A First-Principles Study of Thiol Ligated CdSe Nanoclusters

1. A First-Principles Study of ThiolLigatedCdSeNanoclusters Shanshan Wu1 Aug 1st, 2012 • Advisor: James Glimm1,2 • Collaborators: Michael McGuigan2, • Stan Wong1,2, Amanda Tiano1 • Stony Brook University • Brookhaven National Laboratory

2. Outline • Introduction • Computational Model • Results and Discussions • Conclusions and Prospects

3. Outline • Introduction • Computational Model • Results and Discussions • Conclusions and Prospects

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

5. 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 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. IntroductionResearch 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. 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. Outline • Introduction • Computational Model • Results and Discussions • Conclusions and Prospects

8. Computational ModelDesign of Simulation Model • CdSe Quantum Dots (Cd: cyan, Se: yellow) • Ligands(HS-R-COOH) (S: orange, N: blue, C: gray, O: red, H: white) Wurtzite Bulk1 Cys HSCH(NH2)COOH Reduced Length MPA HSCH2COOH 1. Wyckoff, R.W.G., Crystal Structures. 2nd ed. Vol. 1. 1963, New York: Interscience Publishers. 85-237.

9. Computational ModelDensity Functional Theory • Time-independent Schrödinger Equation • The Kohn-Sham Approach • The Ground State Density 9

10. Computational ModelDensity Functional Theory (cont.) • The Ground State Total Energy • Exchange-correlation Functional Hybrid Functional (B3LYP) 10

11. Computational ModelLinear Combinations of Atomic Orbitals (LCAO) • Linear Combinations of Atomic Orbitals • Basis Functions • Local (Gaussian) Basis Sets • Effective Core Potential

12. Computational ModelAlgorithm of Geometry Optimization • Minimum-energy Configurations • Degrees of Freedom: Bond Lengths, Angles • Quasi-Newton Optimization

13. Computational ModelTime-dependent DFT • Time-dependent Kohn-Sham Scheme where • Time-dependent density: where • Time-dependent XC Potential

14. Computational ModelTime-dependent DFT • The orbital equation is solved iteratively to yield the minimum action solution. • The excitation energies are calculated by linear response theory

15. Computational ModelSimulation Methodology and Model Verification • LANL2DZ/6-31G* (CdSe/ligands) basis sets, B3LYP XC functional are used with NWCHEM 6.0 package • 1% difference to the reference data1 for bond length and energy gap 1. Yang, P. et al., J. of Cluster Science, 2011. 22(3): p. 405-431.

16. Computational ModelModel Validation Absorption Peak of Cys-capped Cd33Se33 Experiment ~422nm Simulation ~413 -- 460nm • Less than10% Difference with Experimental Results 1 1. Nevins, J.S. et al., ACS Applied Materials & Interfaces, 2011. 3(11), 4242.

17. Outline • Introduction • Computational Model • Results and Discussions • Conclusions and Prospects

18. 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

19. Results and DiscussionsMagic vs. Non-magic size QDs • Non-magic size QDs process weaker “self-healing” ability than Magic size ones.

20. Results and DiscussionsMagic vs. Non-magic size QDs (cont.) • Non-magic size QD has a smaller gap value and is less stable than the magic size ones. • Ligandpassivation cannot fundamentally improve the poor properties of non-magic size QDs.

21. 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.

22. 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.

23. Results and DiscussionsLigand Effects on QDs (cont.) Bare QDs vs. Passivated QDs: • Front orbitals mainly originates from CdSe, while the ligandorbitals localizing deep inside the valence and conduction band. • Surface passivation causes concentration of front CdSeorbitals. 3.14 eV 3.39 eV

24. 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.

25. Results and DiscussionsLigand Effects on QDs (cont.) Bare QDs vs. Passivated QDs: • The orbitals involved in the main transitions are unchanged by passivation.

26. Results and DiscussionsLigand Effects on QDs (cont.) Bare QDs vs. Passivated QDs: • Excited electrons are concentrated on CdSe, not on ligands.

27. 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.

28. 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.

29. 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.

30. 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. NH2Me OPMe3 1. Kilina, S., et al., J. of the Am. Chem. Soc., 2009. 131(22): p. 7717-7726.

31. Outline • Introduction • Computational Model • Results and Discussions • Conclusions and Prospects

32. 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.

33. 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

34. Thank you!