Highly air-stabilized black phosphorus on disposable paper substrate as a tunnelling effect-based highly sensitive piezoresistive strain sensor

This report demonstrates synthesis of black phosphorus (BP) from red phosphorus using sonochemistry and the direct growth of highly air-stable BP on cellulose paper using hydrothermal method with PDMS as passivation and its utilization in human motion monitoring system. PDMS not only hinders the oxidation of BP but also provides flexibility to the device, thereby allowing the use of paper-based device as strain sensor. Fabricated device exhibits gauge factor of 6.1, which is comparable with the devices fabricated using sophisticated cleanroom techniques, and device is tested for 2,000 bending cycles revealing excellent reliability and repeatability. Device displayed excellent hysteresis suggesting that tunnelling effect is responsible for the current change of the device upon the external strain. Mathematical model is formulated to describe tunnelling conduction which matched the experimental results. Further, device exhibits excellent air stability for 98 days wherein negligible change in resistance and strain sensing performance is observed. Fabricated device is integrated onto different parts of the human body for its real-time application in human motion monitoring. Successful demonstration of the direct growth of highly air-stable BP on flexible and biodegradable cellulose paper opens up new avenues of research in optoelectronics, health care, security etc.