Mar 5, 2009
Annealing refreshes nanowire sensors
Tin oxide has long been used as the basis for chemical sensors because its conductance is very sensitive to the surrounding environment. Tin oxide nanowires offer a route to improved device performance thanks to their large surface to area ratio and research is underway in several groups to improve the quality of both the material and the resulting next-generation devices.
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.
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.
About the author
The work was performed by the Nanoengineering Research Group at the University of Illinois at Chicago in collaboration with the Centre for Nanotechnology, NASA Ames Research Centre. Ayan Kar is a graduate student at UIC working towards his PhD thesis on oxide nanowires. Jianyong Yang defended his PhD thesis in 2008. Dr Mitra Dutta is a UIC distinguished professor and the head of Electrical and Computer Engineering with an appointment in physics. Dr Michael A Stroscio is the Richard and Loan Hill professor of engineering holding appointments in bioengineering, electrical and computer engineering and physics. Jyoti Kumari is a masters student interning at Ames and Dr M Meyyappan is the director of the Centre of Nanotechnology at Ames Research Centre.