Enhanced Sensitivity of Iontronic Graphene Tactile Sensors Facilitated by Spreading of Ionic Liquid Pinned on Graphene Grid

Abstract

Iontronic graphene tactile sensors (i-GTS) composed of a top floating graphene electrode and an ionic liquid droplet pinned on a bottom graphene grid, which can dramatically enhance the performance of capacitive-type tactile sensors, are presented. When mechanical stress is applied to the top floating electrode, the i-GTS operates in one of the following three regimes: air-air, air-electric double layer (EDL) transition, or EDL-EDL. Once the top electrode contacts the ionic liquid in the i-GTS, the spreading behavior of the ionic liquid causes a capacitance transition (from a few pF to over hundreds of pF). This is because EDLs are formed at the interfaces between the electrodes and the ionic liquid. In this case, the pressure sensitivity increases to approximate to 31.1 kPa(-1) with a gentle touch. Under prolonged application of pressure, the capacitance increases gradually, mainly due to the contact line expansion of the ionic liquid bridge pinned on the graphene grid. The sensors exhibit outstanding properties (response and relaxation times below 80 ms, and stability over 300 cycles) while demonstrating ultimate signal-to-noise ratios in the array tests. The contact-induced spreading behavior of the ionic liquid is the key for boosting the sensor performance.