Ming-Shien Hu

Selective-hydrogen sensing at room temperature with Pt-coated InN nanobelts

The hydrogen sensing characteristics of multiple InN nanobelts grown by metalorganic chemical vapor deposition were investigated. Pt-coated InN sensors could selectively detect hydrogen at the tens of ppm level at 25 °C, while uncoated InN showed no detectable change in current when exposed to hydrogen under the same conditions. Upon exposure to various concentrations of hydrogen (20–300 ppm) in N2 ambient, the relative resistance change increased from 1.2% at 20 ppm H2 to 4% at 300 ppm H2. Approximately 90% of the initial InN resistance was recovered within 2 min by exposing the nanobelts to air. Temperature-dependent measurements showed larger resistance change and faster response at high temperature compared to those at room temperature due to increase in catalytic dissociation rate of H2 as well as diffusion rate of atomic hydrogen into the Pt/InN interface. The Pt-coated InN nanobelt sensors were operated at low power levels (∼0.5 mW).

Surface optical Raman modes in InN nanostructures

Raman spectroscopic investigations are carried out on one-dimensional nanostructures of InN, such as nanowires and nanobelts synthesized by chemical vapor deposition. In addition to the optical phonons allowed by symmetry A1, E1, and E2 (high) modes, two additional Raman peaks are observed around 528 and 560 cm−1 for these nanostructures. Calculations for the frequencies of surface optical (SO) phonon modes in InN nanostructures yield values close to those of the new Raman modes. A possible reason for large intensities for SO modes in these nanostructures is also discussed.

Gold nanoparticle-modulated conductivity in gold peapodded silica nanowires.

We report the enhanced electrical conductivity properties of single gold-peapodded amorphous silica nanowires synthesized using microwave plasma enhanced chemical vapor deposition. Dark conductivity of the gold-peapodded silica nanowires can be adjusted by controlling the number of incorporated metal nanoparticles. The temperature-dependent conductivity measurement reveals that the band tail hopping mechanism dominates the electron transport in the gold-peapodded silica nanowires. The high conductivity in the nano-peapodded nanowires with more embedded gold-nanoparticles can be explained by the higher density of hopping states and shorter hopping distance. These Au-embedded amorphous silica nanowires have provided a new approach to enhance not only the electron conduction, but also the chemical-sensor response/sensitivity.

Pd-catalyzed hydrogen sensing with InN nanobelts

The use of Pd coatings on multiple InN nanobelts is shown to enhance their sensitivity for hydrogen sensing at hundreds of ppm level at 25°C. Without the metal coating to catalyze dissociation of the hydrogen molecules, the InN nanobelts with Ohmic contacts at either end showed no detectable change in current when exposed to hydrogen under the same conditions. Moreover, the Pd-coated InN showed no response to CO2, C2H6, NH3, and O2 (all in N2 ambient). The relative resistance change in the Pd-coated sensors was not linearly dependent on the hydrogen concentration at dilute levels, i.e., 8% at 100ppm H2 and 9.5% at 1000ppm H2. The recovery characteristics of the sensors at room temperature after hydrogen sensing were also examined and ∼50% of the initial InN resistance was recovered 10min after sensor exposure to air. At higher temperatures, larger resistance changes and faster response and recovery were obtained. Pd-coated InN nanobelt sensors displayed much higher relative response than Pt-coated sensors.

Anisotropic surface plasmon excitation in Au/silica nanowire

The surface plasmon (SP) excitations of gold/silica nanowire, investigated by electron energy-loss spectroscopy in conjunction with scanning transmission electron microscopy, are found to be anisotropic with stronger SP intensities observed along the transverse direction of the nanowire. This indicates that the charge carriers generated near the surface of the nanowires by the decay of SP resonance play a significant role to the enhanced photoconductivity. This conclusion is reaffirmed by the polarization dependent photoconductivity measurement.

Growth and luminescence properties of one-dimensional InN and InGaN nanostructures

Growth and luminescence properties of InN nanobelts (InNNBs) and InGaN nanowires (NWs) by MOCVD and thermal CVD will be presented, along with their relation and difference to thin film counterparts. While there is a growing acceptance of the low band gap (0.6-0.7 eV) of InN, the optical properties of the actual samples still suffered, presumably due to the difficulty in obtaining high-quality samples and/or controlling their defect and carrier concentrations. However, the free-standing nanobelts can be nearly defect-free, allowing an excellent opportunity for fundamental investigations on unique dimensionality. InNNBs show photoluminescence (PL) in IR with peak width of 14 meV, the sharpest reported to date for InN. Interestingly, with increasing excitation intensity, InNNBs reveal an anomalously large blueshift in PL, compared to thin films; along with a decrease in the phonon frequencies as evident by Raman measurements. Surface band bending, converse piezoelectric effect, and photoelastic effects are employed to explain these behaviors. As for InGaN NWs, both In-rich and Ga-rich ternary nanowires have been synthesized by simply varying growth temperature. Morphological and structural characterizations reveal a phase-separated microstructure wherein the isovalent heteroatoms are self-aggregated, forming self assembled quantum dots (SAQDs) embedded in NWs. The SAQDs are observed to dominate the emission behavior of both In-rich and Ga-rich nanowires, which has been explained by proposing a multi-level band schema.