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Chemical sensors based on individual single-walled carbon nanotubes (SWNTs) are demonstrated. Upon exposure to gaseous molecules such as NO, or NH,, the electrical resistance of a semiconducting SWNT is found to dramatically increase or decrease. This serves as the basis for nanotube molecular sensors. The nanotube sensors exhibit a fast response and a substantially higher sensitivity than that of existing solid-state sensors at room temperature. Sensor reversibility is achieved by slow recovery under ambient conditions or by heating to high temperatures. The interactions between molecular species and SWNTs and the mechanisms of molecular sensing with nanotube molecular wires are investigated.
Carbon nanotubes are molecular-scale wires with high mechanical stiffness and strength. A SWNT can be metallic, semiconducting, or semimetallic, depending on its chirality (1). Utilization of these properties has led to applications of individual nanotubes or ensembles of nanotubes as scanning probes (2, 3), electron field emission sources (4), actuators (5), and nanoelectronic devices (6). Here, we report the realization of individual semiconducting-SWNT (S-SWNT)-based chemical sensors capable of detecting small concentrations of toxic gas molecules.
Sensing gas molecules is critical to environmental monitoring, control of chemical processes, space missions, and agricultural and medical applications (7). The detection of N02, for instance, is important to monitoring environmental pollution resulting from combustion or automotive emissions (8). Detection of NH3 is needed in industrial, medical, and living environments (9). Existing electrical sensor materials include semiconducting metal oxides (7-9), silicon devices (10, 11), organic materials (12, 13), and carbon black-polymer composites (14). Semiconducting metal oxides have been widely used for N02 and NH3 detection (7-9). These sensors operate at high temperatures (200 deg to 600 deg C) in order to achieve enhanced chemical reactivity between molecules and the sensor materials for substantial sensitivity (7). Conducting polymers (12) and organic phthalocyanine semiconductors (12, 13) have also been investigated for N02 or NH, sensing. The former exhibit limited sensitivity (12), whereas the latter tend to have very high resistivity (sample resistance of >10 gigohms) (13). In this report, we show that the electrical resistance of individual semiconducting SWNTs change by up to three orders of magnitude within several seconds of exposure to NO2 or NH3 molecules at room temperature. Miniaturized chemical sensors based on individual SWNTs are thus demonstrated. Furthermore,...