Dissertations / Theses on the topic 'Atomically thin'
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Bandurin, Denis. "Electron transport in atomically thin crystals." Thesis, University of Manchester, 2017. https://www.research.manchester.ac.uk/portal/en/theses/electron-transport-in-atomically-thin-crystals(e184d9d8-ad44-41e0-8be9-bd381d6a21d6).html.
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This work is dedicated to electron transport in atomically thin crystals. We explore hydrodynamic effects in the electron liquid of graphene and perform a comprehensive study of electronic and optical properties of a novel 2D semiconductor - indium selenide(InSe). Graphene hosts a high quality electron system with weak phonon coupling such that electron-electron scattering can be the dominant process responsible for the establishment of local equilibrium of the electronic system above liquid nitrogen temperatures. Under these conditions, charge carriers are expected to behave as a viscous fluid with a hydrodynamic behaviour similar to classical gases or liquids. In this thesis, we aimed to reveal this hydrodynamic behaviour of the electron fluid by studying transport properties of high-quality graphene devices. To amplify the hydrodynamic effects, we used a special measurement geometry in which the current was injected into the graphene channel and the voltage was measured at the contact nearest to the injector. In this geometry we detected a negative signal which is developed as a result of the viscous drag between adjacent fluid layers, accompanied by the formation of current vortices. The magnitude of the signal allowed us to perform the first measurement of electron viscosity. In order to understand how an electron liquid enters the hydrodynamic regime we studied electron transport in graphene point contacts. We observed a drop in the point contact resistance upon increasing temperature. This drop was attributed to the interaction-induced lubrication of the point contact boundaries that was found to be strong enough to prevent momentum relaxation of charge carriers. The viscosity of the electron fluid was measured over a wide range of temperatures and at different carrier densities. Experimental data was found to be in good agreement with many-body calculations. In this work we also studied transport properties of two-dimensional InSe. We observed high electron mobility transport, quantum oscillations and a fully developed quantum Hall effect. In optical studies, we revealed that due to the crystal symmetry a monolayer InSe features suppressed recombination of electron-hole pairs.
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Pearce, Alexander James. "Electromechanical properties of atomically thin materials." Thesis, University of Exeter, 2014. http://hdl.handle.net/10871/15294.
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We discuss the effect of elastic deformations on the electronic properties of atomically thin materials, with a focus on bilayer graphene and MoS2 membranes. In these materials distortions of the lattice translate into fictitious gauge fields in the electronic Dirac Hamiltonian that are explicitly derived here for arbitrary elastic deformations, including in-plane as well as flexural (out-of-plane) distortions. We consider bilayer graphene, where a constant fictitious gauge field causes a dramatic reconstruction of the low energy trigonally warped electronic spectrum inducing topological transitions in the Fermi surface. We then present results of ballistic transport in trigonally warped bilayer graphene with and without strain, with particular focus on noise and the Fano factor. With the inclusion of trigonal warping the Fano factor at the Dirac point is still F = 1/3, but the range of energies which show pseudo diffusive transport increases by orders of magnitude compared to the results stemming out of a parabolic spectrum and the applied strain acts to increase this energy range further. We also consider arbitrary deformations of another two-dimensional membrane, MoS2. Distortions of this lattice also lead to a fictitious gauge field arising within the Dirac Hamiltonian, but with a distinct structure than seen in graphene. We present the full form of the fictitious gauge fields that arise in MoS2. Using the fictitious gauge fields we study the coupling between electronic and mechanical degrees of freedom, in particular the coupling between electrons and excited vibrational modes, or vibrons. To understand whether these effects may have a strong influence on electronic transport in MoS2 we calculate the dimensionless electron-vibron coupling constant for all vibron modes relevant for electronic transport. We find that electron-vibron coupling constant is highly sample specific and that the longitudinal stretching mode is the vibron with the dominant coupling. This however reaches maximum values which are lower than those observed in carbon nanostructures.
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Baugher, Britton William Herbert. "Electronic transport in atomically thin layered materials." Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/91393.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2014.125
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 101-110).
Electronic transport in atomically thin layered materials has been a burgeoning field of study since the discovery of isolated single layer graphene in 2004. Graphene, a semi-metal, has a unique gapless Dirac-like band structure at low electronic energies, giving rise to novel physical phenomena and applications based on them. Graphene is also light, strong, transparent, highly conductive, and flexible, making it a promising candidate for next-generation electronics. Graphene's success has led to a rapid expansion of the world of 2D electronics, as researchers search for corollary materials that will also support stable, atomically thin, crystalline structures. The family of transition metal diclialcogenides represent some of the most exciting advances in that effort. Crucially, transition metal dichalcogenides add semiconducting elements to the world of 2D materials, enabling digital electronics and optoelectronics. Moreover, the single layer variants of these materials can posses a direct band gap, which greatly enhances their optical properties. This thesis is comprised of work performed on graphene and the dichalcogenides MoS 2 and WSe2. Initially, we expand on the family of exciting graphene devices with new work in the fabrication and characterization of suspended graphene nanoelectromnechanical resonators. Here we will demonstrate novel suspension techniques for graphene devices, the ion beam etching of nanoscale patterns into suspended graphene systems, and characterization studies of high frequency graphene nanoelectromechanical resonators that approach the GHz regime. We will then describe pioneering work on the characterization of atomically thin transition metal dichalcogenides and the development of electronics and optoelectronics based on those materials. We will describe the intrinsic electronic transport properties of high quality monolayer and bilayer MoS 2 , performing Hall measurements and demonstrating the temperature dependence of the material's resistivity, mobility, and contact resistance. And we will present data on optoelectronic devices based on electrically tunable p-n diodes in monolayer WSe2 , demonstrating a photodiode, solar cell, and light emitting diode.
by Britton William Herbert Baugher.
Ph. D.
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Ye, Fan. "HIGHLY TUNABLE ATOMICALLY THIN RESONANTNANOELECTROMECHANICAL SYSTEMS (NEMS)." Case Western Reserve University School of Graduate Studies / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=case1589392684740436.
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Farrokhi, M. Javad. "ELECTRONIC PROPERTIES OF ATOMICALLY THIN MATERIAL HETEROSTRUCTURES." UKnowledge, 2019. https://uknowledge.uky.edu/physastron_etds/67.
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There is a movement in the electronic industry toward building electronic devices with dimensions smaller than is currently possible. Atomically thin 2D material, such as graphene, bilayer graphene, hBN and MoS2 are great candidate for this goal and they have a potential set of novel electronic properties compare to their bulk counterparts due to the exhibition of quantum confinement effects. To this goal, we have investigated the electric field screening of multilayer 2D materials due to the presence of impurity charge in the interface and vertical electric fifield from back gate. Our result shows a dramatic difference of screening behavior in high and low charging limit, which depends on the number of layers as well. We also have an extensive study on quantum tunneling effect in graphene and bilayer graphene heterojunctions. The peculiar electronic properties of graphene lead to an unusual scattering effect of electron in graphene n-p junction. We implement the cohesive tunneling effect to explain the nonlinear electron transport in ultrashort channel graphene devices. This nonlinear behavior could make them tremendously useful for ultra-fast electronic applications.
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Reale, Francesco. "Chemical vapour deposition of atomically thin tungsten disulphide." Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/56620.
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Transition metal dichalcogenides (TMDs) are van der Waals layered materials that have been known and studied for decades in their bulk form for applications such as solid-state lubricants and catalysts. However, TMDs were largely unexplored in their two-dimensional, atomically thin form. Their unique optical and electrical properties due to their reduced dimensionality started to emerge only in the last decade. In particular, group VI-TMDs (i.e., MoS2, WS2, MoSe2, WSe2) have been recently attracting tremendous attention as emerging post-graphene materials owing to their direct band-gap in the visible range which opens many prospects for diverse optoelectronic devices. Among them, atomically thin tungsten disulphide (WS2) has emerged as unique candidate for future nanotechnologies due to its superior optical, electrical and thermal properties. Any envisioned application of WS2 requires high-quality and large-area material obtainable via a scalable synthesis method. The chemical vapour deposition (CVD) of group VI-TMDs holds promise for the synthesis of high quality monolayered material extended over wafer-size areas. Nevertheless, the CVD growth of high-quality atomically thin WS2 layers requires further development since film continuity and thickness uniformity are normally limited to a few tens of micron-sized areas. The most extensively used CVD method to synthesize WS2 entails co-evaporation of tungsten oxide (WO3) and sulphur (S) powders. This choice of precursors is mainly dictated by their low toxicity compared to halides/organic precursors and effective replacement of O by S. However, due to the high sublimation temperature of WO3, the growth has to be carried out at high temperatures in between 950-1070°C and low pressures. In this work we demonstrate a novel facilitated CVD growth of high-quality atomically thin WS2 by using a novel molecular precursor approach. This strategy involves reagents which are carbon free, volatile and easily decomposable at low temperatures, allowing for the growth of mono- and bi-layered WS2 crystals with lateral sizes of 300µm at temperatures as low as 750°C, and with superior optical and electrical properties than those of naturally occurring materials. The charge carrier mobilities that we report are also higher compared to the ones obtained using innovative synthesis procedures, such as CVD growth on reusable gold substrates or metalorganic CVD on different oxides. Further we demonstrate tunability of the optical and electrical properties of WS2, inducing atomic doping with Indium.
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Venanzi, Tommaso. "Optical and infrared properties of atomically thin semiconductors." Technische Universität Dresden, 2020. https://tud.qucosa.de/id/qucosa%3A73364.
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Two-dimensional semiconductors are a topic of intense research and very attractive materials for new developments in different elds of semiconductor technology. These materials are promising candidates to satisfy the demand for faster and more compact electronics. They make new technological possibilities feasible, such as the realization and the commercial development of exible and semitransparent electronics. For these purposes, a deep knowledge of their electronic and optical properties is required. Besides the technological interest, numerous discoveries of fundamental physics were made with many others still to come. Recently for instance, superconductivity has been achieved in twisted bilayer graphene and high-temperature exciton condensation was observed in transition metal dichalcogenide heterostructures 1. The scope of this PhD is to investigate the infrared and optical properties of different two-dimensional semiconductor systems. To this end, various spectroscopic and time-resolved investigations on transition metal dichalcogenide monolayers and few-layer InSe crystals will be presented. First of all, the fabrication of exfoliated samples and van derWaals heterostructures has been successfully carried out and is described in detail. With this knowledge, the exciton physics of MoSe2 monolayer was studied. In particular, the effects of adsorbed gas molecules on the monolayer surface is discussed. It has been demonstrated that these adsorbates can localize excitons at low temperatures and that laser irradiation can release the binding of the physisorbed gas molecules. These results are of fundamental interest for spectroscopic investigations as well as relevant for opto-electronic devices as for instance gas sensors. Thereafter, several experiments were carried out with the use of the infrared free-electron laser FELBE. The general idea was to investigate the response of transition metal dichalcogenide monolayers in the far-infrared frequency range. An effect that was observed is a redshift of the trion induced by non-resonant infrared absorption. In fact, after the absorption of infrared radiation by free carriers, the energy and the momentum of the heated electron gas are transferred to the trion population, leading to a redshift of the trion resonance. By measuring the dynamics of this process, the cooling time of the electron-hole population and the far-infrared absorption of MoSe2 monolayer were extrapolated. The experiments conducted on few-layer InSe will be also presented in this thesis. The effects of hBN-encapsulation on the optical properties of InSe are discussed. The encapsulation in hBN does not only prevent the material from degradation, but also improves the optical quality by reducing the disorder potential in the crystal. Furthermore, the photoluminescence dynamics was investigated as a function of layer thickness and temperature. A bi-exponential decay was observed and the two contributions are attributed to the direct bandgap electron-hole transition and the defect assisted radiative recombination. Because of the direct-to-indirect bandgap crossover driven by the sample thickness, the dynamics gets slower while decreasing the number of layers. In particular, the fast component, i.e. the direct bandgap recombination, tends to disappear for thin InSe samples. Moreover, the photoluminescence lifetime decreases at high temperatures as a consequence of more effcient non-radiative recombination. Finally, heterostructures of MoSe2/WSe2 monolayer were fabricated and the rst spectroscopic results are presented. The interlayer exciton was observed and its dynamics was investigated.
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Manca, Marco. "Study of the optoelectronic properties of atomically thin WSe2." Thesis, Toulouse, INSA, 2019. http://www.theses.fr/2019ISAT0030.
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: Les dichalcogénures de métaux de transition (TMDs) constituent une famille de matériaux lamellaires riches de potentialités en optique et en électronique. La caractérisation des TMDs a permis la découverte de leurs propriétés physiques exceptionnelles : amincis à l’état de mono-feuillets, les TMDs semi-conducteurs deviennent des matériaux à bande interdite directe, donc très efficaces pour l’absorption ou l’émission de lumière. Le gap direct de ces semi-conducteurs est situé aux points K, à la frontière de la zone de Brillouin. Les propriétés optiques sont dominées par les excitons paires électron-trou liées par l’attraction de Coulomb et l’interaction lumière-matière y est extrêmement forte, l’absorption d’un faisceau lumineux pouvant atteindre 20% par monocouche. Outre l’existence du gap, les TMDs se différencient du graphène par leur fort couplage spin-orbite, ainsi que par la rupture de la symétrie d’inversion. En conséquence, les règles de sélection pour les transitions optiques à travers le gap ont un caractère chiral. Les états de spin opposés dans les bandes de valence et de conduction sont significativement clivés en énergie du fait de l’interaction spin-orbite. Cette propriété permet d’exciter optiquement des états de spin et de vallée spécifiques dans l’espace réciproque et de suivre leur comportement dynamique. Ainsi, les TMDs mono-feuillets constituent-ils des systèmes modèles très attractifs pour l’étude de la physique des états de spin et de valléeTransition Metal Dichalcogenides (TMDs) are a family of layered materials with potential applications in optics and electronics. Following the discovery of graphene, TMDs were characterized and extraordinary physical properties were discovered: when thinned down to a monolayer, TMDs become direct band gap materials, therefore strongly facilitating light emission. The direct bandgap of these semiconductors is situated on the edge of the Brillouin zone, at the K-point. This is different from standard semiconductors for optoelectronics like GaAs where the bandgap is in the centre of the Brillouin zone. The optical properties are dominated by excitons, and light-matter interaction is extremely strong with up to 20% of light absorption per monolayer. In addition to a bandgap, TMDs present strong spin-orbit coupling and broken inversion symmetry. As a result, the optical transitions across the bandgap have chiral selection rules. The spin states in the valence and conduction bands are well separated in energy by the spin-orbit interaction. This makes it possible to optically address specific spin and valley states in momentum space and monitor their dynamics. As a result monolayer TMDs are exciting model systems for spin and valley physics: these research fields are termed spintronics and valleytronics. This motivated our work on the exact understanding of the optical transitions, their polarization selections rules and the different exciton states
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Lorchat, Étienne. "Optical spectroscopy of heterostructures based on atomically-thin semiconductors." Thesis, Strasbourg, 2019. http://www.theses.fr/2019STRAE035.
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Au cours de cette thèse, nous avons fabriqué et étudié par spectroscopie optique, des hétérostructures de van der Waals, composées de monofeuillets semi-conducteurs (dichalcogénures de métaux de transition, DMT) couplés à une monocouche de graphène ou à un résonateur plasmonique. Nous avons observé des modifications importantes de la dynamique des états excités optiquement dans le DMT (excitons) lorsque celui-ci est en contact avec le graphène. Le graphène neutralise la couche de DMT et permet un transfert non-radiatif d’excitons en moins de quelques picosecondes. Ce transfert d’énergie peut s’accompagner d’un photodopage extrinsèque considérablement moins efficace. La réduction de la durée de vie des excitons du DMT en présence de graphène a été exploitée pour montrer que leur pseudo-spin de vallée maintenait un degré de polarisation et de cohérence important jusqu’à température ambiante. Enfin, en couplant fortement les excitons d’un DMT aux modes d’un résonateur plasmonique à phase géométrique, nous avons mis en évidence, à température ambiante, le verrouillage du pseudo-spin de vallée sur la direction de propagation des polaritons chiraux (chiralitons) issus du couplageDuring this thesis, we have fabricated and studied by optical spectroscopy, van der Waals heterostructures composed of semiconductor monolayers (transition metal dichalcogenides, TMD) coupled to a graphene monolayer or to a plasmonic resonator. We have observed significant changes in the dynamics of the TMD optically excited states (excitons) when it is in direct contact with graphene. Graphene neutralizes the TMD monolayer and enables non-radiative transfer of excitons within less than a few picoseconds. This energy transfer process may be accompanied by a considerably less efficient, extrinsic photodoping. The reduced lifetime of TMD excitons in the presence of graphene has been exploited to show that their valley pseudo-spin maintains a high degree of polarization and coherence up to room temperature. Finally, by strongly coupling TMD excitons to the modes of a geometric phase plasmonic resonator, we have demonstrated, at room temperature, that the momentum of the resulting chiral polaritons (chiralitons) is locked to their valley pseudo-spin
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Hudson, David Christopher. "Two dimensional atomically thin materials and hybrid superconducting devices." Thesis, University of Exeter, 2014. http://hdl.handle.net/10871/16034.
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In this thesis a variety of topics concerning 2D materials that have been separated from bulk layered crystals are discussed. Throughout the thesis, single and few layers of graphene, fluorinated graphene, MoS2 and WS2 are used. Two new methods of freely suspending 2D materials are presented as well as a method of removing the background from optical images. This aids contrast measurements for the determination of the number of layers. Fluorinated graphene is found to be sensitive to beta radiation; the resistance of fluorinated graphene transistors is shown to decrease upon exposure to the radiation. This happens due to the carbon-fluorine bond breaking. The sp3 hybridised structure of the fluorinated graphene is reduced back into the sp2 hybridised structure of pristine graphene. The superconducting properties of molybdenum-rhenium are characterised. It is shown to have a transition temperature of 7.5 K. It is also discovered that the material has a resistance to hydrofluoric acid; the acid etches nearly all other superconducting materials. This makes MoRe a possible candidate to explore superconductivity in conjunction with high mobility suspended graphene. To see if the material is compatible with graphene, a supported Josephson junction is fabricated. A proximity induced super current is sustained through the junction up to biases of ∼ 200 nA. The temperature dependence of the conductivity is measured for both suspended MoS2 and WS2 on a hexagonal boron nitride substrate. The dominant hopping mechanism that contributes to the conductivity at low temperatures is found to be Mott variable range hopping, with the characteristic T−1/3 dependence. The hopping transport is due to impurities that are intrinsic to the crystals, this is confirmed by comparing the results with those of supported devices on SiO2.
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Han, Kyung-Eun. "Transport of n-alkanes through graphene nanoporous atomically thin membrane." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/123294.
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Thesis: S.B., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2019Cataloged from PDF version of thesis.
Includes bibliographical references (pages 67-68).
Accurately characterizing molecular transport through a nanoporous atomically thin graphene membrane is important in determining potential for graphene's application as a filter. Amongst many factors that affect the transport, the effect of size of molecules is analyzed in this paper. Diffusion of n-alkanes between C10 and C18 was analyzed to isolate the effect of molecule size in transport trends. The n-alkanes are chosen as the solutes for their similar long-chains but differing lengths. Differently branched structures cause variable interactions with graphene between molecules. Thus, this structural consistency in n-alkanes make them optimal solutes. Additionally, these molecules are comprised of only carbon and hydrogen, allowing the same functional groups and polarities. However, their distinct boiling points allow detection in the gas chromatography-mass spectrometry (GCMS).
A diffusion cell with feed and permeate chambers, separated by a semipermeable membrane, was used to induce diffusion with difference in solute concentration between the chambers. Diffusion concentration and rates were calculated using GCMS analysis of samples taken over 6-hour period during the diffusion. The calculations were done for diffusion with and without graphene membrane for comparison. When integrating the GCMS peaks, two types of integration methods - wide-peak and narrow-peak integrations - were used to estimate the error due to difficulties in identifying peak boundaries. These results were further compared to the inherent diffusivity coefficients of the molecules, in order to quantify the selectivity imparted by graphene for diffusion. The diffusion trends from each data set were compared to the diffusion trend from inherent diffusivity coefficients, which shows that diffusiyity should decrease with larger and heavier n-alkanes.
The experimentally obtained data shows that smaller molecules diffused at faster rates, and there was a noticeable drop in the diffusion transport between C 12 and C 14. This is consistent with the expected trend. Studies that minimize sources of errors are recommended to further understand the transport of alkanes through graphene.
by Kyung-Eun Han.
S.B.
S.B. Massachusetts Institute of Technology, Department of Mechanical Engineering
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Unal, Selim. "Field-effect transistors and optoelectronic devices based on emerging atomically thin materials." Thesis, University of Exeter, 2017. http://hdl.handle.net/10871/27140.
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Development of field-effect transistors and their applications is advancing at a relentless pace. Since the discovery of graphene, a single layer of carbon atoms, the ability to isolate and fabricate devices on atomically thin materials has marked a paradigm shift in the timeline of transistor technologies. In this thesis, electrical and optical properties of atomically thin structures of graphene and tungsten disulfide (WS2) are investigated. Transport in graphene side-gated transistors and contact resistance at the metal-WS2 interface are presented. Finally, the optoelectronic performance of the hybrid graphene-WS2 devices is examined. Presently, atomically thin semiconductors grown by chemical vapour deposition are of growing interest by a broad scientific community. For this work of thesis, an air stable material which requires non-toxic gases for the growth such as WS2 is selected. A considerable contact resistance at the metal/WS2 interface is found to hamper the electrical performance of WS2 transistors. The possible origin of this contact resistance is presented in this thesis. The graphene field-effect transistors with graphene side gates are fabricated by a single step of electron beam lithography and an O2 etching procedure. A comparative study of the electrical transport properties as a function of a bias applied to the side and back gate is conducted. The side gates allow for a much more efficient modulation of the charge density in the graphene channel owing to the larger maximum electric field which can experimentally be accomplished. Furthermore, the leakage between the side gate and the graphene channel is studied in a vacuum environment. It is found that the transport between graphene and the side gate is associated with Fowler-Nordheim tunnelling and Frenkel-Poole transport. More specifically, for voltages less than 60 V, the Frenkel-Poole transport dominates the transport, whereas the Fowler-Nordheim tunnelling governs the transport at higher bias. Finally, optoelectronic properties of graphene-WS2 heterostructure are explored. An ionic polymer is used as a top gate to enhance the screening of long-lived trap charges. Responsivities as large as 10^6 A/W under illumination with 600 nm wavelength of light are demonstrated at room temperature. The fall and rise time are in the order of milliseconds due to the screening of the traps by the ionic polymer. This study is the first presentation of the transition metal dichalcogenide (TMDC)-graphene hybrid heterostructure with such a high photoresponsivity and fast response times.
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Malina, Evan. "Mechanical Behavior of Atomically Thin Graphene Sheets Using Atomic Force Microscopy Nanoindentation." ScholarWorks @ UVM, 2011. http://scholarworks.uvm.edu/graddis/145.
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Graphene, an atomically-thin layer of hexagonally bonded carbon atoms, is the strongest material ever tested. The unusual electrical and mechanical properties of graphene are particularly useful for next-generation transparent touch screens, flexible electronic displays, and photovoltaics. As such applications arise, it is critically important to characterize the resistance of this material under impact and deformation by nanoscale contact. The objective of this thesis is to study the physics of deformation in graphene sheets on a flat substrate under nanoindentation, as a function of number of graphene layers and applied force. In this work, the nanoindentation behavior of single and few layer graphene sheets was investigated by using atomic force microscopy (AFM). Graphene was created by mechanical exfoliation and deposited on a flat SiO2 substrate. The system of graphene on SiO2 simulates many of graphene’s applications, but its characterization by nanoindentation is not fully understood. Here, it was found that the deformation of the atomically-thin film remains purely elastic during nanoindentation, while the amorphous substrate deforms plastically. Also, both modulus of elasticity and contact stiffness were found to increase by 18% when few layer graphene sheets were added to a SiO2 substrate. However, no pronounced change in nanohardness was observed in the substrate with and without the addition of graphene. Furthermore, three modes of deformation were observed including purely elastic deformation, plastic deformation and an abnormal force-depth step mechanism. Each of these mechanisms was analyzed in detail using force-displacement curves and AFM images, and a deformation mechanism map, as a function of number of graphene layers and contact force, was developed. In addition to nanomechanical experiments, computer simulations by finite element analysis (FEA) were conducted in order to better understand the nanonindentation process and underlying deformation mechanisms in this system.
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Kozawa, Daichi. "Behavior of photocarrier in atomically thin two-dimensional semiconducting materials for optoelectronics." Kyoto University, 2015. http://hdl.handle.net/2433/199420.
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Nicolas, Rana. "Squeezing light in nanoparticle-film plasmonic metasurface : from nanometric to atomically thin spacer." Thesis, Troyes, 2015. http://www.theses.fr/2015TROY0028/document.
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Les plasmons polaritons de surface (SPP) et les plasmons localisés de surface (LSP) font l’objet de nombreuses investigations du fait de leur fort potentiel technologique. Récemment, une attention particulière a été portée à des systèmes supportant ces deux types de résonances en déposant des nanoparticules (NPs) métalliques sur des films minces métalliques. Plusieurs études ont mis en évidence le couplage et l’hybridation entre modes localisés et délocalisés. Cependant, une compréhension en profondeur des propriétés optiques et du potentiel de ces interfaces est toujours manquante. Nous avons mené ici une étude de systèmes NPs/film couplés. Nous avons étudié à la fois expérimentalement et théoriquement l’influence d’une couche séparatrice ultra-mince en SiO2 ainsi que l’évolution des différents modes plasmoniques pour différentes épaisseurs. Nous avons ainsi mis en lumière que de tels systèmes couplés offrent des propriétés optiques exaltées et une large accordabilité spectrale. Nous avons aussi cherché à diminuer l’épaisseur de la couche séparatrice vers le cas ultime monoatomique en utilisant le graphène. Du fait du caractère non-diélectrique de celui-ci, nous avons mis en évidence un comportement optique inattendu de la résonance plasmonique. Nous avons expliqué celui-ci par la mise en évidence du dopage du graphène par les NPs, ce qui est un premier pas en direction de dispositifs optoélectroniques à base de graphène. Enfin, après avoir amélioré notre compréhension théorique de ces systèmes, nous avons évalué leur potentiel comme capteurs SERS ou LSPSurface plasmon polariton (SPP) and Localized surface plasmon (LSP) have attracted numerous researchers due to their high technological potential. Recently, strong attention was paid to the potential of SPP and LSP combinations by investigating metallic nanoparticles (NPs) on top of metallic thin films. Several studies on such systems have shown the coupling and hybridization between localized and delocalized modes. In this work, we propose a full systematic study on coupled NP/film systems with Au NPs and Au films. We investigate both experimentally and theoretically the influence of an ultra-thin SiO2 dielectric spacer layer, as well as the evolution of the plasmonic modes as the spacer thickness increases. We show that coupled systems exhibit enhanced optical properties and larger tunability compared to uncoupled systems. We also compare these results with those measured for coupled interfaces using graphene as a non-dielectric sub-nanometer spacer. Introducing graphene adds complexity to the system. We show that such coupled systems also exhibit enhanced optical properties and larger tunability of their spectral properties compared to uncoupled systems as well as unexpected optical behavior. We explain this behavior by evidencing graphene doping by metallic NPs, which can be a first step towards graphene based optoelectronic devices. After establishing a deep understanding of coupled systems we perform both SERS and RI sensing measurements to validate the high potential of these plasmonic interfaces
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Sundararajan, Abhishek. "A STUDY ON ATOMICALLY THIN ULTRA SHORT CONDUCTING CHANNELS, BREAKDOWN, AND ENVIRONMENTAL EFFECTS." UKnowledge, 2015. http://uknowledge.uky.edu/physastron_etds/27.
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We have developed a novel method of producing ultra-short channel graphene field effect devices on SiO2 substrates and have studied their electrical transport properties. A nonlinear current behavior is observed coupled with a quasi-saturation effect. An analytical model is developed to explain this behavior using ballistic transport, where the charge carriers experience minimal scattering. We also observe multilevel resistive switching after the device is electrically stressed. In addition, we have studied the evolution of the electrical transport properties of few-layer graphene during electrical breakdown. We are able to significantly increase the time scale of break junction formation, and we are able to observe changes occurring close to breakdown regime. A decrease in conductivity along with p−type doping of the graphene channel is observed as the device is broken. The addition of structural defects generated by thermal stress caused by high current densities is attributed to the observed evolution of electrical properties during the process of breakdown. We have also studied the effects of the local environment on graphene devices. We encapsulate graphene with poly(methyl methacrylate) (PMMA) polymer and study the electrical transport through in situ measurements. We have observed an overall decrease in doping level after low-temperature annealing in dry-nitrogen, indicating that the solvent in the polymer plays an important role in doping. For few-layer encapsulated graphene devices, we observe stable n−doping. Applying the solvent onto encapsulated devices demonstrates enhanced hysteretic switching between p and n−doped states.
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Neumann, Andre [Verfasser], and Alexander [Akademischer Betreuer] Högele. "Cryogenic hyperspectroscopy of nanoemitters and atomically thin semiconductors / Andre Neumann ; Betreuer: Alexander Högele." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2017. http://d-nb.info/1160876320/34.
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Sperber, Jared L. "Investigations of hexagonal boron nitride: bulk crystals and atomically-thin two dimensional layers." Thesis, Kansas State University, 2016. http://hdl.handle.net/2097/32509.
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Master of ScienceDepartment of Chemical Engineering
James H. Edgar
Hexagonal boron nitride has been used as an inert, refractory material with excellent resistance to thermal decomposition and oxidation for more than fifty years. In the past few years, hBN has been targeted for potential electrical and optical devices such as neutron detectors, ultraviolet light emitters, deep ultraviolet light detectors, and substrates for graphene and other atomically-thin two-dimensional materials. All of these potential applications benefit from high quality, single crystals, with thicknesses varying from nanometers to microns. This research was undertaken to investigate four aspects of hBN crystal growth and recovery. (1) In an effort to optimize hBN crystal growth from a nickel-chromium flux, a series of stepped cooling experiments were undertaken. The temperature profile was stepped in a way as to promote growth in both the a and c directions, at their optimal growth conditions. Crystals were found to be typically 100-500 µm across and thickness of approximately 20-30 µm with a pyramid-like crystal habit. (2) A method for the removal of hBN crystals prior to freezing of the metal flux was demonstrated using a specialized hot pressed boron nitride crucible capable of removing hBN crystals from the flux in situ. (3) Growth of isotopically pure hBN crystals was undertaken. By modifying the crucible material for solution growth, enrichment of hBN crystals over 90% was accomplished. (4) Exfoliation of hBN has many potential applications, specifically as graphene-hBN heterostructures where layers approaching thicknesses of single atoms are most effective surface to interact with graphene as an electronic device. Several methods were tested toward exfoliating a single crystal resulting in few-layered hexagonal boron nitride nanosheets. As a result of these investigations a greater understanding of hBN bulk growth, its isotopic enrichment, its recovery, and its exfoliation was obtained.
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Trainer, Daniel Joseph. "INVESTIGATION OF THE QUASIPARTICLE BAND GAP TUNABILITY OF ATOMICALLY THIN MOLYBDENUM DISULFIDE FILMS." Diss., Temple University Libraries, 2019. http://cdm16002.contentdm.oclc.org/cdm/ref/collection/p245801coll10/id/559773.
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PhysicsPh.D.
Two dimensional (2D) materials, including graphene, hexagonal boron nitride and layered transition metal dichalcogenides (TMDs), have been a revolution in condensed matter physics and they are at the forefront of recent scientific research. They are being explored for their unusual electronic, optical and magnetic properties with special interest in their potential uses for sensing, information processing and memory. Molybdenum disulfide (MoS2) has been the flagship semiconducting TMD over the past ten years due to its unique electronic, optical and mechanical properties. In this thesis, we grow mono- to few layer MoS2 films using ambient pressure chemical vapor depositions (AP-CVD) to obtain high quality samples. We employ low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) to study the effect of layer number on the electronic density of states (DOS) of MoS¬2. We find a reduction of the magnitude of the quasiparticle band gap from one to two monolayers (MLs) thick. This reduction is found to be due mainly to a shift of the valence band maxima (VBM) where the conduction band minimum (CBM) does not change dramatically. Density functional theory (DFT) modeling of this system shows that the overlap of the interfacial S-pz orbitals is responsible for shifting the valence band edge at the Γ-point toward the Fermi level (EF), reducing the magnitude of the band gap. Additionally, we show that the crystallographic orientation of monolayer MoS2 with respect to the HOPG substrate can also affect the electronic DOS. This is demonstrated with five different monolayer regions having each with a unique relative crystallographic orientation to the underlying substrate. We find that the quasiparticle band gap is closely related to the moiré pattern periodicity, specifically the larger the moiré periodicity the larger the band gap. Using DFT, we find that artificially increasing the interaction between the film and the substrate means that the magnitude of the band gap reduces. This indicates that the moiré pattern period acts like a barometer for interlayer coupling. We investigate the effect of defects, both point and extended defects, on the electronic properties of mono- to few layer MoS¬2 films. Atomic point defects such including Mo interstitials, S vacancies and O substitutions are identified by STM topography. Two adjacent defects were investigated spectroscopically and found to greatly reduce the quasiparticle band gap and arguments were made to suggest that they are Mo-Sx complex vacancies. Similarly, grain boundaries were found to reduce the band gap to approximately ¼ of the gap found on the pristine film. We use Kelvin probe force microscopy (KPFM) to investigate the affect of annealing the films in UHV. The work function measurements show metastable states are created after the annealing that relax over time to equilibrium values of the work function. Scanning transmission electron microscopy (STEM) is used to show that S vacancies can recombine over time offering a feasible mechanism for the work function changes observed in KPFM. Lastly, we report how strain affects the quasiparticle band gap of monolayer MoS2 by bending the substrate using a custom built STM sample holder. We find that the local, atomic-scale strain can be determined by a careful calibration procedure and a modified, real-space Lawler Fujita algorithm. We find that the band gap of MoS2 reduces with strain at a rate of approximately 400 meV/% up to a maximum strain of 3.1%, after which the film can slip with respect to the substrate. We find evidence of this slipping as nanoscale ripples and wrinkling whose local strain fields alter the local electronic DOS.
Temple University--Theses
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Golalikhani, Maryam. "Structure and electronic properties of atomically-layered ultrathin nickelate films." Diss., Temple University Libraries, 2015. http://cdm16002.contentdm.oclc.org/cdm/ref/collection/p245801coll10/id/353844.
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PhysicsPh.D.
This work presents a study on stoichiometry and structure in perovskite-type oxide thin films and investigates the role of growth–induced defects on the properties of materials. It also explores the possibility to grow thin films with properties close or similar to the ideal bulk parent compound. A novel approach to the growth of thin films, atomic layer-by-layer (ALL) laser molecular beam epitaxy (MBE) using separate oxide targets is introduced to better control the assembly of each atomic layer and to improve interface perfection and stoichiometry. It also is a way to layer materials to achieve a new structure that does not exist in nature. This thesis is divided into three sections. In the first part, we use pulsed laser deposition (PLD) to grow LaAlO3 (LAO) thin films on SrTiO3 (STO) and LAO substrates in a broad range of laser energy density and oxygen pressure. Using x-ray diffraction (θ-2θ scan and reciprocal space mapping), transmission electron microscopy (TEM) and x-ray fluorescence (XRF) we studied stoichiometry and structure of LAO films as a function of growth parameters. We show deviation from bulk–like structure and composition when films are grown at oxygen pressures lower than 10-2 Torr. We conclude that the discussion of LAO/STO interfacial properties should include the effects of growth–induced defects in the LAO films when the deposition is conducted at low oxygen pressures, as is typically reported in the literature. In the second part, we describe a new approach to atomically layer the growth of perovskite oxides: (ALL) laser MBE, using separate oxide targets to grow materials as perfectly as possible starting from the first atomic layer. We use All laser MBE to grow Ruddlesden–Popper (RP) phase Lan+1NinO3n+1 with n = 1, 2, 3 and 4 and we show that this technique enables us to construct new layered materials (n=4). In the last and main section of this thesis, we use All laser MBE from separate oxide targets to build the LaNiO3 (LNO) films as near perfectly as possible by depositing one atomic layer at a time. We study the thickness dependent metal-insulator transition (MIT) in ultrathin LNO films on an LAO substrate. In LNO, the MIT occurs in thin films and superlattices that are only a few unit cells in thickness, the understanding of which remains elusive despite tremendous effort devoted to the subject. Quantum confinement and structure distortion have been evoked as the mechanism of the MIT; however, first-principle calculations show that LaNiO3 remains metallic even at one unit cell thickness. Here, we show that thicknesses of a few unit cells, growth–induced disorders such as cation stoichiometry, oxygen vacancies, and substrate-film interface quality will impact the film properties significantly. We find that a film as thin as 2 unit cells, with LaO termination, is metallic above 150 K. An oxygen K-edge feature in the x-ray absorption spectra is clearly inked to the transition to the insulating phase as well as oxygen vacancies. We conclude that dimensionality and strain are not sufficient to induce the MIT without the contribution of oxygen vacancies in LNO ultrathin films. Dimensionality, strain, crystallinity, cation stoichiometry, and oxygen vacancies are all indispensable ingredients in a true control of the electronic properties of nanoscale strongly–correlated materials.
Temple University--Theses
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Matoba, Tomohiko. "Fabrication of transition-metal oxide thin films with atomically smooth surface for spintronics application." 京都大学 (Kyoto University), 2013. http://hdl.handle.net/2433/174948.
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Venanzi, Tommaso [Verfasser], Manfred [Gutachter] Helm, and Rudolf [Gutachter] Bratschitsch. "Optical and infrared properties of atomically thin semiconductors / Tommaso Venanzi ; Gutachter: Manfred Helm, Rudolf Bratschitsch." Dresden : Technische Universität Dresden, 2021. http://d-nb.info/1231845791/34.
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Nur, Baizura Binti Mohamed. "Study on photoluminescence quantum yields of atomically thin-layered two-dimensional semiconductors transition metal dichalcogenides." Kyoto University, 2018. http://hdl.handle.net/2433/233854.
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Del, Pozo Zamudio Osvaldo. "Optics of atomically thin films and van der Waals heterostructures made from two-dimensional semiconductors." Thesis, University of Sheffield, 2015. http://etheses.whiterose.ac.uk/11975/.
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This thesis discusses optical investigations of two-dimensional metal-chalcogenide semiconductor materials and their heterostructures. Topics include a study of continuous wave (cw) and time-resolved photoluminescence (PL) of GaTe and GaSe thin films. Based on experimental evidence, we propose a model explaining the strong PL intensity decrease for thin films as a result of non-radiative carrier escape via surface states. We investigate the stability of thin films of InSe and GaSe using a combination of PL and Raman spectroscopies. By comparing signal intensities in films exposed to ambient conditions for up to 100 hours, we find notable degradation in GaSe and high stability of InSe. We continue our study with the investigation of optical properties of light emitting diodes (LED) made of van der Waals (vdW) heterostructures comprising graphene as transparent contacts, hexagonal boron nitride as tunnel barriers and transition metal dichalcogenides (TMDC), MoS2 and WS2, as the semiconductor active regions. Single and multiple 'quantum well' structures were fabricated with an aim to enhance the external quantum efficiency (EQE) under electrical injection. We also present PL characterisation of LEDs based on vdW heterostructures comprising WSe2 and MoSe2 as active layers. Temperature dependent experiments show unusual enhancement of the EQE with temperature in WSe2 in contrast to MoSe2, where both electroluminescence and PL are reduced with temperature. A theoretical approach to explain this behaviour is proposed, which is based on the strong spin-orbit interaction present in both materials.
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Lindlau, Jessica [Verfasser], and Alexander [Akademischer Betreuer] Högele. "Optical spectroscopy of charge-tunable atomically thin semiconductors at cryogenic temperatures / Jessica Lindlau ; Betreuer: Alexander Högele." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2019. http://d-nb.info/1196009414/34.
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Ghosh, Sujoy. "SYNTHESIS, ELECTRONIC AND OPTO-ELECTRONIC TRANSPORT PROPERTIES OF ATOMICALLY THIN 2D LAYERS OF MoS2, WSe2 and CuIn7Se11." OpenSIUC, 2016. https://opensiuc.lib.siu.edu/dissertations/1308.
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The recent emergence of a new class of two dimensional layered materials (2DLMs) have not only opened up the potential for exciting new technological opportunities but also established a new platform to explore exciting new fundamental physics and chemistry at the limit of atomic thickness. Among several of these newly rediscovered 2DLMs, transition metal dichalcogenides (TMDCs) as well as other elemental combinations of Group III and Group VI represent a large family of 2D layered materials, which can be isolated into few atomic layers. These materials show remarkable promise for future electronic and opto-electronics applications. The scope of this dissertation, thus, broadly covers the electronic and opto-electronic properties of such few layered 2D materials. Extensive investigation of electronic and opto-electronic transport phenomena of charge carriers in few layer MoS2 synthesized using a variety of methods such as Chemical Vapor Deposition (CVD), liquid phase exfoliation and mechanical exfoliation as well as CVT grown mechanically exfoliated WSe2 and ternary alloy of CuIn7Se11 is reported. Specifically, it is shown that in case of MoS2, the ac conductance (σ(ω); measured in the range of 10mHz < ω < 0.1 MHz) of atomically thin 2D layers of chemical vapor deposited (CVD) Molybdenum Disulphide (MoS2) as well as thin films of exfoliated flakes of MoS2, show "universal" power law behavior (with σ(ω) ~ ωs). The temperature dependence of 's' indicate that the mechanism of ac transport in CVD MoS2 is due to electron hopping by quantum mechanical tunneling (QMT) process whereas the ac transport in exfoliated MoS2 films is due to correlated barrier hoping (CBH) mechanism. The ac conductivity also show scaling behavior, manifested by collapse of the ac conductivity data for both the samples at various temperatures to one single master curve. The T-γ dependence of the d.c conductance suggests that in case of the CVD – grown and mechanically exfoliated MoS2, γ=1/3 which corresponds to the Mott’s variable range hopping (VRH) transport where as in case of liquid phase exfoliated MoS2, γ=1 which relates to thermally activated Arrhenius type transport mechanism. Opto-electronic measurements were also performed in a variety of 2DLM samples. From the field effect transport measurements on the mechanically exfoliated samples of few layers of MoS2, WSe2 and CuIn7Se11, we found at room temperature the charge carrier mobility is ~ 47 cm2/V.s, 80 cm2/V.s and 37 cm2/V.s for MoS2, WSe2 and CuIn7Se11 respectively. The photoconductivity measurements performed on these samples show that it is possible to achieve photo-responsivities values~50 μA/W, 0.2 A/W, 1 A/W and 51 A/W at room temperature for liquid exfoliated MoS2, mechanically exfoliated MoS2, WSe2 and CuIn7Se11 based devices respectively. Mechanisms of photoconduction in these materials were explained on the basis of intensity dependent photo-current measurements. From the intensity dependent photo-current along with the low temperature photoconduction measurements we found that in case of liquid phase exfoliated MoS2 thin film devices the trap states are continuously distributed within the mobility gap in these thin film of MoS2, and play a vital role in influencing the overall photo response. On the other hand for CVT grown mechanically exfoliated WSe2 based devices bimolecular recombination mechanism is the most dominant process for photoconduction. The result obtained are discussed and compared with the available literature on the subject.
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Pachuta, Kevin. "Compositional Changes Associated with the Exfoliation of Lithium Cobalt Oxide into Atomically Thin CoO2 Nanosheets." Case Western Reserve University School of Graduate Studies / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=case1522969523294786.
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Huffstutler, Jacob Danial. "AN ANALYSIS OF ELECTROCHEMICAL ENERGY STORAGE USING ELECTRODES FABRICATED FROM ATOMICALLY THIN 2D STRUCTURES OF MOS2, GRAPHENE AND MOS2/GRAPHENE COMPOSITES." OpenSIUC, 2014. https://opensiuc.lib.siu.edu/theses/1583.
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The behavior of 2D materials has become of great interest in the wake of development of electrochemical double-layer capacitors (EDLCs) and the discovery of monolayer graphene by Geim and Novoselov. This study aims to analyze the response variance of 2D electrode materials for EDLCs prepared through the liquid-phase exfoliation method when subjected to differing conditions. Once exfoliated, samples are tested with a series of structural characterization methods, including tunneling electron microscopy, atomic force microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy. A new ionic liquid for EDLC use, 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate is compared in performance to 6M potassium hydroxide aqueous electrolyte. Devices composed of liquid-phase exfoliated graphene / MoS2 composites are analyzed by concentration for ideal performance. Device performance under cold extreme temperatures for the ionic fluid is presented as well. A brief overview of by-layer analysis of graphene electrode materials is presented as-is. All samples were tested with cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy, with good capacitive results. The evolution of electrochemical behavior through the altered parameters is tracked as well.
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Murray, Clifford David Hughes [Verfasser], Thomas [Gutachter] Michely, and Paul van [Gutachter] Loosdrecht. "Local spectroscopy of atomically thin MoS2: electronic states at 1D defects, charge transfer and screening / Clifford David Hughes Murray ; Gutachter: Thomas Michely, Paul van Loosdrecht." Köln : Universitäts- und Stadtbibliothek Köln, 2020. http://d-nb.info/1225478510/34.
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Cantley, Lauren. "Biomimetic nanopores from atomically thin membranes." Thesis, 2017. https://hdl.handle.net/2144/23567.
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Biological cells are filled with a variety of pores and channels that transport ions and molecules across the cell membrane. These passageways are vital to cell function and remarkably effective due to their high selectivity, high flux, and sensitivity to environmental stimuli. This level of control is extremely attractive for applications ranging from biotechnology to energy and the environment. In this thesis, the unique properties of two dimensional materials are utilized to create solid-state nanopores that closely mimic the function of biological ion channels.
Ionic conductance measurements were used to demonstrate that nanopores introduced into graphene membranes exhibit K+/Na+ selectivity and can modulate the ionic current with an applied gate voltage. These devices are shown to respond to low gate voltages (<500 mV) at biologically relevant concentrations (up to 1M). Cation-anion selectivity, concentration dependence, and pH dependence were also investigated. We propose the observed behavior is dependent on the presence of surface adsorbates that modify the surface energy of the membrane and near the pore, creating a gaseous barrier that is modulated via electro-wetting. Additionally, we work toward creating light responsive MoS2 nanopores operating in solution, by monitoring the current through a MoS2 nanopore while the device is exposed to a focused laser beam.2018-07-09T00:00:00Z
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Kao, R. H., and 高榮輝. "Ultra-thin Oxide with Atomically Smooth Interfaces." Thesis, 1997. http://ndltd.ncl.edu.tw/handle/15934356417162502798.
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碩士中華大學
電機工程研究所
85
Native oxide is an important issue for ultra-thin oxide, which is strongly related to the gate oxide integrity such as QBD, interface scattering, etc. We have designed a leak-tight low-pressure oxidation system to desorb the native oxide in-situ. Atomically flat interfaces between oxide and Si are obtained for oxide thickness of 11 and 38 A. Because of the smooth interface and good thickness uniformity of oxide, both high-field electron mobility and oxide breakdown behavior are much improved, significant mobility improvement is obtained from these oxides with smoother interface than that from conventional furnace oxidation. Mobility reduction in ultra-thin oxide has been observed for the first time, which may be due to the remote coulomb scattering from gate electrode. In our study, the thickness variation of a 23 A N2O-oxide, grown on a 4-inch substrate, is less than 1 A, that is attributed to the increased mean-free-path of N2O-molecules under low pressure environment, only one Si atomic plane distorted beneath the N2O-oxide/Si interface suggests that thermal stress is not the limiting factor to obtain the atomically smooth interface. Direct relationship of electron mobility to interface roughness was obtained from the measured mobility of MOSFET and high resolution TEM.
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Zhang, Xiaoxiao. "Excitonic Structure in Atomically-Thin Transition Metal Dichalcogenides." Thesis, 2016. https://doi.org/10.7916/D8639Q01.
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The strong and distinctive excitonic interactions are among one of the most interesting aspects of the newly discovered family of two-dimensional semiconductors, monolayers of transition metal dichalcogenides (TMDC). In this dissertation, we explore two types different types of excitonic states in these materials beyond the isolated exciton in its radiative ground state.
In the first part of this thesis, we examine higher-order excitonic states, involving correlations between more than a single electron and hole in the usual configuration of an exciton. In particular, we demonstrate the existence of four-body correlated or biexciton states in monolayer WSe₂. The biexciton is identified as a sharply defined state in photoluminescence spectra at high exciton density. The biexciton binding energy, i.e., the energy required to separate it into to isolated excitons, is found to be 52 meV , which is more than an order of magnitude greater than that in conventional quantum-well structures. Such high binding energy arises not only from the two-dimensional carrier confinement, but also from reduced and non-local dielectric screening. These results open the way for the creation of new correlated excitonic states linking the degenerate valleys in TMDC crystals, as well as more complex many-body states such as exciton condensates or the recently reported dropletons.
In the second part of this thesis, two chapters are devoted to the identification and characterization of intrinsic lower-energy dark excitonic states in monolayer WSe₂. These optically forbidden transitions arise from the conduction band spin splitting, which was previously neglected as it only arises from higher-order spin-orbit coupling terms. First, by examining light emission using temperature-dependent photoluminescence and time-resolved photoluminescence, we indirectly probe and identify the existence of dark states that lies ~30 meV below the optically bright states. The presence of the dark state is manifest in pronounced quenching of the bright exciton emission observed at reduced temperature. To extract exact energy levels and actually utilize these dark states, as the second step, we sought direct spectroscopic identification of these states. We achieve this by applying an in-plane magnetic field, which mixes the bright and spin forbidden dark excitons. Both neutral and charged dark excitonic states have been identified in this fashion, and their energy levels are in good agreement with ab-initio calculations using GW-BSE approach. Moreover, due to the protection from their spin structure, much enhanced emission and valley lifetime were observed for these dark states. These studies directly reveal the excitonic spin manifolds in this prototypical two-dimensional semiconductor and provide a new route to control the optical and valley properties of these systems.
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Lin, Che-Yi, and 林哲儀. "Explore intrinsically electrical characteristics of atomically thin SnS2 flake." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/57669934010113348197.
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碩士國立中興大學
物理學系所
103
Here, fakes made of a few layers of SnS2 were obtained by mechanical exfoliation of a semiconducting SnS2 bulk crystal grown by chemical vapor transport and then deposited on a heavily doped Si substrate covered with a 285-nm-thick SiO2 layer. The number of layers was quickly determined by examining the difference in the contrast of the color images and the grayscale images. To study the electrical properties of SnS2 flakes, field-effect transistors (FETs) were fabricated using standard e-beam lithography and thermal evaporation. Atomically thin SnS2 FETs displayed a clear n-type demeanor in charge transport with a current modulation of up to 105 and mobility of ~3.2 cm2V-1s-1. Through careful analysis of temperature dependent resistance between two- and four-terminal FETs, we found the contact resistance extracted was small than ~5 % of total FET resistance, implying the contact resistance can be eliminated in our device fabrication process. Besides, low-frequency noise of intrinsic SnS2 flakes can be uncovered. Our result not only gives atomic insights into the electrical properties of SnS2 FETs for the first time, but also bring a big impact to the development of 2D optoelectronics.
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Chuang, Zong-Ying, and 莊宗穎. "Mechanical Exfoliation of Atomically Thin, Large Area Transition Metal Dichalcogenides." Thesis, 2019. http://ndltd.ncl.edu.tw/handle/4t93g3.
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碩士國立交通大學
光電工程研究所
107
The discovery of the two-dimensional (2D) layered semiconductors, such as transition metal dichalcogenides (TMDCs), has gained interests owing to their remarkable electronic and optical properties. While the ML TMDC flakes prepared by mechanical exfoliation method has a good crystal quality compared to those fabricated by chemical vapor deposition (CVD), atomically thin, large-area sample is difficult to obtain. In this work, we employed an ultrathin gold film to efficiently exfoliate the TMDC layers and successfully obtained a large-area ultrathin TMDC with excellent optical properties. These large-area ultrathin layers can also be transferred to any substrate without any cracks. The effect of thermal annealing in vacuum on the optical and electrical properties of TMDC samples is investigated by using the luminescence spectroscopy, atomic force microscopy, and the micro-four-point probe technique. Annealing in vacuum not only ameliorates the threshold voltage, but also enhances the contact between the electrode and samples, increasing the current and carrier mobility. The concomitant sample transfer to the substrates on a hot plate can effectively reduce wrinkles and further improve the threshold voltage, but the minimal changes of current and carrier mobility are observed.
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"Electronic Transport Studies of Atomically Thin van der Waals Materials." Tulane University, 2018.
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acase@tulane.eduSince the discovery of unconventional superconductivity in iron pnictides and iron chalcogenides, they are of great interest for both fundamental physics and high-field applications. Among the iron-based superconductors, the layered iron chalcogenide Fe(Te1-xSex) is structurally the simplest, and present unusual superconducting and magnetic properties. In particular, the existence of weak van der Waals (vdW) bonding between charge-neutral layers makes it easy to exfoliate Fe(Te1-xSex) crystals down to atomically thin sheets like graphene and transition metal dichalcogenides. In this thesis work, we discovered a series of new phenomenons and implemented electronic transport studies on these novel vdW materials. For iron chalcogenide superconductor Fe(Te1-xSex), it has been speculated from bulk studies that nanoscale inhomogeneous superconductivity may inherently exist in this system. However, this has not been directly observed from nanoscale transport measurements. Thus we prepared Fe(Te0.5Se0.5) nanoflakes with various thickness and systematically studied the correlation between the thickness and superconducting phase transition through micromechanical exfoliation and high-precision low-energy ion milling thinning. Our result revealed a systematic thickness-dependent evolution of superconducting transition. When the thickness of the Fe(Te0.5Se0.5) thin flake is reduced to less than the characteristic inhomogeneity length (around 12nm), both the superconducting current path and the metallicity of the normal state in the Fe(Te0.5Se0.5) atomic sheets are suppressed. This observation provides the first transport evidence for the nanoscale inhomogeneity nature of superconductivity in Fe(Te1-xSex). Our follow-up studies on inhomogeneous superconductivity in another ratio of Te/Se for thinner flakes further revealed the underlying physics behind the broadened phase transition. With the reduction of thickness (d < 9nm), strain free Fe(Te0.7Se0.3) nanoflakes have exhibited characters of two dimensional (2D) superconductivity according to Ginzburg – Landau (GL) fluctuation. We also observed the typical Berezinskii-Kosterlitz-Thouless (BKT) signatures which lead to the broadening of phase transition. With the combination measurements of temperature-dependent resistance (R-T) and magnetoresistance (R-H), it can be clearly identified that four distinct transition regions, including GL fluctuation, BKT transition, intrinsic inhomogeneity and superconducting region, exist in the atomically thin Fe(Te1-xSex) sheets at different temperature range. Furthermore, the dynamic evolution of random-resistor-network due to the inhomogeneous superconductivity has been explicitly revealed through I – V measurements. In addition, we found an exotic switching effect which may be associated with the induced Joule heating in the Fe(Te0.7Se0.3) nanoflakes by performing I – V sweep. We also expanded our study on the newly discovered topological nodal-line semimetal (TNLSM) ZrSiSe which can be treated as another vdW material with vdW bonding between adjacent Se layers. In this thesis work, we systematically studied the thickness dependence of quantum oscillations in ZrSiSe nanoflakes. With the reducing thickness below 50nm, an additional quantum oscillation emerges. This new quantum oscillation corresponds to a 2D surface state, evidenced by the angular dependence of cos(θ) in magnetoresistance measurements. The analysis of Landau level (LL) fan diagram and the directly fitting of Lifshitz-Kosevich (LK) formula both suggest a trivial surface state. In addition, the estimated size of Fermi surface is in good agreement with the reported angular-resolved photoemission spectroscopy (ARPES) result. Our study on the ZrSiSe surface state verifies exceptional case of bulk-edge correspondence principle in TNLSM. And also our experimental demonstrates a new way of probing surface state in TNLSM with nanoscale transport.
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Chunlei Yue
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Lin, B. C., and 林柏村. "From low temperature Si epitaxy to atomically smooth ultra-thin oxide." Thesis, 1999. http://ndltd.ncl.edu.tw/handle/72013835653885263096.
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博士國立交通大學
電子工程系
87
Abstract: We have designed a leak-tight low pressure hot wall furnace system, which can be used to grow Si epitaxy layer, selective epitaxial Si and ultra-thin silicon dioxide (<30A). The residual native oxide not only degrades the quality of epitaxial material but also extremely important for ultra-thin gate oxide. In contrast to previous ultra-high-vacuum chemical-vapor-deposition or molecular beam epitaxy, we have used a leak-tight design and hydrogen bake to reduce the background moisture and oxygen. We have first successfully grown epitaxial Si at 550oC and the quality of epitaxial film has been found comparable to that of Si substrate. The low temperature of 550 oC is especially chosen because it is suitable for future SiGe epitaxy. The selective epitaxy is achieved at low temperatures by using Dichlorosilane (SiH2Cl2) and a minimum temperature of 750 oC is achieved that is low enough for process integration consideration. The native oxide can strongly influence the gate oxide integrity. By removing the native oxide and re-growing thermal oxide, atomically smooth oxide-Si interface can be achieved. Significant mobility improvement was obtained from these oxides than that from conventional furnace oxidation. The trap generation rate and stress-induced leakage current (SILC) are also much reduced using the atomically smooth oxide. The gate oxide quality of ultra-thin oxide can be further improved by using deuterium annealing instead of traditional forming gas annealing. A factor of five times reduction of SILC is obtained by deuterium annealing.
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Chen, Hui-Yuan, and 陳薈元. "Probing Elastic Properties of Atomically-thin Interfacial Layer by Femtosecond Acoustics." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/91772253942514860460.
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碩士國立臺灣大學
光電工程學研究所
103
While the developing trend of modern electronic devices is toward scaling down, heat removal is a critical issue since thermal effect becomes seriously significant as dimensions shrink. Besides, many complex and denser structures such as 3D integrated circuits or FinFET were designed to exploit the limited space and optimize the overall device performance, which is a trend well-characterized by Moore’s law. However, it is well-known that an interfacial layer (IL) is usually formed between two adjacent heterogeneous materials. The existence of this unavoidable interfacial layer might hinder the heat conduction in those fabricated devices, and thus certainly diminishes their operational lifetime. Since heat is mainly carried by acoustic phonons, elastic property of materials is among the essential information for thermal management. Unfortunately, a proper technology to probe the elastic property of this atomically-thin interfacial layer has not yet been documented. In this thesis, we designed an interfacial layer (IL) model system between bulk GaN and Al2O3 film, and conducted femtosecond acoustic measurement to obtain the elastic properties of the IL. The acoustic impedance, mass density and a cross-plane elastic constant of the IL were successfully obtained. We further evaluated a 16% reduction in thermal energy transmission owing to the IL from a theoretical calculation. With the capability of probing the elastic properties across layers of only several atoms thick, our demonstration could be deemed as the first step to deal with heat dissipation issue stemming from the ILs. Hopefully, our approach will provide a better thermal management for nano-scaled devices in the future.
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Fuchigami, Kenji. "Atomically resolved STM studies of the perovskite manganite thin-film surfaces." 2009. http://etd.utk.edu/2009/Spring2009Dissertations/FuchigamiKenji.pdf.
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Motmaen, Dadgar Abdollah. "Strain Engineering, Quantum Transport and Synthesis of Atomically-thin Two-dimensional Materials." Thesis, 2017. https://doi.org/10.7916/D8XD1D8G.
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Two-Dimensional (2D) materials such as graphene, Transition Metal Dichalcogenides (TMDs) and Metal Monochalcogenides (MMs) are the next generation of smart devices because of their outstanding novel properties. Monolayer (one molecule thick.) of famous TMDs such as MoS2, MoSe2, WS2 and WSe2 exhibit phenomenal physical properties including but not limited to low-energy direct bandgap and large piezoelectric responses. These have made them potential candidates for cutting-edge electronic and mechanical devices such as novel transistors and PN-junctions, on-chip energy storage and piezoelectric devices which could be applied in smart sensors and actuators technologies. Additionally, reversible structural phase transition in these materials from semiconducting phase (1H) to metallic phase (1T') as a function of strain, provide compelling physics which facilitates new era of sophisticated flexoelectric devices, novel switches and a giant leap in new regime of transistors.
One iconic characteristics of monolayer 2D materials is their incredible stretchability which allows them to be subjected to several percent strains before yielding. In this thesis I provide facile techniques based on polymer encapsulation to apply several percent (6.5%) controllable, non-destructive and reproducible strains. This is the highest reproducible strain reported so far. Then I show our experimental techniques and object detection algorithm to verify the amount of strain. These followed up by device fabrication techniques as well as in-depth polarized and unpolarized Raman spectroscopy. Then, I show interesting physics of monolayer and bilayer TMDs under strain and how their photoluminescence behaviors change under tensile and compressive strains. Monolayers of TMDs and MMs exhibit 1-10 larger piezoelectric coefficients comparing to bulk piezo materials. These surprising characteristics together with being able to apply large range strains, opens a new avenue of piezoelectricity with enormous magnitudes higher than those commercially available. Further on 2D materials, I show our transport experiments on doped and pristine graphene micro devices and unveil the discoveries of magneto conductance behaviors. To complete, we present our computerized techniques and experimental platforms to make these 2D materials.
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LIN, CHUNG-WEI, and 林忠緯. "Atomically thin metal oxide Titania as electrontransporting layer for Perovskite Solar Cells." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/31629514907756147120.
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碩士國立臺灣大學
材料科學與工程學研究所
105
A recently emerging class of solid-state hybrid organic–inorganic perovskite-based solar cells,using CH3NH3PbX3(X=Cl,Br,I) as light harvesting materials, had demonstrated remarkably high power conversion efficiencies of nearly 21%. Most state-of-the-art perovskite solar cells typically have a device structure that is based on a hightemperature sintered metal oxide(compact TiO2) as electron transporting layer(ETL) which may cause the limitation of perovskite solar cells to be deposited on flexible substrates and affect their compatibility with fabrication processes in multi-junction solar cells. In this work, the utilization of atomically thin titania (atomic Ti0.87O2) deposited at room temperature as an ultra-thin electron transporting layer in perovskite solar cell was demonstrated.Through Langmuir-Blodgett deposition process at room temperature,atomic Ti0.87O2 was conformally deposited on FTO substrate with a high coverage and eliminated the requirement of high temperature process (over 500C) to deposit compact TiO2. The incorporation of multi-layer Ti0.87O2 (around 5 nm) effectively decreased the recombination of electron and hole and leaded to a reduced leakage current. This resulted in a promising device performance (14.05%) that is compatible to the device fabricated using high-temperature sintered metal oxide as electron selection layer. More importantly, we find devices using atomic Ti0.87O2 as electron transporting layer have a better stability in atomsphere. After 30 days, the atomic Ti0.87O2 devices remain about 70% of their original efficiency, unlike compact TiO2 devices, which remain 10% of original efficiency. With the atomic Ti0.87O2 electron transporting layer, we can successfully make a whole low temperature solution process, an atomically thin film ETL, and a stable deivces.
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Lloyd, David. "Engineering with atomically thin materials: making crystal grains, strains, and nanoporous membranes." Thesis, 2020. https://hdl.handle.net/2144/41023.
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Monolayer molybdenum disulfide (MoS2) is a three-atom-thick direct band gap semiconductor, which has received considerable attention for use as a channel material in atomically thin transistors, photodetectors, excitonic LED’s, and many other potential applications. It is also a mechanically exceptional material with a large stiffness and flexibility, and can withstand very large strains (11%) before rupture. In this dissertation we investigated the mechanics of the stiffness and adhesion forces in atomically thin MoS2 membranes, and how biaxial strains can be used to induce large modulations in the band structure of the material.
First, we used chemical vapor deposition (CVD) to grow MoS2 crystals that are highly impermeable to gas, and used a pressure difference across suspended membranes to induce large biaxial strains. We demonstrated the continuous and reversible tuning of the optical band gap of suspended monolayer membranes by as much as 500 meV, and induced strains of as much as 5.6% before rupture. We observed the effect of strain on the energy and intensity of the peaks in the photoluminescence (PL) and Raman spectra and found their linear strain tuning rates, then report evidence for the strain tuning of higher level optical transitions.
Second, we determined the Young’s modulus and works of separation and adhesion of MoS2 membranes, and found that adhesion hysteresis is an important effect in determining the behavior of our systems.
Finally, we investigated the use of atomically thin materials as nanofiltration membranes, by perforating the material with nanopores which selectively permit the transport of smaller molecules while rejecting larger ones. We studied ion transport through nanopores in graphene membranes and demonstrate that in-situ atomic force microscope measurements in liquid are a powerful way to reveal occlusions and contaminants around the pores - work which will aid future researchers in further unveiling the properties of these fascinating systems.
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Christopher, Jason Woodrow. "Riveting two-dimensional materials: exploring strain physics in atomically thin crystals with microelectromechanical systems." Thesis, 2018. https://hdl.handle.net/2144/27857.
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Two dimensional (2D) materials can withstand an order of magnitude more strain than their bulk counterparts, which results in dramatic changes to electrical, thermal and optical properties. These changes can be harnessed for technological applications such as tunable light emitting diodes or field effect transistors, or utilized to explore novel physics like exciton confinement, pseudo-magnetic fields (PMFs), and even quantum gravity. However, current techniques for straining atomically thin materials offer limited control over the strain field, and require bulky pressure chambers or large beam bending equipment. This dissertation describes the development of micro-electromechanical systems (MEMS) as a platform for precisely controlling the magnitude and orientation of the strain field in 2D materials. MEMS are a versatile platform for studying strain physics. Mechanical, electrical, thermal and optical probes can all be easily incorporated into their design. Further, because of their small size and compatibility with electronics manufacturing methods, there is an achievable pathway from the laboratory bench to real-world application. Nevertheless, the incorporation of atomically thin crystals with MEMS has been hampered by fragile, non-planer structures and low friction interfaces. We have innovated two techniques to overcome these critical obstacles: micro-structure assisted transfer to place the 2D materials on the MEMS gently and precisely, and micro-riveting to create a slip-free interface between the 2D materials and MEMS. With these advancements, we were able to strain monolayer molybdenum disulfide (MoS2) to greater than 1\% strain with a MEMS for the first time. The dissertation develops the theoretical underpinnings of this result including original work on the theory of operation of MEMS chevron actuators, and strain generated PMFs in transition metal dichalcogenides, a large class of 2D materials. We conclude the dissertation with a roadmap to guide and inspire future physicists and engineers exploring strain in 2D systems and their applications. The roadmap contains ideas for next-generation fabrication techniques to improve yield, sample quality, and add capabilities. We have also included in the roadmap proposals for experiments such as a speculative technique for realizing topological quantum field theories that mimics recent theoretical wire construction methods.
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Tsai, Yung-Han, and 蔡詠涵. "Two-dimensional atomically thin perovskite oxide as electron transport layer for perovskite solar cells." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/as64kv.
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碩士國立臺灣大學
材料科學與工程學研究所
106
Two-dimensional (2D) oxides are a large group of 2D materials. These 2D oxides can be divided into two subgroups: 2D metal oxides and 2D perovskite oxides. They are rich in structural diversity, electronic properties, and have novel physical and chemical properties from quantum confinement or surface effects comparing to their bulk states. 2D oxides are widely applied in the nanocapacitors, secondary batteries, and photocatalysts fields. Among the 2D perovskite oxides, Ca2Nb3O10 (CNO) atomic sheet is an n-type wide bandgap semiconductor. It has well aligned conduction band minimum with that of the lead halide perovskite, which is an efficient light absorber for solar cell application. These properties make CNO a promising electron transport material to extract electrons and block holes from lead halide perovskite light absorber. On the other hand, comparing to the conventional high temperature (> 500 ˚C) sintered compact-TiO2 electron transport layer, CNO can be deposited with relative low temperature (< 150 ˚C) solution process. In this work, we deposited CNO with low temperature Langmuir-Blodgett deposition method as electron transport layer to fabricate perovskite solar cell. The resultant devices showed best efficiency of 14.10%, which is compatible to the conventional high-temperature sintered compact-TiO2 device (14.07%). Moreover, the CNO based devices showed better electron transport ability than the conventional ones. Our work showed that CNO atomic sheet is a highly promising electron transport material for low-temperature solution processed all perovskite structure solar cells.
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"Photovoltaics large and small: atomically thin semiconductor growth and kilowatt-scale transmissive photovoltaic systems." Tulane University, 2019.
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archives@tulane.eduThis dissertation describes several key developments in semiconductor devices and technologies designed for solar power conversion and other applications. The first development is of two new growth techniques for producing large-area two-dimensional molybdenum disulfide (MoS2). Such two-dimensional materials have the potential to miniaturize photovoltaic volume and mass by orders of magnitude without sacrificing performance. While large-scale 2D-material-based photovoltaics have not yet been realized, large-area growths such as those described in this dissertation provide meaningful progress toward that goal. The described techniques enable 2D MoS2 thickness control on the order of angstroms and increase 2D MoS2 growth speed by two orders of magnitude relative to the current state of the art. Furthermore, the grown materials are developed into preliminary optoelectronic devices, with performance characterization, as a step toward more advanced photovoltaic devices. The second development presented in this dissertation is the design, fabrication, test, and analysis of a kW-scale hybrid spectrum-splitting photovoltaic module. The module is designed to be transmissive to incident infrared radiation, allowing for infrared light to be separately collected by a thermal receiver, while simultaneously collecting high-energy visible and ultraviolet light via photovoltaics. A system is built and tested on an outdoor testbed and shows 75% total power conversion efficiency (thermal and electric) of the incident solar spectrum, surpassing the capability of conventional photovoltaics. This high efficiency and combination of electrical and thermal power accelerates solar energy penetration into new applications requiring multiple power streams. Across these varied length scales, this dissertation gives glimpses into new innovations throughout the photovoltaic and semiconductor fields and aims to share this knowledge and outlook with the next generation of researchers.
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John Robertson
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Sharma, Ankur. "Engineering the Exciton Dynamics and Transport in Atomically Thin Organic and Inorganic Semiconductor Materials." Phd thesis, 2020. http://hdl.handle.net/1885/197201.
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Two-dimensional (2D) semiconductor nanomaterials have recently gathered significant attention due to their remarkable optical and electrical properties. Quantum confinement as resultant of reduced dimensions results in exciting optoelectronic phenomenon, which have never been reported before. This makes them promising candidates for future optoelectronics and photonics device applications. Many body interactions between fundamental particles in 2D semiconductors are strongly enhanced compared with those in bulk semiconductors, because of the reduced dimensionality and thus reduced dielectric screening. These enhanced many body interactions lead to the formation of robust quasi-particles, such as excitons, trions and bi-excitons, which are extremely important for the optoelectronics device applications of 2D semiconductors, such as light emitting diodes, lasers and optical modulators, etc. This thesis focusses on experimentally demonstrating properties of these many body interactions/quasi-particles and engineering them for future device optoelectronic device applications.
After the discovery of graphene, similar other inorganic 2D materials such as transition metal dichalcogenides (TMDCs) have been extensively researched. But the research is still in its early stages and there have been few attempts to study and develop organic 2D materials. First half of this thesis focusses on the interesting optical properties and exciton dynamics from a new class of organic 2D materials grown through low-temperature vapor deposition process. I observed an exciting phenomenon of superradiance/supertransport experimentally from organic 2D materials which has several potential applications in ultrafast excitonic circuits, OLEDs, high-efficiency solar cells, quantum circuits and exciton-polariton devices such as Bose-Einstein condensates at room temperature. The thesis also explores the dynamics and spatial transport mechanism of excitons and trions in inorganic TMDC materials using near-field and time-resolved PL imaging. In the thesis, I have demonstrated an external engineering control for the exciton transport in monolayer TMDCs using back gate voltage, substantiating their use for faster long-range exciton transport applications such as excitonic transistors, quantum logic gates.
The thesis further demonstrates a 1D organic nanowire with exciting optical properties that has potential applications in organic display screens, OFETs and lasers. The exciton dynamics have been studied and reported at various temperatures to understand the nature of excitons as the dimensionality changes from 2D to 1D. The thesis reports the first demonstration of Shpol' skii spectra and zero phonon lines from a solid state system. This was achieved through high crystalline and an ordered growth of organic molecules over hBN adsorbent surfaces. The thesis also demonstrates a functional FET device made incorporating organic 1D wire to demonstrate external engineering control of the Shpol' skii spectra valuable for future nanowire based device applications.
Finally, the thesis demonstrates a first of its kind 2D organic-inorganic hybrid semiconductor heterojunction, which has high efficiency for light to electricity conversion ratio. The organic 2D material reported earlier in the thesis were used to form a heterojunction with inorganic 2D TMDC material to report interesting optical properties. The organic and inorganic part are both atomically thin and that resulted in exciting optical properties that have not been demonstrated before.
In sum, this thesis is focused on exploring the fundamental excitonic dynamics and properties in novel 2D/1D organic, inorganic atomically thin semiconductor materials for their applications in next generation miniaturized optoelectronic, electronic and photonics devices.
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Wurdack, Matthias. "Strong light-matter coupling and room temperature exciton polaritons in atomically-thin WS2 crystals." Phd thesis, 2022. http://hdl.handle.net/1885/266953.
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Monolayer transition metal dichalcogenide crystals (TMDCs) hold great promise for semiconductor optoelectronics because their bound electron-hole pairs (excitons) are stable at room temperature and interact strongly with light. When TMDCs are embedded in an optical microcavity, their excitons can hybridise with cavity photons to form exciton polaritons (polaritons herein), which inherit useful properties from their constituents. For example, the low effective mass inherited from the photonic component enables polaritons in principle to macroscopically occupy their ground state at room temperature, i.e., to form a polariton condensate. Such a polariton condensate can behave like a superfluid and hence, polariton condensation in TMDCs might be a route towards dissipationless transport of information carriers on a microchip. However, because of the low effective interactions between the excitons in TMDCs, spontaneous formation of a polariton condensate in a single monolayer is challenging and clear evidence was not demonstrated yet. In other material systems it was shown that maximising the polariton lifetimes and increasing their density through strong confinement can help to reach this regime. A prevailing approach to generate polaritons with large lifetimes in conventional semiconductors are all-dielectric planar microcavities, in which polaritons can be spatially confined by patterning the cavity spacer. However, integrating monolayer TMDCs in these high-quality structures remains a challenge since they are notoriously fragile and their excitonic properties are extremely sensitive to many nanofabrication techniques. This thesis presents experimental work performed with the aim to integrate TMDCs in high-quality all-dielectric microcavities and create optimal conditions for driving the system to the regime of bosonic condensation at room temperature. In particular, we focus on monolayer WS2, which has a high exciton quantum yield and displays strong light-matter interaction at room temperature. First, the challenge presented of integrating monolayer WS2 into high-Q microcavities without causing damage to the monolayer is overcome by mechanical assembly. We show that WS2 polaritons in such a microcavity can propagate ballistically over tens of micrometres in the thermal regime, before the onset of condensation, and possess enhanced macroscopic coherence due to strong motional narrowing and weak inter-particle interactions. However, the mechanical assembly process of the microcavities is intrinsically non-scalable. To enable the integration of TMDC monolayers into functional devices on larger scales, we developed a new passivation and protection technology utilising liquid-metal printed, ultrathin Ga2O3 glass. We show that the Ga2O3 film strongly suppresses exciton-exciton annihilation in monolayer WS2, which prohibits large exciton densities in blank TMDC monolayers, and that it provides excellent protection against dielectric material deposition. The latter allows us to integrate the monolayer into all-dielectric environments with conventional deposition techniques while maintaining its high optical performance. Finally, we engineer an effective trapping potential for WS2 polaritons at room temperature by placing a WS2/Ga2O3/WS2 structure with a small top layer inside a microcavity. This design allows us to compare the properties of trapped and free polaritons. Remarkably, the ground state emission and the macroscopic coherence is strongly enhanced when trapped due to efficient energy relaxation, long photon lifetimes and strong motional narrowing. Overall, this thesis provides significant insights into the properties and dynamics of free and trapped WS2 polaritons in the thermal regime at room temperature, which can guide future work towards demonstrating unambiguous signatures of polariton condensation in this novel material class.
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Vutukuru, Mounika. "Straining the flatland: novel physics from strain engineering of atomically thin graphene and molybdenum disulfide." Thesis, 2021. https://hdl.handle.net/2144/43097.
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2D materials like graphene and MoS_2 are atomically thin, extremely strong and flexible, making them attractive for integration into strain engineered devices. Strain on these materials can change physical properties, as well as induce exotic physics, not typically seen in solid-state systems. Here, we probe the novel physics arising from distorted lattices of 2D materials, strained by nanopillars indentation and microelectromechanical systems (MEMS), using Raman and photoluminescence (PL) spectroscopy. From nanopillars strained multilayer MoS_2, we observe exciton and charge carrier funneling due to strain, inducing dissociation of excitons in to free electron-hole pairs in the indirect material. Using MEMS devices, we were able to dynamically strain monolayer and multilayer graphene. Multilayer graphene under MEMS strain showed signatures of loss in Bernal stacking due to shear of the individual layers, indicating that MEMS can be used to tune the layer commensuration with tensile strain. We further explore simulation of pseudo-magnetic fields (PMFs) generated in monolayer graphene strained by MEMS, using machine learning, to accelerate and optimize the strength and uniformity of the PMF in new graphene geometries.
Nanopillars provide non-uniform, centrally biaxial strain to multilayer MoS_2 transferred on top. Raman E^1_2g and PL redshift across the pillar confirms 1-2% strain in the material. We also observe a softening in the A_1g Raman mode and an enhancement in the overall PL with an increase in radiative trions, under strain. The changes in these charge-dependent features indicates funneling of charge carriers and neutral excitons to the apex of the pillar, as strain locally deforms the band structure of the conduction and valence bands. DFT calculations of the band structure in bilayer MoS_2 under biaxial strain shows the conduction band is lowered, further increasing the indirectness of multilayer MoS_2. This should cause the PL intensity to decrease, whereas we observe an increase in MoS_2 PL intensity under strain. We theorize that this is due to a dissociation of excitons into free electron-hole pairs. The increase in charge carrier densities due to strain leads to a renormalization of the local band structure and increased dielectric screening, supporting free electron-hole recombination at the K-point without momentum restrictions. In turn, electron-hole recombination occurs around the K-point inducing a high intensity PL, which opens attractive opportunities for utilization in optoelectronic devices.
MEMS chevron actuators can dynamically strain 2D materials, which we demonstrate through uniaxial strain in CVD and exfoliated graphene. We use a novel microstructure assisted transfer technique which can deterministically place materials on non-planar surfaces like MEMS devices. Building on previously reported 1.3% in monolayer MoS2 from our group, we report tunable 0.3% strain in CVD monolayer graphene and 1.2% strain in multilayer exfoliated graphene using MEMS chevron actuators, detected by Raman spectroscopy. The asymmetric-to-symmetric strain evolution of the 2D phonon line shape in multilayer graphene is evidence of changes in interlayer interactions, caused by shearing between layers. This demonstrates that MEMS can be used to tune the commensuration in few layer 2D materials, which is a promising avenue towards Moiré engineering. Using machine learning, we also simulate optimal monolayer graphene geometries for generating strong, uniform pseudo-magnetic fields by MEMS strain. The coupled use of finite-element methods, variational auto-encoder, and auxiliary neural network accelerates the search for PMFs in strained graphene, while optimizing the graphene shape for fabrication through electron-beam lithography. Our experimental and simulated work creates a road-map for rapid advancement in zero-field quantum Hall effect devices using graphene-integrated MEMS actuators.
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Mecouch, William J. "Preparation and characterization of thin, atomically clean GaN(0001) and AlN(0001) films and the deposition of thick GaN films via iodine vapor phase growth." 2005. http://www.lib.ncsu.edu/theses/available/etd-06142005-212822/unrestricted/etd.pdf.
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