Adsorbed Molecules as Interchangeable Dopants and Scatterers with a Van der Waals Bonding Memory in Graphene Sensors

Molecular adsorption-induced doping and scattering play a central role in the detection mechanism of graphene gas sensors. However, while the doping contributions in electric field-enhanced gas sensing is well studied, an understanding of the effects of scattering is still lacking. In this work, the scattering contribution of the graphene-molecule van der Waals (vdW) complex is studied under various electric fields and the associated vdW bonding retention in the complex is investigated. We show that contrary to the generally opined view, doping does not always dominate the graphene-molecule vdW complex interaction and consequently the conductivity response in graphene sensors, rather the vdW complex interaction only shows doping-dominated interaction at zero electric fields while scattering increases with electric field modulation. The experimentally observed electric field-dependent scattering response agrees with electron difference density analysis from density functional theory (DFT) calculations, which shows that scattering is directly dependent on the electric field-induced molecular reorientation as well as the redistribution and delocalization of charge in the graphene-gas molecule vdW complex. Furthermore, "vdW bonding memory", i.e., retention of electric field-induced vdW bonding states after turning off the electric field, is observed and shown to result from the high binding energies of the vdW complexes, which are an order of magnitude higher than the sensing measurement thermal energy. This vdW bonding memory in the graphene-molecule complexes is important for the molecular identification of adsorbed gases based on their tunable charge transfer characteristics.