Chung-Jen Chien

High-performance ZnO nanowire field effect transistors

ZnO nanowires with high crystalline and optical properties are characterized, showing strong effect of the surface defect states. In order to optimize the performance of devices based on these nanowires, a series of complementary metal-oxide semiconductor compatible surface passivation procedures is employed. Electrical transport measurements demonstrate significantly reduced subthreshold swing, high on/off ratio, and unprecedented field effect mobility.

Finite size effect in ZnO nanowires

To clarify the size effect in semiconductor nanowires with decreasing diameters but not yet reaching the quantum confinement region, single crystalline zinc oxide nanowires with diameters around 10nm are synthesized. Electrical transport measurements of these thin nanowires show significant increase in conductivity accompanied by diminished gate modulation and reduced mobility. This phenomenon is a result of the enrichment of surface states owing to the increased surface-to-volume ratio. The enhanced surface effect is confirmed by the temperature dependent photoluminescence measurements and contributes to the “anomalous” blueshift. This study shows that surface states play a dominant role in the electrical and optical properties of quasi-one-dimensional materials.

Electrical and photoconductive properties of vertical ZnO nanowires in high density arrays

High density vertical zinc oxide nanowire arrays were fabricated using highly ordered channels in anodic alumina membranes via chemical vapor deposition assisted by electrochemical deposition methods. Using conductive atomic force microscopy, the electrical transport and photoconduction of individual vertical nanowires were investigated. A negative photoconductivity was observed as a result of electron trapping in the alumina membrane. In contrast, positive photoconductivity was observed using a thermally annealed anodic alumina membrane as the nanowire growth template. These studies provide a pathway for constructing highly integrated nanoscale electronic and optoelectronic circuits, such as logic circuits, light emitting diodes, solar cells, and ultrahigh resolution imaging sensors.

Optical size effects in ultrathin ZnO nanowires

Single crystal ZnO wurtzite nanowires grown along the c-axis with diameters down to 4 nm were synthesized by a catalytic vapor transport technique. Photoluminescence spectra of these wires indicate a blue shift of the free exciton by 19 meV due to confinement. This result was obtained by analyzing the line shape of the blue-shifted LO phonon replica of the free exciton. In addition, a surface-related excitonic luminescence feature centered at 3.366 eV was observed with a strongly elevated thermal activation energy.

Core–shell CdTe–TiO2 nanostructured solar cell

A prototype core–shell nanostructured solar cell with a p–n CdTe–TiO2 radial junction is constructed by pulse potential electrodeposition of CdTe into a TiO2 nanotube array. 1 : 1 stoichiometry of CdTe is achieved with a high filling rate. Measurements are done to characterize the photovoltaic properties of the as-fabricated solar cell with CdTe nanowire array as the absorber layer.

Flexible Symmetric Supercapacitors Based on TiO

Highly conducting carbon nanotube has been widely studied as electrode in double-layer-effect supercapacitors. In this paper, we introduce a hybrid-electrode design to further improve the specific capacitance by incorporating TiO2 nanotubes with carbon nanotubes. Vertically oriented TiO2 nanotube arrays are fabricated by anodization process and used as porous pseudocapacitive electrode with compact density and large surface area. Conductive multiwall carbon nanotube networks are coated on the TiO2 nanotubes to form the hybrid electrode. With 1M H2SO4 as electrolyte, the specific capacitance can be enhanced by about 30% as compared to pure carbon-nanotube electrode. Electrochemical impedance spectroscopy shows good capacitive behavior for the device. This hybrid-electrode technique demonstrates also a promising application for flexible energy devices and electronics.