Tactile pressure dependent conductivity of multiwall carbon nanotube network

Abstract

The conductivity of a network of carbon nanotubes (CNTs) depends heavily on the density of interconnecting nanotubes. For a thick film of nanotubes, the density should be beyond the 3-dimensional (3D) percolation threshold limit to show metallic conduction behavior. If tactile pressure was applied vertically and the nanotubes formed a two dimensional sheet, the conductivity would increase despite the density remaining unchanged. This effect may be used to measure the tactile pressure applied to a thin film surface. Here, we prepared a thick sheet of multi-walled carbon nanotube (MWCNT) network film on poly-dimethylsiloxane (PDMS) substrate and measured its conductivity depending on applied tactile pressure to film surface. Powders of MWCNT were dispersed in DI water with 0.1 % sodium dodecylbenzene sulfonate (NaDDBS) and then the solution was bath-sonicated for 15 hours. The uniform density MWCNT sheets were formed, using vacuum filtration, on ~200 nm pore cellulose membrane filter. Then stretched PDMS was used to `pick-up` the MWCNT sheet by stamping. The relaxing tension in the PDMS surface acts to locally compress and pick-up the nanotube sheets. Since pick-up force is applied to only the top surface of MWCNT sheet, the dense sheet unravels reducing the original interconnectivity. The MWCNT sheet plane of the sample was faced pre-deposited electrodes on a slide glass and silver paste used to improve the adhesion between MWCNT sheet and electrodes. The various forces were applied to the back side of the PDMS and we observed a dramatic decrease in resistance. The resistance of nanotube sheet decreased approximately 50 % at 5 N and the results were repeatable for several cycles. The pressure sensitivity also increases with reduced MWCNT density, which we attribute to the 3D percolation having greater dependence on nanotube density resulting in higher increase in conductivity when low density sheets are compressed.