Gas sensors operate by a variety of fundamentally different mechanisms. Ionization sensors work by fingerprinting the ionization characteristics of distinct gases, but they are limited by their huge, bulky architecture, high power consumption and risky high-voltage operation. Here we report the fabrication and successful testing of ionization microsensors featuring the electrical breakdown of a range of gases and gas mixtures at carbon nanotube tips. The sharp tips of nanotubes generate very high electric fields at relatively low voltages, lowering breakdown voltages several-fold in comparison to traditional electrodes, and thereby enabling compact, battery-powered and safe operation of such sensors. The sensors show good sensitivity and selectivity, and are unaffected by extraneous factors such as temperature, humidity, and gas flow. As such, the devices offer several practical advantages over previously reported nanotube sensor systems. The simple, low-cost, sensors described here could be deployed for a variety of applications, such as environmental monitoring, sensing in chemical processing plants, and gas detection for counter-terrorism.

Nanotechnology Nanotechnology V 1505Miniaturized Gas Ionization Sensors Using Carbon Nanotubes. -Ionization microsensors featuring the electrical breakdown of a range of gases and gas mixtures at carbon nanotube tips are fabricated. The compact, battery-powered sensors show good sensitivity and selectivity, and are unaffected by extraneous factors such as temperature, humidity, and gas flow. The simple, low-cost sensors could be useful for a variety of applications, such as environmental monitoring, sensing in chemical processing plants, and gas detection for counter-terrorism. -(MODI*, A.; KORATKAR, N.; LASS, E.; WEI, B.; AJAYAN, P. M.; Nature (London, UK) 424 (2003) 6945, 171-174;

Electrical transport in vertically aligned films of multiwalled carbon nanotubes has been investigated in the -150 degrees C to 300 degrees C temperature range (all the tests were conducted in air at atmospheric pressure). In all the cases, the nanotube film exhibited a semi-conducting behavior, with the film resistance decreasing with increasing temperature. Removal of amorphous carbon contamination (via plasma etching) significantly improved the nanotube film's sensitivity to temperature changes (particularly in the 20 degrees C to 200 degrees C temperature range). All the of films tested in this study showed a consistent, repeatable behavior that was independent of the nanotube film length. The temperature sensitivity of the nanotube films was also found to be independent of the heating/cooling rates and without hysteresis. Because of the excellent repeatability and stability of the results, it is conceived that miniaturized temperature sensors could be designed using such aligned multiwalled nanotube films.

The influence of electric fields on carbon nanotubes is experimentally demonstrated. Alignment of nanotubes along field lines, directed motion of nanotubes between electrodes separated by several thousand micrometers, and impressive solid-state actuation behavior of nanotube-embedded structures are demonstrated, taking into account the polarization and charging of the nanotubes. These effects are reported for long strands of nanotubes, nanotubes dispersed on substrates, and nanotube-embedded polymer strips. The relative magnitude of the field responsible for polarization and directed motion was found to be dependent on the morphology of the nanotubes used. These observations may foreshadow novel electromechanical applications for nanotube elements.

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