Report

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 ) rr ' 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 Bx 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