One of the fastest growing fields in condensed matter physics is that of two-dimensional materials; compounds that promise to revolutionize nanotechnology due to the ability to easily isolate clean atomically thin sheets of conducting material for use in atomic-scale circuits. Since the initial demonstration of the electric-field effect in nanocircuits fabricated from mechanically exfoliated graphene, the number of available compounds that can be isolated and used in atomically thin circuits has exponentially grown to include diverse electrical properties from metals and insulators to superconductors and magnets. The bulk compounds from which flakes are isolated are known as van der Waals materials named for their intrinsic structural anisotropy resulting in weak van der Waals chemical bonds in one dimension. Since this field is relatively young, there are a multitude of branching opportunities for experimental advancement. In this work, we begin by addressing a significant technical challenge within the two-dimensional community; contacting and measuring air-sensitive two-dimensional materials. We developed a novel technique for embedding metal electrodes in atomically thin insulating flakes used to simultaneously contact and preserve a wide-array of air-sensitive two-dimensional materials. Using this technique, we proceed to explore the properties of a diverse set of van der Waals compounds in both three dimensions and two dimensions. We investigate the nature of superconductivity in the two-dimensional limit by quantifying the fragility of the superconducting state in a single atomic sheet of NbSe2. In combination with theoretical time-dependent Ginzburg-Landau simulations, we show that the dissipation in two-dimensional NbSe2 can be accurately described by vortex dynamics, including the poorly understood low-temperature metallic-like state. We examine how superconductors proximitize with normal metals through measurements on atomic-scale normal metal/insulator/superconductor tunnel junctions fabricated from van der Waals materials, demonstrating agreement with Blonder- Tinkham-Klapwijk theory. In addition, in junctions fabricated from graphene and NbN, a high-critical- field superconductor, we gain an understanding of Andreev processes in graphene under large magnetic fields. Finally, we provide a detailed characterization Re6Se8Cl2 and CrSBr, two new van der Waals compounds. In Re6Se8Cl2, we develop a novel strategy for doping in van der Waals compounds with labile ligands, demonstrating a semiconducting to superconducting transition upon electron doping. In CrSBr, we discover a well-developed semiconducting gap along with strong coupling between magnetic order and transport properties, unique among van der Waals magnets. Further, we find the semiconducting and magnetic properties persist down to 2 layers of CrSBr, with the observation of air-stability, establishing it as a promising material platform for increasing the applicability of van der Waals magnets.

Nonlinear frequency conversion provides essential tools for generating new colors and quantum states of light. Conventional nonlinear crystals have the problem of relative lower nonlinear susceptibilities, which result in the large footprint of devices and low efficient. Transition metal dichalcogenides possess huge nonlinear susceptibilities; further, 3R-stacked transition metal dichalcogenide crystals possess aligned layers with broken inversion symmetry, representing ideal candidates to boost the nonlinear optical gain with minimal footprint. Here we report the second-order nonlinear processes of 3R-MoS2 along the ordinary and extraordinary directions. Along the ordinary axis, by measuring the thickness-dependent second-harmonic generation, we present the first measurement of the second harmonic-generation coherence length of 3R-MoS2 and achieve record nonlinear optical enhancement from a van der Waals material, >104 stronger than a monolayer. It is found that 3R-MoS2 slabs exhibit similar conversion efficiencies of lithium niobate, but within 100-fold shorter propagation lengths. Furthermore, along the extraordinary axis, we achieve broadly tunable second-harmonic generation from 3R-MoS2 in a waveguide geometry, revealing the coherence length in such a structure. We characterize the full refractive-index spectrum and quantify its birefringence with near-field nanoimaging. In order to bring 3R-MoS2 into the application field, we have developed two fabrication methods: low-cost femtosecond laser etching and cleanroom nanolithography-based processes. The femtosecond laser writing setup offers a rapid, residue-free, and in-situ method for patterning grating structures. On the other hand, the cleanroom process can provide structures with higher resolution. The cleanroom fabrication process is based on SF6 RIE and E-beam lithography, which can narrow down the minimum linewidth to ~120nm. To achieve mode matching in waveguiding second-order nonlinear conversion, we utilized the mode dispersion relation calculated by an anisotropic model to find the overlapping of wavevectors among different photon energies. We proposed a molybdenum disulfide on silicon nitride structure (MOSS) to further unleash the potential of 3R-MoS2 in optical parametric conversion. Photonic structure optimization was performed using the Lumerical FDTD simulator, achieving a 90% coupling efficiency from SiN to 3R-MoS2 with a taper structure. With a taper length of 50μm, we successfully maintained a single mode of excitation wave in MoS2, which could provide a monotonoic mode source for nonlinear conversion. Our work highlights the potential of 3R-stacked transition metal dichalcogenides for integrated photonics, providing critical parameters, developing high-resolution fabrication processes, and offering initial designs for highly efficient on-chip nonlinear optical devices including periodically poled structures, optical parametric oscillators and amplifiers, and quantum circuits.

Nowadays, the assembly of small (<1mm) components has become an industrial reality. Many domains like MEMS, surgery, telecommunications, car industry, etc. now have large use of micro-parts. At this scale, predominant forces are different than in macroworld. The pieces often undergo adhesion problems. The adhesion forces can be splitted in different components :van der Waals, electrostatics and capillary condensation. This work focuses on capillary condensation as it often can be the major component of the adhesion force. <p><p>The first part of this work details a review of literature of different fields involved in capillary condensation. A simulation tool is then implemented and theoretically validated in the second part of the work. Finally, a test bed is presented; this bed is then used to experimentally validate the simulation results.<p><p>Experiments and simulation results are shown to concord. Therefore, the simulation tool can be used to model the force due to capillary condensation. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Published Article / This paper investigates the challenges of employing Van der Waals forces in micro-material handling since these forces are dominant in micro-material handling systems. The problems include the creation of a dust-free environment, accurate measurement of the micro-force, and the efficient picking and placing of micro-work pieces. The use of vacuum suction, micro-gripper's surface roughness, geometrical configuration and material type are presented as alternatives to overcome the challenges. An atomic force microscope is proposed for the accurate measurement of the Van der Waals force between the gripper and the micro-work piece.

Published Article / This paper explores the manipulation of Van der Waals' forces by geometrical parameters in a micro-material handling system. It was observed that the flat-flat interactive surfaces exerted the highest intensity of Van der Waals' forces followed by cone-flat, cylinder-flat, sphere-flat and sphere-sphere interactive surfaces, respectively. A conical micro-gripper proved to be versatile in manipulating the Van der Waals' forces efficiently in a 'picking up' and 'releasing' mechanism of micro-work parts. It was deduced that the pick-up position should be rough and spherical, and the placement position should be smooth and flat for an effective 'pick-and-place' cycle to be realised.

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.

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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Bibliography: leaves 140-145. / by Walter Peter Lapatovich. / Ph.D.

<p>This work presents first-principles calculations of long-range intermolecular dispersion energies between two atoms or molecules as expressed in terms of the C₆ dipole-dipole dispersion coefficients. In a series of publications, it has been shown by us that the complex linear polarization propagator method provides accurate <em>ab initio</em> and first-principles density functional theory values of the C₆ dispersion coefficients in comparison with those reported in the literature. The selected samples for the investigation of dispersion interactions in the electronic ground state are the noble gases, <em>n</em>-alkanes, polyacenes, azabenzenes, and C₆₀. It has been shown that the proposed method can also be used to determine dispersion energies for species in their respective excited electronic states. The C₆dispersion coefficients for the first <em>π</em> → <em>π*</em> excited state of the azabenzene molecules have been obtained with the adopted method in the multiconfiguration self-consistent field approximation. The dispersion energy of the <em>π</em> → <em>π*</em> excited state is smaller r than that of the ground state. It is found that the characteristic frequencies ω₁ defined in the London approximation of <em>n</em>-alkanes vary in a narrow range and that makes it possible to construct a simple structure-to-property relation based on the number of -bonds for the dispersion interaction in these saturated compounds. However, this simple approach is not applicable for the interactions of the <em>π</em>-conjugated systems since their characteristic frequencies <em>ω</em>₁ vary strongly depending on the systems.</p> / Report code: LIU-TEK-LIC-2006:2