THE ELECTRONIC AND OPTICAL PROPERTIES OF COLLOIDAL LEAD-SELENIDE SEMICONDUCTOR NANOCRYSTALS

Abstract

Quantum dots of the IV-VI semiconductors, and specifically lead selenide, strongly confine both electrons and holes, leading to a dramatic modification of the bulk semiconductor properties. This dissertation is devoted to the study of the electronic and optical properties of colloidal lead-selenide nanocrystals or quantum dots. We begin by discussing the synthesis and characterization of high-quality colloidal lead-selenide nanocrystals with a narrow size distribution and well-passivated surfaces. With diameters between 3 and 8 nanometers, these lead-selenide quantum dots exhibit size-quantized transitions in the infrared region of the electromagnetic spectrum and exhibit bright band-edge photoluminescence tunable from approximately 1000 to 2000 nanometers. These properties are extremely promising for applications. The current theoretical understanding of the electronic states of IV-VI semiconductor quantum dots is based on envelope function approaches and tight-binding methods. While successful in explaining many features of the electronic structure, all current calculations fail to explain the presence of additional peaks in the optical absorption spectrum of lead-selenide and lead-sulfide quantum dots. We re-examine the leading explanations for these unexplained transitions and also consider a new possibility, that of enhanced electric quadrupole transitions. In addition, the degeneracy of the lowest optical transition in IV-VI quantum dots is predicted to split by the intervalley coupling of the 4 equivalent L-valleys in the first Brillouin zone. Low-temperature photoluminescence and size-selective photoluminescence experiments reveal, for the first time, a splitting in the emission spectra of lead-selenide and lead-sulfide nanocrystals. These observations are consistent with a theoretical treatment of the splitting of the lowest transition in lead-selenide quantum dots due to intervalley coupling.The dynamics of electrons and holes are crucially influenced by quantum confinement. In the strong confinement limit, a dramatic reduction in the excited state (or intraband) relaxation rate of carriers is predicted to occur. With its sparse electronic states and simple energy spectra, lead-selenide quantum dots represent an ideal material system in which to study the intraband carrier relaxation. We present the first measurements to directly time-resolve the intraband relaxation of electrons and holes in lead-selenide nanocrystals. Prior theories cannot explain the observed picosecond time-scale intraband relaxation and we discuss several possible explanations.