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Menon, Vaidehi. "Stiffness and Strain Sensitivity of Graphene-CNT van der Waals Heterostructures: Molecular Dynamics Study". University of Akron / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=akron1595938261814484.
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Boddison-Chouinard, Justin. "Fabricating van der Waals Heterostructures". Thesis, Université d'Ottawa / University of Ottawa, 2018. http://hdl.handle.net/10393/38511.
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The isolation of single layer graphene in 2004 by Geim and Novoselov introduced a
method that researchers could extend to other van der Waals materials. Interesting and new properties arise when we reduce a crystal to two dimensions where they are often different from their bulk counterpart. Due to the van der Waals bonding between layers, these single sheets of crystal can be combined and stacked with diferent sheets to create novel materials.
With the goal to study the interesting physics associated to these stacks, the focus of this work is on the fabrication and characterization of van der Waals heterostructures.
In this work, we first present a brief history of 2D materials, the fabrication of heterostructures, and the various tools used to characterize these materials. We then give a description of the custom-built instrument that was used to assemble various 2D heterostructures followed by the findings associated with the optimization of the cleanliness of the stack's interface and surface. Finally, we discuss the results related to the twisting of adjacent layers of stacked MoS2 and its relation to the interlayer coupling between said layers.
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Mauro, Diego. "Electronic properties of Van der Waals heterostructures". Master's thesis, Alma Mater Studiorum - Università di Bologna, 2016. http://amslaurea.unibo.it/10565/.
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L’interazione spin-orbita (SOI) nel grafene è attualmente oggetto di intensa ricerca grazie alla recente scoperta di una nuova classe di materiali chiamati isolanti topologici.
Questi materiali, la cui esistenza è strettamente legata alla presenza di una forte SOI, sono caratterizzati dall’interessante proprietà di avere un bulk isolante ed allo stesso tempo superfici conduttrici. La scoperta teorica degli isolanti topologici la si deve ad un lavoro nato con l’intento di studiare l’influenza dell’interazione spin-orbita sulle proprietà del grafene. Poichè questa interazione nel grafene è però intrinsecamente troppo piccola, non è mai stato possibile effettuare verifiche sperimentali. Per questa ragione, vari lavori di ricerca hanno recentemente proposto tecniche volte ad aumentare questa interazione. Sebbene alcuni di questi studi abbiano mostrato un effettivo aumento dell’interazione spin-orbita rispetto al piccolo valore intrinseco, sfortunatamente hanno anche evidenziato una consistente riduzione della qualità del grafene.
L’obbiettivo che ci si pone in questa tesi è di determinare se sia possibile aumentare l’interazione spin-orbita nel grafene preservandone allo stesso tempo le qualità. La soluzione proposta in questo lavoro si basa sull’utilizzo di due materiali semiconduttori, diselenio di tungsteno WSe2 e solfuro di molibdeno MoS2, utilizzati da substrato su cui sopra verrà posizionato il grafene formando così un’eterostruttura -nota anche di “van der Waal” (vdW)-.
Il motivo di questa scelta è dovuto al fatto che questi materiali, appartenenti alla famiglia dei metalli di transizione dicalcogenuri (TMDS), mostrano una struttura reticolare simile a quella del grafene, rendendoli ideali per formare eterostrutture e ancora più importante, presentano una SOI estremamente grande. Sostanzialmente l’idea è quindi di sfruttare questa grande interazione spin-orbita del substrato per indurla nel grafene aumentandone così il suo piccolo valore intrinseco.
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Marsden, Alexander J. "Van der Waals epitaxy in graphene heterostructures". Thesis, University of Warwick, 2015. http://wrap.warwick.ac.uk/77193/.
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Graphene — a two-dimensional sheet of carbon atoms — has surged into recent interest with its host of remarkable properties and its ultimate thinness. However, graphene combined with other materials is starting to attract more attention. These heterostructures can be important for production routes, incorporating graphene into existing technologies, or for modifying its intrinsic properties. This thesis aims to examine the role of van der Waals epitaxy within these heterostructures. First, the graphene-copper interaction during chemical vapour deposition of graphene is investigated. Graphene is found to grow with a mismatch epitaxy of 8 relative to the [001] direction of the Cu(100) surface, despite a mismatch in symmetry and lattice parameter between two. Further, the electronic structure of both graphene and copper is unchanged by the interaction. This highlights the weak interaction between the two, owing to its van der Waals nature. Functionalised graphene is another important heterostructure, and is intensively studied for both graphene production routes and for altering graphene’s properties. Here, it is the change to the homogeneous graphene surface that makes it interesting for van der Waals epitaxy. The effect of functionalisation of graphene with atomic oxygen and nitrogen is presented next. In both cases, only small amounts of functionalisation ( 5 at%) is sufficient to significantly deteriorate the -band structure of the graphene through localisation. For small amounts of nitrogen functionalisation, and greater amounts of oxygen functionalisation, extended topological defects are formed in the graphene lattice. Unlike epoxide oxygen groups, these disruptions to the pristine graphene are found to be irreversible by annealing. Next, the interaction between graphene and the organic semiconducting molecule vanadyl-phthalocyanine (VOPc) is presented. As a result of the van der Waals nature of the graphene surface, VOPc molecules can form crystals microns in size when deposited onto a substrate with an elevated temperature of 155 C; at ambient temperatures, the crystals are only tens of nanometres across. In contrast, the functionalised graphene oxide surface prevents large crystal growth, even at elevated temperatures, because surface functionalities inhibit molecule diffusion. This highlights the importance of graphene as a substrate for molecular crystal growth, even when the growth is not epitaxial. Finally, the supramolecular assembly of trimesic acid (TMA) and terephthalic acid (TPA) is presented. Despite their chemical similarity they display different behaviour as they transition from monolayers to three-dimensional structures: for TMA, the epitaxial chicken wire structure seen at a monolayer templates up through the layers as molecules stack, until a thickness of 20 nm, when random in-plane orientations appear; on the other hand, TPA forms a brickwork structure at the monolayer, which quickly transitions to fibre-like crystals with a bulk structure for the thin films. However, the TPA orientation is still determined by the epitaxy with the graphene substrate, although this is significantly weaker than for TMA.
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Coy, Diaz Horacio. "Preparation and Characterization of Van der Waals Heterostructures". Scholar Commons, 2016. http://scholarcommons.usf.edu/etd/6212.
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In this dissertation different van der Waals heterostructures such as graphene-MoS2 and MoTe2-MoS2 were prepared and characterized. In the first heterostructure, polycrystalline graphene was synthesized by chemical vapor deposition and transferred on top of MoS2 single crystal. In the second heterostructure, MoTe2 monolayers were deposited on MoS2 by molecular beam epitaxy.
Characterization of graphene-MoS2 heterostructures was conducted by spin and angle resolve spectroscopy which showed that the electronic structure of the bulk MoS2 and graphene in this van der Waals heterostructures is modified. For MoS2 underneath the graphene, a band structure renormalization and spin polarization are observed. The band structure of MoS2 is modified because the graphene induces screening which shifts the Г-point ~150 meV to lower binding compared to the sample without graphene. The spin polarization is explained by the dipole arising from band bending which breaks the symmetry at the MoS2 surface. For graphene, the band structure at lower binding energy shows that the Dirac cone remains intact with no significant doping. Instead, away from the Fermi level the formation of several gaps in the pi-band due to hybridization with states from the MoS2 is observed.
For the heterostructures made depositing monolayer of MoTe2 on MoS2, the morphology, structure and electronic structure were studied. Two dimensional growth is observed under tellurium rich growth conditions and a substrate temperature of 200 °C but formation of a complete monolayer was not achieved. The obtained MoTe2 monolayer shows a high density of the mirror-twins grain boundaries arranged in a pseudo periodic wagon wheel pattern with a periodicity of ~2.6 nm. These grain boundary are formed due to Te-deficiency during the growth. The defect states from these domain boundary pin the Fermi level in MoTe2 and thus determine the band alignment in the MoTe2-MoS2 heterostructures.
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Ma, Qiong Ph D. Massachusetts Institute of Technology. "Optoelectronics of graphene-based Van der Waals heterostructures". Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/104523.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.Cataloged from PDF version of thesis.
Includes bibliographical references.
Research on van der Waals (vdW) materials (homo- or hetero-) is a rapidly emerging field in condensed matter physics. They are layered structures with strong chemical bonding within layers and relatively weak van der Waals force to combine layers together. This unique layer-bylayer nature makes it easy to exfoliate layers out and at the same time to re-assemble in arbitrary sequences with different combinations. The versatility, flexibility, and relatively low cost of production make the scientific community enthusiastic about their future. In this thesis, I investigate the fundamental physical processes of light-matter interactions in these layered structures, including graphene, boron nitride, transition metal dichalcogenides and heterostructures formed from these materials. My research involves state-of-the-art nanoscale fabrication and microscale photocurrent spectroscopy and imaging. In Chapter 1, 1 will briefly discuss basic physical properties of the vdW materials involved in this thesis and introduce the main nanofabrication and measurement techniques. Chapter 2-4 are about hot electron dynamics and electron-phonon coupling in intrinsic graphene systems, among which Chapter 2 is focusing on the generation mechanism of the photocurrent at the p-n interface, which is demonstrated to have a photothermoelectric origin. This indicates a weak electron-phonon coupling strength in graphene. Chapter 3 is a direct experimental follow-up of the work in Chapter 2 and reveals the dominant electron-phonon coupling mechanism at different temperature and doping regimes. In Chapter 4, I present the observation of anomalous geometric photocurrent patterns in various devices at the charge neutral point. The spatial pattern can be understood as a local photo-generated current near edges being collected by remote electrodes. The anomalous behavior as functions of change density and temperature indicates an interesting regime of energy and charge dynamics. In Chapter 5 and 6, 1 will show the photoresponse of graphene-BN heterostuctures. In graphene-BN stack directly on SiO₂, we observed strong photo-induced doping phenomenon, which can be understood as charge transfer from graphene across BN and eventually trapped at the interface between BN and SiO₂. By inserting another layer of graphene between BN and SiO₂ , we can measure an electrical current after photoexcitation due to such charge transfer. We further studied the competition between this vertical charge transfer and in-plane carrier-carrier scattering in different regimes. In Chapter 7, I will briefly summarize collaborated work with Prof. Dimitri Basov's group on near-field imaging of surface polariton in two-dimensional materials. This technique provides a complementary tool to examine the intriguing light-matter interaction (for large momentum excitations) in low-dimensional materials. Chapter 8 is the outlook, from my own point of view, what more can be done following this thesis.
by Qiong Ma.
Ph. D.
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Khestanova, Ekaterina. "Van der Waals heterostructures : fabrication, mechanical and electronic properties". Thesis, University of Manchester, 2018. https://www.research.manchester.ac.uk/portal/en/theses/van-der-waals-heterostructures-fabrication-mechanical-and-electronic-properties(047ce24b-7a58-4192-845d-54c7506f179f).html.
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The fast progress in the exploration of 2D materials such as graphene became possible due to development of fabrication techniques that allowed these materials to be protected from e.g. undesirable doping and gave rise to new functionalities realized within van der Waals heterostructures. Attracted by van der Waals interaction the constituent layers of such heterostructures preserve their exceptional electronic quality and for example in graphene allow for high electron mobility to be achieved. However, the studies of atomically thin layers such as NbSe2 that exhibit metallic behavior have been impeded by their reactivity and hence oxidation during exposure to ambient or oxidizing agents such as solvents. In this thesis, the existing heterostructure assembly technique was improved by the introduction of exfoliation and re-stacking by a fully motorized system placed in an inert atmosphere. This approach allowed us to overcome the problem of environmental degradation and create Hall bars and planar tunnel junctions from atomically thin superconducting NbSe2. Furthermore, this versatile approach allowed us to study the thickness dependence of the normal and superconducting state transport properties of NbSe2, uncovering the reduction of the superconducting energy gap and transition temperature in the thinnest samples. On the other hand, 2D materials being just 1-3 atoms thick represent an ultimate example of a membrane - thin but laterally extended object. Consisting of such atomically thin membranes the van der Waals heterostructures can be used for purposes other than the studies of electronic transport. In this work, ubiquitous bubbles occurring during van der Waals heterostructure assembly are employed as a tool to explore 2D materials' mechanical properties and mutual adhesion. This allowed us to measure Young's modulus of graphene and other 2D materials under 1-2% strain and deduce the internal pressure that can reach up to 1 GPa in sub-nanometer size bubbles.
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Yu, Geliang. "Transport properties of graphene based van der Waals heterostructures". Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/transport-properties-of-graphene-based-van-der-waals-heterostructures(5cbb782f-4d49-42da-a05e-15b26606e263).html.
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In the past few years, led by graphene, a large variety of two dimensional (2D) materials have been discovered to exhibit astonishing properties. By assembling 2D materials with different designs, we are able to construct novel artificial van der Waals (vdW) heterostructures to explore new fundamental physics and potential applications for future technology. This thesis describes several novel vdW heterostructures and their fundamental properties. At the beginning, the basic properties of some 2D materials and assembled vdW heterostructures are introduced, together with the fabrication procedure and transport measurement setups. Then the graphene based capacitors on hBN (hexagonal Boron Nitride) substrate are studied, where quantum capacitance measurements are applied to determine the density of states and many body effects. Meanwhile, quantum capacitance measurement is also used to search for alternative substrates to hBN which allow graphene to exhibit micrometer-scale ballistic transport. We found that graphene placed on top of MoS2 and TaS2 show comparable mobilities up to 60,000cm2/Vs. After that, the graphene/hBN superlattices are studied. With a Hall bar structure based on the superlattices, we find that new Dirac minibands appear away from the main Dirac cone with pronounced peaks in the resistivity and are accompanied by reversal of the Hall effects. With the capacitive structure based on the superlattices, quantum capacitance measurement is used to directly probe the density states in the graphene/hBN superlattices, and we observe a clear replica spectrum, the Hofstadter-butterfly fan diagram, together with the suppression of quantum Hall Ferromagnetism. In the final part, we report on the existence of the valley current in the graphene/hBN superlattice structure. The topological current originating from graphene’s two valleys flows in opposite directions due to the broken inversion symmetry in the graphene/hBN superlattice, meaning an open band gap in graphene.
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Tomarken, Spencer Louis. "Thermodynamic and tunneling measurements of van der Waals heterostructures". Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/123567.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2019Cataloged from PDF version of thesis.
Includes bibliographical references (pages 201-212).
In certain electronic systems, strong Coulomb interactions between electrons can favor novel electronic phases that are difficult to anticipate theoretically. Accessing fundamental quantities such as the density of states in these platforms is crucial to their analysis. In this thesis, I explore the application of two measurement techniques towards this goal: capacitance measurements that probe the thermodynamic ground state of an electronic system and planar tunneling measurements that access its quasiparticle excitation spectrum. Both techniques were applied to van der Waals materials, a class of crystals composed of layered atomic sheets with weak interplane bonding which permits the isolation of single and few-layer sheets that can be manually assembled into heterostructures. Capacitance measurements were performed on a material system commonly known as magic-angle twisted bilayer graphene (MATBG).
When two monolayers of graphene, a single sheet of graphite, are stacked on top of one another with a relative twist between their crystal axes, the resultant band structure is substantially modified from the cases of both monolayer graphene and Bernal-stacked (non-twisted) bilayer graphene. At certain magic angles, the low energy bands become extremely flat, quenching the electronic kinetic energy and allowing strong electron-electron interactions to become relevant. Exotic insulating and superconducting phases have been observed using conventional transport measurements. By accessing the thermodynamic density of states of MATBG, we estimate its low energy bandwidth, Fermi velocity, and interaction-driven energy gaps. Time-domain planar tunneling was performed on a heterostructure that consisted of monolayer graphene and hexagonal boron nitride (serving as the dielectric and tunnel barrier) sandwiched between a graphite tunneling probe and metal gate.
Tunneling currents were induced by applying a sudden voltage pulse across the full parallel plate structure. The lack of in-plane charge motion allowed access to the tunneling density of states even when the heterostructure was electrically insulating in the quantum Hall regime. These measurements represent the first application of time-domain planar tunneling to the van der Waals class of materials, an important step in extending the technique to new material platforms.
by Spencer Louis Tomarken.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Physics
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Luo, Yuanhong Ph D. Massachusetts Institute of Technology. "Twist angle physics in graphene based van der Waals heterostructures". Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/119050.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged student-submitted from PDF version of thesis.
Includes bibliographical references (pages 121-131).
In this thesis, I present my experimental work on twisted bilayer graphene, a van der Waals heterostructure consisting of two graphene sheets stack on top of each other. In particular, the twist angle is a new degree of freedom in this system, and has an important effect in the determination of its transport properties. The work presented will explore the twist-dependent physics in two regimes: the large twist angle and small twist angle regimes. In the large-twist angle limit, the two sheets have little interlayer interactions and are strongly decoupled, allowing us to put independent quantum Hall edge modes in both layers. We study the edge state interactions in this system, culminating in the formation of a quantum spin Hall state in twisted bilayer graphene. In the small twist angle limit, interlayer interactions are strong and the layers are strongly hybridized. Additionally, a new long-range moiré phenomenon emerges, and we study the effects of the interplay between moiré physics and interlayer interactions on its transport properties.
by Yuanhong Luo.
Ph. D.
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Yankowitz, Matthew Abraham. "Local Probe Spectroscopy of Two-Dimensional van der Waals Heterostructures". Diss., The University of Arizona, 2015. http://hdl.handle.net/10150/594649.
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A large family of materials, collectively known as "van der Waals materials," have attracted enormous research attention over the past decade following the realization that they could be isolated into individual crystalline monolayers, with charge carriers behaving effectively two-dimensionally. More recently, an even larger class of composite materials has been realized, made possible by combining the isolated atomic layers of different materials into "van der Waals heterostructures," which can exhibit electronic and optical behaviors not observed in the parent materials alone. This thesis describes efforts to characterize the atomic-scale structural and electronic properties of these van der Waals materials and heterostructures through scanning tunneling microscopy measurements. The majority of this work addresses the properties of monolayer and few-layer graphene, whose charge carriers are described by massless and massive chiral Dirac Hamiltonians, respectively. In heterostructures with hexagonal boron nitride, an insulating isomorph of graphene, we observe electronic interference patterns between the two materials which depend on their relative rotation. As a result, replica Dirac cones are formed in the valence and conduction bands of graphene, with their energy tuned by the rotation. Further, we are able to dynamically drag the graphene lattice in these heterostructures, owing to an interaction between the scanning probe tip and the domain walls formed by the electronic interference pattern. Similar dragging is observed in domain walls of trilayer graphene, whose electronic properties are found to depend on the stacking configuration of the three layers. Scanning tunneling spectroscopy provides a direct method for visualizing the scattering pathways of electrons in these materials. By analyzing the scattering, we can directly infer properties of the band structures and local environments of these heterostructures. In bilayer graphene, we map the electrically field-tunable band gap and extract electronic hopping parameters. In WSe₂, a semiconducting transition metal dichalcogenide, we observe spin and layer polarizations of the charge carriers, representing a coupling of the spin, valley and layer degrees of freedom.
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Schwarz, Stefan. "Microcavity-enhanced light-matter interaction in van der Waals heterostructures". Thesis, University of Sheffield, 2016. http://etheses.whiterose.ac.uk/12278/.
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The recently emerging layered two-dimensional materials provide a new material class for novel opto-electronic devices. These materials have a unique crystal structure with strong intra-layer bonding and weak van der Waals inter-layer bonding. This allows to thin down the crystal to single atomic layer thickness using an adhesive tape. With the discovery of this method to produce stable monolayer sheets of graphene and the observation of its remarkable properties, a new research area started to develop. Besides graphene there is a whole class of two-dimensional materials with similar crystal structure. One of the most prominent are transition-metal dichalcogenides, molybdenum and tungsten selenide and sulphide. They are semiconducting materials that experience an indirect-to-direct bandgap transition when the material is thinned down to monolayer thickness. This change of the bandstructure leads to a remarkable increase in the emission efficiency of those materials in monolayer form. Strong spin-orbit coupling, inversion symmetry breaking, large exciton binding energy and large oscillator strength means that this class of materials are very promising for future room temperature opto-electronic devices. In this work monolayer sheets of transition-metal dichalcogenides, as well as vertically stacked heterostructure of two-dimensional materials, are coupled to microcavity structures in order to study lightmatter interaction of these materials. A tunable open-acces microcavity structure has been developed to have full control of the light-matter interaction. In this system monolayer sheets of molybdenum disulphide have been studied, where the weak coupling regime with a Purcell enhancement of a factor of 10 has been observed. Monolayer sheets of molybdenum diselenide have been investigated where the first conclusive demonstration of strong exciton-photon coupling is demonstrated. Finally, a light emitting diode, produced by a heterostructure consisting of graphene, boron nitride and tungsten diselenide has been embedded in a microcavity structure where a significant change in the emission pattern of photo- and electroluminescence has been demonstrated.
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Constantinescu, Gabriel Cristian. "Large-scale density functional theory study of van-der-Waals heterostructures". Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/274876.
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Research on two-dimensional (2D) materials currently occupies a sizeable fraction of the materials science community, which has led to the development of a comprehensive body of knowledge on such layered structures. However, the goal of this thesis is to deepen the understanding of the comparatively unknown heterostructures composed of different stacked layers. First, we utilise linear-scaling density functional theory (LS-DFT) to simulate intricate interfaces between the most promising layered materials, such as transition metal dichalcogenides (TMDC) or black phosphorus (BP) and hexagonal boron nitride (hBN). We show that hBN can protect BP from external influences, while also preventing the band-gap reduction in BP stacks, and enabling the use of BP heterostructures as tunnelling field effect transistors. Moreover, our simulations of the electronic structure of TMDC interfaces have reproduced photoemission spectroscopy observations, and have also provided an explanation for the coexistence of commensurate and incommensurate phases within the same crystal. Secondly, we have developed new functionality to be used in the future study of 2D heterostructures, in the form of a linear-response phonon formalism for LS-DFT. As part of its implementation, we have solved multiple implementation and theoretical issues through the use of novel algorithms.
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Zhao, Liang. "Optical properties of two-dimemsional Van der Waals crystals: from terahertz to visible". Case Western Reserve University School of Graduate Studies / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=case1433378350.
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Nagler, Philipp [Verfasser], i Tobias [Akademischer Betreuer] Korn. "Exciton spectroscopy of van der Waals heterostructures / Philipp Nagler ; Betreuer: Tobias Korn". Regensburg : Universitätsbibliothek Regensburg, 2019. http://d-nb.info/1183376073/34.
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Zheng, Zhikun, Xianghui Zhang, Christof Neumann, Daniel Emmrich, Andreas Winter, Henning Vieker, Wei Liu, Marga Lensen, Armin Gölzhäuser i Andrey Turchanin. "Hybrid van der Waals heterostructures of zero-dimensional and two-dimensional materials". Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-188567.
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van der Waals heterostructures meet other low-dimensional materials. Stacking of about 1 nm thick nanosheets with out-of-plane anchor groups functionalized with fullerenes integrates this zero-dimensional material into layered heterostructures with a well-defined chemical composition and without degrading the mechanical properties. The developed modular and highly applicable approach enables the incorporation of other low-dimensional materials, e.g. nanoparticles or nanotubes, into heterostructures significantly extending the possible building blocks.
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Hadland, Erik. "Thin Film van der Waals Heterostructures containing MoSe2 from Modulated Elemental Precursors". Thesis, University of Oregon, 2019. http://hdl.handle.net/1794/24520.
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Transition metal dichalcogenides (TMDs) are naturally occurring layered materials that have attracted immense research interest due to their high degree of chemical tunability. In particular, MoSe2 has been the focus of significant investigation stemming from reports that it converts to a direct band gap semiconductor material at ultralow dimensions. Yet, as more and more is learned about increasingly thin MoSe2, efforts are now aimed at imparting the novel functionality of MoSe2 into van der Waals heterostructures. This dissertation focuses on synthesis and characterization of novel MoSe2-based nanolaminate structures that have been self assembled from modulated elemental precursors.
The first section describes a new treatment of x-ray fluorescence spectroscopy data and its use as a powerful probe for determining the absolute composition per unit area of a thin film with sub-monolayer accuracy. While this has widespread application in the thin film world, it is particularly useful for MER synthesis in the calibration of modulated elemental precursors. In order to crystallize a target structure, it is imperative to deposit the correct number of atoms, which is now possible with greater precision.
The second section shows the importance of rotational (i.e. “turbostratic”) disorder on lowering cross-plane thermal conductivity in two systems—MoSe2 and the (SnSe2)1(MoSe2)1.32 heterostructure. The binary systems exhibits ultralow thermal conductivity that rivals that of WSe2, yet some interlayer atomic registry was noted in TEM images. By interleaving layers of MoSe2 with SnSe2—which also possesses hexagonal symmetry, but has a significantly larger basal plane—the cross-plane thermal
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conductivity was depressed to the lowest reported value in the literature for a fully dense solid.
The final section presents the synthesis and characterization of a new, ternary phase of Bi|Mo|Se. The structure consists of alternating layers of a “puckered” rock salt BiSe lattice and nanosheets of MoSe2. Notably, the MoSe2 sublattice consists of a mixture of the semiconducting 2H phase (~60%) and the metallic 1T phase (~40%). This is the result of electron injection from the BiSe into the conduction band of the MoSe2, which is known to undergo a rearrangement upon reduction.
This dissertation includes previously published and unpublished coauthored materials.2021-04-30
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Lorchat, Étienne. "Optical spectroscopy of heterostructures based on atomically-thin semiconductors". Thesis, Strasbourg, 2019. http://www.theses.fr/2019STRAE035.
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Au cours de cette thèse, nous avons fabriqué et étudié par spectroscopie optique, des hétérostructures de van der Waals, composées de monofeuillets semi-conducteurs (dichalcogénures de métaux de transition, DMT) couplés à une monocouche de graphène ou à un résonateur plasmonique. Nous avons observé des modifications importantes de la dynamique des états excités optiquement dans le DMT (excitons) lorsque celui-ci est en contact avec le graphène. Le graphène neutralise la couche de DMT et permet un transfert non-radiatif d’excitons en moins de quelques picosecondes. Ce transfert d’énergie peut s’accompagner d’un photodopage extrinsèque considérablement moins efficace. La réduction de la durée de vie des excitons du DMT en présence de graphène a été exploitée pour montrer que leur pseudo-spin de vallée maintenait un degré de polarisation et de cohérence important jusqu’à température ambiante. Enfin, en couplant fortement les excitons d’un DMT aux modes d’un résonateur plasmonique à phase géométrique, nous avons mis en évidence, à température ambiante, le verrouillage du pseudo-spin de vallée sur la direction de propagation des polaritons chiraux (chiralitons) issus du couplageDuring this thesis, we have fabricated and studied by optical spectroscopy, van der Waals heterostructures composed of semiconductor monolayers (transition metal dichalcogenides, TMD) coupled to a graphene monolayer or to a plasmonic resonator. We have observed significant changes in the dynamics of the TMD optically excited states (excitons) when it is in direct contact with graphene. Graphene neutralizes the TMD monolayer and enables non-radiative transfer of excitons within less than a few picoseconds. This energy transfer process may be accompanied by a considerably less efficient, extrinsic photodoping. The reduced lifetime of TMD excitons in the presence of graphene has been exploited to show that their valley pseudo-spin maintains a high degree of polarization and coherence up to room temperature. Finally, by strongly coupling TMD excitons to the modes of a geometric phase plasmonic resonator, we have demonstrated, at room temperature, that the momentum of the resulting chiral polaritons (chiralitons) is locked to their valley pseudo-spin
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Adrian, Marlene [Verfasser]. "Energy transfer in free-standing van der Waals heterostructures after optical excitation / Marlene Adrian". Kassel : Universitätsbibliothek Kassel, 2019. http://d-nb.info/1187165239/34.
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Li, Wei. "Ultrasensitive NO2 gas sensors based on layered van der Waals MoO3 and its heterostructures". Thesis, Queensland University of Technology, 2022. https://eprints.qut.edu.au/232666/1/Wei_Li_Thesis.pdf.
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The state of the art NO2 gas sensors suffer from a trade-off between sensitivity, reversibility, and selectivity. This thesis focuses to develop highly sensitive NO2 gas sensors with superior selectivity based on layered molybdenum trioxide and its heterostructures. Through a combination of advanced materials synthesis and characterisation techniques, ultrasensitive NO2 gas sensors based on different morphologies of molybdenum trioxide, including nanoribbons, large-sized single crystalline flake and heterostructures have been demonstrated. This thesis lays the foundation for developing a potential sensing platform based on molybdenum trioxide to enable monolithic, scalable and integrable sensing technologies.
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Henck, Hugo. "Hétérostructures de van der Waals à base de Nitrure". Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLS319/document.
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Le sujet de cette thèse est à l’interface entre l’étude de composés à base de nitrure et des structures émergeantes formées par les matériaux bidimensionnels (2D) d’épaisseur atomique. Ce travail se consacre sur l’hybridation des propriétés électriques et optiques des semi-conducteurs à larges bandes interdites que sont les nitrures et des performances mécaniques, électriques et optiques des matériaux lamellaires, récemment isolé à l’échelle d’un plan atomique, qui sont aujourd’hui considérées avec attention aux regards de futures applications et d’études plus fondamentales. En particulier, une étude des propriétés électroniques, optiques et structurelles d’hétérostructures composées de plusieurs matériaux lamellaires et d’interfaces entre matériaux 2D et 3D a été réalisé par des moyens de microscopie et de spectroscopie tel que la spectroscopie Raman, de photoémission et d’absorption.Ce manuscrit traite dans un premier temps des propriétés structurelles et électroniques du nitrure de bore hexagonal (h-BN), matériau isolant aux propriétés optiques exotiques et essentiel dans la future intégration de ce type de matériaux 2D permettant de mettre en valeur leurs propriétés intrinsèques.En utilisant le graphène comme substrat les problèmes de mesures par photoémission rencontrés pour des matériaux isolant ont pu être surmonté dans le cas du h-BN et une étude des défauts structurels a pu être réalisée. Par conséquent, les premières mesures directes de la structure de bande électronique de plusieurs plans de h-BN sont présentées dans ce manuscrit.Dans un second temps, une approche d’intégration de ces matériaux 2D différente a été étudiée en formant une hétérostructure 2D/3D. L’interface de cette hétérojonction, composée d’un plan de disulfure de molybdène (MoS2) de dopage intrinsèque N associé à 300 nm de nitrure de gallium (GaN) intentionnellement dopé P à l’aide de magnésium, a été caractérisée. Un transfert de charge du GaN vers le MoS2 a pu être identifié suggérant un contrôle des propriétés électroniques de ce type de structure par le choix de matériaux.Ces travaux ont permis de révéler les diagrammes de bandes électroniques complet des structures étudiées a pu être obtenu permettant une meilleur compréhension de ces systèmes émergeantsThis thesis is at the interface between the study of nitride based compounds and the emerging structures formed by atomically thin bi-dimensional (2D) materials. This work consists in the study of the hybridization of the properties of large band gap materials from the nitride family and the mechanical, electronic and optical performances of layered materials, recently isolated at the monolayer level, highly considered due to their possible applications in electronics devices and fundamental research. In particular, a study of electronics and structural properties of stacked layered materials and 2D/3D interfaces have been realised with microscopic and spectroscopic means such as Raman, photoemission and absorption spectroscopy.This work is firstly focused on the structural and electronic properties of hexagonal boron nitride (h-BN), insulating layered material with exotic optical properties, essential in in the purpose of integrating these 2D materials with disclosed performances. Using graphene as an ideal substrate in order to enable the measure of insulating h-BN during photoemission experiments, a study of structural defects has been realized. Consequently, the first direct observation of multilayer h-BN band structure is presented in this manuscript. On the other hand, a different approach consisting on integrating bi-dimensional materials directly on functional bulk materials has been studied. This 2D/3D heterostructure composed of naturally N-doped molybdenum disulphide and intentionally P-doped gallium nitride using magnesium has been characterised. A charge transfer from GaN to MoS2 has been observed suggesting a fine-tuning of the electronic properties of such structure by the choice of materials.In this work present the full band alignment diagrams of the studied structure allowing a better understanding of these emerging systems
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Froehlicher, Guillaume. "Optical spectroscopy of two-dimensional materials : graphene, transition metal dichalcogenides and van der Waals heterostructures". Thesis, Strasbourg, 2016. http://www.theses.fr/2016STRAE033/document.
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Au cours de ce projet, nous avons utilisé la microspectroscopie Raman et de photoluminescence pour étudier des matériaux bidimensionnels (graphène et dichalcogénures de métaux de transition) et des hétérostructures de van der Waals. Tout d’abord, à l’aide de transistors de graphène munis d’une grille électrochimique, nous montrons que la spectroscopie Raman est un outil extrêmement performant pour caractériser précisément des échantillons de graphène. Puis, nous explorons l’évolution des propriétés physiques de N couches de dichalcogénures de métaux de transition semi-conducteurs, en particulier de ditellurure de molybdène (MoTe2) et de diséléniure de molybdène (MoSe2). Dans ces structures lamellaires, nous observons la séparation de Davydov des phonons optiques au centre de la première zone de Brillouin, que nous décrivons à l’aide d’un modèle de chaîne linéaire. Enfin, nous présentons une étude toute optique du transfert de charge et d’énergie dans des hétérostructures de van der Waals constituées de monocouches de graphène et de MoSe2. Ce travail de thèse met en évidence la riche photophysique de ces matériaux atomiquement fins et leur potentiel en vue de la réalisation de nouveaux dispositifs optoélectroniquesIn this project, we have used micro-Raman and micro-photoluminescence spectroscopy to study two-dimensional materials (graphene and transition metal dichalcogenides) and van der Waals heterostructures. First, using electrochemically-gated graphene transistors, we show that Raman spectroscopy is an extremely sensitive tool for advanced characteri-zations of graphene samples. Then, we investigate the evolution of the physical properties of N-layer semiconducting transition metal dichalcogenides, in particular molybdenum ditelluride (MoTe2) and molybdenum diselenide (MoSe2). In these layered structures, theDavydov splitting of zone-center optical phonons is observed and remarkably well described by a ‘textbook’ force constant model. We then describe an all-optical study of interlayer charge and energy transfer in van der Waals heterostructures made of graphene and MoSe2 monolayers. This work sheds light on the very rich photophysics of these atomically thin two-dimensional materials and on their potential in view of optoelectronic applications
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Chen, Xi. "The Hofstadter spectrum of monolayer and bilayer graphene van der Waals heterostructures with boron nitride". Thesis, Lancaster University, 2015. http://eprints.lancs.ac.uk/76231/.
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In this thesis, we consider the electronic properties of materials created by stacking two-dimensional graphene layers. The first material is a heterostructure created by placing a graphene layer on top of a layer of hexagonal boron nitride. The energy bands are determined as well as the energy spectrum in the presence of a magnetic field applied in the direction perpendicular to the layers. There is a miniband structure that includes gaps and secondary Dirac points as well as a fractal structure of magnetic minibands known as Hofstadter's butterfly. The second material is multilayer graphene, which consists of a small number of graphene layers stacked on top of one another. We determine the effect on the low-energy electronic band structure by applying a magnetic field in the direction parallel to the layers, and find that the parallel field can induce a dramatic change in the band structure, which is known as a Lifshitz transition. Furthermore, depending on the magnitude and the direction of the field within the plane of the graphene layers, it is possible to access different phase regions of the band structure. We also model the electronic transport properties of multilayer graphene. We use both analytical mode-matching and the numerical recursive Green function methods to study the transport properties of electrons in multilayer graphene in the vicinity of zero energy, zero temperature and zero magnetic field.
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Del, Pozo Zamudio Osvaldo. "Optics of atomically thin films and van der Waals heterostructures made from two-dimensional semiconductors". Thesis, University of Sheffield, 2015. http://etheses.whiterose.ac.uk/11975/.
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This thesis discusses optical investigations of two-dimensional metal-chalcogenide semiconductor materials and their heterostructures. Topics include a study of continuous wave (cw) and time-resolved photoluminescence (PL) of GaTe and GaSe thin films. Based on experimental evidence, we propose a model explaining the strong PL intensity decrease for thin films as a result of non-radiative carrier escape via surface states. We investigate the stability of thin films of InSe and GaSe using a combination of PL and Raman spectroscopies. By comparing signal intensities in films exposed to ambient conditions for up to 100 hours, we find notable degradation in GaSe and high stability of InSe. We continue our study with the investigation of optical properties of light emitting diodes (LED) made of van der Waals (vdW) heterostructures comprising graphene as transparent contacts, hexagonal boron nitride as tunnel barriers and transition metal dichalcogenides (TMDC), MoS2 and WS2, as the semiconductor active regions. Single and multiple 'quantum well' structures were fabricated with an aim to enhance the external quantum efficiency (EQE) under electrical injection. We also present PL characterisation of LEDs based on vdW heterostructures comprising WSe2 and MoSe2 as active layers. Temperature dependent experiments show unusual enhancement of the EQE with temperature in WSe2 in contrast to MoSe2, where both electroluminescence and PL are reduced with temperature. A theoretical approach to explain this behaviour is proposed, which is based on the strong spin-orbit interaction present in both materials.
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Förg, Michael [Verfasser], i Alexander [Akademischer Betreuer] Högele. "Confocal and cavity-enhanced spectroscopy of semiconductor van der Waals heterostructures / Michael Förg ; Betreuer: Alexander Högele". München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2020. http://d-nb.info/1218466251/34.
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Zollner, Klaus [Verfasser], i Jaroslav [Akademischer Betreuer] Fabian. "Proximity-induced spin-orbit and exchange coupling in van der Waals heterostructures / Klaus Zollner ; Betreuer: Jaroslav Fabian". Regensburg : Universitätsbibliothek Regensburg, 2021. http://d-nb.info/1227039603/34.
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Cohen, Liam Augustus. "Fabricating Van der Waals heterostructures with air sensitive materials : a study of flake Bi₂Sr₂CaCu₂08₊x". Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/118024.
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Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, June 2018.Cataloged from PDF version of thesis.
Includes bibliographical references (pages [96]-[97]).
by Liam Augustus Cohen.
S.B.
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Bradford, Jonathan. "Growth and characterisation of two-dimensional materials and their heterostructures on sic". Thesis, Queensland University of Technology, 2019. https://eprints.qut.edu.au/134400/1/Jonathan_Bradford_Thesis.pdf.
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Atomically thin two-dimensional materials and their hybrids represent an elegant approach to designing and synthesizing functional nanomaterials and are expected to find applications across a broad range of new technologies. This project explored scalable synthesis of various two-dimensional layered materials and their hybrid counterparts on silicon carbide, an industrially relevant device substrate. It demonstrates the integration of graphene, hexagonal boron nitride and transition metal dichalcogenide layers which were characterised by high resolution scanning probe microscopy and electron spectroscopy. The procedures developed in this work are expected to facilitate a route towards large-scale synthesis of novel nanoscale devices directly on-chip.
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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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Tenasini, Giulia. "Quantum transport in monolayer WTe2". Master's thesis, Alma Mater Studiorum - Università di Bologna, 2017. http://amslaurea.unibo.it/14897/.
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Il ditellurio di tungsteno (WTe2) appartiene alla classe dei dicalcogenuri di metalli di transizione (TMDs), che rappresentano attualmente i materiali più promettenti, insieme al grafene, nel campo di ricerca dei cristalli bidimensionali (2D). Grazie ad una caratteristica struttura stratificata, con differenti piani atomici legati da forze di van der Waals, mediante esfoliazione è possibile isolare strati di spessore quasi-atomico di TMDs, detti “monostrati”, con proprietà spesso molto diverse dal materiale bulk originario. Il WTe2 nella sua forma a monostrato, è stato recentemente oggetto di interesse scientifico, in quanto teoricamente predetto essere un isolante topologico (TI) bidimensionale. Un TI è un materiale che internamente si comporta come un isolante elettrico ma che sulla superficie manifesta stati conduttivi. Lo scopo di questa tesi è studiare le proprietà si trasporto di monostrati di WTe2 in micro-dispositivi realizzati con opportune tecniche di nanofabbricazione. L'ossidazione della superficie esterna del WTe2, dovuta ad una non-perfetta stabilità in aria, influenza significativamente il trasporto elettronico in cristalli costituiti da pochi strati atomici ed è causa di una transizione metallo-isolante. Una possibile soluzione per evitare la degradazione del materiale consiste nell' “incapsulamento” di un monostrato di WTe2 fra materiali 2D chimicamente inerti, come il nitruro di boro esagonale. A tale proposito, si è sviluppata una tecnica di “trasferimento” che permette di sollevare e allineare con precisione micrometrica strati di spessore atomico di differenti materiali, assemblando eterostrutture di van der Waals. Campioni selezionati sono studiati mediante misure di magneto-transporto a bassa temperatura (fino a 0.250 K). I dati analizzati evidenziano l'esistenza di un gap di energia in monostrati di WTe2 e la presenza di una corrente localizzata ai bordi del sistema, coerentemente con l'ipotesi di un isolante topologico 2D.
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Tesař, Jan. "Příprava a charakterizace atomárně tenkých vrstev". Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2020. http://www.nusl.cz/ntk/nusl-417143.
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Tato práce se zabývá oblastí dvourozměrných materiálů, jejich přípravou a analýzou. Pravděpodobně nejznámějším zástupcem dvourozměrných materiálů je grafen. Tento 2D allotrop uhlíku, někdy nazývaný „otec 2D materiálů“, v sobě spojuje neobyčejnou kombinaci elektrických, tepelných a mechanických vlastností. Grafen získal mnoho pozornosti a byl také připraven mnoha metodami. Jedna z těchto metod však stále vyniká nad ostatními kvalitou produkovaného grafenu. Mechanická exfoliace je ve srovnání s jinými technikami velmi jednoduchá, takto připravený grafen je však nejkvalitnější. Práce je také zaměřena na optimalizaci procesu tvorby heterostruktur složených z vrstev grafenu a hBN. Dle prezentovaného postupu bylo připraveno několik van der Waalsových heterostruktur, které byly analyzovány Ramanovskou spektroskopií, mikroskopií atomových sil a nízkoenergiovou elektronovou mikroskopií. Měření pohyblivosti nosičů náboje bylo provedeno v GFET uspořádání. Získané hodnoty pohyblivosti prokázaly vynikající transportní vlastnosti exfoliovaného grafenu v porovnání s grafenem připraveným jinými metodami. V práci popsaný proces přípravy je tedy vhodný pro výrobu kvalitních heterostruktur.
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Panadés-Barrueta, Ramón Lorenzo. "Full quantum simulations of the interaction between atmospheric molecules and model soot particles". Thesis, Lille 1, 2020. http://www.theses.fr/2020LIL1R022.
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Nous visons à simuler avec des arguments purement quantiques (noyaux et électrons) les processus d’adsorption et de photoréactivité du NO2 adsorbé sur des particules de suie (modélisées comme de grands hydrocarbures aromatiques polycycliques, HAP) dans les conditions atmosphériques. Une description détaillée de ces processus est nécessaire pour comprendre le comportement différentiel (jour-nuit) de la production de HONO, qui est un précurseur du radical hydroxyle (OH). En particulier, le mécanisme spécifique de l’interconversion entre NO2 et HONO par la suie n’est pas encore totalement compris. En raison de sa pertinence particulière dans ce contexte, nous avons choisi le systèmePyrène-NO2.La première étape de cette étude a consisté à déterminer les configurations stables (états de transition et minima) du système Pyrène-NO2 . À cette fin, nous avons utilisé la méthode van der Waals Transition State Search using Chemical Dynamics Simulations (vdW-TSSCDS), la généralisation de l’algorithme TSSCDS récemment développée dans notre groupe. Ainsi, le présent travail représente la première application devdW-TSSCDS à un grand système (81D). Partant d’un ensemble de géométries d’entrée judicieusement choisies, la méthode susmentionnée permet de caractériser la topographie d’une surface d’énergie potentielle intermoléculaire (SEP), ou en d’autres termes, de déterminer les conformations les plus stables du système, de manière entièrement automatisée et efficace.Les informations topographiques recueillies ont été utilisées pour obtenir une description globale (fit) du potentiel d’interaction, nécessaire à l’élucidation dynamique de l’interaction intermoléculaire (physisorption), des propriétés spectroscopiques et de la réactivité des espèces adsorbées. Pour atteindre ce dernier objectif, nous avons développé deux méthodologies différentes ainsi que les progiciels correspondants. La première d’entre elles est l’algorithme SRP-MGPF (Specific Reaction Parameter Multigrid POTFIT), qui est implémenté dans le progiciel SRPTucker. Cette méthode calcule des SEPs (intermoléculaires) chimiquement précis par reparamétrage de méthodes semiempiriques, qui sont ensuite tenseur-décomposées sous forme Tucker à l’aide de MGPF.Ce logiciel a été interfacé avec succès avec la version Heidelberg du paquet MCTDH (Multi-configuration Time-Dependent Hartree). La seconde méthode permet d’obtenir la SEP directement sous la forme mathématique requise par MCTDH, d’où son nom de Sum-Of-Products Finite-Basis-Representation (SOP-FBR). La SOP-FBR constitue une approche alternative aux méthododes d’ajustement NN. L’idée la sous-tend est simple : à partir d’une expansion Tucker low rank sur la grille, nous remplaçons les fonctions de base basées sur la grille par une expansion en termes de polynômes orthogonaux. Comme dans la méthode précédente, l’intégration avec la MCTDH a été assurée.Les deux méthodes ont été testées avec succès à un certain nombre de problèmes de référence, à savoir : le Hamiltonian Hénon-Heiles, la SEP global du H2O, et la SEP d’isomérisation HONO (6D)We aim at simulating full quantum mechanically (nuclei and electrons) the processes of adsorption and photoreactivity of NO2 adsorbed on soot particles (modeled as large Polycyclic Aromatic Hydrocarbons, PAHs) in atmospheric conditions. A detailed description of these processes is necessary to understand the differential day-nighttime behavior of the production of HONO, which is a precursor of the hydroxyl radical (OH). In particular, the specific mechanism of the soot-mediated interconversion between NO2 and HONO is to date not fully understood. Due to its particular relevance in this context, we have chosen the Pyrene-NO2 system. The first stage in this study has consisted in the determination of the stable configurations (transition states and minima) of the Pyrene-NO2 system. To this end, we have used the recently developed van der Waals Transition State Search using Chemical Dynamics Simulations (vdW-TSSCDS) method, the generalization of the TSSCDS algorithm developed in our group. In this way, the present work represents the first application of vdW-TSSCDS to a large system (81D). Starting from a set of judiciously chosen input geometries, the aforementioned method permits the characterization of the topography of an intermolecular Potential Energy Surface (PES), or in other words the determination of the most stable conformations of the system, in a fully automated and efficient manner. The gathered topographical information has been used to obtain a global description (fit) of the interaction potential, necessary for the dynamical elucidation of the intermolecular interaction (physisorption), spectroscopic properties and reactivity of the adsorbed species. To achieve this last goal, we have developed two different methodologies together with the corresponding software packages. The first one of them is the SpecificReaction Parameter Multigrid POTFIT (SRP-MGPF) algorithm, which is implemented in the SRPTucker package. This method computes chemically accurate (intermolecular) PESs through reparametrization of semiempirical methods, which are subsequently tensor decomposed into Tucker form using MGPF. This software has been successfully interfaced with the Heidelberg version of the Multi-configuration Time-DependentHartree (MCTDH) package. The second method allows for obtaining the PES directly in the mathematical form required by MCTDH, thence its name Sum-Of-Products Finite-Basis-Representation (SOP-FBR). SOP-FBR constitutes an alternative approach to NN-fitting methods. The idea behind it is simple: from the basis of a low-rank Tucker expansion on the grid, we replace the grid-based basis functions by an expansion in terms of a orthogonal polynomials. As in the previous method, an smooth integration with MCTDH has been ensured. Both methods have been successfully benchmarked with a number of reference problems, namely: the Hénon-Heiles Hamiltonian, a global H2O PES, and the HONO isomerization PES (6D)
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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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Hsu, Chi-Chang, i 徐啟昌. "Light interaction in InSe/GaSe van der Waals heterostructures". Thesis, 2018. http://ndltd.ncl.edu.tw/handle/4wc8ju.
Pełny tekst źródłaStreszczenie:
碩士國立中山大學
物理學系研究所
106
The two-dimensional heterostructures are bonded by van der Waals forces at the interface, different from three-dimensional materials which are connected by covalent bonds. These heterostructures lack dangling bonds and lattice mismatch. In addition, ultra-fast charge transfer in van der Waals heterostructures takes place within 50 fs. When incident light passes through the heterostructures, it engages in multiple reflections within the underlying substrates, producing interferences that lead to enhancement or attenuation of Raman intensities and photoluminescence intensities. Thus, the multiple boundaries and thickness of each material play a key role in the interaction of light and heterostructures and strongly affect the device performance. Gallium selenide (GaSe) and indium selenide (InSe) of IIIA-VIA groups have direct band gaps in few layers and multi-color photoresponse ranging from ultraviolet to near infrared. Therefore, in this study, InSe/GaSe and GaSe/InSe heterostructures were fabricated on wafers with silicon dioxide of different thicknesses. The interaction between the light and heterostructures was investigated by employing Raman and photoluminescence spectroscopy. From the results, the Raman intensities of the individual InSe, individual GaSe and junction are the strongest when the thickness of silicon dioxide is 270 nm. The Raman intensities of InSe and GaSe at the lower layer of the junction are higher than the upper layer. The photoluminescence intensity of GaSe at the lower layer of the junction is stronger than the upper layer. It is found that when the excitation wavelength is close to the band gap of GaSe, a resonance phenomenon occur at interface resulting in enhancement of Raman intensities. Based on the interference model, a strategy to modulate the photon and photoelectric properties of InSe/GaSe heterostructures and GaSe/InSe heterostructures is proposed which may provide new ways to improve the performance of optoelectronic devices such as LEDs and solar cells.
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Kuiri, Manabendra. "Quantum capacitance and noise measurements in van der Waals heterostructures". Thesis, 2019. https://etd.iisc.ac.in/handle/2005/5092.
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The discovery of graphene has revolutionized the field of condensed matter physics and opened new prospects for a wide range of other two-dimensional materials. Further with the advancement of fabrication techniques, one can reassemble atomically thin layers in van der Waals heterostructures, which shows exotic physics like Hofstadter’s butterfly, Valley Hall effect, Coulomb drag, Mott insulator, even denominator fractional quantum Hall effect and superconductivity. The transport properties in these systems are governed by the electronic band dispersion; therefore, it is desirable to directly probe the electronic density of states, which is beyond the conventional transport measurement. In this thesis, we have carried out the quantum capacitance measurements to probe the thermodynamic density of states of van-der Waals heterostructures based on two-dimensional materials. In the first part of my thesis, we have investigated the band structure renormalization in monolayer graphene in presence of tunable one-dimensional super-lattice potential at both zero and finite magnetic fields and supported by our theoretical calculations. Furthermore, we have employed the magneto- capacitance spectroscopy to study the energetics of bilayer graphene in presence of electric and magnetic fields. Our results directly capture the phase transition between the different ground states of zeroth Landau level in bilayer graphene. At higher electric fields, we also observe the collapsing of the Landau levels, which was consistent with the existing theoretical predictions. As a part of the thesis, we study the anisotropic band dispersion of Black phosphorus, a two-dimensional material, using the above technique and could also probe the localized states near the band edge. In the second part of the thesis, we have investigated the transport and optoelectronic properties of transition metal chalcogenides (MoTe2) of different phases (2H and 1T’). The low-frequency 1/f noise measurements in our dual gated devices allow us to extract out the noise contribution originating from both the channel as well as contacts. We show that the origin of noise in the MoTe2 channel is due to carrier number fluctuation. The transport measurement of 1T’ MoTe2 shows two transitions depending on the thickness of the flakes, where the transition at low temperature is due to MIT (metal-insulator) transition and the transition at higher temperature is attributed to a structural phase transition. Finally, we measure the photo-responsivity in MoTe2-graphene van der Waals heterostructures and show that the photo-responsivity at the junction is largely enhanced as compared to bare MoTe2. By measuring photo-voltaic response of the junction using the ionic liquid gating, we also estimate the work function of MoTe2
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Finney, Nathan Robert. "Symmetry engineering via angular control of layered van der Waals heterostructures". Thesis, 2021. https://doi.org/10.7916/d8-h1jk-ha03.
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Crystal symmetry and elemental composition play a critical role in determining the physical properties of materials. In layered van der Waals (vdW) heterostructures, a two-dimensional (2D) material layer can be influenced by interactions between adjacent layers, dictating that the measured properties of the combined system will be in part derived from the geometric structure within the active layers. This thesis examines active crystal symmetry tuning in composite heterostructures of two-dimensional (2D) materials, engineered via nanomechanically assisted twist angle control, and designed by careful consideration of lowest energy stacking configurations. The material systems, devices, and experimental setups described in this thesis constitute a platform featuring highly programmable properties that are on-demand and reversible.
Two prototypical systems are discussed in detail. The first is graphene encapsulated between boron nitride (BN) crystals, wherein the alignment state between the three layers is controlled. The second is the same system, but with no graphene between the encapsulating BN layers. In both systems, a long-wavelength geometric interference pattern, also known as a moiré pattern, forms between the adjacent crystals as a consequence of lattice-constant mismatch and twist angle. The moiré pattern caries its own symmetry properties that are also demonstrated to be tunable, and can be thought of as an artificially constructed superlattice of periodic potential with wavelength much greater than the lattice constants of the constituent layers. In the BN-encapsulated graphene system we show drastic tunability of band gaps at primary and secondary Dirac points (PDP and SDPs) indicating reversible on-demand inversion symmetry breaking, as well as evidence of dual coexisting moiré superlattices and additional higher-order interference patterns that form between them. The all-BN system shows substantial enhancement and suppression of second harmonic generation (SHG) response from the vdW interface between the BN crystals when the quadrupole component of the SHG response is engineered to be minimal, by controlling for total layer number and layer number parity.
Changes in the physical properties of each composite system are measured with a combination of electronic transport measurements, and optical measurements (Raman and SHG), as well as piezo-force microscopy (PFM) measurements that give direct imaging of the moiré pattern. A number of invented and adapted fabrication and actuation techniques for controlling the twist angle of a bulk vdW crystal are discussed, and in the latter portion of this thesis these techniques are extended to include actuation of monolayer flakes of 2D crystals. In this discussion several case studies are discussed, including twist angle control for a single sample monolayer tungsten diselenide on monolayer molybdenum diselenide, as well as twist angle control for twisted bilayer graphene and graphene on BN. Additionally, a novel in-plane bending mode for graphene on BN is demonstrated using similar techniques. Further discussion of actuation via traditional electrostatic MEMS techniques is also included, illustrating complete on-chip control for on-demand nanomechanical actuation of 2D materials in vdW heterostructures.
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(9597326), Maithilee Motlag. "Laser shock nanostraining of 2D materials and van der Waals heterostructures". Thesis, 2021.
Znajdź pełny tekst źródłaStreszczenie:
Since the successful exfoliation of graphene, two-dimensional (2D) materials have attracted a lot of scientific interest due to their electronic, chemical, and mechanical properties. Due their reduced dimensionality, these 2D materials exhibit superior mechanical and optoelectronic properties when compared to their bulk counterparts. Within the family of 2D materials, the ultrathin transition metal dichalcogenides (TMDs) such as Tungsten diselenide and Molybdenum disulphide have gained significant attention due to their chemical versatility and tunability. Furthermore, it is possible to leverage the distinct characteristic properties of these 2D materials, which are held together by van der Waals forces, by stacking different 2D layers on top of each other resulting in van der Waals (vdW) heterostructures. Due to the absence of feasible methods to effectively deform the crystal structures of these 2D materials and vdW heterostructures, their mechanical properties have not been thoroughly understood. The atomistic simulations can effectively capture the material behavior at the nanoscale level and help us not only not only understand the mechanical properties of these materials but also aid in the development of tailored processes to tune the material properties for the design of novel metamaterials. Using atomistic simulations, we develop the process - property relationships which can guide the direction of experimentation efforts, thereby making the process of discovering and designing new metamaterials efficient.
In this work, we have used laser shock nanostraining technique which is a scalable approach to modulate the optomechanical properties of 2D materials and vdW materials for practical semiconductor industry applications. The deformation mechanisms of 2D materials such as graphene, boron nitride (BN) and TMDs such as WSe2 and MoS2 are examined by employing a laser shocking process. We report studies on crystal structure deformation of multilayered WSe2 and monolayer graphene at ultra-high strain rate using laser shock . The laser shocking process generates high pressure at GPa level, causing asymmetric 3D straining in graphene and a novel kinked-like locking structure in multilayered WSe2. The deformation processes and related mechanical behaviors in laser shocked 2D materials are examined using atomistic simulations. Moiré heterostructures can be obtained by introducing a twist angle between these 2D layers, which can result into vdW materials with different properties, thereby adding an additional degree of freedom in the process-property design approach. We were able to successfully create a tunable stain profile in 2D materials and vdW heterostructures to modulate the local properties such as friction, and bandgap by controlling the level of laser shock, twist angle between the 2D layers and by applying appropriate laser shock pressure . We thus extend this knowledge to further explore the pathways of strain modulation using a combination of laser shocking process, moiré engineering, and strain engineering in 2D materials consisting of graphene, BN, and MoS2 and to develop the process - property relationships in vdW materials.
In summary, this research presents a systematic understanding of the effect of laser shocking process on the van der Waals materials and demonstrates the modulation of mechanical and opto-electronic property using laser nanostraining approach. This understanding provides us with opportunities for deterministic design of 2D materials with controllable properties for semiconductor and nanoelectronics applications.
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Paul, Tathagata. "Physics and application of charge transfer in van der Waals heterostructures". Thesis, 2019. https://etd.iisc.ac.in/handle/2005/4503.
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Since the discovery of graphene, the field of 2D materials has garnered massive interest from a materials science, basic physics and device application point of view. This
results from the diverse range of electronic and transport properties observed in these
systems. For example, graphene, which has a gapless Dirac fermionic band structure with
extremely high carrier mobility, shows low photoresponse due to the absence of a band
gap. However, layered transition metal dichalcogenides (TMDCs) such as MoS2, which
possess a semiconducting band structure with disorder dominated hopping like transport
mechanism and low carrier mobility, demonstrates high photoresponse due to the presence
of a band gap. One of the major benefits of 2D materials is the possibility of stacking
together isolated atomic planes of different materials in a layer by layer manner forming an atomic Lego or a van der Waals heterostructure. Proximity induced interaction
between two or more 2D crystals with varied crystal structure and electronic properties leads to a plethora of possibilities for the emergence of new physics and/or device
functionality. Consequently, van der Waals heterostructures have been utilized to design
devices for a wide variety of applications such as electronic, piezoelectric, thermoelectric,
optoelectronic and non-volatile information storage to name a few. For optoelectronic
and memory-based applications, charge transfer between the constituent layers of the van
der Waals heterostructure has proven to be of immense importance. There are reports
of excellent photodetectors based on MoS2 graphene heterostructures where a transfer of
photogenerated carriers from the MoS2 to graphene layer leads to high responsivity figures
of ∼ 5 × 108 AW−1 at room temperature. In this thesis, we study the effect of vertical
charge transfer in TMDC based van der Waals heterostructures aimed at non-volatile
memory, memristor and bio-inspired synaptic applications.
For this purpose, we use a trilayer stack of MoS2, hBN and graphene. Here hBN acts
as a tunnel barrier separating the MoS2 channel from the graphene floating gate (FG).
This design is motivated by our investigations into the ON/OFF switching mechanism in
back gated TMDC FETs where we observed clear signatures of percolative switching in
a disordered channel with low subthreshold slopes. An improvement in the subthreshold
slope is brought about by capacitance engineering via extension of the FG, leading tohigh quality MoS2 FETs with near-ideal subthreshold slope (≈ 80 mV/decade) maintained for almost four decades of change in conductance. The device also demonstrates a
large anti-hysteresis in the transfer characteristics due to the transfer of charges from the
channel to the FG. This, coupled with a low OFF state current makes the MoS2 FG device
ideal for energy efficient memory applications. The charge transfer process also leads to a
hysteresis in the output characteristics which is indicative of a memristor like behaviour.
Furthermore, the quanta of charge transferred can be controlled using short time period
pulses at the gate and drain terminal. This leads to a multi-state memory device with repeated increase and decrease of the channel conductance resulting from the accumulation
or depletion of electronic charges on the graphene FG. Pulsed charge transfer mediated
changes in device conductance is analogous to the pulsed potentiation and depression of a
biological synapse which is mediated via controlled release of neurotransmitters into the
synaptic cleft. In addition to pulsed potentiation and depression, the device successfully
replicates other synaptic properties such as paired pulse facilitation (PPF) and spike time
dependent plasticity (STDP) while maintaining a low power dissipation (∼5 fJ per pulse),
making it ideal for future neuromorphic applications.
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Chen, Ciao-Fen, i 陳巧芬. "Multifunctional Logic-gate Realizations in van der Waals Heterostructures via Rapid Thermal Annealing". Thesis, 2019. http://ndltd.ncl.edu.tw/cgi-bin/gs32/gsweb.cgi/login?o=dnclcdr&s=id=%22107NCHU5198003%22.&searchmode=basic.
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碩士國立中興大學
物理學系所
107
In this work, we produced a field-effect transistor based on a Molybdenum ditelluride(MoTe2)/ Tin sulfide (SnS2) structure by mechanical exfoliation and dry transferred. SnS2 is a typical n-type semiconductor, and MoTe2 exhibits ambipolar behavior which the thickness is between 3 to 10 nm. By the process of rapid thermal annealing in the dry air environment, the polarity of MoTe2 could change to p-type because of oxygen doping. Because of this process, the position of Fermi level would be down-shifted to valence band. As for SnS2, we chose the thicker thickness of SnS2 to prevent the polarity change from annealing. The band structure of heterojunction in various annealing temperature condition and gate voltage was different, so the rectification ratio would be positive or negative. In terms of applications in logic circuits, we observed the binary inverter in the condition that was without annealing process. We hope that we can obverse the multistate logic inverter by the method of annealing.
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Tsai, Meng-Yu, i 蔡孟宇. "Nonvolatile Optoelectronic Memory with Fully Visible Spectral Distinction Based on van der Waals Heterostructures". Thesis, 2019. http://ndltd.ncl.edu.tw/cgi-bin/gs32/gsweb.cgi/login?o=dnclcdr&s=id=%22107NCHU5759017%22.&searchmode=basic.
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碩士國立中興大學
奈米科學研究所
107
Various functional wearable-device are facing the hidden problem of cost, power consumption and efficacy performance. However, with the miniaturization of electronic component size, portable and low-power devices become more and more popular. In this research, we apply two-dimensional layered materials to the structure of nonvolatile memory. Through the excellent electrical properties of 2-D layered materials and the structural arrangement of floating-gate memory, we start to explore the photoelectric characteristics and the application value of ReSe2/h-BN/graphene 2-D structure. In terms of experiment, mechanical exfoliation was chosen to complete multi-functional 2-D components. To research the memory ability of two-dimensional structure, we selected ReSe2 material as semiconductor, h-BN material as insulator, and graphene as the metal of floating-gate, expecting that this component will show non-volatile performance with low power consumption as well as erasable-programmable property. Combining the ambipolar electrical characteristics of ReSe2 with the storage ability of graphene floating gate not only shows excellent memory retention, which maintains nearly 6 orders of storage space for a long time, but also shows the durability of cycling endurance test. Therefore, it can be developed into a functional application of memristor with multi-resistive-state performance. However, there are unexpected results in the illumination experiment. We found that, through photon injection, electrons and holes will be erased at the same time. Besides, this device not only exhibits rapid photoreaction time of 1 sec, but also discriminates fully visible light and different optical power without external power supply, which can achieve the function of distinguishing color and brightness. There are lots of potential to create functional applications, such like power-saving nanoscale light detector or digital camera. As a nanoscale electronic component, this non-volatile ReSe2/h-BN/graphene device exhibits great memory performance as expected, and achieves characteristics of low cost, low power consumption and multi-function. The outstanding performance creates highly applied value in the future.
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Pandey, Mrityunjay. "Scanning Probe microscopy of van der Waals heterostructures and non-equilibrium magnetotransport in graphene". Thesis, 2022. https://etd.iisc.ac.in/handle/2005/6006.
Pełny tekst źródłaStreszczenie:
Graphene is a two-dimensional semimetal that has linear dispersion in energy-momentum
space. When graphene is subjected to a perpendicular magnetic field, the dispersion is no
longer linear, resulting in discrete energy levels because of the formation of cyclotron orbits
of different energies. This energy discretization leads to quantum oscillations in longitudinal
magnetoresistance known as Shubhnikov de-Haas oscillations which provide a plethora of
properties, including the effective mass of charge carriers and topological properties like Berry
phase. Furthermore, the transverse resistance in the magnetic field is quantized, making it
useful for resistance metrology. The quantization effects have been realized in graphene in
the ohmic regime, i.e., with a small current density < 0.01 A/m passing through the channel.
Non-equilibrium magnetotransport studies in two-dimensional electron gas systems based on
GaAs-AlGaAs quantum wells have been intensively investigated under high current densities,
demonstrating the effect of carrier heating, magnetophonon-oscillations, and Hall field-induced
magneto-oscillations in longitudinal resistance. However, the effect of high current densities on
magnetotransport in graphene has not been thoroughly investigated. In this thesis, we have explored the magnetotransport in graphene Hall bar devices under non-equilibrium conditions by
introducing a high current density (> 1 A/m) through the channel, which produces a strong Hall
field across the channel and results in tilting of the Landau levels. For the experiments aimed at
realizing electron transitions between two cyclotron orbits in the presence of a magnetic field,
the width of the channel becomes crucial. We have fabricated large-width Hall bar devices,
which ensures the number of cyclotron orbits in the bulk is significant, and edge scattering will
have less contribution, making it more sensitive to magnetotransport in the bulk of the channel.
Making extra-large width devices becomes a significant step that requires a sizeable clean
xii
area of graphene to ensure high mobility. The dry pick-up and transfer method to fabricate
hexagonal boron nitride (hBN)- encapsulated graphene is a standard technique to achieve
high-quality devices. Sandwiching graphene between hBN often leads to folding, wrinkling,
and the formation of air pockets between hBN and graphene, which limit sample quality. There fore, it becomes essential to identify the geometrical extent of clean graphene. Here we have
developed a non-invasive sub-surface electrical scanning probe technique to identify a clean and
significant area of graphene encapsulated by 20-30 nm thick hBN. We have used Electrostatic
Force Microscopy (EFM) to identify the region of interest. This method reveals the effect of
substrate and ambient environment on the doping of graphene. We have conducted elaborate
measurements on various encapsulated layered materials and observed that the EFM phase acts
as a clear fingerprint of the constituent layered materials in complex heterostructures involving
graphene, hBN, and transition metal dichalcogenides. In addition to providing visually striking
images of buried layers, the technique is also useful in probing the electrical properties of the
constituent layers. We have extended the technique to other van der Waals heterostructures
of transition metal dichalcogenides such as MoS2 and WSe2 encapsulated in hBN. We expect
our findings to advance reliable and high throughput device architectures for various nano and
optoelectronics applications.
To explore the non-equilibrium transport properties in graphene we have exploited the EFM
technique to identify the homogeneous and residue-free region of graphene encapsulated by
hBN. Hall bars with device widths ranging between 12 µm to 18 µm were made to investigate
the non-equilibrium magnetotransport. In addition to expected carrier heating effects, we
observe two branches of novel magnetoresistance oscillations near the charge neutrality point
when plotted as a function of carrier density and dc current at magnetic field ranging between
1 T to 5 T. These oscillations show linear dispersion as a function of dc current and carrier
density. The drift velocity of carriers associated with dispersion matches well with the TA, and
LA phonon modes in graphene, indicating phonon-assisted intra-Landau level transitions aid in
these oscillations. The novelty of these results are expected to stimulate further studies that
can help unravel a unified picture of the various resonant processes in this regime, not only in
graphene but also in related Moiré heterostructures
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Murali, Krishna. "Engineering van der Waals Heterojunctions for Electronic and Optoelectronic Device Applications". Thesis, 2020. https://etd.iisc.ac.in/handle/2005/4778.
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Efficient preparation and characterization of layered materials and their van der Waals
heterojunctions lay the foundation for various opportunities in both fundamental studies
and device applications. The vast library of 2D materials displays a range of electronic
properties, including conductors, semiconductors, insulators, semimetal, and superconductors, and shows strong light-matter interaction. The fact that each layer in the layered
material is bonded via van der Waals interaction opens up the possibility of assembling
different layers arbitrarily without any consideration over the precision of lattice match-
ing. This unique stacking with one-atomic-plane precision can unfold diverse van der
Waals heterostructure devices by efficiently engineering its energy band alignment. This
paves a path to design novel devices such as solar cells, photodetectors, light-emitting
diodes and transistors.
In this thesis, our motivation is to explore the electronic and optoelectronic characteristics of 2D materials and their heterojunctions. We focus on designing 2D heterostructures for the multi-functional devices including electronic (diode/transistor)
and optoelectronic (highly sensitive photodetection) applications. As the initial step, we
realized SnSe2 based photoconductor which shows a very high responsivity of 10^3 A/W
at 1 mV voltage bias. We investigated the role of trap states present at the channel-
substrate interface on the observed gain mechanism in typical planar 2D photoconductors.
Next, in order to improve the speed for a photodetector, we designed a heterostructure composed of ITO/WSe2/SnSe2 vertical heterojunction. This novel design helped us to achieve a large responsivity at near IR region while maintaining high operational speed. We achieved a high responsivity of more than 1100 A/W and fast transient response time in the order of 10 us. Considering the interest of broad band detection, we then fabricated a graphene-absorption-based photodetector where graphene can act as
the absorbing medium, utilizing its zero-band gap nature. The absorbed photo-carriers
are vertically transported in a fast time scale to a floating MoS2 quantum well, providing photo-gating. This structure exhibited the responsivity of 4.4 * 10^6 A/W at 30 fW
incident power which is higher than that of any reported graphene absorption-based photodetectors. As a continuation of the study of heterostructure transport characteristics,
we realized a backward diode with WSe2/SnSe2 structure which exhibits an ultra-high reverse recti cation ratio of 2.1 *10^4 with an impressive curvature coefficient of 37 V^(-1).
Finally, we proposed a novel methodology for the extraction of Schottky Barrier Height
(SBH) using a vertical heterojunction of multilayer transition metal dichalcogenide with
asymmetric contacts which allow easy and direct quantitative evaluation of SBH for two
contacts simultaneously.Visvesvarayya PhD Scheme
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Roy, Kallol. "Optoelectronic Properties of Graphene Based Van-der-Waals Hybrids". Thesis, 2017. http://etd.iisc.ac.in/handle/2005/4142.
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Light matter interactions in atomically thin van der Waals materials have attracted significant attention in recent day [1–6]. Although the thickness does not exceed few nano-meters, such an atomically thin materials alone or in combination with other nanostructures show exciting and unexpected photodetection properties [7–16].
Fabrication of atomically sharp junctions can be achieved with 2D van der Waals heterostructures, which significantly enhances the scope to design new type of physical systems, where novel phenomena can be studied [15, 17, 18]. Heterostructures combine properties of dissimilar materials resulting improved device performances and hence, can be applied to multiple fields [19–21].
This thesis encompasses photo-response study of various atomically thin heterostructures made of graphene, bilayer-graphene (BLG) and MoS2. A graphene-on-MoS2 heterostructure, made of monolayer graphene and few atomic layers of MoS2, combine superior electronic transport properties of graphene with the optical properties of MoS2. Such hybrids exhibit enormous photo-responsivity, with values as high as ∼ 1010 A W−1 at ∼ 130 K and ∼ 108 A W−1 at room temperature, which make these the most photo-responsive material available till date. Presence of tunable persistent photo-response allows these to function as optoelectronic memory devices; where the persistent state shows near perfect charge retention within the experimental time scale of operation (∼ 12 hrs).
Noise-free large gain (109 − 1010) mechanism is one of the salient features of graphene-MoS2 hybrids. Devices made from BLG-on-MoS2 hybrids further help in improving the photoresponsive gain in these devices, and a large photoresponsivity (∼ 109 A W−1) is maintained even when operating these devices at low channel bias (VDS < 50 mV), or at a low range of channel current (IDS < 10 nA). In an optimized operating condition, where circuit noise is lower than the signal from a single photoelectron, BLG-on-MoS2 devices function as a number resolved single photon detector. High specific detectivity and low noise equivalent power of these devices, allow investigation of photon noise present in an optical source.
Along with the optoelectronic property study, various optical and electrical character-izations are adapted that explain the interface properties of graphene-MoS2 heterostructures. For example, Raman spectroscopy and photoluminicence study at the interface suggest strong interlayer coupling and efficient dissociation of excitons respectively, which play a key role in attaining large photoresponse. Interfacial barrier characteristics are also investigated in a vertical graphene-MoS2 geometry, which shows that the barrier height can be tuned by applying an electrostatic field.
Various experimental techniques and instruments, such as heterostructure fabrication technique and setup, optical cryostat etc., were developed in house to accomplish experimental investigation, which are discussed in details.
Results of photo-response study in van der Waals materials have opened up the possibility of designing a new class of photosensitive devices which can be utilized in various optoelectronic applications such as in biomedical sensing, astronomical sensing, optical communications, optical quantum information processing and in applications where low intensity photodetection and number resolved single photon detection attracts tremendous interest.
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Paul, Arup Kumar. "Exploring carrier dynamics in van-der-Waals heterostructures with shot noise spectroscopy, thermoelectricity and opto-electronic study". Thesis, 2021. https://etd.iisc.ac.in/handle/2005/5627.
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In the last two decades, there has been tremendous progress in two-dimensional (2D) - material research. It is mainly
due to the availability of a large selection of 2D-materials and their van-der-Waals heterostructures. The 2D-material
based heterostructures, in particular, have significant fundamental and technological importance due to their tunable
electrical and optical properties. In this thesis, we have studied three types of 2D-heterostructures: graphene pn
junction, magic-angle twisted bilayer graphene, and MoTe2-MoS2 based hetero-pn junction. We investigate the carrier
transport and its dynamics using cutting-edge probes like shot noise spectroscopy, thermoelectricity, and optoelectronic
studies.
In the first part of the thesis, we will present our research on equilibration dynamics of quantum Hall (QH)
edge states in graphene pn junction (PNJ). Graphene pn junction with co-propagating spin-valley polarized QH edges
is a promising platform for studying electron interferometry. Though several conductance measurements have been
attempted for such PNJs, the edge dynamics of the spin-valley symmetry broken edge states remain unexplored. In this
study, we present the measurements of conductance together with shot noise, an ideal tool to unravel the dynamics,
in dual graphite gated hexagonal boron nitride encapsulated high mobility graphene devices. The conductance data
show that the symmetry broken QH edges at the PNJ follow spin selective equilibration. The shot noise results as a
function of both p and n side filling factors reveal the unique dependence of the scattering mechanism. Remarkably, the
scattering is found to be fully tunable from incoherent to a coherent regime with the increasing number of QH edges
at the PNJ, shedding crucial insights into the velocity-dependent phase coherence of the QH edges at a pn junction.
Furthermore, we study the bias-dependent tunneling between the ν = ±1 QH edges of the pn junction. At zero bias,
we observe finite tunneling (t ∼ 0.5), which remains almost constant up to a bias energy of few hundreds of µV . The
tunneling sharply falls with further increasing bias voltage and exhibits repeated smaller peaks at discrete energies
before completely vanishing. The tunneling at zero bias is anomalous as it is expected to be zero between ν = ±1
edges due to their orthogonal spin polarization. Thus, from tunneling to fully blockade regime suggests bias-controlled
switching of spin polarization at graphene pn junction.
In the second part we will discuss the interaction driven resetting of the band-structure of a Magic-angle twisted
bilayer graphene (MtBLG) using thermoelectricity as a probe. MtBLG has proven to be an extremely promising
new platform to realize and study a host of emergent quantum phases arising from the strong correlations in its
narrow bandwidth flat band. In this regard, thermal transport phenomena like thermopower, in addition to being
coveted technologically, is also sensitive to the particle-hole (PH) asymmetry, making it a crucial tool to probe the
underlying electronic structure of this material. In this study, we have carried out thermopower measurements of
MtBLG as a function of carrier density, temperature and magnetic field, and report the observation of an unusually
large thermopower reaching up to a value as high as ∼ 100µV/K at a low temperature of 1K. Surprisingly, our observed thermopower exhibits peak-like features in close correspondence to the resistance peaks around the integer
Moire fillings, including the Dirac Point, violates the Mott formula. We show that the large thermopower peaks and
their associated behaviour arise from the emergent highly PH asymmetric electronic structure due to the cascade of
Dirac revivals.
In the last part, we will present the optoelectronic properties of MoTe2-MoS2 hetero pn junction. MoS2 and
MoTe2 are, respectively, n-type and p-type transition metal di-chalcogenides with band-gaps of ∼ 1.8 eV and 0.8 eV.
Over and above the antiambipolar transfer characteristics observed similar to other hetero pn junction, our experiments
reveal a unique feature as a dip in the transconductance near the maximum. We further observe that the modulation of
the dip in the transconductance depends on the doping concentration of the 2D flakes and also on the power density
of the incident light. We also demonstrate high photo-responsivity of ∼ 105A/W at room temperature for a forward
bias of 1.5V. We explain these new findings based on interlayer recombination rate-dependent semi-classical transport
model.
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Dandu, Medha. "Tailoring optical and electrical characteristics of layered materials through van der Waals heterojunctions". Thesis, 2021. https://etd.iisc.ac.in/handle/2005/5623.
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The feasibility of isolation of layered materials and arbitrary stacking of different materials provide plenty of opportunities to realize van der Waals heterostructures (vdWhs) with desired characteristics. In this thesis, we experimentally demonstrate the tunability of optical and electrical characteristics of transition metal dichalcogenides (TMDs), a class of layered materials, using their vdWhs. Monolayer (1L) TMDs exhibit remarkable light-matter interaction by hosting direct bandgap, strongly bound excitonic complexes, ultra-fast radiative decay, many-body states, and coupled spin-valley degrees of freedom. However, their sub-nm thickness limits light absorption, impairing their viability in photonic and optoelectronic applications.
The physical proximity of layers in vdWhs drives strong interlayer dipole-dipole coupling resulting in nonradiative energy transfer (NRET) from one layer (donor) to another (acceptor) under spectral resonance. Motivated by the high efficiency of NRET in vdWhs, we study the prospect of enhancement of optical properties of a 1L-TMD stacked on top of strongly absorbing, non-luminescent, multilayer SnSe2 whose direct bandgap is close to exciton emission of 1L-TMDs – MoS2 and WS2. We show that NRET enhances both single-photon and two-photon luminescence by one order of magnitude in such vdWhs. We also demonstrate a new technique of Raman enhancement driven by NRET in vdWhs. We achieve a 10-fold enhancement in the Raman intensity, enabling the observation of the otherwise invisible weak Raman modes.
We establish the evidence for NRET-aided photoluminescence (PL) and Raman enhancement by modulating the degree of enhancement by systematically varying multiple parameters - donor material, acceptor material, their thickness, physical separation between donor and acceptor by insertion of spacer layer (hBN), sample temperature, and excitation wavelength. We also use the above parameters to decouple the effects of charge transfer and optical interference from NRET and establish a lower limit of the NRET-driven enhancement factor. We significantly modulate the strength of NRET by controlling the spectral overlap between 1L-TMD and SnSe2 through temperature variation. We show a remarkable agreement between such temperature-dependent Raman enhancement and the NRET-driven Raman polarizability model. We emphasize the advantages of using SnSe2 as a donor and elucidate the impact of various parameters on the PL enhancement using a rate equation framework. This NRET-driven enhancement can be used in tandem with other techniques and thus opens new avenues for improving quantum efficiency, coupling the advantages of uniform enhancement accessible across the entire junction area of vdWhs.
Further, we study the role of NRET in photocurrent generation across vdWhs by designing a vertical junction of SnSe2/multilayer-MoS2/TaSe2. We report the observation of an unusual negative differential photoconductance (NDPC) behaviour arising from the existence of NRET across the SnSe2/MoS2 junction. The modulation of NRET-driven NDPC characteristics with incident optical power results in a striking transition of the photocurrent's power law from sublinear to a superlinear regime. These observations highlight the nontrivial impact of NRET on the photoresponse of vdWhs and unfold possibilities to harness NRET in synergy with charge transfer.
The stacking angle between the individual layers in vdWhs provides another knob to tune their properties. The emergence of moiré patterns in twisted vdWhs creates superlattices where electronic bands fold into a series of minibands, inducing new phenomena. We experimentally demonstrate the PL emission from the moiré superlattice-induced intralayer exciton minibands in twisted TMD homobilayers using artificially stacked 1L-MoS2 layers at minimal twist angles. We also show the electrical tunability of these moiré excitons and the evolution of distinct moiré trions. We experimentally discern the localized versus delocalized nature of individual moiré peaks through different regimes of gating and optical excitation. Further, we discuss the gate-controlled valley coherence and resonant Raman scattering of moiré excitons. These experimental results provide unique insights into the moiré modulated optical properties of twisted bilayers.
Next, we focus on tuning the electrical characteristics of vdWhs to realize ambipolar injection, which is useful for LED and CMOS applications. vdW contacts offer atomically smooth and pristine interfaces without dangling bonds, coupled with a weak interaction at the interface. Such contacts help to achieve a completely de-pinned contact close to the Schottky-Mott limit. We demonstrate the weakly pinned nature of a vdW contact (TaSe2) by realizing improved ambipolar carrier injection into few-layer WS2 and WSe2 channels (compared to Au). Backward diodes offer a superior high-frequency response, temperature stability, radiation hardness, and 1/f noise performance than a conventional diode. We demonstrate a vdWh based backward diode by exploiting the giant staggered band offsets of the WSe2/SnSe2 junction. The diode exhibits an ultra-high reverse rectification ratio of ~2.1*10^4 up to a substantial bias of 1.5 V, with an excellent curvature coefficient of ~37 V^{-1}, outperforming existing backward diode reports. We efficiently modulate the carrier transport by varying the thickness of the WSe2 layer, the type of metal contacts employed, and the external gate and drain bias. We also show that the effective current transfer length at the vertical junction in vdWhs can be as large as the whole interface, which is in sharp contrast to the smaller transfer length (~100 nm) in typical metal-layered semiconductor junctions.
The results from this thesis widen the horizon for practical electronic, photonic, and optoelectronic applications of vdWhs.
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Ghosh, Priyadarshini. "Controlled Nanoscale Growth of 2D Materials and Their Heterostructures". Thesis, 2017. http://etd.iisc.ac.in/handle/2005/4227.
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Unique ranges of properties of two dimensional (2D) materials foster new opportunities in many scientific and technological fields. Beyond pristine 2D materials, their heterostructures provide more possibilities in real application as the stack can be tailored according to desired properties. Besides, exotic combination can give rise to new functionalities. Despite experimental success in many lab-grade devices, large scale commercialization of 2D technology is still being actively worked on. This requires easy and cost effective synthesis technique which can give high quality sample in large scale. In this thesis, we discuss the scalable, application specific growth of 2D materials and their heterostructures. The easiest preparation method (micromechanical cleavage) does not offer a very good route for commercial use and alternative approaches can offer large scale production. A brief survey on the various production methods have been carried out in Chapter 2. It is shown that through bottom up approach a balance between growing large scale and high quality can be achievable and CVD is one such controllable growth platform. However to control materials at such atomic level require detail understanding of nucleation, surface science and defect control. Next in Chapter 3, taking graphene growth as an example, a CVD based growth method is first established which can also be applied for other 2D materials.
The grain size of a CVD grown monolayer large area graphene film is key to its performance. Microstructural design for the desired grain size requires a fundamental understanding of graphene nucleation and growth. Hence, first the CVD growth phenomenon of graphene has been exploited. In this course, two ultimate levers for controlling the nucleation density are identified and they are substrate defect density, which are the active nucleation sites and “gas-phase supersaturation”. It is observed that defects on copper surface, namely dislocations, grain boundaries, triple points and rolling marks, initiate the nucleation of graphene. It has been shown that among these defects, dislocations are the most potent nucleation sites, as they get activated at lowest supersaturation. Defects in graphene can be made useful in functionalization or making heterostructures which can be useful in chemical sensing or energy generating fields. Specifically, combination of graphene with plasmonic nanostructures would allow for making surface enhanced Raman spectroscopy (SERS) based sensing platform. Graphene being atomically thin can ideally be placed between plasmonic metal dimers to create precise sub-nm gap. Such hybrid structure of metal dimers with graphene spacer increases the Raman signal by several orders by near-field enhancement resulting from strong electromagnetic coupling between the particles. Besides these hybrids have applications in other different areas such as optical switch, displays, and photodetectors.
In Chapter 4, wet-chemistry-based method is applied to fabricate a SERS substrate with 7x10cm Au nanoparticle dimers, separated by a single graphene layer in which each dimer can act as a plasmonic enhancer. The method involves selective growing of Au particle on top of a graphene covered another Au particle. It is shown that desired density of graphene separated dimers can be obtained effortlessly by controlling the nucleation and growth of the Au particles. A 35x enhancement in graphene spectra, seen from a single such dimer indicates that this could find applications in identifying ppb concentration of Raman active substances. Furthermore, using R6G as a probe molecule, it is shown that these substrates can be efficient reliable SERS substrates in low cost SERS based sensing application.
In Chapter 5 heterostructure of graphene and Sb2Te3 has been demonstrated. A very simple wet chemical transfer method is applied to make this heterostructure. Structural properties of the individual in the heterostructure have been investigated throughly by microscopic and spectroscopic analysis to verify the preservation of the properties. Further, graphene has been found to increase the stability of Sb2Te3 in ambient condition.
In Chapter 6 a new facile method to grow Bi2Te3 –Sb2Te3 heterostructure in large scale is described. This method is based on an extended liquid phase exfoliation methodology, which has been applied to many oxide layered
0.52−−
materials such as GO, Ti0.87O2 , and Ca2Nb3O10 nanosheets. Analysis of optimized dispersion of heterostructure was conducted using SEM, Raman and TEM-EDS, where the presence of both the materials-Bi2Te3 and Sb2Te3 were discovered. Current thermoelectric application requires mass production of such superlattice and this method can provide that in cost-effective way.
In summary, starting with controllable growth of 2D materials, we have discussed formation of hybrid structures of 2D materials and explored some of the applications thereof.
47
Sahoo, Anindita. "Electrical Transport in the Hybrid Structures of 2D Van Der Waals Materials and Perovskite Oxide". Thesis, 2016. http://etd.iisc.ac.in/handle/2005/2948.
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Perovskite oxides have provided a wide variety of exotic functionalities based on their unique physical and chemical properties. By combining different perovskite oxides, interesting physical phenomena have been observed at the interfaces of perovskite heterostructures. The most interesting among these phenomena is the formation of two dimensional electron gas at the interface of two perovskite materials SrTiO3 and LaAlO3 which led to a number of fascinating physical properties such as metal-insulator transition, super-conductivity, large negative magnetoresistance and so on. This has raised the interest in exploiting the interface of various hybrids structures built on the perovskite oxide backbone. On the other hand, the two dimensional (2D) van der Waals materials such as graphene, MoS2, boron nitride etc. represent a new paradigm in the 2D electron-ics. The functionalities of these individual materials have been combined to obtain new enriched functionalities by stacking different materials together forming van der Waals heterostructures. In this work, we present a detailed study of the interface in hybrid structures made of vander Waals materials (graphene and MoS2) and their hybrids with a perovskite material namely, SrTiO3 which is known as the building block of complex oxide heterostructures.
In graphene-MoS2 vertical heterostructure, we have carried out a detailed set of investigations on the modulation of the Schottky barrier at the graphene-MoS2 interface with varying external electric field. By using different stacking sequences and device structures, we obtained high mobility at large current on-off ratio at room temperature along with a tunable Schottky barrier which can be varied as high as ∼ 0.4 eV by applying electric field. We also explored the interface of graphene and SrTiO3 as well as MoS2 and SrTiO3 by electrical transport and low frequency 1/f noise measurements. We observed a hysteretic feature in the transfer characteristics of dual gated graphene and MoS2 field effect transistors on SrTiO3. The dual gated geometry enabled us to measure the effective capacitance of SrTiO3 interface which showed an enhancement indicating the possible existence of negative capacitance developed by the surface dipoles at the interface of SrTiO3 and the graphene or MoS2 channel. Our 1/f noise study and the analysis of higher order statistics of noise also support the possibility of electric field-driven reorient able surface dipoles at the interface.
48
Sahoo, Anindita. "Electrical Transport in the Hybrid Structures of 2D Van Der Waals Materials and Perovskite Oxide". Thesis, 2016. http://etd.iisc.ernet.in/handle/2005/2948.
Pełny tekst źródłaStreszczenie:
Perovskite oxides have provided a wide variety of exotic functionalities based on their unique physical and chemical properties. By combining different perovskite oxides, interesting physical phenomena have been observed at the interfaces of perovskite heterostructures. The most interesting among these phenomena is the formation of two dimensional electron gas at the interface of two perovskite materials SrTiO3 and LaAlO3 which led to a number of fascinating physical properties such as metal-insulator transition, super-conductivity, large negative magnetoresistance and so on. This has raised the interest in exploiting the interface of various hybrids structures built on the perovskite oxide backbone. On the other hand, the two dimensional (2D) van der Waals materials such as graphene, MoS2, boron nitride etc. represent a new paradigm in the 2D electron-ics. The functionalities of these individual materials have been combined to obtain new enriched functionalities by stacking different materials together forming van der Waals heterostructures. In this work, we present a detailed study of the interface in hybrid structures made of vander Waals materials (graphene and MoS2) and their hybrids with a perovskite material namely, SrTiO3 which is known as the building block of complex oxide heterostructures.
In graphene-MoS2 vertical heterostructure, we have carried out a detailed set of investigations on the modulation of the Schottky barrier at the graphene-MoS2 interface with varying external electric field. By using different stacking sequences and device structures, we obtained high mobility at large current on-off ratio at room temperature along with a tunable Schottky barrier which can be varied as high as ∼ 0.4 eV by applying electric field. We also explored the interface of graphene and SrTiO3 as well as MoS2 and SrTiO3 by electrical transport and low frequency 1/f noise measurements. We observed a hysteretic feature in the transfer characteristics of dual gated graphene and MoS2 field effect transistors on SrTiO3. The dual gated geometry enabled us to measure the effective capacitance of SrTiO3 interface which showed an enhancement indicating the possible existence of negative capacitance developed by the surface dipoles at the interface of SrTiO3 and the graphene or MoS2 channel. Our 1/f noise study and the analysis of higher order statistics of noise also support the possibility of electric field-driven reorient able surface dipoles at the interface.
49
Ghawri, Bhaskar. "Investigation of twisted bilayer graphene using electrical and thermoelectric transport". Thesis, 2022. https://etd.iisc.ac.in/handle/2005/5954.
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The ability to tune the twist angle between di erent layers of two-dimensional materials has opened up a new
dimension to band engineering in van der Waals (vdW) heterostructures. By taking advantage of the formation
of a moir e superlattice arising from a small lattice mismatch or twist angle between two adjacent atomic layers,
one can create materials with tailored electronic, optical, mechanical, thermal and optoelectronic properties. In
particular, twisted bilayer graphene (tBLG) has attracted considerable scienti c interest owing to exceptional band
tunability. The coupling between the two graphene layers depends strongly on the twist angle, leading to angledependent
electronic and phononic hybridization. In addition, when the relative rotation is close to the magic angle
( m = 1:1 ), the low-energy electronic bands are nearly
at, leading to a multitude of interaction-driven phases. This
includes correlated insulators, superconductivity, magnetism, non-trivial band topology, nematicity and signatures
of non-Fermi liquid (NFL) excitations with linear-in-temperature resistivity that persists down to temperatures well
below the Bloch{Gr uneisen temperature. Although signi cant progress has been made in understanding the vast
phase diagram of tBLG, there is no consensus on the origin of the superconductivity, metallic states and the role
of electron-phonon coupling at very low temperatures ( 1 K). In this thesis, we study the in-plane and cross-plane
electrical and thermoelectric properties of tBLG with varying twist angles to understand the nature of metallic states
and the role of layer breathing phonon modes at low twist angles.
The cross-plane thermoelectric transport in large angle tBLG is driven by the scattering of electrons and inter-layer
breathing phonon modes. However, the relevance of layer hybridized phonons in thermoelectric transport remains
unclear when the electronic hybridization of the two layers becomes strong at low . In the rst part of the thesis, we
show out-of-plane thermoelectric measurements across the vdW gap in tBLG, which exhibits an interplay of twistdependent
interlayer electronic and phononic hybridization. We show that at large twist angles, the thermopower is
entirely driven by a novel phonon-drag e ect at the subnanometer scale. In contrast, the electronic component of the
thermopower is recovered only when the misorientation between the layers is reduced to < 6 . Our experiment shows
that cross-plane thermoelectricity at low angles is exceptionally sensitive to the nature of band dispersion.
Although the T-linear resistivity in tBLG at low temperatures has been attributed to the absence of a well-de ned
quasiparticle spectrum, experimentally, the manifestation of NFL e ects in transport properties of twisted bilayer
graphene remains ambiguous. In the next part of the thesis, we have performed simultaneous measurements of
electrical resistivity ( ) and thermoelectric power (S) in tBLG for several twist angles between 1:0 1:7 . We
observe an emergent violation of the semiclassical Mott relation (MR) in the form of excess S close to half- lling for
1:6 that vanishes for & 2 . In addition, for a device with 1:24 , excess S is observed at fractional band
lling. The combination of non-trivial electrical transport and violation of Mott relation provides strong evidence of
NFL physics intrinsic to tBLG.
Next, we study the electrical and thermoelectric transport in marginally tBLG ( 0:5 ), where the electronic band
structure at a low twist angle is expected to become qualitatively di erent as compared to magic-angle because of large
moir e unit cell and strain-accompanied lattice reconstruction. We observe a strong metallic behaviour accompanied
by a T-linear and an emergent violation of the semi-classical Mott relation in the vicinity of van Hove singularities
(vHSs). Our experiments show that the thermopower is exceptionally sensitive to the band dispersion in small-angle
tBLG even at high temperatures, and the low-T transport is governed by a network of topological channels formed
at domain boundaries between AB and BA regions.
Finally, we have demonstrated the working of a thermoelectric generator consisting of dual-junction tBLG. We
show that the thermopower in cross-plane tBLG can be enhanced by using a dual-junction device. Further, using
an external resistor enables us to measure the current-voltage characteristics of the device and estimate the power
generated in the system.
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Maity, Nikhilesh. "Computational Modeling of two Dimensional Heterostructures for Optoelectronic and Catalytic Applications". Thesis, 2023. https://etd.iisc.ac.in/handle/2005/6052.
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Two-dimensional van der Waals (2D-vdW) materials have attracted significant attention for their unique and excellent properties. The properties of the 2D-vdW materials can be precisely engineered using various techniques for the desired applications. We carried out a study of 2D-vdW materials and their heterostructures for optoelectronic and catalytic applications using state of the art ab-initio modeling such as density functional theory (DFT), many-body perturbation theory (MBPT), and density functional perturbation theory (DFPT). We report the generation of linearly polarized, anisotropic, intra and interlayer excitonic bound states in GeSe/SnS vdW heterostructure using GW and Bethe-Salpeter equation simulations (BSE), addressing the current demand of optical polarizers. A dramatic variation in excitonic binding energy and optical band gap is observed upon applying biaxial strain, which is attributed to the change in effective dielectric constant and band dispersion. Building upon the concept of optical and excitonic properties, we discuss the Z-scheme mechanism in C3N3/C3N4 vdW heterostructure for water splitting catalysts. The spontaneous redox reactions for the water splitting combined with band alignment, presence of higher-order interlayer excitons, fast electron-hole recombination, and high charge mobility facilitate the Z-scheme mechanism compared to the type II mechanism. For optoelectronic applications, the stacking order plays a crucial role in 2D materials. Rhenium disulfide (ReS2) is one of the most potential candidates for optoelectronic properties; however, extremely weak interlayer coupling strength makes it challenging to determine the stacking order in multilayer ReS2. We successfully identify two distinct stacking orders (AA & AB) by the potential energy profile and the vibrational Raman modes. We extend this study to determine the stacking-order-driven optical and excitonic properties. By symmetry analysis, we also explore the origin of extra Raman modes and splitting of Raman modes in multilayer ReS2, which is another debatable topic. The extra modes and the splitting in Raman spectra are attributed to the layer parity-dependent breaking of inversion symmetry. Due to the weak coupling strength between the layers, multilayer ReS2 is designed with a proper doping strategy for the layer-independent deep center defects. The thermodynamic study confirms that S_Re is the best possible deep isolated defect for a single photon emitter. This study highlights the importance of heterostructuring, stacking-order, and strain engineering to study the extraordinary properties of 2D materials and also paves the path to overcome critical challenges in optoelectronic research applications.