Fermi-Level Modulation in Graphene Sensors Revealed by Voltage-Dependent Raman Spectroscopy and 3D Waterfall Response Mapping

Abstract

The electronic properties of graphene sensors are governed by the position of the Fermi level, which determines the carrier concentration and consequently controls the electrical conductivity and sensing performance. Although Fermi-level modulation is widely recognized as the fundamental mechanism of graphene-based sensing, its direct experimental correlation with the dynamic sensing response has remained insufficiently explored. In this work, we demonstrate that Raman spectroscopy provides a direct, nondestructive method for quantifying the relative Fermi-level modulation through systematic shifts of the G and 2D Raman bands associated with changes in carrier concentration. The Raman analysis reveals reversible electrical tuning of the graphene electronic structure under source-drain voltage without introducing structural defects, allowing quantitative estimation of the Fermi-level relative displacement. To investigate the dynamic behavior of the same electronic states during sensor operation, low-frequency time–frequency waterfall spectroscopy was employed under chemical and optical stimulation. The results demonstrate that external stimuli induce transient perturbations of the graphene Fermi level, producing characteristic conductivity fluctuations that depend on the magnitude and kinetics of carrier-density modulation. The combination of static (Raman spectroscopy) and dynamic (waterfall fluctuation) analyses establishes a unified sensing mechanism linking Fermi-level modulation → carrier concentration → conductivity fluctuations → sensor response. These findings demonstrate that evaluation of the Fermi level is essential not only for understanding the intrinsic electronic structure of graphene but also for interpreting and optimizing its real-time chemical and optical sensing performance, providing a comprehensive framework for the design of next-generation graphene sensors.