The one-dimensional side gate based on graphene edges shows a significant capability of reducing the channel length of field-effect transistors, further increasing the integration density of semiconductor devices. The nanoscale electric field distribution near the edge provides the physical limit of the effective channel length; however, its imaging under ambient conditions is still lacking, which is a critical aspect for the practical deployment of semiconductor devices. Here, we used scanning nitrogen-vacancy (N-$V$) microscopy to investigate the electric field distribution near edges of single-layer graphene. Real-space scanning maps of photocharged floating graphene flakes were acquired with a spatial resolution of approximately 10 nm, and the electric edge effect was quantitatively studied by analyzing the N-$V$ spin energy-level shifts due to the electric Stark effect. Since the graphene flakes are isolated from external electric sources, we brought out a theory based on the photothermionic effect to explain the charge transfer from graphene to the oxygen-terminated diamond probe with a disordered distribution of charge traps. Real-time tracing of electric fields detected the photothermionic emission process and the recombination process of the emitted electrons. This study provides a perspective for graphene-based one-dimensional gates and optoelectronics with nanoscale real-space imaging and, moreover, offers a method to tune the chemical environment of diamond surfaces based on optical charge transfer.

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• Electron emission
• NV centers
• Optoelectronics
• Quantum sensing
• Field-effect transistors
• Graphene
• Optically detected magnetic resonance
• Scanning probe microscopy