A team of scientists from the University of Manchester and Graphenea have produced pressure sensors based on graphene/polymer heterostructures that compete with existing technology. Two types of sensors were made, the first sustaining large maximum load up to 250 kPa, the other yielding unprecedented pressure sensitivity of 123 aF Pa-1 mm2 over a pressure scale of 80 kPa.
The sensors fall into the capacitive type, a common design that measures the motion of a freestanding membrane that moves under pressure towards a fixed electrode. Such sensors find their applications in a vast industrial space that includes biomedical sensing (ex. blood pressure monitoring), altitude sensing, depth sensing, and gas or liquid system leak testing.
The sensors are made by coating graphene with a polymer layer, for optimal trade-off between device performance and membrane yield – free-standing graphene has a tendency to stick to the substrate during the fabrication process, which is avoided with polymer reinforcement. The present devices have 100% fabrication yield. Optical and mechanical inspection reveals that the quality of the graphene is excellent after the two-step transfer fabrication process.
Figure: Sketch, photograph, and AFM image of graphene/polymer pressure sensors. Reproduced from Nanoscale 2017, Advance Article , DOI: 10.1039/C7NR04621A with permission from The Royal Society of Chemistry.
The first sensor type was shown to sustain an up to 250 kPa pressure difference. The sensor reacted to pressure changes and had a relaxation time of several seconds, on par with commercial polymer-based sensors. The second type of sensor was made by straining the membrane during fabrication, which resulted in better sensitivity. Suspended over a gap just 50 nm deep, the high-performance device yielded a sensitivity of 4.2 aF Pa-1, which is in the same ballpark as state-of-the-art commercial silicon- and polymer-based pressure sensors. The full pressure scale was 100 kPa, significantly outperforming metal-polymer sensors. Owing to the 2D nature of graphene, the sensitivity per unit area of the new device was over 30 times higher than for silicon pressure sensors and 5 times higher than for polymer-based sensors. Furthermore, graphene/polymer sensors should not suffer from brittle fatigue that silicon-based sensors experience over time and in humid conditions.
The work, published in the journal Nanoscale, highlights the potential of such graphene/polymer heterostructures for use in highly sensitive pressure sensors with a large operating range and excellent reliability.