Chemically Induced Surface Potential Modulation at Pd|Al₂O₃|Graphene Field Effect Transistors: Implications for Enhanced H₂ Sensing

Surface potential is an important parameter for the development and optimization of high-performance charge/potential sensitive chemosensors. In this work, we demonstrate a technique of measuring surface potential modulation (SPM) due to chemoselective absorption in functional material (FM), at FM|dielectric (D) interface using graphene-heterostructure-field-effect-transistor (GHFET) made of FM|D|graphene (G). In the GHFET, chemoselective FM acts as a floating gate, where gate voltage is modulated chemically and causes resistive response in graphene. We have developed an analytical model of the GHFET and find that the SPM can be measured as the sum of potential modulation across the dielectric and the Fermi energy modulation of graphene. We experimentally demonstrate the methodology of measuring SPM in GHFET made of Pd|Al2O3|G layers for H2 gas absorption in Pd layer. We find that the SPM shows saturating nature with increasing H2 concentration under N2 gas environment and estimate the value of saturated SPM as 308 ± 21 meV. In air environment, no such saturation of the SPM with H2 gas concentration is observed. Notably, the GHFET of Pd|Al2O3|G layers shows better H2 gas sensitivity response (28 ± 1% at 0.4% H2) under air environment compared to other reported graphene based H2 sensors. Our methodology has an implication for enhancing H2 gas sensing.