The sensing ability of individual SnO(2) nanowires and nanobelts configured as gas sensors was measured before and after functionalization with Pd catalyst particles. In situ deposition of Pd in the same reaction chamber in which the sensing measurements were carried out ensured that the observed modification in behavior was due to the Pd functionalization rather than the variation in properties from one nanowire to another. Changes in the conductance in the early stages of metal deposition (i.e., before metal percolation) indicated that the Pd nanoparticles on the nanowire surface created Schottky barrier-type junctions resulting in the formation of electron depletion regions within the nanowire, constricting the effective conduction channel and reducing the conductance. Pd-functionalized nanostructures exhibited a dramatic improvement in sensitivity toward oxygen and hydrogen due to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion of the resultant atomic species to the oxide surface.

na powder (P172SB, PØchiney) with a specific area of 10.2 m 2 g ±1 , a refractive index of 1.70, and a mean diameter of 0.5 lm. A dispersant (phosphoric ester) is also used to decrease the viscosity and to increase the stability of the suspension. The final mixture is prepared with 80 wt.-% (50 vol.-%) of alumina, dispersant (1.5 wt.-% with respect to alumina) and photoinitiator (5.56 wt.-% with respect to the monomer) mixed in a ball mill for 20 min at 350 rpm.The scraper is based on a 10 mm long scalpel. It is moved at a speed of 1.2 mm s ±1, which corresponds to a shear rate varying from 60 to 24 s ±1 , depending on the layer thickness (from 20 to 50 lm, respectively).The debinding step consists of several heating steps. First, the sample is heated to 120 C (degradation temperature of the polymer) at 60 C h ±1. A slower second temperature gradient of 6 C h ±1 is applied up to 500 C and the temperature is maintained for 30 min. The quick heating of the sintering step is applied by increasing the temperature at 900 C h ±1 to 1550 C. The object to be sintered is kept at this temperature for 5 h, and is then cooled to ambient temperature. Detection of CO and O 2 Using Tin Oxide Nanowire Sensors** By Andrei Kolmakov, Youxiang Zhang, Guosheng Cheng, and Martin Moskovits* Solid-state gas sensors play a major role in semiconductor processing, medical diagnosis, environmental sensing, personal safety, and national security, with economic impact in agriculture, medicine, and in the automotive and aerospace industries.[1±3] Most sensors operate on the basis of the modification of the electrical properties of an active element, normally a metal oxide film, brought about by the adsorption of an analyte on the surface of the sensor. A current major goal in gas sensing is massive parallelism, wherein many sensors, each with its unique chemical properties, are operated together and their outputs processed simultaneously, perhaps using learning strategies like neural networks, so that the overall device operates ªintelligentlyº, mimicking, for example, the complex operation of a mammalian nose and its corresponding brain function. [4] This task implies the need for further miniaturization of the active elements with the simultaneous sensitivity increase to compensate for surface area loss. Here, conventional thin film technology faces its fundamental limits. Promising strategies for achieving the above goal of using many sensing elements restricted to a small volume will likely come out of nanoscience and technology and, specifically, out of a subset of technologies amenable to parallelism and array fabrication that do not sacrifice sensitivity and selectivity. This challenge necessitates several design features to be achieved simultaneously, including the development of new materials, innovation in structure and architecture, and the development of highly sensitive, responsive, and selective, yet ultra-small active elements arranged so as to minimize inter-element cross-talk. [5,6] The high surface-to-volume ratio of nano...

Metal-oxide nanowires can function as sensitive and selective chemical or biological sensors, which could potentially be massively multiplexed in devices of small size. The active nanowire sensor element in such devices can be configured either as resistors whose conductance is altered by charge-transfer processes occurring at their surfaces or as field-effect transistors whose properties can be controlled by applying an appropriate potential onto its gate. Functionalizing the surface of these entities offers yet another avenue for expanding their sensing capability. In turn, because charge exchange between an adsorbate and the nanowire can change the electron density in the nanowire, modifying the nanowire's carrier density by external means, such as applying a potential to the gate, could modify its surface chemical properties and perhaps change the rate and selectivity of catalytic processes occurring at its surface. Although research on the use of metal-oxide nanowires as sensors is still in early stages, several encouraging experiments have been reported that are interesting in their own right and indicative of a promising future.

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Using temperature driven sharp metal-insulator phase transition in single crystal VO(2) nanowires, the realization of a novel gas sensing concept has been tested. Varying the temperature of the nanowire close to the transition edge, the conductance of the nanowire becomes extremely responsive to the tiny changes in molecular composition, pressure, and temperature of the ambient gas environment. This gas sensing analog of the transition edge sensor radiometry used in astrophysics opens new opportunities in gas sensorics.