Stabilizing Degenerate Dopants in Colloidal Semiconductor Nanocrystals

Abstract

Semiconductor nanocrystals possessing excess delocalized charge carriers are garnering considerable attention for their applications in a wide range of emerging technologies. In order to realize their incorporation into practical devices, a detailed understanding of their electronic structural properties is essential. Precise control over the introduction and removal of degenerate dopants from semiconductor nanostructures presents exciting opportunities for careful study of their electronic structures and associated capabilities. This thesis explores the careful incorporation and characterization of the stability of degenerate dopants in semiconductor nanomaterials. Chapter 1 provides an introduction to degenerately doped semiconductor nanocrystals, briefly describing methods to incorporate and confirm the presence of excess charge carriers. We explore factors that influence charge carrier stability in a variety of semiconductor nanocrystal lattices, and consider what limits the extent of stable carrier incorporation. Chapter 2 investigates the stability of degenerate dopants in n-doped CdSe/CdS core/shell quantum dots and explores their luminescent signatures. These studies reveal that CdS shell deposition greatly slows electron trapping, and we characterize the luminescence from a photogenerated four-carrier negative tetron (three electrons and one hole). Chapter 3 applies in situ spectroelectrochemical potentiometry to quantify the Fermi-level energy in p-doped plasmonic copper-sulfide nanocrystals. We reversibly tune the plasmon absorption band using redox chemistry, and demonstrate identical absorption bands in low-chalcocite copper-sulfide nanocrystals with and without cation vacancies. This work demonstrates that cation vacancies internal to the nanocrystal lattice are far more effective at stabilizing free carriers than anions at the nanocrystal surfaces. Chapter 4 applies magnetic circular dichroism (MCD) spectroscopy to identify classical cyclotron splittings in plasmonic semiconductor nanocrystals, and compares their magnetic signatures with those exhibited by plasmonic metal nanoparticles. We demonstrate that the sign of the MCD signal is sensitive to the sign of the carriers (n or p), and that the magnitude of the cyclotron splitting reflects the carrier effective masses for the delocalized carriers. Overall, this work focuses on stable incorporation of degenerate dopants into semiconductor nanocrystals as a method to improve our understanding of the fundamental electronic structural properties of these materials to work toward their implementation into practical device architectures.