Dissertations / Theses on the topic 'Aimants van der Waals'
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1
Wang, Hangtian. "Interfacial Engineering of the Magnetism in 2D Magnets, Topological Insulators, and Their Heterostructures." Electronic Thesis or Diss., Université de Lorraine, 2023. http://www.theses.fr/2023LORR0206.
Abstract:
Alors que le nœud critique des circuits intégrés (CI) entre dans la phase 1 nm, les matériaux tridimensionnels traditionnels ne peuvent pas conserver leurs propriétés physiques d'origine et ne peuvent donc pas répondre aux besoins des processus de fabrication des circuits intégrés. Parallèlement, la diminution de la largeur des lignes entraîne également une augmentation inévitable de la consommation d'énergie statique. Par conséquent, la recherche de nouveaux matériaux et de nouvelles technologies pour briser le « mur de taille » et le « mur de puissance » est devenue une direction cruciale dans l'industrie des circuits intégrés. En tant que nouveau membre de la famille des matériaux bidimensionnels (2D), les aimants 2D peuvent maintenir leur ordre magnétique à longue portée à l'échelle atomique avec leurs propriétés physiques facilement contrôlées par des stimuli externes, ce qui constitue une plate-forme idéale pour la haute densité et les dispositifs spintroniques de faible puissance. Cependant, en raison de l'effet dimensionnel, le magnétisme 2D ne peut pas exister à haute température. Bien que plusieurs méthodes puissent améliorer la température de Curie (Tc) des aimants 2D (comme le dopage, l'intercalation ionique ou le pompage laser), elles sont loin d'être faciles à contrôler et à haut rendement. Plus important encore, la méthode de préparation largement utilisée par exfoliation mécanique abandonne le mérite de l'effet interfacial 2D, qui s'est avéré être une approche importante pour une manipulation magnétique 2D efficace. Par conséquent, l'étude de l'effet interfacial dans les aimants 2D épitaxiaux est considérée comme un domaine clé pour obtenir un ordre ferromagnétique 2D stable, à grande échelle, à haute Tc, facile à contrôler. L'isolant topologique (TI) est un autre matériau 2D avec un fort couplage spin-orbital. Les états de surface protégés par la topologie ont fourni à TI de nombreux effets fascinants liés au spin, tels que le verrouillage de l'impulsion de spin, l'effet d'échange de spin, etc., ce qui fait de ce matériau un candidat potentiel pour fabriquer des dispositifs spintroniques efficaces. De plus, le TI peut être intégré à des aimants 2D pour former une hétérostructure 2D, dans laquelle non seulement le magnétisme peut être amélioré via l'effet d'interface, mais également les propriétés liées au spin de l'hétérostructure peuvent être manipulées grâce aux avantages de ces aimantsWith the critical node of integrated circuits (IC) entering the 1 nm stage, traditional three-dimensional materials cannot maintain their original physical properties, and thus cannot meet the needs of IC manufacturing processes. Meanwhile, the shrinking line width also introduces an inevitable increase in static power consumption. Therefore, researching new materials and new technologies to break through the "Size Wall" and "Power Wall" has become a crucial direction in the IC industry. As a new member of the two-dimensional (2D) material family, the 2D magnets can maintain its long-range magnetic order at the atomic scale with its physical properties easily controlled by external stimuli, which provides an ideal platform for the high-density and low-power spintronic devices. However, due to the dimensional effect, 2D magnetism cannot exist at high temperatures. Although several methods can enhance the Curie temperature (Tc) of 2D magnets (such as doping, ion intercalation, or laser pumping), they are far from easy-controllability and high-efficiency. More importantly, the widely-used preparation method via mechanical exfoliation abandons the merit of 2D interfacial effect, which was proved to be an important approach to efficient 2D magnetic manipulation. Therefore, studying the interfacial effect in epitaxial 2D magnets is regarded as a key field to achieving large-scale, high-Tc, easy-controlling, and stable 2D ferromagnetic order. Topological insulator (TI) is another 2D material with strong spin-orbital coupling. The topology-protected surface states provided TI with numerous fascinates spin-related effects, such as spin-momentum locking, spin exchange effect, etc., which makes this material a potential candidate to fabricate effective spintronic devices. In addition, the TI can be integrated with 2D magnets to form a 2D heterostructure, in which not only the magnetism can be enhanced via the interfacial effect, but also the spin-related properties of the heterostructure can be manipulated due to the advantages of these two materials
2
Bezzi, Luca. "Materiali 2D van der Waals." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020.
Abstract:
Dalla scoperta del grafene, molte ricerche sono state condotte sui cosiddetti “materiali 2D”. Questo elaborato si focalizza sulle proprietà strutturali, elettroniche, ottiche ed eccitoniche di due materiali bidimensionali, ossia il grafene il disolfuro di molibdeno (MoS2-1H), quest’ultimo un importante semiconduttore. Le proprietà di questi materiali sono diverse rispetto alla loro controparte massiva (bulk) grafite e MoS2-2H, e un loro confronto è stato preso in considerazione. Come metodo di indagine sono state scelte simulazioni quanto- meccaniche ab initio dei sistemi in esame, un approccio che, negli ultimi decenni, sta avendo un impatto sempre più importante sulla fisica, sulla chimica dello stato solido e sulla scienza dei materiali, promuovendo non solo una comprensione più profonda, ma anche la possibilità di contribuire in modo significativo alla progettazione di materiali per nuove tecnologie. Questo importante passo avanti è stato possibile grazie a: (i) una descrizione migliorata ed efficiente degli effetti elettronici a molti corpi (many-body) nella teoria del funzionale della densità (DFT), nonché lo sviluppo di metodi post-DFT per lo studio di proprietà specifiche; (ii) un’accurata implementazione di questi metodi in software altamente efficienti, stabili e versatili, capaci di sfruttare il potenziale delle architetture informatiche moderne. Tra i possibili software ab initio basati su DFT, abbiamo scelto il pacchetto di simulazione di Vienna ab initio VASP, considerato un gold standard per questo tipo di indagini. I risultati ottenuti per le varie proprietà di bulk e di superficie (bidimensionale) dei materiali scelti sono in ottimo accordo con dati ottenuti in precedenza, sia a livello teorico, sia sperimentale. Questo elaborato getta quindi le basi per futuri studi nel campo dei materiali 2D per comprendere, analizzare, ingegnerizzare nuovi materiali con proprietà desiderabili e per sviluppare nuove applicazioni degli stessi.
3
Boddison-Chouinard, Justin. "Fabricating van der Waals Heterostructures." Thesis, Université d'Ottawa / University of Ottawa, 2018. http://hdl.handle.net/10393/38511.
Abstract:
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.
4
Tiller, Andrew R. "Spectra of Van der Waals complexes." Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.333415.
5
Mauro, Diego. "Electronic properties of Van der Waals heterostructures." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2016. http://amslaurea.unibo.it/10565/.
Abstract:
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.
6
Klein, Andreas. "Energietransferprozesse in matrixisolierten van-der-Waals-Komplexen." [S.l. : s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=962344761.
7
Odeyemi, Tinuade A. "Numerical Modelling of van der Waals Fluids." Thèse, Université d'Ottawa / University of Ottawa, 2012. http://hdl.handle.net/10393/22661.
Abstract:
Many problems in fluid mechanics and material sciences deal with liquid-vapour flows. In these flows, the ideal gas assumption is not accurate and the van der Waals equation of state is usually used. This equation of state is non-convex and causes the solution domain to have two hyperbolic regions separated by an elliptic region. Therefore, the governing equations of these flows have a mixed elliptic-hyperbolic nature.
Numerical oscillations usually appear with standard finite-difference space discretization schemes, and they persist when the order of accuracy of the semi-discrete scheme is increased. In this study, we propose to use a Chebyshev pseudospectral method for solving the governing equations. A comparison of the results of this method with very high-order (up to tenth-order accurate) finite difference schemes is presented, which shows that the proposed method leads to a lower level of numerical oscillations than other high-order finite difference schemes, and also does not exhibit fast-traveling packages of short waves which are usually observed in high-order finite difference methods. The proposed method can thus successfully capture various complex regimes of waves and phase transitions in both elliptic and hyperbolic regimes
8
Marsden, Alexander J. "Van der Waals epitaxy in graphene heterostructures." Thesis, University of Warwick, 2015. http://wrap.warwick.ac.uk/77193/.
Abstract:
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.
9
Connelly, James Patrick. "Microwave studies of Van der Waals complexes." Thesis, University of Oxford, 1993. http://ora.ox.ac.uk/objects/uuid:3865eb1d-d288-44c9-8d42-84f7ff2c0608.
Abstract:
This thesis describes the commissioning and development of a pulsed supersonic nozzle, Fourier-transform microwave spectrometer and its application to the study of several weakly bound van der Waals complexes. A pulsed supersonic expansion, Fourier-transform microwave spectrometer based on the Flygare design with a number of modifications has been constructed with an operating range of 6-18 GHz. A homodyne detection circuit mixing signals to modulus values between dc and 1 MHz is used, requiring two measurements to determine absolute transition frequencies. Transition frequencies are measured from the power spectrum by determining the first derivative zero crossing point in a least squares fitting procedure. Semiautomation of many of the spectrometer operations has been achieved allowing unattended data collection over scans of up to 300 MHz. The microwave spectrum of Ar2-OCS and Ar2-OC34S has been observed and analysed using conventional Watson S reduction hamiltonian parameters. Effective structural parameters are derived and used in a harmonic force field analysis, based on the centrifugal distortion constants, to compare the trimer interations with a model based on the sum of dimer interactions. A series of complexes containing the nitrogen molecule undergoing tunnelling motions have been studied. Hyperfine matrix elements for the first order nuclear quadrupole interaction are derived for the coupled identical nuclei case appropriate to the rapid tunnelling motions observed. The microwave spectrum of N2-OCS is described. Tunnelling and nuclear spin statistical effects for two symmetry states are observed arising from the interchange of nitrogen nuclei. Rotational and quadrupole constants are derived; an accidental near degeneracy of two rotational levels allows the off-diagonal quadrupole coupling constant to be determined from second order effects. A tunnelling hamiltonian fitting the quadrupole coupling constants to an angular potential has been used to calculate the tunnelling frequency and barrier to N2 rotation. The microwave spectrum of N2-O3 and a preliminary spectrum of N2-SO3 have been observed. Rotation-inversion motions of the O3 and SO2 moieties must be considered in addition to the N2 tunnelling to fit the spectrum. Tunnelling frequencies for the O3/SO2 and geared motions with the N2 are derived as well as structural parameters. Modifications for production of refractory molecules and complexes by laser ablation have been made. A modified nozzle employing rods of material is used with the ablation process taking place in the nozzle throat. Modifications to obtain an expansion along the axis of the microwave cavity employ a hemispherical Fabry-Perot cavity configuration. The system has been tested on a number of diatomic molecules including PbS and CuCl.
10
Wright, Nicholas J. "Bound states of Van der Waals trimers." Thesis, Durham University, 1998. http://etheses.dur.ac.uk/5048/.
Abstract:
A method for calculating the energy levels and wave functions of floppy tri- atomic systems such as rare-gas trimers has been developed. It is based upon a potential-optimized discrete variable representation and takes into account the wide-amplitude vibrations that occur in such systems. The quadrature error that occurs in DVR calculations is analysed and a method of correction implemented. The diagonalisation procedure is based upon a combination of successive diagonalisation and truncation and a Lanczos diagonaliser. Using this method the wave functions of the Ar(_3) Van der Waals trimer have been calculated. The wave functions for the low-lying states show very regular behaviour. Above the barrier to linearity, most of the wave functions are irregular but some have simple nodal patterns that suggest localization along periodic orbits. In addition to the "horseshoe" states previously described for H(^+)(_3), localized features corresponding to symmetric and antisym metric stretching vibrations around a linear configuration have been identified. The different localized modes can be combined to form more complex states in a manner analogous to normal modes. A preliminary study of the rotational states of Ar(_3) has also been performed. The rotational constants for the low lying states of Ar(_3) reflect the increasing average size of Ar(_3) with increasing vibrational excitation. The rotational constants are obtained from two methods, expectation values and energy level differences. The results for the levels above the barrier to isomerisation reveal that the simple models used for obtaining the rotational constants are no longer valid and indicate that a more sophisticated treatment is necessary.
11
Bryan, Robert. "Theoretical studies of Van der Waals clusters." Thesis, Durham University, 1997. http://etheses.dur.ac.uk/4712/.
Abstract:
The vibrational energy levels of various rare gas trimers, Ar(_3), Ne(_3), He(_3), Ar(_2)Ne and Ne(_2)Ar, have been calculated using a coupled channel approach. We have compared results obtained with previous calculations. The existence of Efi-mov states in He(_3) has been investigated, and no evidence of their existence has been found. The affect of the Eckart conditions on embedding axis into a rotating-vibrating system has been investigated for several rare gas systems. A wide range of rare gas trimers have been studied, Ar(_3), He(_2)Ar, Ar(_2)He, Ar(_2)Ne and Ne(_2)Ar. For each trimer the full range of molecular motion is investigated. The low energy minima for the Ar(_n)N(_2) and Ne(_n)N(_2) systems have been found using simulated annealing search, and a gradient based minimisation technique, of a pairwise potential energy surface. Clusters with n ≥ 12 have been studied, and first solvation shells for both systems have been proposed. For each value of n, for n = 1 - 12, the first few low energy minima of the potential energy surface have been found. From these studies, we have gained a detailed understanding of the interplay of forces that determine the low energy structures for these systems. The affect of three-body interactions on the low energy minima both rare gas-N(_2) systems has been studied. In both system, rare gas-rare gas and rare gas- threebody interactions have been taken into account. This study has shown that the three-body forces have a small affect on the low energy structures of each system.
12
Tulegenov, Akyl S. "SIMPER method for van der Waals complexes." Thesis, University of Nottingham, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.431329.
13
McDowell, Sean Alistair Courtney. "Theoretical studies of Van der Waals molecules." Thesis, University of Cambridge, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.259733.
14
Le, Sueur Catherine Ruth. "Induction effects in Van der Waals complexes." Thesis, University of Cambridge, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.385523.
15
Willberg, Dean Michael Zewail Ahmed H. "Picosecond spectroscopy of van der Waals clusters /." Diss., Pasadena, Calif. : California Institute of Technology, 1994. http://resolver.caltech.edu/CaltechETD:etd-04042008-110156.
16
Columberg, Gieri. "Mikrowellen-Spektroskopie T-förmiger Van der Waals Komplexe /." [S.l.] : [s.n.], 1996. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=11636.
17
Coy, Diaz Horacio. "Preparation and Characterization of Van der Waals Heterostructures." Scholar Commons, 2016. http://scholarcommons.usf.edu/etd/6212.
Abstract:
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.
18
Lawrence, Stuart John. "High-resolution spectroscopy of van der Waals molecules." Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318824.
19
Kettley, J. C. "Van der Waals complexes of large aromatic molecules." Thesis, University of Nottingham, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.371128.
20
Althorpe, Stuart C. "Bound state calculations for van der Waals dimers." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.319937.
21
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.
Abstract:
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.
22
Waage, Magnus Heskestad. "Radiative corrections to van der Waals interaction in fluids." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for fysikk, 2012. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-18872.
Abstract:
The van der Waals interaction energy is derived for a homogenous fluid of polarizable particles. Low-temperature corrections to the van der Waals interaction energy are computed numerically for three cases: Dilute media with radiative interactions, arbitrarily dense media without radiative interactions, and arbitrarily dense media with radiative interactions. The validity of the model used to calculate the corrections at arbitrary densities is argued for, by comparison with the results for low densities.
23
DelRio, Frank William. "Van der Waals and capillary adhesion in microelectromechanical systems." Diss., Connect to online resource, 2006. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3239374.
24
Peet, Andrew Charles. "Vibrational predissociation of Van der Waals complexes containing ethylene." Thesis, University of Cambridge, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.329168.
25
Howson, Joanna M. M. "Obtaining potential energy surfaces of Van der Waals molecules." Thesis, Durham University, 1999. http://etheses.dur.ac.uk/4488/.
Abstract:
Two different methods were used to obtain a potential energy surface for the Arco molecule. One involved choosing a functional form for the repulsion and dispersion energies whose parameters were determined by a fit to experimental data. A physically justified potential that agreed with experiment could not be obtained. The other method was based on calculating ab initio interaction energies at different configurations of the molecule and interpolating between them. The resulting surface was scaled in the energy and the co-ordinates. Improved agreement was achieved for most observed bound states. Errors in the surface may have been due to an inadequate density of ab initio energies. So, how the molecular configurations chosen for interaction energy calculations affected the potential obtained was investigated. Both the co-ordinate system and the interpolation scheme also significantly affected the quality of surface obtained. The best compromise between accuracy and number of configurations, was points distributed on a regular grid in elliptic co-ordinates with Gauss-Legendre quadrature points in the angular co-ordinate. This information was employed to obtain a potential energy surface for the weakly bound HeOCS molecule which was in close agreement with experiment. A co-ordinate and energy scaling, guided by experiment, was applied to the ab initio surface. Only three scaling parameters were required due to the high quality of the initial surface.
26
Sanz-Garcia, Aranzazu. "Modelling the dispersion energy for Van der Waals complexes." Thesis, University of Nottingham, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.252013.
27
Musgrave, Adam. "Electronic spectroscopy of Van der Waals clusters and complexes." Thesis, University of Nottingham, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.445684.
28
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.
Abstract:
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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Schofield, Robert Christopher. "Raman studies of 2-dimensional van der Waals materials." Thesis, University of Sheffield, 2018. http://etheses.whiterose.ac.uk/21313/.
Abstract:
Presented herein are results of optical studies with emphasis on the Raman response, providing significant contribution to the knowledge of the field. In Mox W(1−x) S2 , confirmation of the behaviour of the excitonic properties is made. Raman measurements performed in this system allow investigation with unprecedented resolution, highlighting deviations in the high frequency A1g optical phonon mode from theoretical predictions, and previous experimental studies. In the low frequency, data confirms the trends in the shear and breathing interlayer modes in alloys between WS2 and MoS2 are well described by the modification in the square density. Resonant excitation for [Mo] < 0.4, highlights new evidence for the understanding of the hitherto unexplained ‘Peak X’ resonant feature. Diverse indium-selenium compounds isolated by novel means are studied. The ULF Raman modes of PDMS exfoliated InSe are documented for the first time, demonstrating the ε-phase with ABA stacking, with flake of thickness N manifesting (N − 1) shear modes owing to resonant excitation of few layer samples. InSe flakes encapsulated in hexagonal boron nitride manifest different stacking orders to those of PDMS exfoliated InSe, and were found to have significant contamination, with crystalline degradation of the monolayer flake, and peaks corresponding N2 & O2 rotational modes present. In2Se3 films grown epitaxially on GaSe display substrate-selective polymorphism, where α-, β-, & γ- phases are identified, in addition to regions of InSe. Laser photo-annealing is shown to drive a phase change from the groundstate β → α phase, which is against the thermodynamic gradient. MoSe2/WS2 twisted hetero-bi-layer structures are studied, where shear modes showing a linear softening from AA′ stacking towards the AB at 60° indicating reduced interlayer coupling, as expected from the difference in interlayer spacing of AA′ and AB ordering. High frequency modes in the heterobilayer also demonstrate some sensitivity in the relative angle, and are analysed in detail.
30
David, Lamuel Abraham. "Van der Waals sheets for rechargeable metal-ion batteries." Diss., Kansas State University, 2015. http://hdl.handle.net/2097/32796.
Abstract:
Doctor of PhilosophyDepartment of Mechanical and Nuclear Engineering
Gurpreet Singh
The inevitable depletion of fossil fuels and related environmental issues has led to exploration of alternative energy sources and storage technologies. Among various energy storage technologies, rechargeable metal-ion batteries (MIB) are at the forefront. One dominant factor affecting the performance of MIB is the choice of electrode material. This thesis reports synthesis of paper like electrodes composed for three representative layered materials (van der Waals sheets) namely reduced graphene oxide (rGO), molybdenum disulfide (MoS₂) and hexagonal boron nitride (BN) and their use as a flexible negative electrode for Li and Na-ion batteries. Additionally, layered or sandwiched structures of vdW sheets with precursor-derived ceramics (PDCs) were explored as high C-rate electrode materials. Electrochemical performance of rGO paper electrodes depended upon its reduction temperature, with maximum Li charge capacity of 325 mAh.g⁻¹ observed for specimen annealed at 900°C. However, a sharp decline in Na charge capacity was noted for rGO annealed above 500 °C. More importantly, annealing of GO in NH₃ at 500 °C showed negligible cyclability for Na-ions while there was improvement in electrode's Li-ion cycling performance. This is due to increased level of ordering in graphene sheets and decreased interlayer spacing with increasing annealing temperatures in Ar or reduction at moderate temperatures in NH₃. Further enhancement in rGO electrodes was achieved by interfacing exfoliated MoS₂ with rGO in 8:2 wt. ratios. Such papers showed good Na cycling ability with charge capacity of approx. 225.mAh.g⁻¹ and coulombic efficiency reaching 99%. Composite paper electrode of rGO and silicon oxycarbide SiOC (a type of PDC) was tested as high power-high energy anode material. Owing to this unique structure, the SiOC/rGO composite electrode exhibited stable Li-ion charge capacity of 543.mAh.g⁻¹ at 2400 mA.g⁻¹ with nearly 100% average cycling efficiency. Further, mechanical characterization of composite papers revealed difference in fracture mechanism between rGO and 60SiOC composite freestanding paper. This work demonstrates the first high power density silicon based PDC/rGO composite with high cyclic stability. Composite paper electrodes of exfoliated MoS₂ sheets and silicon carbonitride (another type of PDC material) were prepared by chemical interfacing of MoS₂ with polysilazane followed by pyrolysis . Microscopic and spectroscopic techniques confirmed ceramization of polymer to ceramic phase on surfaces on MoS₂. The electrode showed classical three-phase behavior characteristics of a conversion reaction. Excellent C-rate performance and Li capacity of 530 mAh.g⁻¹ which is approximately 3 times higher than bulk MoS₂ was observed. Composite papers of BN sheets with SiCN (SiCN/BN) showed improved electrical conductivity, high-temperature oxidation resistance (at 1000 °C), and high electrochemical activity (~517 mAh g⁻¹ at 100 mA g⁻¹) toward Li-ions generally not observed in SiCN or B-doped SiCN. Chemical characterization of the composite suggests increased free-carbon content in the SiCN phase, which may have exceeded the percolation limit, leading to the improved conductivity and Li-reversible capacity. The novel approach to synthesis of van der Waals sheets and its PDC composites along with battery cyclic performance testing offers a starting point to further explore the cyclic performance of other van der Waals sheets functionalized with various other PDC chemistries.
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Gée, Christelle. "Reactions chimiques isolees sur agregats de van der waals." Paris 11, 1997. http://www.theses.fr/1997PA112092.
Abstract:
Cette these porte sur l'etude des collisions reactives ou non reactives en presence d'un milieu materiel: un agregat forme de plusieurs centaines d'atomes ou molecules. L'agregat par sa taille finie permet d'isoler un couple de reactifs qui peuvent migrer sur l'agregat, entrer en collision et eventuellement declencher une reaction chimique. Notre dispositif experimental permet de generer un faisceau d'agregats homogenes (d'argon, de methane et d'azote) et d'y deposer les reactifs en faible quantite, par une technique collisionnelle. Nous etudions ces reactions chimiques isolees a la fois par des diagnostics optiques et par spectrometrie de masse. Les travaux realises sur les systemes reactionnels (ba/n#2o), (ba/co#2), (ba/sf#6) et (ba/ch#4) font appel a une technique d'analyse introduite dans cette these: la methode c. I. C. R. Acronyme de cluster isolated chemical reaction. Sa puissance reside dans la capacite a isoler une reaction sur un agregat pour en determiner la stoechiometrie. Nous avons suivi 2 orientations dans cette these. La premiere orientation porte sur l'effet de solvatation en observant l'influence de l'agregat sur une reaction chimique. L'agregat est parfois spectateur (reaction ba+n#2o), mais il peut aussi perturber considerablement, voire inhiber la reaction comme pour les paires 1 ba+1 sf#6 et 1 ba+1 co#2 ou l'agregat bloque la reaction en voie d'entree. Le deuxieme theme aborde est la reactivite de petits agregats de baryum. Nous avons etudie l'evolution de leur reactivite en fonction de leur taille et du caractere oxydo/reducteur du reactif moleculaire (cl#2, n#2o, o#2, co#2, sf#6, ch#4). A cette occasion, l'etude des systemes ba#2+co#2 et ba#2+n#2o a permis d'assigner un spectre de chimiluminescence jusque la non assigne: celui de ba#2o. La technique c. I. C. R. A montre toute sa force sur cet exemple en permettant l'identification du produit de reaction grace a la comprehension de la stoechimetrie de la reaction
32
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.
Abstract:
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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Gani, Yohanes Satrio. "Electronic Properties of Two-Dimensional Van Der Waals Systems." W&M ScholarWorks, 2019. https://scholarworks.wm.edu/etd/1563899012.
Abstract:
In this dissertation we study the electronic structure of van der Waals systems. A van der Waals systems is a heterostructure in which the different constituents are held together by van der Waals forces. We study two different types of van der Waals systems: van der Waals systems formed by graphene and a monolayer of NbSe2, van der Waals systems obtained by placing graphene nanoribbons on a two-dimensional crystal. For the first type of systems we build a continuous low-energy effective model that takes into account the presence of a twist angle between graphene and NbSe2, and of spin-orbit coupling and superconducting pairing in NbSe2. We then obtain how the superconducting pairing induced by proximity into the graphene layer depends on the twist angle. For the second type of systems we obtain using ab-initio methods the electronic structure of graphene nanoribbons placed on hexagonal boron nitride, and of graphene nanoribbons placed on monolayers of transition metal dichalcogenide. For both cases we show how the electronic structure depends on the stacking configuration.
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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.
Abstract:
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
35
Henck, Hugo. "Hétérostructures de van der Waals à base de Nitrure." Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLS319/document.
Abstract:
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
36
Scheele, Iris. "Hochauflösende Infrarot-Spektroskopie an schwach gebundenen Van-der-Waals-Systemen." [S.l.] : [s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=963558668.
37
Quayle, Christopher John Kendrick. "Alignment effects in the photodissociation of van der Waals molecules." Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.357431.
38
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.
Abstract:
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.
39
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.
Abstract:
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.
40
Matope, Stephen. "Application of Van-der-Waals forces in micro-material handling." Thesis, Stellenbosch : Stellenbosch University, 2012. http://hdl.handle.net/10019.1/71608.
Abstract:
Thesis (PhD)--Stellenbosch University, 2012.This doctoral dissertation focuses on the application of Van-der-Waals’ forces in micromaterial handling. A micro-material handling system consists of four main elements, which include: the micro-gripper, the micro-workpart, the picking up position and the placement position. The scientific theoretical frameworks of Van-der-Waals’ forces, presented by Van der Waals, Hamaker, London, Lifshitz, Israelachvilli, Parsegian, Rumpf and Rabinovich, are employed in exploring the extent to which these forces could be applied in a micromanufacturing situation. Engineering theoretical frameworks presented by Fearing, Bohringer, Sitti, Feddema, Arai and Fukuda, are employed in order to provide an in-depth synthesis of the application of Van-der-Waals’ forces in micro-material handling. An empirical or pragmatic methodology was adopted in the research. The Electron Beam Evaporation (e-beam) method was used in generating interactive surfaces of uniform surface roughness values. E-beam depositions of copper, aluminum and silver on silicon substrates were developed. The deposition rates were in the range of 0.6 – 1.2 Angstrom/s, at an average vacuum pressure of 2 x 10-6 mbar. The topographies were analysed and characterised using an Atomic Force Microscope and the corresponding rms surface roughness values were obtained. The Rumpf-Rabinovich equation, which gives the relationship of the exerted Van-der-Waals’ forces and the rms surface roughness values, is used to numerically model the results. In the final synthesis it is observed that the e-beam depositions of copper are generally suited for the pick-up position. Aluminum is suited for the micro-gripper and silver is suited for the placement position in an optimised micro-material handling system. Another Atomic Force Microscope was used in order to validate the numerically modelled results of the exerted Van- der-Waals’ forces. The aim was to measure the magnitude of Vander- Waals’ forces exerted by the e-beam depositions and to evaluate their applicability in micro-material handling operations. The measurements proved that Van-der-Waals’ forces exerted by the samples could be used for micro-material handling purposes on condition that they exceeded the weight of the micro-part being handled. Three fundamental parameters, ie: material type, geometrical configuration and surface topography were used to develop strategies of manipulation of micro-materials by Van-der- Waals’ forces. The first strategy was based on the material type variation of the interactive surfaces in a micro-material handling operation. This strategy hinged on the fact that materials have different Hamaker coefficients, which resulted in them experiencing a specific Van-der- Waals’ forces’ intensity during handling. The second strategy utilised variation in the geometrical configuration of the interacting surfaces. The guiding principle in this case was that, the larger the contact area was, the greater the exerted Van-der-Waals’ forces would be In the analytical modelling of Van-der-Waals’ forces with reference to geometrical configuration, a flat surface was found to exert more force than other configurations. The application of the design, for purposes of manufacturing and assembling (DFMA) criteria, also proved that flat interactive surfaces have high design efficiency. The third strategy was based on surface roughness. The rougher the topography of a given surface was, the lesser the Van-der-Waals’ forces exerted were. It was synthesised that in order for a pick-transfer-place cycle to be realised, the root-mean-square (rms) interactive surface roughness values of the micro-part (including the picking position, the micro-gripper, and the placement position) should decrease successively. Hybrid strategies were also identified in this research in order to deal with some complex cases. The hybrids combined at least two of the aforementioned strategies.
41
Economides, George. "Investigations of open-shell open-shell Van der Waals complexes." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:e27330e0-2eaa-4181-af30-70e8b7a3a692.
Abstract:
The question posed in this work is how one would model and predict the rotational spectrum of open-shell open-shell van der Waals complexes. There are two secondary questions that arise: the nature of radical-radical interactions in such systems and the modelling of the large amplitude motion of the constituent molecules. Four different systems were studied in this work, each providing part of the answer to the main question. Starting with the large amplitude motion, there are two theoretical approaches that may be adopted: to either model the whole complex as a semi-rigid molecule, or to perform quantum dynamical calculations. We recorded and analysed the rotational spectrum (using Fourier transform microwave spectroscopy) of the molecule of tertiary butyl acetate (TBAc) which exhibits a high degree of internal rotation; and of the weakly-bound complex between a neon atom and a nitrogen dioxide molecule (Ne-NO2). We used the semi-rigid approach for TBAc and the quantum dynamical approach for Ne-NO2. We also explored the compatibility of these two approaches. Moreover, we were able to predict and analyse the fine and hyperfine structure of the Ne-NO2 spectrum using spherical tensor operator algebra and the results of our dynamics calculations. To explore the nature of the interactions in an radical-radical van der Waals complex we calculated the PESs of the possible states that the complex may be formed in, when an oxygen and a nitrogen monoxide molecule meet on a plane using a number of high level ab initio methods. Finally, our conclusions were tested and applied when we performed the angular quantum dynamics to predict the rotational spectrum of the complex between an oxygen and a nitrogen dioxide molecule, and account for the effect of nuclear spin statistics in that system.
42
Dhont, Guillaume. "Spectroscopie Renner-Teller dans des complexes van der Waals chargés." Université de Marne-la-Vallée, 2003. http://www.theses.fr/2003MARN0175.
Abstract:
Les complexes de van der Waals formés d'une diatomique à couche ouverte et d'un atome de gaz rare offrent une diversité de types de liaison et de dynamique bien plus large que les complexes à couche fermée. Pour les diatomiques dans un état électronique à moment angulaire non nul, la situation se complique par l'effet Renner-Teller qui nécessite de prendre en compte simultanément deux fonctions d'énergie potentielle. Les hamiltoniens permettant l'étude quantique de ces systèmes sont présentés dans deux systèmes d'axes liés à la molécule. Chaque orientation du repère possède un modèle sous-jacent : dans un cas, le mouvement de rotation de la diatomique est perturbé par la présence de l'atome de gaz rare, l'autre cas est bien adapté à une analyse du complexe comme une molécule triatomique quasi-rigide. Un nouveau programme permettant le calcul variationnel des états rovibroniques a été développé pour chaque sytème d'axes. Les fonctions d'énergie potentielle bi- et tri-dimensionnelle de l'état électronique fondamental et du premier état excité de HeHCl+ ont été déterminées. Ces calculs ont montré que le mouvement de rotation de la diatomique est relativement peu gêné. Pour HeHF+, les premiers états rovibroniques de ce système peuvent, au contraire, être interprétés dans le cadre de l'effet Renner-Teller d'une triatomique quasi-rigide. La profondeur de puits des fonctions d'énergie potentielle de ArNH- est intermédiaire entre HeHCl+ et HeHF+, mais sa dynamique se rapproche beaucoup de celle de HeHF+. Des surfaces tridimensionnelles de moment dipolaire et de moment dipolaire de transition ont aussi été calculées pour le système HeHCl+ et des expressions analytiques sont présentées.
43
BENSLIMANE, MOHAMED. "Collisions agregats de van der waals surface aux energies thermiques." Palaiseau, Ecole polytechnique, 1995. http://www.theses.fr/1995EPXX0041.
Abstract:
Le travail decrit dans ce memoire porte sur l'etude experimentale et la modelisation de la dynamique des collisions entre agregats de van der waals (d'argon ou d'azote) et une surface de graphite pyrolytique aux energies thermiques (60 mev/atome). Le dispositif experimental comporte un generateur de faisceau moleculaire et une chambre ultravide contenant l'echantillon de graphite et deux systemes de detection des particules diffusees (par spectrometrie de masse et par rempi), pouvant tourner autour de l'echantillon sur 340. La taille des agregats incidents (de 300 a 10000 monomeres par agregat) est controlee par une methode originale. Les donnees experimentales sont constituees de distributions angulaires de flux et de temps de vol des particules diffusees. Ces distributions comportent une composante tres etroite, rasant la surface (pic a 85) et due a des agregats residuels, une composante supra-speculaire plus large et une composante diffuse dominante aux angles de diffusion negatifs (i. E. Du meme cote de la normale que le faisceau incident). Ces deux dernieres sont dues essentiellement a des monomeres. Ces donnees experimentales sont reproduites quantitativement a partir du modele suivant: les agregats incidents subissent une collision fortement inelastique au cours de laquelle ils se thermalisent et perdent leur vitesse de translation dans la direction normale a la surface. Ils vont donc glisser sur celle-ci en evaporant des petits fragments a l'origine de la composante supra-speculaire. Les monomeres de l'agregat pris en sandwich entre celui-ci et la surface se detachent de l'agregat et subissent un processus de quasi-piegeage suivi de desorption. Ceux d'entre eux qui ne reintegrent pas le restant de cet agregat sont a l'origine de la composante diffuse. Les autres vont transferer a cet agregat une quantite de mouvement dans la direction normale, ce qui permet a celui-ci de quitter la surface. La composante rasante est alors observee si l'agregat residuel quitte la surface avant evaporation complete
44
Schwarz, Stefan. "Microcavity-enhanced light-matter interaction in van der Waals heterostructures." Thesis, University of Sheffield, 2016. http://etheses.whiterose.ac.uk/12278/.
Abstract:
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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Lonij, Vincent P. A. "ATOM OPTICS, CORE ELECTRONS, AND THE VAN DER WAALS POTENTIAL." Diss., The University of Arizona, 2011. http://hdl.handle.net/10150/145119.
Abstract:
This dissertation describes new measurements of the van der Waals (vdW) potential energy for atoms near a surface. The measurements presented here were accomplished by studying diffraction a beam of atoms transmitted through a nanograting. I will describe how we improved precision by a factor of 10 over previous diffraction measurements by studying how different types of atoms interact with the same surface. As a result of this new precision, we were able to show for the first time the contribution of atomic core electrons to the atom-surface potential, and experimentally test different atomic structure calculation methods.In addition, this dissertation will describe how changing the width of the grating bars to achieve a particular "magic" grating bar width or rotating a grating to a particular "magic" angle allows us to determine both the atom-surface potential strength and the geometry of the grating. This represents an improvement over several recent studies where uncertainties in the nanograting geometry limited precision in the measurements of the vdW potential.For a complementary measurement, also discussed in this dissertation, we collaborated with the Vigue group in Toulouse, France. In this collaboration we used an atom interferometer to measure the phase shift due to transmission through a nanograting. By combining diffraction data from Tucson with interferometry data from Toulouse we improved the precision of interferometry measurements of the atom-surface potential of a single atomic species by almost a factor of 10 over previous interferometric measurements of the vdW potential. These interferometry measurements also serve to measure the shape of the vdW potential and set a limit on non-Newtonian gravitational interactions at 1-2 nm length scales.Finally, this dissertation will discuss how nanogratings with optimized geometry can improve atom interferometers, for example, with blazed gratings. We discuss next generation atom-surface potential measurements and examine new ways of analyzing diffraction data.
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Hermann, Jan. "Towards unified density-functional model of van der Waals interactions." Doctoral thesis, Humboldt-Universität zu Berlin, 2018. http://dx.doi.org/10.18452/18706.
Abstract:
Van der Waals-Wechselwirkungen (vdW) sind allgegenwärtig und spielen eine zentrale Rolle in einer großen Anzahl biologischer und moderner synthetischer Materialien. Die am weitesten verbreitete theoretische Methode zur Berechnung von Materialeigenschaften, die Dichtefunktionaltheorie (DFT) in semilokaler Näherung, vernachlässigt diese Wechselwirkungen jedoch größtenteils, was zur Entwicklung vieler verschiedener vdW-Modelle führte. Die hier vorgestellte Arbeit ebnet den Weg hin zu einem vereinheitlichten vdW-Modell welches die besten Elemente der unterschiedlichen Klassen von vdW-Modellen vereint. Zu diesem Zweck haben wir einen vereinheitlichten theoretischen Rahmen geschaffen, der auf dem Reichweite-separierten Adiabatischer-Zusammenhang-Fluktuations-Dissipations-Theorem aufbaut und die meisten existierenden vdW-Modelle umfasst. Wir analysieren die MBD-korrelierte Wellenfunktion am prototypischen Beispiel von π–π-Wechselwirkungen in supramolekularen Komplexen und stellen fest, dass diese Wechselwirkungen größtenteils durch delokalisierte kollektive Ladungsfluktuationen entstehen. Um zu dem langreichweitigen vdW-Modell ein ausgewogenes kurzreichweitiges Dichtefunktional zu identifizieren, präsentieren wir eine umfassende Untersuchung zum Zusammenspiel der kurz- und langreichweitigen Energiebeiträge in acht semilokalen Funktionalen und drei vdW-Modellen für eine große Spanne von Systemen. Die Bindungsenergieprofile vieler der DFT+vdW-Kombinationen unterscheiden sich sowohl quantitativ als auch qualitativ stark voneinander. Schließlich untersuchen wir die Performance des Vydrov–Van Voorhis-Polarisierbarkeitsfunktionals über das Periodensystem der Elemente hinweg und identifizieren eine systematische Unterschätzung der Polarisierbarkeiten und vdW-C₆-Koeffizienten für s- und d-Block-Elemente. Als Lösung entwickeln wir eine orbitalabhängige Verallgemeinerung des Funktionals.The ubiquitous long-range van der Waals interactions play a central role in nearly all biological and modern synthetic materials. Yet the most widely used theoretical method for calculating material properties, the density functional theory (DFT) in semilocal approximation, largely neglects these interactions, which motivated the development of many different vdW models. The work in this thesis paves way towards a unified vdW model that combines best elements from the different classes of the vdW models. To this end, we developed a unified theoretical framework based on the range-separated adiabatic-connection fluctuation--dissipation theorem that encompasses most existing vdW models. We analyze the MBD correlated wave function on the prototypical case of π–π interactions in supramolecular complexes and find that these interactions are largely driven by delocalized collective charge fluctuations. To identify a balanced short-range density functional to accompany the long-range vdW model, we present a comprehensive study of the interplay between the short-range and long-range energy contributions in eight semilocal functionals and three vdW models on a wide range of systems. The binding-energy profiles of many of the DFT+vdW combinations differ both quantitatively and qualitatively, and some of the qualitative differences are independent of the choice of the vdW model. Finally, we investigate the performance of the Vydrov—Van Voorhis polarizability functional across the periodic table, identify systematic underestimation of the polarizabilities and vdW C₆ coefficients for s- and d-block elements, and develop an orbital-dependent generalization of this functional to resolve the issue.
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Ramilowski, Jordan Aleksander. "Dynamics of van der Waals Clusters: Theoretical and Computational Studies." DigitalCommons@USU, 2010. https://digitalcommons.usu.edu/etd/719.
Abstract:
The marriage of two very powerful techniques - cryogenic matrix isolation spectroscopy and seeded supersonic molecular beams - has led to the development of a novel type of cryogenic matrix isolation spectroscopy in ultracold, near 0 K, He droplets. The technique known as helium nanodroplet isolation (HENDI) has seen tremendeous experimental interest over the past 20 years; this in turn has resulted in the availability of spectroscopic data for many molecules and clusters embedded in He clusters. The experimental findings havemotivated a large number of theoretical calculations. This dissertation focuses on theoretical and computational studies of the rotational dynamics of weakly bound van der Waals clusters with its main theme being the dynamics of molecules and small molecular dimers embedded in superfluid He-4 nanodroplets. The single molecular dopant systems studied were clusters of HCN-(He)N, HX-(He)N, where X = F, Cl, Br as well as NH3-(He)N, with N = 1 - 20. Ground and excited state calculations were performed using the rigid body diffusion Monte Carlo (RBDMC) algorithm. For the excited state calculations a new approach was developed: adiabatic-node DMC (ANDMC). The ANDMC method was used to study the renormalization of molecular rotational constants in He droplets. It revealed that the dynamics depend on a delicate interplay between the gas phase rotational constant value and the anisotropies in the potential energy interaction between the He atom and the dopant. Also presented are the results of the first DMC simulations of the ammonia dimer doped into a small droplet of He-4. Further, a new approach to finding nodal surfaces for DMC simulations was developed that involved using a genetic algorithm (GA). This method was implemented to systematically and automatically compute nodal surfaces of excited states of the HCN-He complex and of the interchange tunneling splitting in the hydrogen-bonded HCl-HCl complex. The classical rotational dynamics of HX-He complexes with X = F, Cl, Br, CN were studied to gain insight into quantum simulations and revealed highly chaotic dynamics for states with J > 0. Fractal Weyl law behavior in an open, chaotic Hamiltonian system is the subject of the final chapter.
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MacKenzie, Valerie Jane. "Photophysics and spectroscopy of tropolone and its van der Waals complexes." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape7/PQDD_0011/NQ37898.pdf.
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Perreault, John D. "Using Atom Optics to Measure van der Waals Atom-Surface Interactions." Diss., Tucson, Arizona : University of Arizona, 2005. http://etd.library.arizona.edu/etd/GetFileServlet?file=file:///data1/pdf/etd/azu%5Fetd%5F1317%5F1%5Fm.pdf&type=application/pdf.
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Thornley, Alice E. "Quantum mechanics of Van der Waals complexes : rare gas-hydrocarbon systems." Thesis, Durham University, 1994. http://etheses.dur.ac.uk/5139/.
Abstract:
The coupled channel approach has been used to study the Van der Waals complexes Ar-C(_2)H(_2), Ar-CH(_4) and Ar-C(_2)H(_4). The Ar-C(_2)H(_2) study employs a pairwise additive, atom-atom potential energy surface first without, and then with, angular anisotropy in some of the carbon atom parameters. The complex is found to be a nearly free internal rotor and the correlation between the complex and the acetylene monomer energy levels is clear. The Ar-CH(_4) study uses two potentials. The first includes the isotropic V(_o) dependence and the first angularly anisotropic V(_3) term. This is then modified to give a second potential which includes a V(_4) term as well. The role of these anisotropic terms in splitting the triply degenerate bending states of the complex, when the methane monomer is in the ground vibrational state, is discussed. The energy level pattern is found to be best described in terms of a hindered rotor model. For Ar-C(_2)H(_4) a pairwise additive, atom-atom potential including angular anisotropy in the carbon atom parameters is again used. The ethylene monomer in the complex is nearly free to rotate about the C-C axis but steric considerations make end-over-end rotation restricted. A method has been implemented to extract wavefunctions from coupled channel calculations. The utility of this technique has been illustrated by providing insights into the Ar-HF, He-CO(_2) and Ar-C(_2)H(_4) systems via calculated spectra and direct visualisation of the wavefunctions.