Graphene-based wearable temperature sensor and infrared photodetector on a flexible polyimide substrate

This paper describes an approach to the fabrication of flexible electronics i.e., a wearable temperature sensor and infrared (IR) photodetector on flexible polyimide (PI) substrate. Solar exfoliated reduced graphene oxide (SrGO) and graphene flakes are used as the sensing materials for developing the sensors on a PI substrate. PI, apart from being flexible and compatible with microfabrication processes, also helps in reducing the mobility and recombination of the photo-generated electrons of graphene due of its dielectric nature, thus enabling IR detection. Current responsivity and external quantum efficiency of IR photodetector for graphene flake-and SrGO-based devices were found to be 0.4 AW(-1), 16.53% and 0.8 AW(-1), 33.06% respectively which are higher than those of commercially available photodetectors. In addition, we demonstrate an ultrasensitive wearable human body temperature sensor in the temperature range of 35 degrees C to45 degrees C, wherein both graphene flake-and SrGO-based devices exhibited a negative temperature coefficient of -41.30x10(-4) degrees C-1 and -74.29x10 -4 degrees C-1 respectively, which are higher than commercially available counterparts. Plausible underlying mechanisms to both IR sensing and temperature sensing have been studied. Furthermore, as a proof of concept, we investigated the effect of IR radiation emitted by a human hand on the device. Interestingly it was found that the device was very sensitive to it, indicating that the sensor can be used for motion detection which has potential applications in security, surveillance etc. The strategy presented here provides a new, simple, cost effective approach for the fabrication of next-generation wearable and bio-implantable devices based on a polyimide substrate that can be easily integrated onto the surface of a leaf, skin, paper, clothes etc owing to its versatile nature.