Nano-FTIR identification of functionalization in two-photon oxidized graphene

Graphene oxide (GO) plays an important role in next-generation electronic, photonic, and sensing technologies due to its tunable chemical functionality and unique electronic properties. However, characterizing the spatial and chemical heterogeneity of GO at the nanoscale remains a persistent challenge, due to the limitations of conventional spectroscopy in resolving localized functional groups. This is especially true for GO modified by femtosecond laser-induced two-photon oxidation (TPO), which creates spatially confined chemical environments that bulk techniques struggle to resolve. Herein, we employ Fourier transform infrared nanospectroscopy (nano-FTIR) to achieve highly localized, nanoscale chemical characterization of two-photon produced GO. Using tip-enhanced spectroscopy, we resolve the vibrational fingerprints of key functional groups with sub-diffraction spatial resolution. Nano-FTIR analysis reveals that epoxide groups dominate the oxidation, with a strong vibrational feature consistently appearing near 1225 cm⁻¹. Laser writing parameters are systematically varied to understand dose-dependent oxidation behavior. The resulting chemical contrasts are validated by Raman spectroscopy, AFM topography, and comparison with commercial GO. Our findings demonstrate that nano-FTIR not only maps chemical heterogeneity with unprecedented precision but also reveals nonlinear oxidation dynamics. This work highlights the utility of nano-FTIR as a powerful non-destructive tool for spatially resolved chemical analysis of laser-induced graphene or other 2D-materials.