Conventional thermal conductivity gauges (e.g. Pirani gauges) lend themselves to applications such as leak detectors, or in gas chromatographs for identifying various gas species. However, these conventional gauges are physically large, operate at high power, and have a slow response time.

A schematic (a) of the CNT Gas Pressure or Chemical Sensor. Au/Cr electrodes anchor the tube during exposure to 10:1 BHF for removing SiO2 beneath the tubes. Critical point drying in an IPA bath is used for the final release. (b) The comparison of conductance for an unsuspended and suspended tube. The suspended tube shows a negative differential conductance (NDC) regime. The Inset shows the current is still linear up to a current as large as 8 μA for the unsuspended tube.
A single-walled carbon-nanotube (SWNT)-based chemical sensing gauge relies on differences in thermal conductance of the respective gases surrounding the CNT as it is voltage-biased, as a means for chemical identification. Such a sensor provides benefits of significantly reduced size and compactness, fast response time, low-power operation, and inexpensive manufacturing since it can be batch-fabricated using Si integrated-circuit (IC) process technology.

The mechanism by which the sensing occurs is enabled by the reduced dimensionality for phonon scattering in 1D systems — in particular, suspended SWNTs — which can cause unique effects to arise at large bias voltages and power. Such effects are completely absent in 2D or 3D conductors such as within the filament of a conventional Pirani gauge. In suspended SWNTs at high fields, a large non-equilibrium optical phonon population exists, and their long relaxation times result in non-isothermal conditions along the length of the tube. In unsuspended tubes, the I-V characteristic increases monotonically at high voltages suggestive of isothermal conditions, since the substrate facilitates in the relaxation of optical phonons emitted through electron scattering. In contrast, the current in the suspended tube saturates and a negative differential conductance (NDC) regime is encountered, which cannot be explained by velocity saturation (at –5 kV/cm). This is a signature of the effective temperature rise within the tube that can be used to enhance sensitivity for gas detection. The large optical phonon density in suspended SWNTs at high fields with long lifetimes may play an important role in determining the rate of temperature rise in the tubes, which can be exploited maximally for their utility as thermal conductivity-based gas sensors.

The method used to form suspended long (>5 μm) SWNTs is novel and has not been reported in the past. A dry release technique is used based on critical-point drying. In the past, the reduced surface tension of various solvents has been used; however, these are all based on wet processes that will inherently suffer to a larger extent from capillary forces upon release.

The SWNT-based chemical sensor described here could be useful for future NASA astrobiology missions to detect specific biomarkers in the gaseous phase or to decipher biological activity by measuring outgassing rates; for example, within microcavities. The sensor could be used in future NASA instruments that require gas chromatographs to identify chemical species in planetary atmospheres, as well as future Lander missions.

The sensor could also be used aboard NASA spacecraft or instruments as a low-power, low-mass, compact leak detector with a fast response time compared to conventionally used thermal-conductivity-based leak detectors such as the Pirani gauge.

This work was done by Anupama B. Kaul of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it..

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