Y. X. Liang

Oxygen sensing characteristics of individual ZnO nanowire transistors

Individual ZnO nanowire transistors are fabricated, and their sensing properties are investigated. The transistors show a carrier density of 2300μm−1 and mobility up to 6.4cm2∕Vs, which are obtained from the ISD−VG curves. The threshold voltage shifts in the positive direction and the source-drain current decreases as ambient oxygen concentration increases. However, the opposite occurs when the transistors are under illumination. Surface adsorbates on the ZnO nanowires affect both the mobility and the carrier density. Our data are helpful in understanding the sensing mechanism of the gas sensors.

Electronic transport through individual ZnO nanowires

Electronic transport through individual ZnO nanowires has been investigated. The current increases linearly with the bias and the conductance jumps upon ultraviolet illumination. The increase rate upon the illumination is much faster than the decrease rate as the light is off. The decrease rate under vacuum is slower than that in air. These phenomena are related to the surface oxygen species and further confirmed by in situ current–voltage measurements as a function of oxygen pressure at room temperature. Also, the conductance increases greatly as the temperature is raised. These results demonstrate that the surface oxygen species dominate the transport process through individual ZnO nanowires, which indicates their potential application to room temperature gas sensors.

Low-resistance gas sensors fabricated from multiwalled carbon nanotubes coated with a thin tin oxide layer

Gas sensors have been fabricated from multiwalled carbon nanotubes (MWNTs) coated with a thin tin oxide layer, and have been used to detect oxidizing and reducing gases down to a ppm level. The barriers between the tin oxide nanocrystal grains on the MWNTs dominate the sensor resistance in different gases, and the conducting carriers in the MWNTs have a low resistance, which make the resistance of the sensors much lower than that of SnO2 nanobelt sensors. The resistance is 130kΩ in air, 230kΩ in 2ppm NO2, and 2.8MΩ in 50ppm NO2, so that impedance matching with amplifying circuits can be easily achieved.

Field-emission from long SnO2 nanobelt arrays

We report on field emission from SnO2 nanobelt arrays with the length of about 90 μm grown on silicon substrates. The turn-on field of the nanobelt arrays at the current density of 1μA∕cm2, is 4.5, 3.0, 2.4, and 2.3V∕μm as the distance between anode and cathode (d) is 0.1, 0.2, 0.35, and 0.5 mm, respectively. The current density rapidly reaches 2.1mA∕cm2 at the electrical field of 4.4V∕μm at d=0.35mm. The current density is higher than or comparable to those of the carbon nanotubes and other one-dimensional nanostructured materials. We also discuss the mechanism of high current densities and estimate the enhancement factor according to both the Fowler–Nordheim law and the reported model on micrometer-long of carbon nanotubes.

Nonlinear characteristics of the Fowler–Nordheim plot for field emission from In2O3 nanowires grown on InAs substrate

Nonlinear characteristics of the Fowler–Nordheim (F–N) plot for field emission from In2O3 nanowires (NWs) is investigated. The field emission from the aligned and nonaligned In2O3 NWs are measured and a stable emission with fluctuations less than 10% was obtained for the aligned In2O3 NWs. It is found that the nonaligned In2O3 NWs with a longer length have higher turn-on and threshold electric fields. Their F–N plots, showing pronounced nonlinear characteristics, are divided into several regions based on physical origins. Field penetration competes with surface states in influencing the field emission of the In2O3 NWs with increasing the electric field, which is responsible for the nonlinear characteristics of the F–N plots.

Electric-field-aligned vertical growth and field emission properties of In2O3 nanowires

Vertically aligned In2O3 nanowires are grown on InAs substrates by an electric field in the plasma sheath. The In2O3 nanowires are single crystalline with diameters less than 10nm. Field emission results show that the aligned In2O3 nanowires have lower turn-on and threshold electric fields than nonaligned ones. This is discussed in terms of orientation, emitter shapes, and density of the In2O3 nanowires.

Current saturation in multiwalled carbon nanotubes by large bias

Transport properties of a single multiwalled carbon nanotube (MWNT) have been investigated in vacuum at room temperature. The MWNTs show the large current carrying capacity at large bias due to the ballistic transport. The conductance for a single MWNT around zero bias is 0.4G0, and increases almost linearly with the applied voltage until it reaches its acmes. Being the signature of the ballistic transport for MWNTs, the conductance acmes are observed at the bias of ±5.8 V equal to ±2γ0/e, where γ0 is the π bonding energy for carbon nanotubes. Our calculation shows a similar curve to our experimental results, which further indicates the ballistic transport through the single MWNT.

Applied field Mossbauer study of shape anisotropy in Fe nanowire arrays

In order to investigate the magnetic anisotropy of Fe nanowire arrays from microscopic point of view, Fe57 Mossbauer spectra were measured at various magnetic fields along and perpendicular to the nanowire axis, respectively. On the basis of the absorption intensities of the second and the fifth lines of sextet, the orientation of Fe magnetic moments can be detected. It was found that the shape anisotropy dominates the overall magnetic anisotropy in the Fe nanowires. Furthermore, the longitudinal and transverse demagnetizing fields of Fe nanowire, respectively, were deduced from the effective hyperfine field at Fe57 nuclei as a function of applied field. The chain-of-spheres model in conjunction with symmetric fanning mechanism was adopted to interpret the domain structure and the parallel coercivity of magnetic nanowire arrays.

In2O3 nanowires grown from Au/In film on glass

Two kinds of In2O3 nanowires (NWs), i.e., straight and tapered ones, are grown from 20-nm-thick Au∕300-nm-thick In and 20-nm-thick Au∕1-μm-thick In films on glass at 400°C by a one-step annealing method, respectively. All the NWs are single crystalline. The growth of the NWs is initiated by Au catalyst particles via vapor-liquid-solid growth mechanism, and an additional side growth might be responsible for the nonuniform diameters of the tapered NWs. It is revealed that a certain content of oxygen in the In film facilitates the large-scale growth of the In2O3 NWs. The field-emission studies show that the In2O3 NWs on glass have a low turn-on electric field of about 4.3V∕μm. Our growth method has potential applications in the in situ fabrication and integration of the In2O3 NWs-based devices, especially with glass as substrates.