Spectral properties of semiconductor nanocrystals and their applications

Abstract

The ability to engineer the optical properties of the semiconductor nanocrystals by controlling their growth - size, shape, materials, coatings, etc - makes them appealing for many optical applications. Despite the impressive development of nanocrystal manufacturing capabilities, there are still many basic questions about how to model nanocrystals that have yet to be adequately answered. This thesis investigates three important optical properties: 1) the temperature dependence of the bandedge absorption energy Eabs(T), 2) the temperature dependence of the Stokes shift, and 3) the homogeneous linewidth. We relate these properties to various nanocrystal applications with particular focus on nanocrystal based microbead barcodes. We present measurements of the temperature dependence of the absorption and emission spectra from 5 sizes of CdSe/ZnS nanocrystal ensembles. Our measurements show that dEabs(T)/dT is similar to the value for bulk CdSe for all sizes of nanocrystals, in contrast with previous experiments. We develop a model that can explain measured values of dEabs(T)/dT in both epitaxial quantum dots and colloidal nanocrystals of different materials. We interpret our measurements of the temperature dependence of the Stokes shift and linewidth, along with single nanocrystal fluorescence, from the perspective of two models based on different physical processes: 1) the fine structure of the bandedge exciton and 2) exciton-acoustic phonon scattering. We find that neither theory is able to adequately explain our measurements in isolation. We conclude that a comprehensive model that includes both physical mechanisms is required to explain our experimental results.

(cont.) We present a detailed analysis of nanocrystal based microbead barcodes for high throughput biological screening. We make design decisions for how such a system would operate, develop a Monte Carlo simulation of the expected noise, and investigate different coding architectures. We investigate this system from the perspective of information and coding theory. We develop a Monte Carlo code generation algorithm to evaluate the information capacity of this system.