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
2
Outline
3
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
7
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
8
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
)()()(21
)(21
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)
][)()(21)(][][][
'
'' nE
rrrnrndrdrrnndrVnTnE 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 Potential11
,
, )(p
pipi RrC
)()()( rYcrcr nlm
N
i
rfainn
ninedr1
22
)(
Computational ModelAlgorithm of Geometry Optimization
12
Minimum-energy Configurations
Degrees of Freedom: Bond Lengths, Angles Quasi-Newton Optimization
0)()(
RRERF
xBxxxfxfxxf TTkkk
21)()()(
Computational ModelTime-dependent DFTTime-dependent Kohn-Sham Scheme
whereTime-dependent density:where Time-dependent XC Potential
13
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
14
Computational ModelTime-dependent DFT
spectraVntrntrVext
ext ,),,(),(
),(),(),()(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 ValidationAbsorption 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
17
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
18
Results and DiscussionsMagic vs. Non-magic size QDs
19
Non-magic size QDs process weaker “self-healing” ability than Magic size ones.
Results and DiscussionsMagic vs. Non-magic size QDs (cont.)
20
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 QDsWhen 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.
21
Results and DiscussionsLigand Effects on QDsBare QDs vs. Passivated QDs: CdSe structures are almost preserved after
saturation. An opening of energy gap by 7%~10% is
observed by passivation.
22
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.
23
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.
24
Results and DiscussionsLigand Effects on QDs (cont.)Bare QDs vs. Passivated QDs: The orbitals involved in the main
transitions are unchanged by passivation.
25
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) — LCd13Se13 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.
26
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.
27
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.
28
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.
29
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
31
Conclusions and ProspectsConclusionsConclusions: 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. 32
Conclusions and ProspectsProspectsProspects: The effect of ligands as the linker between
CdSe and TiO2 The effect of the gold cluster to the CdSe-
TiO2 devices
33
THANK YOU!
34
