RESUMEN
It is a well-known fact that a sphere offers less surface area, and thus less surface energy, than any other arrangement of the same volume. From this perspective, all other shapes are metastable objects. In this paper, we present and discuss a manifestation of this metastability: the spontaneous alignment of free-standing amorphous nanowires towards, and ultimately parallel to, a flux of directional ion irradiation. The behavior expected from surface energy reduction is the opposite of that predicted by both theory and experiment regarding defect generation in crystalline nanowires, but is consistent with other observations on non-crystalline materials. We verify our expectations by bending and aligning finely stranded amorphous silica nanowires, noting that such nanostructures are particularly susceptible to bending through ion-induced surface energy reduction. We offer support for this mechanism through bending rate studies, thermal annealing experiments and mathematical modeling. Experimentally, we also demonstrate selective reorientation of nanowires in patterned areas, as well as conformal coating of reoriented arrays with functional materials. These capabilities offer the prospect of exploiting engineered surface anisotropies in optical, fluidic and micromechanical applications.
RESUMEN
We evaluate the probe forming capability of a JEOL 2200FS transmission electron microscope equipped with a spherical aberration (Cs) probe corrector. The achievement of a real space sub-Angstrom (0.1 nm) probe for scanning transmission electron microscopy (STEM) imaging is demonstrated by acquisition and modeling of high-angle annular dark-field STEM images. We show that by optimizing the illumination system, large probe currents and large collection angles for electron energy loss spectroscopy (EELS) can be combined to yield EELS fine structure data spatially resolved to the atomic scale. We demonstrate the probe forming flexibility provided by the additional lenses in the probe corrector in several ways, including the formation of nanometer-sized parallel beams for nanoarea electron diffraction, and the formation of focused probes for convergent beam electron diffraction with a range of convergence angles. The different probes that can be formed using the probe corrected STEM opens up new applications for electron microscopy and diffraction.
RESUMEN
Vertically aligned silicon oxide nanowires can be synthesized over a large area by a low-temperature, ion-enhanced, reactive vapour-liquid-solid (VLS) method. Synthesis of these randomly ordered arrays begins with a thin indium film deposited on a Si or SiO(2) surface. At the processing temperature of 190 degrees C, the indium film becomes a self-organized seed layer of molten droplets, receiving atomic silicon from a DC magnetron sputtering source rather than from the gaseous precursors used in conventional VLS growth. Simultaneous vigorous ion bombardment aligns the objects vertically and expedites mixing of oxygen and silicon into the indium. Silicon oxide precipitates from each droplet in the form of multiple thin strands having diameters as small as 5 nm. These strands form a single loose bundle growing normal to the surface, eventually consolidating to form one nanowire. The vertical rate of growth can reach 300 nm min(-1) in an environment containing argon, hydrogen, and traces of water vapour. This paper discusses the physical and chemical factors leading to the formation of the nanostructures. It also demonstrates how the shape of the resulting nanostructures can be further controlled by sputtering, during both VLS growth and post-VLS processing. Key technological advantages of the developed process are nanowire growth at low substrate temperatures and the ability to form aligned nanostructure arrays, without the use of lithography or templates, on any substrate onto which a thin silicon film can be deposited.