Exploring the Photophysics of Engineered and Intrinsic Charge-Carrier Trapping Processes in Semiconductor Nanocrystals

Abstract

Colloidal semiconductor nanocrystals (NCs) have been a topic of extensive research over the past few decades, as the materials’ photoluminescence (PL) properties are particularly interesting for a variety of applications. Emissive materials are only of interest, however, if the highest PL efficiencies can be attained—something that has yet to be achieved for a variety of NCs. Low PL efficiencies can often be blamed on unwanted charge-carrier trapping processes that limit band edge recombination. This thesis describes methods of addressing charge-carrier trapping, including incorporating an efficient “engineered” trap to control charge-carrier movement as well as targeted synthetic and spectroscopic studies to eliminate surface trapping in semiconductor NCs. Chapter 1 provides a general overview of these topics accompanied by relevant literature studies on both engineered traps and surface trapping in semiconductor NCs. Chapter 2 presents ultrafast spectroscopic data for Cu+:CdSe/CdS NCs, a system in which copper serves as an efficient hole trap. The data include time-resolved photoluminescence (TRPL) spectroscopy to study hole localization at copper after photoexcitation and transient absorption (TA) spectroscopy to observe transitions involving excited-state Cu2+. Chapter 3 details the synthesis and spectroscopic results for a series of Ag1–xCuxInS2 NCs, a material of interest due to the spectroscopic similarities of CuInS2 NCs to the copper-doped materials. Through creating a synthetic bridge from AgInS2 to CuInS2 NCs, we can monitor the PL energy as a function of increasing copper content. This aims to help support previous assignments that the PL in CuInS2 NCs arises due to a self-trapped exciton. Chapter 4 discusses the importance of electron and hole trapping in determining the PLQY of InP NCs, a material with inherently low PL efficiencies. Studying a sample series of InP NCs with five different surface chemistries using both TRPL and TA allows us to investigate how different surface treatments affect charge-carrier trapping processes. Together, these studies help elucidate the role charge-carrier trapping plays in determining the PL efficiencies of colloidal semiconductor nanocrystals and will help us design targeted synthetic strategies for obtaining materials with 100% PL efficiencies.