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1
Gorini, Lorenzo. "Electrical contact properties of ultrathin transition metal dichalcogenide sheets". Master's thesis, Alma Mater Studiorum - Università di Bologna, 2018. http://amslaurea.unibo.it/16884/.
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The graphene discovery led to advances in exfoliation and synthetic techniques, and the lack of a bandgap in graphene has stimulated the research for new 2D semiconducting materials. Transition metal dichalcogenides (TMDCs), semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), have recently been isolated. TMDCs exhibit a unique combination of atomic-scale thickness, strong spin–orbit coupling and favourable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, valleytronics and optoelectronics.
According to optical measurements, single-layer WS2 sheets exhibit a direct band gap of at least 2.0 eV. Because of its strong spin-orbit coupling induced valence band splitting, WS2 shows spin-valley coupling, even in few-layer sheets , which may allow easier observation of the valley Hall effect than in the other TMDCs.
The thesis reviews the theoretical background of TMDCs and their optoelectronic properties. It also reports on the fabrication of field-effect transistors based on few-layer sheets of WS2 and the investigation of their electronic transport properties. Particularly the project focuses on improving the interface between the metal contact and WS2 sheet, where annealing improves the contact transparency. Together with van der Pauw geometry, annealing allows four-terminal measurements to be performed and the pristine properties of the material to be recovered at room temperature, where the devices show n-type behaviour and a linear I-V curve.
The promising improvements and the electronic properties shown in this thesis make WS2 interesting for future applications in valleytronic devices.
2
Choukroun, Jean. "Theoretical sStudy of In-plane Heterojunctions of Transition-metal Dichalcogenides and their Applications for Low-power Transistors". Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLS557/document.
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La miniaturisation des MOSFET a permis une forte diminution des transistors et des puces, ainsi qu’une augmentation exponentielle des capacités de calcul. Cette miniaturisation ne peut néanmoins continuer ainsi: de nos jours, un microprocesseur peut contenir des dizaines de milliards de transistors et la chaleur dégagée par ces composants peut fortement détériorer ses performances. De plus, du fait de leur principe même de fonctionnement, la tension d’alimentation des MOSFET ne peut être réduite sans en impacter les performances. De nouvelles architectures telles que le TFET -basé sur l’effet tunnel bande-à-bande et pouvant fonctionner à des tensions d’alimentation très basses- ainsi que de nouveaux matériaux pourraient donc apporter une alternative au MOSFET silicium. Les monocouches de dichalcogènures de métaux de transitions (TMDs) -des semiconducteurs à bande interdite directe d’environ 1 à 2 eV- possèdent un fort potentiel pour l’électronique et la photonique. De plus, dans le cas de contraintes appropriées, ils peuvent conduire un alignement de bandes présentant un broken-gap; cette configuration permet de surpasser les limites habituelles du TFETs, à savoir de faibles courants dus à l’effet tunnel sur lequel ces dispositifs reposent. Dans ce travail de thèse, des hétérojonctions planaires de TMD sont modélisées via une approche atomistique de liaisons fortes, et une configuration broken-gap est observée dans deux d’entre elles (MoTe2/MoS2 et WTe2/MoS2). Leur potentiel dans le cadre de transistors à effet tunnel (TFETs) est évalué au moyen de simulations de transport quantique basées sur un modèle TB atomistique ainsi que la théorie des fonctions de Green hors-équilibre. Des TFETs type-p et type-n basés sur ces hétérojonctions sont simulés et présentent des courants ON élevés (ION > 103 µA/µm) ainsi que des pentes sous-seuil extrêmement raides (SS < 5 mV/dec) à des tensions d’alimentation très faibles (VDD = 0.3 V). Plusieurs architectures novatrices basées sur ces TFETs et découlant de la nature 2D des matériaux utilisés sont également présentées, et permettent d’atteindre des performances encore plus élevéesNowadays, microprocessors can contain tens of billions of transistors and as a result, heat dissipation and its impact on device performance has increasingly become a hindrance to further scaling. Due to their working mechanism, the power supply of MOSFETs cannot be reduced without deteriorating overall performance, and Si-MOSFETs scaling therefore seems to be reaching its end. New architectures such as the TFET, which can perform at low supply voltages thanks to its reliance on band-to-band tunneling, and new materials could solve this issue. Transition metal dichalcogenide monolayers (TMDs) are 2D semiconductors with direct band gaps ranging from 1 to 2 eV, and therefore hold potential in electronics and photonics. Moreover, when under appropriate strains, their band alignment can result in broken-gap configurations which can circumvent the traditionally low currents observed in TFETs due to the tunneling mechanism they rely upon. In this work, in-plane TMD heterojunctions are investigated using an atomistic tight-binding approach, two of which lead to a broken-gap configuration (MoTe2/MoS2 and WTe2/MoS2). The potential of these heterojunctions for use in tunnel field-effect transistors (TFETs) is evaluated via quantum transport computations based on an atomistic tight-binding model and the non-equilibrium Green’s function theory. Both p-type and n-type TFETs based on these in-plane TMD heterojunctions are shownto yield high ON currents (ION > 103 µA/µm) and extremely low subthreshold swings (SS < 5 mV/dec) at low supply voltages (VDD = 0.3 V). Innovative device architectures allowed by the 2D nature of these materials are also proposed, and shown to enhance performance even further
3
Plumadore, Ryan. "Study of Two Dimensional Materials by Scanning Probe Microscopy". Thesis, Université d'Ottawa / University of Ottawa, 2019. http://hdl.handle.net/10393/38637.
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This thesis explores structural and electronic properties of layered materials at the nanometre scale. Room temperature and low temperature ultrahigh vacuum scanning probe microscopy (scanning tunneling microscopy, scanning tunneling spectroscopy, atomic force microscopy) is used as the primary characterization method. The main findings in this thesis are: (a) observations of the atomic lattice and imaging local lattice defects of semiconducting ReS2 by scanning tunneling microscopy, (b) measurement of the electronic band gap of ReS2 by scanning tunneling spectroscopy, and (c) scanning tunneling microscopy study of 1T-TaS2 lattice and chemically functionalizing its defects with magnetic molecules.
4
Zeng, Xiaoling [Verfasser], Veit [Akademischer Betreuer] Wagner, Veit [Gutachter] Wagner, Thomas [Gutachter] Heine i Marko [Gutachter] Marinkovic. "Solution Processed 2D Transition Metal Dichalcogenides and Electrical Properties of TMD Thin Film Transistors / Xiaoling Zeng ; Gutachter: Veit Wagner, Thomas Heine, Marko Marinkovic ; Betreuer: Veit Wagner". Bremen : IRC-Library, Information Resource Center der Jacobs University Bremen, 2017. http://d-nb.info/1148104011/34.
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Těšík, Jan. "Luminiscence polovodičů studovaná rastrovací optickou mikroskopií v blízkém poli". Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2017. http://www.nusl.cz/ntk/nusl-320110.
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This work is focused on the study of luminescence of atomic thin layers of transition metal chalkogenides (eg. MoS2). In the experimental part, the work deals with the preparation of atomic thin layers of semiconducting chalcogenides and the subsequent manufacturing of plasmonic interference structures around these layers. The illumination of the interference structure will create a standing plasmonic wave that will excite the photoluminescence of the semiconductor. Photoluminescence was studied both by far-field spectroscopy and near-field optical microscopy.
6
Desgué, Eva. "Control of structural and electrical properties of bilayer to multilayer PtSe₂ films grown by molecular beam epitaxy for high-performance optoelectronic devices". Electronic Thesis or Diss., université Paris-Saclay, 2024. http://www.theses.fr/2024UPASP170.
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Le PtSe₂ est un matériau 2D de la famille des dichalcogénures de métaux de transition (TMDs) qui présente des propriétés intrinsèques exceptionnelles : mobilité des porteurs de charge élevée (200 - 450 cm².(V.s)⁻¹), gap électronique ajustable en fonction du nombre de monocouches (MLs), absorption optique large bande et excellente stabilité à l'air. Ces propriétés sont idéales pour des applications (opto)électroniques. Cependant, la croissance de PtSe₂ de haute qualité cristalline sur un substrat à bas coût et isolant reste un enjeu majeur. Ici, la synthèse de PtSe₂ bicouche à multicouche (< 20 MLs) par épitaxie par jets moléculaires (MBE) est optimisée sur un substrat de saphir. Les caractérisations systématiques comprennent la diffraction électronique (RHEED), la spectroscopie Raman, la spectroscopie de rayons X à dispersion d'énergie (EDS) et des mesures électriques de conductivité. Pour les films épais de PtSe₂ semi-métallique, on démontre que des températures élevées de croissance (520 °C) et de recuit (690 °C), ainsi qu'un fort flux de sélénium (Ф(Se) = 0,5 Å.s⁻¹ ; Ф(Se)/Ф(Pt) ~ 170), permettent d'obtenir une haute qualité cristalline et une haute conductivité électrique. L'impact du recuit post-croissance sur les propriétés structurelles des films épais est particulièrement étudié par diffraction des rayons X (XRD) et microscopie électronique à transmission (STEM). Les films de PtSe₂ non recuits consistent en une distribution 3D de domaines superposés ayant différentes orientations dans le plan, tandis que les films recuits consistent en un réseau 2D de domaines monocristallins selon l'axe c. En d'autres termes, les films non recuits ont des domaines d'épaisseur plus faible que celle du film et sont constitués de phases semi-conductrices et semi-métalliques, entraînant une faible conductivité (0,5 mS). Au contraire, les films recuits sont composés uniquement de domaines quasi-monocristallins et semi-métalliques, et présentent une très haute conductivité, jusqu'à 1,6 mS. On montre également que l'indicateur de qualité cristalline couramment utilisé, qui est la largeur à mi-hauteur (FWHM) du pic Raman Eg, n'est valide que s'il est étudié conjointement avec la FWHM du pic Raman A1g. On démontre que plus la FWHM des pics Eg et A1g est faible, plus la qualité cristalline des films de PtSe₂ dans le plan et hors du plan, respectivement, est élevée, et plus la conductivité électrique augmente. Concernant les films bicouches de PtSe₂ semi-conducteur, on obtient des films de haute qualité cristalline, dont la FWHM des pics Eg et A1g est comparable à celle des cristaux exfoliés, en effectuant une synthèse avec un flux périodique de Pt (periodic supply epitaxy). Les films de PtSe₂ bicouches à multicouches ne sont pas monocristallins mais présentent une texture de fibre selon l'axe c, ce qui est typique sur un substrat de saphir. On démontre pour la première fois l'épitaxie d'un film épais de PtSe₂ sur des surfaces vicinales (marches) de saphir. Pour finir, nous avons fabriqué des dispositifs optoélectroniques fonctionnant à 1,55 µm, la longueur d'onde typique des télécommunications par fibre optique. Ils sont à base de PtSe₂ épais semi-métallique, présentant une haute conductivité électrique et une bonne absorption optique à 1,55 µm, qui est directement synthétisé sur un substrat de saphir 2 pouces. On montre des photodétecteurs à base de PtSe₂ avec une largeur de bande record de 60 GHz et le premier mélangeur optoélectronique à base d'un TMD présentant, de plus, une largeur de bande supérieure à 30 GHzPtSe₂ is a 2D material from the transition metal dichalcogenide (TMD) family that exhibits outstanding intrinsic properties: high charge carrier mobility (200 - 450 cm².(V.s)⁻¹), tunable bandgap with the number of monolayers (MLs), broadband optical absorption and excellent air stability. These properties are ideally suited for (opto)electronic applications. However, the growth of high crystalline quality PtSe₂ on low-cost and insulating substrates remains a major challenge. Here, the synthesis of bilayer to multilayer PtSe₂ films (< 20 MLs) by molecular beam epitaxy (MBE) is optimized on a sapphire substrate. The systematic characterizations include electron diffraction (RHEED), Raman spectroscopy, energy dispersive X-ray spectroscopy (EDX) and electrical conductivity measurements. For thick semimetallic PtSe₂ films, we demonstrate that high growth (520°C) and annealing (690°C) temperatures, combined with a high selenium flux (Ф(Se) = 0.5 Å.s⁻¹; Ф(Se)/Ф(Pt) ~ 170), leads to high crystalline quality and high electrical conductivity. In particular, the effect of the post-growth annealing on the structural properties of the thick films is investigated using X-ray diffraction (XRD) and transmission electron microscopy (STEM). We show that non-annealed PtSe₂ films consist of a 3D random distribution of superimposed domains with different in-plane orientations, while the annealed films consist of a 2D network of single-crystalline domains along the c-axis. In other words, non-annealed films have domains with a thickness smaller than that of the film and are composed of both semiconducting and semimetallic phases, resulting in low electrical conductivity (0.5 mS). In contrast, the annealed films are composed solely of quasi-single-crystalline and semimetallic domains, and exhibit high conductivity, up to 1.6 mS. We also show that the commonly used crystalline quality indicator, which is the full width at half maximum (FWHM) of the Eg Raman peak, becomes a reliable metric only when it is studied in conjunction with the FWHM of the A1g Raman peak. We demonstrate that the lower the FWHM of both the Eg and A1g peaks, the higher the crystalline quality of the in-plane and out-of-plane PtSe₂ films, respectively, and the higher the electrical conductivity. For semiconducting PtSe₂ bilayer films, high crystalline quality films with Eg and A1g FWHM values comparable to those of exfoliated crystals are obtained using a periodic Pt flux (periodic supply epitaxy). The bilayer to multilayer PtSe₂ films are not monocrystalline but present a fiber texture along the c-axis, which is typical on a sapphire substrate. The epitaxy of a thick PtSe₂ film on vicinal sapphire surfaces (steps) is demonstrated for the first time. Finally, we fabricated optoelectronic devices operating at 1.55 µm, the typical wavelength of optical fiber telecommunications. They are based on thick semi-metallic PtSe₂, exhibiting high electrical conductivity and good optical absorption at 1.55 µm, which is directly synthesized on a 2-inch sapphire substrate. We demonstrate PtSe₂-based photodetectors with a record bandwidth of 60 GHz and the first TMD-based optoelectronic mixer with, in addition, a bandwidth larger than 30 GHz
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Hart, Lewis. "Novel transition metal dichalcogenide semiconductors and heterostructures". Thesis, University of Bath, 2018. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.760986.
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Rhenium diselenide and rhenium disulphide are layered semiconductors that belong to the transition metal dichalcogenide (TMD) family. Like graphene and other TMDs, these materials can be exfoliated down to a few atomic layers. However, unlike other TMDs, the rhenium dichalcogenides are only stable in a triclinic structure that exhibits in-plane anisotropy. This anisotropy manifests itself in the vibrational, optical and electronic transport properties ofthese crystals. Ab initio calculations and experimental results are presented to describe the Raman spectra of the rhenium dichalcogenides. From Raman spectroscopy the anisotropy of these crystals can be observed. Flipping a flake (a C2 rotation about an axis in the layer plane) is not a symmetry of the system. Therefore, there are two non-equivalent vertical orientations. Raman spectroscopy can be used to identify whether a flake is facing "up" or "down". The latticedynamics of these crystals are described using a simple ball and spring model. It is shown that low mass impurities, such as sulphur, in ReSe2 can occupy four non-equivalent positions of the unit cell; there are four local vibrational modes corresponding to these four positions and Raman spectroscopy can be used to find them. An unusual experimental geometry (edge-on excitation) helps enhance these signals. The electronic band structures of bulk ReSe2 and ReS2 are explored using angle-resolved photoemission spectroscopy (ARPES). From the measurements and complementary DFT calculations it is shown that: (i) there is anisotropy in the electronic dispersions; (ii) the valence band maxima are not located along any of the high symmetry directions; and (iii) both of these crystals have indirect band gaps. The rhenium dichalcogenides were thought to act as electronically decoupled monolayers; it is demonstrated that this is not the case and that thereis signicant electronic coupling between the layers. Finally, ARPES results of a monolayer of ReSe2 are presented; again, anisotropy in the electronic band structure is observed.
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McCormick, Elizabeth Joan McCormick. "Optical Properties of Two Dimensional Semiconductors". The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1531907387651019.
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Morell, Bennasser Nicolás. "Optomechanical resonators based on transition metal dichalcogenide monolayers". Doctoral thesis, Universitat Politècnica de Catalunya, 2018. http://hdl.handle.net/10803/664927.
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Suspended monolayer transition metal dichalcogenides (TMD) aremembranes that combine ultralow mass and exceptional optical prop-erties, making them intriguing materials for opto-mechanical applica-tions. However, the low measured quality factor of TMD resonatorshas been a roadblock so far. In this thesis, we first show an ultra-sensitive optical readout of monolayer TMD resonators that allows usto reveal their mechanical properties at cryogenic temperatures. Wefind that the quality factor of monolayer WSe2resonators greatly in-creases below room temperature, reaching values as high as 16000 at liquid nitrogen temperature and 47000 at liquid helium temper-ature. This surpasses the quality factor of monolayer graphene res-onators with similar surface areas. Upon cooling the resonator, the res-onant frequency increases significantly due to the thermal contractionof the WSe2lattice. These measurements allow us to experimentallystudy the thermal expansion coefficient of WSe2 monolayers for thefirst time. High Q-factors are also found in resonators based on MoS2 and MoSe2 monolayers. The high quality-factor found in this workopens new possibilities for coupling mechanical vibrational states totwo-dimensional excitons, valley pseudospins, and single quantumemitters and for quantum opto-mechanical experiments based on theCasimir interaction.The sensing capabilities offered by these high Q-factor nanome-chanical oscillators are also of interest for studying thermodynamicproperties in condensed matter regimes that are difficult to access. Inthe second part of the thesis, we use optomechanical systems basedon a MoSe2 monolayer to probe the thermal properties of phononsin two-dimensional lattices. We measure the thermal conductivityand the specific heat capacity down to cryogenic temperature. Thephonon transport crossovers from the diffusive to the ballistic regimewhen lowering the temperature below~100 K. The temperature de-pendence of the specific heat capacity approaches a quadratic depen-dence, the signature of two-dimensional lattices. Both the thermalconductivity and the specific heat capacity measurements are consis-tent with predictions based on first-principles. Our result establishes anew strategy to investigate thermal transport in two-dimensional ma-terials, and allows for exploring the phonon hydrodynamic regime,the anomalous heat conduction, and the phase transitions of electronicmany-body collective phenomena in monolayersLos dicalcogenuros de metal de transición (TMD) monocapa suspendidos combinan una masa ultrabaja y propiedades ópticas excepcionales, lo que los convierte en materiales intrigantes para aplicaciones opto-mecánicas. Sin embargo, el bajo factor de calidad Q medido en los resonadores de TMD ha sido un obstáculo hasta ahora. En esta tesis, primero mostramos una lectura óptica ultra sensible de resonadores TMD de monocapa que nos permite revelar sus propiedades mecánicas a temperaturas criogénicas. Encontramos que el factor de calidad de los resonadores WSe2 monocapa aumenta considerablemente por debajo de la temperatura ambiente, alcanzando valores tan altos como 1.6 x 104 en temperatura de nitrógeno líquido y 4.7 x 104 en temperatura de helio líquido. Esto supera el factor de calidad de los resonadores de grafeno monocapa con áreas de superficie similares. Al enfriar el resonador, la frecuencia de resonancia aumenta significativamente debido a la contracción térmica la red del cristal de WSe2. Estas mediciones nos permiten estudiar experimentalmente el coeficiente de expansión térmica de las monocapas de WSe2 por primera vez. Los altos factores Q también se encuentran en los resonadores basados en las monocapas de MoS2 y MoSe2. El alto factor de calidad que se encuentra en este trabajo abre nuevas posibilidades para acoplar estados vibracionales mecánicos a excitones bidimensionales, valley pseudo-spins y emisores cuánticos únicos y para experimentos opto-mecánicos cuánticos basados en la interacción de Casimir. Las capacidades de detección ofrecidas por este nano-resonador mecánico de alto factor Q también son interesantes para estudiar propiedades termodinámicas en regímenes de la materia condensada a los que es difícil acceder. En la segunda parte de la tesis, utilizamos sistemas optomecánicos basados en una monocapa de MoSe2 para probar las propiedades térmicas de los fonones en redes de cristales bidimensionales. Medimos la conductividad térmica y la capacidad calorífica específica hasta temperaturas criogénicas. Los régimenes de transporte de fonones pasan de el difuso al balístico al bajar la temperatura por debajo de 100 K. La dependencia de la temperatura de la capacidad calorífica específica se aproxima a una dependencia cuadrática, lo cual es la firma de las redes bidimensionales. Tanto la conductividad térmica como las mediciones de la capacidad calorífica específica son coherentes con las predicciones basadas en primeros principios. Nuestro resultado establece una nueva estrategia para investigar el transporte térmico en materiales bidimensionales y permite explorar el régimen hidrodinámico de fonones, la conducción de calor anómala y las transiciones de fase de los fenómenos colectivos de cuerpos electrónicos en monocapas.
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Ilic, Stefan. "Quantum coherent phenomena in disordered transition metal dichalcogenide monolayers". Thesis, Université Grenoble Alpes (ComUE), 2019. http://www.theses.fr/2019GREAY038.
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Les monocouches de dichalcogénures de métaux de transition (TMDC) sont des matériaux bidimensionnels découverts récemment. Ils possèdent un fort couplage spin-orbite (SOC) intrinsèque qui agit comme un champ Zeeman effectif perpendiculaire, mais avec des orientations opposées dans chaque vallée située autour des points +K et -K de la zone Brillouin. En présence de désordre, ce SOC influence fortement les phénomènes quantiques cohérents dans les TMDC. Dans cette thèse, nous étudions deux de ces phénomènes : la supraconductivité et les corrections à la conductance dues aux interférences quantiques, telles que la localisation ou l’anti-localisation faible, ainsi que les fluctuations universelles de la conductance.Une supraconductivité a été identifiée expérimentalement dans plusieurs TMDC, aussi bien dans les régimes dopés n (MoS2, WS2) que p (NbSe2, TaS2). Dans ces matériaux, le SOC intrinsèque provoque un "appariement d'Ising" inhabituel des paires de Cooper. En effet, celles-ci sont formées avec des électrons provenant de vallées opposées, donc leurs spins sont figés perpendiculairement à la couche. Un champ magnétique appliqué parallèlement à la couche n’est donc pas efficace pour briser les paires de Cooper par l'effet paramagnétique, ce qui entraîne une augmentation considérable du champ critique dans le plan. C’est la signature principale de la supraconductivité d'Ising. Dans la première partie de ce travail, nous calculons le champ critique et la densité des états dans les TMDC supraconducteurs désordonnés. Nous montrons que la diffusion intra-vallée n'affecte pas ces propriétés. En revanche, elles dépendent fortement de la diffusion inter-vallée qui produit un mécanisme de brisure des paires de Cooper. Dans les supraconducteurs Ising dopés p, dans lesquels plusieurs bandes croisent le niveau de Fermi, nous identifions la diffusion inter-bande comme un autre mécanisme important de brisure des paires. Nous montrons qu'une faible diffusion inter-vallée ou inter-bande peut expliquer les observations expérimentales dans les supraconducteurs TMDC dopés n ou p, respectivement.Dans la deuxième partie de ce travail, nous calculons les corrections à la conductance dues aux interférences quantiques dans les TMDC métalliques. Leur mesure peut servir de sonde indépendante pour identifier la nature du SOC et du désordre. En raison de l'interaction entre la structure de la vallée et le SOC, ces matériaux présentent un riche comportement de localisation (ou anti-localisation) faible et des fluctuations universelles de la conductance, qui sont qualitativement différents des autres systèmes bidimensionnels, comme les métaux conventionnels ou le graphène. Nos résultats peuvent également être utilisés pour décrire les hétéro-structures graphène/TMDC, dans lesquelles le SOC est induit dans la couche de graphène. Nous discutons différents régimes de paramètres qui permettent d’interpréter des expériences récentes et d’évaluer l’intensité du SOC et du désordre. En outre, nous montrons qu'un champ Zeeman dans le plan peut être utilisé pour distinguer les contributions de différents types de SOC à la localisation ou l’anti-localisation faibleTransition metal dichalcogenide monolayers (TMDCs) are recently discovered two-dimensional materials. They host a strong intrinsic spin-orbit coupling (SOC), that acts as an effective Zeeman field with opposite, out-of-plane orientations in the +K and –K corners of the Brillouin zone (valleys). This SOC, and its interplay with disorder, strongly influences the behavior of quantum coherent phenomena in TMDCs. In this thesis, we investigate two such phenomena: superconductivity and interference corrections to the conductance, which include weak (anti-) localization and universal conductance fluctuations.Several superconducting TMDCs have been experimentally found in both n-doped (MoS¬2, WS2) and p-doped (NbSe2, TaS2) regimes. Here, the intrinsic SOC causes unusual “Ising pairing” of the Cooper pairs, formed of electrons from opposite valleys with strongly pinned out-of-plane spins. In-plane magnetic fields are thus not efficient in breaking the Cooper pairs by the paramagnetic effect, which results in a large enhancement of the in-plane upper critical field – the main signature of Ising superconductivity. In the first part of this work, we calculate the upper critical field as well as the density of states of disordered superconducting TMDCs. We show that intravalley scattering does not affect these properties, but that they strongly depend on intervalley scattering, which provides a depairing mechanism. In p-doped Ising superconductors, where multiple bands cross the Fermi level, we identify interband scattering as another important mechanism. We show that weak intervalley and interband scattering can explain experimental observations in n- and p-doped TMDC superconductors, respectively.In the second part of this work, we calculate the interference corrections to the conductance in the normal state of TMDCs, which can serve as an independent probe of SOC of disorder. Because of the interplay between valley structure and SOC, these materials exhibit a rich behavior of weak (anti-) localization and universal conductance fluctuations, which is qualitatively different from other two-dimensional systems such as conventional metals or graphene. Our results can also be used to describe graphene/TMDC heterostructures, where SOC is induced in the graphene sheet. We discuss parameter regimes that can be used to interpret recent experiments and assess the strength of SOC and disorder. Furthermore, we show that an in-plane Zeeman field can be used to distinguish contributions of different kinds of SOC to the weak (anti-) localization
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Kuba, Jakub. "Studium fotoluminiscence tenkých vrstev MoS2". Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2016. http://www.nusl.cz/ntk/nusl-254284.
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The thesis deals with study of thin layers of transition metal dichalcogenides, especially of molybdenum disulfide. Nanostructures were fabricated on two-dimensional crystals of MoS2 and WSe2. Within followed analysis attention was paid to the photoluminescence properties. In the thesis transition metal dichalcogenides are reviewed and description of the modified process of preparation by micromechanical exfoliation is given.
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Sawant, Ronit Prasad. "COMSOL Multi-physics model for Transition Metal Dichalcogenides (TMD’s)-Nafion composite Based Electromechanical Actuators". Digital WPI, 2018. https://digitalcommons.wpi.edu/etd-theses/1261.
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The ability to convert electrical energy into mechanical motion is of significant interest in many energy conversion technologies. For more than a decade Ionic polymer-metal composite (IPMC) as an electroactive smart polymer material has been extensively studied and has shown great potential as soft robotic actuators, artificial muscles and dynamic sensors in the micro-to-macro size range. IPMC consists of an ion exchange polymer membrane sandwiched between two noble metal electrodes on either side of the membrane. Under applied potential, the IPMC actuator results in bending deformation because of ion migration and redistribution across its surface due to the imposed voltage. Nafion are highly porous polymer materials which have been extensively studied as the ion exchange membrane in IPMC. Nafion has also been mixed with carbon nanotubes, graphene, and metallic nanoparticles to improve actuation and bending characteristics of electro-mechanical actuators. For the first time, liquid phase exfoliated Transition Metal Dichalcogenides (TMDs)-Nafion nanocomposite based electro-mechanical actuators has been studied and demonstrate the improvement in the electromechanical actuation performance.
In this thesis, we create a 2D model of the TMD-Nafion based electromechanical actuator in COMSOL Multi-physics software. The behavior of the model is examined at different electric potentials, frequencies, and actuation lengths. The simulation results were compared with the experimental data for validation of the model. The data showed improvement in the actuation for TMD-Nafion actuator when compared with pure Nafion actuator. The improvement in the actuation was due to the increase in diffusivity of the TMD-Nafion actuator in comparison with pure Nafion actuator. This increase in the diffusivity as seen in the model is because of the new proton conducting pathways being established with the addition of TMDs. The model also shows an increase in the stress and strain values with the incorporation of TMDs. With the same length of the actuator we were able to obtain more stress and strain with the addition of TMDs. This helps in improving the performance of the actuator as it would be able to handle more stress cycles which also increases the life of the actuator.
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Park, Juhong. "Fabrication of Large-Scale and Thickness-Modulated Two-Dimensional Transition Metal Dichalcogenides [2D TMDs] Nanolayers". Thesis, University of North Texas, 2019. https://digital.library.unt.edu/ark:/67531/metadc1505271/.
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This thesis describes the fabrication and characterization of two-dimensional transition dichalcogenides (2D TMDs) nanolayers for various applications in electronic and opto-electronic devices applications. In Chapter 1, crystal and optical structure of TMDs materials are introduced. Many TMDs materials reveal three structure polytypes (1T, 2H, and 3R). The important electronic properties are determined by the crystal structure of TMDs; thus, the information of crystal structure is explained. In addition, the detailed information of photon vibration and optical band gap structure from single-layer to bulk TMDs materials are introduced in this chapter. In Chapter 2, detailed information of physical properties and synthesis techniques for molybdenum disulfide (MoS2), tungsten disulfide (WS2), and molybdenum ditelluride (MoTe2) nanolayers are explained. The three representative crystal structures are trigonal prismatic (hexagonal, H), octahedral (tetragonal, T), and distorted structure (Tʹ). At room temperature, the stable structure of MoS2 and WS2 is semiconducting 2H phase, and MoTe2 can reveal both 2H (semiconducting phase) and 1Tʹ (semi-metallic phase) phases determined by the existence of strains. In addition, the pros and cons of the synthesis techniques for nanolayers are discussed. In Chapter 3, the topic of synthesized large-scale MoS2, WS2, and MoTe2 films is considered. For MoS2 and WS2 films, the layer thickness is modulated from single-layer to multi-layers. The few-layer MoTe2 film is synthesized with two different phases (2H or 1Tʹ). The all TMDs films are fabricated using two-step chemical vapor deposition (CVD) method. The analyses of atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and Raman spectroscopy confirm that the synthesis of high crystalline MoS2, WS2, and MoTe2 films are successful. The electronic properties of both MoS2 and WS2 exhibit a p-type conduction with relatively high field effect mobility and current on/off ratio. In Chapter 4, vertically-stacked few-layer MoS2/WS2 heterostructures on SiO2/Si and flexible polyethylene terephthalate (PET) substrates is presented. Detailed structural characterizations by Raman spectroscopy and high-resolution/scanning transmission electron microscopy (HRTEM/STEM) show the structural integrity of two distinct 2D TMD layers with atomically sharp van der Waals (vdW) heterointerfaces. Electrical transport measurements of the MoS2/WS2 heterostructure reveal diode-like behavior with current on/off ratio of ~ 104. In Chapter 5, optically uniform and scalable single-layer Mo1-xWxS2 alloys are synthesized by a two-step CVD method followed by a laser thinning. Post laser treatment is presented for etching of few-layer Mo1-xWxS2 alloys down to single-layer alloys. The optical band gap is controlled from 1.871 to 1.971 eV with the variation in the tungsten (W) content, x = 0 to 1. PL and Raman mapping analyses confirm that the laser-thinning of the Mo1-xWxS2 alloys is a self-limiting process caused via heat dissipation to SiO2/Si substrate, resulting in fabrication of spatially uniform single-layer Mo1-xWxS2 alloy films.
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Skrypka, Oleksandr. "Optical properties of transition metal dichalcogenide monolayers, heterostructures and alloys". Thesis, University of Sheffield, 2018. http://etheses.whiterose.ac.uk/22939/.
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Recent progress in the fabrication of two-dimensional (2D) materials attracts our attention to the study of such structures. Most of them are typically van-der-Waals-like with weak interlayer and strong intralayer bonding. Nowadays, single-layer semiconducting transition metal dichalcogenides (TMDCs), namely molybdenum and tungsten selenides and sulphides, have the biggest interest in optoelectonics due to the remarkable optical properties: the giant spin-orbit coupling, the inversion symmetry breaking, the large exciton binding energy and large oscillator strength. This thesis discusses the investigations in the samples based on the above compounds using photoluminescence (PL), photoluminescence excitation (PLE) and reflectance contrast (RC) spectroscopies. We observe prominent changes in the PL spectral parameters, such as integrated intensities, linewidths, energy positions of peaks and distances between peak maxima, with the alloy composition in the mono-, bi- and trilayer MoxW1-xS2 crystals at room temperature. The bowing parameters from parabolic dependences of the band-edge PL and RC on the Mo concentration are extracted. The modifications of the PL and RC spectral peaks with the number of layers and the indirect-to-direct band-gap crossover are also first demonstrated in those materials. Analysing gate-voltage- and temperature-dependent PL spectra, we make evaluations of exciton-electron and exciton-phonon interactions in the single-layer MoSe2. Additionally, the influence of localization and recharging processes is established. In the MoSe2/WSe2 heterostructures (HSs), the behaviour of localized interlayer excitonic (IX) complexes is investigated by means of different experimental techniques, specifically polarization-resolved, power-, gate-voltage-, temperature-dependent PL and PLE. The impact of the HfO2/hBN encapsulation on the structure and polarization of the IX emission is studied for the first time. Finally, the specific band alignment of the MoSe2/WS2 HS appears to cause the extraordinary PL signal emitted from the states which are likely originated from the interlayer electron hybridization.
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Xie, Aozhen. "Exfoliation of Transitional Metal Dichalcogenides (TMDS) and the Application of Co-Exfoliation of MoS2/Natural Graphite in Lithium Ion Battery (LIB)". University of Akron / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=akron1396966667.
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Hoshyargar, Faegheh [Verfasser]. "Tailor synthesis of 0D, 1D and 2D transition metal dichalcogenide nanostructures / Faegheh Hoshyargar". Mainz : Universitätsbibliothek der Johannes Gutenberg-Universität Mainz, 2015. http://d-nb.info/1225296269/34.
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Lourenço, Pedro. "Experimental and numerical study of ion irradiation impacts on Transition Metal Dichalcogenide layers". Electronic Thesis or Diss., Sorbonne université, 2022. http://www.theses.fr/2022SORUS078.
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Dans cette thèse, je présente l’étude de défauts générés artificiellement par irradiation ionique à la surface de dichalcogénures de métaux de transition (TMDC), plus particulièrement de cristaux de disulfure de tungstène (WS2) et de diséléniure de tungstène (WSe2). Je présente également l’analyse structurelle des films WS2 obtenus par pulvérisation magnétron réactive (RMS) et je compare les défauts structurels observés aux défauts générés artificiellement sur les cristaux en volume. Cette thèse est composée de six chapitres. Dans le premier chapitre, une introduction à la structure et aux propriétés des dichalcogénures de métaux de transition est présentée, suivie d’une discussion des études précédentes sur la génération de défauts par irradiation ionique. Les développements récents dans les méthodes de fabrication de couches minces de TMDC, comme la pulvérisation magnétron réactive, sont également abordés. Dans le chapitre 2, je décris en détail les techniques expérimentales et les méthodes d’analyse utilisées pour caractériser les matériaux TMDC et dans le chapitre 3, je décris les méthodes utilisées pour la simulation numérique de l’irradiation ionique des TMDC. Dans le chapitre 4, je présente mon travail sur la conception des expériences et la calibration d’une source d’ions qui a ensuite été utilisée pour produire des ions de faible énergie afin de générer artificiellement des défauts sur les surfaces des TMDCs. Dans le chapitre 5, je présente mes résultats de caractérisation des films WS2 synthétisés par RMS, qui ont été obtenus à Uppsala par l’équipe de Tomas Nyberg. Dans le chapitre 6, je présente la génération artificielle de défauts sur les surfaces de TMDCs en utilisant la source d’ions décrite dans le chapitre 4. En outre, je présente les études de dynamique moléculaire que j’ai réalisées pour comprendre le mécanisme de production de défauts dans les TMDC par irradiation ionique. Les dichalcogénures de métaux de transition (TMDCs) sont une grande famille de matériaux lamellaires couvrant une large gamme de propriétés électroniques, chimiques et physiques. Ils peuvent être métalliques, semi-conducteurs ou semi-métalliques. Ces matériaux sont composés de feuilles constitués d’une tricouche de formule MX2 (où M un métal de transition, X un atome de chalcogène, c’est-à-dire S, Se ou Te) et ont un réseau atomique hexagonal [...]In this thesis, I present the study of artificially generated defects by ion irradiation on the surface of Transition Metal Dichalcogenides (TMDC), more specifically of tungsten disulphide (WS2) and tungsten diselenide (WSe2) crystals. I also present the structural analysis of WS2 films grown by Reactive Magnetron Sputtering (RMS) and compare the observed structural defects to the artifically generated defects on the bulk crystals This thesis is composed of six chapters. In the first chapter, an introduction to the transition metal dichalcogenide structure and properties is discussed, followed by a discussion of previous studies about the defect generation by ion irradiation. Recent developments in the fabrication methods of TMDC thin-films such as reactive magnetron sputtering are also discussed. In chapter 2 I describe in detail the experimental techniques and the analysis methods used to characterize the TMDC materials and in chapter 3 I describe the methods used for numerical simulation of ion irradiation of TMDCs. In chapter 4, I present my work on the design of the experiments and the calibration of a ion source which was later used to produce low energy ions to artificially generate defects on TMDCs surfaces. Chapter 5 I present my characterization results of WS2 films grown by RMS, which were grown in Uppsala by the team of Tomas Nyberg. In chapter 6 I present the artificial generation of defects on TMDC surfaces using the ion source described in chapter 4. Furthermore, I present the molecular dynamics studies which were performed to have an understanding of the defect production mechanism in TMDCs by ion irradiation
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Reifler, Ellen Sarah. "Investigation of Intrinsic and Tunable Properties of Two-Dimensional Transition-Metal Dichalcogenides for Optical Applications". Research Showcase @ CMU, 2018. http://repository.cmu.edu/dissertations/1182.
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Since the scotch-tape isolation of graphene, two-dimensional (2D) materials have been studied with increasing enthusiasm. Two-dimensional transition-metal dichalcogenides are of particular interest as atomically thin semiconductors. These materials are naturally transparent in their few-layer form, have direct band gaps in their monolayer form, exhibit extraordinary absorption, and demonstrate unique physics, making them promising for efficient and novel optical devices. Due to the two-dimensional nature of the materials, their properties are highly susceptible to the environment above and below the 2D films. It is critical to understand the influences of this environment on the properties of 2D materials and on the performance parameters of devices made with the materials. For transparent optical devices requiring electrical contacts and gates, the effect of transparent conducting oxides on the optical properties of 2D semiconductors is of particular importance. The ability to tune the optical properties of 2D transition-metal dichalcogenides could allow for improved control of the emission or absorption wavelength of optical devices made with the materials. Continuously tuning the optical properties of these materials would be advantageous for variable wavelength devices such as photodetectors or light emitters. This thesis systematically investigates the intrinsic structural and optical properties of two-dimensional transition-metal dichalcogenide films, the effect of substrate-based optical interference on the optical emission properties of the materials, and demonstrates methods to controllably tune the luminescence emission of the materials for future optical applications. This thesis advances the study of these materials toward integration in future efficient and novel optical devices. The specific transition metal dichalcogenides investigated here are molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2). The thickness-dependence of the intrinsic in-plane crystal structure of these materials is elucidated with high-resolution transmission electron microscopy; thickness-dependent optical properties are studied using Raman and photoluminescence spectroscopies. This thesis investigates the optical interference effects from substrates with transparent conducting oxide layers on the optical properties of few-layer MoS2 films. An understanding of these effects is critical for integrating MoS2 into efficient optical devices. We predict contributions of optical interference effects to the luminescence emission of few-layer MoS2 films. The predictions are experimentally verified. We also demonstrate the use of optical interference effects to tune the wavelength and intensity of the luminescence emission of few-layer MoS2. This thesis explores the use of electric fields applied perpendicular to the films to continuously and reversibly tune the band gap of few-layer MoS2 for future variable wavelength devices. To facilitate integration into devices, we demonstrate electric fieldinduced band gap tuning by applying electric fields with a pair of transparent or semitransparent conducting layers, and without the need for direct electrical contact to the MoS2 films. The observed band gap tuning is attributed to the Stark Effect. We discuss challenges to maximizing the effect of electric field-induced band gap tuning. We demonstrate that optical interference effects do not prevent observation of band gap tuning via applied electric fields. We successfully combine two luminescence emission tuning methods: optical interference effects and electric field effects.
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Racke, David. "Measuring and Controlling Energy Level Alignment at Hybrid Organic/Inorganic Semiconductor Interfaces". Diss., The University of Arizona, 2015. http://hdl.handle.net/10150/556212.
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In this dissertation, I present the results of my research regarding hybrid semiconductor interfaces between organic and inorganic semiconductors. Using photoemission spectroscopy, I elucidate the important role of defect-induced electronic states within the inorganic semiconductor phase. These states significantly affect both the energy level alignment and the charge carrier dynamics at the hybrid interface. I demonstrate that the behavior of these hybrid semiconductor interfaces is complex and not well characterized by current models for organic semiconductor interfaces. Specifically, I show that hybrid interfaces host unique electronic phenomena that depend sensitively on the surface structure of the inorganic semiconductor. I also demonstrate new applications of photoemission spectroscopies that enable the direct analysis of important properties of inorganic semiconductors, including charge carrier behavior near hybrid interfaces and the electronic character of defect-induced energy levels. The research presented here focuses on two different n-type inorganic semiconductors, tin disulfide (SnS₂) and zinc oxide (ZnO). SnS₂ is a layered transition metal dichalcogenide that presents an atomically flat and inert surface, ideal for sensitively probing electronic interactions at the hybrid interface. To probe the electronic structure of the SnS₂ surface, I used a variety of organic molecules, including copper phthalocyanine, vanadyl naphthalocyanine, chloro-boron subphthalocyanine, and C₆₀. ZnO has a complex surface structure that can be modified by simple experimental procedures; it was therefore used as a tunable semiconductor substrate where the effects of altered electronic structure can be observed. By carefully studying the origin of hybrid interfacial interactions, these research projects provide a first step in explicitly elucidating the fundamental mechanisms that determine the electronic properties of hybrid interfaces.
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Ali, Azmat. "Elucidating interplay, stability and charge transfer dynamics at lead halide perovskite nanocrystal / 2D transition metal dichalcogenide interface for solar cell applications". Electronic Thesis or Diss., Sorbonne université, 2023. http://www.theses.fr/2023SORUS375.
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Les pérovskites d'halogénure de plomb sont considérées comme des candidats sérieux pour la prochaine génération de photovoltaïques, mais l'instabilité intrinsèque des pérovskites d'halogénures reste un obstacle à la commercialisation de cette technologie. Les propriétés remarquables des pérovskites d'halogénure, CsPbBr3 entièrement inorganique, utilisées dans cette thèse, lorsqu'elles sont combinées aux propriétés exceptionnelles des nanocristaux (NC), peuvent donner lieu à un matériau qui possède les attributs des deux. Cependant, les propriétés fondamentales des NCs de pérovskite sont fortement modifiées à l'interface avec les couches de transport de charge et par l'exposition à la lumière. Cette thèse étudie, à l'aide de la spectroscopie de photoélectrons (PES), les processus dynamiques qui se produisent à la surface et dans les interfaces des NCs CsPbBr3 sur les métaux, les semi-conducteurs et les oxydes, lorsqu'ils sont exposés à la lumière infrarouge (IR) et à la lumière ultraviolette (UV). La décomposition de la pérovskite sur tous les substrats sous illumination UV donne un produit de dégradation commun de plomb métallique (Pb0) et de bromure gazeux (Br2(g)). Cependant, pour les NCs CsPbBr3 sur l'or (Au), l'irradiation par la lumière UV et les rayons X intenses conduit non seulement au produit de dégradation anticipé, Pb0 et Br2(g), mais donne également lieu à une nouvelle espèce chimique, qui est associée au dépôt sous-potentiel de plomb (PbUPD) sur la surface de l'Au. En outre, il est démontré que l'UPD de Pb ne se produit que lorsque la structure pérovskite se brise et qu'un contact direct entre la pérovskite et l'Au est établi. De plus, la dégradation des NC de pérovskite par la lumière UV en ce qui concerne la formation de plomb métallique se produit dans une moindre mesure sur ITO et MoOx que sur MoS2 et Au, ce qui révèle que les substrats avec des bandes interdites plus larges empêchent la décomposition de la pérovskite. De même, on observe des effets différents des NCs à la lumière IR sur Au, MoS2 et MoOx. L'illumination IR affecte les NCs de pérovskite sur Au de la même manière que la lumière UV et les rayons X intenses, mais aucun gaz de bromure n'est formé. L'illumination IR des NCs CsPbBr3 sur MoS2 et MoOx entraîne un phénomène photoélectrique de surface intra-bande. Cette tension provient des défauts profonds situés au milieu de la bande interdite. Cependant, aucune décomposition de la pérovskite n'est observée. Les résultats de cette thèse mettent en évidence les propriétés de la pérovskite qui dépendent du substrat et leur influence lorsqu'elle est exposée à la lumière IR et UVLead halide perovskites are considered strong contenders for the next generation of photovoltaics, nonetheless the intrinsic instability of halide perovskite remains a bottleneck for the commercialization of this technology. The striking properties of halide perovskites, fully inorganic CsPbBr3, used in this thesis, when combined with the exceptional properties of nanocrystals (NCs), can result in a material that possesses the attributes of both. Yet, the fundamental properties of perovskite NCs are strongly modified at the interface with charge transport layers and exposure to light. In this thesis, using photoelectron spectroscopy (PES), the dynamical processes that occur at the surface and in the interfaces of CsPbBr3 NCs on metals, semiconductors and oxides, when exposed to infrared (IR) and ultraviolet (UV) light are investigated. The decomposition of perovskite on all the substrates under UV illumination gives a common degradation product of metallic lead (Pb0) and bromide gas (Br2(g)). However, for the CsPbBr3 NCs on gold (Au), both UV light and intense x-rays irradiation not only leads to the anticipated degradation product, Pb0 and Br2(g), but also gives rise to a new chemical specie, which is associated with the underpotential deposition of lead (PbUPD) on the Au surface. Furthermore, UPD of Pb is shown to occur only when the perovskite structure breaks and a direct contact of perovskite and Au is made. Moreover, UV light degradation to the perovskite NCs with regard to metallic lead formation occurs to a lesser extent on ITO and MoOx than on MoS2 and Au, revealing that substrates with wider bandgaps prevent the decomposition of the perovskite. Similarly, different effects of NCs to IR light is observed on Au, and MoS2 and MoOx. IR illumination affects the perovskite NCs on Au in the same way as the UV light and intense x-rays do, but no bromide gas is formed. IR illumination of the CsPbBr3 NCs on MoS2 and MoOx results in intraband surface photovoltage. This photovoltage stems from the deep defects-states located in the middle of the bandgap. However, no decomposition of the perovskite is observed. The findings of this thesis emphasize the substrate dependent properties of the perovskite and their influence when exposed to IR and UV light
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Skiöld, Nyberg Harald. "Formation and Function of Low-Friction Tribofilms". Doctoral thesis, Uppsala universitet, Tillämpad materialvetenskap, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-233712.
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The use of low-friction coatings on machine elements is steadily increasing, and they are expected to play an important role in the reduction of fuel consumption of future motorized vehicles. Many low-friction coatings function by transformation of the outermost coating layer into tribofilms, which then cover the coating surface and its counter surface. It is within these tribofilms that sliding takes place, and their properties largely determine the performance. The role of the coating is then not to provide low friction, but to supply support and constituents for the tribofilm. In this thesis, the formation of such tribofilms has been studied for a number of different low-friction coatings. The sensitivity of the tribofilm formation towards changes in the tribological system, such as increased surface roughness, varied surrounding atmosphere and reduced availability of the tribofilm constituents has been given special attention. For TaC/aC coatings, the formation of a functioning tribofilm was found to be a multi-step process, where wear fragments are formed, agglomerated, compacted and eventually stabilized into a dense film of fine grains. This formation is delayed by a moderate roughening of the coated surface. Coatings based on tungsten disulphide (WS2) are often able to provide exceptionally low friction, but their use is restricted by their poor mechanical properties and sensitivity to humidity. Large improvements in the mechanical properties can be achieved by addition of for example carbon, but the achievable hardness is still limited. When titanium was added to W-S-C coatings, a carbidic hard phase was formed, causing drastically increased hardness, with retained low friction. Titanium oxides in the tribofilms however caused the friction to be high initially and unstable in the long term. In a study of W-S-N coatings, the effects of humidity and oxygen were studied separately, and it was found that the detrimental role of oxygen is larger than often assumed. Low friction tribofilms may form by rearrangement of coating material, but also by tribochemical reactions between constituents of the coating and its counter surface. This was observed for Ti-C-S coatings, which formed WS2 tribofilms when sliding against tungsten counter surfaces, leading to dramatic friction reductions.
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König, Andreas. "Charge-Density Waves and Collective Dynamics in the Transition-Metal Dichalcogenides: An Electron Energy-Loss Study". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2013. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-126887.
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In this thesis, we present a detailed investigation of the electronic properties of particular transition-metal dichalcogenides. Applying electron-energy loss spectroscopy, the connection between the negative plasmon dispersion of tantalum diselenide and the occurrence of a charge-density wave state (CDW) in this compound as well as related materials is observed. Our studies include doping experiments with alkali metal addition altering the charge density of the compounds. This is known to suppress the CDW. We show that it further changes the plasmon dispersion from negative to positive slope. To estimate the doping rate of the investigated tantalum diselenide samples, a density functional theory approach is introduced, giving reliable results for a quantitative analysis of our findings. We refer to a theoretical model to describe the connection of the charge ordering and the plasmon dynamics. Investigations of the non-CDW compound niobium disulfide give further insights into the proposed interaction. Experimental results are further evaluated by a Kramers-Kronig-analysis. A structural analysis, by means of elastic electron scattering, shows the CDW to be suppressed upon doping, giving space for an emerging superstructure related to the introduced K atomsIn der vorliegenden Arbeit wird eine detaillierte Untersuchung der elektronischen Eigenschaften von ausgewählten Übergangsmetall-Dichalcogeniden präsentiert. Unter Anwendung von Elektronenenergieverlust-Spektroskopie wird die Verbindung der negativen Plasmomendispersion in Tantaldiselenid zum Auftreten eines Ladungsdichtewelle-Zustands (CDW) in diesem und in verwandten Materialien untersucht. Die Untersuchungen schließen Dotierungsexperimente mit dem Zusatz von Alkalimetallen ein, die die Ladungsdichte der Proben beeinflussen. Einerseits unterdrückt dies die CDW. Es wird außerdem gezeigt, dass sich der Anstieg der Plasmonendispersion von negativ zu positiv ändert. Ein Dichtefunktional-Theorie-Zugang zur Abschätzung der Dotierungsraten der untersuchten Tantaldiselenid-Proben wird genutzt, um verlässliche Ergebnisse für die quantitative Analyse unserer Messungen zu erhalten. Ein theoretisches Modell wird einbezogen, welches die Verbindung der Ladungsordung zur kollektiven Anregung der Ladungsdichte beschreibt, Untersuchungen der nicht-CDW Substanz Niobdisulfid geben weitere Einblicke in die Verbindung der beiden Phänomene. Die experimentellen Resultate werden weiterhin mit einer Kramers-Kronig-Analyse ausgewertet. Strukturelle Untersuchungen mit elastischer Elektronenstreuung zeigen, wie die CDW unterdrückt wird und einer auftauchenden Überstruktur, verursacht von den interkalierten K-Atomen, Raum gibt
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Lee, Edwin Wendell II. "Growth and Nb-doping of MoS2 towards novel 2D/3D heterojunction bipolar transistors". The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1480686917234143.
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Zibouche, Nourdine [Verfasser], Thomas [Akademischer Betreuer] [Gutachter] Heine, Veit [Gutachter] Wagner, Thomas [Gutachter] Niehaus i Agnieszka [Gutachter] Kuc. "Structural, Electronic and Mechanical Properties of One- and Two-Dimensional Transition Metal Dichalcogenide Materials / Nourdine Zibouche ; Gutachter: Thomas Heine, Veit Wagner, Thomas Niehaus, Agnieszka Kuc ; Betreuer: Thomas Heine". Bremen : IRC-Library, Information Resource Center der Jacobs University Bremen, 2014. http://d-nb.info/1128906163/34.
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Sundberg, Jill. "Triboactive Low-Friction Coatings Based on Sulfides and Carbides". Doctoral thesis, Uppsala universitet, Oorganisk kemi, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-230989.
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For sustainable development, it is highly important to limit the loss of energy and materials in machines used for transportation, manufacturing, and other purposes. Large improvements can be achieved by reducing friction and wear in machine elements, for example by the application of coatings. This work is focused on triboactive coatings, for which the outermost layer changes in tribological contacts to form so-called tribofilms. The coatings are deposited by magnetron sputtering (a physical vapor deposition method) and thoroughly chemically and structurally characterized, often theoretically modelled, and tribologically evaluated, to study the connection between the composition, structure and tribological performance of the coatings. Tungsten disulfide, WS2, is a layered material with the possibility of ultra-low friction. This work presents a number of nanocomposite or amorphous coatings based on WS2, which combine the low friction with improved mechanical properties. Addition of N can give amorphous coatings consisting of a network of W, S and N with N2 molecules in nanometer-sized pockets, or lead to the formation of a metastable cubic tungsten nitride. Co-deposition with C can also give amorphous coatings, or nanocomposites with WSx grains in an amorphous C-based matrix. Further increase in coating hardness is achieved by adding both C and Ti, forming titanium carbide. All the WS2-based materials can provide very low friction (down to µ<0.02) by the formation of WS2 tribofilms, but the performance is dependent on the atmosphere as O2 and H2O can be detrimental to the tribofilm functionality. Another possibility is to form low-friction tribofilms by tribochemical reactions between the two surfaces in contact. Addition of S to TiC/a-C nanocomposite coatings leads to the formation of a metastable S-doped carbide phase, TiCxSy, from which S can be released. This enables the formation of low-friction WS2 tribofilms when a Ti-C-S coating is run against a W counter-surface. Reduced friction, at a moderate level, also occurs for steel counter-surfaces, likely due to formation of beneficial iron sulfide tribofilms. The studied coatings, whether based on WS2 or TiC, are thus triboactive, with the ability to form low-friction tribofilms in a sliding contact.
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Mosconi, Dario. "Crashing flatland: defective and hybrid 2D-materials for (Electro) catalysis". Doctoral thesis, Università degli studi di Padova, 2018. http://hdl.handle.net/11577/3426844.
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This Ph.D. project is aimed to discover new strategies to develop materials to utilize in the fields of Green Energy and Green Chemistry and it was directed at the application of 2D Materials in particular. This thesis is divided into five main chapters where we presented five exemplary systems in which we focused our attention on different aspects of materials design. Each chapter comprises an introduction and a conclusion section, in which we tried to go into the details of each targeted application and of the specific design strategy employed. However, at the beginning and at the end of the thesis, the reader can find an Introduction and a Conclusion section where we tried to collocate the goals and challenges of this work within a broader context of materials science and catalysis/electrocatalysis.
In our studies in the Green Energy area, we focused on the use of MoS2-based materials in water splitting cathodic half-reaction in order to obtain the best possible performance in hydrogen generation in different conditions. To do that, different strategies were developed to drive the original material to adapt to specific application. In detail, in Chapter Two we investigated the design of three-dimensional MoS2 structures doped with different amount of Ni in order to activate MoS2 for the Hydrogen Evolution Reaction (HER) performed in alkaline environment, which typically hinder this reaction. We carried out an extensive structural characterization in order to establish the role of each type of active sites formed on the material in the HER activity and kinetics. In Chapter Three, we developed an electrodeposition method for preparing amorphous MoS2/Ag2S hybrid using recycled DVD as the support; this revealed as a viable opportunity to turn an abundant waste into an added-value material. After a suitable investigation to understand what kind of material was formed upon electrodeposition, MoS2/Ag2S/DVD was tested for HER in acidic medium. In Chapter Four another kind of hybrid was prepared by designing a one-pot solvothermal synthesis of MoS2(1-x)Se2x nanosheets grown on N-doped reduced Graphene Oxide (N-rGO). The goal was the control of the optoelectronic properties of the final material, since the combination of MoS2(1-x)Se2x and N-rGO allows to form p-n nanojunctions, which induce an enhancement of HER activity upon illumination with visible light. Then we used different techniques to prove what was the best Se:S ratio to optimize both the absolute performances in HER and the enhancement upon light irradiation.
Regarding Green Chemistry area, we used Graphene Acid (GA) as starting material and we exploited its uniform surface functionalization to prepare materials for heterogeneous catalysis for different reactions, comparing them with the benchmark Graphene Oxide (GO), modified with the same protocol. In Chapter Five, we synthesized a heterogeneous catalyst by covalently grafting Ferrocene (Fc) moieties to –COOH surface groups of GA and GO. The resulting Fc-modified graphene derivatives have been tested as heterogeneous catalysts for the C-H insertion of aryl diazonium salts into several arene substrates. The tests revealed a strong influence of the support, which we could attribute the intrinsic properties of GA. In Chapter Six, we have grown Pd nanoparticles on GA to prepare a catalyst for Suzuki-Miyaura cross coupling reaction. We have studied the effect of surface functionalization on the nanoparticles formation process and on the derived capability on the controlling the size distribution. The catalysts were tested in Suzuki cross coupling in green conditions and we could highlight the influence of nanoparticles size on activity. Moreover, we studied the same catalysts also for boronic acid homocoupling reaction, that can provide similar final products, but in a more atom economically way.Questo progetto di dottorato è mirato alla scoperta di nuove strategie per lo sviluppo di materiali da utilizzare nei campi della Green Energy e della Green Chemistry ed è rivolto all’applicazione dei materiali 2D in particolare. Questa tesi è divisa in cinque capitoli principali dove presentiamo cinque sistemi esemplificativi in cui ci siamo focalizzati su diversi aspetti del design del materiale. Ogni capitolo comprende una sezione di introduzione e una di conclusione, in cui abbiamo provato ad andare nel dettaglio di ogni applicazione e della specifica strategia di design utilizzata. In ogni caso, all’inizio e alla fine della tesi, il lettore può trovare una sezione di Introduzione e una di Conclusione dove abbiamo provato a collocare gli obbiettivi e le sfide di questo lavoro in un contesto più ampio della scienza dei materiali e della catalisi/elettrocatalisi. Nei nostri studi nell’area della Green Energy, ci siamo focalizzati sull’utilizzo di materiali a base MoS2 per la riduzione dell’acqua così da ottenere le migliori performance possibile nella generazione di idrogeno in diverse condizioni. Abbiamo sviluppato diverse strategie per indurre il materiale originale ad adattarsi alla specifica applicazione. Nel Capitolo Due abbiamo investigato il design di strutture 3D di MoS2 drogato con diverse quantità di Ni, con lo scopo di attivare il MoS2 per Hydrogen Evolution Reaction (HER) in ambiente alcalino, che di solito ostacola la reazione. Abbiamo eseguito un’estensiva analisi strutturale per stabilire il ruolo di ogni tipo di sito attivo formato sul materiale nell’attività e nella cinetica della HER. Nel Capitolo Tre, abbiamo sviluppato un metodo di elettrodeposizione per preparare un ibrido MoS2/Ag2S amorfo usando DVD riciclati come supporto, rivelandosi un’ottima strada per ridare valore a un materiale di scarto. Dopo un’adeguata analisi per capire il tipo di materiale formato, MoS2/Ag2S/DVD è stato testato per la HER in ambiente acido. Nel Capitolo Quattro abbiamo preparato un ibrido ottimizzando una sintesi solvotermale di nanofogli di MoS2(1-x)Se2x su Grafene Ossido ridotto drogato-N (N-rGO). L’obiettivo era il controllo delle proprietà optoelettroniche del materiale, dato che la combinazione di MoS2(1-x)Se2x e N-rGO permette di formare nanogiunzione p-n, che inducono un aumento dell’attività HER sotto illuminazione. Abbiamo utilizzato differenti tecniche per provare quale fosse il miglior rapporto Se:S per ottimizzare sia la performance assoluta in HER sia l’incremento dovuto all’irradiamento. Riguardo all’area della Green Chemistry, abbiamo utilizzato il Grafene Acido (GA) come materiale di partenza e abbiamo sfruttato la sua funzionalizzazione superficiale uniforme per preparare materiali per catalisi eterogenea di diverse reazioni, comparandoli con il riferimento Grafene Ossido (GO), modificato con la stessa procedura. Nel Capitolo Cinque, abbiamo sintetizzato un catalizzatore eterogeneo attaccando unità di Ferrocene (Fc) a GA e GO. I risultanti derivati grafenici modificati con Fc sono stati testati come catalizzatori eterogenei per l’inserimento di sali di diazonio aromatici in substrati arenici. I test hanno rivelato una forte incidenza del supporto, attribuibile alle proprietà intrinseche del GA. Nel Capitolo Sei, abbiamo cresciuto nanoparticelle di Pd sul GA per preparare un catalizzatore per la reazione di cross coupling Suzuki-Miyaura. Abbiamo studiato gli effetti della chimica superficiale sul processo di formazione delle nanoparticelle e sulla conseguente capacità di controllare la taglia. I catalizzatori sono stati testati nella Suzuki-Miyaura in condizioni green e abbiamo potuto evidenziare l’influenza della taglia delle nanoparticelle sull’attività. In aggiunta, abbiamo studiato gli stessi catalizzatori anche per la reazione di homocoupling di acidi boronici, la quale può fornire simili prodotti finali, ma con un migliore economia atomica.
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Bawden, Lewis. "A spin- and angle-resolved photoemission study of coupled spin-orbital textures driven by global and local inversion symmetry breaking". Thesis, University of St Andrews, 2017. http://hdl.handle.net/10023/12049.
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The effect of spin-orbit coupling had once been thought to be a minor perturbation to the low energy band structure that could be ignored. Instead, a surge in recent theoretical and experimental efforts have shown spin-orbit interactions to have significant consequences. The main objective of this thesis is to investigate the role of the orbital sector and crystal symmetries in governing the spin texture in materials that have strong spin- orbit interactions. This can be accessed through a combination of spin- and angle-resolved photoemission spectroscopy (ARPES and spin-ARPES), both of which are powerful techniques for probing the one-electron band structure plus interactions, and supported by density functional theory calculations (DFT). We focus first on a globally inversion asymmetric material, the layered semiconductor BiTeI, which hosts a giant spin-splitting of its bulk bands. We show that these spin-split bands develop a previously undiscovered, momentum-space ordering of the atomic orbitals. We demonstrate this orbital texture to be atomic element specific by exploiting resonant enhancements in ARPES. These orbital textures drive a hierarchy of spin textures that are then tied to the constituent atomic layers. This opens routes to controlling the spin-splitting through manipulation of the atomic orbitals. This is contrasted against a material where inversion symmetry is globally upheld but locally broken within each monolayer of a two layer unit cell. Through our ARPES and spin-ARPES measurements of 2H-NbSe2, we discover the first experimental evidence for a strong out-of-plane spin polarisation that persists up to the Fermi surface in this globally inversion sym- metric material. This is found to be intrinsically linked to the orbital character and dimensionality of the underlying bands. So far, previous theories underpinning this (and related) materials' collective phases assume a spin- degenerate Fermi sea. We therefore expect this spin-polarisation to play a role in determining the underlying mechanism for the charge density wave phase and superconductivity. Through these studies, this thesis then develops the importance of global versus local inversion symmetry breaking and uncovers how this is intricately tied to the underlying atomic orbital configuration.
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PIATTI, ERIK. "Electrochemical gating for superconductivity engineering in materials towards the 2D limit". Doctoral thesis, Politecnico di Torino, 2017. http://hdl.handle.net/11583/2669688.
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In this thesis work we explored the capability of electrochemical gating to reliably control the ground state of several chosen materials, with a specific focus on the engineering of the superconducting state. We also experimented with different electrolyte compositions in order to best match the electrochemical requirements of the various materials under study (e.g., chemical stability). In the presentation of the results, we will move from the thicker, bulk-like materials down to the truly two-dimensional properties of thin exfoliated single crystals.
Chapter 1 presents a general analysis of the field-effect technique based on an electrolytic gate. We discuss the basic principle that allows for the existence of ultrahigh electric fields at the device surface, together with the several pratical limitations and criticalities the technique entails. In particular, we consider the critical distinction between purely electrostatic gating and the regimes where various types of electrochemical interactions are activated between the sample and the electrolyte. We also discuss in detail a purely electrochemical measurement that can be performed on the complete devices in order to determine the amount of charge accumulated in the electric double layer.
Chapter 2 shows a selection of our results on superconducting thin films. We analyze extensively the response of conventional BCS superconductor niobium nitride to EDL gating as a function of film thickness (∼ 40−10 nm), and we interpret our data in the framework of a bulk control of the superconducting transition mediated by proximity effect. We then extend our analysis to more complex materials. We show preliminary results on state-of-the-art thin films (∼ 20 nm) of two-gap superconductor magnesium diboride. Finally, we consider thin films of iron-based superconductor barium iron arsenide and show how its Tc can be modulated by the electric field only in the smallest thicknesses available by state-of-the-art growth techniques (∼ 10 nm).
Chapter 3 presents our results on thin flakes (∼ 5−10 nm) of transition metal dichalcogenides. We explore via EDL gating the valley occupation in the conduction and of semiconducting molybdenum and tungsten disulphides at high carrier densities. We show preliminary evidence linking the emergence of EDL-induced superconductivity with the population of secondary minima in the bandstructure for molybdenum disulphide. We also exploit electrochemical gating beyond the electrostatic regime to perform field-assisted intercalation of molybdenum disulphide with alkali ions, in an effort to demonstrate both surface and bulk gate-controlled superconductivity in the same device architecture. We find preliminary evidence for the onset of a possible Charge-Density-Wave phase at very high ion doping.
Chapter 4 is entirely devoted to our results on few-layer graphene. While we did not observe any gate-induced superconductivity (down to T= 3.5 K) even at the highest induced carrier densities ∼ 6 · 1014 cm-2, we were able to extensively study the dominant scattering mechanisms both in the high and low temperature regimes; in particular, we showed that inelastic scattering for T . 90 K is dominated by electron-electron collisions, in contrast with what was found in the literature for single-layer graphene. Moreover, we observed the emergence of quantum coherence phenomena (weak localization) for T . 20 K in these previously unreached conditions of ultrahigh carrier doping.
Finally, in the Conclusions we summarize the most significant results obtained during this thesis work together with the questions that are still left open. Furthermore, we consider some perspectives and future lines of research that could be pursued in the framework of electrolyte gating.
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Islam, Md Samiul. "Coherent ultrafast spectroscopy of excitons in Van der Waals materials". Electronic Thesis or Diss., Strasbourg, 2024. http://www.theses.fr/2024STRAE011.
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Dans cette thèse, basée sur un développement original de microscopie de mélange à quatre ondesultrarapide, la première mesure directe de la dynamique de cohérence excitonique dans le disulfurede rhénium a été obtenue. Ces résultats ont démontré une robustesse unique de la cohérenceexcitonique par rapport à d'autres matériaux dichalcogénures de métaux de transition(TMD). Le potentiel de contrôle des propriétés intrinsèques des excitons dans les matériaux van der Waals (vdW) a été exploré dans des assemblages bidimensionnels innovants. En particulier, l'impact du graphène sur l'environnement excitonique d'une hétérostructure et sur les propriétés dynamiques de ces excitons a été étudié. Enfin, une avancée significative vers la compréhension et l'ingénierie des propriétés optiques des défauts émetteurs de photons uniques dans le nitrure de bore hexagonal (hBN) a été réaliséeIn this thesis, based on an original development of ultrafast four wave mixing microscopy, the firstdirect measurement of excitonic coherence dynamics in rhenium disulfide was obtained. Theseresults demonstrated a unique robustness of excitonic coherence compared to other Transition metal dichalcogenide (TMD) materials. The potential for controlling the intrinsic properties of excitons in van der Waals (vdW) materials was explored in innovative two-dimensional assemblies. In particular, the impact of graphene in the excitonic environment of a heterostructure on the dynamic properties of these excitons has been investigated. Finally, a significant step towards understanding and engineering the optical properties of single photon emitting defects in hBN has been achieved
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Mouafo, Notemgnou Louis Donald. "Two dimensional materials, nanoparticles and their heterostructures for nanoelectronics and spintronics". Thesis, Strasbourg, 2019. http://www.theses.fr/2019STRAE002/document.
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Cette thèse porte sur l’étude du transport de charge et de spin dans les nanostructures 0D, 2D et les hétérostructures 2D-0D de Van der Waals (h-VdW). Les nanocristaux pérovskite de La0.67Sr0.33MnO3 ont révélé des magnétorésistances (MR) exceptionnelles à basse température résultant de l’aimantation de leur coquille indépendamment du coeur ferromagnétique. Les transistors à effet de champ à base de MoSe2 ont permis d’élucider les mécanismes d’injection de charge à l’interface metal/semiconducteur 2D. Une méthode de fabrication des h-VdW adaptés à l’électronique à un électron est rapportée et basée sur la croissance d’amas d’Al auto-organisés à la surface du graphene et du MoS2. La transparence des matériaux 2D au champ électrique permet de moduler efficacement l’état électrique des amas par la tension de grille arrière donnant lieu aux fonctionnalités de logique à un électron. Les dispositifs à base de graphene présentent des MR attribuées aux effets magnéto-Coulomb anisotropiquesThis thesis investigates the charge and spin transport processes in 0D, 2D nanostructures and 2D-0D Van der Waals heterostructures (VdWh). The La0.67Sr0.33MnO3 perovskite nanocrystals reveal exceptional magnetoresistances (MR) at low temperature driven by their paramagnetic shell magnetization independently of their ferromagnetic core. A detailed study of MoSe2 field effect transistors enables to elucidate a complete map of the charge injection mechanisms at the metal/MoSe2 interface. An alternative approach is reported for fabricating 2D-0D VdWh suitable for single electron electronics involving the growth of self-assembled Al nanoclusters over the graphene and MoS2 surfaces. The transparency the 2D materials to the vertical electric field enables efficient modulation of the electric state of the supported Al clusters resulting to single electron logic functionalities. The devices consisting of graphene exhibit MR attributed to the magneto-Coulomb effect
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(7046639), Feng Zhang. "Transition Metal Dichalcogenide Based Memory Devices and Transistors". Thesis, 2019.
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Silicon based semiconductor technology is facing more and more challenges to continue the Moore's law due to its fundamental scaling limitations. To continue the pace of progress of device performance for both logic and memory devices, researchers are exploring new low-dimensional materials, e.g. nanowire, nanotube, graphene and hexagonal boron nitride. Transition metal dichalcogenides (TMDs) are attracted considerable attention due their atomically thin nature and proper bandgap at the initial study. Recently, more and more interesting properties are found in these materials, which will bring out more potential usefulness for electronic applications. Competing with the silicon device performance is not the only goal in the potential path finding of beyond silicon. Low-dimensional materials may have other outstanding performances as an alternative materials in many application realms.
This thesis explores the potential of TMD based devices in memory and logic applications. For the memory application, TMD based vertical devices are fully studied. Two-terminal vertical transition metal dichalcogenide (TMD) based memory selectors were firstly built and characterized, exhibiting better overall performance compared with some traditional selectors. Polymorphism is one of unique properties in TMD materials. 2D phase engineering in TMDs attracted great attention. While electric switching between semiconductor phase to metallic phase is the most desirable. In this thesis, electric field induced structural transition in MoTe2 and Mo1-xWxTe2 is firstly presented. Reproducible bipolar resistive random access (RRAM) behavior is observed in MoTe2 and Mo1-xWxTe2 based vertical devices. Direct confirmation of a phase transition from a 2H semiconductor to a distorted 2Hd metallic phase was obtained after applying an electric field. Set voltage is changed with flake thickness, and switching speed is less than 5 ns. Different from conventional RRAM devices based on ionic migration, the MoTe2-based RRAMs offer intrinsically better reliability and control. In comparison to phase change memory (PCM)-based devices that operate based on a change between an amorphous and a crystalline structure, our MoTe2-based RRAM devices allow faster switching due to a transition between two crystalline states. Moreover, utilization of atomically thin 2D materials allows for aggressive scaling and high-performance flexible electronics applications. Both of the studies shine lights on the new application in the memory field with two-dimensional materials.
For the logic application, the ultra thin body nature of TMDs allows for more aggressive scaling compared with bulk material - silicon. Two aspects of scaling properties in TMD based devices are discussed, channel length scaling and channel width scaling. A tunability of short channel effects in MoS2 field effect transistor (FET) is reported. The electrical performance of MoS2 flakes is governed by an unexpected dependence on the effective body thickness of the device which in turn depends on the amount of intercalated water molecules that exist in the layered structure. In particular, we observe that the doping stage of a MoS2 FET strongly depends on the environment (air/vacuum). For the channel width scaling, the impact of edge states in three types of TMDs, metallic Td-phase WTe2 as well as semiconducting 2H-phase MoTe2 and MoS2 were explored, by patterning thin flakes into ribbons with varying channel widths. No obvious charge depletion at the edges is observed for any of these three materials, which is different from what has been observed in graphene nanoribbon devices.
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Smaili, Idris. "Applications of Magnetic Transition Metal Dichalcogenide Monolayers to the Field of Spin-orbitronics". Diss., 2021. http://hdl.handle.net/10754/670961.
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Magnetic randomaccess memory (MRAM) devices have been widely studied since the
1960s. During this time, the size of spintronic devices has continued to decrease. Conse quently, there is now an urgent need for new lowdimensional magnetic materials to mimic
the traditional structures of spintronics at the nanoscale. We also require new effective
mechanisms to conduct the main functions of memory devices, which are: reading, writ ing, and storing data.
To date, most research efforts have focused on MRAM devices based on magnetic tun nel junction (MTJ), such as a conventional fielddriven MRAM and spintransfer torque
(STT)MRAM devices. Consequently, many efforts are currently focusing on new alterna tives using different techniques, such as spinorbit torque (SOT) and magnetic skyrmions (a
skyrmion is the smallest potential disruption to a uniform magnet required to obtain more
effective memory devices). The most promising memory devices are SOTMRAMs and
skyrmionbased memories.
This study investigates the magnetic properties of 1Tphase vanadium dichalcogenide (VXY)
Janus monolayers, where X, Y= S, Se, or Te (i.e., monolayers that exhibit inversion symme try breaking due to the presence of different chalcogen elements). This study is developed
along four directions: (I) the nature of the magnetism and the SOT effect of Janus mono layers; (II) the Dzyaloshinskii Moriya interaction (DMI); (III) investigation of stability en hancement by adopting practical procedures for industry; and (IV) study of the effect of a
hexagonal boron nitride (hBN) monolayer as an insulator on the magnetism of the VXY
monolayer. This study provides a clear perspective for the next generation of memory de vices, such as SOTMRAMs based on transition metal dichalcogenide monolayers.
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Kuruva, Hemanjaneyulu. "Addressing the Performance and Reliability Bottlenecks in 2D Transition Metal Dichalcogenide (TMD) Based Transistor Technology". Thesis, 2021. https://etd.iisc.ac.in/handle/2005/5716.
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In this thesis, we presented different contributions towards the development of
2D material technology. Firstly the realization of desired dimensions over singlecrystal
high-quality MoS2 material through dry etching techniques. SF6 plasma induces
large residue over the material, inhibiting the application despite its advantage
over SiO2 etch selectivity. On the other hand, CHF3 plasma is shown to give a
well-controlled etching process with its relatively lower etch rate than SF6 plasma.
However, under over-etch conditions, plasma is observed to introduce two significant
challenges. The first is the doping induced by high-energy fluorine radicals diffused
through resist and the TMD material. The second one is the crystal damage caused
by plasma from the side walls elimination of these two challenges required highly
controlled etching. Optimized and controlled etching using CHF3 plasma resulted
in transistors’ fabrication without compromising the performance compared to reference
transistors. The same controlled etching process is observed to apply to other
TMDs as well. Transistors implemented with such an approach have shown no degradation
in performance metrics than standard devices, thus generalizing the process
applicability to all TMDs.
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"Fundamental Toxicology Studies of 2D Transition Metal Dichalcogenides". Master's thesis, 2019. http://hdl.handle.net/2286/R.I.55629.
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abstract: Two-dimensional quantum materials have garnered increasing interest in a wide
variety of applications due to their promising optical and electronic properties. These
quantum materials are highly anticipated to make transformative quantum sensors and
biosensors. Biosensors are currently considered among one of the most promising
solutions to a wide variety of biomedical and environmental problems including highly
sensitive and selective detection of difficult pathogens, toxins, and biomolecules.
However, scientists face enormous challenges in achieving these goals with current
technologies. Quantum biosensors can have detection with extraordinary sensitivity and
selectivity through manipulation of their quantum states, offering extraordinary properties
that cannot be attained with traditional materials. These quantum materials are anticipated
to make significant impact in the detection, diagnosis, and treatment of many diseases.
Despite the exciting promise of these cutting-edge technologies, it is largely
unknown what the inherent toxicity and biocompatibility of two-dimensional (2D)
materials are. Studies are greatly needed to lay the foundation for understanding the
interactions between quantum materials and biosystems. This work introduces a new
method to continuously monitor the cell proliferation and toxicity behavior of 2D
materials. The cell viability and toxicity measurements coupled with Live/Dead
fluorescence imaging suggest the biocompatibility of crystalline MoS2 and MoSSe
monolayers and the significantly-reduced cellular growth of defected MoTe2 thin films
and exfoliated MoS2 nanosheets. Results show the exciting potential of incorporating
kinetic cell viability data of 2D materials with other assay tools to further fundamental
understanding of 2D material biocompatibility.Dissertation/Thesis
Masters Thesis Materials Science and Engineering 2019
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Shamshiri, Mohammadreza. "Influence of laser structural patterning on the tribological performance of C-alloy TMD coatings". Master's thesis, 2019. http://hdl.handle.net/10316/93635.
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Dissertação de Mestrado Conjunto Europeu em Tribologia de Superficies e Interfaces apresentada à Faculdade de Ciências e TecnologiaIntroductionRising consumption of infinite energy resources because of economic growth and finding out how to generate growth with fewer resources have become the crucial issues throughout the world. Scientists have been investigating energy dissipation and material loss, particularly in relation to friction and wear, for more than 300 years. In fact, about one-third of world energy resources in the present use, appears as friction in one form or another. These two concepts, namely friction and wear, are considerably connected with the field of science called tribology [ ].Tribology is the technology and science of interacting surfaces in relative motion. The word tribology is derived from the Greek word ‘Tribos’ which means rubbing, so tribology would be the science of rubbing. Estimates show that the ignorance of tribology in the U.S. results in losses, equal to about 6% of its gross national product or about $200 billion per year. Thus, saving energy and materials using reducing friction and wear is a beneficial solution, also resulting in an increased lifetime of components [ , ]. Using solid lubricants is one the effective solutions to reduce the coefficient of friction and wear. These lubricants could be very beneficial in the systems where oil lubrication is not possible as the vacuum applications or the sliding contacts where the presence of contaminants must be prevented such as in food industry. In addition, most of the liquid lubricants are environmentally harmful as the European Union has been placing some restrictions on use of these materials [ ]. The liquid lubricants such as oils could also be evaporated in elevated temperatures, resulting in damage to surfaces. The tribofilms generated by these lubricants can maintain a steady thickness which remains unaffected by a load, temperature and the like. In solid lubricating, tribological contacts lead to a transferring a thin layer of material from the surface of the coating to the counterface, usually known as a transfer film or tribofilm. Due to chemical reactions with the surrounding environment, the wear surfaces can show different chemistry, microstructure, and crystallographic texture from those of the bulk coating; so, these coatings illustrate different characteristics in different environments. For example, a typical solid lubricant can give extremely low friction and long wear life in one environment and fail in a different environment [4].Diamond-like carbons (DLCs), transition metal dichalcogenides (TMDs), polymeric composite coatings and the like are among the solid lubricants which are commonly utilized. Usage of some surface and subsurface analytical techniques such as X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and so on has provided the fundamental understanding of synthesis-structure tribology relationships in solid lubricant materials [ , ]. The present research work is aimed at investigating the effect of laser structural patterning on the tribological performance of W-S-C coatings.
IntroductionRising consumption of infinite energy resources because of economic growth and finding out how to generate growth with fewer resources have become the crucial issues throughout the world. Scientists have been investigating energy dissipation and material loss, particularly in relation to friction and wear, for more than 300 years. In fact, about one-third of world energy resources in the present use, appears as friction in one form or another. These two concepts, namely friction and wear, are considerably connected with the field of science called tribology [ ].Tribology is the technology and science of interacting surfaces in relative motion. The word tribology is derived from the Greek word ‘Tribos’ which means rubbing, so tribology would be the science of rubbing. Estimates show that the ignorance of tribology in the U.S. results in losses, equal to about 6% of its gross national product or about $200 billion per year. Thus, saving energy and materials using reducing friction and wear is a beneficial solution, also resulting in an increased lifetime of components [ , ]. Using solid lubricants is one the effective solutions to reduce the coefficient of friction and wear. These lubricants could be very beneficial in the systems where oil lubrication is not possible as the vacuum applications or the sliding contacts where the presence of contaminants must be prevented such as in food industry. In addition, most of the liquid lubricants are environmentally harmful as the European Union has been placing some restrictions on use of these materials [ ]. The liquid lubricants such as oils could also be evaporated in elevated temperatures, resulting in damage to surfaces. The tribofilms generated by these lubricants can maintain a steady thickness which remains unaffected by a load, temperature and the like. In solid lubricating, tribological contacts lead to a transferring a thin layer of material from the surface of the coating to the counterface, usually known as a transfer film or tribofilm. Due to chemical reactions with the surrounding environment, the wear surfaces can show different chemistry, microstructure, and crystallographic texture from those of the bulk coating; so, these coatings illustrate different characteristics in different environments. For example, a typical solid lubricant can give extremely low friction and long wear life in one environment and fail in a different environment [4].Diamond-like carbons (DLCs), transition metal dichalcogenides (TMDs), polymeric composite coatings and the like are among the solid lubricants which are commonly utilized. Usage of some surface and subsurface analytical techniques such as X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and so on has provided the fundamental understanding of synthesis-structure tribology relationships in solid lubricant materials [ , ]. The present research work is aimed at investigating the effect of laser structural patterning on the tribological performance of W-S-C coatings.
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Zheng, Chun-Teng, i 鄭峻騰. "Investigation and Analysis of Non-planar 2-D Transition-Metal-Dichalcogenide (TMD) FETs, Stacked Planar Hybrid Si/TMD Dual-Channel FETs, and Stacked Nanowire FETs for SRAM Applications". Thesis, 2017. http://ndltd.ncl.edu.tw/handle/phgvpq.
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碩士國立交通大學
電子研究所
106
This thesis comprehensively investigates the 2D Transition-Metal- Dichalcogenide (TMD) de-vices targeting the ITRS 2028 (5.9nm) technology node with the aid of TCAD numerical sim-ulation. We benchmark the difference between non-planar and planar 2D TMD devices for standard 6T SRAM cells. Our study indicates that the non-planar 2D SRAM can possess higher cell stability and performance than the planar one. We have also explored and evaluated the feasibility of a novel hybrid Si/TMD stacked dual-channel CMOS technology. A novel 4T SRAM cell is proposed with reduced the cell ar-ea and comparable read stability and superior write-ability as compared with the standard 6T SRAM. In addition, we have evaluated the vertically stacked nanowires for SRAM applications. Our study indicates that the contact resistance may significantly degrade the read/write stabil-ity and the back-end-of-line parasitic capacitance needs to be carefully considered for the stacked nanowire SRAM.
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Raju, Geet. "Tribological study of TMD coatings for rubber applications". Master's thesis, 2015. http://hdl.handle.net/10316/39086.
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Dissertação de Mestrado em Engenharia de Materiais apresentada à Faculdade de Ciências e Tecnologia da Universidade de Coimbra.Os revestimentos de Dicalcogenetos de Metais de Transição (DMT) pertencem à classe de materiais com propriedades auto-lubrificantes. Em condições favoráveis podem apresentar coeficientes de atrito extremamente baixos. O uso destes materiais na indústria de moldagem de componentes plásticos pode tornar-se relevante, onde uma das maiores dificuldades é a desmoldagem das peças do molde, o que leva a um aumento das perdas e diminuição da produção. Para o estudo foram depositados três revestimentos do sistema W-S-C para estudar o seu comportamento tribológico em deslizamento conta borracha acrilo-nitrilo butadieno (NBR) como contra corpo. Os revestimentos foram depositados com teores crescentes em carbono 0, 49 e 64 % atómico de carbono. Os revestimentos foram caracterizados em ensaios de pino disco a diferentes temperaturas, temperatura ambiente, 100 e 200 ºC. O coeficiente de atrito para o revestimento de WSx sem carbono apresentou um comportamento idêntico para todas as temperaturas testadas. No entanto, para ambos os revestimentos dopados com carbono à temperatura ambiente e a 100ºC apresentaram coeficientes de atrito bastante elevados. Com o aumento da temperatura para 200ºC observou-se uma redução acentuada do coeficiente de atrito. Uma vez que os revestimentos apresentaram valores de atrito elevados, os revestimentos foram também testados com um maior número de ciclos e uma carga normal superior. Novamente o revestimento W-S sem carbono foi o que apresentou o melhor desempenho. A composição química, estrutura e morfologia foram analisadas e posteriormente correlacionadas com as propriedades mecânicas. A dopagem dos revestimentos com carbono origina um aumento substancial da dureza para valores de 7-7,5 GPa, sendo o valor máximo obtido para o teor de carbono de 64 % at. Os padrões de difracção de raios X mostram que com o aumento do teor em carbono dos revestimentos há uma perda de cristalinidade, com os revestimentos dopados com carbono a apresentarem espectros típicos de estruturas amorfas. As pistas e partículas de desgaste foram analisadas por Microscopia Electrónica de Varrimento para observar possíveis transformações estruturais ocorridas pelo aumento da carga aplicada. Os resultados do coeficiente de atrito foram comparados com os modelos existentes para os DMT puros, tendo-se concluído que os mecanismos de deslizamento para os revestimentos do sistema W-S-C, nas condições testadas, é distinto do comportamento para os revestimentos de DMT puros quando utilizado contra NBR.
Transition metal dichalcogenides (TMDs) are a promising solid, self-lubricating family of thin films. They possess the property of “superlubricity” in favorable conditions. Their use may become relevant in the polymer molding industry, where a lot of material is wasted due to problems of mold release and fouling. Three different W-S based coatings were deposited in order to check their tribological behaviour against acrylonitrile butadiene rubber (NBR) as the counter body. Two of these coatings were alloyed with 49 at.% and 64 at.% of carbon. The deposited samples were then tested with a pin-on-disk test rig at room temperature (RT), 100 ⁰C and 200 ⁰C. The friction coefficient for the pure W-S coating showed a similar behaviour across all temperatures. However, for both the C-alloyed coatings, the friction coefficient obtained was very high for both RT and 100 ⁰C. A marked improvement in friction was only observed at 200 ⁰C for both these coatings. Durability tests for these coatings were also conducted on the pin-on-disk with a longer number of cycles and a higher load. Again, the pure W-S performed better than the C-alloyed coatings. The chemical composition, structure and morphology of the coatings were analyzed and correlated with the mechanical properties. Alloying W-S films with carbon led to a substantial increase in the hardness of the coatings to around 7-7,5 GPa; the maximum hardness was obtained for the coatings with carbon contents close to 64 at.%. XRD diffraction patterns showed that there was a loss of crystallinity with the increase of the carbon content in the film. The wear tracks and wear debris were also analyzed by SEM to understand the structural transformations induced by the increasing load. The friction results were compared with existing models for pure transition metal dichalcogenides (TMD), and it could be concluded that the friction mechanisms of W-S-C coatings fundamentally differ from those of pure TMD when used against NBR.
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(7046831), Yi-Tse Hung. "EXPERIMENTAL PROBING OF CHARGE AND VALLEY COUPLED SPIN DEGREES OF FREEDOM IN TWO-DIMENSIONAL TRANSITION METAL DICHALCOGENIDES". Thesis, 2019.
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Charge degree of freedom has been successfully manipulated in the semiconductor industry over the past few decades. The trend of doubling the number of transistors every two years in each chip was observed by Gordon Moore at 1965 and this observation was named after him, Moores law. People have kept up with the prediction fairly well till very recently when the fundamental physics limitations has reached in the conventional Si-based devices. All variety of materials and different degrees of freedom are being explored intensively to make novel device designs to overcome this challenge. In this dissertation, we will focus on two-dimensional transition metal dichalcogenides (TMDs) materials and explore not only charge but also valley and spin degrees of freedom. 2D TMDs have attracted a lot of attention for many reasons and one of them is their superior electrostatic control due to the lowering of dimensionality from 3D to 2D. Such reduction of the dimensionality besides the easiness of doping, on the other hand, makes good metal contact harder to achieve due to its inert surface comparing to the existing Si technology. To evaluate the possibility of being one of the promising candidates of post-CMOS (complementary metal oxide semiconductor) devices, the access to both electrons (conduction band) and holes (valence band) is required in order to make CMOS devices. Fermi-level pinning in these materials, however, severely limits the tunability of the Fermi level alignment between metal and semiconductor by choosing different metal work functions. In Chapter 2, we will discuss our results on making good contact by lowering the Schottky barrier height and having atomically precise doping layer control and its associated doping level where we also achieved the record high hole branch current at the bias volt- age of -1V. Besides the manipulation of charge degree of freedom, we also explored and demonstrated the unique valley degree of freedom that can be electrically generated and detected for the first time in Chapter 3. Many fascinating properties of valley physics can be analogized to spin physics, such as, zero dissipation pure spin/valley current and binary nature (spin +1/2 and -1/2, valley K and K’). Due to the unique lattice structure in TMDs, monolayer particularly, the inversion symmetry is intrinsically broken which lifts the Kramers degeneracy and leads to non-zero Berry curvature. As a result, it possesses valley Hall effect. Even more interestingly, when the transport carriers are in the valence band of monolayer TMDs, spin and valley are locked and it is called spin-locked valley Hall effect. Owing to the nature of being 2D materials, these spins’ polarization is out-of-plane unlike the conventional spin Hall effect materials, such as Pt, Ta, and W, where spins are polarized in the surface plane. This out-of-plane polarization is particularly favorable in the SOT-magnetic random access memory (SOT-MRAM) applications due to the lowering of critical switching current and consequently the reducing of power consumption. We directly observed this spin-locked valley Hall effect for the first time and we will discuss it in Chapter 4.
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Mishra, Pawan. "III-nitrides, 2D transition metal dichalcogenides, and their heterojunctions". Diss., 2017. http://hdl.handle.net/10754/623463.
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Group III-nitride materials have attracted great attention for applications in high efficiency electronic and optoelectronics devices such as high electron mobility transistors, light emitting diodes, and laser diodes. On the other hand, group VI transition metal dichalcogenides (TMDs) in the form of MX2 has recently emerged as a novel atomic layered material system with excellent thermoelectric, electronic and optoelectronic properties. Also, the recent investigations reveal that the dissimilar heterojunctions formed by TMDs and III-nitrides provide the route for novel devices in the area of optoelectronic, electronics, and water splitting applications. In addition, integration of III-nitrides and TMDs will enable high density integrated optoelectronic circuits and the development of hybrid integration technologies.
In this work, we have demonstrated kinetically controlled growth processes in plasma assisted molecular beam epitaxy (PAMBE) for the III-nitrides and their engineered heterostructures. Techniques such as Ga irradiation and nitrogen plasma exposure has been utilized to implement bulk GaN, InGaN and their heterostructures in PAMBE. For the growth of III-nitride based heterostructures, the in-situ surface stoichiometry monitoring (i-SSM) technique was developed and used for implementing stepped and compositionally graded InGaN-based multiple quantum wells (MQWs). Their optical and microstrain analysis in conjunction with theoretical studies confirmed improvement in the radiative recombination rate of the graded-MQWs as compared to that of stepped-MQWs, owing to the reduced strain in graded-MQWs.
Our achievement also includes the realization of the p-type MoS2 by engineering pristine MoS2 layers in PAMBE. Mainly, Ga and nitrogen plasma irradiation on the pristine MoS2 in PAMBE has resulted in the realization of the p-type MoS2. Also, GaN epitaxial thin layers were deposited on MoS2/c-sapphire, WSe2/c-sapphire substrates by PAMBE to study the band discontinuity at GaN/TMDs heterointerface. The determination of band offset parameters for both GaN/MoS2 and GaN/WSe2 heterostructures revealed realization of type-II band alignment.
Also, heterojunctions such as AlGaN/MoS2 is implemented to achieve type-I heterojunction. This work may open up a new avenue towards photonic quantum devices based on the integration of III-nitrides with 2D TMDs.
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Lin, Wei Cheng, i 林威丞. "Scalable Synthesis of Transition Metal dichalcogenide". Thesis, 2016. http://ndltd.ncl.edu.tw/handle/7599td.
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碩士國立清華大學
材料科學工程學系
105
In this research, we used chemical vapor deposition to synthesize monolayer MoS2. Sulfurization and tellurization was used to synthesize scalable size of MoS2、MoTe2 by sulfurizing or tellurizing pre-deposited MoO3 on the substrate.The influence of growth parameters was investigated, such as concentration of precursor vapor、growth temperature and pressure, trying to realize scalable synthesis of two-dimensional semiconductor. The advantages of sulfurization and tellurization were its high uniformity、good controbility. But the mobilities of MoS2 synthesized by Sulfurization were relatively low compared to MoS2 synthesized by chemical vapor deposition.
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Huang, Kuan-Hua, i 黃冠華. "Synthesis and Characterization of Transition Metal Dichalcogenide Heterostructures". Thesis, 2017. http://ndltd.ncl.edu.tw/handle/4d4sh9.
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Arumugam, Manikandan, i 曼尼. "1D-Transition Metal/ Transition Metal Phosphide, and 3D-Transition Metal Dichalcogenide as Electrocatalyst for Hydrogen Evolution Reaction". Thesis, 2018. http://ndltd.ncl.edu.tw/handle/nc37wj.
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Shih, En-Min. "Low-Temperature Transport Study of Transition Metal Dichalcogenide Heterostructures". Thesis, 2020. https://doi.org/10.7916/d8-05sk-s245.
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The electron-electron interaction is the origin of many interesting phenomena in condensed matter. These phenomena post challenges to theoretical physics and can lead to important future applications. Transition metal dichalcogenide heterostructures provide excellent platforms to study these phenomena because of the two-dimensional nature, large effective mass and tunable bandwidth with moiré potential. As electron bands become narrower such that the Coulomb interaction energy becomes comparable to the bandwidth, interactions can drive new quantum phases. This dissertation describes the realization of this platform and probing of correlated phenomena with low- temperature transport measurements.
As the first step, the electrical contact problem of few-layer transition metal dichalcogenides, which prohibits low-temperature transport measurements, needs to be solved. Two different contact schemes have been used to attack this problem. For p-type transition metal dichalcogenide, prepatterned platinum is used to bottom contact transition metal dichalcogenides. This method prevents channel from deterioration due to electron beam evaporation and the high workfunction platinum can place the Fermi level underneath the material valence band. Alternatively, for n-type transition metal dichalcogenides, a single layer of boron nitride is put on transition metal dichalcogenide before cobalt evaporation. This way, the boron nitride layer protects the transition metal dichalcogenide from the process of evaporation and can decrease the work function of cobalt thus putting Fermi level above the conduction band. With these contact methods, Ohmic contacts can be achieved at cryogenic temperature and probing the transition metal dichalcogenide heterostructures with transport measurements become accessible.
Then, the magnetotransport properties of monolayer molybdenum disulphide and bilayer tungsten diselenide encapsulated with boron nitride with graphite dual-gate were measured. There are three unique features underlie this two dimensional electron gas system. First, the system is strong correlated. The Landau level spectrum reveals strong correlated signatures, such as enhanced spin-orbit coupling splitting and enhanced effective g-factor. Second, the longitudinal resistance/conductance at half-filling of Landau levels are found to depend on the spin orientation. The minority spin Landau level become totally localized at higher magnetic field. Third, in bilayer device the two layers are weak coupled and can be independently controlled by two gates. All this features establish transition metal dichalcogenide a unique platform for studying correlated physics.
Finally, to achieve higher level of correlation, two layers of tungsten diselenide are stacked together with a small twist angle. With the help of moiré potential and layer hybridization, the bandwidth can be continuously tuned by the twist angle. In the range of 3 degree to 5.1degree, with moderate correlation strength, correlated insulating states are shown at half-filled flatband and are highly tunable with vertical electric field.
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Peng, Chih-Cheng, i 彭志誠. "Crystal Growth and Characterization of Mo1-xNbxS2 Transition Metal Dichalcogenide". Thesis, 2014. http://ndltd.ncl.edu.tw/handle/61319414714727634755.
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碩士國立臺灣科技大學
電子工程系
102
Single crystals of Mo1-xNbxS2 have been grown by chemical vapor transport method using Iodine as a transport agent. These series platelets up to 1.5×1.5 cm2 surface area and 0.2~0.3 cm in thickness be obtained. X-ray diffraction patterns show two-layered hexagonal primitive unit cell (2H) for molybdenum disulfide and three-layered rhombohedral primitive unit cell (3R) for niobium disulfide. The effect for all niobium-doped samples are an increase in lattice a and a decrease in lattice c, which led to a increase of the cell volumes. The co-existence on basal plane for both 2H-type and 3R-type vibration active-mode were observed by polarization dependent Raman scattering. With substituted Nb concentration increase, all vibration active-mode of series compounds are shifting to low-frequency, which reveal parabolic relation. Molybdenum disulfide belongs to semiconductor whereas niobium disulfide belongs to metallic compound. It is found that linear trend of conductivity due to doped-Nb concentration increase. Temperature dependent resistivity for each compound shows the progress of semiconductor-metal transition. The critical composition happened on x ≈ 0.1 to transfer from semiconductor to metallic characteristic.
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Wu, Pei-Ying, i 吳佩穎. "Kerr rotation spectroscopy of transition metal dichalcogenide monolayers and heterostructures". Thesis, 2016. http://ndltd.ncl.edu.tw/handle/62343930726413724912.
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碩士國立交通大學
電子物理系所
104
Two-dimension layered materials, such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDCs), have opened up a new field of heterojunction material. These kinds of van der Waals hybrid materials' electronic and optical properties could be tuned by changing the constituent layered materials or controlling the stacking structures. The novel spin-valley physics in monolayer TMDCs further makes them very promising for developing the future valleytronic device. In this work, we have first measured the process of valley depolarization by time-resolved Kerr rotation (TRKR) spectroscopy in tungsten diselenide (WSe2) monolayers. The correlation between Kerr signal and resident hole density has been investigated by the power dependence of pump beam. Moreover, the anticorrelation between strong PL intensity and large Kerr rotation reveal the importance of defect and resident hole density in the sample. The energy dependence of Kerr signal, including Kerr rotation and Kerr ellipticity, are further studied by Kerr spectra. We found the Kerr signals are mainly originated from the reduced oscillator strength by pump laser. Second, the spin transfer and dynamics of spin-valley polarization in WSe2-MoSe2 vertical heterostrutures have been investigated by two-color TRKR. We found both of them are determined by the stacking symmetry.
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Chiu, Yi-Lun, i 邱益綸. "Ultrafast Carrier Dynamics of Few-layer Transition Metal Dichalcogenide MoSe2". Thesis, 2017. http://ndltd.ncl.edu.tw/handle/xx3nj5.
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碩士國立交通大學
光電工程研究所
106
Ultrafast dynamic properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) molybdenum diselenide (MoSe2) films were investigated using femtosecond pump-and-probe technique. Strong in-plane covalent bonding and weak van der Waals coupling between TMDC layers enables MoSe2 to have layered structure. Bulk and multilayer MoSe2 is known to have an indirect bandgap, while monolayer MoSe2 is a direct bandgap semiconductor. Its unique property of strong quantum confinement leads to the formation of tightly bound excitons with extremely large binding energy for atomically thin TMDCs. In this work, we have studied the ultrafast dynamic evolution of A-excitons in multilayer (2–4 layers) MoSe2 grown by chemical vapor deposition (CVD). The transient transmission shows the initial negative signals around time zero for both far below and above the A-exciton absorption edge, whereas it shows the positive signals near the exciton transition peak. The photoexcited carriers relax quickly within 0.7 ps, which can be attributed to either carrier cooling via carrier-phonon scattering or defect capturing. The fast relaxing negative (positive) signals change its sign to positive (negative) instead of simple exponential decay to equilibrium and slowly relax within 20 – 30 ps. This secondary absorption may be defect-induced absorption related to the CVD deposition process. The band-broadening due to carrier collision in closely neighboring A and B excitons may be responsible for the sign flipping of initial photo-induced absorption.
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Nauman, Mudassar. "Enhanced Nonlinear Light-matter Interactions in Transition-Metal-Dichalcogenide Metasurfaces". Phd thesis, 2022. http://hdl.handle.net/1885/270049.
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Nonlinear light sources are central to a myriad of applications, driving a quest for their miniaturisation down to the nanoscale. In this quest, nonlinear metasurfaces hold a great promise, as they enhance nonlinear effects through their resonant photonic environment and high refractive index, such as in high-index dielectric metasurfaces. However, there is no empirical evidence of any dielectric metasurface to date, that may operate as an ultrathin nonlinear mirror to underpin the novel nonlinear light sources. To address the aforementioned challenge, this thesis primarily focuses on the study of nonlinear elastic light-matter interactions in bulk high-index Transition-metal-dichalcogenide (TMDC) metasurfaces via theoretical modelling and validation by experimental techniques. Firstly, by employing the electron beam lithography (EBL) method, we developed our fabrication recipe to realize arrays of TMDC metaatoms on low index transparent substrate. Leveraging the high index of TMDCs (Mo- and W- based sulphides and selenides), we have modelled and fabricated (by employing EBL technique) subwavelength arrays of TMDCs metaatoms on low index transparent Sapphire substrate. The proposed metasurfaces comprise different periodicity and shapes of metaatoms, that support electric and magnetic type Mie-resonances and high-Q resonances bound state in continuum (BIC), in the visible and near-infrared spectrum, respectively.
Secondly, all the dielectric metasurfaces reported to date, operate in the diffractive regime for nonlinear emissions. Therefore, the ability to funnel nonlinear emissions into the zeroth-order beam to trigger the control to tune the directionality of second harmonic emissions from forward to backward direction remained as elusive as ever. To address this challenge, here we demonstrate a single-crystalline high index subwavelength TMDC MoS2 metasurfaces that exhibit enhanced single-beam second-and third-harmonic generation in the visible to the near-infrared regime. The highest refractive index > 4 of the proposed arrays of metaatoms allows us to induce multipolar resonances at the second harmonic (SH) wavelengths. These resonances can be tailored all optically (either by excitation wavelength or incident polarisation) to trigger the tuning of the unidirectional emission of SH light in a forward or backward direction. Moreover, the interference of these resonances with the SH light enables us to modulate the polrisation resolved SH response.
Thirdly, in the quest of tunable nonlinear metasurfaces to underpin next-gen photonic devices, we proposed tunable enhanced second harmonic generation (SHG), enabled by exploiting the quantum interference effects between pronounced optical resonances such as the quasi-bound state in continuum (q-BIC) and thermally tunable excitons in high index subwavelength WS2 metasurfaces. Moreover, theoretical modelling and experimental studies were performed to engineer the novel combined effect of harmonic of q-BIC and exciton that enable us to observe the remarkable SHG enhancement and efficiency in the visible spectrum (600-650 nm), where WS2 is opaque.
Finally, an additional study has been performed on inelastic light-matter interactions in MXene Quantum Dots-Monolayer WS2 Heterostructure. Highly enhanced photoluminescence (PL) is observed, in atomically thin WS2 sitting over the QDs arrays of MXene, by increasing the laser power at room temperature.
In sum, this thesis aimed at engineering the nonlinear elastic light-matter interactions in high index TMDC metasurfaces. The intriguing results of the proposed metasurfaces may open new opportunities for metasurface-based nonlinear light sources, including ultrathin nonlinear mirrors and entangled-photon generation.
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Debnath, Rahul. "Study on optical and electrical transport properties of twisted bilayer transition metal dichalcogenides". Thesis, 2022. https://etd.iisc.ac.in/handle/2005/5921.
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Van der Waals (vdW) heterostructures, where dissimilar atomically thin vdW crystals are vertically assembled, have initiated a new paradigm to create flexible multifunctional devices. Despite the weak nature of vdW interactions, unusually strong interlayer coupling and hybridization in these heterostructures lead to novel physical phenomena ranging from interfacial stress fields to modification of electronic band structure. In twisted van der Waals heterostructures (vdWHs), the angular mismatch between two similar lattices generates a large-scale interference pattern, known as the moiré pattern, which strongly impacts the electronic band structure of the superlattice. The moiré patterns in vdWHs create a periodic potential for electrons and excitons to yield many interesting phenomena such as Hofstadter butterfly spectrum, moiré excitons, tunable Mott insulator phases, unconventional superconductivity. In this thesis, we study the effects of moiré patterns on twisted TMDC bilayers by using Raman and PL measurements and try to probe the modified electronic properties in moiré superlattice through transport measurements.
The relative rotation between the adjacent layers or the twist angle between them plays a crucial role in changing the electronic band structure of the superlattice. The first part of the thesis attempts to create such twisted TMDC bilayers with highly accurate twist angle. The assembly of multi-layers of precisely twisted two-dimensional layered materials requires knowledge of the atomic structure at the edge of the flake. Here, we demonstrate a simple and elegant transfer protocol using only optical microscope as an edge identifier tool, using which controlled transfer of twisted homobilayer and heterobilayer transition metal dichalcogenides is performed with close to 100 % yield. The fabricated twisted van der Waals heterostructures have been characterized by SHG, Raman spectroscopy, and photoluminescence spectroscopy, confirming the desired twist angle within 0.50 accuracy. The presented method is reliable, and quick, and prevents the use of invasive tools, which is desirable for reproducible device functionalities.
Next, we study the phonon renormalization in twisted bilayer MoS2, which adds insight into the moiré physics. The interlayer coupling in these heterostructures is sensitive to twist angles (θ) and is key to controllably tuning several exotic properties. We demonstrate a systematic evolution of the interlayer coupling strength with twist angle in bilayer MoS2 using a combination of Raman spectroscopy and classical simulations. At zero doping, we show a monotonic increment of the separation between the A1g and E2g mode frequencies as θ decreases from 100 to 10, which saturates to that for a bilayer at small twist angles. Furthermore, we use doping-dependent Raman spectroscopy to reveal the θ-dependent softening and broadening of the A1g mode, whereas the E2g mode remains unaffected. Using first principles-based simulations, we demonstrate large (weak) electron-phonon coupling for the A1g (E2g) mode, explaining the observed trends. Our study provides a non-destructive way to characterize the twist angle and the interlayer coupling and establishes the manipulation of phonons in twisted bilayer MoS2 (twistnonics).
Besides the closely aligned moiré lattice, intermediate misorientation (twist angles > 150) bilayers also offer a unique opportunity to tune excitonic behavior within these concurrent physical mechanisms but are seldom studied. To explore the light-matter interaction at an intermediate angle, we measure many-body excitonic complexes in monolayer (ML), natural bilayer (BL), and twisted bilayer (tBL) WSe2. Neutral biexciton (XX) is observed in tBL for the first time while being undetected in non-encapsulated ML and BL, demonstrating the unique effects of disorder screening in twisted bilayers. The XX, as well as charged biexciton (XX-), are robust to thermal dissociation and are controllable by electrostatic doping. Vanishing of momentum indirect interlayer excitons with increasing electron doping is demonstrated in tBL, resulting from the near-alignment of Q'-K and K-K valleys. Intermediate misorientation samples offer a high degree of control of excitonic complexes while offering possibilities for studying exciton-phonon coupling, band alignment, and screening.
Finally, we investigate the electrical transport in Gr/tWSe2 heterostructure, using graphene as a sensing layer to probe the electronic effects of the underlying twisted TMDC structure on monolayer graphene. Unlike graphene, TMDC materials show a massive contact resistance. We tried to solve this issue by using different work function materials to reduce the Schottky barrier across the metal-semiconductor junction. However, getting an ohmic contact between the metal-semiconductor junction is a difficult technological challenge. We resolve this issue by using graphene as a sensing layer in monolayer graphene/twisted bilayer WSe2-based heterostructures. We observe the ferroelectricity in the sample, which can be understood by the presence of moiré ferroelectric domains in twisted TMDC lattice. We find that the polarization switching can be controlled through the vertical electric field. We also find a huge nonlocal signal in graphene at a zero magnetic field that can't be explained via classical contribution. Both the nonlocal and local resistance can be controlled through the electric field. We further explore the magnetotransport properties of the system and find that the magnetoresistance of the sample increases with an in-plane magnetic field.
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Kim, Suk Hyun. "Probing Transition Metal Dichalcogenide Monolayers and Heterostructures by Polarization-Resolved Spectroscopy". Thesis, 2018. https://doi.org/10.7916/D8GF218M.
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The goal of this dissertation is to introduce my study on exotic materials in two dimensional world, not only to the well-trained researchers in this field but also to the beginners of condensed matter experiment. I hope this material to be a good guide for those of who paves the way of spintronics and valleytronics
The first chapter will give you the introduction to two dimensional materials - Graphene and Monolayer Transition Metal DiChalcogenide (TMDC). The second chapter introduces some toolkits on optical techniques on condensed matter experiment, from very basics for everyone to the advanced for main projects of this work. They include Reflection Contrast, Raman Spectroscopy, Photoluminescence, and Pump Probe Spectroscopy. Chapter three will be review on several literature which are prerequisites for understanding and getting inspiration for this work. They are on the spin-valley indexes of carriers in TMDC, interlayer charge transfer in TMDC heterostructre, valley Hall effect, etc.
Chapter four will focus on the first half of main project, “Charge and Spin-Valley Transfer in Transition Metal Dichalcogenide Heterostructure”. Starting from the fabrication of heterostructure samples for our playground, we investigate the Interlayer Charge Transfer in our Heterostructure sample by ultrafast pump probe spectroscopy. We bring the polarization resolved version of the technique to study the Spin-Valley indexes conservation in the interlayer transferred charge, and analyze its physical meaning. We study which one is the dominantly preserved quantity among spin and valley by using the broadband pump probe spectroscopy which covers A and B excitonic energy in TMDC material. As all the measurement here are taken under room temperature condition, this work paves the way for possible real device application.
Chapter five will cover the second half of main project, “Electrical control of spin and valley Hall effect in monolayer WSe2 transistors near room temperature”. Valley Hall effect device in praevious studies will be briefly revisited, and our new device is presented, using hole as carrier rather than electron for the robustness of valley index conservation, followed by optical experiment setting and results. Quantitative analyze on valley polarized carrier concentration and its depolarization time constant will follow. Chapter six will be a summary and direction to the future work.
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Ardelean, Jenny V. "Optical Characterization of Charge Transfer Excitons in Transition Metal Dichalcogenide Heterostructures". Thesis, 2019. https://doi.org/10.7916/d8-6wyb-gr91.
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Two-dimensional materials such as graphene, boron nitride and transition metal dichalcogenides have attracted significant research interest due to their unique optoelectronic properties. Transition metal dichalcogenides (TMDCs) are a family of two-dimensional semiconductors which exhibit strong light-matter interaction and show great promise for applications ranging from more efficient LEDs to quantum computing. One of the most intriguing qualities of TMDCs is their ability to be stacked on top of one another to tailor devices with specific properties and exploit interlayer phenomena to develop new characteristics. One such interlayer interaction is the generation of charge transfer excitons which span the interface between two different TMDC monolayers.
This work aims to study the intrinsic optical properties of charge transfer excitons in TMDC heterostructures. We must first start by investigating methods to protect and isolate our sample of interest from its chemical and electrostatic environment. We demonstrate that near intrinsic photoluminescence (PL) linewidth and exciton emission homogeneity from monolayer TMDCs can be achieved using a combination of BN encapsulation and passivation of substrate hydroxyl groups. Next, we develop clean stacking techniques and incorporate low defect density source crystals to maintain intrinsic properties and ensure a sufficiently high quality heterostructure interface to study characteristics of charge transfer excitons in 2D TDMCs. Strong photoluminescence emission from charge transfer excitons is realized and is shown to persist to room temperature. Charge transfer exciton lifetime is measured to be two orders of magnitude longer than previously reported. Using these high quality heterostructures, we study the behavior of charge transfer excitons under high excitation density. We observe the dissociation of charge transfer excitons into spatially separated electron-hole plasmas under optical excitation. We then probe properties of charge transfer exciton emission enhancement due resonant coupling to surface plasmon modes of gold nanorods.