1,720,981 research outputs found
Gallium nitride nanostructures for light-emitting diode applications
No full textThis review summarizes recent research on GaN nanostructures for light-emitting diode (LED) applications. GaN nanostructure fabrication methods are first discussed, followed by a brief explanation of the basic components of the LED structure based on nitride nanostructures. Various device architectures of nanostructured GaN LEDs, as the main focus of the review, are then presented, covering research from the early LEDs based on a single GaN nanostructure to the most advanced LEDs based on GaN nanostructure arrays on flexible substrates. The research discussed in this review will promote novel applications of GaN LEDs that exploit the advantages of nanostructures. (C) 2012 Elsevier Ltd. All rights reserved.N
High-Resolution Observation of Nucleation and Growth Behavior of Nanomaterials Using a Graphene Template
No full textBy using graphene as an electron beam-transparent substrate for both nanomaterial growth and transmission electron microscopy (TEM) measurements, we investigate initial growth behavior of nanomaterials. The direct growth and imaging method using graphene facilitate atomic-resolution imaging of nanomaterials at the very early stage of growth. This enables the observation of the transition in crystal structure of ZnO nuclei and the formation of various defects during nanomaterial growth. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.N
Position- and Morphology-Controlled ZnO Nanostructures Grown on Graphene Layers
No full textPosition- and morphology-controlled ZnO nanostructures are grown on an oxygen plasma-treated selective area of graphene layers using metal-organic vapor-phase epitaxy. The structural and optical characteristics examined by electron microscopy, cathodoluminescence and photoluminescence techniques indicate that high-quality nanostructures are prepared on graphene layers. This approach to grow the controlled ZnO nanostructures selectively on graphene layers enables us to fabricate various nanodevices including GaN/ZnO coaxial nanotube LED microarrays.N
Nearly Perfect Polycrystalline, Large-Grained Silicon Arrays Formed at Low-Temperature Ambient by Local Pyrolysis
No full textWe report low-temperature ambient synthesis of high-quality, several micrometer thick polycrystalline silicon arrays on soda lime glass substrates by local pyrolysis, where SiH4 gas is locally decomposed near and condensed on the resistively heated microheater arrays with an average growth rate of 50 nm/s. The silicon arrays had nearly perfect crystallinity and a minimum grain size larger than 0.2 mu m, as determined by spatially resolved Raman spectroscopy and transmission electron microscopy. Boron-doped silicon arrays by the local pyrolysis was further fabricated to yield concentric p-i-n heterojunction arrays with rectifying current voltage or photovoltaic characteristics.N
Microstructural defects in GaN thin films grown on chemically vapor-deposited graphene layers
No full textMicrostructural defects in GaN thin films grown on graphene produced via chemical vapor deposition have been investigated using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). EBSD analysis reveals the preferred orientations of the GaN films. We further examined the microstructural defects such as grain boundaries and threading dislocations present in the films using TEM. Plan-view TEM analysis showed presence of both high-and low-angle grain boundaries and the threading dislocations mostly bound to those grain boundaries. Moreover, the characteristics and behavior of the threading dislocations were also investigated using cross-section TEM analysis. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790385
Heterointerface effects in the electrointercalation of van der Waals heterostructures
No full textMolecular-scale manipulation of electronic and ionic charge accumulation in materials is the backbone of electrochemical energy storagel(1-4). Layered van der Waals (vdW) crystals are a diverse family of materials into which mobile ions can electrochemically intercalate into the interlamellar gaps of the host atomic lattice(5,6). The structural diversity of such materials enables the interfacial properties of composites to be optimized to improve ion intercalation for energy storage and electronic devices(7-12). However, the ability of heterolayers to modify intercalation reactions, and their role at the atomic level, are yet to be elucidated. Here we demonstrate the electrointercalation of lithium at the level of individual atomic interfaces of dissimilar vdW layers. Electrochemical devices based on vdW heterostructures(13) of stacked hexagonal boron nitride, graphene and molybdenum dichalcogenide (MoX2;X=S, Se) layers are constructed. We use transmission electron microscopy, in situ magnetoresistance and optical spectroscopy techniques, as well as low-temperature quantum magneto-oscillation measurements and ab initio calculations, to resolve the intermediate stages of lithium intercalation at heterointerfaces. The formation of vdW heterointerfaces between graphene and MoX2 results in a more than tenfold greater accumulation of charge in MoX2 when compared to MoX2/MoX2 homointerfaces, while enforcing a more negative intercalation potential than that of bulk MoX2 by at least 0.5 V. Beyond energy storage, our combined experimental and computational methodology for manipulating and characterizing the electrochemical behaviour of layered systems opens new pathways to control the charge density in two-dimensional electronic and optoelectronic devices.N
Measuring the local twist angle and layer arrangement in Van der Waals Heterostructures
Full text linkThe properties of Van der Waals (VdW) heterostructures are determined by the twist angle and the interface between adjacent layers as well as their polytype and stacking. Here, the use of spectroscopic low energy electron microscopy (LEEM) and micro low energy electron diffraction (µLEED) methods to measure these properties locally is described. The authors present results on a MoS2/hBN heterostructure, but the methods are applicable to other materials. Diffraction spot analysis is used to assess the benefits of using hBN as a substrate. In addition, by making use of the broken rotational symmetry of the lattice, the cleaving history of the MoS2 flake is determined, that is, which layer stems from where in the bulk. Quantum Matter and Optic
Growth and characterizations of GaN micro-rods on graphene films for flexible light emitting diodes
No full textWe report the growth of GaN micro-rods and coaxial quantum-well heterostructures on graphene films, together with structural and optical characterization, for applications in flexible optical devices. Graphene films were grown on Cu foil by means of chemical vapor deposition, and used as the substrates for the growth of the GaN micro-rods, which were subsequently transferred onto SiO2/Si substrates. Highly Si-doped, n-type GaN micro-rods were grown on the graphene films using metal–organic chemical vapor deposition. The growth and vertical alignment of the GaN micro-rods, which is a critical factor for the fabrication of high-performance light-emitting diodes (LEDs), were characterized using electron microscopy and X-ray diffraction. The GaN micro-rods exhibited promising photoluminescence characteristics for optoelectronic device applications, including room-temperature stimulated emission. To fabricate flexible LEDs, InxGa1–xN/GaN multiple quantum wells and a p-type GaN layer were deposited coaxially on the GaN micro-rods, and transferred onto Ag-coated polymer substrates using lift-off. Ti/Au and Ni/Au metal layers were formed to provide electrical contacts to the n-type and p-type GaN regions, respectively. The micro-rod LEDs exhibited intense emission of visible light, even after transfer onto the flexible polymer substrate, and reliable operation was achieved following numerous cycles of mechanical deformation
Understanding luminescence properties of grain boundaries in GaN thin films and their atomistic origin
No full textWe report our findings on the optical properties of grain boundaries in GaN films grown on graphene layers and discuss their atomistic origin. We combine electron backscatter diffraction with cathodoluminescence to directly correlate the structural defects with their optical properties, enabling the high-precision local luminescence measurement of the grain boundaries in GaN films. To further understand the atomistic origin of the luminescence properties, we carefully probed atomic core structures of the grain boundaries by exploiting aberration-corrected scanning transmission electron microscopy. The atomic core structures of grain boundaries show different ordering behaviors compared with those observed previously in threading dislocations. Energetics of the grain boundary core structures and their correlation with electronic structures were studied by first principles calculation. Published by AIP Publishing
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