2,4,6‐Trinitrotoluene (TNT) Chemical Sensing Based on Aligned Single‐Walled Carbon Nanotubes and ZnO Nanowires

2010 WILEY-VCH Verlag Gmb Chemical sensors based on one-dimensional (1D) nanostructures have attracted a great deal of attention because of their exquisite sensitivity and fast response to the surrounding environment. In addition, both carbon nanotubes and metal oxide nanowires are promising candidates for building an electronic nose (e-nose) system. Among these materials, semiconductor single-walled carbon nanotubes (SWNTs) are molecular-scale wires composed entirely of surface atoms, which should be ideal for the direct electrical detection and are expected to exhibit excellent sensitivity to surrounding chemical and biological species. Kong et al. initially utilized SWNT field-effect transistors (FETs) to detect nitrogen dioxide (NO2) and ammonia (NH3), and demonstrated a detection limit of 2 ppm for NO2 and 0.1% for NH3. [11] Subsequently, such SWNT-based chemical sensors have been applied to detect a wide variety of chemicals and the detection limits have been significantly improved. Qi et al. fabricated large arrays of functionalized SWNTsensors, and the detection limit of NO2 was lowered to 100 ppt. [12] In addition, metal oxide nanowires have been widely studied and demonstrated with great potential for chemical sensing applications. Recently, due to the threat of terrorism and the need for homeland security, significant progress has been achieved in the detection of both explosives and nerve agents, such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), hexogen (DRX), and dimethyl methylphosphonate (DMMP). One of the leading candidates is 1D nanostructure-based chemoresistors or FETs. Snow et al. and Wang et al. have reported the detection of DMMP at ppb level by using SWNT and SnO2 nanowire-based chemical sensors, respectively. However, to our knowledge, there were only a few reports on the use of 1D nanostructure-based chemoresistors and FETs for detecting explosives, and the detection mechanism is still unclear. In addition, electronic devices fabricated on mechanically flexible substrates have recently attracted enormous attention, due to the proliferation of handheld applications in portable electronics, aerospace science, and civil engineering. Currently, conventional microfabrication techniques or printing methods can be applied to SWNTs on plastic substrates to form devices, allowing inexpensive mass-production and conformable electronics. In this paper, we report the transfer of aligned semiconductor SWNTs onto cloth fabric and successful fabrication of flexible SWNTchemical sensors, which have great potential for wearable electronics. These SWNT chemical sensors exhibited good sensitivity of trace chemical vapors, including 8 ppb TNT and 40 ppb NO2, at room temperature. Besides, to realize the concept of an electronic nose (e-nose) system for explosives, we also fabricated ZnO nanowire-based chemical sensors, which showed a detection limit of 60 ppb for TNT molecules at room temperature. To our knowledge, this is the first TNT sensor built on the basis of metal oxide nanowires. In addition, the detection limit of our chemical sensors is close to the limit of 1.5 ppb TNT set by the U.S. Occupational Safety and Health Administration. The flexible TNT sensors can find immediate applications in systems that demand mechanical flexibility, light weight, and high sensitivity. The fabrication of flexible SWNTchemical sensors started with the synthesis of SWNTs on quartz substrates using a chemical vapor deposition (CVD) method, which have been reported by us and other groups. After growth, we adapted a facile method to transfer the aligned nanotubes from the original substrate to fabric. In brief, a 100-nm-thick gold film was first deposited on the original substrate with aligned SWNTs, followed by applying a thermal tape to peel off the gold film and nanotubes from the growth substrate. The gold film with SWNTs on the thermal tape were pressed against a piece of textile fabric, which was pre-coated with polyethylene at elevated temperature and then transferred from thermal tape onto textile fabric, which had a 50-nm Ti layer as back-gate electrode and 2-mm-thick SU-8 as gate dielectric layer. The thermal tape was released, and KI/I2 gold etchant was then applied to remove gold films. Finally, Ti (0.5 nm) and Pd (40 nm) were deposited on the transferred SWNTs as source/drain electrodes. A schematic diagram of a flexible SWNT chemical sensor is shown in Figure 1a. Figure 1b shows an optical photograph of flexible aligned SWNT FETs on a textile fabric. It can be clearly seen from the SEM image (right) that the nanotubes bridge the two electrodes. Figure 1c displays the current–gate-voltage (I–Vg) characteristics of a typical flexible transistor on fabric before and after electrical breakdown. The device showed significant improvement for the