P. Feng

Contact-controlled sensing properties of flowerlike ZnO nanostructures

Contact-controlled sensing is realized from flowerlike ZnO structures composed of rods. The rods are around 150 nm in diameter and up to a few micrometers in length. When they are exposed to air, a depletion region formed below the surface of the rods arising from the adsorption of oxygen. Surface depletion does not greatly reduce either the carrier density or the mobility in the rods but significantly modify the potential barrier of the contacts between the rods. Both the large diameter and the long length result in a low sensor resistance down to 104kΩ in air at 300 °C. The exponential increase of the tunneling rate with the thickness and height of the contact barrier leads to a high sensitivity up to 14.6 for 100 ppm ethanol. These results indicate that contact-controlled sensing can be used to fabricate high-performance sensors with both high sensitivity and low resistance.

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.

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.

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.

Photocurrent characteristics of individual ZnGa2O4 nanowires

ZnGa2O4 nanowires were grown via a low-pressure chemical vapor deposition method, and the electrical transport properties of these nanowires were investigated. It was found that the current across individual nanowires was several picoamperes at a bias of 30V, and the current was insensitive to oxygen and temperature. These behaviors still maintained as the ZnGa2O4 nanowires were exposed to below-band-gap irradiation. In contrast, upon exposure to 254nm ultraviolet light, the current across the nanowire increased a lot. With decreasing oxygen pressure or increasing temperature, the photocurrent increased evidently; this could be understood from the Langmuir model and the adsorption isobar, respectively. The present results demonstrate that surface-related processes especially oxygen chemisorption have significant effects on the photoelectric properties of nanostructures. The optically driven oxygen and temperature sensing as found in the ZnGa2O4 nanowires may find promising applications in functional devices.

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.

Anomalous electrorheological behavior of ZnO nanowires

We observe an anomalous electrorheological (ER) behavior of suspensions composed of ZnO nanowires and silicone oil. In contrast to the usual ER behavior, a decrease in viscosity of the suspensions is observed. Such an anomalous behavior results from the migration of ZnO nanowires to the electrodes under a dc electric field. The migration leads to a relatively pure silicone oil zone between the electrodes, as confirmed by optical microscope observations. The occurrence of the electrophoresis is proposed as the origin of the decrease in shear stress.

Extremely high oxygen sensing of individual ZnSnO3 nanowires arising from grain boundary barrier modulation

Extremely high oxygen sensing is realized from individual ZnSnO3 nanowires with abundant grain boundaries. The current across one single ZnSnO3 nanowire increases by about six orders of magnitude, from 1.20×10−7to3.78×10−1μA, as the oxygen pressure decreases from 3.7×104to1.0×10−4Pa. Such a drastic sensing is ascribed to grain boundary barrier modulation. This interpretation is confirmed by the sensing experiments under UV illumination. The results demonstrate a promising approach to realize miniaturized and highly sensitive oxygen sensors.

Electrical transport through individual nanowires with transverse grain boundaries

V2O4∙0.25H2O nanowires are synthesized via hydrothermal route. The nanowires are of metastable phase, and transverse grain boundaries are observed in their microstructures. Transport through individual V2O4∙0.25H2O nanowires shows nonlinear current-voltage (I-V) characteristics in the bias range of −3to3V. The resistance rapidly decreases from 2.54to0.5MΩ as the bias is raised from 0to1V. Such behaviors can be attributed to the presence of the barrier at the transverse grain boundary. By analyzing the I-V curves at various temperatures, the effective barrier height is estimated to be about 0.13eV. Our results provide important information about how the microstructure mismatch affects the electrical properties.