Jun 23, 2009
Carbon nanotubes wired for pressure sensing
Carbon nanotube (CNT)-based pressure sensors offer the advantages of ultra-low-power operation, wide dynamic range and ease of integration with microcavities for vacuum microelectronics, compared with conventional thermal conductivity gauges such as Pirani or thermocouple devices.
A thermal conductivity gauge works on the principle of thermal exchange between a current carrying wire and the surrounding gas. As the voltage-biased wire is heated, surrounding gases remove heat from the wire. Because the rate of heat removal varies with pressure, changes in the system pressure can be deduced by monitoring the current response of the wire. This mechanism can also be used for chemical identification because the degree of heat removal is also dependent on the thermal conductivity of the particular gas surrounding the wire.
Researchers at the Jet Propulsion Laboratory, California Institute of Technology, US, have been investigating CNT-based thermal conductivity gauges in more detail. The team found that the devices can operate at ultra-low-power (nW–µW) and exhibit a wide dynamic range (760–10–6 Torr). In addition, the sensor's compact design suits vacuum-based microcavity applications such as vacuum microelectronics and RF micro-electro-mechanical-systems (MEMS).
In contrast, Pirani gauges are physically large, dissipate a large amount of power and are slow to respond. While miniaturized, low-power MEMS-based pressure sensors have emerged, obtaining a wide-dynamic range with such sensors has been challenging. The need for a wide dynamic range can occur when operating in a microcavity. Here, out-gassing can cause large pressure changes over short time intervals due to the small volumes that are involved.
The enhanced pressure sensitivity of CNT-based sensors, especially at large bias voltages, was attributed to the one-dimensional nature of electrical transport within the suspended tubes, as well as the contacts. These artifacts in suspended tubes were manifested by the presence of a negative differential conductance (NDC) regime in the device's IV curve (see figure). The feature was absent in the profile for unreleased tubes (see figure). The presence of the NDC suggests a large optical phonon density in suspended tubes at large biases, which affects tube temperatures and can be exploited to enhance the sensitivity of CNT-based pressure sensors.
The team published its work in Nanotechnology.
About the author
Anupama B Kaul is a task manager at the Jet Propulsion Labs, where she is leading the development of nanoscale materials and devices for their potential application in space electronics. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration and was funded by a Defense Advanced Research Project Agency seedling fund (Task Order NMO#715839 under NAS7-03001).