Shanshan Wu1
Aug 1st, 2012
Advisor: James Glimm1,2
Collaborators: Michael McGuigan2,
Stan Wong1,2, Amanda Tiano1
1. Stony Brook University
2. Brookhaven National Laboratory
Introduction
 Computational Model
 Results and Discussions
 Conclusions and Prospects

2
Introduction
 Computational Model
 Results and Discussions
 Conclusions and Prospects

3



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.
1. Renewables 2011 Global Status Report. REN21, 2011: p. 17-18.
4



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
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.
5



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.
1. 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.
6
Introduction
 Computational Model
 Results and Discussions
 Conclusions and Prospects

7

CdSe Quantum Dots (Cd: cyan, Se: yellow)
Wurtzite Bulk 1

Ligands (HS-R-COOH) (S: orange, N: blue, C: gray, O: red, H: white)
Cys
MPA
Reduced Length
HSCH(NH2)COOH
HSCH2COOH
1. Wyckoff, R.W.G., Crystal Structures. 2nd ed. Vol. 1. 1963, New
York: Interscience Publishers. 85-237.
8

Time-independent Schrödinger Equation
H  i ( r )   i i ( r )

The Kohn-Sham Approach
H  
1
  V KS ( r )
2
2
 
1
2

  V ext ( r )  V Hartree ( r )  V XC ( r )
2
The Ground State Density
N
n(r)  2  i (r)
2
i 1
9

The Ground State Total Energy
E KS [ n ]  T [ n ]   drV ext [ n ]n ( r ) 

1
2
'
 drdr
'
n ( r )n ( r )
rr
'
 E xc [ n ]
Exchange-correlation Functional
E xc [ n ] 
LDA
GGA
3
E
[
n
]

d
d
r
n
(
r
)

(
n
)
xc
xc
 r n ( r ) xc ( n ,  n )

3
Hybrid Functional (B3LYP)
10

Linear Combinations of Atomic Orbitals
i 
 C p ,  ( r  R p )
i
p ,

Basis Functions
 (r)   c  (r)   c






Y lm  n ( r )

Local (Gaussian) Basis Sets
N
 n (r) 
d
2 2
in
e
 a in f n r
i 1

Effective Core Potential
11

Minimum-energy Configurations
F (R)  


E ( R )
R
0
Degrees of Freedom: Bond Lengths, Angles
Quasi-Newton Optimization
f ( xk  x )  f ( xk )   f ( xk ) x 
T
1
x Bx
T
2
12

Time-dependent Kohn-Sham Scheme
 2

i  j  r , t   H ( r , t ) j  r , t    
 V s  r , t  
t
2



where
j
r , t 
V s [ n ]r , t   V ext ( r , t )  V Hartree [ n ]( r , t )  V xc [ n ]( r , t )
N

Time-dependent density: n  r , t   2   j  r , t 
where

j 1


2
gs




r 
r
,
t

a
t

gs
 jk
j
k


r  ,

r
,
t


j
0
j
k 1
j  1,..., N
Time-dependent XC Potential
V xc n  r , t   V xc n ( t )  r 
gs
13

The orbital equation is solved iteratively to yield the
minimum action solution.
A( n ) 

T
t0
dt  j ( r , t ) i  t  H ( r , t )  j ( r , t )
A( n )
n ( r , t )

0
The excitation energies are calculated by linear
response theory
 V ext ( r , t )   n ( r , t ),
n
 V ext
, spectra
14


LANL2DZ/6-31G* (CdSe/ligands) basis sets, B3LYP
XC functional are used with NWCHEM 6.0 package
1% difference to the reference data
and energy gap
1
for bond length
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)
1. Yang, P. et al., J. of Cluster Science, 2011. 22(3): p. 405-431.
15
Absorption Peak of Cys-capped Cd33Se33
1
Experiment ~422nm

Simulation ~413 -- 460nm
Less than10% Difference with Experimental Results
1. Nevins, J.S. et al., ACS Applied Materials & Interfaces, 2011. 3(11), 4242.
16
Introduction
 Computational Model
 Results and Discussions
 Conclusions and Prospects

17

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

Non-magic size QDs process weaker
ability than Magic size ones.
“self-healing”
19


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.
20
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.
21
Bare QDs vs. Passivated QDs:
 CdSe structures are almost preserved after saturation.
 An opening of energy gap by 7%~10% is observed by
passivation.
22
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.
3.14 eV
3.39 eV
23
Bare QDs vs. Passivated QDs:
 Passivation gives doubled intensity of absorption
spectrum with a blue shift by ~0.2 eV.
24
Bare QDs vs. Passivated QDs:
 The orbitals involved in the main transitions are
unchanged by passivation.
Composition of Main Transitions
from TDDFT Calculation
System
Energy
(eV)
Oscillator
Strength
Cd13Se13+Cys
2.90
0.0865
H-2 (Se 4p) — L (Cd 5s, Se4p)
3.25
0.2272
H-9 (Se 4p) — L
2.72
0.0637
H-2 (Se 4p) — L (Cd 5s, Se 5s)
3.02
0.1042
H-9 (Se 4p) — L
Cd13Se13
Excited-State
Composition
25
Bare QDs vs. Passivated QDs:
 Excited electrons are concentrated on CdSe, not on
ligands.
26
Effects of Length and Function Group (NH2):
 Varying the length of ligands has only a minor effect
on the structure and energy gap.
27
Effects of Length and Function Group (NH2):
 Cys- and MPA-capped QDs
obtain rather close structures
and energy gaps.
28
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
Compare Thiol with Amine and Phosphine:
 Thiol opens the HOMO-LUMO gap by 11% vs.
NH2Me by 7% and OPMe3 by 5% 1.
NH2Me
OPMe3
1. Kilina, S., et al., J. of the Am. Chem. Soc., 2009. 131(22): p. 7717-7726.
30
Introduction
 Computational Model
 Results and Discussions
 Conclusions and Prospects

31
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.
32
Prospects:
 The effect of ligands as the linker between CdSe and
TiO2
 The effect of the gold cluster to the CdSe-TiO2 devices
33
34