GaN nanowires have been the subject of intense research lately, due to the many potential ultraviolet applications and
interesting properties that they possess. Because GaN has an anisotropic wurtzite crystal structure, many of its properties
are dependent upon crystal orientation. For example, the photoluminescence (PL) of GaN nanowires with growth
direction along the a-axis is blue-shifted relative to the PL of wires with growth direction along the c-axis. However, the
origin of the difference in PL between nanowire samples of different growth directions remains unclear. To determine if
surface states play a role in the dependence of GaN nanowire photoluminescence on crystal orientation, we use time-integrated
photoluminescence (TIPL) and time-resolved photoluminescence (TRPL) to study the PL from GaN nanowire
samples of different crystallographic orientations. We observe temporal dynamics of the blue-shifted PL feature in the a-axis
GaN nanowires that is suggestive of a surface trapping process occurring, where some fraction of electron-hole
pairs are prevented from recombining via the band edge emission process because carriers diffuse to the surface where
they are trapped before carrier relaxation to the band edge is complete. Once a carrier is trapped and localized at a
surface trap state, light emission primarily occurs only when the complementary carrier diffuses to the same surface trap.
We envision that a thin oxide layer forming at the surface introduces surface traps that cause the blue emission, and that
the surfaces of the a-axis GaN nanowires are more susceptible to this oxidation than the c-axis GaN nanowire surfaces.
Here, we report a direct synthesis approach for obtaining GaN nanowires with control on growth directions: <0001> or
c-direction, and <10-10> or a-direction, on amorphous substrates. The direct nitridation of Ga droplets using either
dissociated ammonia or N2/H2 plasma resulted in GaN nanowires with <0001> growth direction; and the vapor transport
of controlled (low) amounts of Ga flux in the presence of dissociated ammonia resulted in GaN nanowires with <10-10>
growth direction. In both cases, the resulting GaN nanowires have diameters as small as 20 nm and lengths exceeding
one hundred microns. Photoluminescence measurements showed that the bandgap of <10-10> wires blue-shifted by 50
meV from the wires with <0001> direction. Homo-epitaxial growth studies onto the pre-synthesized a-direction GaN
nanowires led to belt or ribbon shaped morphologies. Homo-epitaxial growth onto c-direction wires developed micro
hexagonal prism morphologies. The island growth morphologies observed on the hundred micron long, sub 30 nm size
nanowires suggest that the surface transport of adatoms on c-direction wires exhibit ballistic transport or "one-dimensional"
transport with mean distances over several tens of microns.
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 N2/O2 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.