Hybrid systems and the role of the linker molecule in charge transfer heterostructures : a first principles study

Abstract

Hybrid systems are composed of materials like nanorods and sheets that have reduced dimensionality. These hybrid systems have the potential to revolutionize the way we harvest energy and design systems at the nanoscale. One such example---colloidal quantum dots---are a 0D material, and show remarkable properties useful for photovoltaics and photocatalysis. The ligand molecules that tether these QDs to another surface possess a role that is not well understood. In this thesis, first principles calculations are employed to study these hybrid systems. First, density functional theory is used to understand how a model linker molecule (cysteine) chemically links the QD to the surface. We find configurations for cysteine in various protonation states linked to CdSe(100) and CdSe(001) via a Monte Carlo simulation. Next, we investigate what effect this linker molecule has on the charge transfer between the QD and the surface using the novel three-way-heterostructure from the previous work. We find that the linker molecule, when interacting with both sides, undergoes an intramolecular charge transfer, and the resulting system has greatly enhanced charge transfer based on Marcus Theory. Then, a dynamical simulation of this heterostucture was performed using time-dependent DFT (TD-DFT) in order to explore how the ligand affect the charge transfer at the ultra-fast timescale. We find that the ligand molecule and increased ionic temperature, enhance the charge transfer at this timescale, and that electrons are preferentially transferred from CdS to MoS2 as band alignment would predict. Finally, the study of hybrid systems is brought to a twisted bilayer of 2-dimensional h-BN, where the possible Moire patterns are analyzed and enumerated, and then a study of intercalated transition metal defects is conducted.