Conductivity Preservation of Aryl Diazonium-Functionalized Bilayer Graphene Probed by Operando Hall

Abstract

Covalent functionalization of graphene is a means to achieve robust immobilization of functional groups, but it results in a significant loss of electrical conductivity in monolayer graphene (MonoG) due to the introduction of strong scattering by point defects. This work presents gate-activated covalent functionalization of bilayer graphene (BiG) integrated with an operando Hall characterization to measure the charge carrier density and mobility in real-time. Using an integrated Ag/AgCl gate electrode to modulate the BiG Fermi level, we achieve precise control over the covalent grafting of aryl diazonium groups on BiG. We demonstrate that BiG preserves the majority of its conductivity after functionalization by losing only 20% of its conductivity, whereas MonoG experiences 80% reduction in conductivity under similar conditions. BiG enables a significantly wider tuning range for surface coverage while preserving the conductivity. Operando Hall measurements of BiG functionalization reveal that the observed conductivity decrease is primarily driven by a reduction in charge mobility due to short-range scattering, while the charge carrier density changes only modestly. Furthermore, we characterize the impact of covalent attachments on the graphene density of states (DOS) and interfacial charge storage through Hall-derived quantum capacitance and electrochemical impedance spectroscopy (EIS). Finally, we demonstrate the application of carboxyphenyl functionalized BiG for pH sensing and present a site-binding model that describes the electrostatic coupling between site-binding coverage and the charge density within the conducting channel. This architecture provides a robust and tunable platform for graphene FET sensors.