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.
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
Finite elements calculations have been performed of the surface enhanced Raman (SERS) activity of Ag coated
dielectric nanowires. It is shown that the SERS fields and the angle of the peak field from intersecting nanowires can be
changed through the angle of the nanowires. In addition, it is shown that the strength of the SERS enhancement and its
spatial profile depend on whether the nanowires are in free space or on a substrate. Experimental data for benzene thiol
on dielectric coated nanowires is shown to support the calculations. These results demonstrate the importance of
geometry and local environment on electric field hot spots in the SERS process.
We have recently shown that dielectric/metal composite nanowires can exhibit very strong surface
enhanced Raman (SERS) signals, when arranged in a random 3D geometry. Since we believe that
the intersections of nanowires are critical in generating the high electric fields necessary for this
enhancement, we are investigating this effect under more controlled conditions. Thus, we will
discuss the formation of nanowire arrays by in-situ growth, achieved by the control of nanowire
material/substrate combination, as well as ex-situ nanowire array formation involving e-beam
lithography. The effects of nanowire geometry and the resulting SERS behavior show the
importance of the dielectric/metal configuration, as well as the importance of nanowire geometry in
the SERS effect.
The origins of the surface-enhanced Raman (SERS) effect have been widely studied and are generally accepted as
understood. However, there is still a need to provide a satisfactorily complete model which addresses the well known
phenomena in which some molecules exhibit weak or even no SERS response at all. The relative intensities of
vibrational modes observed in SERS can depend strongly on the mechanism of surface adsorption, such as bond type
(covalent/noncovalent) and number of covalent bonds. Thus, in this experimental study the role of surface adsorption in
surface-enhanced Raman scattering is investigated. A simple group of Benzene thiols was chosen to facilitate
comparison with theoretical models. Experimental results with consideration towards surface bond strengths and
reduction in degrees of freedom due to single and multiple surface bonds is presented and their effect on the relative
intensities and positions of observed vibrational modes observed in the SERS spectra are discussed. The relative
stability of molecules in the presence of nanostructures exhibiting strong and weak local electric fields will also be
presented and discussed.
Self-assembled GaN quantum dots are characterized using Raman techniques. The electrical and optical properties of these GaN quantum dots are modeled in light of optoelectronic applications. Strain-induced changes in the phononic properties of these nanostructures are modeled and the strain-induced frequency shifts are compared with Raman measurements. Acoustic phonons in colloidal GaN quantum dots are modeled using a quantized elastic continuum model. Shifts observed in the Raman signatures for different excitation wavelengths provide evidence the Raman signatures of GaN quantum dots are observed.
A variety of colloidal semiconductor quantum dots and related quantum-wire structures are characterized using absorption and photoluminescence measurements. The electronic properties of these structures are modeled and compared with experiment. The characterization and application of ensembles of colloidal quantum dots with molecular interconnects are considered. The chemically-directed assembly of ensembles of colloidal quantum dots with biomolecular interconnects is demonstrated with quantum dot densities in excess of 10<sup>+17 </sup>cm<sup>-3</sup>. Non-charge transfer processes for switching based on dipole-dipole interactions - Forester interactions - are examined for colloidal quantum dots linked with biomolecules. Charge transport in biomolecules is studied using indirect-bandgap colloidal nanocrystals linked with biomolecules.