The major difference between semiconductor quantum dots and bulk semiconductors is in the quantum confinement effect. It results the controllable exciton¡¦s absorption and emission spectra by tuning the size of the quantum dot. Moreover, multi-exciton states are reported to be observed in the highly symmetric quantum dot systems. In this dissertation, we use the single molecule fluorescence measurement to study the power dependence of multi-exciton state in single CdSe/ZnS semiconductor quantum dots. At low excitation fluence, anti-bunching behavior, and nearly single exponential relaxation dynamics are observed. By increasing the laser power, bi-exponential fluorescence decay dynamics as well as bunching behaviors from the same QD indicate the fast PL dynamics due to the relaxation from multi-exciton. The results indicate certain threshold energy level for multi-exciton generation. In addition, the multiple step cascade radiative relaxation processes are observed. Besides, we modulate linear polarization light to study the excitation orientation dependence. The results indicate the emission dipole of multi-exciton is similar to the single exciton, having a two dimensional transition dipole plane with c-axis symmetry. However, the absorption dipole of multi-exciton exhibits different orientation dependence from the single exciton.

Neste trabalho foram estudados os processos de migração e transferência de energia em derivados de polifluoreno, de estrutura totalmente conjugada e de conjugação confinada. A dinâmica de estado excitado dos derivados PF-BNI, PFOPPV e PFPPV foi estudada em solução diluída de diferentes solventes através de espectroscopia eletrônica de alta resolução temporal. O derivado PF-BNI, contendo N-(2-benzotiazol)-1,8-naftalimida (BNI) como terminador da cadeia de poli(9,9\'dioctilfluoreno), foi sintetizado pela rota de Yamamoto. O material apresenta alto rendimento quântico de fluorescência e suas propriedades de emissão são moduladas pela polaridade do solvente. As medidas de fluorescência resolvida no tempo mostraram a migração do exciton singlete ao longo da cadeia polimérica e a formação de um estado excitado de transferência de carga intracadeia (ICCT). Os derivados PFOPPV e PFPPV são copolímeros de fluoreno contendo unidades fluoreno-vinileno-fenileno no segmento cromofórico. No PFOPPV o segmento cromofórico é confinado entre segmentos alifáticos (-(CH2)8-) flexíveis, no PFPPV a cadeia principal é totalmente conjugada. A dinâmica de estado excitado dos derivados, PFOPPV e PFPPV, é caracterizada pela presença de segmentos cromofóricos contendo isômeros cis e trans. No PFOPPV, a sua estrutura segmentada permite a transferência de energia entre os segmentos cromofóricos, via interações dipolo-dipolo. A transferência acarreta a depolarização da emissão de fluorescência. No caso do PFPPV a migração de energia ocorre em menos de 20 ps e o decaimento de fluorescência decorre da emissão de segmentos contendo isômeros cis e trans, já na condição estacionária. / This work reports the study of energy transfer and migration processes in fully conjugated and segmented polyfluorene derivatives. The excited-state dynamics of the derivatives, PF-BNI, PFOPPV and PFPPV was studied in diluted solution of different solvents by means of ultrafast time-resolved spectroscopy. Poly(9,9\'-dioctylfluorene) end-capped with N-(2-benzothiazole)-1,8-naphthalimide, named PF-BNI, was prepared via Yamamoto-coupling reaction. This derivative is a highly fluorescence material with emission modulated by solvent polarity. Time-resolved fluorescence measurements showed the singlet exciton migration through the polymer backbone and the formation of an intrachain charge transfer excited-state (ICCT). PFOPPV and PFPPV are both fluorene copolymers containing fluorene-vinylene-phenylene moieties in the backbone. Whereas the PFPPV backbone is fully conjugated, the chromophore segment in PFOPPV backbone is confined between aliphatic (-(CH2)8-) flexible segments. The excited-state dynamics of both copolymers is characterized by the presence of conjugated moieties containing cis and trans isomers. The segmented structure of PFOPPV allows the resonant energy transfer between the chromophores, which is provided by dipole-dipole interactions. The energy transfer process leads to the depolarization of PFOPPV fluorescence emission. For PFPPV the energy migration occurs in less than 20 ps and the fluorescence decay is ascribed to the emission of chromophore segments containing cis and trans, already in a photostationary condition.

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.

Indiana University-Purdue University Indianapolis (IUPUI) / In this thesis, we focus on two broad categories of problems, exciton condensation and bound states, and two complimentary approaches, real and momentum space, to solve these problems. In chapter 2 we begin by developing the self-consistent mean field equations, in momentum space, used to calculate exciton condensation in semiconductor heterostructures/double quantum wells and graphene. In the double quantum well case, where we have one layer containing electrons and the other layer with holes separated by a distance $d$, we extend the analytical solution to the two dimensional hydrogen atom in order to provide a semi-quantitative measure of when a system of excitons can be considered dilute. Next we focus on the problem of electron-electron screening, using the random phase approximation, in double layer graphene. The literature contains calculations showing that when screening is not taken into account the temperature at which excitons in double layer graphene condense is approximately room temperature. Also in the literature is a calculation showing that under certain assumptions the transition temperature is approximately \unit{mK}. The essential result is that the condensate is exponentially suppressed by the number of electron species in the system. Our mean field calculations show that the condensate, is in fact, not exponentially suppressed. Next, in chapter 3, we show the use of momentum space to solve the Schr\"{o}dinger equation for a class of potentials that are not usually a part of a quantum mechanics courses. Our approach avoids the typical pitfalls that exist when one tries to discretize the real space Schr\"{o}dinger equation. This technique widens the number of problems that can presented in an introductory quantum mechanics course while at the same time, because of the ease of its implementation, provides a simple introduction to numerical techniques and programming in general to students. We have furthered this idea by creating a modular program that allows students to choose the potential they wish to solve for while abstracting away the details of how the solution is found. In chapter 4 we revisit the single exciton and exciton condensation in double layer graphene problems through the use of real space lattice models. In the first section, we once again develop the equations needed to solve the problem of exciton condensation in a double layer graphene system. In addition to this we show that by using this technique, we find that for a non-interacting system with a finite non-zero tunneling between the layers that the on-site exciton density is proportional to the tunneling amplitude. The second section returns to the single exciton problem. In agreement with our momentum space calculations, we find that as the layer separation distance is increased the bound state wave function broadens. Finally, an interesting consequence of the lattice model is explored briefly. We show that for a system containing an electron in a periodic potential, there exists a bound state for both an attractive as well as repulsive potential. The bound state for the repulsive potential has as its energy $-E_0$ where $E_0$ is the ground state energy of the attractive potential with the same strength.

The exciton diffusion length, which is the distance an exciton can diffuse in its lifetime, is an important parameter that has a critical impact on the operation of many organic optoelectronic devices, including organic solar cells, light emitting diodes and lasers. Knowledge of the exciton diffusion length can be a powerful aid for the design and optimisation of these devices. This thesis details the development of techniques based on time-resolved fluorescence for measuring the exciton diffusion in organic semiconductors. Two main methods were used to investigate exciton diffusion in the conjugated polymers P3HT, MEH-PPV and F8BT: the surface quenching technique and exciton-exciton annihilation. In particular, the surface quenching technique was adapted to avoid some of the potential pitfalls that have plagued earlier measurements. Using a titania quencher, measurements were performed using the surface quenching technique and fitted with an exciton diffusion model, allowing the calculation of the exciton diffusion length. Results from measurements of the exciton-exciton annihilation rate, which is a diffusion controlled process, where in good agreement with those from surface quenching, confirming the robustness of this twofold approach. A novel method for the control of the β-phase conformation in PFO films was used to produce films containing varying concentrations of β-phase. Exciton-exciton annihilation was used to investigate exciton diffusion in these films, revealing a gradual rise with increasing β-phase fraction due to improved interconnectivity. This work demonstrates how simple processing techniques can be used to control both film morphology and the exciton diffusion. The thickness dependence of the photoluminescence lifetime in conjugated polymers is a phenomenon that has so far received little attention and, thus, remained unexplained. This study demonstrates that it is not due to exciton quenching by external factors, but can be explained by a change in the morphology with decreasing film thickness.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

This thesis presents studies of exciton diffusion in organic semiconductors measured using exciton-exciton annihilation and the measurements were performed on materials important for organic solar cells. In the conjugated polymer poly(3-hexylthiophene) (P3HT), the effect of molecular weight (4-76 kgmol⁻¹) was explored. Using exciton-exciton annihilation measurements, the highest diffusion coefficient was observed in the intermediate molecular weight region and was correlated with long conjugation lengths, higher fraction of aggregated states and more delocalised excitons within the aggregate. The results demonstrated that the molecular weight dependence is due to a complex relationship between intermolecular interactions, aggregate size and Boltzmann statistics. This thesis also includes an investigation of exciton diffusion in diketopyrrolopyrrole(DPP)-based small molecules as a function of molecular structure. Significant changes in photophysical and exciton diffusion properties were observed due to minor changes in molecular structure. Long conjugation lengths, bulky side chains or reduced steric hindrance due to absence of end alkyl chains correlated with reduced film crystallinity and reduced diffusion coefficients. The increase in disorder observed due to large conformational torsions resulted in inhomogeneous broadening of density of states and as a result exciton diffusion becomes dispersive. In this case, a slowdown of exciton diffusion is observed. This study demonstrates that enhanced exciton diffusion can be achieved by designing more rigid and planar conjugated backbones with smaller conjugation lengths. Finally, exciton diffusion measurements were used to rationalise the performance of T3 truxene oligomers as explosive sensors. Side chain lengths were found to have a subtle influence on exciton diffusion. Time-resolved PL quenching measurements were used to estimate the quencher concentration. Differences in quencher concentration were observed suggesting different interaction strengths of the quencher with the truxene oligomer which help explain the explosive sensing performance.

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.