Structure dependent piezoresistivity of singlewalled carbon nanotube thin films

Individual single-walled carbon nanotubes (SWCNTs) possess excellent piezoresistive property, which is manifested by their significant electrical resistance change when subject to mechanical deformation. However, in comparison to individual tubes, the SWCNT thin films-a twodimensional network formed by a random assembly of individual tubes typically show much lower piezoresistive sensitivity. To understand this problem and develop SWCNT thin film piezoresistive sensor with high sensitivity, in this study, we systematically investigated the processing-structure-piezoresistivity relationship of SWCNT thin films that were fabricated by spray coating technique. The structures of SWCNTs used for fabricating the thin film sensors have been tailored by varying the sonication conditions and characterized by preparative ultracentrifuge method (PUM), dynamic light scattering (DLS) and scanning electron microscopy (SEM). Coupled mechanical and electrical measurements were performed to understand the effects of SWCNT size (length and diameter) and film thickness on the piezoresistive sensing performance of SWCNT thin film sensors. It was found that the sensors made of shorter tube (< ∼ 700 nm) have higher gauge factor than the ones made of long tubes (∼> 1000 nm). In addition, when the tube length is greater than ∼ 1000 nm, the film thickness has insignificant influence on the sensitivity of the SWCNT thin film sensor. However, when the tube length is less than ∼ 700 nm, the piezoresistive sensitivity of SWCNT thin films is inversely proportional to the film thickness.