Unravelling chemical heterogeneity and dual emission pathways in graphene quantum dots via single-particle infrared spectroscopy†

Understanding the relationship between the local chemical structure and photoluminescence (PL) in graphene quantum dots (GQDs) and nitrogen-functionalized GQDs (N-GQDs) is critical for their advancement in optoelectronics, sensing, and bioimaging. Ensemble measurements mask the structural and functional heterogeneity intrinsic to these quasi-zero-dimensional systems. Here, we employed single-particle photo-induced force microscopy (PiFM) to chemically map individual GQDs and N-GQDs, revealing diverse surface functional groups and bonding architectures that are obscured in bulk analyses. PiFM-IR spectra correlate well with vibrational modes predicted by density functional theory (DFT) on model structures incorporating oxygen and nitrogen functionalities. While ensemble characterization techniques such as Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy support the findings of single-particle analysis, the latter offers significantly superior spatial and chemical resolution. Optical features of the GQDs and the N-GQDs show size- and chemical structure-dependent behaviour such as excitation-dependent emission thresholds and biexponential decay dynamics. These observations support a dual recombination mechanism involving band-edge-to-band-edge transitions and surface-/dopant-mediated transition pathways. By integrating these methods, we established a robust framework for connecting a structure with optical behaviour, highlighting the importance of single-particle studies for rational design of carbon-based quantum materials.