ABSTRACT
Cantilever and beam structures have been widely used as sensing devices due to their distinct advantages which have been reviewed in literature.1-3 Several transduction methods can be applied to cantilever and beam type devices making them usable for wide variety of applications. Some of the commonly used transduction techniques are (i) frequency change due to additional mass loading, (ii) bending of bimetallic cantilever for temperature sensing, (iii) stress sensing by measuring the cantilever bending due to stress applied on one side, and (iv) chemical sensing by change in resistivity of the cantilever. The evolution of cantilever and beam structures from macro-to micromachined structures and now nanowire (NW) and other nanostructure-based devices have
made them further attractive for sensing applications. Nanoscale cantilever and beam devices are widely used for sensing applica-tions because of large surface area, miniature size, low power con-sumption, and nanomoles-per-mole (ppb) sensitivity.4 This chapter focuses on chemiresistive-type NW and hybrid nanoparticle (NP)/NW devices that have been recently demonstrated.5,6In a chemiresistor-type NW sensor, which is fabricated as a two-terminal electrical device, the sensitivity and selectivity depend on the interaction of different chemical analytes with the NW surface. Constrained by the surface properties of the NW material, most NW sensors can detect only specific type of analytes. In order to make a nanosensor array for a wide range of analytes, there is a need to tune the device sensitivity and selectivity toward different chemicals. In the hybrid NP/NW devices discussed in this chapter, GaN NWs with relatively inactive surface properties (i.e., with no chemiresistive sensitivity to different classes of organic vapors) have been used and have been functionalized with analyte-dependent active metal-oxides. Photoconductive metal-oxide-semiconductors are attractive as a functionalizing material due to their active surface properties and possible. 12.1 Metal Oxides for Gas SensingMetal oxides have long been used for gas sensing. An extensive review of various metal oxides used for sensing different gases is available in literature.7-9 Semiconducting metal oxides thin films are used for chemical sensing7,10 because their transport properties readily respond to the adsorbed analytes. SnO2 is one of the most widely used semiconducting metal oxide for gas sensing followed by ZnO.9 Gas-sensing properties of SnO2 and ZnO have been widely studied.11,12 Nowadays, nanowires and nanostructures of these semiconducting metal oxides are being heavily used for sensing applications. Recently, nanowires and other nanostructures of these materials have been shown to exhibit even better sensing characteristics due to increased surface area, reduction in size and power consumption, and nmol/mol ppb sensitivity.13,14 For this reason, there has been vast research in the growth and properties of nanostructures of ZnO15 and SnO2 along with other metal oxides for the fabrication of nano-noses16 and sensor arrays.
