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Nano Lett ; 11(10): 4200-6, 2011 Oct 12.
Artigo em Inglês | MEDLINE | ID: mdl-21877708


By the virtue of the nature of the vapor-liquid-solid (VLS) growth process in semiconductor nanowires (NWs) and their small size, the nucleation, propagation, and termination of stacking defects in NWs are dramatically different from that in thin films. We demonstrate germanium-silicon axial NW heterostructure growth by the VLS method with 100% composition modulation and use these structures as a platform to understand how defects in stacking sequence force the ledge nucleation site to be moved along or pinned at a single point on the triple-phase circumference, which in turn determines the NW morphology. Combining structural analysis and atomistic simulation of the nucleation and propagation of stacking defects, we explain these observations based on preferred nucleation sites during NW growth. The stacking defects are found to provide a fingerprint of the layer-by-layer growth process and reveal how the 19.5° kinking in semiconductor NWs observed at high Si growth rates results from a stacking-induced twin boundary formation at the NW edge. This study provides basic foundations for an atomic level understanding of crystalline and defective ledge nucleation and propagation during [111] oriented NW growth and improves understanding for control of fault nucleation and kinking in NWs.

ACS Nano ; 4(7): 3831-8, 2010 Jul 27.
Artigo em Inglês | MEDLINE | ID: mdl-20536187


Near-field scanning microwave microscopy is employed for quantitative imaging at 4 GHz of the local impedance for monolayer and few-layer graphene. The microwave response of graphene is found to be thickness dependent and determined by the local sheet resistance of the graphene flake. Calibration of the measurement system and knowledge of the probe geometry allows evaluation of the AC impedance for monolayer and few-layer graphene, which is found to be predominantly active. The use of localized evanescent electromagnetic field in our experiment provides a promising tool for investigations of plasma waves in graphene with wave numbers determined by the spatial spectrum of the near-field. By using near-field microwave microscopy one can perform simultaneous imaging of location, geometry, thickness, and distribution of electrical properties of graphene without a need for device fabrication.