pH Sensing Performances of CVD Graphene-Based ISFETs with High Stability

This work aims to investigate the pH sensing performances of CVD graphene, develop highly stable graphene-based ISFETs with reduced drift and hysteresis effects and further improve the sensing performance by applying plasma treatments on graphene. The advantage of graphene is its robust honeycomb ring structure formed by tightly bonded sp² carbons, which makes it a super barrier against diffusion species like water. This property addresses the stability issues of ISFETs, which are mainly caused by the slow penetration of water and water-related species into the gate sensing oxide in traditional ISFET structures. As a result, graphene-based ISFETs exhibit low drift rates, normally below 1 mV/hr, and small hysteresis widths within 3 mV, which is competitive among most metal oxides. Besides, a fresh polycrystalline CVD-grown graphene with minor oxidation is less sensitive to pH with about 5 mV/pH, however, we could tune up the sensitivity by surface oxygen plasma treatments, resulting in as high as 40 mV/pH with high stability and reliability. Lastly, we have also demonstrated a reference-electrode-free ISFET that comprises two graphene sensing films with different sensitivities, exhibiting a drift rate as low as 0.38mV/hr, which can be an example of future development of graphene-based ISFETs.