Inorganic nanowires are expected to play a central role in the re-engineering of products with applications in composites, thin films, nanodispersions, energy conversion devices, sensors, nanoelectronic devices and optics. The synthesis of materials at the nanoscale might also help in the discovery of new phases with interesting properties. However, the synthesis strategies for inorganic nanowires is quite limited and have not reached the level of maturity needed for either bulk manufacturing or for controlling nanowire characteristics such as sub 10 nm diameters and different growth directions. In this regard, we report several synthesis strategies that potentially offer in-situ control over the resulting nanowire characteristics such as size, growth direction and an ability to form two-dimensional networks. The techniques described here could be scaled up easily for bulk production of various nanostructures. Our preliminary results suggest that the nanowires form stable dispersions in both organic and aqueous solvents compared to nanoparticles of the same material.
We present the synthesis of two novel morphologies for carbon tubular structures: Nanopipettes and Micropipes. The synthesis procedures for both these structures are both unique and different from each other and the conventional methods used for carbon nanotubes.
Carbon nanopipettes, open at both ends, are made up of a central nanotube (~10-20 nm) surrounded by helical sheets of graphite. Thus nanopipettes have an outer conical structure, with a base size of about a micron, that narrows down to about 10-20 nm at the tip. Due to their unique morphology, the outer walls of the nanopipettes continuously expose edge planes of graphite, giving a very stable and reversible electrochemical response for detecting neurological compounds such as dopamine. The synthesis of carbon nanopipettes is based on high temperature nucleation and growth of carbon nanotubes under conditions of hydrogen etching during growth.
Carbon micropipes, on the other hand, are tubular structures whose internal diameters range from a few nanometers to a few microns with a constant wall thickness of 10-20 nm. In addition to tuning the internal diameters, the conical angles of these structures could also be changed during synthesis. Due to their larger inner diameters and thin walls, both the straight and conical micro-tubular structures are suitable for microfluidic devices such as throttle valves, micro-reactors, and distribution channels. The synthesis of carbon micro-tubular structures is based on the wetting behavior of gallium with carbon during growth. The contact angle between gallium and the carbon wall determines the conical angle of the structure. By varying the contact angle, one can alter the conical angles from 400 to -150, and synthesize straight tubes using different N<sub>2</sub>/O<sub>2</sub> dosing compositions. An 'n-step' dosing sequence at various stages of growth resulted in 'n-staged' morphologies for carbon micro-tubular structures such as funnels, tube-on-cone, Y-junctions and dumbbells.