances for the charge carriers to travel and be

collected. The delicate thin films of these pyrite nanowires (NWs) make it challenging for optical

bandgap determination with conventional absorption spectroscopy. Figure 5.3(e) shows the spectrally

resolved transient SPV measurements of pyrite NWs grown on stainless steel (spacer: two 75 μm thick

Teflon sheets, sense: semitransparent Cr electrode, made by evaporating 5 nm Cr on glass, 40x

amplifier). This figure plots the peak area calculated from each transient versus wavelength. The inset

shows the raw data at λ = 1000 nm. The surface sensitive, non-contact nature of the SPV measurement

made it possible for us to extract the bandgap of these NWs and show that they matched the known

values of pyrite. Additionally, from the negative sign of the transient signal, these NWs were determined

to exhibit p-type doping. This p-type behaviour is commonly seen in thin films of pyrite, but not in

natural single crystals of pyrite (which are n-type). We also observed signals at sub-bandgap

illumination, indicating defect or trap states on the surface of these NWs.9

Extracting timescales with this technique is not straightforward, as there are various RC time

constants associated with the SPV system. Figure 5.4(a) shows an equivalent circuit model of the SPV

measurement. The transient SPV signal is represented as a battery with a time-controlled switch. The

battery simulates the illumination-induced generation of photocurrent from the material, while the

switch terminates this current after a specified period of time (tSW) simulating the transient light pulse

96 and recombination of the separated charges. The semiconducting material has a space charge layer,

represented as a capacitance, Csc, and a charge carrier recombination resistance, Rrecom. The space

charge capacitance and recombination resistance make up an RC time constant, which we will call the

recombination time constant, tr = Rrecom*Csc. Additionally, there are external contact resistances and the

sample internal resistance; these values are collectively represented as Rc in the diagram. The capacitor-

like arrangement of the SPV measurement here (from the MIS structure formed by the sample,

insulating gap, and sense electrode) is shown as Cgap. Finally, we have the impedance of the oscilloscope

(either 50 Ω or 1 MΩ), shown as Rmeas. Another RC time constant is established outside of the

semiconductor space charge layer by the MIS structure. We will call this tMIS = (Rc + Rmeas)*Cgap.

Simulations of the transient signal were performed with TINA-TI v9 (Texas Instruments). For

simplicity, we make the switch time, tSW to be equal to τr, and redefine it as tSPV (= tSW τr) in all cases

shown here. Cgap depends on the spacer used, and can be calculated using C εrε0A/d, where εr is the

dielectric constant, ε0 is the vacuum permittivity = 8.854 x 10−12 Fm-1, A is the overlap area of the two

plates, and d is the separation between plates. For εr 1 (for air), A π*(0.5 cm)2, and d 100 μm, we

have C ~ 7 pF. For the simulation (and simplicity), we will set Cgap = 10 pF, and also assume Rc 100 Ω.

Using these values, we obtain tMIS = 1.5 ns at an oscilloscope impedance of 50 Ω, and tMIS 10 μs at

oscilloscope impedance of 1 MΩ.

Figures 5.4(b) – (d) show the results of the simulations at a range of SPV timescales: tSPV = 100

ps, 1, 10, and 100 ns. Qualitatively, we observed the positive rise and negative recovery in the signal,

coming from the charging and discharging of the capacitive currents in the circuit. Next to each trace in

the figures, the exponential time constants, τ, from fitting the negative part of the signal for the

corresponding tSPV are shown. Several observations can be made: First, if tSPV << tMIS, as shown in the tSPV

100 ps case, τ tMIS, the recovery time constant is just dictated by the discharge of the capacitor-like

MIS setup. Second, if tSPV ≈ tMIS, τ is a complicated sum of both tSPV and tMIS. This is illustrated in the tSPV =

97

Figure 5.4: (a) Equivalent circuit model of the transient SPV setup. (b) Simulations of the

transient SPV signal at various tSPV. The time constant, τ from exponential fits of the negative

recovery of each curve is shown accordingly in the plot (c) Simulation of the transient SPV at

tSPV = 100 ns at a larger time window. Inset is an enlarged section, showing the small negative

decay (d) Plot of fitted τ of various simulated transients vs. tSPV. The dotted line is an artificial

line to guide the eye. (e) Experimental transient signals of two n-Si samples of different doping

levels (and different carrier dynamics) at oscilloscope impedance of 50 Ω. (f) The same

transient signals of the Si samples, but at oscilloscope impedance of 1 MΩ.

τ=100ns

SPV signal

Csc

Rrecom

CgapRc

Rmeas V

t

50 mV

τ=1.5ns

τ=2ns

τ=10ns

(a)

(b) (c) (d)

0.02

0.01

0.00

3020100

tSPV = 100ps 1ns10ns 100ns

Sign

al (

V)

Time (ns)

0.010

0.005

0.000

4002000

-2

-1

0

4000

x10-4

0.1

1

10

100

τ(n

s)

0.1 1 10 100tSPV (ns)

τ

Time (ns)

nSi(0.001)

nSi(10)

(e) Rmeas= 50 Ω

(f) Rmeas= 1 MΩ

nSi(0.001)

nSi(10)

0.002

0.001

0.000

-0.001

3020100-10

0.02

0.01

0.00

3020100-10

0.004

0.002

0.000

-0.002

3002001000-100

0.010

0.000

6004002000

Time (ns) Time (ns)

Time (ns) Time (μs)

Sign

alSi

gnal

98 1 ns case; the fitted τ was ~2 ns. Third, if tSPV >> tMIS, τ tSPV, as shown in the tSPV, = 10 and 100 ns traces.

Figure 5.4(d) shows a plot of fitted τ vs. tSPV obtained from a range of simulated transients. This shows

that to experimentally obtain the true tSPV, we must make sure that the SPV timescale is at least 10x

larger than tMIS. However, to maintain electrical neutrality, the peak areas of the positive rise and

negative recovery must remain equal. Therefore, a large tSPV would have a small negative peak height, as

illustrated in Figure 5.4(c) for the tSPV = 100 ns case (as the negative feature is more spread out in time).

The negative peak is barely visible at full scale, but the enlarged section in the inset clearly shows the

small negative decay, with τ 100ns.

We can experimentally see this effect of SPV timescales by comparing n-type Si of two different

doping levels: sample (1) has resistivity of ~0.001 Ω·cm (‘nSi_0.001’) and sample (2) resistivity of ~10

Ω·cm (‘nSi_10’). Figures (c) and (d) show the transient signal of the Si samples. As expected, the highly

doped sample has a shorter lifetime, as we can observe the negative signal immediately after the

positive rise. For the less doped sample, we only observed the rise, but we could not detect the negative

decay, even at longer times. We can further verify this by changing the oscilloscope impedance to 1 MΩ.

The tMIS is estimated to be ~10 μs at Rc = 100 Ω. The transient signals are shown in Figures (e) and (f). For

the highly doped Si, only oscillations due to mismatched sample/scope impedances were observed.

However, for the less-doped sample, we observed a positive decay in the microsecond region, and a

small negative recovery. Since the contact resistance of this sample is unknown, it is unclear whether

this positive decay is from the SPV process or from the MIS time constant.

In conclusion, this transient SPV technique with a nanosecond-pulsed laser is an alternative

method to obtain spectrally resolved information, and can give better signal sensitivity when compared

to the steady-state SPV measurement. However, it may be challenging to extract timescales from this

technique, due to the presence of two time constants (tSPV and tMIS) that affects the observed transient.

One would also have to accurately know the sample and contact resistances, as well as take into

99 account the resolution limits of the instrument (for example, the bandwidth of the oscilloscope and the

pulse width of the laser).

5.4 Ultrafast SPV Experiments

Limitations in the nanosecond SPV technique (as discussed in the previous section) can be

overcome by changing our measurement setup. Instead of directly measuring transient signals from an

oscilloscope (which would not be possible anyway for very fast carrier dynamics, in the regime below 1

ns), we employ two ultrafast (~50 femtosecond) laser pulses separated by a tunable delay. The

measurement concept is illustrated in Figure 5.5(a). We measure a time averaged, gated electronic

signal, for example with a lock-in amplifier or a boxcar averager. The reason we can measure changes in

the time averaged signal as a function of delay is because the SPV phenomenon is inherently a sublinear

process (except at very small light intensities). For example, as shown in the SPV signal vs. light intensity

plot (‘power curve’) in Figure 5.5(b), the magnitude of the SPV signal at intensity of 2I (point A) is less

than twice of the magnitude of the SPV signal at I (point B). If we have two light pulses of equal

intensity, the signal is expected to be B when pulses are overlapped in time, but exponentially increase

to 2A when pulses are far apart in time. Scanning the delay between these pulses would generate a plot

of SPV signal vs. delay depicted in Figure 5.5(c). The timescale observed depends on the recombination

dynamics of the material at the surface. This two-pulse time delay SPV measurement method has been

previously done with an STM tip10,11 or through photoemission spectroscopy12,13 in an ultra-high vacuum

chamber, but has not been performed under ambient conditions.

The ultrafast laser used in this experiment was a 1 kHz Ti:sapphire regenerative amplifier

(Spitfire Pro, Spectra Physics) pumped by a 528 nm diode laser (Empower-30, Spectra Physics) with a

central wavelength of 800 nm. The energy of the laser pulse was adjusted with a silver-coated glass

gradient neutral density filter, before the pulse was split into two with a 50:50 beamsplitter (the actual

ratio, as determined experimentally was 1 : 1.14). The time interval between pulses was controlled by

100

Figure 5.5: (a) An illustration of the ultrafast SPV measurement utilizing

two pulses delayed in time from one another, and the expected signal

from the various regimes of delay time (b) Plot of SPV signal vs. light

intensity, showing the sublinear nature of the SPV phenomenon (c) The

signal that we would expect as a function of delay (d) Schematic of the

ultrafast SPV measurement setup

t2 = t1 t2 > t1 t2 >> t1

t1

t2

t1

t2

t t t

signal = 2Asignal = B B < signal < 2A

t1

t2

2A

B

delay

Sign

al

0

SPV

sign

al

Intensity

A

B

I 2I

(a)

(b) (c)

(d)

boxcarampsample

sensediode

∆t

101 directing one of the beams into an optical delay line composed of a retroreflector mounted on a 300

mm-long linear translation stage (Thorlabs LTS300). Both beams were then collected and directed

through the transparent sense electrode and into the sample. Auto-correlation of the two pulses with a

piece of BBO crystal yielded a Gaussian width of about 150 fs.

As shown in Figure 5.5(d), the capacitor-like setup of the sample-sense electrodes was first

connected to pre-amplifiers (two 10x Fast ComTec Model TA2000B-1 connected in series, resulting in

100x gain), then a fast diode (Model 203A from Krytar, 10 MHz – 20 GHz zero bias Schottky detector

operating at negative polarity) before connecting to a boxcar averager system (Stanford Research

Systems Model SR250). The diode was needed since this technique relies on a time-averaged detection

method; the raw transient signal has capacitive charge-discharge peaks that would average out to zero.

Figures 5.6(a) and (b) show the transient signal on the oscilloscope with and without the diode

respectively on an n-type InP sample. Since this is a negative bias diode, all connections in this ultrafast

SPV section were made such that we obtain negative signal rectification (for n-type semiconductor,

sense electrode to ground, but for p-type semiconductor, sample to ground). We note that no signal (as

observed on the scope) was detected if the electrodes were connected the other way. All data was

taken using a piece 127 μm-thick Teflon piece as the spacer, and a piece of ITO glass (30 – 60 Ω/sq,

Sigma-Aldrich product 703184) as the sense electrode. To make sure the contacts were ohmic, Pt foil

was used as the back contact for p-type samples, while Zr foil was used for n-type samples.

We successfully demonstrated fast dynamics in a few highly-doped semiconductors with this

technique. First we must make sure that the sum of light intensity of the two pulses does not lie within

the linear regime of the power curve. Figure 5.6(c) shows the SPV signal (as measured by the boxcar

averager) vs. the power of a single pulse for the n-InP sample. Figures 5.6(d) shows the delay scan of the

n-InP sample with one of the beams at 3 mW power, and the other at 3.4 mW (as measured by a

ThorLabs S120VC Si-photodiode detector). Fitting the positive half of the delay to an exponential yielded

102 a time constant of 870 ps. We also measured a p-type GaAs sample, as shown in Figure 5.6(e), with 4

and 4.6 mW pulse powers. The p-GaAs delay scan has a time constant of 1.9 ns. Both crystals were

obtained from Institute of Electronic Materials Technology (ITME). The n-type InP(100), doped with S,

has a resistivity of 6.5 x 10-4 Ω·cm and a carrier concentration of 8.3 x 1018 cm-3. The p-type GaAs(100),

doped with Zn+In, has a resistivity of 0.0525 - 0.0738 Ω·cm and a carrier concentration (4.7-7.3) x 1017

cm-3. These resistivity and carrier concentration values were obtained from the manufacturers.

We have utilized this technique to monitor the effect of surface treatment on a semiconductor.

Iron pyrite (FeS2), a cheap, earth abundant semiconductor is currently studied as a possible material for

harvesting solar energy for photoelectrochemical applications. However, one of the major challenges in

practical utilization of pyrite is the fast recombination lifetimes due to the large concentration of defect

and trap states in the material. Surface treatments can change carrier lifetimes of pyrite and gaining an

understanding of how different surface treatment affect the carrier lifetime is important for the

development of practical pyrite devices.4 This ultrafast SPV technique can provide a convenient

technique to measure carrier dynamics of pyrite. Here we have successfully measured the SPV

recombination lifetimes of single crystal natural pyrite.

Slices of pyrite sample (~2 mm thick) were cut from the faces of a single crystal natural

pyrite(100) cube with a rotating diamond saw. The samples were then cleaned with deionized water (DI

H2O), acetone, carbon tetrachloride and dichloromethane via sonication. From the sign of the transient

SPV signal on the oscilloscope, the natural pyrite samples exhibited n-type behaviour. This doping type is

consistent with what was previously observed by others using non-SPV measurement techniques.14

Figure 5.6(f) – (h) show the scans of SPV signal of a pyrite sample as a function of delay between pulses,

at 10 and 11.4 mW laser powers, before and after various treatments. The as-prepared sample on Figure

(f) shows a fitted exponential time constant of 420 ps. The sample was then subjected to a polishing

procedure: 2 min with 0.05 μm-sized suspension of alumina particles, then 3 min with 0.02 μm-sized

103

Figure 5.6: (a) Transient signal without diode (b) Transient signal with diode, showing

that only the negative signal was detected (c) Plot of integrated signal from the boxcar

vs. power of one of the pulses (d)-(h) Signal plotted as a function of delay of (d) the n-

InP(100) sample (e) p-GaAs(100) sample (f) as-prepared natural pyrite (g) polished pyrite

(h) electrochemically treated pyrite. The natural pyrite sample showed n-type behaviour

in all cases.

τ=870ps

n-InP(100)

τ=1.9ns

p-GaAs(100)

(a) (b) (c)

(d) (e)

(f) As-prepared (g) Polished (h) Electrochemically treated

-0.4

-0.2

0.0

0.2

40200

-0.02

-0.01

0.00

40200

-0.15

-0.10

-0.05

0.00

6420

Sign

al

Bo

xcar

sign

al

Time (ns) Time (ns) Power (mW)

Without diode

With diode

n-InP n-InP

τ=420ps τ=120ps τ=440ps

-0.30

-0.29

-0.28

8004000-400

Sign

al

-0.56

-0.54

-0.52

10005000Time (ps) Time (ps)

-0.24

-0.22

-0.20

10005000

-0.28

-0.26

-0.24

10005000

-0.22

-0.20

10005000Time (ps) Time (ps) Time (ps)

104 suspension of silica particles. The sample was rinsed thoroughly and sonicated with DI H2O in between

steps and after polishing. As shown in Figure (g), the polished sample exhibited a lifetime of 120 ps, a

decrease from the freshly prepared sample. This decrease in lifetime could indicate that the polishing

procedure introduced more defect and trap states into the crystal, thus decreasing effective carrier

lifetime of the sample. We subsequently performed an electrochemical treatment on the same sample

that was previously found to passivate pyrite crystals. This procedure involves reducing H2SO4 to H

atoms that can diffuse into the pyrite crystal to passivate defects.15-18 Figure (h) shows the resulting

delay scan of the treated pyrite sample. We obtained a time constant of 440 ps, an increase from the

polished sample (back to within ~5 % of the original value), indicating some passivation of the sample

has occurred. Using the ultrafast technique, we can track the effects of surface treatment on the carrier

lifetimes of pyrite. We found that the polishing treatment was detrimental to the crystal, and led to a 71

% decrease in the carrier lifetime. However, the electrochemical treatment led to a recovery of the

lifetime, as a result of passivation of the defects in the crystal.

5.5 Conclusions

SPV is a non-contact, non-destructive technique to conveniently measure spatial separation of

charges within a material, or across the interface of a heterojunction. Steady-state SPV experiments can

provide information on the bandgap of a semiconductor, and the evidence of charge transfer between a

sensitized system. Transient experiments can be similarly used to inform us about the spectral

dependence of the SPV signal. Additionally, we can also find out which type of charge is localized on the

surface from the sign of the transient signal. Obtaining SPV timescales from a single-pulse experiment is

potentially challenging, as the RC time constant from the MIS capacitive measurement setup plays a

large role in the timescales measured from the oscilloscope. We have further developed an ultrafast SPV

technique to overcome the limitations of the single-pulse experiments that involves irradiating the

sample with two ultrafast pulses delayed from one another. With this technique, we were able to

105 measure SPV recombination timescales of highly doped n-type InP and p-type GaAs single crystals, as

well as natural iron pyrite crystals. We further measured the effect of surface treatments on these pyrite

samples, and found that polishing the sample introduces more defects to the sample, but the defects

were potentially passivated with an electrochemical treatment.

5.6 References

1. Kronik, L.; Shapira, Y. Surface photovoltage phenomena: Theory, Experiment, and Applications, Surf. Sci. Rep. 1999, 37, 1-206.

2. Zhang, J. Z. Interfacial Charge Carrier Dynamics of Colloidal Semiconductor Nanoparticles, J. Phys. Chem. B 2000, 104, 7239-7253.

3. Katz, M. J.; Riha, S. C.; Jeong, N. C.; Martinson, A. B. F.; Farha, O. K.; Hupp, J. T. Toward Solar Fuels: Water Splitting with Sunlight and "Rust"?, Coord. Chem. Rev. 2012, 256, 2521-2529.

4. Steinhagen, C.; Harvey, T. B.; Stolle, C. J.; Harris, J.; Korgel, B. A. Pyrite Nanocrystal Solar Cells: Promising, or Fool's Gold?, J. Phys. Chem. Lett. 2012, 3, 2352-2356.

5. Kronik, L.; Shapira, Y. Surface photovoltage spectroscopy of semiconductor structures: at the crossroads of physics, chemistry and electrical engineering, 2001, 31, 954-965.

6. Strehlow, W. H.; Cook, E. L. Compilation of Energy Band Gaps in Elemental and Binary Compound Semiconductors and Insulators, J. Phys. Chem. Ref. Data 1973, 2, 163-193.

7. Kamat, P. V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters, J. Phys. Chem. C 2008, 112, 18737-18753.

8. Li, L.; Yu, Y.; Meng, F.; Tan, Y.; Hamers, R. J.; Jin, S. Facile Solution Synthesis of α-FeF3·3H2O Nanowires and Their Conversion to α-Fe2O3 Nanowires for Photoelectrochemical Application, Nano Lett. 2012, 12, 724-731.

9. Caban-Acevedo, M.; Faber, M. S.; Tan, Y.; Hamers, R. J.; Jin, S. Synthesis and Properties of Semiconducting Iron Pyrite (FeS2) Nanowires, Nano Lett. 2012, 12, 1977-1982.

10. Terada, Y.; Yoshida, S.; Takeuchi, O.; Shigekawa, H. Real-Space Imaging of Transient Carrier Dynamics by Nanoscale Pump-Probe Microscopy, Nat. Photonics 2010, 4, 869-874.

11. Hamers, R. J.; Cahill, D. G. Ultrafast Time Resolution in Scanned Probe Microscopies, Appl. Phys. Lett. 1990, 57, 2031-2033.

12. Tokudomi, S.; Azuma, J.; Takahashi, K.; Kamada, M. Ultrafast time dependence of surface photo-voltage effect on p-type GaAs(100) surface, J. Phys. Soc. Jpn. 2008, 77.

106 13. Sezen, H.; Ozbay, E.; Aktas, O.; Suzer, S. Transient Surface Photovoltage in n- and p-GaN as

Probed by X-Ray Photoelectron Spectroscopy, Appl. Phys. Lett. 2011, 98.

14. Ennaoui, A.; Fiechter, S.; Pettenkofer, C.; Alonsovante, N.; Buker, K.; Bronold, M.; Hopfner, C.; Tributsch, H. Iron Disulfide for Solar-Energy Conversion, Sol. Energy Mater. Sol. Cells 1993, 29, 289-370.

15. Alonsovante, N.; Chatzitheodorou, G.; Fiechter, S.; Mgoduka, N.; Poulios, I.; Tributsch, H. Interfacial Behavior of Hydrogen-Treated Sulfur Deficient Pyrite (FeS2-x), Sol. Energy. Mater. 1988, 18, 9-21.

16. Buker, K.; Alonsovante, N.; Scheer, R.; Tributsch, H. Influence of Electrochemical Activation and Surface Orientation on the Photoresponse of Single-Crystalline Pyrite Electrolyte and Pyrite Metal Junctions, Ber. Bunsen. Phys. Chem. 1994, 98, 674-682.

17. Bungs, M.; Tributsch, H. Electrochemical and photoelectrochemical insertion and transport of hydrogen in pyrite, Ber. Bunsen. Phys. Chem. 1997, 101, 1844-1850.

18. Bronold, M.; Buker, K.; Kubala, S.; Pettenkofer, C.; Tributsch, H. Surface Preparation of FeS2 via Electrochemical Etching and Interface Formation with Metals, Phys. Status Solidi A 1993, 135, 231-243.

107

Chapter 6

Conclusions and Outlook

There is great potential for nanostructured materials such as chalcogenide quantum dots or

earth abundant iron pyrite for applications in photovoltaic or photoelectrochemical devices to enable

cheap, efficient energy conversion from the sun. However, there are still a lot of challenges that needs

to be faced. In this thesis, we have only attempted to address a part of the problem.

We first explored the issue of chemical instability of chalcogenide quantum dots (QDs) and its

relationship to the electronic structure of the capping ligands. Ligands play a large role in the

photostability and electronic properties of quantum dots. We surprisingly found that the use of small

conjugated organic ligands can passivate QDs, much more so with ligands bearing electron-donating

substituents. There are many opportunities to which our work can be expanded. As briefly studied here,

one is to further explore more ligand designs, especially those that could be used for PbS and PbSe QDs.

Furthermore, our work has largely been of fundamental science; we have yet to utilize this ligand

passivation idea fully into a working solar cell. I imagine that we could incorporate these QDs with

electron-donating ligands into a polymer or a solid state solar cell to enhance the photostability of these

devices.

The other part of my thesis is development of measurement methods to enable the study of

these nanostructured materials. Transient surface photovoltage (SPV) methods, particularly ultrafast

SPV, have a lot of potential for the measurement of fast carrier dynamics in materials and

heterostructures that are relevant for photovoltaic or photoelectrochemical cells. The ultrafast SPV

work done in this thesis has only explored carrier relaxation times, but one could possibly extend this

method to measure fast charge injection dynamics across heterostructures. I am also especially excited

about the proposed future work of combining the ultrafast SPV method with an AFM to allow for

108 nanometer spatial resolution – we can potentially measure charge dynamics from individual quantum

dots!

109

Appendix

A1 Molecular Coverage Calculations from XPS Data for Ligands on Nanoparticles

Standard coverage calculation models of a thin film on a substrate from XPS assume a flat, thick

underlying substrate such that the sample is considered laterally identical. The signal contribution from

a thin film is only from the top of the sample closest to the detector. In the planar sample case, the

photoelectrons from the underlying substrate are only attenuated and scattered through the thin film in

one direction. However, this calculation of a planar sample may not be accurate for samples of a

different geometry, such as a very small nanoparticle coated with a thin film of organic ligands. If the

nanoparticle on the order of the electron attenuation length, the standard method will yield a large

overestimation of the thin film coverage since the sample cannot be treated infinitely identical in the

lateral dimension. The organic shell surrounding the nanoparticle in the x and y direction will also

contribute to the measured signal coming from the shell.

To account for the geometry of the small nanoparticle we used

direct numerical integration to establish the relationship between XPS

peak areas and molecular coverage of the core-shell system. We modeled

the system with a CdSe sphere with 1.6 nm radius surrounded by an

organic shell of thickness t. We then used numerical integration to

theoretically calculate the expected carbon to cadmium ratio as a

function of thickness of the organic layer t. The C/Cd values can be considered as the peak area ratios

AC/ACd obtained from XPS.

We used cylindrical coordinates to describe the nanoparticle sphere, but we began the

numerical integration with a two-dimensional slice of the sphere. We first created a region of space and

divided the space into a two-dimensional array of r- and z-coordinates. In the z-direction, zero is set at

110 the bottom of the defined space farthest from the detector. We then described a circle, and assigned

the region within the circle either as the nanoparticle core or as the organic shell. All space not confined

within the circle is assigned as vacuum. Starting from z = 0 we then integrated the expected signal from

each element along the z-direction. For example, to calculate the Cd signal at a specific r-coordinate, we

first determined whether the coordinate point is comprised of CdSe core or the organic shell. If the

point is CdSe, the Cd signal is:

( ) ( )

The first term is the total Cd signal coming from a previous (z-1) element which then undergoes

a small scattering loss e-∆z/λ in the current z element. λCd,CdSe is the ineleastic mean free path (IMFP) of

the photoelectrons of Cd through CdSe. The second term is the new Cd signal from the current z-

coordinate. In this term, ρCd,CdSe is the density of Cd in CdSe.

If the current coordinate consists of the organic layer, no new Cd photoelectrons are generated,

but the total Cd signal from the previous (z-1) element is still scattered by the organic layer. The Cd

signal is then:

( ) ( )

A similar procedure is applied for the C signal, with the appropriate IMFPs. Once we obtained

the signal at the top of the defined space in the z-direction, we then integrated along the azimuthal

direction Φ at each r-coordinate, and added up the values at all r-coordinates to obtain the total Cd and

C signal from the three-dimensional core-shell structure.

From the plot of C/Cd values vs. t calculated from the numerical integration, we used the

experimentally obtained AC/ACd values from XPS (after normalizing the appropriate sensitivity factors) to

obtain an effective thickness (t) of the organic layer. This value is essentially an effective thickness and

111 not the actual thickness of the molecular layer. The molecular coverage (molecules/cm2) is a product of

the density (molecules/cm3) and the thickness t.

Density values of the organic compounds were obtained from the manufacturers. The electron

attenuation lengths in polystyrene (a good model for aromatics) is 3.0 nm for Cd(3d) electrons at 1077

eV kinetic energy, and 3.6 nm for C(1s) electrons at 1200 eV kinetic energy.1 The attenuation length for

Cd(3d) electrons in CdSe is 1.5 nm.2 The IMFPs for other energies were calculated using this equation

λ(E2) λ(E1)*(E2/E1)0.85, where λ(E1) and λ(E2) are IMFPs of electrons of kinetic energies E1 and E2

respectively in a specific material.

For example, the plot of C/Cd signal vs. thickness below was generated from numerical integration

with a spherical CdSe with radius of 1.6 nm coated with an organic layer of thiophenol, using these

values:

Density of CdSe: 5.82 g/cm3

Molar mass of CdSe: 191.36 g/mol

Number density of Cd in CdSe: 0.0304 mol/cm3 =

1.83 x 1022 CdSe/cm3 = 1.83 x 1022 Cd atoms/cm3

Density of thiophenol: 1.08 g/cm3

Molar mass of thiophenol (C6H6S): 110.19 g/mol

Number density of thiophenol is thus: 0.009801 mol/cm3 = 5.9 x 1021 molecules/cm3

Number density of carbon (6 Cs per molecule): 3.54 x 1022 C atoms/cm3

λCd,CdSe (IMFP of cadmium photoelectrons in CdSe): 1.5 nm

λC,CdSe (IMFP of carbon photoelectrons in CdSe): 1.7 nm

λCd,Org (IMFP of cadmium photoelectrons in organic layer): 3.0 nm

λC,Org (IMFP of carbon photoelectrons in organic layer): 3.6 nm

6

4

2

0

C/C

d s

ign

al

0.80.60.40.20.0

thickness (nm)

112

The experimentally obtained Ac/ACd peak area ratio from XPS was 4.5, after correction to the

appropriate atomic sensitivity factors. From the plot above, we obtained an equivalent thickness of 0.43

nm. This thickness gives a carbon density of:

(3.54 x 1022 C/cm3)*(0.43 nm)*(10-7 cm/nm) = 1.52 x 1015 C/cm2

The molecular coverage of thiophenol on CdSe is then:

(1/6)*(1.52 x 1015 C/cm2) = 2.5 x 1014 molecules/cm2.

XPS analysis of a core-shell nanoparticle with an additional interface layer

There are situations in which the C signal from the XPS data cannot

be reliably used to obtain molecular coverage (e.g. adventitious carbon

contamination or the presence of multiple ligand types). If the ligand of

interest has a non-carbon binding head group, we can use the particular

element specific to the binding head group for quantitative XPS analysis.

Here we describe a CdSe core coated with an organic ligand that is bound to the CdSe with a sulfur-

containing functional group (e.g. thiol). In the case above, with no interface layer, we defined a region of

space divided into a two-dimensional array of r- and z-coordinates and subsequently assigned a specific

coordinate to the core, to the shell, or to vacuum. In this case, we defined an additional interface layer

between the core and the shell. We are specifically interested in how the S/Cd ratio varies as a function

of the (total) thickness of the organic layer (t).

In the case of the core-shell interface analysis, we first corrected for the fact

that the volume of the interface layer, VS, does not linearly increase with the volume of

the carbon layer, VC, i.e. the signal Sig(t) ≠ Sig(torg - tS) + Sig(tS) for a volumetric

integration. We used mass conservation, ρCVC NρSVS, to obtain the relationship between VS and VC,

where ρC is the density of carbon in a typical carbon material, ρS is the density of sulfur in a typical

113 sulfur material, and N is the stoichiometric carbon to sulfur atom ratio in the molecule. For density

values, we used the densities of polystyrene (PS) and sulfur S8 to calculate for ρC and for ρS respectively.

Using a density of 1.05 g/cm3 and a molar mass (of a monomer) of 104.144 g/mol for PS, we obtained ρC

= 4.77833 x 1026 C/cm3. Using a density of 2.07 g/cm3 and a molar mass of 8*(32.07) g/mol for sulfur, we

obtained ρS = 5.97 x 1026 S/cm3.

The relationship between VC and VS was then needed to proceed with the numerical

integration of the interface system. We first rewrote VC and VS as:

( )

( )

( )

where rCdSe is the radius of the CdSe nanoparticle core, torg is the thickness of the organic layer, and tS

is the thickness of the sulfur interface layer. We solved for VC/VS, using the mass conversation constraint

described above. Substituting VC and VS from above into the mass conservation equation:

( )

( )

( )

After getting this expression, the next step was to obtain the relationship between tS and torg, since the

numerical integration needed to be performed as a function of thickness t. The way to obtain this

relationship was to rearrange the above equation into this convenient form: c0x3 + c1x2 + c2x + c3 = 0,

in which the x variable is tS. We grouped the constants together and redefined it as A NρS/ρC. After

expanding the cubic expressions from the above equation with some TI-89 calculator magic, we

refactored the equation into the form: c0x3 + c1x2 + c2x + c3 = 0. The coefficients obtained were:

114

Once we properly factored the VC/VS equation, the cubic roots were found. Taking only the real

roots, we solved tS as a function of torg at each thickness value of the organic layer. After taking the

account of this volumetric correction, we performed similar numerical integration as described in the

previous section to obtain Cd(r,z) and S(r,z). In this case however, we needed to account for scattering

from three separate layers. We then used the relationship between S/Cd and thickness t to calculate

the molecular coverage, using a similar calculation procedure detailed above.

References

1. Seah, M. P.; Spencer, S. J. Attenuation Lengths in Organic Materials, Surf. Interface Anal. 2011, 43, 744-751.

2. Katari, J. E. B.; Colvin, V. L.; Alivisatos, A. P. X-ray Photoelectron Spectroscopy of CdSe Nanocrystals with Applications to Studies of the Nanocrystal Surface, J. Phys. Chem. 1994, 98, 4109-4117.

115 A2 Technical Design and Drawing of the Spectroelectrochemistry Cell

116

117

118 A3 Technical Design and Drawing of the SPV Cell

119

A4

Additional Data on Surface Photovoltage (SPV) Measurement

A4.1 Steady-State SPV Measurement

A4.1.1 CdSe Quantum Dot (QD) Functionalized on Doped Single Crystal Rutile (SCR) TiO2(110)1

Sample Preparation: SCR TiO2(110) samples (10x10x0.5 mm, 1 side polished, MTI corporation) were

doped by heating in a reducing environment (5% H2 in N2 gas or ‘forming gas’) at 600 C for 1 h. Samples

turned from white to greyish blue in colour. The samples were then cleaned by exposing them to the UV

lamp for 30 min. One of the sample (labeled A) were soaked in a 0.1 M 3-Mercaptopropionic Acid in

anhydrous acetonitrile (ACN) for 6 h in the dark. Sample A were then rinsed with anhydrous ACN, and

soaked in CdSe QD solution in toluene (QD diameter 3.25 nm, concentration 25 μM) overnight in the

dark. The sample was rinsed with toluene, then heptane, and was subsequently transferred into an Ar-

filled glovebox for storage. The other sample (Sample B) was designated as the bare TiO2 sample; no

further manipulation was done after the UV cleaning procedure. After cleaning, sample A was

transferred into the glovebox as well.

SPV setup: Samples (A and B) were assembled into SPV cells in the glovebox, each with a Kapton spacer

(25 μm thick) and a piece of fluorine-doped tin oxide (FTO) coated glass as the sense electrode. A piece

of Cu foil was used as the back contact for the samples. After assembly, we attempted to obtain the

absorption spectra of the samples (in reflection mode, see section 4.3.3 for optical reflection setup) but

were unsuccessful due to the low coverage of the CdSe QDs. The assembled SPV cells were placed in an

N2 purged cell during experiments to prevent CdSe photooxidation. A 250 W tungsten-halogen lamp

coupled to a monochromator served as the light source. A 400 nm cut-on filter was used. The light was

chopped at 590 Hz. Signals were pre-amplified (Femto at x105 V/A gain, low speed) before going into a

lock-in amplifier (see section 5.3 for more details).

120 Results: The figures below show the results of the SPV measurements. The data was normalized to the

photon flux of the light source (obtained from a pyroelectric detector). In Figure A4.1(a), we show the

SPV signal vs. wavelength. We observed a peak at ~550 nm in the SPV spectrum of the sample

functionalized with the QDs which was not present in the bare TiO2 sample. This peak in the SPV

spectrum is most likely a result of the spatial separation generated from the electron transfer from the

CdSe QD to the TiO2. Below ~420 nm, the SPV signal for both samples greatly increased. The bandgap of

rutile is 3.1 eV;2 therefore the signal at short wavelengths is due to the bandgap absorption of the rutile

TiO2 sample. Figure A4.1(b) shows the phase component from the lock-in measurement plotted against

wavelength. When there is no SPV signal, the phase is random. The phase then settles to a constant

value when there is some SPV signal. Therefore, we observed the onset of constant phase at ~600 nm

for CdSe-TiO2 sample, and at ~420 nm for the bare TiO2 sample.

Figure A4.1: (a) Spectrally-resolved steady-state SPV spectra of CdSe QD functionalized SCR TiO2(110) along with a bare TiO2 sample (b) The phase part of the signal for both samples

-50

0

50

Ph

ase

(deg

rees

)

700600500400Wavelength (nm)

SPV

sig

nal

(ar

b. u

nit

s)

700600500400

Wavelength (nm)

CdSe-TiO2

Bare TiO2

(a) (b)

121 A4.2 Transient SPV Measurement

A4.2.1 Bulk Diamond

The diamond SPV results presented below were each performed with a piece of Kapton spacer

as the gap (25 μm thick). In contrast to almost all other SPV data shown in this work in which air or Ar

gas is the material between the electrodes, in this case the open region of the gap was filled with 0.5 M

H2SO4 in Ar-degassed H2O, as previously done by others.3 The setup of conductive space between the

electrodes makes it less of a photocapacitance measurement, but makes it more of a direct

photocurrent measurement. A piece of Pt foil served as the back contact to the diamond sample, and

the sense electrode was Pt mesh. A piece of fused silica window was placed on top of the mesh for

mechanical integrity. A 40x pre-amplifier was used (FastComTec, model TA2000B-3). The signals were

recorded with a digital oscilloscope (Agilent Model DSO5054A) at 50 Ω input impedance.

Black Diamond4

The black diamond sample (purchased from Element Six) was hydrogen terminated (H-

terminated) on both sides with the hydrogen plasma chamber and kept in N2-degassed isopropanol.

Light intensity was adjusted to 1 mJ (or 5.1 mJ/cm2, area of illumination is a circle with a diameter of 0.5

cm) with a polarizer. However, note that the light intensity on the sample is actually ½ of that, since it

passes through Pt mesh sense electrode before hitting the sample. The intensity of light at the sample

was therefore 2.6 mJ/cm2. At wavelengths below 300 nm, the polarizer could not be used, so the light

intensity could not be adjusted. The data was then normalized to the power of the laser to match the

intensities at other wavelengths. Figure A4.2 below shows the peak amplitude of the transients

extracted from exponential fits as a function of wavelength. The inset shows the raw signals from the

experiments at three different wavelengths. We observed transient signals at all wavelengths used in

this experiment (210 – 700 nm), indicating there are sub-bandgap processes resulting in SPV signals. The

amplitude of the signal increased sharply when excitations were above the bandgap of diamond (5.5 eV

122 or 225 nm). The SPV sign of all transient signals collected here indicates an electron accumulation at the

surface, consistent with the negative electron affinity (NEA) effect of a H-terminated diamond material.

Figure A4.2: Peak amplitudes extracted from transient SPV signals from a black diamond sample plotted as a function of wavelength. The scale on the y-axis is reversed such that larger negative signals yield an upwards trend. Inset: Raw transient signals at wavelengths of 450, 300, and 225 nm

Yellow Diamond5

The yellow diamond sample was H-terminated only on one side (the top side) with the hydrogen

plasma chamber and kept in N2-degassed isopropanol. Light intensity was adjusted to 1 mJ (or 2.6

mJ/cm2 at sample, see ‘black diamond’ section above for more details) for wavelengths of 420 – 700 nm.

We could not adjust intensities at other wavelengths. The data shown below was normalized to the

excitation intensities. The SPV of yellow diamond was observed to be more complicated than of the

black diamond. We observed different signs at different wavelength regions. A few representative raw

transient spectra are as shown in Figure A4.3(a) below. Figure A4.3(b) shows a plot of peak heights

plotted vs. wavelength. In this case, due to the changing peak shape across different wavelength, peak

-0.08

-0.06

-0.04

-0.02Pea

k A

mp

litu

de

(arb

. un

its)

700600500400300

Wavelength (nm)

-0.015

-0.010

-0.005

0.000

Sign

al (

V)

151050time (μs)

450 nm300225

123 heights were obtained from taking the maximum of the first observable peak. Negative charge

accumulation at the surface (according to the SPV sign) only occurs at 450 – 700 nm; in this (visible)

region, the SPV signal peaks at 500nm. At wavelengths below 420 nm, the sign of the SPV indicates a

positive charge (or hole) accumulation, even at super-bandgap excitations. This observation is not

consistent with an NEA model of H-terminated diamond, in which negative charge is accumulated at the

surface.

Figure A4.3: (a) Raw transient signals of a yellow diamond sample at wavelengths of 220, 360, 560, and 740 nm (b) Peak heights derived from the transient signals plotted as a function of wavelength. The scale on the y-axis is reversed such that larger negative signals yield an upwards trend

Boron-doped “Blue” Diamond6

Sample description: Type IIB (cleavage), From Dr. J.E. Butler, (111), 1 off

This sample was H-terminated only on one side (the top side) with the hydrogen plasma

chamber. Light intensities were not controlled in this case, but all data was normalized to the same

intensity. Figure A4.4 shows the integrated areas of the sample plotted as a function of wavelength. The

inset shows the integrated transient spectra at excitations of 220 and 230 nm. On this sample, negative

charge (electron) accumulation was observed at wavelengths below the bandgap (< 225 nm). The

observation of electron accumulation at super-bandgap excitations is consistent with an NEA effect. At

-0.10

-0.05

0.00

0.05

Sign

al (

V)

1050

time (ns)

220 nm360560740

0.4

0.2

0.0

-0.2

Pea

k H

eigh

t (a

rb. u

nit

s)

800600400

Wavelength (nm)

(a) (b)

124 wavelengths > 225 nm, we observed small signals with the opposite sign, indicating some sub-bandgap

processes leading to SPV, with hole accumulation at the surface.

-1000

-500

0

Inte

grat

ed A

rea

(arb

. un

its)

320280240

Wavelength (nm)

-80

-40

0

Inte

grat

ed S

ign

al

3020100Time (ns)

220 nm230

Figure A4.4: Peak areas extracted from transient SPV signals plotted as a function of wavelength of a boron-doped diamond sample. The scale on the y-axis is reversed such that larger negative signals yield an upwards trend. Inset: Integrated transient signals at wavelengths of 220 and 230 nm

125 A4.2.2 N719 Dye on Porous Nanocrystalline TiO2 Films7

Sample Preparation: Porous TiO2 nanocrystalline thin film was screen-printed onto fluorine-doped thin

oxide (FTO) coated glass and annealed as described in Chapter 2 of the thesis. The film was then

additionally cleaned at 500 C for 15 min to remove any adsorbed water and organic contaminants. A

saturated solution of N719 dye (Solaronix) was made in a mixture of anhydrous acetonitrile (ACN) and

anhydrous tert-butanol (50:50 by volume). The solution was kept in the dark. The clean TiO2 film was

immersed in the N719 dye solution (while the film was still warm to the touch) and left in the dark

overnight. After dye adsorption, the film was rinsed with anhydrous ACN.

SPV Measurement Setup: The sample was sandwiched with a Kapton spacer (25 μm thick) and another

piece of FTO glass into an SPV cell. The open region as defined by the Kapton spacer was filled with air.

The 40x preamplifier and Faraday cage were used. Optical excitation from the ns-laser was set to 500

nm and the intensity was adjusted with a polarizer to be 50 μJ ( 128 μJ/cm2 at sample, after accounting

for Faraday cage and a laser beam of ~0.5 cm in diameter).

Results: Figure A4.5 shows the transient signal of the dyed TiO2 sample. The positive signal indicates

electron transfer from the N719 dye into the TiO2 film, therefore leaving positive charges on the surface.

Sign

al (

arb

. un

its)

150100500Time (ns)

Figure A4.5: Transient SPV signal at 500 nm of a sample of a nanocrystalline TiO2 film dyed with N719

126 A4.2.3 TiO2 (Single Crystal Rutile)

Sample Preparation: A sample of single crystal rutile (SCR) TiO2 (110) was purchased from CrysTec GmbH

(both side polished, 10x10x5 mm). The sample was first cleaned in HF (48 %) for 15 min. It was

subsequently annealed in an ‘oven’ made out of a TiO2 sputter target (Kurt J. Lesker) at 900 C for 1 h.

Before use, the sample was additionally heated at 500 C for a few minutes to remove surface organic

contaminants and adsorbed water.

Measurement Setup: The SCR TiO2 sample was mounted on a piece of fluorine-doped tin oxide (FTO)

coated glass with (double-sided) conductive carbon tape. A SPV sandwich cell was assembled in an Ar-

filled glovebox with the TiO2 sample, a Kapton spacer (25 μm thick), and another piece of FTO glass as

the sense electrode. The space between the electrodes is therefore Ar gas. The 40x amplifier was used

and transient signals were recorded with the Agilent oscilloscope. The excitation wavelength used was

380 nm at intensities of 3, 30, and 100 μJ ( 7.7, 77, and 255 μJ/cm2 at the sample respectively)

Results: Figure A4.6 below shows the resulting transient SPV signals at three different intensities. At the

small light intensity, the signal was observed to exhibit n-type behaviour. As we increased the intensity

of the excitation, we observed a sign reversal of the SPV signal. The reason for this sign reversal is not

fully known, but to interpret the SPV signals correctly, we must use the signal obtained at the weakest

intensity. At higher optical powers, the light pulse penetrates deeper into the bulk, and could induce

other processes outside of the space charge layer such as trap states from defects in the bulk, and the

Dember effect.

127

Figure A4.6: Transient SPV signal at 380 nm of a single crystal rutile TiO2 (110) sample at three different intensities

Sign

al (

arb

.un

its)

3020100

Time (ns)

3 μJ30100

128 A4.2.4 ZnO (Single Crystal Sample and Porous Nanocrystalline Film)8

Sample Preparation: The ZnO (10-10) single crystal (SC) sample (CrysTec GmbH) was annealed at 1050 C

for 3 h.9 The porous ZnO nanocrystalline (NC) film was doctor-bladed onto a piece of fluorine-doped tin

oxide (FTO) coated glass from a paste containing ZnO nanopowder (100nm particle size, Sigma-Aldrich)

and organic binders. The film was then sintered following a procedure described in chapter 2 of this

thesis.

Measurement Setup: The ZnO SC sample was mounted on FTO glass with (double-sided) conductive

carbon tape. Both samples were sandwiched into the SPV cells, each with a Kapton spacer (25 μm thick)

and another piece of FTO glass as the sense electrode. The space between the electrodes is air. Samples

were illuminated with 375 nm light from the ns-laser. The intensity was adjusted to 5 μJ ( 12.8 μJ/cm2

on sample, after correcting to the fact that the beam has to travel through the Cu mesh of the Faraday

cage, as well as taking the area of illumination into account). The 40x fast pre-amplifier was used and

the transient signals were recorded with the Agilent DSO5054A oscilloscope.

Results: Figures A4.7 below show the transient SPV signals of the ZnO samples: (a) SC and (b) NC film.

The positive sign indicate that both ZnO samples exhibit n-type behaviour, in which holes are

accumulated on the surface. While the NC sample showed a simple transient with a single peak, the SC

sample had a long-live oscillation after the main signal. ZnO is a piezoelectric material; therefore this

observed oscillation is due to the induced piezoelectric effect in the SC sample. The oscillation was not

observed in the NC sample, since there was no long-range crystallinity in this sample for the

piezoelectric effect to persist.

129

Figure A4.7: Transient SPV signal at 375 nm of ZnO: (a) single crystal and (b) nanocrystalline thin film

806040200-20

Time (ns)

806040200-20

Time (ns)

Sign

al (

arb

. un

its)

(a) (b)

130 A4.2.5 Iron Pyrite (FeS2)

10

Natural Single Crystal Pyrite (100)

Sample Preparation and Measurement Setup: A ~2 mm thick slice was cut from a natural pyrite (100)

cube (Ward’s Science, sample mined from Spain) with a diamond saw. The sample was then cleaned

with deionized water, acetone, carbon tetrachloride, dichloromethane via sonication. The pyrite sample

was assembled into an SPV cell with a Teflon spacer (127 μm thick) and a piece of tin-doped indium

oxide (ITO) coated glass as the sense electrode. No amplifier was used, and transient signals were

recorded with an oscilloscope (Agilent DSO9404A) at 50 Ω input impedance. Excitation of the sample

was performed at 800 nm and at an intensity of ~0.1 mJ (0.26 mJ/cm2 at sample).

Results: The figures below show the resulting raw transient and integrated signals. The sample shows n-

type behaviour in which holes are localized on the surface.

Figure A4.8: Raw and integrated transient signal of natural pyrite (100)

Nanostructured Pyrite Thin Film on CoS/Ti/glass

Sample Preparation and Measurement Setup: The pyrite thin film was prepared by Miguel Caban-

Acevedo from the Jin group. Briefly, a paste containing iron fluoride nanowires was doctor-bladed onto

a piece of CoS/Ti on borosilicate glass and dried. The iron fluoride film was then converted to iron pyrite

following a procedure similar to a published paper.11 The sample was assembled into an SPV cell with a

Sign

al (

arb

. un

its)

Inte

grat

ed S

ign

al

(arb

. un

its)0.002

0.000

-0.002

3020100-10Time (ns)

0.08

0.06

0.04

0.02

3020100-10Time (ns)

131 Teflon spacer (127 μm thick) and a piece of ITO glass as the sense electrode. The 40x amplifier was used,

and transient signals were recorded with the same oscilloscope as above. The sample was excited at 800

nm at an intensity of ~0.65 mJ (1.1 mJ/cm2 at sample).

Results: The figures below show the resulting raw transient and integrated signals from the SPV

experiment. The transient signals were very weak; therefore they were only detected when higher laser

intensities (than the natural pyrite sample) and the 40x amplifier were used. The sample exhibited a p-

type behaviour, in contrast to what was observed in the natural pyrite sample.

Sign

al (

arb

. un

its)

Inte

grat

ed S

ign

al

(arb

. un

its)

-0.010

-0.005

0.000

100500

-0.2

0.0

0.2

8006004002000Time (ns) Time (ns)

Figure A4.9: Raw and integrated transient signal of pyrite thin film

132 A4.2.6 Layered Chalcogenide Nanostructures of MoS2, WS2, and WSe2

12

Measurement Setup: The nanostructured samples were synthesized by Mark Lukowski (Jin group) on

two substrates: Al foil or fluorine-doped tin oxide (FTO) coated glass. They were each sandwiched into

an SPV cell with a spacer and a piece of FTO glass as the sense electrode. Optical excitation of the

sample was set to 700 nm with the ns-laser, at intensities of approximately 1 mJ (= 2.6 mJ/cm2 at

sample; area of beam = 0.196 cm2, and the beam also passed through the Cu mesh of the Faraday cage)

The 40x amplifier was used, and the transient signals were recorded with an oscilloscope (Agilent

DSO9404A) at 50 Ω input impedance. All samples exhibited p-type behaviour.

Figure A4.10: Integrated transient signals of nanostructured (a) MoS2 (b) WS2 (c) WSe2 on FTO glass, and (d) WSe2 on Al foil

-0.4

-0.2

0.0

6040200-20

Time (ns)

-20

-10

0

3210-1

Time (μs)

-0.10

-0.05

0.00

2000

Time (ns)

-0.8

-0.6

-0.4

-0.2

0.0

2000

Time (ns)

Inte

grat

ed S

ign

al(a

rb.u

nit

s)In

tegr

ated

Sig

nal

(arb

.un

its)

MoS2

WS2

WSe2/FTO

WSe2/Al

(a) (b)

(c) (d)

133 A4.2.7 Sensitized SnO2

Ru-polypyridyl complex sensitized SnO2 nanoparticle film13

This SPV data is as published in ACS Appl. Mater. Interfaces, 2011, 3, 3110–3119

Figure A4.11: Raw transient SPV signals of (a) bare SnO2 film and (b) Ru-complex sensitized SnO2 (c) The amplitude extracted from the transient SPV signals of the sensitized film plotted against excitation wavelength

(a) (b)

(c)

134 SnO2 nanorods sensitized with TiO2 nanoparticles (SnO2-TiO2 heterostructures)14

This SPV data is as published in J. Mater. Chem., 2012, 22, 11561-11567

Figure A4.12: (a) Illustration of the assembled TiO2 nanoparticle – SnO2 nanorod heterostructure (b) Raw transient SPV spectra of the heterostructure and a control sample with no TiO2 functionalized (c) The integrated peak areas of the SPV transients plotted against wavelength

A4.2.8 Hematite (α-Fe2O3) Nanowires on FTO15

This SPV data is as published in Nano Lett., 2012, 12, 724-731

(a)

Figure A4.13: Raw transient signals of hematite nanowires (converted from iron fluoride nanowires)

135 A4.3 Ultrafast SPV

Please refer to Chapter 5, section 5.4 for a more complete explanation of the scientific background and

experimental setup.

A4.3.1 Ultrafast SPV of n-GaAs

Sample description: Si-doped (n-type) single crystal wafer of GaAs(100) was purchased from MTI Corp.

From the specification sheet, the sample has a carrier concentration of (1.0 - 1.8) x 1018 cm-3.

Measurement Setup: The sample (with a Zr foil as the back contact) was sandwiched into an SPV cell

with a Teflon spacer (127 μm thick) and a piece of tin-doped indium oxide (ITO) coated glass (30 – 60

Ω/sq, Sigma-Aldrich product 703184) as the sense electrode. The sample was first verified to exhibit

spatial charge separation characteristic of an n-type semiconductor (holes are localized on surface) by

the sign of the transient signal as recorded on the oscilloscope. For the ultrafast delay scan, the sense

electrode was connected to ground so that we obtain a negative transient signal. The transient signal is

shown in Figure A4.14(a).16 This was done because the diode operated in the negative bias

configuration. The sample was illuminated through the sense electrode from a Ti:sapphire regenerative

amplifier (Spitfire Pro, Spectra-Physics) with a central wavelength of 800 nm at 1 kHz repetition rate. The

pulses were split into two to perform a delay scan and were adjusted to 3.0 and 3.4 mW (= 15 and 17

mW/cm2 respectively, the size of the beam was approximately 0.5 cm in diameter) with a neutral

density filter, as measured by a Si-detector (Thorlabs S120VC Si-photodiode detector). The signal was

then pre-amplified (100x amplification, from two 10x Fast ComTec Model TA2000B-1 connected in

series) before going through a fast diode (Model 203A from Krytar, 10 MHz – 20 GHz zero bias Schottky

detector operating at negative polarity) and recorded with a boxcar averager (Stanford Research

Systems Model SR250). Figure A4.14(b) shows the transient signal after the diode, and the gate width of

the boxcar averager.16

136 Results: A power curve was obtained prior to the delay scans to determine the intensity of light required

to generate a non-linear SPV behaviour. For the power curve, the sample was illuminated with only one

of the two pulses, and the signal was recorded with the boxcar at various light intensities. The resulting

power curve is shown in Figure A4.14(c).17 Figure A4.14(d) shows the signal as a function of delay.18 An

exponential fit of the dynamics yielded a recombination time of 1.2 ± 0.28 ns. The error bar presented

was obtained from the fitting procedure.

-0.3

-0.2

-0.1

0.0

Bo

xcar

Sig

nal

6420Laser Power (mW)

-0.360

-0.355

-0.350

-0.345

10005000

τ = 1.2 0.28 ns

Delay (ps)

Bo

xcar

Sig

nal

(V

)

-0.05

0.00

0.05

6040200

SignalGate

Time (ns)

-0.6

-0.4

-0.2

0.0

0.2

3020100-10Time (ns)

Osc

illo

sco

pe

sign

al (

V)

(a) (b)

(c) (d)

137 A4.3.2 Ultrafast SPV of Synthetic Single Crystal Iron Pyrite19

Sample Preparation: The crystal was grown by chemical vapour transport from iron disulfide powder

(Alfa Aesar, 99.9 % pure) using FeCl2 (VWR, 99.999 %) as the transport agent. Sample was grown by

Miguel Caban-Acevedo (Jin group).

Measurement Setup: The sample was embedded in epoxy on the sides, with the top and bottom of the

crystal exposed. The non-flat side of the crystal was made to contact to a flexible steel foil to function as

the back contact. The sample, with the flat side exposed for light excitation, was assembled into an SPV

cell with a Teflon spacer (127 μm thick) and a piece of ITO glass. From the sign of the transient

spectrum, the sample was found to exhibit SPV signal typical of an n-type semiconductor. Other details

of the laser and electronic measurement are as described above in the n-GaAs section. The laser

intensity in this experiment was adjusted to 1.0 and 1.1 mW (5.1 and 5.6 mW/cm2 respectively) with a

neutral density filter.

Results: Figure A4.15 below shows the resulting delay scan from the experiment. The dynamics was fit

to an exponential function and yielded a recombination time of 177 ± 27 ps. The Error bar presented

here was obtained from the fitting procedure.

-0.30

-0.25

-0.20

10005000-500

τ =177 27 ps

Delay (ps)

Bo

xcar

Sig

nal

(V

)

Figure A4.15: Delay scan of the synthetic single crystal pyrite

138 A4.4 References

1. Notebook Entry: YU049, SPV of CdSe/OA on SCR110dTiO2/MPA with Cu Back Contact in Cell

2. Linsebigler, A. L.; Lu, G.; Yates, J. T. Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results, Chem. Rev. 1995, 95, 735-758.

3. Pleskov, Y. V. Photochemistry of Synthetic Diamond Electrodes. Diamond and Diamond-Like Film Applications; Technomic Publishing Company: Lancaster, Pennsylvania; 1996; pp 90-96.

4. Notebook Entry: YU090, Transient PV of Black Diamond (with faster amplifier)

5. Notebook Entry: YU103, Transient PV and UV-Vis of Yellow Diamond (w/ NIR wavelengths)

6. Notebook Entry: YU108, Photovoltage (Transient) of Boron-Doped Diamond

7. Notebook Entry: YU139, Transient SPV of N719 on TiO2nc, Varying Intensity

8. Notebook Entry: YU233, Transient SPV of nSi, ZnOsc w/ C tape, ZnOnc, Use of Slow Current Amp

9. Chen, J. X.; Ruther, R. E.; Tan, Y. Z.; Bishop, L. M.; Hamers, R. J. Molecular Adsorption on ZnO(10(1)over-bar0) Single-Crystal Surfaces: Morphology and Charge Transfer, Langmuir 2012, 28, 10437-10445.

10. Notebook Entry: YV205, Transient SPV of Natural Pyrite and Pyrite Thin Film

11. Faber, M. S.; Park, K.; Cabán-Acevedo, M.; Santra, P. K.; Jin, S. Earth-Abundant Cobalt Pyrite (CoS2) Thin Film on Glass as a Robust, High-Performance Counter Electrode for Quantum Dot-Sensitized Solar Cells, J. Phys. Chem. Lett 2013, 4, 1843–1849.

12. yizheng_archive3\For Jin Group\Mark\SPV for mark 082012

13. Benson, M. C.; Ruther, R. E.; Gerken, J. B.; Rigsby, M. L.; Bishop, L. M.; Tan, Y.; Stahl, S. S.; Hamers, R. J. Modular "Click" Chemistry for Electrochemically and Photoelectrochemically Active Molecular Interfaces to Tin Oxide Surfaces, ACS Appl. Mater. Interfaces 2011, 3, 3110-3119.

14. Shah, S.; Benson, M. C.; Bishop, L. M.; Huhn, A. M.; Ruther, R. E.; Yeager, J. C.; Tan, Y.; Louis, K. M.; Hamers, R. J. Chemically assembled heterojunctions of SnO2 nanorods with TiO2 nanoparticles via "click" chemistry, J. Mater. Chem. 2012, 22, 11561-11567.

15. Li, L.; Yu, Y.; Meng, F.; Tan, Y.; Hamers, R. J.; Jin, S. Facile Solution Synthesis of α-FeF3·3H2O Nanowires and Their Conversion to α-Fe2O3 Nanowires for Photoelectrochemical Application, Nano Lett. 2012, 12, 724-731.

16. Notebook Entry: YV212, Ultrafast SPV

17. Notebook Entry: YV191, Ultrafast SPV

18. Notebook Entry: YV198, Ultrafast SPV

139 19. Notebook Entry: YV185, Ultrafast SPV