Alcohol Vapor Sensors Based on Single-Walled Carbon Nanotube Field Effect Transistors

We have measured conductance of single-walled semiconducting carbon nanotubes in field-effect transistor (FET) geometry and investigated the device response to alcoholic vapors. We observe significant changes in FET drain current when the device is exposed to various kinds of alcoholic vapors. These responses are reversible and reproducible over many cycles of vapor exposure. Our experiments demonstrate that carbon nanotube FETs are sensitive to a wide range of alcoholic vapors. Chemical sensors1,2 based on carbon nanotubes have recently attracted a great deal of attention. Nanotubes can be expected to exhibit excellent properties as transducers since they have large surface area and are known to exhibit charge-sensitive conductance. For these applications, semiconducting nanotubes play an important role and therefore field-effect transistor geometry3,4 is very convenient. Indeed, a number of researchers have reported that nanotube transistors are responsive to several gaseous agents.2 In particular, Hongjie Dai and co-workers have shown that nanotube transistors are very sensitive to NO2 and NH3. In addition, the conductance of nanotubes is known to be sensitive to ambient environments, especially to oxygen and/or oxygen-containing gaseous species.5,6 For practical sensor application, however, it is necessary to investigate and to understand characteristics such as reversibility, reproducibility, sensitivity, and selectivity to various gaseous analytes. We report here an investigation of the influence of alcohols on the characteristics of FET devices fabricated from singlewalled carbon nanotubes. In particular, we have synthesized single-walled carbon nanotubes by chemical vapor deposition (CVD) and have fabricated field-effect transistor (FET) structures with channel lengths of 2.5 μm and 5 μm. For a given applied gate voltage, we have measured the change in drain current which can be observed when the whole device is exposed to alcoholic vapors, causing changes in the observed operating characteristics (threshold voltage and saturation current) of the FET device. The structure for our FET-based sensor and the corresponding experimental geometry are schematically shown in Figure 1a. Single-walled carbon nanotubes are synthesized by CVD with patterned Fe/Mo catalysts on heavily doped Si substrates capped with 100 nm thick SiO2. Jing Kong, Hongjie Dai, and co-workers7 originally developed this approach. CVD growth is carried out in a quartz tube (1 in. in diameter) at 850 °C for 4 min under the flow of mixed gases of 500 mL/min of argon, 50 mL/min of hydrogen, and 500 mL/min of methane. Then, nanotube-bearing substrates are loaded into a conventional vacuum evaporator system with metal shadow masks to form source (S) and drain (D) electrodes consisting of a 5 nm thick chromium adhesion layer and a 25 nm thick gold contact. The spacing between S/D electrodes is 2.5 μm or 5 μm. This resist-free process allows us to obtain good contact resistance without annealing. The fabrication procedure may be found in refs 8 and 9 in detail. The specific growth conditions are adjusted so that a single carbon nanotube bridges the S/D electrodes as shown