Bidimensional versus tridimensional oxygen vacancy diffusion in SnO(2-x) under different gas environments.

Metal oxides present oxygen defects that induce different chemical and physical properties. Experiments performed in SnO(2-x) sensors show that the dynamics of these vacancies are strongly affected by the presence of different gases in the environment. Experimentally, the electrical resistance of individual metal oxide SnO(2-x) nanowires shows modulation: when the environment is oxygen rich long term drifts (hours) are observed indicating extended vacancy dynamics. Instead, if CO is present, drifts disappear in minutes. Density functional theory indicates that changes in resistance follow the extension of reoxidation. For oxygen-poor environments, oxygen vacancy excorporation and healing are confined to the near-surface layer of SnO(2-x) (bidimensional or near-surface diffusion), and completed in short times. Under oxygen-rich conditions, tridimensional diffusion of oxygen vacancies towards the surface takes place at room temperature. In this case, a push-pull mechanism allows bulk-to-surface diffusion and as a consequence resistance drifts are longer and the vacancy quenching is more extensive.

The effects of electron-hole separation on the photoconductivity of individual metal oxide nanowires.

The responses of individual ZnO nanowires to UV light demonstrate that the persistent photoconductivity (PPC) state is directly related to the electron-hole separation near the surface. Our results demonstrate that the electrical transport in these nanomaterials is influenced by the surface in two different ways. On the one hand, the effective mobility and the density of free carriers are determined by recombination mechanisms assisted by the oxidizing molecules in air. This phenomenon can also be blocked by surface passivation. On the other hand, the surface built-in potential separates the photogenerated electron-hole pairs and accumulates holes at the surface. After illumination, the charge separation makes the electron-hole recombination difficult and originates PPC. This effect is quickly reverted after increasing either the probing current (self-heating by Joule dissipation) or the oxygen content in air (favouring the surface recombination mechanisms). The model for PPC in individual nanowires presented here illustrates the intrinsic potential of metal oxide nanowires to develop optoelectronic devices or optochemical sensors with better and new performances.

Portable microsensors based on individual SnO2 nanowires.

Individual SnO(2) nanowires were integrated in suspended micromembrane-based bottom-up devices. Electrical contacts between the nanowires and the electrodes were achieved with the help of electron- and ion-beam-assisted direct-write nanolithography processes. The stability of these nanomaterials was evaluated as function of time and applied current, showing that stable and reliable devices were obtained. Furthermore, the possibility of modulating their temperature using the integrated microheater placed in the membrane was also demonstrated, enabling these devices to be used in gas sensing procedures. We present a methodology and general strategy for the fabrication and characterization of portable and reliable nanowire-based devices.

Water vapor detection with individual tin oxide nanowires.

Individual tin oxide nanowires (NWs), contacted to platinum electrodes using focused ion beam assisted nanolithography, were used for detecting water vapor (1500-32 000 ppm) in different gaseous environments. Responses obtained in synthetic air (SA) and nitrogen atmospheres suggested differences in the sensing mechanism, which were related to changes in surface density of the adsorbed oxygen species in the two cases. A model describing the different behaviors has been proposed together with comparative evaluation of NW responses against sensors based on bulk tin oxide. The results obtained on ten individual devices (tested >6 times) revealed the interfering effect of water in the detection of carbon monoxide and illustrated the intrinsic potential of nanowire-based devices as humidity sensors. Investigations were made on sensitivity, recovery time and device stability as well as surface-humidity interactions. This is the first step towards fundamental understanding of single-crystalline one-dimensional (1D) tin oxide nanostructures for sensor applications, which could lead to integration in real devices.

Template synthesis and forming electrical contacts to single Au nanowires by focused ion beam techniques.

Metallic Au nanowires were electrochemically synthesized in 20 µm thick ion track etched polycarbonate membranes with the nominal pore diameter of 200 nm. Scanning and transmission electron microscopy analysis and x-ray diffraction of samples revealed that the nanowires are dense with a fcc [Formula: see text] texturing. The I-V characteristics of a single Au nanowire were investigated using a four-point microprobe set-up. The Au nanowire was placed in electrical contact with electrodes patterned on planar substrates using a dual-beam focused ion beam technique. The resistivity of the Au nanowires was found to be 2.8 × 10(-4) Ω cm.