T. H. Wang

Individual β-Ga2O3 nanowires as solar-blind photodetectors

Individual β-Ga2O3 nanowires as solar-blind photodetectors are investigated. The detectors show encouraging advantages to 254nm light. The dark current is on the order of pA. The conductance of the nanowire increases by about three orders of magnitude under 254nm ultraviolet illumination. The upper limits of the response and recovery time are 0.22 and 0.09s, respectively. These results indicate that β-Ga2O3 nanowires have potential applications in realizing future miniaturized solar-blind photodetectors.

Surface accumulation conduction controlled sensing characteristic of p-type CuO nanorods induced by oxygen adsorption

P-type CuO nanorods were synthesized by a hydrothermal method and the ethanol-sensing properties of sensors based on CuO were investigated. The sensor resistance increased when it was exposed to ethanol and decreased in the air, which is contrary to the case for sensors realized from n-type semiconductor. The resistance of the CuO-based sensor was about 2 kΩ in air and 6 kΩ in ethanol vapour with concentration of 2000 ppm. Such a sensing property is attributed to surface accumulation conduction. Sensors based on CuO nanorods have potential applications in detecting ethanol in low concentration.

Fast humidity sensors based on CeO2 nanowires

Fast humidity sensors are reported that are based on CeO2 nanowires synthesized by a hydrothermal method. Both the response and recovery time are about 3 s, and are independent of the humidity. The sensitivity increases gradually as the humidity increases, and is up to 85 at 97% RH. The resistance decreases exponentially with increasing humidity, implying ion-type conductivity as the humidity sensing mechanism. A model based on the morphology and surface energy of the nanowires is given to explain these results further. Our experimental results indicate a pathway to improving the performance of humidity sensors.

Achieving fast oxygen response in individual β-Ga2O3 nanowires by ultraviolet illumination

The authors report a route to realize very quick oxygen response and demonstrate it by using individual β-Ga2O3 nanowires. The current across the nanowire is at a low level and varies slightly with changing the oxygen pressure. In contrast, under 254nm ultraviolet illumination, the current increases rapidly to a value that reflects the level of the oxygen pressure around the nanowire. The illumination gives rise to the oxygen sensing. This optically driven oxygen sensing is the origin of the fast response. The results demonstrate a promising approach to realize fast-response gas sensors.

Bandgap narrowing and ethanol sensing properties of In-doped ZnO nanowires

Indium doping effects on the optical and electrical properties of ZnO nanowires are investigated. The abnormal Raman spectrum shows only a peak centred at 439 cm−1 related to high E2 mode, which is due to In doping. The acceptor binding energy is estimated to be 93 meV from the results of temperature-dependent photoluminescence spectra. The redshift of the bandgap edge is attributed to a merging of donor and conduction bands. The sensitivity of the sensors fabricated from In-doped ZnO nanowires is about 3–1 ppm ethanol, and increases nearly linearly up to 27 as the ethanol concentration is raised to 100 ppm. Our results indicate that the In-doped ZnO nanowires have potential applications in fabricating optoelectrical devices and gas sensors.

Room-temperature oxygen sensitivity of ZnS nanobelts

Room-temperature oxygen sensing is realized from individual ZnS nanobelts. Under UV illumination the current through ZnS nanobelt increases from 0.265to2.26nA as the oxygen pressure decreases from 1×105to3×10−3Pa. The conductance of ZnS nanobelt exhibits a logarithmic dependence on oxygen pressure, which is in agreement with theoretical prediction. The sensing is based on the enhanced modulation of ZnS nanobelts conductance by adsorbed oxygen under illumination. These results demonstrate an approach to in situ precisely detect oxygen at room temperature.

Extremely stable field emission from AlZnO nanowire arrays

Extremely stable electric field emission from well-aligned AlZnO nanowire arrays is realized. The emission current density is up to 6.5mA∕cm2, and no current saturation is observed. The turn-on field is 2.9V∕μm as d (distance between the nanowire emitters and anode) is 0.64mm. After aging for two days, the emission current is extremely stable with the fluctuations of±0.4%. The high stability arises from the high crystal quality with few surface states of the nanowires and the in situ fabrication of cathodes. The field emission behaviors are in excellent agreement with Fowler–Nordheim theory, and the relationship between the field enhancement factor β and d follows a universal equation. Our results imply that AlZnO nanowire arrays are promising candidates for field emission displays.

Ultralow threshold field emission from ZnO nanorod arrays grown on ZnO film at low temperature

ZnO nanorod arrays have been synthesized on silicon substrate covered with ZnO film by thermal evaporation of zinc particles at a low temperature of 550 °C. Their field emission has been investigated: the turn-on electric field (at the current density of 1 µA cm−2) is about 3.8 V µm−1, and the threshold electric field (at the current density of 1 mA cm−2) is 6.3 V µm−1 at the working distance of 100 µm. In comparison, the turn-on and threshold electric fields of the not well-aligned ZnO nanorod arrays and ZnO film are 9.8, 15.8 V µm−1 and 13.7, 26.0 V µm−1 at 100 µm, respectively. These behaviors indicate that such an ultralow threshold field emission is attributed to the aligned structure, the good electric contact with the conducting substrate where they grow, and weaker field-screening effect. Our results demonstrate that well-aligned nanorod arrays with excellent field-emission performance grown at such a low temperature can provide the possibility of application in glass-sealed flat panel displays.

Anomalous photoconductivity of CeO2 nanowires in air

The conductance of the CeO2 nanowire film is found to decrease by about two orders of magnitude in air under ultraviolet illumination. Such a drastic decrease in conductance is attributed to light-induced desorption of H2O from the nanowire’s surface. When exposed in air, the surface conductivity of the nanowire increases significantly due to the adsorption of H2O. Considering the large surface-to-volume ratio of the nanowire, the conductance of the nanowire film is mainly controlled by surface conduction. Upon ultraviolet illumination, desorption of H2O results in the decrease of the conductance of the nanowire film, thus leading to the anomalous photoconductivity.

Electronic transport characteristics through individual ZnSnO3 nanowires

Composite ZnSnO3 nanowires are synthesized via a one-step thermal evaporation method. The nanowires are of core-shell structures with the presence of grain boundary and out-of-phase boundaries. Transport through individual nanowires shows nonlinear current-voltage (I-V) characteristics in the range of the voltage from −3to3V. Such a behavior can be attributed to the presence of the barrier at the grain boundary, and the effective barrier height is estimated to be about 0.22eV by analyzing the I-V curves at various temperatures. The current at −3V jumps from 0.12to6.0μA within 30s at 300K as exposed to UV illumination. Such jump can be well explained in terms of effective barrier height and depletion width.