The improved electromechanical sensitivity of polymer thin films containing carbon clusters produced in situ by irradiation with metal ions

Greatly enhanced electromechanical sensitivities were observed in ion beam irradiated polyimide films, where high resolution transmission electron microscopy and atomic force microscopy studies showed that 5.5 MeV Cu³⁺ ions produce randomly dispersed and highly oriented conducting nanochannels containing carbon clusters in polyimide with considerable overlap and narrow tunnel gap distributions between neighbouring nanochannels. Selected area electron diffraction studies confirmed the graphitic structure of these carbon clusters and also showed that the graphitic layers had preferential orientation parallel to the ion beam direction arising from relaxation of the graphitic layers within the nanochannels relieving local surface stresses in the polyimide matrix generated during irradiation as well as alignment of graphitic basal layers by volatile gases formed during irradiation escaping to the surface minimising resistance to gas flow. Electron energy loss spectroscopy also confirmed the graphitic structure of these carbon clusters within the nanochannels. Film electrical resistance increased exponentially with applied strain demonstrating the dominant role of tunnel gap modulation under strain. Gauge factors >1000 were achieved in these films at >3000 μstrains. Narrow distribution of tunnel gaps between the overlapping nanochannels is responsible for the high electromechanical sensitivities observed in these films compared to carbon nanotube–polymer (～50) films.