A First-Principles Study of Thiol Ligated CdSe

Nanoclusters

Shanshan Wu1

Aug 1st, 2012

Advisor: James Glimm1,2

Collaborators: Michael McGuigan2, Stan Wong1,2, Amanda Tiano1

1. Stony Brook University2. Brookhaven National Laboratory

Outline

Introduction Computational Model Results and Discussions Conclusions and Prospects

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Outline

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Introduction Computational Model Results and Discussions Conclusions and Prospects

IntroductionSurvey on Renewable Energy 1

Renewable energy provides 19.4% of global electricity production, 2010. Solar PV provides 0.5% of global electricity demand. Solar PV has a 49% growth rate during the last 5 years.

41. Renewables 2011 Global Status Report. REN21, 2011: p. 17-18.

IntroductionProfile of Quantum Dot(QD) Sensitized Solar Cell

Advantage 1

• Tailor the absorption spectrum by size control.

• Low-cost production method 12% experimental efficiency 2

Research Interests• Size and Shape Control of QDs • Surface Passivation• Attachment and Electron Transmission

to the TiO2 5

1. Rühle, S., et al., ChemPhysChem, 2010. 11(11): p. 2290-2304.2. Robel, I., et al., J. Am. Chem. Soc, 2006. 128(7): p. 2385-2393.

Introduction Research Motivations

Thiol (Cysteine/MPA) replaces amine or phosphine oxide as the surfactant for CdSe-TiO2 composites 1, 2. Cysteine allows generation of 2 nm ultra-stable CdSe QDs with intensive absorption peak 2. No systematic investigation for MPA or Cys capped CdSe QDs by the DFT and TDDFT method.

61. Robel, I., et al., J. Am. Chem. Soc, 2006. 128(7): p. 2385-2393.2. Nevins, J.S. et al., ACS Applied Materials & Interfaces, 2011. 3(11), 4242.

Outline

Introduction Computational Model Results and Discussions Conclusions and Prospects

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CdSe Quantum Dots (Cd: cyan, Se: yellow)

Ligands (HS-R-COOH) (S: orange, N: blue, C: gray, O: red, H: white)

Computational ModelDesign of Simulation Model

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Cys

MPA

Reduced Length

HSCH(NH2)COOH

HSCH2COOH

Wurtzite Bulk 1

1. Wyckoff, R.W.G., Crystal Structures. 2nd ed. Vol. 1. 1963, New York: Interscience Publishers. 85-237.

Computational ModelDensity Functional Theory

Time-independent Schrödinger Equation

The Kohn-Sham Approach

The Ground State Density

)()( rrH iii

)()()(2

1

)(2

1

2

2

rVrVrV

rVH

XCHartreeext

KS

N

ii rrn

1

2)(2)(

9

Computational ModelDensity Functional Theory (cont.)

The Ground State Total Energy

Exchange-correlation Functional

Hybrid Functional (B3LYP)

][)()(

2

1)(][][][

'

'' nE

rr

rnrndrdrrnndrVnTnE xcextKS

10

)()(][ 3 nrnrdnE xcLDAxc ),()(][ 3 nnrnrdnE xc

GGAxc

Computational ModelLinear Combinations of Atomic Orbitals (LCAO)

Linear Combinations of Atomic Orbitals

Basis Functions

Local (Gaussian) Basis Sets

Effective Core Potential

11

,

, )(p

pipi RrC

)()()( rYcrcr nlm

N

i

rfainn

ninedr1

22

)(

Computational ModelAlgorithm of Geometry Optimization

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Minimum-energy Configurations

Degrees of Freedom: Bond Lengths, Angles Quasi-Newton Optimization

0)(

)(

R

RERF

xBxxxfxfxxf TTkkk

2

1)()()(

Computational ModelTime-dependent DFT

Time-dependent Kohn-Sham Scheme

where

Time-dependent density:

where

Time-dependent XC Potential

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Njt gsjj ,...,1,r,r 0

ttVttrHtt

i jsjj ,r,r2

,r),(,r2

N

jj ttn

1

2,r2,r

),]([),]([),(,r][ trnVtrnVtrVtnV xcHartreeexts

1

r,rk

gskjkj tat

r)(,r tnVtnV gsxcxc

The orbital equation is solved iteratively to yield the minimum action solution.

The excitation energies are calculated by linear response theory

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Computational ModelTime-dependent DFT

spectraV

ntrntrV

extext ,),,(),(

),(),(),()(0

trtrHitrdtnA jtjTt

0),(

)(

trn

nA

LANL2DZ/6-31G* (CdSe/ligands) basis sets, B3LYP XC functional are used with NWCHEM 6.0 package

1% difference to the reference data 1 for bond length and energy gap

Computational ModelSimulation Methodology and Model Verification

System Cd-Se Bond Length (Å)(intra / inter layer)

HOMO-LUMO Gap

(eV)Cd6Se6 2.699 / 2.862 (2.670 / 2.864) 3.14 (3.14)

Cd13Se13 2.710 / 2.801 (2.704 / 2.785) 3.06 (2.99)

151. Yang, P. et al., J. of Cluster Science, 2011. 22(3): p. 405-431.

Computational ModelModel Validation

Absorption Peak of Cys-capped Cd33Se33

Experiment ~422nm Simulation ~413 -- 460nm

Less than10% Difference with Experimental Results

161. Nevins, J.S. et al., ACS Applied Materials & Interfaces, 2011. 3(11), 4242.

1

Outline

Introduction Computational Model Results and Discussions Conclusions and Prospects

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Results and DiscussionsSummary

Magic vs. Non-magic Size QDs

Size Effects of QDs

Ligand Effects on QDs

• Bare QDs vs. Passivated QDs

• Effects of Length and Function Group (NH2)

• Compare Thiol with Amine and Phosphine

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Results and DiscussionsMagic vs. Non-magic size QDs

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Non-magic size QDs process weaker “self-healing” ability than Magic size ones.

Results and DiscussionsMagic vs. Non-magic size QDs (cont.)

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Non-magic size QD has a smaller gap value and is less stable than the magic size ones.

Ligand passivation cannot fundamentally improve the poor properties of non-magic size QDs.

Results and DiscussionsSize Effects of QDs

When increasing the size of QDs: The stability is increased with descending

energy gaps. The absorption intensity is doubled with a

5% red shift for the highest absorption peak.

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Results and DiscussionsLigand Effects on QDs

Bare QDs vs. Passivated QDs: CdSe structures are almost preserved after

saturation. An opening of energy gap by 7%~10% is

observed by passivation.

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Results and DiscussionsLigand Effects on QDs (cont.)

Bare QDs vs. Passivated QDs: Front orbitals mainly originates from CdSe,

while the ligand orbitals localizing deep inside the valence and conduction band.

Surface passivation causes concentration of front CdSe orbitals.

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3.14 eV

3.39 eV

Results and DiscussionsLigand Effects on QDs (cont.)

Bare QDs vs. Passivated QDs: Passivation gives doubled intensity of

absorption spectrum with a blue shift by ~0.2 eV.

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Results and DiscussionsLigand Effects on QDs (cont.)

Bare QDs vs. Passivated QDs: The orbitals involved in the main

transitions are unchanged by passivation.

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Composition of Main Transitions from TDDFT Calculation

System Energy (eV)

Oscillator Strength

Excited-State Composition

Cd13Se13+Cys

2.90 0.0865 H-2 (Se 4p) — L (Cd 5s, Se4p)

3.25 0.2272 H-9 (Se 4p) — L

Cd13Se13 2.72 0.0637 H-2 (Se 4p) — L (Cd 5s, Se 5s)

3.02 0.1042 H-9 (Se 4p) — L

Results and DiscussionsLigand Effects on QDs (cont.)

Bare QDs vs. Passivated QDs: Excited electrons are concentrated on

CdSe, not on ligands.

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Results and DiscussionsLigand Effects on QDs (cont.)

Effects of Length and Function Group (NH2): Varying the length of ligands has only a

minor effect on the structure and energy gap.

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Results and DiscussionsLigand Effects on QDs (cont.)

Effects of Length and Function Group (NH2): Cys- and MPA-capped QDs obtain rather close structures and energy gaps.

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Results and DiscussionsLigand Effects on QDs (cont.)

Effects of Length and Function Group (NH2): Varying length and including the amine

group of ligand show nearly no effect on the active absorption peaks.

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Results and DiscussionsLigand Effects on QDs (cont.)

Compare Thiol with Amine and Phosphine: Thiol opens the HOMO-LUMO gap by 11%

vs. NH2Me by 7% and OPMe3 by 5% 1.

301. Kilina, S., et al., J. of the Am. Chem. Soc., 2009. 131(22): p. 7717-7726.

NH2Me OPMe3

Outline

Introduction Computational Model Results and Discussions Conclusions and Prospects

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Conclusions and ProspectsConclusions

Conclusions: Neither “self-healing” nor passivation

fundamentally improves the properties. When increasing the size, the absorption is

enhanced with a red shift. A doubled intensity and a blue shift are

observed on the absorption by passivation; Varying length and including the amine group in the thiol have minimal effect; Thiol shows a better ability to improve the band gap opening than amine or phosphine oxide ligands.

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Conclusions and ProspectsProspects

Prospects: The effect of ligands as the linker between

CdSe and TiO2 The effect of the gold cluster to the CdSe-

TiO2 devices

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THANK YOU!

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