Experimental and Computational Investigation of Colloidal Semiconductor Nanocrystals with Excess Charge Carriers: Effects of Surface Interactions on Electronic Properties

Abstract

Band engineering in nanomaterials facilitates wide variety of applications in semiconductor electronics. Understanding properties that enable and govern band engineering is critical for implementing the functionalities that electronically doped nanocrystals provide. The aim of this work is to better understand the effects of surface interactions on electronic properties of semiconductor nanocrystals, explore the emergence of supercapacitance, and examine the effects of band engineering on electron transfer in complex nanocrystal systems. To achieve this aim, experimental methods for nanocrystal synthesis and characterization are combined with a computational framework for modeling of nanocrystal-ligand interfaces. Unique energy storage properties of ZnO and ZnO-related colloidal nanocrystals are explored with focus on supercapacitance and band edge potential tuning. It is shown that ZnO acquires a surprisingly high capacitance under various conditions, such as aluminum doping, interactions with Li+, and shape/media changes, due to electrostatic interactions on the nanocrystal surface and within the tunneled ZnO crystal structure. Modulation of band edge potentials by perturbations in surface chemistry is explored on colloidal PbS, CdS, and CdSe quantum dots. CdS presents a particularly interesting system to investigate, as it exhibits an apparent change in electrochemical properties when mixed with ZnO nanocrystals, forming an electron transfer pair. Experimental evidence and modeling of CdS-ligand interactions support the hypothesis that a shift in band edge potentials happens upon mixing of ZnO and CdS nanocrystals due to change in total dipole caused by new ligands present in the mixture, thus enabling CdS-to-ZnO electron transfer. More broadly, it is shown in this dissertation that ligands have a profound effect on electronic properties of various colloidal nanocrystal materials. The developed computational approach generalizes this phenomenon and offers an opportunity to predict, through modeling, the effects of ligand-nanocrystal interactions on nanocrystal properties and resulting behavior of complex nanomaterial systems. Overall, this work provides new insights into properties of colloidal semiconductor nanocrystals with excess charge carriers and informs rational design and tuning of electronic properties for next-generation nanomaterials and applications.