Date of Award

1-24-2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

Arindam Chakraborty

Keywords

Green's Functions;Inverse Photoemission;Quantum Confined Stark Effect;Quantum Dots;SSE-MO;Stratified Stochastic Enumeration

Subject Categories

Chemistry | Computational Chemistry | Physical Sciences and Mathematics

Abstract

There are three main focuses of this work. First, the theoretical details of the Stratified Stochastic Enumeration of Molecular Orbitals (SSE-MO) method is presented, along with its application for calculating ionization potentials (IPs) of quantum dots. The SSE-MO method can readily be applied for the purpose of efficiently and accurately calculating ionization potentials, by constructing the frequency-dependent self-energy operator and then subsequently, solving the associated Dyson equation. Constructing the frequency-dependent self-energy operator is challenging because the scaling of the computational cost with respect to system size, becomes prohibitive for large systems, such as quantum dots. This is due to the large number of 2particle-1hole (2p1h) and 1particle-2hole (1p2h) terms that must be computed. The key strategy of the SSE-MO method is to utilize a stratified stochastic enumeration scheme in order to efficiently construct the 2p1h and 1p2h terms of the self-energy operator, while maintaining high accuracy. Second, a computational and theoretical investigation into the inverse photoemission processes in a variety of quantum dots (CdS, CdSe, PbS, and PbSe) is presented. Inverse photoemission occurs when an incident electron is captured by a material in one of the high energy unoccupied states. This captured electron then subsequently de-excites to a lower energy unoccupied state, resulting in the emission of a photon. We investigated the inverse photoemission (IPE) processes in these dots, both in the absence of an external electric field and when the dots are in the presence of a Stark field. In order to construct the spectra for the CdS, CdSe, PbS, and PbSe dots, we combined the Frequency-Dependent Geminal-Screened interaction kernel method (FD-GSIK) with time-dependent perturbation theory. Studying the inverse photoemission processes in quantum dots, can provide insight that is valuable for a variety of applications including, but not limited to, the development of scintillators and for achieving a greater understanding of the surface chemistry of materials. Additionally, investigation into the impact of Stark fields on the IPE processes in these materials can provide important information that can aid in the identification of materials that are useful for electroluminescence applications and for the development of new highly controllable photon sources. Furthermore, investigating the effects of the direction and magnitude of Stark fields on the IPE spectra of materials can provide a route to systematically enhance inverse photoemission transition probabilities and alter the energy of the emitted photon. The third focus of this work, is a theoretical and computational investigation into field-assisted photoionization in PbS clusters and quantum dots. In particular, the results from this investigation into how the IPs, for ionization from the HOMO, of Pb4S4, Pb44S44, and Pb140S140 are impacted by the application of non-ionizing Stark fields, of differing strengths and directions. The ability to control the position of the energy levels in quantum dots is highly relevant for the optimization of optoelectronic devices. In order to obtain a first-order approximation to the field-dependent Green's function IPs, of these PbS systems, we employed the recently developed SSE-MO method accompanied by the use of a Padé approximation. The results presented in this chapter indicate that the ionization potentials of PbS clusters and quantum dots can be manipulated by carefully fine-tuning the magnitude and direction of applied static electric fields.

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