Reduced and Oxidized Colloid Quantum Dots

I. Introduction on colloidal quantum dots, spectroscopy, dynamics, microscopy

II. Charges, conduction, lasing

Philippe Guyot-Sionnest

The University of Chicago

I. Colloidal Quantum DotsPerceived applications:Screen-printed flat panel displays of large area. Better than OLED.Phosphors for white light LED conversion. Mix at will.Photovoltaic energy conversion. Bandgap optimized.Biolabels. A bit bigger but much better than dyes.Infrared tags for night vision. No organic alternative.Laser “dyes” for infrared (near IR and Atmosphere windows).Nanoelectronic and spintronics self-assembled components. Colloidal molecules.

e-

h+

hν Many start-up companies, Nanosys, Q.dot, Evident tech….

Ø150 M$ of venture capital.

Ø1B$ perceived value

Research: Fabrication, Spectra-size, carrier dynamics, trapping, energy relaxation, carrier transport…

Nanocrystal quantum dots:

The colloid synthesis:1982: Precipitation, ionic precursors, Aqueous solutions. Brus, Henglein, Nozik1986: Micelle “nanoreactor”. Pileni, Brus. Water/micelle in Oil.1993 “organometallic approach”: Purely organic environment, high temperatures and surfactants. Murray and Bawendi2000 “Greener” reagents for II-VI. Peng

The pioneers:

Ekimov and Efros, 1980. Effective Mass Approximation applied to CuCl aggregates and excitonic spectra.

QDs from the Near IR to UV:

1200 2000 2800

Abs

orba

nce

(arb

. uni

ts)

Wavelength (nm)300 400 500

Abs

orba

nce

(arb

. uni

ts)

Photolum

inescence Intensity (arb. units)

Wavelength (nm)

400 500 600 700

Abs

orba

nce

(arb

. uni

ts)

Photolum

inescence Intensity

Wavelength (nm)

CdSe ZnSe ZnOPbSe

300 400 500 600

Abs

orba

nce

(arb

. uni

ts)

Photolum

inescence Intensity (arb. units)

Wavelength (nm)

IR Material Visible Material UV Material

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�� � � Hines JPC 98 Shim JACS 01Wehrenberg JPC 02

Spectroscopy

Continuously size-tunable spectra. Excitonic peaks assigned to transitions between

“particle in the box” quantum states

Murray and Bawendi, CdSe, 1993

1S-1S3/2

1S-2S3/2

1P-1P3/2

Bawendi, Murray, Norris, Efros, 93-96

Some parity rules seen in Linear and nonlinear optical spectra. (and LARGE two-photon cross-section)

Trapping and recombination center

Band-edge fluorescence

Type I Core/shell: CdSe/ZnS, CdSe/CdS, InAs/CdSe, CdSe/ZnSe/ZnS, PbSe/CdSe, etc…

Surface capping molecules or inorganic shell to “passivate” the surface

CdSe: Alkyl amines and alkylphosphine/oxide enhance luminescence. Thiols and pyridine reducePL by orders of magnitude. (different for CdTe)

Phosphors. Lighting, Light-emitting diodes. , Displays, Lasers Fluorescence:

Other materials and shapes e.g. PbSe

Kinetic size and shape control: => sphere, cubes, rods, stars…

Small changes in surfactant compositions lead to large effects on final shape and size monodispersivity.

1200 2000 2800

Abs

orba

nce

(arb

. uni

ts)

Wavelength (nm)

For PbSe nanocrystals, 80 % QY, small shift and long (~0.9 µs) lifetimes at RT.

2

1

12

32

���

����

� +=ε

εεradTT

Role of dielectric confinement in lengthening the lifetime:

ε2 ε1

With εPbSe~ 24, ε1~ 2,

T~ 20Trad ~ 0.4 µs.

JPCB 2002

IntrabandSpectroscopy

Colloid QDs are soluble mid-IR material for linear and nonlinear optic,

light emission, etc….

Carrier dynamics

• Multicarrier effects: Auger.• Intraband relaxation.• Linewidths.

Auger processShort biexciton lifetimes

PRB 60, R2181, 1999, and unpublished

the Auger process is a three-body process,

Its bulk rate is: dn/dt~ γn3, with γ~ 10-29/10-30 cm6s-1

0

0.05

0.1

0.15

0 200 400 600 800∆α

(O.D

.)Delay (ps)

0.14 mJ/cm2

1.15 mJ/cm2

1

10

2 3γ (x

10-3

0 cm

6 s-1)

Radius (nm)

R4

Klimov showed that γ is size dependent ~ R3, Science 287, 1011, 2000

Typical time scales for “biexciton” Auger relaxation:

~ 20 ps for 3 nm diameter, and ~ 500 ps for 9 nm diameter. => much faster than fluorescence. => A significant “colloid” issue for lasing.

1Se-1Pe relaxation rate?• Klimov et al, PRL 1998: 100 fs-

0.5 ps Interband bleach recovery.

• Too fast for the understood phonon-mechanism, ∆E~ 10 ωLO(phonon bottleneck)

• Explanation: electron-hole Auger relaxation, Singh (APL 1994)

150 ps; Efros (Sol. State. Comm. 1995) ~ 2 ps, Zunger (nanolett. 2004)~ 100s of ps.

• An open debate.

Intraband relaxation

Interband linewidths and Acoustic side band by hole-burning

CdSe, Palinginis, Wang et al, PRB 67, 201307 2003

~ 10 µeV observed at low power high rep-rate or cw-hole burning.

Linewidths:

10Kn-ZnO

Shim, PRB 64, 345432, 2001

intraband linewidths and LO-phonon replicas.

Weak Coupling to LO phonon• In polar semiconductors, polar cell motions, ( Cd2+�Se2- Longitudinal

Optical Phonons) can couple to changes in charge distribution.• Moderate magnitude and ~1/R size dependence (larger coupling for

smaller sizes) consistent with the bulk electron-LO coupling.

CdSe-

- -

Moderate Coupling to

acoustic phonons

CdSe

Intraband Photon Echo

InP

T2=8ps;170µ V

)/(sinh20 kTTg LOLOacoustic ωγ �++Γ=Γ

PRB 2001

Coupling to Acoustic Phonons

( ) 221

21

2 /1~)(~

~

RrDg

Tlinewidth

PSe ∆Ψ−Ψ�

γEffective FWHM

Deformation potential: Acoustic phonon shift valence and conduction band energies.

Small particle=> strong overlap of deformation amplitude and electronic wavefunction

Takagahara:

53

22

~~

~~−

−�Rg

Rg ii

ωγ

ωγ �

Brus et al:

Because 1/2k∆(r)2R3~ hν~1/R, so ∆(r)~1/R2

?

Two photon microscopy of single nanocrystal. Blanton et al, Chem. Phys. Lett. 229, 317 (1994), APL 1996.Biological imaging. Webb et al, Science 300, 1434 (2003)

Observation of intensity and spectral wandering.

Single dot microscopyAPL 1996

One-Photon Microscopy

• Visible to the eye.• Narrow emission ~

100 µeV.• Linear Stark effect

Demonstrated.• Spectral and intensity

fluctuations. • Blinking: Nirmal, Brus,

Bawendi. Power law Statistics, Kuno and Nesbitt.

Presumed to be due to charge moving on the surface, ionization, or dynamic surface reorganization.

Empedocles and Bawendi, 1996

Blinking

Time bins

A nanocrystal mystery:

Dots blink on and off with Tonν and Toff

µ.,

ν~ 1.5 ~ µ. Power law is independent of T, Radius, material. Can be seen in ensemble fluorescence (Pelton, APL), like 1/f noise of resistors. Bawendi, PRB 63, 205316, 2001

II. Colloidal QDs and the role of charges.

1. Some Possible Applications.

2. Effect of charges on color.

3. Effect of charges on Transport in close-packed QD films.

4. Effect of charges on Fluorescence

Applications: Solar cells with Colloid Quantum dots.

Alivisatos, Nature 2002 State of the art: 1.5 % efficiency at A.M. 1.5

Light Emitting Diodes

Bawendi, Nature 2002.

State of the art efficiency: 0.52%

Still in question: is it genuine e-h recombination or is it simply energy transfer?

Reduced or Oxidized QDs?

• Nozik, Henglein, Kamat, mid-80’s- mid-90’

• Brus, “A simple model for the ionization potential, electron affinity, and aqueous redox potentials of small semiconductor crystallites” , J. Chem. Phys. 79, 5566-5571 (1983)

µ

0

0.5

1

1.5

2

2.5

3

0 0.5 1 1.5 2 2.5 3

Abs

orba

nce

Energy (eV)

Charge and color

0

0.5

1

1.5

2

2.5

3

0 0.5 1 1.5 2 2.5 3

Abs

orba

nce

Energy (eV)

1Se

1Pe

visible absorption

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n-type!

Before charge transfer After charge transfer

dramatic changes in optical properties.

Shim, Nature 407, 981 (2000).

IR absorption

visible bleach

1Se

1Pe

Electrochromic response:

0

0.2

0.4

0.6

0.8

1

0.2 0.3 0.4 0.5

Abs

orb

an

ce

Energy (eV)1.8 1.9 2 2.1 2.2 2.3 2.4

0

0.2

0.4

0.6

0.8

1

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 0.2 0.4 0.6

No

rma

lized

Ab

sorb

an

ce (

arb

. un

its)

Energy (eV)

1Se

1Pe

1S3/2

2S3/2

2(3)S1 /2

1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

−∆

α/α

Wang, Appl. Phys. Lett. 80, 4 (2002). Science 291, 2390 (2001).

0 V

−1.170 V

Thin films of nanocrystals change color with an applied electrochemical potential.

0

0.1

0.2

0.3

0.4

0

0.1

0.2

0.3

0.4

0.5

0.6

0 400 800 1200 1600

No

rmal

ize

d A

bso

rba

nce

(a

rb.

un

its)

−∆α

/α

Time (sec)

OffOff

OffOff

On On On On

Spectral changes resulting form 1Se occupation:

?

P-state charging at more negative potentials

Charges and Conductione− e−

Organic surfactant layer insulates the nanocrystal

σσσσ~10−14 S/cm below 200 K. 1

µ~ 10−4−10−6 cm2/Vs only at very high fields of 107

V/m. 2

1) M. Drndic et al., J. Appl. Phys. 92, 7498 (2002); C. A. Leatherdale et al., Phys. Rev. B 62, 2669 (2000);

2) D. S. Ginger et al., J. Appl. Phys. 87, 1361 (2000).

Nanocrystal solids have been reported to be “excellent” insulators!

Electrochemical tuning of carrier density in nanocrystals:

~5 �m

CdSe NanocrystalSolution

~40 mV

UV/Vis Source

UV/Vis Detector

1

2

3H2N NH2

H2N NH2

H2NNH2

Shell to shell conduction:

10-3

10-2

10-1

100

-1 -0.5 0

10-7

10-6

10-5

10-4

Op

tica

l Ble

ach

(O.D

.)

Potential (V)

a)

Co

ndu

ctance (S

)

10-7

10-6

10-5

10-4

-1 -0.5 0

10-3

10-2

10-1

100

Co

ndu

ctance (S

)

Potential (V)

b)

1Se

1Pe

Op

tica

l Ble

ach

(O

.D.)

1Se

1Pe

Conductivity peak at half filling ~ x(1−x) where x is the filling factor.

6.4 nm CdSe 5.4 nm CdSe

-0.04

-0.02

0

400 500 600 700Op

tica

l Ble

ach

(O

.D.)

W avelength (nm)

Further improvement of conduction by modifying linker:

10-8

10-7

10-6

10-5

10-4

10-3

10-2

-1 -0.5 0

Con

duct

ivity

(S/c

m)

Potential (V)

TOPO/1,7-heptadiamine

Pyridine/1,7-heptadiamine

Pyridine/1,4-phenylenediamine

6.4 nm CdSe

σ � � µ � � �

10-7

10-6

10-5

10-4

10-3

10-2

0.1 1C

ondu

ctan

ce (S

)

# e- in 1Se state

µ~ 10-2cm2/V/s

µ~ 0.5.10-5cm2/V/s

2

Yu, Science, 300, 1277 (2003)

-5

0

5

10

15

0.05 0.1 0.15 0.2 0.25 0.3

ln (

G/n

S)

T-1/2(K-1/2)

-5

0

5

10

15

0 0.05 0.1ln

(G

/nS

)

1/T(K-1)

))/*(exp( 2/1TTG −∝ , Τ∗∼ 5300Κ

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100K

10K

Variable Range Hopping

Coulomb gap

Efros and Shklovskii model of VR-> LnG ~ (T*/T) 1/2

Energy randomness

Mott’s model of VRH -> LnG ~ T-1/4

B0

2*

48.2

ake

Tπεε

=

2

0B0

4

C)4( πεεkageT =

!

ε

Extremely nonlinear I/V: 9 decades for one decade of V

0.001

0.1

10

1000

105

0 0.001 0.002 0.003

4.3K 10K

15K

22K

36K

53K

G (n

S)

E-1/2(V/m)-1/2

"�# $

0.0001

0.01

1

100

104

106

0.1 1 10 100

Cur

rent

(nA

)

Bias (V)

52.5 K

36K

22K

15K10K

4.3K

I ~ V9 ???

EEAG *exp−=

High-Field dependence

0.001

0.1

10

1000

105

0 0.001 0.002 0.003

4.3K10K

15K

22K

36K53K

Con

duct

an

ce (

nS

) (A)

E -1/2(V/m)-1/2

0

2

4

6

8

0 4 8 12 16

Cu

rre

nt

(nA

)

4.3K10K

15K

22K

36K(B)

E (105V/m)

0

0.5

1

1.5

2

Cu

rre

nt

(nA

)

10K

15K

22K

36K(D)

4.3K0.001

0.1

10

1000

105

Co

nd

uct

anc

e(n

S)

4.3K 10K

15K

22K

36K53K

(C)

0

2

4

6

8

0 0.002 0.004 0.006

1/E1/2(V/m)-1/2

r/d

(E)

4.3K

10K15K22K

36K

53K0

2

4

6

8

0 1 2 3 4

r/d

E (105V/m)

22K15K

10K

4.3K(F)

36K

)8

exp(1

)8

2exp(

B

*B

*

TkeEr

ra

TT

TkeEr

ra

TT

ar

AG+−+

+−−=

Simulation --------

%

&eaTk

E2

*B* =

Dielectric constant effect: T*~1/εPbSe: ε~ 300 CdSe: ε~10,

0 0.001 0.002 0.003

G(n

S)

E-1/2 (V/m)-1.2

4.3 K

40K105

1000

10

0.1

0.001

PbSe, 7 nm

0.001

0.1

10

1000

105

0 0.001 0.002 0.003

4.3K10K

15K

22K

36K

53K

G (n

S)

E-1/2(V/m)-1/2

CdSe, 7 nm

Τ∗∼ 600Κ Τ∗∼ 5300Κ

Charge, Fluorescence and Lasing

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Lower ASE threshold of (QD)2-:

0.0

0.4

0.8

1.2

600 620 640 660

PL

Inte

nsity

Wavelength (nm)

0

100

200

300

400

500

600

0 0.5 1 1.5 2

PL

and

AS

E In

tens

ity (a

rb. u

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)

Pump Fluence (mJ/cm2)

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50

100

150

200

-1000 0 1000 2000 3000 4000 5000

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nm

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Time (ms)

−1.3 V

−1.4 V

−0.7 V

−1.2 V−1.5 V

−1.6 V

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3.5

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−∆α/

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500 550 600 650 700

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Wavelength (nm)

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Charges in Colloidal Quantum Dots :

III. Reduced lasing-threshold in the conducting state.

• Sean Blanton (1992-1997)• Mark Schmidt (post-doc,1995-1997)• Margaret (Peggy) Hines(1993-1998)• Moonsub Shim (UIUC)(1998-2001) • Congjun Wang(2000-2004)• Brian Wehrenberg(2000-)• Dong Yu(2001-)

PRL 92, 216802 (2004)JPCB 0489830(2004)JACS, 125, 7806, (2003)Science, 300, 1277 (2003)

•Single dot microscopy

•Two-photon spectroscopy

•Dipole moment•CdSe/ZnS•ZnSe•IntrabandSpectroscopy

JPCB, 107, 7355 (2003).APL, 80, 4 (2002). Science, 201, 2390, (2001)JPCB, 104, 1494, (2001)Nature, 407, 981 (2001)

Charges and colloid quantum dots, the work:

Alamin Dhirani ,92-97, STM-molecular electronics, U.Toronto.

Pao-Hong Lin, 94-00, Vib. Dyn. And Mol. Elec. (ITRI Taiwan)

Uwe Schroeder, post-doc 96-98, UHV Vib. Dyn. SFG. Siemens

Chris Matranga, 98-02, Vib. Dyn., UHV, SFG. DOE lab.

Herdis Adams, 01-, STM, mol. Elec.

Mingzhao Liu, 02-, plasmonics

Jiasen Ma, 03- Mol. Elec.

Matt Pelton, 03- post-doc plasmonics