A tunable palladium nanoparticle film-based strain sensor in a Mott variable-range hopping regime

Metal nanoparticle (NP) film, an important artificial material that exhibits electronic transport properties different from bulk metals, has often been employed to fabricate micro/nano electronic devices. In this paper, Pd NP film was formed on the surface of flexible polyethylene terephthalate (PET) sheet in order to fabricate a strain sensor by means of gas-phase cluster beam deposition. Three Pd NP film-based strain sensors with different NP coverages were fabricated, and their electronic transport properties and strain sensing behaviors were investigated. Despite the differences in inter-particle coupling strength induced by different NP coverages, the Mott variable-range hopping (VRH) transport was found to be dominant in all three sensors. Sensors made of Pd NP films with lower NP coverage were found to give strain gauges with greater sensitivity, and their strain sensing calibration curves deviated from the exponential law due to variations in the gauge factor during strain loading, which was originated from the topology change of the percolation network. Sensitivity could be optimized by regulating the NP coverage, which is readily tuned during device fabrication. The tunable sensitivity, up to 1000, as well as the variety in flexible substrate materials available make them extremely promising for developing micro- and nano-sized electromechanical devices. Besides, the investigations about the reliability and hysteresis performance of the sensors were carried out. Our strain sensors show an excellent performance to the long-term bending-unbending cyclic test.