Browsing by Subject "Exciton"
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Item Electron interactions in 2D materials : excitons and quantum hall effect(2016-08) Wu, Fengcheng; MacDonald, Allan H.; Li, Xiaoqin; Fiete, Gregory A.; Niu, Qian; Tutuc, EmanuelThis dissertation presents studies of the electron interaction effects in two-dimensional materials. In particular, excitonic effect in transition metal dichalcogenides and quantum Hall effect in graphene have been investigated. The common thread that passes through the two topics is the interplay between electron interactions and spin and valley degrees of freedom. Chapter 1 is a brief introduction to the thesis. Chapter 2 addresses the energy and wave function of excitons in monolayer MoS$_2$. It reveals several interesting features, which can be important for exciton dynamics. Chapter 3 describes a theory of spatially indirect exciton condensates in transition metal dichalcogenide heterostructures. A systematic approach is developed to construct an effective exciton model with exciton-exciton interactions. The effective exciton model provides a useful guidance to construct the condensate phase diagram of excitons with multiple flavors. Chapter 4 identifies an SO(5) symmetry in the quantum Hall effect in graphene. The enlarged SO(5) symmetry unifies the spin antiferromagnetic order and valley $XY$ order. The physics of the SO(5) symmetry is explored using exact diagonalization and low-energy effective theory. Chapter 5 speculates about possible SU(3) and SU(4) singlet fractional quantum Hall states at a filling factor $\nu=2/3$ based on finite-size exact diagonalization study. These singlets are surprising because they are not captured by the composite fermion approach. The shift quantum number and the pair correlation function of the new states are presented.Item Exciton behaviour at organic solar cell interfaces(2015-12) Dinica, Olivia; Henkelman, Graeme; Rossky, Peter J; Vanden Bout, David A; Makarov, Dmitrii E; Mullins, Charles BOrganic photovoltaics (OPVs) have emerged as a promising class of materials in the production of flexible and cheap solar cells. Polymer OPVs are typically composed of a blend of a semiconducting electron donating poly- mer with an electron accepting fullerene derivative. This blend leads to a high donor-acceptor interfacial surface area where excitons are split apart to create free charges. The generation of free charges after photo-excitation is a main factor influencing solar cell efficiency. However, the mechanisms of charge transfer and the competing process of charge recombination at the interface are not completely clear. The understanding of these processes is essential for the rational design of materials that can maximize photovoltaic conversion efficiencies. The focus of this dissertation is on the influence that electric fields and chemical structure have on exciton dissociation and recombination at the interface of donor-acceptor materials. In particular, we use mixed quantum/classical dynamical simulations and electronic structure calculations to investigate several oligomer-fullerene systems. In order to study the potential energy surfaces guiding the dynamics of electron transfer, the nuclear and electron dynamics of large systems need to be simulated. To make these calculations computationally feasible, a mixed quantum classical molecular dynamics (MQCMD) approach was taken. This technique is based on the QCFF/PI formalism first described by Warshel and Karplus and was further developed by Lobaugh and Rossky for the simulation of betaine-30. This approach divides the conjugated system into a classical and a quantum subsystem. The quantum treatment is reserved for the π electronic system described by the Pariser-Parr-Pople (PPP) Hamiltonian. The classical potential describes a fully flexible molecular backbone and is modeled using an empirical molecular mechanics force field. In the first part of the dissertation we are examining the effect of an external electric field on charge transfer pathways and rates at sexithiophene/fullerene interfaces. In the second part, we develop a rigorous parametrization technique that allows us to model push-pull polymers. These polymers include PCDTBT and KP115 which have a more complex molecular structure than homo-polymers like the one considered in the first part. We use the QCFF/PI method as well as electronic structure calculations to investigate the influence of molecular structure and donor-acceptor orientation on charge transfer and recombination. The pathways linking exciton formation, charge transfer and thermal relaxation are explored, particularly in the context of dependence in the morphology of the donor molecules as well as the non-adiabatic coupling between excited states.Item Novel tools for ultrafast spectroscopy(2011-12) Jarvis, Thomas William; Li, Elaine; Fink, Manfred; Keto, John; Lim, Sang-Hyun; Shih, Chih-Kang; Sitz, GregExciton dynamics in semiconductor nanostructures are dominated by the effects of many-body physics. The application of coherent spectroscopic tools, such as two-dimensional Fourier transform spectroscopy (2dFTS), to the study of these systems can reveal signatures of these effects, and in combination with sophisticated theoretical modeling, can lead to more complete understanding of the behaviour of these systems. 2dFTS has previously been applied to the study of GaAs quantum well samples. In this thesis, we outline a precis of the technique before describing our own experiments using 2dFTS in a partially collinear geometry. This geometry has previously been used to study chemical systems, but we believe these experiments to be the first such performed on semiconductor samples. We extend this technique to a reflection mode 2dFTS experiment, which we believe to be the first such measurement. In order to extend the techniques of coherent spectroscopy to structured systems, we construct an experimental apparatus that permits us to control the beam geometry used to perform four-wave mixing reflection measurements. To isolate extremely weak signals from intense background fields, we extend a conventional lock-in detection scheme to one that treats the optical fields exciting the sample on an unequal footing. To the best of our knowledge, these measurements represent a novel spectroscopic tool that has not previously been described.Item Transparent carbon electrodes for spectroelectrochemical studies(2012-08) Walker, Erin Kate; Stevenson, Keith J.; Vanden Bout, David A.; Crooks, Richard M.; Webb, Lauren J.; Korgel, Brian A.This dissertation describes the assessment and use of carbon optically transparent electrodes (C-OTEs) based on pyrolyzed photoresist films (PPFs) as a platform for spectroelectrochemical investigations. C-OTEs are examined for use in UV-Vis spectroelectrochemistry and electrogenerated chemiluminescence and compared to non-transparent glassy carbon (GC) and the conventional transparent electrode indium tin oxide (ITO). Chapter 1 provides a general overview of transparent electrodes, carbon electrodes, and spectroelectrochemistry. Chapter 2 details a UV-Vis spectroelectrochemical investigation of electrogenerated graphitic oxides (EGO) on the surface of the C-OTE in the presence of KCl. X-ray photoelectron spectroscopy and time of flight secondary ion mass spectroscopy are used to determine EGO composition. Several supporting electrolytes are investigated to determine the mechanism of EGO formation. Chapter 3 details experiments to electrochemically access the exciton emission from self-assembled double-walled tubular J-aggregates via electrogenerated chemiluminescence (ECL). Optimization of ECL intensity with respect to the coreactant concentration and the supporting electrolyte pH is performed on opaque glassy carbon electrodes. ECL and fluorescence spectra are compared, and C-OTEs are utilized to determine the source of disagreement between the spectra. Chapter 4 describes the preparation and characterization (i.e. transparency, thickness, sheet resistance, rms roughness, and electroactive surface area) of C-OTEs and explores C-OTEs for general use in ECL under a variety of conditions. Simultaneous cyclic voltammograms and ECL transients are obtained for three thicknesses of PPFs and compared to non-transparent GC and the conventional transparent electrode ITO in both front face and transmission electrode cell geometries. Despite positive potential shifts in oxidation and ECL peaks, attributed to the internal resistance of the PPFs that result from their nanoscale thickness, the PPFs display similar ECL activity to GC, including the low oxidation potential observed for amine coreactants on hydrophobic electrodes. Overall, C-OTEs are promising electrodes for spectroelectrochemical applications because they yield higher ECL than ITO in both oxidative-reductive and reductive-oxidative ECL modes, are more stable in alkaline solutions, display a wide potential window of stability, and have tunable transparency for more efficient detection of light in the transmission cell geometry. Future directions for this research are discussed in Chapter 5, which outlines several approaches to designing and improving spectroelectrochemical sensors.