A gas sensor that relies on a single nanowire for its extremely high sensitivity has been created by a team of scientists led by Zhong Lin Wang of the Georgia Institute of Technology in the US in collaboration with colleagues at the National Tsing-Hua and Tamkang universities in Taiwan.1

Fig. 1: The resistivity of a zinc-oxide nanowire held between platinum electrodes is extremely sensitive to gases such as carbon monoxide.

The team created the device using nanowires of zinc oxide positioned across a pair of platinum electrodes (Fig. 1). They used a focused beam of gallium ions to fix the two ends of the nanowire in place, forming an ohmic contact. Gallium changes the electrical properties of the nanowire, allowing current to flow with less resistance. When a gas molecule sticks to the surface of the nanowire, it changes the wire's ability to conduct electrons, and this change can be used to sense the presence of gasses such as carbon monoxide or oxygen.

In a different approach, the group fabricated a device with a sensitivity to carbon monoxide of more than 10,000 times that achieved by other devices. After fixing one end of the nanowire to the platinum electrode with gallium ions, they left the other end to make physical contact with the electrode without using the ion beam. This resulted in a ‘Schottky’ contact with a much higher resistance to electrical current.

The Schottky device is much more sensitive because it relies on gas molecules sticking to the contact area between the nanowire and the platinum electrode, where they have a much greater effect on the flow of current through the nanowire than in the ohmic device, making it a much more sensitive detector.

The team tested their Schottky device over a range of temperatures, and found that it was most sensitive at about 275 °C, detecting concentrations of carbon monoxide as low as 50 parts per million. Further development could improve the sensitivity further, says Wang. “We think that even a few molecules of gas can be detected using this concept of Schottky device.” The team now hopes to develop a device that could actually attract the gas, further increasing its sensitivity.