Anisotropic Dielectric Breakdown Strength of Single Crystal Hexagonal Boron Nitride

Dielectric breakdown has historically been of great interest from the perspectives of fundamental physics and electrical reliability. However, to date, the anisotropy in the dielectric breakdown has not been discussed. Here, we report an anisotropic dielectric breakdown strength (EBD) for h-BN, which is used as an ideal substrate for two-dimensional (2D) material devices. Under a well-controlled relative humidity, EBD values in the directions both normal and parallel to the c axis (EBD+c & EBD//c) were measured to be 3 and 12 MV/cm, respectively. When the crystal structure is changed from sp3 of cubic-BN (c-BN) to sp2 of h-BN, EBD+c for h-BN becomes smaller than that for c-BN, while EBD//c for h-BN drastically increases. Therefore, h-BN can possess a relatively high EBD concentrated only in the direction parallel to the c axis by conceding a weak bonding direction in the highly anisotropic crystal structure. This explains why the EBD//c for h-BN is higher than that for diamond. Moreover, the presented EBD value obtained from the high quality bulk h-BN crystal can be regarded as the standard for qualifying the crystallinity of h-BN layers grown via chemical vapor deposition for future electronic applications

Charge transport and mobility engineering in two-dimensional transition metal chalcogenide semiconductors

Two-dimensional (2D) van der Waals semiconductors represent the thinnest, air stable semiconducting materials known. Their unique optical, electronic and mechanical properties hold great potential for harnessing them as key components in novel applications for electronics and optoelectronics. However, the charge transport behavior in 2D semiconductors is more susceptible to external surroundings (e.g. gaseous adsorbates from air and trapped charges in substrates) and their electronic performance is generally lower than corresponding bulk materials due to the fact that surface and bulk coincide. In this article, we review recent progress on the charge transport properties and carrier mobility engineering of 2D transition metal chalcogenides, with a particular focus on the markedly high dependence of carrier mobility on thickness. We unveil the origin of this unique thickness dependence and elaborate the devised strategies to master it for carrier mobility optimization. Specifically, physical and chemical methods towards the optimization of the major factors influencing the extrinsic transport such as electrode/semiconductor contacts, interfacial Coulomb impurities and atomic defects are discussed. In particular, the use of \textit{ad-hoc} molecules makes it possible to engineer the interface with the dielectric and heal the vacancies in such materials. By casting fresh light onto the theoretical and experimental works, we provide a guide for improving the electronic performance of the 2D semiconductors, with the ultimate goal of achieving technologically viable atomically thin (opto)electronics.Comment: 33 pages, 19 figures and 6 table

A Composite BRDF Model for Hazy Gloss

International audienceWe introduce a bidirectional reflectance distribution function (BRDF) model for the rendering of materials that exhibit hazy reflections, whereby the specular reflections appear to be flanked by a surrounding halo. The focus of this work is on artistic control and ease of implementation for real-time and off-line rendering. We propose relying on a composite material based on a pair of arbitrary BRDF models; however, instead of controlling their physical parameters, we expose perceptual parameters inspired by visual experiments [VBF17]. Our main contribution then consists in a mapping from perceptual to physical parameters that ensures the resulting composite BRDF is valid in terms of reciprocity, positivity and energy conservation. The immediate benefit of our approach is to provide direct artistic control over both the intensity and extent of the haze effect, which is not only necessary for editing purposes, but also essential to vary haziness spatially over an object surface. Our solution is also simple to implement as it requires no new importance sampling strategy and relies on existing BRDF models. Such a simplicity is key to approximating the method for the editing of hazy gloss in real-time and for compositing

Gradient Optics of subwavelength nanofilms

Propagation and tunneling of light through subwavelength photonic barriers, formed by dielectric layers with continuous spatial variations of dielectric susceptibility across the film are considered. Effects of giant heterogeneity-induced non-local dispersion, both normal and anomalous, are examined by means of a series of exact analytical solutions of Maxwell equations for gradient media. Generalized Fresnel formulae, visualizing a profound influence of gradient and curvature of dielectric susceptibility profiles on reflectance/transmittance of periodical photonic heterostructures are presented. Depending on the cutoff frequency of the barrier, governed by technologically managed spatial profile of its refractive index, propagation or tunneling of light through these barriers are examined. Nonattenuative transfer of EM energy by evanescent waves, tunneling through dielectric gradient barriers, characterized by real values of refractive index, decreasing in the depth of medium, is shown. Scaling of the obtained results for different spectral ranges of visible, IR and THz waves is illustrated. Potential of gradient optical structures for design of miniaturized filters, polarizers and frequency-selective interfaces of subwavelength thickness is considered

BxDF material acquisition, representation, and rendering for VR and design

Photorealistic and physically-based rendering of real-world environments with high fidelity materials is important to a range of applications, including special effects, architectural modelling, cultural heritage, computer games, automotive design, and virtual reality (VR). Our perception of the world depends on lighting and surface material characteristics, which determine how the light is reflected, scattered, and absorbed. In order to reproduce appearance, we must therefore understand all the ways objects interact with light, and the acquisition and representation of materials has thus been an important part of computer graphics from early days. Nevertheless, no material model nor acquisition setup is without limitations in terms of the variety of materials represented, and different approaches vary widely in terms of compatibility and ease of use. In this course, we describe the state of the art in material appearance acquisition and modelling, ranging from mathematical BSDFs to data-driven capture and representation of anisotropic materials, and volumetric/thread models for patterned fabrics. We further address the problem of material appearance constancy across different rendering platforms. We present two case studies in architectural and interior design. The first study demonstrates Yulio, a new platform for the creation, delivery, and visualization of acquired material models and reverse engineered cloth models in immersive VR experiences. The second study shows an end-to-end process of capture and data-driven BSDF representation using the physically-based Radiance system for lighting simulation and rendering