Graphene–Metal Composite Sensors with Near-Zero Temperature Coefficient of Resistance

This article describes the design of piezoresistive thin-film sensors based on single-layer graphene decorated with metallic nanoislands. The defining characteristic of these composite thin films is that they can be engineered to exhibit a temperature coefficient of resistance (TCR) that is close to zero. A mechanical sensor with this property is stable against temperature fluctuations of the type encountered during operations in the real world, for example, in a wearable sensor. The metallic nanoislands are grown on graphene through thermal deposition of metals (gold or palladium) at a low nominal thickness. Metallic films exhibit an increase in resistance with temperature (positive TCR), whereas graphene exhibits a decrease in resistance with temperature (negative TCR). By varying the amount of deposition, the morphology of the nanoislands can be tuned such that the TCRs of a metal and graphene cancel out. The quantitative analysis of scanning electron microscope images reveals the importance of the surface coverage of the metal (as opposed to the total mass of the metal deposited). The stability of the sensor to temperature fluctuations that might be encountered in the outdoors is demonstrated by subjecting a wearable pulse sensor to simulated solar irradiation.