Exploring excitations and vibrations in semiconductor nanocrystals through fluorescence and Raman spectroscopy

TLDR: In this paper, the authors used Förster theory to understand the photoluminescence and vibrational characteristics of QDs, including spectrally-resolved time-correlated single photon counting measurements to understand QD photon lifetime as a function of emission wavelength and low-frequency Raman spectroscopy to measure acoustic phonons in nanocrystal solids.

Abstract: Semiconductor nanocrystals, also known as quantum dots (QDs) have been used in solid state light emission applications ranging from fluorescent downconverters to LEDs and lasers, as well as energy generation devices such as solar photovoltaics and thermoelectrics. In order to realize these myriad applications, the fundamental physics of both electronic and vibrational energy transfer must be understood to engineer better device performance. This thesis begins with a general introduction to the physics and chemistry of QDs as well as an introduction to lattice vibrations, including a proposed model for understanding thermal conductivity in solid state QD-based devices. It continues with a discussion of the methods used to understand the photoluminescence and vibrational characteristics of QDs, including spectrallyresolved time-correlated single photon counting measurements to understand QD photoluminescence lifetime as a function of emission wavelength, and low-frequency Raman spectroscopy to measure acoustic phonons in nanocrystal solids. These two chapters serve as an introduction to the ideas and methods used throughout the thesis. In Chapter 3, Förster theory is used in conduction with spectrallyand temporallyresolved photoluminescence spectroscopy to understand the rates of excitonic energy transfer in CdSe/CdZnS core/shell QDs through a calculation of the effective dipoledipole coupling distance known as the Förster radius. This work demonstrated energy transfer rates between donor and acceptor QDs between 10-100 times faster than the predictions based on standard applications of Förster theory, corresponding to an effective Förster radius of 8-9 nm in close packed QD films. Several possible effects, including an enhanced absorption cross section, ordered dipole orientations, or dipolemultipole coupling, can explain the observed difference between our measurements and the Förster theory predictions, demonstrating that several standard assumptions commonly used for calculating QD resonant energy transfer rates must be carefully considered when the QDs are in a thin-film geometry. Chapters 4-5 involve the use of low-frequency Raman spectroscopy to probe acoustic phonons in QDs. These low-frequency acoustic vibrations affect the