Transparent, flexible electrodes and sensors based on carbon nanotube thin films

Abstract

We obtained thin films of single-walled carbon nanotubes (CNTs), which may be used as transparent, flexible electrodes in electronic devices, on a substrate using a very simple method. In order to construct reproducible devices with similar properties, in particular with similar impedance Z(ω), it is important to associate the electrical properties with the number of CNTs in a network. We prepared thin CNT networks on transparent, flexible substrates (PPC, polypropylene carbonate) with different CNT densities. The number of CNTs was estimated using a mathematical method based on the data obtained from thermo-gravimetric analysis (TGA). We were able to estimate the relative number of CNTs using optical absorption spectroscopy. These two methods are in good agreement. We also analysed the various samples using electrical impedance measurements at frequencies of up to 110 MHz. Low-density networks are semiconductors, whilst high-density networks behave like metals and are sufficiently good conductors to be used as working electrodes in electrochemical processes. It is thus possible to obtain CNT polymer and metal composite conductors electrochemically. With sensor applications in mind, we used CNT thin films as a working electrode to obtain a composite CNT-conducting polymer. This was performed by electrochemically depositing a conducting polymer polypyrrole or polyaniline on the electrode. The pH dependence of the device was measured by recording its open circuit potential in various buffer solutions. This enabled us to analyse the properties of the device as an electrochemical sensor. The results showed a good sensitivity, linearity and stability in both cases. Thus, the CNT/polypyrrole and CNT/polyaniline devices could have applications as solidstate gas sensors or biosensors when they are deposited on transparent and flexible surfaces of any shape.