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Computing Optical Properties of Ultra-thin Crystals

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 Added by Cihan Bacaksiz
 Publication date 2016
  fields Physics
and research's language is English




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An overview is given of recent advances in experimental and theoretical understanding of optical properties of ultra-thin crystal structures (graphene, phosphorene, silicene, MoS2, MoSe2 , WS2 , WSe2 , h-AlN, h-BN, fluorographene, graphane). Ultra-thin crystals are atomically-thick layered crystals that have unique properties which differ from their 3D counterpart. Because of the difficulties in the synthesis of few-atom-thick crystal structures, which are thought to be the main building blocks of future nanotechnology, reliable theoretical predictions of their electronic, vibrational and optical properties are of great importance. Recent studies revealed the reliable predictive power of existing theoretical approaches based on density functional theory (DFT).



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180 - Martha Lasia , Luis Brey 2014
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We report low-temperature measurements of current-voltage characteristics for highly conductive Nb/Al-AlOx-Nb junctions with thicknesses of the Al interlayer ranging from 40 to 150 nm and ultra-thin barriers formed by diffusive oxidation of the Al surface. In the superconducting state these devices have revealed a strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents across the devices, we conclude that the strong suppression of the subgap resistance comparing with conventional tunnel junctions originates from a universal bimodal distribution of transparencies across the Al-oxide barrier proposed earlier by Schep and Bauer. We suggest a simple physical explanation of its source in the nanometer-thick oxide films relating it to strong local barrier-height fluctuations which are generated by oxygen vacancies in thin aluminum oxide tunnel barriers formed by thermal oxidation.
Control over the electronic spectrum at low energy is at the heart of the functioning of modern advanced electronics: high electron mobility transistors, semiconductor and Capasso terahertz lasers, and many others. Most of those devices rely on the meticulous engineering of the size quantization of electrons in quantum wells. This avenue, however, hasnt been explored in the case of 2D materials. Here we transfer this concept onto the van der Waals heterostructures which utilize few-layers films of InSe as quantum wells. The precise control over the energy of the subbands and their uniformity guarantees extremely high quality of the electronic transport in such systems. Using novel tunnelling and light emitting devices, for the first time we reveal the full subbands structure by studying resonance features in the tunnelling current, photoabsorption and light emission. In the future, these systems will allow development of elementary blocks for atomically thin infrared and THz light sources based on intersubband optical transitions in few-layer films of van der Waals materials.
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158 - Lida Ansari , Farzan Gity , 2016
Structures and electronic properties of rhombohedral [111] and [110] bismuth nanowires are calculated with the use of density functional theory. The formation of an energy band gap from quantum confinement is studied and to improve estimates for the band gap the GW approximation is applied. The [111] oriented nanowires require surface bonds to be chemically saturated to avoid formation of metallic surface states whereas the surface of the [110] nanowires do not support metallic surface states. It is found that the onset of quantum confinement in the surface passivated [111] nanowires occurs at larger critical dimensions than for the [110] nanowires. For the [111] oriented nanowires it is predicted that a band gap of approximately 0.5 eV can be formed at a diameter of approximately 6 nm, whereas for the [110] oriented nanowires a diameter of approximately 3 nm is required to achieve a similar band gap energy. The GW correction is also applied to estimates of the electron affinity, ionisation potentials and work functions for both orientations of the nanowires for various diameters below 5 nm. The magnitude of the energy band gaps that arise in bismuth at critical dimensions of a few nanometers are of the same order as for conventional bulk semiconductors.
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