Engineered Thermopower and Thermal Conductivity Gradients in Fluorinated Graphene Films for Zero-Bias Infrared Sensing under Uniform Illumination

Achieving high-performance photothermoelectric (PTE) devices under uniform illumination is highly desirable for a range of emerging applications. This underscores the need for effective strategies to deliberately engineer nonuniformity in either the temperature profile or the Seebeck coefficient ( S eff ) along the device channel. In this work, a controlled current-induced thermal annealing (CITA) process enables effective modulation of the local annealing temperature distribution, creating a graded fluorine-to-carbon (F/C) atomic ratio in fluorinated graphene (GFG) films. This consequently results in pronounced thermal conductivity ( κ ) and S eff gradients along the film. Under uniform optical heating and zero-bias conditions, a nonzero photovoltage appears across the GFG film, even when both ends are in the same state (i.e., with no temperature difference). The optimized GFG-CITA-68.91 mA device delivers reproducible zero-bias photovoltages under both visible and infrared (IR) illumination and exhibits a linear power dependence and subsecond response time. Moreover, owing to its graded microstructure, the GFG device further exhibits micrometer-scale position-sensitive PTE responses under localized laser heating. The device is able to effectively discriminate infrared stimuli from mechanical perturbations. Finally, our analysis reveals that, beyond establishing S eff asymmetry, the κ gradient generates a locally enhanced temperature gradient that spatially coincides with regions of higher S eff, thereby amplifying the PTE photovoltage under uniform illumination. This monolithic, graded material design enabled by the convenient CITA strategy eliminates the need for heterogeneous material integration, thus preserving mechanical integrity and flexibility, which highlights its potential for application in powerless wearable IR sensors and electronic skin.