Abstract
In this work, nitric oxide (NO) gas sensors with ultrahigh sensitivities based on single ZnO nanowires (NWs) are reported. The sensing principle is because NO is an oxidizing gas and the adsorption of gas molecules on the NW surface causes the NW conductance to decrease. An individual sensor is composed of a single ZnO NW with a diameter of around 300 nm in contact with Ti electrodes of 100, 300 or 500 nm thick created by photolithography. The fabricated devices have been utilized to sense NO gas in a concentration range between 0.1 and 2 ppm at room temperature. For the sensors with electrodes of 100 and 300 nm thick, ultrahigh sensitivities over 104 are achieved for 2 ppm NO sensing. From the I−V curves, it is found the contact is Ohmic type prior to exposure to NO and turns to Schottky type afterwards. For the sensor with electrodes of 500 nm thick, the sensitivity is around 102 and much lower than the previous values. In addition, the contact remains Ohmic type after NO sensing. The results indicate that the contact characteristic change at NW-electrodes interface is the dominant mechanism for the ultrahigh sensitivity of gas sensing and it can be modified by the electrode thickness. The results show the response time decreases as the NO concentration increases due to faster adsorption of NO gas under higher concentration. The response time is defined as time needed for the current to become 10 percent of the initial current. In addition, under the same concentration of 0.1 ppm, the response time of the sensors with 100, 300 and 500 nm thick electrodes are 331, 183 and 26 s, which means the response is faster as the electrode thickness increases. Besides, the adsorption and desorption rate constants of Ohmic type device can be calculated to derive the carrier concentration of ZnO NW. Therefore, we can fabricate a NO gas sensor with high sensitivity or fast response by changing the thickness of electrodes, and it has the advantages of room-temperature functionality and repeatability.