Environmental Gas and Light Sensing Using ZnO Nanowires

In this paper, vertical arrays of zinc oxide (ZnO) nanowires (NWs) are synthesized by a chemical vapor deposition system and then deposited on patterned electrodes using dielectrophoresis. The NW devices illustrate four orders of magnitude increase in conductivity when exposed to ultraviolet (UV) irradiation of 1220 μW/cm². UV response has a fast component, due to electron-hole generation, as well as a slower component, attributed to the release of oxygen species from the surface of NW. Moreover, due to the increased electron density in the presence of UV, the type of oxygen species on the surface of ZnO changes to more reactive negative ions. In addition, when the pressure is decreased to 0.05 mBar, the conductivity of the NWs increases ~3.5 times due to the removal of oxygen molecules from the surface. For the first time, UV irradiation is used to improve the carbon monoxide (CO) sensing properties of ZnO NWs. When exposed to 250 μW/cm² UV irradiation, not only the sensitivity increases more than 75%, but also a repeatable and recoverable response is obtained, which is due to formation of more reactive oxygen ions. For the same reason, when the temperature is elevated, higher sensitivity to CO is achieved. The devices demonstrate exponential sensitivities of more than four decades when the relative humidity (RH) increases to 86% at room temperature.