Magnetoresistance (MR) and doping effects have been investigated in a poly(3-hexylthiophene-2,5-diyl) (P3HT) based organic light emitting diodes. In single device of fixed composition (Au/P3HT/Al as spun and processed in air), the measured MR strongly depends on the drive conditions. The magnetoconductance (MC) varies from negative to positive (-0.4% ≤ MC ≤ 0.4%) with increasing current density, depending on which microscopic mechanism dominates. The negative MC is due to bipolaron based interactions and the positive MC to triplet-polaron based interactions (as confirmed by light emission). Oxygen doping is prevalent in P3HT devices processed in air and the effect of de-doping (by annealing above the glass transition temperature) is investigated on the MC of an Au/P3HT/Al diode. De-doping reduces the current through the device under forward bias by ~3 orders of magnitude, but increases the negative (low current) MC from a maximum of -0.5% pre-annealing to -3% post-annealing. This increased negative MC is consistent with bipolaron theory predictions based on Fermi level shifts and density of states (DoS) changes due to de-doping. The decrease in current density is explained by increased injection barriers at both electrodes also resulting from de-doping. Deliberate chemical doping of P3HT is carried out using pentacene as a hole trap centre. The trapping effect of pentacene is confirmed by reproducible and significant hole mobility-pentacene concentration behaviour, as measured by dark injection (DI) transient measurements. The enhanced carrier injection resulting from the pentacene doping also leads to increased electroluminescence (EL). The resultant MC in pentacene doped devices is strongly dependent on carrier injection and can be significantly enhanced by doping, for example from -0.2% to -0.6% depending on device and drive conditions. Throughout this thesis Lorentzian and non-Lorentzian function fitting is carried out on the measured MC, although the underlying microscopic mechanisms cannot always be discerned.

In ‘clean' confined conductors (the so-called mesoscopic systems), the electronic phase and momentum can be preserved over very long distances compared to the system dimensions. This gives rise to peculiar transport properties, bearing signatures of electron interferences, ballistic electron trajectories, electron-electron interactions, regular-chaotic electron dynamics and (in some cases) spin-orbit coupling. Examples of such effects are the Universal Conductance Fluctuations (UCFs) and the Weak Localization observed in the low-temperature magnetoconductance of many confined electronic systems. Of central importance, the electronic phase coherence time and the spin-orbit coupling time determine the amplitude of these quantum effects. In the first part of this thesis, we use UCFs to extract these characteristic timescales in open ballistic quantum dots (QDs) fabricated from InGaAs heterostructures. We observe an intrinsic saturation of the coherence time at low temperature in the InGaAs QDs. The origin of this phenomenon has been intensely debated during the last decade. Based on our observations and previous experimental data in QDs, we propose an explanation: the dwell time becomes the limiting factor for electron interferences in QDs at low temperature. Then, we report on magnetoconductance measurements in a bismuth ballistic nano-cavity. The cavity is found to be zero-dimensional for phase coherent processes at low temperature. We evidence an anomalous reduction of the phase coherence time in the cavity with respect to data obtained in thin Bi films, while the spin-orbit coupling time is similar in both systems. Finally, we examine the current-voltage characteristics of asymmetric InGaAs nano-junctions in the nonlinear regime. We observe a new tunable rectification effect, whose amplitude and sign are governed by the conductances of the junctions' channels. We show that the effect is ballistic and exhibits new features with respect to predictions of available models.

Open quantum dots (OQDs) are now commonly used as an experimental tool for the investigation of a particular regime of quantum transport where the electron dynamics is both ballistic and coherent. In particular, the Universal Conductance Fluctuations (UCFs), observed in ballistic quantum dots, arise from the complex quantum interferences occurring between electron trajectories that bounce multiple times against the dot walls before escaping through its leads. Central to quantum interference phenomena is the presence of a magnetic field B that breaks the time-reversal symmetry and changes the phase experienced by electrons in the dot. OQDs are typically patterned on top of two-dimensional electron gases (2DEGs). Interestingly, when confined to wide GaAs quantum wells (QWs), 2DEGs are known to exhibit a rich physics arising from the interplay of a strong in-plane magnetic field, multiple subband occupation, and the finite thickness of the electronic wavefunction. In this thesis, we use 2DEGs, confined to wide (WQW) and narrow (NQW) quantum wells with one and two occupied subbands at B = 0 T, respectively, to study the parallel field-induced transport in open quantum dots as a function of the well width and the tilt angle of B with respect to the electron gas. Both the WQW and NQW dots feature a rich spectrum of UCFs at intermediate tilt angles and, quite unexpectedly, under a strictly parallel B. Combined with the observation, in the case of the WQW dot, of a reduction in UCFs amplitude at large parallel B, our data indicates that the finite thickness of the electron layer and the orbital effect are responsible for the in-plane field-induced UCFs. In the second part of this work, we observe a saturation of the UCFs spectral distribution, expressed in terms of an effective tilt angle, as B approaches a strictly parallel configuration, along with the persistence of a limited number of frequency components in the case of the narrow quantum well dot. It is found that the saturation angle strongly depends on the width of the 2DEG confining well. Using the results of self-consistent Poisson-Schrödinger simulations, the magnetoconductance is rescaled as a function of the Fermi level E_F in the 2DEG. A power spectrum analysis of the parallel B UCFs in energy space and its good agreement with theoretical predictions suggest that such a B to E_F mapping is indeed relevant for the interpretation of parallel B-induced UCFs

xvi, 116 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / This dissertation focuses on low-dimensional electron transport phenomena in devices ranging from semiconductor electron 'billiards' to semimetal atomic clusters to gold nanoparticles. In each material system, the goal of this research is to understand how carrier transport occurs when many elements act in concert. In the semiconductor electron billiards, magnetoconductance fluctuations, the result of electron quantum interference within the device, are used as a probe of electron transport through arrays of one, two, and three connected billiards. By combining two established analysis techniques, this research demonstrates a novel method for determining the quantum energy level spacing in each of the arrays. That information in turn shows the extent (and limits) of the phase-coherent electron wavefunction in each of the devices. The use of the following two material systems, the semimetal atomic clusters and the gold nanoparticles, is inspired by the electron billiard results. First, the output of the simple, rectangular electron billiards, the magnetoconductance fluctuations, is quite generally found to be fractal. This research addresses the question of what output one might expect from a device with manifestly fractal geometry by simulating the electrical response of fractal resistor networks and by outlining a method to implement such devices in fractal aggregates of semimetal atomic clusters. Second, in gold nanoparticle arrays, the number of array elements can increase by orders of magnitude over the billiard arrays, all with the potential to stay in a similar, phase-coherent transport regime. The last portion of this dissertation details the fabrication of these nanoparticle-based devices and their electrical characteristics, which exhibit strong evidence for electron transport in the Coulomb-blockade regime. A sketch for further 'off-blockade' experiments to realize magnetoconductance fluctuations, i.e. phase-coherent electron phenomena, is presented. / Committee in charge: Jens Noeckel, Chairperson, Physics; Richard Taylor, Member, Physics; Heiner Linke, Member, Physics; David Strom, Member, Physics; James Hutchison, Outside Member, Chemistry

Electron transport in mesoscopic systems, such as quantum point contacts and Aharonov-Bohm rings are investigated numerically in a tight-binding language with a recursive Green's function algorithm. The simulation reveals among other things the quantized nature of the conductance in point contacts, the Hall conductance, the decreasing sensitivity to scattering impurities in a magnetic field, and the periodic magnetoconductance in an Aharonov-Bohm ring. Furthermore, the probability density distributions for some different setups are mapped, making the transmission coefficients, the quantum Hall effect, and the cyclotron radius visible, where the latter indicates the correspondance between quantum mechanics and classical physics on the mesoscopic scale.