Tensile strain sensors using stretchable carbon nanotube thin-films

Abstract

Recently, it has been reported that lateral strain applied to carbon nanotube networks results in change in the network conductivity [1]. This was attributed to the strain induced lateral elongation of the carbon nanotube lattice along its axis resulting in the change in energy band structure. For a distribution of nanotube bundles forming a percolation network, the applied tensile strain may change the overall conductance by changing the inter-bundle conductivity, by possible telescopic elongation of the nanotube bundle, and by elongation of the nanotube lattice. Also we must consider the transfer of applied strain from the substrate to the nanotube thin-film, which would depend heavily on the nanotube-polymer interface. For possible strain sensor application of carbon nanotube thin-films a detailed study of the strain dependent conductance of the carbon nanotube thin-film on elastomer substrate should be made. Here, we investigate the external strain dependent conductivity of single walled carbon nanotube bundle network thin-film (NTFs) embedded on a stretchable silicone elastomer surface for possible application to tensile strain sensors. The nanotubes were dispersed in 0.1% sodium dodecylbenzene sulfonate solution under ultrasonic agitation. The NTF was patterned in-situ into sensor line structures during its formation using selective vacuum filtration [2]. Here, the device pattern was directly patterned on top of the ~20 nm pore alumina membrane surface and during vacuum filtration, the NTF device patterns formed only on the exposed areas of the filter. To transfer the NTF onto a flexible substrate, poly-dimethylsiloxane (PDMS) was cured directly on the patterned filter [3]. After filter removal, a highly flexible NTF was formed on the PDMS surface, as shown in figure 1(b). Optical image of the NTF line pattern on the filter surface shows that the nanotube bundles only exist inside the patterned area (see figure 2(a) and 2(b)). The SEM image of the NTF after transfer to the PDMS shows that the NTF was embedded within the substrate surface making intimate contact with the PDMS. We measured the change in NTF conductivity depending on the sample elongation. Figure 3 shows the measured change in device conductance with sequentially increasing applied tensile strain from 1% to 36%. It can be seen that the device conductivity does not recover to its original level during the recovery time interval given between each measurement. This may by due to the mechanical recovery characteristics of the PDMS. For 24 % strain applied to the PDMS, a conductivity change of ~10 % was observed. For higher levels of strain the device characteristics were unstable. Since the vacuum filtration process produces nanotube networks with random orientation the net strain applied to the nanotubes reduces from the local strain on the PDMS by a factor of cos2θ, where θ is the orientation of the nanotube relative to the strain direction resulting in reduced depression in conductance at high strain levels. For low tensile strain levels, the decrease in conductance depending on applied strain shows strong correlation, as shown in figure 4. It was even possible to detect a strain level of 0.05% using 5 mg/l low density NTF on PDMS(see inset of figure 4). We will further investigate the characteristics of the strain dependent conductivity of the NTF strain sensor on nanotube density and surface functionalization. Also, we will discuss methods to increase sensor reliability and operational lifetime.