1. Shanshan Wu 1 Aug 1st, 2012 Advisor: James Glimm 1,2 Collaborators: Michael McGuigan 2, Stan Wong 1,2, Amanda Tiano 1 1.Stony Brook University 2.Brookhaven National Laboratory

2.  Introduction  Computational Model  Results and Discussions  Conclusions and Prospects 2

3. 3  Introduction  Computational Model  Results and Discussions  Conclusions and Prospects

4.  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. 4 1. Renewables 2011 Global Status Report. REN21, 2011: p. 17-18.

5.  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 TiO 2 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.

6.  Thiol (Cysteine/MPA) replaces amine or phosphine oxide as the surfactant for CdSe-TiO 2 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. 6 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.

7.  Introduction  Computational Model  Results and Discussions  Conclusions and Prospects 7

8.  CdSe Quantum Dots (Cd: cyan, Se: yellow)  Ligands (HS-R-COOH) (S: orange, N: blue, C: gray, O: red, H: white) 8 Cys MPA Reduced Length HSCH(NH 2 )COOH HSCH 2 COOH Wurtzite Bulk 1 1. Wyckoff, R.W.G., Crystal Structures. 2nd ed. Vol. 1. 1963, New York: Interscience Publishers. 85-237.

9.  Time-independent Schrödinger Equation  The Kohn-Sham Approach  The Ground State Density 9

10.  The Ground State Total Energy  Exchange-correlation Functional Hybrid Functional (B3LYP) 10

11.  Linear Combinations of Atomic Orbitals  Basis Functions  Local (Gaussian) Basis Sets  Effective Core Potential 11

12. 12  Minimum-energy Configurations  Degrees of Freedom: Bond Lengths, Angles  Quasi-Newton Optimization

13.  Time-dependent Kohn-Sham Scheme where  Time-dependent density: where  Time-dependent XC Potential 13

14.  The orbital equation is solved iteratively to yield the minimum action solution.  The excitation energies are calculated by linear response theory 14

15.  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 System Cd-Se Bond Length (Å) (intra / inter layer) HOMO-LUMO Gap (eV) Cd 6 Se 6 2.699 / 2.862 (2.670 / 2.864)3.14 (3.14) Cd 13 Se 13 2.710 / 2.801 (2.704 / 2.785)3.06 (2.99) 15 1. Yang, P. et al., J. of Cluster Science, 2011. 22 (3): p. 405-431.

16. Absorption Peak of Cys-capped Cd 33 Se 33 Experiment ~422nm Simulation ~413 -- 460nm  Less than10% Difference with Experimental Results 16 1. Nevins, J.S. et al., ACS Applied Materials & Interfaces, 2011. 3 (11), 4242. 1

17.  Introduction  Computational Model  Results and Discussions  Conclusions and Prospects 17

18.  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 (NH 2 ) Compare Thiol with Amine and Phosphine 18

19. 19  Non-magic size QDs process weaker “self-healing” ability than Magic size ones.

20. 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.

21. 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

22. 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

23. 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

24. Bare QDs vs. Passivated QDs:  Passivation gives doubled intensity of absorption spectrum with a blue shift by ~0.2 eV. 24

25. 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 Cd 13 Se 13 +Cys 2.900.0865H-2 (Se 4p) — L (Cd 5s, Se4p) 3.250.2272H-9 (Se 4p) — L Cd 13 Se 13 2.720.0637H-2 (Se 4p) — L (Cd 5s, Se 5s) 3.020.1042H-9 (Se 4p) — L

26. Bare QDs vs. Passivated QDs:  Excited electrons are concentrated on CdSe, not on ligands. 26

27. Effects of Length and Function Group (NH 2 ):  Varying the length of ligands has only a minor effect on the structure and energy gap. 27

28. Effects of Length and Function Group (NH 2 ):  Cys- and MPA-capped QDs obtain rather close structures and energy gaps. 28

29. Effects of Length and Function Group (NH 2 ):  Varying length and including the amine group of ligand show nearly no effect on the active absorption peaks. 29

30. Compare Thiol with Amine and Phosphine:  Thiol opens the HOMO-LUMO gap by 11% vs. NH2Me by 7% and OPMe3 by 5% 1. 30 1. Kilina, S., et al., J. of the Am. Chem. Soc., 2009. 131(22): p. 7717-7726. NH 2 Me OPMe 3

31.  Introduction  Computational Model  Results and Discussions  Conclusions and Prospects 31

32. 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

33. Prospects:  The effect of ligands as the linker between CdSe and TiO2  The effect of the gold cluster to the CdSe-TiO2 devices 33

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