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

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 spectrally-resolved 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 spectrally- and temporally-resolved photoluminescence spectroscopy to understand the rates of excitonic energy transfer in CdSe/CdZnS core/shell QDs through a calculation of the effective dipole-dipole 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 dipole-multipole 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 electronic, optical, and thermal properties of semiconductor nanocrystals, and the ability to rationally tune these modes would offer a powerful strategy for controlling phonon-assisted processes. Chapter 4 demonstrates that surface chemistry can be used to manipulate the low-frequency acoustic vibrations of CdSe QDs, and shows in particular that surface-bound ligands modify the resonant vibrational frequencies of the core. This effect is more pronounced for smaller nanocrystals, where the surface ligands constitute a larger fraction of the overall mass. A continuum mechanics model that explicitly includes the ligand shell quantitatively reproduces most of our experimental results. This model can be extended to understand the effect of inorganic shells as well, and we demonstrate that by growing a CdS epitaxial shell we can achieve reduction in acoustic phonon frequencies by more than 70% compared to the CdSe core alone. In Chapter 5, these low frequency phonons are further measured as a function of temperature. Low-temperature measurements allow the unambiguous assignment of overtone modes in large CdSe/CdS nanocrystals to a higher order (n = 2) vibrational mode rather than a multiphonon mode. Additionally, the acoustic phonon frequency is shown to vary with temperature though the linewidth remains constant for a variety of sizes of QDs. This variation of frequency without a corresponding broadening suggests that the pure volume contribution to the temperature-dependent phonon energies dominates over phononphonon interactions through anharmonic coupling.