Fabrication of flexible and transparent single wall carbon nanotube gas sensors by selective vacuum filteration and poly(dimethyl siloxane) mold transfer

Abstract

We report on the fabrication of thin-film gas sensors on poly(dimethyl siloxane) (PDMS) substrates using networked thin-film of single walled carbon nanotube (SWCNT) bundles. The catalytic CVD synthesized 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 substrates, as shown in figure 1. The light regions were covered with resist during filteration preventing SWCNT deposition. The surface of the SWCNT film on PDMS observed by SEM showed that the mold transfer process resulted in the nanotube thin-film having `grass` like surface structure. This may be from the nanotubes being pulled-off from the filter surface during transfer. 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 180o bending. As shown in figure 2, the conductance showed less than 5% reduction demonstrating high flexibility. To test its gas sensing ability, the SWCNT thin film was exposed to 10 ppm NH3 gas. The film conductance showed immediate reduction to the low partial pressure NH3 gas, demonstrating high gas sensitivity(figure 3). We will present flexible thin-film`s SWCNT bundle density and average length dependent optical, mechanical and electrical characteristics. Gas sensing operation of the SWCNT flexible thin-film depending on thin-film preparation will also be presented