Spray Pyrolysis synthesis of Mn doped ZnS Nanoparticles

Ajinkya DigheM. S. Thesis Defense

Department of Chemical and Biological Engineering

Advisor: Prof. Mark T. SwihartCommittee Member: Prof. Marina

Tsianou

Outline Introduction

Semiconductor nanoparticles

Quantum confinement

Surface defects and surface to volume ratio

Mn doped ZnS

Spray pyrolysis

Mechanism of particle formation

Materials and experimental parameters

Results and Discussion

X-Ray diffraction(XRD)

Photoluminescence

Transmission Electron Microscopy

Energy dispersive X-Ray (EDX)Analysis

Conclusion

Semiconductor nanoparticles“0” D structures with scales in the nanometer range are

called Nanoparticles.

Interest in nanoparticles:

Drastic change in optical and electronic properties w.r.t. bulk

materials

Shift to higher band-gap energy.

Changes are due to

Quantum confinement effects,

High surface to volume ratio of atoms

Surface defectsAlivisatos, A.P., Semiconductor clusters, nanocrystals, and quantum dots. Science, 1996. 271(5215): P.933-937

Quantum confinementThe restricted movement of the electron in

certain permissible energy states is termed

as quantum confinement.

Surface to volume ratio and surface defects

Size (nm) Atoms Percentage of atoms at surface

10 3 × 104 20

4 4 × 103 40

2 2.5 × 102 80

1 30 >90

• Imperfect

surfaces posses

bonds and

defects.

• These act as

traps for

electrons and

holes

•Bleaches the

exciton

absorption

Hu, H.; Zhang, W., Synthesis and properties of transition metals and rare-earth metals doped ZnS nanoparticles. Optical Materials 2006, 28 (5), 536-550.

Electron Wave function

Hole Wave

function

Defectsstates

Defectsstates

ExcitationBlue

Emission

Orange

Emission

Mn d-states

Mn d-states

Radiation Less Decay

Conduction Band

Valence Band

Activated

Mn2+ activated ZnS

Doped

ZnS:Mn

= Zn2+

= Mn 2+

= S2-

Mn doped ZnS nanoparticles

• ZnS has wide band gap of 3.72-3.77 eV

• Important semiconductor in the near-UV region with

applications in electronics and as biological sensors

• Electroluminescent material with applications in flat

panel displays.

• 18% increase in Photoluminescence(PL) efficiency.

• PL lifetime shortening by about 5 orders due to

quick relaxation of the electron.Hu, H.; Zhang, W., Synthesis and properties of transition metals and rare-earth metals doped ZnS nanoparticles. Optical Materials 2006, 28 (5), 536-550.

Schematic showing the Mn2+ and Zn2+ atoms tetrahedrally coordinated with the S atoms in doped form.Energy Dissipation pathways in Mn doped ZnS

nanoparticles

Spray Pyrolysis

3-zone tube furnaceCold water

Cold waterDiffusion

Dryer

Atomizer

Precursor solution

Nitrogen

Flowmeter

Collection Filter

Pump

To hood

Schematic Drawing of the reactor system for the synthesis of Mn doped ZnS nanoparticles by spray pyrolysis.

Mechanism of particle formation

Materials

Zinc diethyl dithiocarbamate

Dimanganese decacarbonyl

Manganese ethylene

bis(dithiocarbamate)

Tricarbonylcyclopentadienyl manganese

Mn acetate

Experimental parametersToluene was used as a primary solvent

Pyridine was used to increase the solubility of the

Mn ethylene bis(dithiocarbamate)

Dimethyl formamide(DMF) was used to dissolve Mn

acetate.

Temperature of the furnace: 500 oC to 800 oC

Conc. of the Mn precursor: 5% of the conc. of

Zn precursor

Zn Precursor conc. : 0.005 – 0.01 mol/ L

Capping leads to homogeneous dispersion of

the particles in the solvent

To study the effect of capping, various

capping agents were tested.

Trioctylphosphine oxide(TOPO)

Dodecanethiol and

Oleylamine

Results and DiscussionYield and production:

The yield of the particles as compared to the

amount of precursor is low.

~ 0.27% of the theoretical yield

Possible reasons for low yield:

Deposition of films via chemical vapor deposition

Deposition of particles via thermophoresis and

diffusion on the walls of the furnace.

Modified process to increase the yield

Particles were collected in solution phase

Recovered the particles using reverse

solubility and centrifugation

90% of the theoretical yield

However, particles show organic

contamination

V-8

3-zone tube furnaceCold water

Cold waterDiffusion

Dryer

Atomizer

Precursor solution

Nitrogen

Flowmeter

Collection Filter

Pump

To Hood

Liquid Nitrogen Bath

Cold Trap

XRD studies of structure

20 25 30 35 40 45 50 55 60

2θ (degrees)

Inte

nsi

ty

XRD result for the particles produced using different Mn precursors at 700 °C

111

220

311

101: ZnO

Mn acetate

Mn2(CO)5

C5H5Mn(CO)3

Mn ethylene bis(dithiocarbamate)

Photoluminescence

Intensity of emission varies considerably

depending on the Mn precursor.

Large quantity of ZnO present in the sample.

Conclusion: Mn has not substituted the Zn in

the crystal lattice 350 400 450 500 550 600

0

100

200

300

400

500

600

700

800

900

1000

Wavelength

Inte

nsi

ty

Photoluminescence spectra for particles produced from different Mn precursors

ZnO

Surface defects: ZnS

Zn vacancies: ZnS

Mn acetate

Mn2(CO)5

C5H5Mn(CO)3

Mn ethylene bis(dithiocarbamate)

PL spectra showing the effect of Mn precursor concentration

Mn precursor: C5H5Mn(CO)5

Mn conc. : 1% to 5% atomic.

350 400 450 500 550 6000

100

200

300

400

500

600

700

800

900

1000

Wavelength

Intensity

1.0% Mn

2.5 % Mn

5.o% Mn

Effect of capping agent on the PL spectra

Mn precursor: Mn ethylene

bis(dithiocarbamate)

400 450 500 550 600 6500

50

100

150

200

250

300

Wavelength

Intensity

Oleylamine

Trioctylphosphine oxideDodecanethiol

Mn ethylene bis(dithiocarbamate)

TEM analysis of size and morphology

Crystalline NP are prepared as seen from the

lattice fringes.

Size of the particles: 2 to 5 nm.

Particles are irregular in shape and not

perfect spheres.

EDX studies of composition3 types of Mn microstructures could be

formed

Mn atoms substitutionally occupying Zn sites

Mn clusters and

MnS and Mn oxides.

No Mn substitution- as there is no peak for

Mn in the PL spectra.

XRD result does not give any peak for MnS.

Element Atomic%

O K 33.67

S K 29.03

Mn K 1.43

Zn K 32.75

Element Atomic%

O K 29.65

S K 34.72

Mn K 7.43

Zn K 26.38

Zinc diethyl dithiocarbamate and Mn ethylene bis(dithiocarbamate)

Zinc diethyl dithiocarbamate and

tricarbonylcyclopentadienyl Mn

Form of Mn presentMn ethylene bis(dithiocarbamate)

N2 (99.8% pure) acts a source of O2

MnS formed reacts with O2 to give MnO

MnS + 3/2 O2 MnO +

SO2

ΔHrxn = -469.2 KJ mol-1

Decomposition of C5H5Mn-(CO)3 and (CO)5Mn-

Mn(CO)5 give Mn

Mechanism of Mn precursor decomposition

Mn ethylene bis(dithiocarbamate) and Zn diethyl

dithiocarbamate

Scheme for the decomposition of Zn Precursor

Zn(S2CN(C2H5) 2) 2 ZnS + C2H5NCS + (C2H5)

2NH +

CS2 + C2H4

A similar decomposition mechanism for the Mn

precursor gives MnS

Different chemical reactivities.Thermogravimetric Analysis (TGA) of Manganese ethylene bis(dithiocarbamate)

Thermogravimetric Analysis (TGA) of Zinc diethyl dithiocarbamate

Zinc diethyl dithiocarbamate and tricarbonylcyclopentadienyl manganese

C5H5Mn C2H2+MnC

• Bond dissociation energy

for the Mn-CO bond: 230 KJ

mol-1

•Deposition of Mn heavily

contaminated with carbon

First bond to break

Russell, D. K.; Davidson, I. M. T.; Ellis, A. M.; Mills, G. P.; Pennington, M.; Povey, I. M.; Raynor, J. B.; Saydam, S.; Workman, A. D., Mechanisms of pyrolysis of tricarbonylcyclopentadienyl manganese and tricarbonyl(methylcyclopentadienyl) manganese. Organometallics 1995, 14 (8), 3717-3723.

• BDE for the (CO) 5Mn-Mn(CO)5 bond is 185 KJ mol-1

• BDE for the Mn-CO bond can be assumed to be 230

KJ mol-1

Zinc diethyl dithiocarbamate and dimanganese decacarbonyl

2 Mn + 10 CO

ConclusionMn is present on the surface of the particles or

present as other compounds but not in

incorporated form as is confirmed by the PL

spectra.

Mn precursor decompose before the decomposition

of the ZnS precursor leading to non-incorporation.

Crystalline nanoparticles were produced without

Mn incorporation.

AcknowledgementsI would like to thank my major advisor, Dr. Swihart, for his guidance,

encouragement, suggestions and patience throughout my

graduate study at UB

 I also owe my thanks to Dr. Tsianou for being on my committee and

her suggestions and her constant support and enquiry about the

project.

I would also like to thank to Sha Liu for her great help to get me

started on the project and the numerous meaningful discussions.

I would like to also thank my other group members

Folarin Erogbogbo, William Scharmach, (Ashley) Ching-wen

Chang, (Roger) Chen-An Tien, Nitin and Xin Liu for their help and

friendship.