Polycrystalline grain boundaries can have a strong influence on the properties of graphene films, and also allow the preparation of materials with novel properties [1-5]. Herein, a room temperature friction method is developed for the rapid transfer-free production of polycrystalline graphene on flexible substrates. Starting with low-cost commercially available graphite powders, mono- and few-layer graphene were directly fabricated with an average production time of less than one minute (from raw graphite to graphene on substrates). The applications including strain sensing and humidity sensing were investigated.
Firstly, we studied the flexible polycrystalline graphene films on PET substrates for strain sensing. This type of strain sensing is based on percolating networks of graphene flakes, and the sensing mechanism originates from strain-dependent changes to the film morphology, which was exploited to obtain films with a high strain gauge factor. From the results in the curves describing the change in resistance ΔR as a function of mechanical strain, Δε, it demonstrates a strain gauge factor as high as 300 (calculated per the Ref. 6), which reveals the applicability of this material as a transparent strain sensor.
Secondly, we studied the flexible polycrystalline graphene films as transparent humidity sensors. In this section, the curves of resistance, ΔR, changing as a function of relative humidity, were investigaed. The change in resistance rates was approximately 20% for relative humidity values from 30% to 90%, demonstrating the usefulness of this method for transparent flexible humidity sensing. These results indicate that the PET humidity sensor has the advantage of high flexibility during operation.
Thus, considering the highly efficient, scalable and low-cost fabrication process (the total time from raw materials to final products was within minutes, all the procedures were carried out under room temperature, direct fabrication without any transfer step, avoiding chemical wastes, the raw materials were economically low cost.), such friction method and the flexible polycrystalline graphene films is favorable to flexible electronic applications in the near future.
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