Browsing by Subject "bandgap"
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Item AVS talk: Wide Bandgap Semiconducting graphene by nitrogen seeding(2013-11-14) Cohen, PhilipItem Contamination Induced Continuous-Wave Laser Damage of Optical Elements(2018-06) Brown, AndrewContinuous-Wave laser-induced optical breakdown affects anyone whose work requires tightly focused light, high power sources, or delicate materials. It often occurs unexpectedly and seemingly randomly at optical intensities far lower than those predicted by ultra-short pulse laser experiments. Further complicating the issue is that the majority of laser damage experiments use carefully controlled laboratory conditions with short-pulsed lasers focused to small spots on clean, pristine materials. Continuous-Wave laser damage is usually attributed to contamination, and occurs under radically different conditions. To determine the origin of contamination-induced breakdown, microparticle contaminated optics were stressed using a 17 kW continuous-wave laser. Contamination-induced breakdown occurred at intensity levels many orders of magnitude lower than expected in clean, pristine materials. For both half-wave and high reflectivity coatings, damage thresholds were found to strongly follow the bandgap energy of the film. It is theorized that surface contamination heated by the laser thermally generates free carriers in the films. If the free carrier concentration exceeds a certain threshold, runaway absorption and breakdown will occur. A thermal model incorporating the particle absorption, interfacial heat transfer, and free carrier absorption was developed, and it explains the observed data. The bandgap of the film, the absorption and thermal contact of the contaminant, and the evaporation time of the particle, all determine whether a material can survive. The observed bandgap dependence is in direct contrast to the behavior observed for clean samples under continuous wave and long-pulse illumination, and, unexpectedly, has similarities to ultra-short pulse breakdown for clean samples, albeit with a substantially different physical mechanism. These findings strongly suggest that low bandgap materials are a liability in optics exposed to environmental contamination. Laser conditioning was examined as a means of preventing damage by removing contamination without initiating damage. Absorption measurements taken using photo thermal common-path interferometry show up to a 90% absorption reduction with conditioned samples. Regular laser conditioning at low irradiances can prolong the life of optics that must operate in difficult environmental conditions.Item Nitrogen modification of Epitaxial Graphene formed on SiC(2013-11-14) Conrad, E. H.Item Wave Propagation in Periodic, Configurable Kerfed Metamaterials(2021-08) Widstrand, CalebKerfing, also known as relief cutting, is a subtractive cutting process that enables planar structures to undergo dramatic deformation in the presence of static loads. Starting from flat and rigid sheets, different kinds of kerfing patterns can be used to induce a wide variety of unconventional free-form shapes, making the process especially appealing for architectural applications. Many kerfed structures feature repetitive units, making them inherently periodic structures. Observing this, we investigate the bandgap behavior of certain meandering-type kerf patterns via Bloch analysis, finite element simulations and laser vibrometry experiments. This investigation reveals the existence of phononic bandgaps in the band structure of the kerfed cells. Leveraging the extreme deformability of these kerfed structures, we test the robustness of the bandgap landscape against drastic changes in global shape and we consider the possibility to use the large deformability as a tuning mechanism. Additionally, we briefly explore opportunities for utilizing different densities of cuts in joint finite assemblies in order to leverage bandgap behavior of each cell type. Finally, we touch on some of the other possibilities of kerfed structures as a metamaterial platform for wave control as inspiration for future works.Item Widegap semiconducting graphene from nitrogen seeded SiC(arXiv, 2013-06-18) Wang, F; Liu, G; Rothwell, S; Nevius, M; Tajeda, A; Taleb-Ibrahimi, A; Feldman, L C; Cohen, P I; Conrad, E HAll carbon electronics based on graphene has been an elusive goal. For more than a decade, the inability to produce significant band-gaps in this material has prevented the development of semiconducting graphene. While chemical functionalization was thought to be a route to semiconducting graphene, disorder in the chemical adsorbates, leading to low mobilities, have proved to be a hurdle in its production. We demonstrate a new approach to produce semiconducting graphene that uses a small concentration of covalently bonded surface nitrogen, not as a means to functionalize graphene, but instead as a way to constrain and bend graphene. We demonstrate that a submonolayer concentration of nitrogen on SiC is sufficient to pin epitaxial graphene to the SiC interface as it grows, causing the graphene to buckle. The resulting 3-dimensional modulation of the graphene opens a band-gap greater than 0.7eV in the otherwise continuous metallic graphene sheet.