Detection of NO 2 down to ppb Levels Using Individual and Multiple In 2 O 3 Nanowire Devices

We demonstrate detection of NO 2 down to ppb levels using transistors based on both single and multiple In 2O3 nanowires operating at room temperature. This represents orders-of-magnitude improvement over previously reported metal oxide film or nanowire/nanobelt sensors. A comparison between the single and multiple nanowire sensors reveals that the latter have numerous advantages in terms of great reliability, high sensitivity, and simplicity in fabrication. Furthermore, selective detection of NO 2 can be readily achieved with multiple-nanowire sensors even with other common chemicals such as NH 3, O2, CO, and H2 around. Chemical sensing based on various nanostructures 1-11 has attracted enormous attention, as this is widely perceived as one of the most promising areas for nanotechnology to generate significant impact. Among the chemicals studied, NO2 is one of the most dangerous air pollutants, which plays a major role in the formation of ozone and acid rain. Continued or frequent exposure to NO 2 concentrations higher than the air quality standard (53 ppb) may cause increased incidence of acute respiratory illness in children. 12 The detection and measurement of NO 2 gas are thus of great importance in both environmental protection and human health. Solid-state sensors for NO 2 detection have been under development based on both conventional metal oxide thin films and one-dimensional nanostructures, however, with various limitations. For instance, most existing metal oxide thin-film sensors work at elevated temperatures with sensing limitations around 1 ppm or even higher. 13-17 Nanowires or nanobelts made of metal oxides can deliver better performance because of their enhanced surface-to-volume ratios, and SnO2 nanobelts have been demonstrated to detect 3 ppm NO2 at room temperature. 5 In addition, by operating SnO 2 nanobelt sensors at 400 °C, detection down to 0.5 ppm NO 2 has been successfully achieved. 7 These represent significant advance in the field of solid-state chemical sensing; however, the detection limit does not satisfy the requirement of 53 ppb NO2 set by the U.S. Environmental Control Agency. On the other hand, carbon nanotube mat samples have been reported to reliably detect NO 2 down to 1 ppb or less, with the aid of functional polymer coating to enhance the sticking coefficient.3 In this letter we report the detection of NO 2 down to ppb levels for the first time with metal oxide nanowire transistors. Two types of devices have been studied based on both single and multiple In2O3 nanowires operating at room temperature and without the help of functional polymer coating. This represents orders-of-magnitude improvement over previously reported metal oxide film or nanowire/nanobelt sensors, and rivals the performance of the best nanotube chemical sensors. While single In2O3 nanowire devices exhibited strong gate dependence and nice transistor behavior, sensors based on multiple In2O3 nanowires displayed numerous advantages in terms of great reliability, high sensitivity down to 5 ppb, and simplicity in fabrication. Furthermore, selective detection of NO2 among other common chemicals has been observed with multiple-nanowire sensors without the extra step of polymer coating. These novel devices are a proven new generation of NO2 sensors that could replace the conventional devices, and even surpass the carbon nanotube (CNT) NO 2 sensors 2,3 in terms of their great simplicity in device fabrication. In2O3 nanowires were synthesized by a laser ablation method, and details can be found in our previous publication.18 This method typically produced single-crystalline In2O3 nanowires with diameters ∼10 nm and lengths ∼5 μm. These nanowires are wide band gap semiconductors with bixbyte crystal structure, as shown in Figure 1a. The conductance of In 2O3 nanowires is directly related to the carrier concentration inside, which in turn can be altered by * Corresponding author. E-mail: chongwuz@usc.edu. NANO LETTERS 2004 Vol. 4, No. 1