Friday, February 15, 2008
NO2 gas sensors based on single-wall carbon nanotubes
Researchers successfully fabricated and tested molecular gas sensors made of suspended SWNTs. They showed for two different semiconducting SWNTs a transient chemiresistor like change in conductivity when they are exposed to NO2. The full gate characteristic (−20V:20 V) of the SWNTs is highly dominated by a large hysteresis and it shows three prominent changes in the characteristic after the exposure to NO2. Researchers found a broadened hysteresis, suppression of the n-type conductivity for positive Vg and an increase in the p-type conductivity for negative Vg. These changes are related to dynamically charged and discharged trap states of NO2 molecules on and around the CNTs and additional static effects correlated to NO2 doping of the CNT.
Measurements of the decay times of the charged trap states show very long time constants that are comparable to the constants found for H2O. By waiting a longer time (≈30 min) than the decay times of the trap states before starting to measure, researchers could acquire a gate characteristic close to the intrinsic one. This enabled a coarse separation between dynamic and static effects. The resulting intrinsic gate characteristic measurement shows a static shift of the gate threshold voltage and a modification of the SB transparency. The measurements enable for the first time a quantitative comparison between the gate threshold shift and the saturation current in the transient measurements which leads to the conclusion that the suspended SWNT gas sensors operate as ChemFET rather than chemiresistors. Considering the application the researchers conclude that individual suspended s-SWNTs are highly sensitive to NO2, which can be exploited in applications such as fire detection, with operation at room temperature and at few nanoWatts.
Source: Sensors and Actuators B (2008), article in press
Measurements of the decay times of the charged trap states show very long time constants that are comparable to the constants found for H2O. By waiting a longer time (≈30 min) than the decay times of the trap states before starting to measure, researchers could acquire a gate characteristic close to the intrinsic one. This enabled a coarse separation between dynamic and static effects. The resulting intrinsic gate characteristic measurement shows a static shift of the gate threshold voltage and a modification of the SB transparency. The measurements enable for the first time a quantitative comparison between the gate threshold shift and the saturation current in the transient measurements which leads to the conclusion that the suspended SWNT gas sensors operate as ChemFET rather than chemiresistors. Considering the application the researchers conclude that individual suspended s-SWNTs are highly sensitive to NO2, which can be exploited in applications such as fire detection, with operation at room temperature and at few nanoWatts.
Source: Sensors and Actuators B (2008), article in press
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