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We trace Sn nanoparticles (NPs) produced from SnO2 nanotubes (NTs) during lithiation initialized by high energy e-beam irradiation. The growth dynamics of Sn NPs is visualized in liquid electrolytes by graphene liquid cell transmission electron microscopy. The observation reveals that Sn NPs grow on the surface of SnO2 NTs via coalescence and the final shape of agglomerated NPs is governed by surface energy of the Sn NPs and the interfacial energy between Sn NPs and SnO2 NTs. Our result will likely benefit more rational material design of the ideal interface for facile ion insertion.
The electrical properties of Ni-based ohmic contacts N-face p-type GaN are presented. The specific contact resistance of N-face p-GaN exhibits a liner decrease from 1.01 × cm2 to 9.05 × 10-3 Ω cm2 for the as-deposited and the annealed Ni/Au contacts, respectively, with increasing annealing temperature Furthermore, the specific contact resistance could be decreased by four orders of magnitude to 1.03 × 10-4 Ω cm2 as a result of surface treatment using an alcohol-based (NH4)2S solution. The depth profile data measured by the intensity of O1s core peak in the x-ray photoemission spectra showed that the alcohol-based (NH4)2S treatment was effective in removing of the surface oxide layer of GaN. In addition, a Ga 2p core-level peak showed a red-shift of binding energy by 0.3 eV by alcohol-based (NH4)2S treatment, indicating that the surface Fermi level was shifted toward the valence-band edge. Thus, the low ohmic contact behavior observed in our treated sample might be explained in terms of the removal of the oxide layer and reducing the barrier heights by reduced band-bending effect.
Growth behavior and microstructure of oxide scale formed on MoSi2 coating by cyclic oxidation testing in air at 500 °C were investigated using field emission scanning electron microscopy, cross-sectional transmission electron microscopy, glancing angle x-ray diffraction, and x-ray photoelectron spectroscopy. MoSi2 coating was prepared by chemical vapor deposition of Si on a Mo substrate at 1100 °C for 5 h using SiCl4–H2 precursor gas mixtures. After the incubation period of about 454 cycles, accelerated oxidation behavior was observed in MoSi2 coating and the weight gain increased linearly with increasing oxidation cycles. Microstructural analyses revealed that pest oxide scale was formed in three sequential processes. Initially, nanometer-sized crystalline Mo4O11 particles were formed with an amorphous SiO2 matrix at MoSi2 interface region. Inward diffusing oxygen reacted with Mo4O11 to form Mo9O26 nano-sized particles. At final stage of oxidation, MoO3 was formed from Mo9O26 with oxygen and growth of MoO3 took place forming massive precipitates with irregular and wavy shapes. The internal stress caused by the growth of massive MoO3 precipitates and the volatilization of MoO3 was attributed to the formation of many lateral cracks into the matrix leading to pest oxidation of MoSi2 coating.
Surface acoustic wave (SAW) propagation properties of gallium nitride (GaN) epitaxial layers on sapphire were theoretically and experimentally characterized. GaN thin films were grown on a c-plane sapphire substrate using a metalorganic chemical vapor deposition system. The experimental characterization of SAW propagation properties was performed with a linear array of interdigital transducer structures, while SAW velocities were calculated by matrix methods. Experimentally, we found pseudo-SAW and high-velocity pseudo-SAW modes in the GaN/sapphire structure, which had a good agreement with calculated velocities.
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