Abstract
In this work, the piezoresistive effects of defective graphene used on a flexible pressure sensor are demonstrated. The graphene used was deposited at substrate temperatures of 750, 850 and 1000 °C using the hot-filament thermal chemical vapor deposition method in which the resultant graphene had different defect densities. Incorporation of the graphene as the sensing materials in sensor device showed that a linear variation in the resistance change with the applied gas pressure was obtained in the range of 0 to 50 kPa. The deposition temperature of the graphene deposited on copper foil using this technique was shown to be capable of tuning the sensitivity of the flexible graphene-based pressure sensor. We found that the sensor performance is strongly dominated by the defect density in the graphene, where graphene with the highest defect density deposited at 750 °C exhibited an almost four-fold sensitivity as compared to that deposited at 1000 °C. This effect is believed to have been contributed by the scattering of charge carriers in the graphene networks through various forms such as from the defects in the graphene lattice itself, tunneling between graphene islands, and tunneling between defect-like structures.
Highlights
Graphene, a two-dimensional (2D) carbon material consisting of hexagonally packed carbon atoms bonded by sp[2] bonds is the most robust material known
Pereira et al.[9] proposed that the piezoresistive effect in graphene is related to the graphene lattice distortion, which leads to a modified electronic band structure
The graphene was transferred using a wet transfer method mediated by poly-methyl methacrylate (PMMA) onto interdigitated electrode (IDE) in order to improve the flexibility of the substrate and the electrical properties of the graphene network (Supplementary Information, S1)
Summary
The transferred graphene grown at the 850 °C and 1000 °C substrate temperatures (see Fig. 5(b,c)) do not show this color contrast scheme, indicating less probability of deposited graphene layers overlapping, due to a lower nucleation density. The RMS (root mean square) surface roughness of the resultant transferred graphene was shown to increase by ~50% when the growth temperature decreased from 1000 to 750 °C due to the presence of a larger defect density (see Table 1). Under the application of pressure, the tunneling distortion among overlapped and adjacent graphene islands and defect-like structures became more significant, which in turn increased the density of the tunneling per graphene area This led to a higher probability of electron scattering in the percolating networks, which further reduced the density of the conducting paths, thereby effectively increasing the rate of the resistance change. Different percolation schemes through specific type of defects (i.e. nanoparticle density, overlapping region or ID/IG ratio) may have a significant impact to the piezoresistive effect of graphene-based flexible pressure sensors which can be further study, but these cases exceed the scope of this work
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