The photoluminescence spectrum of phosphorus-doped silicon at 17 _ 3 19-3 dopant concentrations ranging from 1.2 x 10¹⁷ cm⁻³ to 4.0 x 10¹⁹ cm⁻³ is studied as a function of excitation intensity. The spectra are interpreted tn terms of two types of recombination events', one' attributed to the recombination of oppositely charged carriers inside an electron-hole droplet and the other outside due to the recombination -of free holes with electrons in the impurity band. The latter type of event gives rise to a new photoluminescence peak observed for the first time. The line shape of this peak compares very well with a first principle calculation of the impurity band density of states within the Hubbard model. Existing theories for the ground state energy of an electron-hole droplet in n-type heavily doped silicon are reviewed and new numerical results are presented. However, within the present model droplets are not theoretically understood at this time in heavily-doped silicon. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The crux of all modern semiconductor technology is the exciton, the bound electron-hole pair that drives everything from photovoltaics to light emitting diodes to transistors. This dissertation explores how we can develop materials that are able to control the energetics of excitons, by splitting them and combining them. Also explored are the applications allowed by the control of exciton energetics. The topics covered in this thesis are singlet exciton fission, and triplet fusion upconversion. Chapter 1 will introduce these concepts, and provide an overview of these fields. Chapter 2 discusses the singlet fission properties of a fully conjugated tetracene polymer and its derivatives. This chapter includes the synthesis of these materials, their photophysical properties, as well as their incorporation into bilayer semiconducting devices and their properties under an applied magnetic field. Chapter 3 explores the study of an organic-inorganic hybrid singlet fission triplet acceptor complex. A singlet fission capable pentacene dimer was covalently linked to an iron-oxo cluster with an electron affinity appropriate to accept triplets generated from singlet fission. This chapter explores the synthesis and photophysical properties of this hybrid complex, as well as the nature of the triplet pair state generated from intramolecular singlet fission. In Chapter 4, a new design rule for intramolecular singlet fission is studied, the energy sink. A series of pentacene dimers spaced by tetracene bridges are synthesized, and their singlet fission properties are explored via transient absorption spectroscopy and time resolved electron spin resonance spectroscopy. Chapter 5 begins the portion of the thesis focused on triplet fusion upconversion. The lessons learned from previous work in intramolecular singlet fission are applied to synthesize more efficient annihilators for upconversion. A series of tetracene dimers are synthesized, and their upconversion properties are explored. The work demonstrates intramolecular triplet fusion as a method to enhance the performance of existing annihilators. Chapter 6 details the discovery that diketopyrrolopyrroles can be used as upconversion annihilators. The advantages of these materials relative to existing annihilators are explained. Energetic design rules for upconversion annihilators are also discussed. Finally, in Chapter 7 a new application of triplet fusion upconversion is explored: infrared light sensitized photoredox catalysis.

No description available.

A new method is developed for treating atoms and molecules in a magnetic field in a gauge-invariant way using the Rayleigh-Ritz energy variational principle. The energy operator depends on the vector potential which must be chosen in some gauge. In order to adapt the trial wave function to the gauge of the vector potential, the trial wave function can be multiplied by a phase factor which depends on the spatial coordinates. When the energy expectation value is minimized with respect to the phase function, the equation for charge conservation for stationary states is obtained. This equation can be solved for the phase function, and the solution used in the energy expectation value to obtain a gauge-invariant energy. The method is applicable to all quantum mechanical systems for which the variational principle can be applied. It ensures satisfaction of the charge conservation condition, a gauge-invariant energy, and the best upper bound to the ground-state energy which can be obtained for the form of trial wave function chosen.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

Thesis (Ph.D.)-- University of Adelaide, Dept. of Physical and Inorganic Chemistry, 1982. / Typescript (photocopy).

Thesis (Ph. D.)--University of Hong Kong, 1997. / Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1982. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 129-1390).

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy