THE ELECTRONIC AND OPTICAL PROPERTIES OF COLLOIDAL LEAD-SELENIDE SEMICONDUCTOR NANOCRYSTALS
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.
This work is primarily supported by the Center for Nanoscale Systems at Cornell University, under the Nanoscale Science and Engineering Initiative of the National Science Foundation: NSF Award # EEC-0117770
quantum dots; nanocrystals; PbSe; lead selenide; semiconductors
dissertation or thesis