One approach that is being employed by scientists based at the University of Illinois at Chicago and at NASA Ames Research Center is to use a simple rapid thermal annealing technique in the presence of oxygen to refresh nanowire-based designs.

In their paper, published recently in Nanotechnology, the team explains why it is necessary to perform this step and points the way towards future improvements.

Raman, PL and XPS data obtained by the group, demonstrate that the crystalline quality of the nanowires improves as annealing duration is increased, reaching a near-perfect stoichiometry after a two-hour period. These changes have implications in device applications as vacancy concentrations and chemical structure affect electrical and optical properties. The operation of thin-film and nanowire tin-oxide-based chemical sensors depends on a change in conductance of the sensor when exposed to an analyte. Typically, under ambient conditions, these sensors have oxygen-derived adsorbates such as O, O2 and O2– on the tin oxide surface. These deplete the electrons near the surface, leading to a lower conductance state; during sensor operation, when the surface layer is exposed to a reducing gas environment, the conductance increases as electrons are released back to the conduction band from the reducing surface reactions. Annealing of the oxide under an oxygen or vacuum environment alters the oxygen vacancies as seen here and can be used effectively as needed to refresh the sensor between exposures.

Optical option

The same group of researchers has recently obtained ultraviolet emission from these tin oxide nanowires for the first time. The development points towards the use of a simple optical technique to detect the analyte rather than recording changes in the conductance of the nanowire-based device.