Electron-Beam Induced Growth of Silica Nanowires and Silica/Carbon Heterostructures
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Electron-Beam Induced Growth of Silica Nanowires and Silica/Carbon Heterostructures Francisco Sol·, Oscar Resto, Azlin M Biaggi-Labiosa, and Luis F Fonseca Department of Physics, University of Puerto Rico, Rio Piedras Campus, San Juan, 00931, Puerto Rico ABSTRACT A novel synthesis of silica nanowires and silica/carbon heterostructures by electron beam irradiation on porous silicon films was investigated. The method allows us to monitor the growth process in real time at atomic scales. Depending on the electron dose we obtain nanowires with diameters in the range of 15-49 nm and lengths up to 500 nm. We found that the adequate electron dose was between 0.01 Acm-2 and 2 Acm-2. Additional electron dose causes plastic and failure deformations in the silica nanowires. A growth model consistent with our findings is presented that involves the flow of mass from the substrate to the nanowire driven by the local electric fields. Heterostructures showing a nanopalm-like shape are obtained after exposing the silica nanowire to poor vacuum conditions in which carbon aggregation from the surrounding gas is promoted by the local electric fields enhanced at the tip of the silica wires. INTRODUCTION Since the discovery of carbon nanotubes by Iijima [1], one dimensional nanomaterials such as nanotubes and nanorods have fascinated the scientific and technological community. Among these nanomaterials, silica nanorods have attracted a great deal of interest because of their possible applications in nanoscale optical devices. For example, Mazur and coworkers have demonstrated that silica nanorods can be used as waveguides by launching light into them using evanescent coupling [2] while Lou et al. proposed the use of these nanorods for nanosensor applications [3]. The synthesis of silica nanowires has been achieved by several techniques, such as laser ablation [4], chemical vapor deposition [5], and thermally activated gas phase reactions [6]. None of the previous techniques can monitor the growth of silica nanowires during synthesis. Here we present a novel method that allows us to follow the growth process of silica nanowires and silica/carbon heterostructures in real time with a high transmission electron microscope (HRTEM). The process consists of irradiating free standing porous silicon (PSi) films with an electron beam.
EXPERIMENT For the study we prepared several porous silicon (PSi) [7] free standing films with different thicknesses and synthesis parameters by electrochemical anodization of Si wafers (270 µm thick) with crystalline orientation at the polished surface side and dried them by using the critical point drying method [8]. Our preparation procedure is explained elsewhere [9]. Pieces of films of the order of 1 mm2 were attached to beryllium-copper and nickel TEM grids. The sample holder was of beryllium oxide. E-beam irradiations and observations were made in a JEOL 100S TEM at 100kV, LEO-922, FEI TECNAI F20, Carl Zeiss Libra 200 and Philips CM200 HRTEMs at 200KV. X-Ray Photoelectron Spectroscopy analyses
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