Dissertations / Theses: 'Excitonic transport'

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Breitkreiz, Maxim. "Transport Theory for Metals with Excitonic Instabilities." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-190697.

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Metals with excitonic instabilities are multiband systems with significant electron-electron interaction. The electronic transport in such systems is affected by collective fluctuations of the electrons, leading to anomalous features in the measured transport coefficients. Many of these anomalies have not been well understood because the transport mechanisms in these systems tend to be rather complex. The complexity arises, on the one hand, from the multiband nature and, on the other, from the anisotropic scattering of electrons accompanied by emitting or absorbing collective fluctuations. Previous works considering scattering due to collective fluctuations have mainly focused on single-band systems, for example in the context of the normal-state transport in cuprates. The recent discovery of high-temperature superconductivity in iron pnictides has renewed the interest in multiband systems. Exploring the transport mechanisms in multiband systems, I find some interesting new aspects, which do not occur in single-band systems. In particular, anisotropic scattering in a model with electronlike and holelike Fermi surfaces can lead to a negative conductivity contribution of the minority carriers, i.e., in an electric field, the minority carriers drift in the direction opposite of what one would expect based on their charge. I show that this effect can explain a reduced magnetoresistance in connection with an enhanced Hall coefficient, which has been measured in pnictides. Of particular interest are multiband models with hot spots on the Fermi surface, in part because of their relevance for the iron pnictides. Hot spots are states with enhanced scattering and therefore reduced excitation lifetimes. In single-band systems, the hot spots are found to have a much lower contribution to the total conductivity than other parts of the Fermi surface, which leads to the so-called hot-spot structure. I show that in the multiband case, the conductivity contributions are much more isotropic around the Fermi surface so that hot spots contribute to transport with a similar strength as other parts of the Fermi surface. I discuss this effect on the basis of an approximate analytical solution of the transport problem and numerically calculate the temperature dependence of several transport coefficients. It turns out that in the nematic phase of iron pnictides, the unexpectedly strong conductivity contribution of hot spots can explain the puzzling behavior of the resistive anisotropy. I show that the experimental observations can be explained within a scenario in which the anisotropy is mainly due to the broken symmetry of the spin-fluctuation spectrum in the nematic phase. In the spin-density-wave state, strongly anisotropic scattering can arise due to the propagating magnons. Using a two-band model relevant for iron pnictides, I find that this scattering can lead to an unusual interruption of the orbital motion of electrons in the magnetic field. As a consequence, the low-field magnetoresistance is linear with an alternating sign of the slope as a function of the direction of the current. In strong magnetic fields, the interrupted orbital motion makes the system unstable, which is characterized by a drop of the resistivity to zero.

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Chiaruttini, François. "États collectifs et dispositifs basés sur les excitons indirects dans des puits quantiques à grand gap." Thesis, Montpellier, 2020. http://www.theses.fr/2020MONTS029.

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Les excitons indirects dipolaires, sont des quasi-particules bosoniques dans les semi-conducteurs, composés d'un électron et d'un trou spatialement séparés mais toujours liés par interaction coulombienne. Leur grande durée de vie radiative et leur capacité à se déplacer sur de grandes distances avant leur recombinaison en font un système unique qui peut être à la fois optiquement actif mais également électriquement contrôlable. Ce système permet l'étude des propriétés fondamentales de la lumière et de la matière, mais aussi le développement de dispositifs excitoniques conceptuellement nouveaux. Les excitons dans les puits quantiques polaires en nitrure de gallium (GaN) peuvent être considérés comme des excitons naturellement indirects, en raison du fort champ électrique intrinsèque existant dans la direction de croissance cristalline. Cette thèse est consacrée à l'étude expérimentale et une étude des excitons indirects dans des hétérostructures GaN/(Al,Ga)N, ainsi que les états collectifs, depuis la conception et la fabrication jusqu'à la spectroscopie optique de leurs états collectifs.Les principaux résultats de ce travail sont (i) la démonstration du confinement spatial dans le plan et du refroidissement des excitons indirects, lorsqu'ils sont piégés dans le potentiel électrostatique créé par des électrodes semi-transparentes de géométries diverses soigneusement conçues et déposées sur la surface de l'échantillon, qui est une condition préalable à l'étude du diagramme de phase complexe de ces bosons dipolaires à basse température ; (ii) la preuve de principe du contrôle électrique des densités et des flux d'excitons indirects dans le plan du puits quantique. Cela ouvre des perspectives intéressantes pour la réalisation de dispositifs excitoniques ; (iii) les premiers points sur le diagramme de phase des bosons dipolaires, fournissant une première mise en évidence non seulement de l'existence d'un phase fortement corrélée résultant des corrélations induites à forte densité (phase de liquide dipolaire) mais aussi la dissociation (transition de Mott) de ces excitons indirects dans les hétérostructures GaN/(Al,Ga)N
Indirect, or dipolar excitons are bosonic quasi-particles in semiconductors composed of spatially separated but still Coulomb-bound electron and hole. They have long lifetime and can travel over large distances before recombination, offering a unique system that can be both optically active and electrically controllable. It is suitable for studies of fundamental properties of light and matter and for the development of conceptually new excitonic devices. Excitons in polar GaN quantum wells can be considered as naturally indirect excitons, because of the strong built-in electric field in the growth direction. This dissertation describes an experimental realization and investigation of indirect excitons engineered in GaN/(Al,Ga)N heterostructures, and the collective states that these can form. The main results of this work are (i) the demonstration of the in-plane confinement and cooling of indirect excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes carefully designed and deposited on the sample surface, this is a prerequisite for studies of the complex phase diagram of these dipolar bosons at low temperatures ; (ii) The proof-of-principle for electrical control of the indirect exciton densities and fluxes in the plane of the heterostructure, which opens attractive prospects for realization of excitonic devices ; (iii) the first points on the dipolar boson phase diagram, providing first evidence of the density-induced correlated state (dipolar liquid) and dissociation (Mott transition) of the indirect excitons in GaN/(Al,Ga)N heterostructures

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Fruchtman, Amir. "Theory and modelling of energy transport in quantum nanostructures." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:9c00d93c-c839-4342-9dc1-c2917c71a670.

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This thesis is concerned with quantum properties of excitonic energy transport in nanostructures that are embedded in a noisy environment. Of principal interests are ways to exploit this environment to facilitate the transport of energetic excitations. The first research chapter deals with an extension to the 'standard' open quantum system picture, where the Hilbert space is split into three: system, environment, and a wider universe. This division is natural for many biological and artificial nanostructures. A new analytical method, based on a phase space representation of the density matrix, is developed for studying such division. The effects of the wider universe are shown to be captured by a simple correction of the environmental response function. The second research chapter addresses the question: when do second-order perturbative approaches to open quantum systems, which are intuitive and simple to compute, provide adequate accuracy? A simple analytical criterion is developed, and its validity is verified for the case of the much-studied FMO dynamics as well as the canonical spin-boson model. In the third research chapter, an intuitive model of a photocell is studied. The model comprises two light-absorbing molecules coupled to an idealised reaction centre, showing asymmetric dimers are capable of providing a significant enhancement of light-to-current conversion under ambient conditions. This is done by 'parking' the energy of an absorbed photon in a dark state which neither absorbs nor emits light. In the final research chapter, a basic model for what can be thought as a "quantum brachistochrone" problem is investigated. Exotic energy configurations are found to yield considerable enhancement to the exciton's transfer probability, due to similar mechanisms studied in the previous chapter.

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Lafalce, Evan. "Photophysical and Electronic Properties of Low-Bandgap Semiconducting Polymers." Scholar Commons, 2014. https://scholarcommons.usf.edu/etd/5424.

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In this Ph.D. work, we investigate the optoelectronic properties of low-bandgap semiconducting polymers and project the potential for employing these materials in electronic and photonics devices, with a particular emphasis on use in organic solar cells. The field of organic solar cells is well developed and many of the fundamental aspects of device operation and material requirements have been established. However, there is still more work to be done in order for these devices to ultimately reach their full potential and achieve commercialization. Of immediate concern is the low power conversion efficiency demonstrated in these devices so far. In order to improve upon this efficiency, several routes are being explored. Because the optical bandgaps of semiconducting polymers are larger than in inorganic semiconductors, one of the most promising routes currently under exploration is the development of low-bandgap materials. Using polymers with lower band gaps will allow more of the solar irradiance spectrum to be absorbed and converted into electricity and thus possibly boost the overall efficiency. The bandgap of these semiconducting polymers is determined by the chemical structure, and therefore can be tailored through synthesis if the relevant structure-property relationships are well-understood. The materials studied in this work, a new series of Poly(thienylenevinylene) (PTV) derivatives, posses lower band gaps than conventional polymers through a design that incorporates aromatic-quinoid structural disturbances. This type of chemical structure delocalizes the electronic structure along the polymer backbone and reduces the energy of the lowest excited-state leading to a smaller band-gap. We investigate these materials through a variety of techniques including linear spectroscopy such as absorption and photoluminescence, pump-probe techniques like cw-photoinduced absorption and transient photo-induced absorption, and the non-linear electroasborption technique in order to interrogate the consequences of the delocalized electronic structure and its response to optical stimuli. We additionally consider the effects of environmental factors such as temperature, solvents and chemical doping agents. During the course of these investigations, we consider both of the two primary categorical descriptions of structure-property relationships for polymers within the molecular exciton model, namely the role of inter-molecular interactions on the electronic properties through the variation of supermolecular order and the fundamental determination of electronic structure due to specific intra-molecular interaction along the backbone of the polymer chain. We show that the dilution of aromaticity in semiconducting polymers, while being a viable means of reducing the optical band gap, results in a significant increase in the role of electron-electron interactions in determining the electronic properties. This is observed to be detrimental for device performance as the highly polarizable excited state common to polymers gives way to highly correlated state that extinguishes both the emissive properties and more importantly for solar cells, the charge-generating characteristics. This situation is shown to be predominant regardless of the nature of interchain interactions. We therefore show that the method of obtaining low-bandgap polymers here comes along with costly side-effects that inhibit their efficient application in solar cells. Further, we directly probe the efficacy of these materials in the common bulk-heterojunction architecture with both spectroscopy and device characterization in order to determine the limiting and beneficial factors. We show that, while from the point of view of absorption of solar radiation these low-bandgap polymers are more suited for solar cells, the ability to convert the absorbed photons into electron-hole pairs and generate electricity is lacking, due to the internal conversion into the highly correlated state and thus, the absorbed photon energy is lost. For completeness, we fabricate devices and verify that both the charge-transport properties and alignment of charge extraction levels with those of the contacts can not be responsible for the dramatic decrease in efficiency found from these devices as compared to other higher band gap polymers. We thus conclusively determine that the lack of power converison efficiency is governed by the inefficiency of charge-generation resulting from the intrinsic defective molecular structures rendering a low-lying optically forbidden state below the lowest optical allowed state that consumes the majority of the photogenerated excitons. It is emphasized that our means of investigation allow us to truly access the potential of these materials. In contrast, the direct application of these systems in devices and interpretation of the performance is exceedingly complex and may obscure their true potential. In other words, poor performance from a device may be extrinsic in nature and the optimization process may be very costly with respect to both time and materials. The methods used here however, allow us to determine the intrinsic potential. Not only is this beneficial in terms of preserving the resources that would be used on the trial-and-error method for devices, but it also allows us to learn more on a fundamental level about the structure-property relationships and their implications for device performance. The benefits of this increased understanding are two-fold. First, by learning about the fundamental response of a material, a new application may be realized. For example, the rapidly efficient internal conversion process that renders the materials in this study as poor candidates for solar cells may make them useful for photonics applications, as optical switches, for instance. Secondly, this type of investigation has implications for the whole organic electronics community instead of just being limited to the particular material system and the primary application attempted. In this case, we are essentially able to determine a threshold for aromaticty necessary in a structure that will preserve the stability of the ionic excited state that is useful for charge generation in solar cells.

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Breitkreiz, Maxim [Verfasser], Carsten [Akademischer Betreuer] Timm, and Jörg [Akademischer Betreuer] Schmalian. "Transport Theory for Metals with Excitonic Instabilities / Maxim Breitkreiz. Betreuer: Carsten Timm. Gutachter: Carsten Timm ; Jörg Schmalian." Dresden : Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://d-nb.info/1080645284/34.

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Breitkreiz, Maxim [Verfasser], Carsten Akademischer Betreuer] Timm, and Jörg [Akademischer Betreuer] [Schmalian. "Transport Theory for Metals with Excitonic Instabilities / Maxim Breitkreiz. Betreuer: Carsten Timm. Gutachter: Carsten Timm ; Jörg Schmalian." Dresden : Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://d-nb.info/1080645284/34.

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Coulson, Christopher. "Charge transport of exciton-polaritons." Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.648166.

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Abdelmoula, Tarik. "Exciton transport in organic nanostructures." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/31376.

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Excitons are quasi-particles, which are responsible for energy transport in organic semiconductors. Excitons are therefore instrumental in understanding the photophysics of organic opto-electronic devices. The present work focused on describing the dynamics of spin-forbidden, long-lived triplet excitons in archetypal organic materials such as CBP. Triplet excitons lifetime and diffusion length are here estimated from modelling the results of triplet-triplet photoinduced absorption spectroscopy, steady-state photoluminescence spectroscopy and time-resolved photoluminescence measurements. The last two measurements are performed using a modified time-of-flight method, whereby the investigated material is adjacent to a phosphorescent sensing layer and optically excited from the opposite side. As the thickness of the material is increased, the variations of phosphorescence intensity coming from the sensing layer is correlated to the exciton diffusion parameters. We show that for fluorescent materials such as CBP, the near-field component of this emission couples to guided modes supported by the structure and directly excites the sensitizer - here Ir(ppy)3 doped into CBP - which lead to an overestimation of the diffusion length. In addition, we investigate a strategy to mitigate the effect of guided modes by using an optical quenching layer of C6. This results in an estimated triplet exciton lifetime in the ms range and a diffusion length in excess of 30 nm, based on modelling the steady-state and time-resolved emission of the sensing layer when varying CBP thickness.

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Ivanov, Anton [Verfasser], and Heinz-Peter [Akademischer Betreuer] Breuer. "Exciton and electron transport in open quantum systems." Freiburg : Universität, 2016. http://d-nb.info/1125905409/34.

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Bjorgaard, Josiah August. "Exciton Diffusion, Transport, and Localization in Conjugated Polymers." Diss., North Dakota State University, 2013. https://hdl.handle.net/10365/27196.

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Conjugated polymers are wide bandgap semiconductors which have a series of conjugated π-orbitals that extend along the polymer ‘backbone’. The π-orbital conjugation can be disrupted by twisting of the polymer, affecting their optical properties. These materials are very useful for devices, where they are frequently found in semicrystalline thin films. In thin films, Frenkel excitons diffuse on a nanometer scale. However, measurement of the diffusion length of excitons in conjugated polymer films is currently very difficult. Disordered packing and twisting of polymers plays a significant role, but has not been examined in detail. This dissertation presents methods of measuring exciton diffusion length in polymer films and nanoparticles and explains the effect of nuclear disorder on the optical spectra and exciton diffusion in semicrystalline polymer films.

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Kawata, Kentaro. "Exciton transport in a conjugated polymer-TiOâ‚‚ photovoltaic composite." Thesis, University of Oxford, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.426397.

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Akselrod, Gleb M. (Gleb Markovitch). "Exciton transport and coherence in molecular and nanostructured materials." Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/84397.

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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2013.
Title as it appears in MIT degrees awarded booklet, September 2013: Mediating light and matter: excitons in organic and nanostructured materials. Vita. Cataloged from PDF version of thesis.
Includes bibliographical references (pages [151]-165).
Over the past 20 years a new classes of optically active materials have been developed that are composites of nano-engineered constituents such as molecules, polymers, and nanocrystals. These disordered materials have enabled devices such as organic light emitting diodes, color tunable lasers, and low-cost photovoltaics, and hold promise as a platform for all-optical computing. The defining optical and electronic characteristic of molecular and nanostructured materials is the exciton, a bound electron hole pair. Excitons, which can be generated optically or electrically, are the nanoscale carriers of energy, acting as intermediates between photons and electronic excitations. The goal of this thesis is to add to the present understanding of two fundamental aspects of excitons in molecular and nanostructured materials. First we focus on the spatial transport of excitons, which is central to the operation of photovoltaics, LEDs, and potential excitonic transistors. Despite its importance, the precise dynamics of exciton transport and how it relates to disorder, the defining characteristic of molecular and nanostructured materials, remains elusive. Here we develop a technique for direct visualization of exciton transport. We reveal unambiguously that transport occurs by random walk diffusion and that it transitions to sub diffusive as energetic disorder is increased. Furthermore, we harness exciton transport in J-aggregate materials to build a platform for the enhancement of absorption and fluorescence of organic molecules and quantum dots. Second we turn to the interaction of excitons with optical microcavities. Using the thermally stable excitons in molecular materials, it is possible to create strongly coupled states of excitons and photons, known as polaritons. A longstanding research goal has been creating polaritons at high densities in order to study condensation phenomena and as a route to low threshold organic lasers. In this thesis we elucidate that a key mechanism that prevents polariton condensation is exciton-exciton annihilation. In order to circumvent annihilation, we develop a new microcavity architecture with an intracavity excitation scheme and demonstrate room temperature lasing through a polariton mode. Finally, we show super radiant lasing from an organic microcavity, an alternative method over strong coupling that results in a substantially reduced lasing threshold.
by Gleb M. Akselrod.
Ph.D.

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Konishi, Kazuki. "Transport properties of photoexcited carriers and excitons in ultrapure diamond." Doctoral thesis, Kyoto University, 2021. http://hdl.handle.net/2433/263446.

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Tolk, M. "Improving exciton dissociation and charge transport in organic photovoltaic cells." Thesis, University College London (University of London), 2013. http://discovery.ucl.ac.uk/1393274/.

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I have divided this dissertation into three chapters: introduction to organic solar cells, thermo-chemical lithography of a conjugated polymer, and triplet emitters in organic solar cells (OSCs). The first chapter introduces OSCs giving the background necessary to understand the problem of simultaneous optimisation of exciton dissociation and charge transport. The second chapter deals with scanning thermo-chemical lithography (SThL) of PPV on indium-tin oxide (ITO) by means of a thermal AFM, i.e. an AFM that has a hot probe scanning across the surface, to ultimately pattern the active layer of an OSC. I investigate the influence of the thermal conductivity of the substrate on the lithography by combining finite element simulations of the heat transfer and experimental results. The model explains the rather substrate-independent feature size observed during experiments and it is found that for the highest resolution features, there exists a small gap of unconverted polymer near the substrate, which is why SThL is possible on high thermal conductivity substrates such as gold. In the third chapter I report experimental findings regarding the inclusion of triplet emitters in organic photovoltaic cells. The idea is to increase the exciton diffusion length (L) of the primary photoexcitations by converting them into triplet excitons, which are known to have longer lifetimes and hence offer the potential of increased exciton diffusion lengths. Several host systems were chosen, among them P3HT:PC61BM, MDMO-PPV:PC61BM and PBTTT:bis-PC61BM. As phosphorescent molecules I used Cu-complexes and different Ir-complexes. Results on MDMO-PPV:PC61BM blends and bilayer devices showed a promising increase in the short-circuit current density (Jsc) partly supported by an increase in the incident photon to current efficiency peak in the polymer absorption wavelength range. The overall achievability of the idea is critically discussed and a 1D random walk model used to estimate possible improvements of Jsc upon increases in L.

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Fornari, Rocco P. "Modelling charge and exciton transport in polymeric and molecular systems." Thesis, University of Warwick, 2018. http://wrap.warwick.ac.uk/110784/.

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In this thesis some fundamental aspects of charge transport and exciton dynamics in organic semiconductors are explored from a theoretical and computational point of view. After a brief review of the field of organic electronics, the theoretical methods most commonly used to describe exciton dynamics and charge transport are summarised, with an emphasis on the specific methods employed in this thesis (chapter 1). A very general kinetic rate of hopping between electronic states in the incoherent regime is then derived (chapter 2). This rate contains the most commonly used rates (Miller-Abrahams, Marcus, Marcus-Levich-Jortner) as special cases. The excitonic couplings between molecules determine the properties of excited states in biological and artificial molecular aggregates. A large number of excitonic couplings in these systems are computed (chapters 3 and 4) including both the Coulombic and the short-range (non-Coulombic) contributions as well as the thermal fluctuation of the coupling (dynamic disorder). The effect of thermal fluctuations in crystalline materials is found to be important when evaluating exciton dynamics (chapter 3). The short-range component of the coupling needs to be included when the interacting molecules are in close contact (chapter 3). The characteristics of charge transport in disordered polymers depend in principle on many parameters. With the aim of accounting for the complicated nature of these materials, a very general charge transport model is presented here (chapter 5). A detailed electronic structure with variable localization of the electronic states is obtained from a simple model Hamiltonian depending on just a few parameters. Using the hopping rate derived in chapter 2, the charge mobility along disordered polymer chains is computed. The proposed model includes features of both variable range hopping (VRH) and mobility edge (ME) models, but it starts from fewer assumptions. Donor-acceptor copolymers have a narrower transport band which in principle should result in lower mobility. Instead, the narrower band is found to enhance mobility if the other parameters are kept constant. By exploring the large parameter space of this model, the temperature dependence of mobility is found to follow a universal Arrhenius behaviour in agreement with experimental data (chapter 6). The activation energy for transport depends only on the effective electronic disorder of the polymer chain. When the 3D structure of the polymer chains and the role of inter-chain hopping are also considered (chapter 7), the mobility is found to be linearly dependent on the persistence length. The activation energy is found to depend only on the electronic disorder and not on chain rigidity.

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Leonhardt, Karsten. "Interplay of excitation transport and atomic motion in flexible Rydberg aggregates." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-213759.

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Strong resonant dipole-dipole interactions in flexible Rydberg aggregates enable the formation of excitons, many-body states which collectively share excitation between atoms. Exciting the most energetic exciton of a linear Rydberg chain whose outer two atoms on one end are closely spaced causes the initiation of an exciton pulse for which electronic excitation and diatomic proximity propagate directed through the chain. The emerging transport of excitation is largely adiabatic and is enabled by the interplay between atomic motion and dynamical variation of the exciton. Here, we demonstrate the coherent splitting of such pulses into two modes, which induce strongly different atomic motion, leading to clear signatures of nonadiabatic effects in atomic density profiles. The mechanism exploits local nonadiabatic effects at a conical intersection, turning them from a decoherence source into an asset. The conical intersection is a consequence of the exciton pulses moving along a linear Rydberg chain and approaching an additional linear, perpendicularly aligned Rydberg chain. The intersection provides a sensitive knob controlling the propagation direction and coherence properties of exciton pulses. We demonstrate that this scenario can be exploited as an exciton switch, controlling direction and coherence properties of the joint pulse on the second of the chains. Initially, we demonstrate the pulse splitting on planar aggregates with atomic motion one-dimensionally constrained and employing isotropic interactions. Subsequently, we confirm the splitting mechanism for a fully realistic scenario in which all spatial restrictions are removed and the full anisotropy of the dipole-dipole interactions is taken into account. Our results enable the experimental observation of non-adiabatic electronic dynamics and entanglement transport with Rydberg atoms. The conical intersection crossings are clearly evident, both in atomic mean position information and excited state spectra of the Rydberg system. This suggests flexible Rydberg aggregates as a test-bench for quantum chemical effects in experiments on much inflated length scales. The fundamental ideas discussed here have general implications for excitons on a dynamic network.

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Yoon, Yoseob. "Effects of interactions on correlation, thermalization, and transport of exciton-polaritons." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/122718.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2019
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 249-271).
Light-matter interactions are fundamental processes that allow us not only to interrogate material properties but also to coherently control material phases that cannot be reached otherwise. Matter-matter interactions, on the other hand, result in strong correlations and emergent behavior that cannot be explained in terms of single-particle physics. Exciton-polaritons (hereafter "polaritons") are hybrid quasiparticles in a semiconductor quantum-well microcavity that exhibit both light-matter and matter-matter interactions. Polaritons have the effective mass inherited from the ultralight cavity photon mass, which sets polariton transport phenomena to be photon-like and allows macroscopic quantum phenomena such as Bose-Einstein condensation and superfluidity up to room temperature. Meanwhile, the effect of photon dressing only reduces the exciton-exciton interaction strength by the Hopfield coefficient, which sets the polariton-polariton interaction strength to be exciton-like.
Along with the narrow linewidth protected from inhomogeneously broadening, polaritons are an excellent platform to study interaction-induced physics and nonlinear device applications such as ultralow-power optical switches. In this thesis, we investigated the effects of light-matter and matter-matter interactions on various aspects of polaritons. In the first part, we first measured the polariton-polariton interaction strength by tracking the energy blueshift as a function of polariton density. This was enabled by separating and trapping polaritons away from a pumped region, where the measurement of polariton interactions can be obscured by much heavier particles such as a dark exciton reservoir. We provided possible mechanisms that explain the observed anomalously large blueshifts. In the second part, we addressed a long-standing debate on whether polaritons can reach thermal equilibrium.
We used a long-lifetime microcavity structure to achieve Bose-Einstein distributions of polaritons, which was the first demonstration of polaritons in equilibrium. This allowed us to measure equilibrium properties, such as temperature and chemical potential, and to map out the phase diagram of Bose-Einstein condensation in a quasi-two-dimensional system. We further investigated how all-optical trapping and polariton interactions enhance relaxation and thermalization processes. In particular, we found that a significant redistribution of polaritons occurs through the reduced density of states and polariton interactions. In the third part, we studied trapped eigenstates and interference patterns of polariton condensates in various trapping and pump geometries. Competition between eigenstates and selection of one of them have been well explained by the overlap of real-space, monientum-space, and energy distributions between the pump and the eigenstate.
A mismatch between the pump-induced potential profile and the polariton source profile was a key factor in determining the distribution of transported polaritons. In the last part, we extended the polariton physics to study topological and cooperative effects in open quantum systems. We demonstrated bulk Fermi arcs by connecting two exceptional points arising from the engineered non-Hermitian properties of a photonic crystal. In addition, we theoretically showed that a cascaded-cavity system can outperform a single-cavity system in terms of the single-photon indistinguishability and efficiency, which works even with bad quantum emitters and practical cavity quality factors. Our work provides invaluable insights into the fundamental light-matter and matter-matter interactions, as well as many-body physics of condensed matter and photonic systems.
by Yoseob Yoon.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Chemistry

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Solnyshkov, Dmitry. "Exciton-polaritons in planar microcavities." Clermont-Ferrand 2, 2007. http://www.theses.fr/2007CLF21801.

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Cette thèse est consacrée aux propriétés des exciton-polaritons, les particules mixtes formées à partir de la lumière et la matière dans les microcavités de semi-conducteurs dans le régime de couplage fort. D'abord, j'analyse la possibilité de condensation de Bose des exciton- polaritons à température ambiante dans les microcavités de GaN avec les équations de Boltzmann semi-classiques. Puis les effets de polarisation dans le régime d'oscillateur paramétrique sont étudiés avec les équations de Boltzmann semi-classiques avec pseudospin. La deuxième partie de la thèse est consacrée aux propriétés des condensats et modes macrooccupés des exciton-polaritons. Leur polarisation, dispersion des excitations, propagation, localisation et superfluidité sont décrits avec l'équation de Gross-Pitaevskii

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Chuang, Chern. "Spectroscopy, relaxation, and transport of molecular excitons in noisy and disordered environments." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/115803.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 139-150).
In this thesis contribution we theoretically investigate the spectroscopy, relaxation, and transport properties of Frenkel excitons in molecular aggregates, with extensive comparison to or prediction of experimental observables. Particular emphasis is devoted to the effects of thermal noise, static disorder, and system dimensionality. Our key contributions are summarized as the following. We study the spectroscopic signatures of excitonic molecular aggregates of dimensionality larger than unity as functions of temperature and disorder strength. These findings are applied to the determination of essential system characteristics and quantitatively explain the spectroscopic traits seen in experiments where either the temperature or disorder strength is altered. A classification scheme generalized from Kasha's seminal work on J- and H-aggregates is proposed that is compatible with experimental observations previously unexplained. We recognize the importance of long-wavelength approximations in understanding the density of states in two-dimensional excitonic aggregates. And for tubular aggregates this leads to a simple expression for the energy gap between the parallel- and the perpendicular-polarized peaks useful in inferring key system parameters. This long-wavelength approach is then extended to the analysis of 2D excitonic molecular aggregates in general. A universal scaling relation concerning the steady-state diffusive transport of excitons in molecular tubes is predicted and analyzed, where the key order parameter is identified as the ratio between the localization length of the exciton wavefunctions and the tube circumference. A unified theoretical framework is proposed to explain the relaxation of hot excitons generated in emissive conjugated polymers across three orders of magnitude in timescale, with quantitative agreements with experiments.
by Chern Chuang.
Ph. D.

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Valleau, Stephanie. "Theoretical study of exciton transport in natural and synthetic light-harvesting systems." Thesis, Harvard University, 2016. http://nrs.harvard.edu/urn-3:HUL.InstRepos:33493387.

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In the first part of this dissertation, we investigate on the presence of quantum effects in the exciton dynamics of the Fenna-Matthews-Olson photosynthetic complex of green sulfur bacteria using an atomistic Quantum Mechanics / Molecular Mechanics (QM/MM) model combined with open quantum systems methods. Subsequently, we explore the theoretical connection between the atomistic QM/MM approach and the open quantum system methods and propose the correct theoretical expressions to maintain consistency when using both approaches contemporarily. In particular we show that when using the correct prefactor to extract the spectral density - the strength of coupling between excitation and other degrees of freedom - the atomistic results are in good agreement with experimental predictions. We then describe a first atomistic study of the full light-harvesting complex of green sulfur bacteria. The various units are treated atomistically and the full system's exciton dynamics is obtained using a Markovian open quantum system master equation. To conclude the first part, we describe a Machine Learning algorithm which we developed and implemented to learn time-dependent density functional theory energies by using trained neural networks and supplying these with coulomb matrices extracted from molecular dynamics simulations. This approach provides a much more rapid solution to obtaining a QM/MM Hamiltonian and subsequently extracting dynamics. It is particularly useful when multiple identical molecules are found in similar environments as one can train the network on a single molecule and predict all others. We applied this method to the Fenna-Matthews-Olson complex. In the second part of this dissertation we focus on model systems and synthetic aggregates. In particular, we investigate the exciton dynamics in thin-film J-aggregates using a Markovian stochastic Schrödinger equation approach. We derive expressions to obtain diffusion constants from the dynamics and compare a series of different thin-film J-aggregates. The parameters of the model are obtained atomistically. From this model we obtain information on the parameters which lead to optimal exciton diffusion. This can guide the design of new exciton transfer materials.
Chemistry and Chemical Biology

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Hammack, Aaron Tynes. "Studies of transport and thermalization of excitons and the development of techniques for in-situ manipulation of excitons in coupled quantum wells." Diss., [La Jolla] : University of California, San Diego, 2010. http://wwwlib.umi.com/cr/ucsd/fullcit?p3403192.

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Grice, Alan William. "Device physics of conjugated polymer LEDs." Thesis, University of Sheffield, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.286975.

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Taïnoff, Dimitri. "Influence des défauts sur les propriétés optiques et électroniques des nanoparticules de ZnO." Phd thesis, Université Claude Bernard - Lyon I, 2009. http://tel.archives-ouvertes.fr/tel-00507281.

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L'objectif de cette étude est de mieux comprendre le rôle joué par les défauts dans les propriétés optiques et électroniques des nanostructures d'oxyde de zinc. Pour ce faire, nous avons synthétisé des nanoparticules d'oxyde de zinc de 6 à 18 nm de diamètres pouvant être considérées comme modèle en terme de stœchiométrie, de cristallinité et de qualité de surface par une méthode physique originale : la Low Energy Cluster Beam Deposition.La caractérisation optique des défauts présents dans les nanoparticules de ZnO a été faite grâce à l'analyse des spectres d'émission visible et UV à différentes températures [10K-300K]. En particulier la luminescence excitonique à 3,31 eV, qui est un sujet controversé, a été étudiée en comparant la luminescence excitonique d'échantillons structurés à différentes échelles (nanoparticules, microcristaux et monocristal). Les temps de déclins très rapides des défauts donneurs ont été étudiés par spectroscopie à décalage de fréquence au CELIA à Bordeaux révélant une dépendance en fonction de la taille des NPs du type Giant Oscillator Strenght.Les propriétés de transport électronique des couches minces de NPs, naturellement dopées n, ont été caractérisées grâce à des expériences σ(T). Différents scénarios sont proposés pour expliquer les résultats des expériences de conductivité, et discutés en fonction des propriétés optiques des couches et de leur morphologie. En particulier, il est montré que la surface des NPs, très réactive, influence fortement le transport, ce qui laisse entrevoir la possibilité d'utiliser ces films nanostructurés comme capteurs de gaz.

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Hsieh, Chi-Ti. "Carrier transport in optical-emitting and photodetecting devices based on carbon-nanotube field-effect transistors." Diss., Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/34797.

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A theory of the carrier transport, optical emission, and photoconductivity from optoelectronic devices based on ambipolar long-channel carbon-nanotube (CNT) field-effect transistors (FETs) is presented in this dissertation. In optical emitters based on ambipolar long-channel CNT FETs, an analytic diffusive-transport model for various recombination mechanisms is provided for the first time. The relationship and the scaling of emitted light-spot size and emitted optical power are clearly depicted for the first time as well. We also implement a numerical diffusive-transport approach for the light emission, in which the focus is on the effects of radiative and nonradiative recombination in the channel, with the movement of the spatial recombination profile in response to the gate and drain voltages. For the first time, we find that the emitted light-spot size and the emitted optical power depend sensitively on the operative nonradiative recombination mechanisms. We implement a numerical diffusive-transport approach including exciton photogeneration as well for photoconductors based on ambipolar long-channel CNT FETs with uniform and near-field photoexcitation. We show that the photocurrents are typically much smaller than the dark currents, and explain some possible reasons. Moreover, the exciton densities in CNTs are calculated and the effect of exciton diffusion is presented.

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Bellisario, Darin O. "An analysis of exciton transport, electron tunneling, and electromigration in nanotube and nanowire systems." Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/103509.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2016.
"February 2016." Vita. Cataloged from PDF version of thesis.
Includes bibliographical references (pages 204-213).
Herein three systems - electromigration in metal nanowires, electron tunneling in single-molecules, and carbon nanotube photovoltaics - are investigated. In the first area, electromigrative failure of metal nanowires has been shown to form single-molecule tunnel junctions, but the process has remained unpredictable, limiting the yield of devices under current methods. Electromigration in micron diameter and larger wires is well understood as the migration of vacancies in the bulk crystal, but both the quantitative predictions and qualitative features of that mechanism break down at the nanometer scale. We propose, and validate against experimental data, that as the wire diameter falls below a micron,- the increased surface-area-to-volume ratio and the low barrier to surface atom translation shift the dominant mechanism of electromigration from bulk transport to surface transport. We then apply the model to design a process controller to guide gradual electromigration. We then turn to investigating the tunnel junctions themselves. Diverse physical insights have been gained from electron tunneling measurements of single molecules, but to date all observations have been static i.e. subject to long integration times. We performed temporally resolved measurements, revealing underlying molecule dynamics. In particular we find that molecules can stochastically switch between discrete inelastic transport states, suggesting discretized molecule reconfiguration consistent with the body of literature from Scanning Tunneling Microscopy. Finally, we investigate carbon nanotube (CNT) network solar cells. The large parametric space associated with the nanometer-scale heterogeneous material, including the mixture of nanotube length, chirality, orientation, etc., has prevented proof-of-concept devices from revealing a research pathway to practical efficiencies. To address this empirical limitation, we derived a model of CNT photovoltaic steady-state operation from the light absorption and exciton transport behaviors of single and aggregate nanotubes. To do so, we treated single nanotube properties as random variables, describing the nanotube network as distributions of those properties. Applying the model to different solar cell architectures, we predict that efficiencies will be dramatically higher in high density films of verticallyaligned nanotubes. We also show that the film thickness must be at an optimum, and that as a rule of thumb the film thickness should be approximately the exciton diffusion length.
by Darin O. Bellisario.
Ph. D.

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Adams, Michael [Verfasser], and B. S. [Akademischer Betreuer] Richards. "Triplet Exciton Transport in Porphyrin-Based Surface-Anchored Metal-Organic Framework Thin Films / Michael Adams ; Betreuer: B. S. Richards." Karlsruhe : KIT-Bibliothek, 2019. http://d-nb.info/119237360X/34.

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Valente, Gustavo Targino. "Dinâmica de éxcitons e transporte de cargas em heteroestruturas orgânicas." Universidade de São Paulo, 2017. http://www.teses.usp.br/teses/disponiveis/76/76132/tde-31012018-154934/.

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A proposta desse estudo é investigar as propriedades de migração do éxciton, transferência de energia e transporte de cargas em heteroestruturas orgânicas ultrafinas compostas pela integração de um polímero semicondutor com moléculas de clorofila. A sintonização dos estados eletrônicos desses materiais torna possível a obtenção de heteroestruturas com modulação energética capazes de aprisionar éxcitons e cargas apresentando potencialidade de aplicação em Diodos Orgânicos Emissores de Luz (OLEDs). Para tal filmes de polifuoreno (PFO) (camada transportadora de carga) totalmente amorfo e filmes de clorofila (camada ativa na forma de poço de potencial) foram preparados utilizando a técnica e automontagem (LBL) combinada com spin-coating, caracterizados por microscopia confocal por varredura a laser e técnicas espectroscópicas de absorção e emissão. Investigou-se os processos fotofísicos utilizando microscopia confocal e de tempo de vida. Os resultados foram interpretados com base no modelo de transferência de energia de Förster combinado com as taxas de Miller-Abrahams e com a equação de difusão excitônica. Com essa abordagem, obteve-se uma relação entre a migração do éxciton no PFO e a transferência de energia não radiativa deste polímero para as moléculas de clorofila. Observou-se uma eficiente transferência de energia igual a 94% no regime de filmes ultrafinos. Para compreender os mecanismos de transporte de carga, implementamos e validamos o método de simulação de Monte Carlo para o transporte de carga em sistemas orgânicos desordenados. Com essa abordagem investigou-se a dinâmica das cargas em filmes poliméricos desordenados com e sem a camada de poço de potencial. Propriedades elétricas, tais como, mobilidade elétrica e coeficiente de difusão, foram obtidas e estão de acordo com os reportados na literatura. Obteve-se uma taxa de preenchimento de cargas no poço de potencial igual a 1010 buracos/s para campo elétrico de 1 MV/cm e constatou-se que a taxa aumenta com o campo elétrico. Tal abordagem apresenta-se como uma alternativa interessante para auxiliar o planejamento experimental de OLEDs baseados em heteroestruturas orgânicas.
In this study the exciton migration, energy transfer and charge transport in ultrathin organic heterostructure formed by semiconductor polymer and chlorophyll molecules were investigated. The energetic tuning between these materials promotes organic heterostructures with energetic modulation capable of trapping excitons and charges showing an application potential in Organic Light Emitting Diodes (OLEDS). Amorphous polyfluorenes (PFO) and chlorophyll a (chla) were prepared using self-assembly combined with spin-coating methods and characterized by confocal laser scanning microscopy and spectroscopic techniques. Photophysical processes were investigated using confocal and life-time microscopy and the results interpreted from the model of Förster energy combined with the Miller-Abrahams rate as well as the exciton diffusion equation. These results provided a relationship between the exciton migration in the PFO film and the non-radiative energy transfer from polymer to chla molecules. An efficient transfer of energy equal to 94% was observed. Method of the Monte Carlo simulation were implemented to investigate the charge transport in this disordered organic system. Using this method, the charge dynamics with and no potential well layer was studied. Electrical properties obtained, such as electric mobility and diffusion coefficient, are in agreement with literature. It was estimated a charge fill rate in the potential well equal to 1010 holes/s for 1 MV/cm and this parameter increases with the electric field. This approach has been shown to be an interesting alternative for the experimental design of OLEDs composed by organic heterostructure.

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Liess, Andreas [Verfasser], Frank [Gutachter] Würthner, and Jens [Gutachter] Pflaum. "Structure-Property Relationships of Merocyanine Dyes in the Solid State: Charge Transport and Exciton Coupling / Andreas Liess ; Gutachter: Frank Würthner, Jens Pflaum." Würzburg : Universität Würzburg, 2017. http://d-nb.info/1138923346/34.

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Kaiser, Waldemar [Verfasser], Alessio [Akademischer Betreuer] Gagliardi, Alessio [Gutachter] Gagliardi, and David [Gutachter] Egger. "Kinetic Monte Carlo Study of Charge and Exciton Transport in Organic Solar Cells / Waldemar Kaiser ; Gutachter: Alessio Gagliardi, David Egger ; Betreuer: Alessio Gagliardi." München : Universitätsbibliothek der TU München, 2021. http://d-nb.info/1237413273/34.

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Rahman, Hasan Abdur [Verfasser], Ulrich [Akademischer Betreuer] Kleinekathöfer, Ulrich [Gutachter] Kleinekathöfer, Stefan [Gutachter] Kettemann, and Dmitry [Gutachter] Ryndyk. "Time Evolution in Open Quantum Systems - From Exciton Dynamics to Charge Transport / Hasan Abdur Rahman ; Gutachter: Ulrich Kleinekathöfer, Stefan Kettemann, Dmitry Ryndyk ; Betreuer: Ulrich Kleinekathöfer." Bremen : IRC-Library, Information Resource Center der Jacobs University Bremen, 2019. http://d-nb.info/1190888114/34.

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Tignone, Edoardo. "Cavity quantum electrodynamics : from photonic crystals to Rydberg atoms." Thesis, Strasbourg, 2016. http://www.theses.fr/2016STRAF008/document.

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Dans le premier chapitre de la thèse, nous étudions la possibilité d’améliorer le couplage opto- mechanique photon-phonon entre le mode de résonance d’une cavité Fabry-Pérot de haute finesse et les vibrations mécaniques des éléments diélectriques (membranes) à l’intérieur de la cavité. En introduisant un défaut quadratique dans la disposition des membranes, nous montrons que le deux couplages (linéaire et quadratique) augmentent. Enfin, nous proposons un modèle très simple avec lequel on cherche à simuler un cristal photonique quasipériodique. Dans le deuxième chapitre de cette thèse, nous présentons nos résultats de recherche sur le transport d’excitons à travers une cavité visant à augmenter l’efficacité du transport. Le modèle que l’on étudie est une chaîne unidimensionnelle d’atomes froids comprenant chacun deux niveaux énergétiques. Grâce au couplage entre exciton et photon, ces deux quanta s’hybrident et forment deux branches de polariton à l’intérieur de la cavité. Nous avons observé qu’à résonance avec un des deux modes de polariton, on peut transmettre l’exciton via le mode polaritonique dans un temps très court. En outre, le désordre n’affecte la propagation excitonique que de façon algébrique. Dans le troisième chapitre de cette thèse, nous présentons nos résultats de recherche sur la réalisa- tion d’interactions entre photons grâce à la médiation d’atomes ultrafroids piégés dans un réseaux optique unidimensionnelle et placés à l’intérieur d’une fibre à cristaux photoniques. Nous avons détecté un régime dans lequel on peut réaliser le “bunching” photon-photon.Dans le quatrième et dernière chapitre de cette thèse, nous étendons les résultats du chapitre précédent aux atomes de Rydberg
In the first chapter of this thesis, we study a quasiperiodic array of dielectric membranes inside a high-finesse Fabry-Pérot cavity. We work within the framework of the transfer matrix formal- ism. We show that, in a transmissive regime, the introduction of a quadratic spatial defect in the membrane positions enhances both the linear and quadratic optomechanical couplings between optical and mechanical degrees of freedom. Finally, we propose a theoretical model to simulate a one-dimensional quasiperiodic photonic crystal. In the second chapter of this thesis, we consider the problem of the transport of an exciton through a one-dimensional chain of two-level systems. We embed the chain of emitters in a transverse optical cavity and we show that, in the strong coupling regime, a ultrafast ballistic transport of the exciton is possible via the polaritonic modes rather than ordinary hopping. Due to the hybrid nature of polaritons, the transport efficiency is particularly robust against disorder and imperfections in the system. In the third chapter of this thesis, we consider an ordered array of cold atoms trapped in an optical lattice inside a hollow-core photonic crystal fiber. We study photon-photon interactions mediated by hard-core repulsion between excitons. We show that, in spite of underlying repulsive interac- tion, photons in the scattering states demonstrate bunching, which can be controlled by tuning the interatomic separation. We interpret this bunching as the result of scattering due to the mismatch of the quantization volumes for excitons and photons, and discuss the dependence of the effect on experimentally relevant parameters. In the fourth chapter of the thesis, we extend the results of the previous chapter to Rydberg atoms

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Pokhrel, Chandra Prasad. "Crystal growth and charge carrier transport in liquid crystals and other novel organic semiconductors." [Kent, Ohio] : Kent State University, 2009. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=kent1254234736.

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Thesis (Ph.D.)--Kent State University, 2009.
Title from PDF t.p. (viewed April 1, 2010). Advisor: Brett Ellman. Keywords: Laser; Charge generation; Charge transport; Mobility; Trapping; Space charge; Hopping; Tunneling; Lattice vibration; Exciton; Polaron; HUMO; LUMO; Action Spectrum; Quantum efficiency; Crystal Growth; Liquid crystal; Disordered medium. Includes bibliographical references.

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Paul, Sanjoy. "CHARGE TRANSPORT IN LIQUID CRYSTALLINE SMECTIC AND DISCOTIC ORGANIC SEMICONDUCTORS: NEW RESULTS AND EXPERIMENTAL METHODOLOGIES." Kent State University / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=kent1469836810.

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Hultell, (Andersson) Magnus. "Electron-Lattice Dynamics in pi-Conjugated Systems." Licentiate thesis, Linköping University, Linköping University, Department of Physics, Chemistry and Biology, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-7996.

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In this thesis we explore in particular the dynamics of a special type of quasi-particle in pi-conjugated materials termed polaron, the origin of which is intimately related to the strong interactions between the electronic and the vibrational degrees of freedom within these systems. In order to conduct such studies with the particular focus of each appended paper, we simultaneously solve the time-dependent Schrödinger equation and the lattice equation of motion with a three-dimensional extension of the famous Su-Schrieffer-Heeger (SSH) model Hamiltonian. In particular, we demonstrate in Paper I the applicability of the method to model transport dynamics in molecular crystals in a region were neither band theory nor perturbative treatments such as the Holstein model and extended Marcus theory apply. In Paper II we expand the model Hamiltonian to treat the revolution of phenylene rings around the sigma-bonds and demonstrate the great impact of stochastic ring torsion on the intra-chain mobility in conjugated polymers using poly[phenylene vinylene] (PPV) as a model system. Finally, in Paper III we go beyond the original purpose of the methodology and utilize its great flexibility to study radiationless relaxations of hot excitons.

Report code: LiU-TEK-LIC-2007:4.

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Wagner, Markus Raphael [Verfasser], Michael [Gutachter] Kneissl, Janina [Gutachter] Maultzsch, Friedhelm [Gutachter] Bechstedt, Bernard [Gutachter] Gil, Marius [Gutachter] Grundmann, Axel [Gutachter] Hoffmann, Pablo [Gutachter] Ordejon, Jim [Gutachter] Speck, and Martin [Gutachter] Stutzmann. "Phononic and photonic semiconductor nanostructures : transport, coherence, confinement, and dynamics of phonons and excitons in single and periodic nanostructures / Markus Raphael Wagner ; Gutachter: Michael Kneissl, Janina Maultzsch, Friedhelm Bechstedt, Bernard Gil, Marius Grundmann, Axel Hoffmann, Pablo Ordejon, Jim Speck, Martin Stutzmann." Berlin : Technische Universität Berlin, 2020. http://d-nb.info/1223537609/34.

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Nayyar, Iffat. "Prediction of Optical Properties of Pi-Conjugated Organic Materials for Technological Innovations." Doctoral diss., University of Central Florida, 2013. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/5993.

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Organic ?-conjugated solids are promising candidates for new optoelectronic materials. The large body of evidence points at their advantageous properties such as high charge-carrier mobility, large nonlinear polarizability, mechanical flexibility, simple and low cost fabrication and superior luminescence. They can be used as nonlinear optical (NLO) materials with large two-photon absorption (2PA) and as electronic components capable of generating nonlinear neutral (excitonic) and charged (polaronic) excitations. In this work, we investigate the appropriate theoretical methods used for the (a) prediction of 2PA properties for rational design of organic materials with improved NLO properties, and (b) understanding of the essential electronic excitations controlling the energy-transfer and charge-transport properties in organic optoelectronics. Accurate prediction of these electro-optical properties is helpful for structure-activity relationships useful for technological innovations. In Chapter 1 we emphasize on the potential use of the organic materials for these two applications. The 2PA process is advantageous over one-photon absorption for deep-tissue fluorescence microscopy, photodynamic therapy, microfabrication and optical data storage owing to the three-dimensional spatial selectivity and improved penetration depth in the absorbing or scattering media. The design of the NLO materials with large 2PA cross-sections may reduce the optical damage due to the use of the high intensity laser beams for excitation. The organic molecules also possess self-localized excited states which can decay radiatively or nonradiatively to form excitonic states. This suggests the use of these materials in the electroluminescent devices such as light-emitting diodes and photovoltaic cells through the processes of exciton formation or dissociation, respectively. It is therefore necessary to understand ultrafast relaxation processes required in understanding the interplay between the efficient radiative transfer between the excited states and exciton dissociation into polarons for improving the efficiency of these devices. In Chapter 2, we provide the detailed description of the various theoretical methods applied for the prediction as well as the interpretation of the optical properties of a special class of substituted PPV [poly (p-phenylene vinylene)] oligomers. In Chapter 3, we report the accuracy of different second and third order time dependent density functional theory (TD-DFT) formalisms in prediction of the 2PA spectra compared to the experimental measurements for donor-acceptor PPV derivatives. We recommend a posteriori Tamm-Dancoff approximation method for both qualitative and quantitative analysis of 2PA properties. Whereas, Agren's quadratic response methods lack the double excitations and are not suitable for the qualitative analysis of the state-specific contributions distorting the overall quality of the 2PA predictions. We trace the reasons to the artifactual excited states above the ionization threshold. We also study the effect of the basis set, geometrical constraints and the orbital exchange fraction on the 2PA excitation energies and cross-sections. Higher exchange (BMK and M05-2X) and range-separated (CAM-B3LYP) hybrid functionals are found to yield inaccurate predictions both quantitatively and qualitatively. The failure of the exchange-correlation (XC) functionals with correct asymptotic is traced to the inaccurate transition dipoles between the valence states, where functionals with low HF exchange succeed. In Chapter 4, we test the performance of different semiempirical wavefunction theory methods for the prediction of 2PA properties compared to the DFT results for the same set of molecules. The spectroscopic parameterized (ZINDO/S) method is relatively better than the general purpose parameterized (PM6) method but the accuracy is trailing behind the DFT methods. The poor performances of PM6 and ZINDO/S methods are attributed to the incorrect description of excited-to-excited state transition and 2PA energies, respectively. The different semiempirical parameterizations can at best be used for quantitative analysis of the 2PA properties. The ZINDO/S method combined with different orders of multi-reference configuration interactions provide an improved description of 2PA properties. However, the results are observed to be highly dependent on the specific choice for the active space, order of excitation and reference configurations. In Chapter 5, we present a linear response TD-DFT study to benchmark the ability of existing functional models to describe the extent of self-trapped neutral and charged excitations in PPV and its derivative MEH-PPV considered in their trans-isomeric forms. The electronic excitations in question include the lowest singlet (S1) and triplet (T1†) excitons, positive (P+) and negative (P-) polarons and the lowest triplet (T1) states. Use of the long-range-corrected DFT functional, such as LC-wPBE, is found to be crucial in order to predict the physically correct spatial localization of all the electronic excitations in agreement with experiment. The inclusion of polarizable dielectric environment play an important role for the charged states. The particle-hole symmetry is preserved for both the polymers in trans geometries. These studies indicate two distinct origins leading to self-localization of electronic excitations. Firstly, distortion of molecular geometry may create a spatially localized potential energy well where the state wavefunction self-traps. Secondly, even in the absence of geometric and vibrational dynamics, the excitation may become spatially confined due to energy stabilization caused by polarization effects from surrounding dielectric medium. In Chapter 6, we aim to separate these two fundamental sources of spatial localization. We observe the electronic localization of P+ and P- is determined by the polarization effects of the surrounding media and the character of the DFT functional. In contrast, the self-trapping of the electronic wavefunctions of S1 and T1(T1†) mostly follows their lattice distortions. Geometry relaxation plays an important role in the localization of the S1 and T1† excitons owing to the non-variational construction of the excited state wavefunction. While, mean-field calculated P+, P- and T1 states are always spatially localized even in ground state S0 geometry. Polaron P+ and P- formation is signified by the presence of the localized states for the hole or the electron deep inside the HOMO-LUMO gap of the oligomer as a result of the orbital stabilization at the LC-wPBE level. The broadening of the HOMO-LUMO band gap for the T1 exciton compared to the charged states is associated with the inverted bond length alternation observed at this level. The molecular orbital energetics are investigated to identify the relationships between state localization and the corresponding orbital structure. In Chapter 7, we investigate the effect of various conformational defects of trans and cis nature on the energetics and localization of the charged P+ and P- excitations in PPV and MEH-PPV. We observe that the extent of self-trapping for P+ and P- polarons is highly sensitive on molecular and structural conformations, and distribution of atomic charges within the polymers. The particle-hole symmetry is broken with the introduction of trans defects and inclusion of the polarizable environment in consistent with experiment. The differences in the behavior of PPV and MEH-PPV is rationalized based on their orbital energetics and atomic charge distributions. We show these isomeric defects influence the behavior and drift mobilities of the charge carriers in substituted PPVs.
Ph.D.
Doctorate
Physics
Sciences
Physics

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Hernandez, Ramirez Gilberto. "CRISTAUX LIQUIDES DE TYPE DONNEUR-ACCEPTEUR-DONNEUR POUR LA CONVERSION PHOTOVOLTAÏQUE." Phd thesis, Université de Strasbourg, 2010. http://tel.archives-ouvertes.fr/tel-00579777.

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Ce travail de thèse porte sur la synthèse de cristaux liquides semiconducteurs, obtenus à partir de molécules associant des unités à caractère donneur-accepteur-donneur (DAD) et substituées aux deux extrémités par une chaîne oligosiloxane. Le fort pouvoir microségrégeant des oligosiloxanes a pour effet de stabiliser, pour l'ensemble des matériaux, une phase smectique unique (désordonnée) sur une gamme de température remarquablement large (>300°C). Pour des raisons de géométrie, les partie D et A doivent se nanostructurer en phase smectique pour conduire à la formation d'une structure à lamelles D/A alternées, favorable pour des applications photovoltaïques. Les matériaux ont fait l'objet de nombreuses études, notamment pour caractériser leurs propriétés structurales, photophysiques et de transport de charge. Ces matériaux ont ainsi révélé l'existence d'un transport de charge ambipolaire avec des valeurs de mobilité de l'ordre de 10-3 cm2/Vs en phase smectique. Les tests préliminaires de conversion photovoltaïque montre l'existence d'un très faible rendement, qui démontre l'importance d'un travail ultérieur d'optimisation des conditions de dépôts et du contrôle de l'orientation des couches smectiques sur les substrats.

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38

Jambois, Olivier. "Elaboration et étude de la structure et des mécanismes de luminescence de nanocristaux de silicium de taille contrôlée." Phd thesis, Université Henri Poincaré - Nancy I, 2005. http://tel.archives-ouvertes.fr/tel-00011284.

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Ce travail porte sur l'étude des mécanismes de luminescence de nanocristaux de silicium (nc-Si) de taille contrôlée. Les matériaux étudiés sont des couches minces de SiO2 contenant des nc-Si confinés. La structure des films est caractérisée par spectroscopie d'absorption infrarouge, diffraction de rayons X et microscopie électronique à transmission. La distribution en taille des nc-Si est mesurée, montrant que la taille est contrôlée avec une faible dispersion.
Les mécanismes de luminescence sont étudiés par spectroscopie de photoluminescence continue et résolue en temps de 4 K à 300 K. Corrélés à l'étude de structure, les résultats de photoluminescence montrent que la qualité de la matrice et la taille des nc-Si contrôlent les propriétés de luminescence des nc-Si. Les mécanismes de recombinaison des porteurs sont étudiés. Enfin, le transport électrique dans les couches est caractérisé. L'électroluminescence est observée et montre le rôle joué par les nc-Si sur la luminescence.

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39

Chomette, André. "Proprietes electroniques et optiques des superreseaux gaas/gaalas de petites periodes." Paris 6, 1988. http://www.theses.fr/1988PA066146.

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40

Kim, Seyoung 1981. "Electron transport in graphene transistors and heterostructures : towards graphene-based nanoelectronics." Thesis, 2012. http://hdl.handle.net/2152/ETD-UT-2012-05-5420.

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Two graphene layers placed in close proximity offer a unique system to investigate interacting electron physics as well as to test novel electronic device concepts. In this system, the interlayer spacing can be reduced to value much smaller than that achievable in semiconductor heterostructures, and the zero energy band-gap allows the realization of coupled hole-hole, electron-hole, and electron-electron two-dimensional systems in the same sample. Leveraging the fabrication technique and electron transport study in dual-gated graphene field-effect transistors, we realize independently contacted graphene double layers separated by an ultra-thin dielectric. We probe the resistance and density of each layer, and quantitatively explain their dependence on the backgate and interlayer bias. We experimentally measure the Coulomb drag between the two graphene layers for the first time, by flowing current in one layer and measuring the voltage drop in the opposite layer. The drag resistivity gauges the momentum transfer between the two layers, which, in turn, probes the interlayer electron-electron scattering rate. The temperature dependence of the Coulomb drag above temperatures of 50 K reveals that the ground state in each layer is a Fermi liquid. Below 50 K we observe mesoscopic fluctuations of the drag resistivity, as a result of the interplay between coherent intralayer transport and interlayer interaction. In addition, we develop a technique to directly measure the Fermi energy in an electron system as a function of carrier density using double layer structure. We demonstrate this method in the double layer graphene structure and probe the Fermi energy in graphene both at zero and in high magnetic fields. Last, we realize dual-gated bilayer graphene devices, where we investigate quantum Hall effects at zero energy as a function of transverse electric field and perpendicular magnetic field. Here we observe a development of v = 0 quantum Hall state at large electric fields and in high magnetic fields, which is explained by broken spin and valley spin symmetry in the zero energy Landau levels.
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41

"Charge and Exciton Transport in DNA." Tulane University, 2013.

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42

Finck, Aaron David Kiyoshi. "Studies of Exciton Condensation and Transport in Quantum Hall Bilayers." Thesis, 2012. https://thesis.library.caltech.edu/6689/1/finck_2012_thesis.pdf.

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This thesis is a report of the transport properties of bilayer two-dimensional electron systems found in GaAs/AlGaAs double quantum well semiconductor heterostructures. When a strong perpendicular magnetic field is applied so that the total Landau filling factor is equal to one and if the two layers are close enough together, a novel quantum Hall (QH) state with strong interlayer correlations can form. This QH state is often described as an excitonic condensate, in which electrons in one layer pair with holes in the other. As neutral particles, excitons feel no Lorentz force and are not confined to the edges of the bilayer system like charged quasiparticles are. Instead, excitons are expected to be able to move freely through the bulk and even flow without any dissipation under proper conditions (i.e.,~excitonic superfluidity). Counterflow studies that directly probe the bulk verify this exciton transport in the electrically insulating interior. We also report on studies of the phase boundary between the correlated and uncorrelated phases at total Landau filling factor one as the effective interlayer separation is tuned. When both phases are fully spin polarized at high Zeeman energy, the phase transition is much broader than when the uncorrelated phase is incompletely polarized at low Zeeman energy. This suggests a possible change in the nature of the phase transition in the regime of complete spin polarization.

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43

Clark, Katie Ann. "Optoelectronic properties and energy transport processes in cylindrical J-aggregates." Thesis, 2014. http://hdl.handle.net/2152/25919.

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The light harvesting systems of photosynthetic organisms harness solar energy by efficient light capture and subsequent transport of the light’s energy to a chemical reaction center. Man-made optical devices could benefit by mimicking these naturally occurring light harvesting processes. Supramolecular organic nanostructures, composed of the amphiphilic carbocyanine dye 3,3’-bis- (2-sulfopropyl)-5,5’,6,6’-tetrachloro-1,1’- dioctylbenzimida-carbocyanine (C8S3), self assemble in aqueous solution to form tubular, double-walled J-aggregates. These J-aggregates have drawn comparisons to light harvesting systems, owing to their optical and structural similarities to the cylindrical chlorosomes (antenna) from green sulfur bacteria. This research utilizes optical spectroscopy and microscopy to study the supramolecular origins of the exciton transitions and fundamental nature of exciton energy transport in C8S3 artificial light harvesting systems. Two J-aggregate morphologies are investigated: well-separated, double-walled nanotubes and bundles of agglomerated nanotubes. Linear dichroism spectroscopy of flow-aligned nanotubes is used to generate the first quantitative, polarized model for the complicated C8S3 nanotube excitonic absorption spectrum that is consistent with theoretical predictions. The C8S3 J-aggregate photophysical properties are further explored, as the Stokes shift, quantum yield, and spectral line broadening are measured as a function of temperature from 77 – 298 K. The temperature-dependent emission ratios of the C8S3 J-aggregate two-band fluorescence spectra reveal that nanotube emission is well described with Boltzmann partitioning between states, while the bundles’ is not. Finally, understanding energy transport in these materials is critical for the proposed use of artificial light harvesting systems in optoelectronic devices. The spatial extent of energy transfer in individual C8S3 J- aggregate structures is directly determined using fluorescence imaging. We find that aggregate structural hierarchy greatly influences exciton transport distances: impressive average exciton migration distances of ~ 150 nm are measured along the nanotubes, while these distances increase to over 500 nm in the bundle superstructures.
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44

Liess, Andreas. "Structure-Property Relationships of Merocyanine Dyes in the Solid State: Charge Transport and Exciton Coupling." Doctoral thesis, 2017. https://nbn-resolving.org/urn:nbn:de:bvb:20-opus-152900.

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The present thesis demonstrates the importance of the solid state packing of dipolar merocyanine dyes with regard to charge transport and exciton coupling. Due to the charge transport theory for disordered materials, it is expected that high ground state dipole moments in amorphous thin films lead to low mobility values due to a broadening of the density of states. However, due to their inherent dipolarity, merocyanine dyes usually align in antiparallel dimers in an ordered fashion. The examination of twenty different molecules with ground state dipole moments up to 15.0 D shows that by a high dipolarity and well-defined sterics, the molecules pack in a highly regular two-dimensional brickwork-type structure, which is beneficial for hole transport. Utilization of these molecules for organic thin-film transistors (OTFTs) leads to hole mobility values up to 0.21 cm²/Vs. By fabrication of single crystal field-effect transistors (SCFETs) for the derivative showing the highest mobility values in OTFTs, even hole mobilities up to 2.34 cm²/Vs are achieved. Hence, merocyanine based transistors show hole mobility values comparable to those of conventional p-type organic semiconductors and therefore high ground state dipole moments are not necessarily disadvantageous regarding high mobility applications. By examination of a different series of ten merocyanine dyes with the same chromophore backbone but different donor substituents, it is demonstrated that the size of the donor has a significant influence on the optical properties of thin films. For small and rigid donor substituents, a hypsochromic shift of the absorption compared to the monomer absorption in solution is observed due to the card stack like packing of the molecules in the solid state. By utilization of sterical demanding or flexible donor substituents, a zig-zag type packing is observed, leading to a bathochromical shift of the absorption. These packing motifs and spectral shifts with an offset of 0.93 eV of the H- and J-bands comply with the archetype examples of H- and J-aggregates from Kasha’s exciton theory
Im Rahmen der vorliegenden Doktorarbeit wird die Wichtigkeit der Packung von dipolaren Merocyaninfarbstoffen im Festkörper im Hinblick auf Ladungstransport sowie Exzitonenkopplung demonstriert. Aufgrund der Ladungstransporttheorie für ungeordnete Materialien wird erwartet, dass hohe Grundzustandsdipolmomente die Mobilität aufgrund einer Verbreiterung der Zustandsdichte verringern. Allerdings neigen Merocyanine durch ihre Dipolarität normalerweise zur Ausbildung von geordneten antiparallelen Dimeren. Durch Untersuchung von zwanzig verschiedenen Molekülen mit Grundzustandsdipolmomenten bis zu 15.0 D wird gezeigt, dass durch eine hohe Dipolarität sowie eine gut definierte Sterik der Moleküle eine hoch geordnete zweidimensionale Backstein-artige Packung erzielt wird, welche günstig für den Lochtransport ist. Hierdurch werden in organischen Dünnschichttransistoren (OTFTs) Lochmobilitäten bis zu 0.21 cm²/Vs erzielt. Durch Fertigung von Einkristallfeldeffekttransistoren (SCFETs) für das Derivat mit den höchsten Lochmobilitäten in OTFTs werden außerdem Lochmobilitäten bis zu 2.34 cm²/Vs demonstriert. Damit zeigen Merocyanin-basierte Transistoren ähnliche Lochmobilitätswerte wie konventionelle organische p-Halbleiter. Folglich sind hohe Grundzustandsdipolmomente für Anwendungen, welche hohe Mobilitäten erfordern, nicht zwangsläufig von Nachteil. Durch Untersuchung einer weiteren Serie von zehn Merocyaninfarbstoffen mit gleichem Chromophorgrundgerüst und verschiedenen Donorsubstituenten wird außerdem gezeigt, dass die Größe des Donors einen signifikanten Einfluss auf die optischen Eigenschaften von Dünnschichten hat. Für kleine und rigide Donorsubstituenten wird eine hypsochrome Verschiebung der Absorption im Vergleich zum Monomer in Lösung beobachtet, welche durch eine Kartenstapel-artige Packung der Farbstoffe im Festkörper bedingt wird. Bei der Verwendung von sterisch anspruchsvollen oder flexiblen Donorsubstituenten wird eine Zick-Zack-artige Packung beobachtet, welche eine bathochrome Verschiebung der Absorption bewirkt. Diese Packungsmotive und spektralen Verschiebungen mit einem Versatz von 0.93 eV der H- und J-Banden stehen im Einklang zu den typischen Beispielen von H- und J-Aggregaten aus Kashas Exzitonentheorie

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45

Stehr, Vera. "Prediction of charge and energy transport in organic crystals with quantum chemical protocols employing the hopping model." Doctoral thesis, 2015. https://nbn-resolving.org/urn:nbn:de:bvb:20-opus-114940.

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As organic semiconductors gain more importance for application, research into their properties has become necessary. This work investigated the exciton and charge transport properties of organic semiconducting crystals. Based on a hopping approach, protocols have been developed for the calculation of Charge mobilities and singlet exciton diffusion coefficients. The protocols do not require any input from experimental data except for the x-ray crystal structure, since all needed quantities can be taken from high-level quantum chemical calculations. Hence, they allow to predict the transport properties of yet unknown compounds for given packings, which is important for a rational design of new materials. Different thermally activated hopping models based on time-dependent perturbation theory were studied for the charge and exciton transport; i. e. the spectral overlap approach, the Marcus theory, and the Levich-Jortner theory. Their derivations were presented coherently in order to emphasize the different levels of approximations and their respective prerequisites. A short reference was made to the empirical Miller-Abrahams hopping rate. Rate equation approaches to calculate the stationary charge carrier mobilities and exciton diffusion coefficients have been developed, which are based on the master equation. The rate equation approach is faster and more efficient than the frequently used Monte Carlo method and, therefore, provides the possibility to study the anisotropy of the transport parameters and their three-dimensional representation in the crystal. The Marcus theory, originally derived for outer sphere electron transfer in solvents, had already been well established for charge transport in organic solids. It was shown that this theory fits even better for excitons than for charges compared with the experiment. The Levich-Jortner theory strongly overestimates the charge carrier mobilities and the results deviate even stronger from the experiment than those obtained with the Marcus theory. The latter contains larger approximations by treating all vibrational modes classically. The spectral overlap approach in combination with the developed rate equations leads to even quantitatively very good results for exciton diffusion lengths compared to experiment. This approach and the appendant rate equations have also been adapted to charge transport. The Einstein relation, which relates the diffusion coefficient with the mobility, is important for the rate equations, which have been developed here for transport in organic crystals. It has been argued that this relation does not hold in disordered organic materials. This was analyzed within the Framework of the Gaussian disorder model and the Miller-Abrahams hopping rate
Organische Halbleiter gewinnen immer größere Bedeutung für Anwendungen in der Elektronik. In dieser Arbeit wurden deren Eigenschaften bezüglich des Exzitonen- und Ladungstransports untersucht. Diese beiden Prozesse sind wesentlich für viele Bauteile der organischen Elektronik, wie zum Beispiel Solarzellen. Ausgehend von einem Sprungmodell wurden Verfahren zur Berechnung von Ladungsträgerbeweglichkeiten und Diffusionskoeffizienten von Singulettanregungen entwickelt, wofür bis auf die Röntgenstruktur des Kristalls keine weiteren experimentellen Daten benötigt werden, da alle notwendigen Größen durch quantenchemische Rechnungen auf hohem Niveau bestimmt werden können. Dies ermöglicht die Vorhersage der Transporteigenschaften von noch unbekannten Materialien mit bekannter Struktur, was eine Voraussetzung für das Maßschneidern neuer Materialien darstellt. Verschiedene, auf der zeitabhängigen Störungstheorie basierende thermisch aktivierte Sprungmodelle - der spektrale Überlappungsansatz, die Marcus- und die Levich-Jortner-Theorie - wurden für die Anwendung auf den Ladungs- und Energietransport hin untersucht. Ausgehend von Fermis Goldener Regel wurden die Sprunggleichungen konsistent hergeleitet, um die verschiedenen Abstufungen der jeweils vorgenommenen Näherungen und deren Voraussetzungen deutlich zu machen. Zusätzlich dazu wurde ein kurzer Exkurs zur empirischen Miller-Abrahams-Sprungrate und deren Anwendung in amorphen Systemen gemacht. Unter Verwendung der Mastergleichung wurden Ratengleichungsansätze zur Berechnung der stationären Ladungsträgerbeweglichkeiten und Exzitonendiffusionskoeffizienten entwickelt. Die Berechnung der Transportgrößen über Ratengleichungen ist wesentlich schneller und effizienter als die häufig angewendete Monte-Carlo-Simulation. Dies ermöglicht die Analyse der Anisotropie des Transports im Kristall und ihre dreidimensionale Darstellung. Die Marcustheorie, die ursprünglich für Elektronentransfer in Lösungen entwickelt wurde, hat sich auch für Ladungstransport in organischen Festkörpern bewährt. Hier wurde diese Theorie auch auf Exzitonentransport übertragen und gezeigt, daß sie im Vergleich zum Experiment für Exzitonen sogar bessere Ergebnisse liefert als für Ladungsträger. Die Levich-Jortner-Theorie überschätzt die Ladungsträgerbeweglichkeiten im Falle der Acene sehr stark. Ihre Ergebnisse weichen sogar stärker vom Experiment ab als die der Marcustheorie. Letztere enthält deutlich stärkere Näherungen, weil alle Molekülschwingungen klassisch behandelt werden. Der spektrale Überlappungsansatz führt zusammen mit den hier entwickelten Ratengleichungen sogar zu quantitativ guten Ergebnissen für die Exzitonendiffusion. Dieser Ansatz und die Ratengleichungen wurden auch für die Berechnung der Ladungsträgerbeweglichkeiten angepaßt. Für die in dieser Arbeit entwickelten Ratengleichungen ist die Einsteinrelation, welche die Diffusion mit der Drift in Beziehung setzt, von zentraler Bedeutung. Es ist umstritten, ob diese Beziehung auch in amorphen, ungeordneten Materialien gültig ist. Dieser Frage wurde im Rahmen des Gaußschen Unordnungsmodells und der Miller-Abrahams-Sprungrate nachgegangen

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46

(6577541), Long Yuan. "Spatial and Temporal Imaging of Exciton Dynamics and transport in two-dimensional Semiconductors and heterostructures by ultrafast transient absorption microscopy." Thesis, 2019.

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Recently, atomically thin two-dimensional (2D) layered materials such as graphene and transition metal dichalcogenides (TMDCs) have emerged as a new class of materials due to their unique electronic structures and optical properties at the nanoscale limit. 2D materials also hold great promises as building blocks for creating new heterostructures for optoelectronic applications such as atomically thin photovoltaics, light emitting diodes, and photodetectors. Understanding the fundamental photo-physics process in 2D semiconductors and heterostructures is critical for above-mentioned applications.

In Chapter 1, we briefly describe photo-generated charge carriers in two-dimensional (2D) transition metal dichalcogenides (TMDCs) semiconductors and heterostructures. Due to the reduced dielectric screening in the single-layer or few-layer of TMDCs semiconductors, Columbo interaction between electron and hole in the exciton is greatly enhanced that leads to extraordinary large exciton binding energy compared with bulk semiconductors. The environmental robust 2D excitons provide an ideal platform to study exciton properties in TMDCs semiconductors. Since layers in 2D materials are holding by weak van de Waals interaction, different 2D layers could be assembled together to make 2D heterostructures. The successful preparation of 2D heterostructures paves a new path to explore intriguing optoelectronic properties.

In Chapter 2, we introduce various optical microscopy techniques used in our work for the optical characterization of 2D semiconductors and heterostructures. These optical imaging tools with high spatial and temporal resolution allow us to directly track charge and energy flow at 2D interfaces.

Exciton recombination is a critical factor in determining the efficiency for optoelectronic applications such as semiconductor lasers and light-emitting diodes. Although exciton dynamics have been investigated in different 2D semiconductor, large variations in sample qualities due to different preparation methods have prevented obtaining intrinsic exciton lifetimes from being conclusively established. In Chapter 3, we study exciton dynamics in 2D TMDCs semiconductors using ultrafast PL and transient absorption microscopy. Here we employ 2D WS2 semiconductor as a model system to study exciton dynamics due to the low defect density and high quantum yield of WS2. We mainly focus on how the exciton population affects exciton dynamics. At low exciton density regime, we demonstrate how the interlayer between the bright and dark exciton populations influence exciton recombination. At high exciton density regime, we exhibit significant exciton-exciton annihilation in monolayer WS2. When comparing with the bilayer and trilayer WS2, the exciton-exciton annihilation rate in monolayer WS2 increases by two orders of magnitude due to enhanced many-body interactions at single layer limit.

Long-range transport of 2D excitons is desirable for optoelectronic applications based on TMDCs semiconductors. However, there still lacks a comprehensive understanding of the intrinsic limit for exciton transport in the TMDCs materials currently. In Chapter 4, we employ ultrafast transient absorption microscopy that is capable of imaging excitons transport with ~ 200 fs temporal resolution and ~ 50 nm spatial precision to track exciton motion in 2D WS2 with different thickness. Our results demonstrate that exciton mobility in single layer WS2 is largely limited by extrinsic factors such as charge impurities and surface phonons of the substrate. The intrinsic phonon-limited exciton transport is achieved in WS2 layers with a thickness greater than 20 layers.

Efficient photocarrier generation and separation at 2D interfaces remain a central challenge for many optoelectronic applications based on 2D heterostructures. The structural tunability of 2D nanostructures along with atomically thin and sharp 2D interfaces provides new opportunities for controlling charge transfer (CT) interactions at 2D interfaces. A largely unexplored question is how interlayer CT interactions contribute to interfacial photo-carrier generation and separation in 2D heterostructures. In Chapter 5, we present a joint experimental and theoretical study to address carrier generation from interlayer CT transitions in WS2-graphene heterostructures. We use spatially resolved ultrafast transient absorption microscopy to elucidate the role of interlayer coupling on charge transfer and photo-carrier generation in WS2-graphene heterostructures. These results demonstrate efficient broadband photo-carrier generation in WS2-graphene heterostructures which is highly desirable for atomically thin photovoltaic and photodetector applications based on graphene and 2D semiconductors.

CT exciton transport at heterointerfaces plays a critical role in light to electricity conversion using 2D heterostructures. One of the challenges is that direct measurements of CT exciton transport require quantitative information in both spatial and temporal domains. In order to address this challenge, we employ transient absorption microscopy (TAM) with high temporal and spatial resolution to image both bright and dark CT excitons in WS2-tetrance and CVD WS2-WSe2 heterostructure. In Chapter 6, we study the formation and transport of interlayer CT excitons in 2D WS2-Tetracene vdW heterostructures. TAM measurements of CT exciton transport at these 2D interfaces reveal coexistence of delocalized and localized CT excitons. The highly mobile delocalized CT excitons could be the key factor to overcome large CT exciton binding energy in achieving efficient charge separation. In Chapter 7, we study stacking orientational dependent interlayer exciton recombination and transport in CVD WS2-WSe2 heterostructures. Temperature-dependent interlayer exciton dynamics measurements suggest the existence of moiré potential that localizes interlayer excitons. TAM measurements of interlayer excitons transport reveal that CT excitons at WS2-WSe2 heterointerface are much more mobile than intralayer excitons of WS2. We attributed this to the dipole-dipole repulsion from bipolar interlayer excitons that efficiently screen the moiré potential fluctuations and facilitate interlayer exciton transport. Our results provide fundamental insights in understanding the influence of moiré potential on interlayer exciton dynamics and transport in CVD WS2-WSe2 heterostructures which has important implications in optoelectronic applications such as atomically thin photovoltaics and light harvesting devices.

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47

Messerschmidt, Simon. "Recombination dynamics of optically generated small polarons and self-trapped excitons in lithium niobate." Doctoral thesis, 2019. https://repositorium.ub.uni-osnabrueck.de/handle/urn:nbn:de:gbv:700-201907311925.

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Quasi-particles formed in lithium niobate after pulse exposure were investigated by transient absorption and photoluminescence spectroscopy as well as numerical simulations. This includes the formation process, the transport through the crystal, interim pinning on defects during the relaxation process, and the final recombination with deep centers. It was shown that the charge-transport through the crystal can be described by a hopping transport including different types of hops between regular or defective lattice sites, i.e., the transport includes a mixture of free and bound small polarons. Furthermore, the different types of hops connected with varying activation energies and their distribution are responsible for an altered temporal decay curve when changing the crystal composition or temperature. Additionally, it was shown that the hitherto accepted recombination model is insufficient to describe all transient absorption and luminescence effects in lithium niobate under certain experimental conditions, i.e., long-living absorption dynamics in the blue/UV spectral range do not follow the typical polaron dynamics and cannot be described under the assumption of charge compensation. However, similar decay characteristics between self-trapped excitons known from photoluminescence spectroscopy and the unexpected behavior of the transient absorption were found leading to a revised model. This includes, besides the known polaron relaxation and recombination branch, a significant role of self-trapped excitons and their pinning on defects (pinned STEs). Since the consideration of further absorption centers in the relaxation path after pulse exposure might result in misinterpretations of previously determined polaron absorption cross-sections and shapes, the necessity to perform a review became apparent. Therefore, a supercontinuum pump-probe experiment was designed and all measurements applied under the same experimental conditions (temperature, polarization) so that one can extract the absorption amplitudes of the single quasi-particles in a spectral range of 0.7-3.0eV. The detailed knowledge might be used to deconvolve the absorption spectra and transform them to number densities of the involved centers which enables one to obtain an easier insight into recombination and decay dynamics of small polarons and self-trapped excitons. As the hopping transport of quasi-particles and the concept of pinned STEs might be fundamental processes, a thorough understanding opens up the possibility of their exploitation in various materials. In particular, results presented herein are not only limited to lithium niobate and its applications; an extension to a wide range of further strongly polar crystals in both their microscopic processes and their use in industry can be considered.

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48

Abdelmalek, Mina. "Investigating the Factors Governing the Efficiency and the Electroluminescence Stability in Simplified Phosphorescent Organic Light-Emitting Devices Utilizing One Material for Both Hole Transport and Emitter Host." Thesis, 2013. http://hdl.handle.net/10012/8080.

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Organic Light-Emitting Devices (OLEDs) have reached industrial maturity in display technology, since OLEDs provide salient advantages such as high brightness, fast response, wide viewing angle, mechanical flexibility, and low cost manufacturing. Due to the ability of electroluminescence (EL) from triplet excited states as well as singlet excited states, phosphorescent OLEDs (PHOLEDs) have a potential to achieve 100% internal quantum efficiency. Therefore, PHOLEDs can offer a competitive external quantum efficiency. However, the operational stability of PHOLEDs is relatively poor. Several mechanisms have been proposed to address the chemical and physical phenomena associated with intrinsic degradation of PHOLEDs, nevertheless, the reasons behind voltage rise and luminance loss accompanying PHOLEDs long term operation are not yet well understood. The state of the art p-i-n PHOLEDs offer relatively high efficiency and low efficiency roll-off. However, this technology is characterized by structure complexity. Therefore, much of the current research on PHOLEDs focuses on the development of the simplest possible and most easily processed architecture that can deliver the optimal combination of device properties. Simplified PHOLEDs, utilizing one material for both hole transport and emitter host, can be a good candidate for replacement of p-i-n technology. Simplified PHOLEDs offer higher efficiency than the p-i-n PHOLEDs , yet, their EL stability is found to be poor. In this thesis, the role of the ITO/organic interface on simplified PHOLEDs efficiency will be investigated. Furthermore, possible degradation mechanisms at the ITO/organic interface will be explored. Moreover, we will correlate degradation at the ITO/organic interface to PHOLEDs operational stability. Eventually, organic layers modifications including but not limited to emissive layer (EML) will be examined. By studying the indium tin oxide (ITO)/organic interface in simplified PHOLEDs, it was found that this interface is critical to PHOLEDs performance. The study shows that, this interface is critical to the PHOLED overall stability and is considered as one of the limiting factors of the long term operational stability of simplified PHOLEDs. The effect of optical excitation on the ITO/organic interface stability in hole-only devices was investigated. It was found that the ITO/organic interface is susceptible to exciton-induced degradation. This degradation affects the device stability severely compared to current-induced degradation. The exciton-induced degradation can be prevented by doping the hole transport layer (HTL), at the interface with an exciton quencher layer or by blocking the electrons from leaking to the ITO/organic interface that may further recombine with holes to form excitons. Further studies showed that upon combining both electrical stress and optical excitation, the device degradation is even more pronounced which is most likely due to interactions between charges and excitons. By using exciton life-time measurements, a new role of molybdenum trioxide (MoO3) in the electrical stability of PHOLEDs, as an exciton quencher layer, is introduced. Delayed EL (DEL) measurements showed that the simplified PHOLEDs are susceptible to triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ) which might affect the operational stability of simplified PHOLEDs. Finally, EML modifications showed that the recombination zone of simplified PHOLEDs is located near the HTL/EML interface.

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49

Gubaev, Airat. "Light-induced absorption changes in ferroelectric crystals:SrxBa1-xNb2O6:Ce; KTaO3; KTa1-xNbxO3." Doctoral thesis, 2005. https://repositorium.ub.uni-osnabrueck.de/handle/urn:nbn:de:gbv:700-2005122011.

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The aim of the current work was to investigate the photo-induced charge transport at low temperatures, allowing more sensitive, detailed measurements of the first steps in the build-up of space charge fields, which modify the refractive index, leading to modern applications like volume holographic storage. We investigated the light-induced properties of SBN:Ce, KTO and KTN materials like origin of trapping centers which are involved in the charge transport process, characterization of trapping centers, like temperature dependence, illumination intensity dependence, evolution with time, spectral response, activation energies, the basic properties of the electronic excitations and photo-carriers localization based on results of absorption, light-induced absorption, photoluminescence, and photocurrent. The main contributions of this dissertation are summarized as follows: The experimental intensity dependence, temperature dependence, and decay process of the light-induced polaron (NIR) and VIS center absorption can be fitted with the help of a simplified charge transfer model (for SBN). The decay observed of the NIR polaron and the VIS centers is present due to the Fourier spectrometer light. The dissociation of the VIS centers into NIR centers under red light was observed. The model proposed for the VIS-centers in SBN is a triad structure related to the simultaneous bonding of two hole polarons and one electronic polaron.In KTN the emergence of the UV-light induced wide absorption bands in the NIR region with maxima at 0.69 0.8 eV at low temperatures is treated as a manifestation of the localization of photo-induced electrons and the formation of small electron polarons in close-neighbor Nb-Nb pair centers. Also, these properties in KTN can be fitted with the help of the simplified charge transfer model.

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Dissertations / Theses on the topic 'Excitonic transport'

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