We have investigated the growth of ZnO nanowires on curved BaTiO3 retroreflector beads,
as well as growth of ZnO nanowires on flat substrates. Results indicate that the growth of ZnO
aligned nanowire arrays occurs farther away from the Zn source in the retroreflectors, while the
results are opposite for the flat Si substrates. In the case of the ZnO nanowires on flat Si, the
nanowires formed in nearly aligned arrays are short and significantly thicker, suggesting that the
growth occurs both longitudinally and laterally in this process, which is not the case for the growth
on the retroreflector beads. The SERS response of the nanowire arrays on the retroreflectors has
been compared to random nanowires on flat Si substrates, and results show that the signal strength
is 29 times greater in the case of the wires grown on the retroreflectors. Since one would only
expect a factor of 4 enhancement due to the light reflecting properties of the retroreflector, it is
believed that the enhancement in the SERS signal is due to light channeling by the aligned nanowire
arrays.
We demonstrated a fast and easy way to synthesize Ag nanoparticles (NPs) on ZnO
nanowires (NWs) and silicon substrates by an electroless (EL) plating approach. ZnO NWs
used here were grown via vapor-solid (VS) mechanism at 560 °C for 30 min. The stability to
oxidation of these EL-produced homogeneous Ag NPs on ZnO nanowires was investigated by
surface enhanced Raman spectroscopy (SERS), showing that the attachment of thiol to the Ag
surface can slow down the oxidation process, and the SERS signal remains strong for more
than ten days. Furthermore, we examined the surface oxidation kinetics of the Ag NPs as a
function of NPs size and size distribution by monitoring the oxygen amount in the composites
using energy dispersive x-ray (EDX). Results indicate that the EL plated Ag NPs show faster
oxidation rates than those produced by e-beam (EB) evaporation in air. We attribute this to
the fact that the EL produced silver particles are very small, in the 20 nm range, and thus have
high surface energy, thus enhancing the oxidation. These studies provide extensive
information related to the Ag NP oxidation rates, which can help in extending the Ag lifetime
for various applications.
We have investigated the formation of silicon nanowire arrays by the use of a simple
chemical etching approach. The etching characteristics of silicon nanowire arrays using wafers
with diverse doping levels and several orientations have been examined. Furthermore, the
etching solution, etching time and temperature were also considered in order to optimize the
etching conditions to produce thinner and more orderly silicon nanowire arrays in registry with
the substrate. Since this process takes advantage of a silver catalyst, we have also investigated
various ways of forming the initial silver catalyst on the silicon surfaces, and we show that
electroless Ag deposition, as well as e-beam thin film deposition of Ag, results in successful,
highly aligned and ordered Si nanowire arrays after the etching step. In addition, we have also
performed Surfaced Enhanced Raman Scattering (SERS) measurements on the nanowire arrays
and on nanowires removed from the substrate.
There has been significant interest in a variety of nanowire (NW) systems for various sensing
applications. We had developed highly sensitive dielectric core/metal sheath nanowires composites
which serve as surface-enhanced Raman scattering (SERS) substrates. Previously, our composites
were fabricated using e-beam deposition, which has the problem of incomplete coverage. Here we
report an electroless (EL) plating approach to cover the NWs with a silver sheath, producing the
core/metal NW structures for the SERS measurements. In comparison with the common silver
deposition via e-beam evaporation, electroless coating can result in the full metal coverage on NWs.
Therefore, this approach provides a way to fully cover nanostructures with Ag, including NWs arrays,
regardless of the orientations and shapes of the nanostructures. SERS measurements on EL produced
Ag/NWs composites show stronger signals than those produced by e-beam evaporation. Electric field
calculations suggest that the strong SERS signal is due to plasmonic coupling of neighboring closely
spaced islands.
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