Gas sensing characteristics of flexible and transparent carbon nanotube network

Abstract

We report on the ambient gas dependent conductivity of a carbon nanotube network thin-film prepared on a flexible and transparent poly(dimethyl siloxane) (PDMS) surface. The SWCNT bundles were dispersed in 0.1 % sodium dodecyl sulfate (SDS) solution at various ultrasonic power. SWCNT network thin-film was initially formed, using vacuum filteration, on ~20 nm pore alumina membrane filter surfaces. To transfer the SWCNT bundle thin-film onto a flexible substrate, PDMS was cured directly on the filter. After filter removal, a highly flexible SWCNT thin-film was formed on the PDMS surface. By pre-patterning the filter surface, it was possible to selectively filter and directly assemble SWCNTs into device structures and transfer them to the PDMS. The density of the thin film was controlled by limiting the amount of SWCNT dispersed in solution. The thin-film resistance increased from 0.2 kΩ/sq to 2.8 kΩ/sq as the SWCNT density was reduced by four fold, while the optical transmittance was increased from 40 to 80 % at 400~700 nm wavelength range. Mechanical stability of the flexible SWCNT device was tested by measuring conductance before and during 180 degree bending which showed less than 5% reduction demonstrating high flexibility. To test its gas sensing ability, the SWCNT thin film was exposed to NH3 gas. The film conductance showed immediate reduction to the low partial pressure NH3 gas, demonstrating high gas sensitivity. The sensitivity to gas exposure increased with reduction of nanotube density, and increase in sensor length. Since the gas exposure results in carrier doping or enhanced scattering, we may attribute the dependence on nanotube density and device length to increased scattrering induced mobility reduction affecting conduction within the nanotube film.