There has been an argument on carbon nanotube (CNT) based gas detectors with a field-effect transistor (FET) geometry: do the response signals result from charge transfer between adsorbed gas molecules and the CNT channel and/or from the gas species induced Schottky barrier modulation at the CNT/metal contacts? To differentiate the sensing mechanisms, we employed three CNTFET structures, i.e., (1) the entire CNT channel and CNT/electrode contacts are accessible to NH(3) gas; (2) the CNT/electrode contacts are passivated with a Si(3)N(4) thin film, leaving the CNT channel open to the gas and, in contrast, (3) the CNT channel is covered with the film, while the contacts are open to the gas. We suggest that the Schottky barrier modulation at the contacts is the dominant mechanism from room temperature to 150 degrees C. At higher temperatures, the charge transfer process contributes to the response signals. There is a clear evidence that the adsorption of NH(3) on the CNT channel is facilitated by environmental oxygen.

Single-wall carbon nanotubes (SWNTs) suspended in isopropyl alcohol are placed between two cross-structured electrodes using an ac dielectrophoresis technique. The SWNTs are found to attach to the electrodes along the direction of the ac external electric field. The SWNTs predominately bridge the shortest gap between the two electrodes and the spatial distribution of the tubes becomes wider for a long manipulation time (say, greater than 300 s). The observed phenomenon is analyzed in terms of the dielectrophoresis-induced torque and force on the SWNTs. Our simulation shows that the time for rotating SWNTs to the direction of the electric field is much smaller than that for translating SWNTs. We also found that metallic SWNTs are forced along the gradient direction of spacial distribution of the electric field strength while semiconducting SWNTs are forced in the opposite direction.

Influences of ac electric field on the spatial distribution of single wall carbon nanotubes (SWCNTs) between two adjacent electrodes have been studied. The SWCNTs are found to be well aligned with the electric field direction and the number density of the SWCNTs attached to the electrodes is increased with the magnitude of the electric field. Induced ac dielectrophoresis force and torque on the SWCNTs are analyzed. It is suggested that the SWCNTs rotate to align with the external field direction almost instantaneously once the electric field is applied. In contrast, the translational motion along the field gradient takes a much longer time. Our results show that it is possible to separate metallic and semiconducting SWCNTs by frequency tuning. Taking the influences of frequency and viscosity into consideration, we simulate the distributions of SWCNTs between different electrode structures. Both theoretical and experimental results show that perpendicular electrodes have better control over the SWCNT’s location and direction than parallel electrodes.

Uniform SnO(2) nanorods were grown by inductively coupled plasma-enhanced chemical vapor deposition without catalysts and additional heating. The SnO(2) nanorods were aligned on a pair of Au/Ti electrodes by the dielectrophoresis method. SnO(2) single-nanorod gas sensors were fabricated by connecting individual SnO(2) nanorods to a pair of Au/Ti electrodes with Pt stripes deposited by a focused ion beam. The sensing properties of the SnO(2) single-nanorod sensor were studied. The SnO(2) single-nanorod sensor could detect 100 ppm H(2) at room temperature with repeated response and showed a large change of resistance, fast response time and good reversibility at an elevated operating temperature of 200 degrees C. The optimal sensing performance of the sensor is achieved at the operating temperature of around 250 degrees C.

No abstract