Chapter 8 - Chemical and Biological Sensors

Abstract

Near the threshold voltage, the charge in the channel of a carbon nanotube field-effect transistor depends linearly on the gate voltage, but the dependence of the conductance is exponential. Thus, the conductance is extremely sensitive to the charge in the channel, and this is the basis for utilizing carbon nanotube field-effect transistors as ultrasensitive sensors. One of the early experiments, that supported the charge transfer model, consisted in measuring the resistance of networks of carbon nanotubes in vacuum and in oxygen-rich environments. Charge transfer effects can also arise when the work function of metal clusters on carbon nanotubes is modified by analytes. For example, it is well-known that hydrogen diffuses readily into palladium (Pd), which can lead to a change in its work function. Indeed, this mechanism forms the basis of hydrogen gas sensor technologies. It is possible to take advantage of this mechanism in carbon nanotube devices by decorating the nanotube surface with nanoparticles of palladium. This is accomplished by first assembling a carbon nanotube electronic device, followed by electron beam evaporation of Pd on the whole device, leading to Pd nanoparticles decorating the nanotube sidewalls.