Ultralow-Power Alcohol Vapor Sensors Using Chemically Functionalized Multiwalled Carbon Nanotubes

Alcohol sensors, batch fabricated by forming bundles of chemically functionalized multiwalled carbon nanotubes (f-CNTs) across Au electrodes on SiO₂/Si substrates using an AC electrophoretic technique, were developed for alcohol vapor detection using an ultralow input power of ~ 0.01 - 1 muW, which is lower than the power required for most commercially available alcohol sensors by more than four orders of magnitude. The multiwalled carbon nanotubes (MWCNTs) have been chemically functionalized with the COOH groups by oxidation. We found that the sensors are selective with respect to flow from air, water vapor, and alcohol vapor. The sensor response is linear for alcohol vapor concentrations from 1 to 21 ppm with a detection limit of 0.9 ppm. The transient response of these sensors is experimentally shown to be ~1 s and the variation of the responses at each concentration is within 10% for all of the tested sensors. The sensors could also easily be reset to their initial states by annealing the f-CNTs sensing elements at a current of 100-200 muA within ~ 100-200 s. We demonstrated that the response of the sensors can be increased by one order of magnitude after adding the functional group COOH onto the nanotubes, i.e., from ~0.9% of a bare MWCNTs sensor to ~9.6% of an f-CNTs sensor with a dose of 21 ppm alcohol vapor.