Multichannel Photoluminescence of Graphene Quantum Dots Across Femtosecond to Cryogenic Timescales

Graphene quantum dots (GQDs) exhibit complex photoluminescence (PL) originating from intrinsic sp² carbon domains, surface functional groups, and structural defects. Yet the spectral overlap among these emissive channels hinders clear identification of their recombination pathways. Here, we investigate multichannel PL dynamics of commercial GQDs using time-resolved and cryogenic PL spectroscopy. PL spectra reveal three distinct peaks: Peak I (443 nm) from π-π* transitions, Peak II (520 nm) from surface-dominated contribution functional states, and Peak III (583 nm) from pyrrolic N-related defects. Time-correlated single-photon counting detects only a 460 nm emission linked to graphitic N traps, indicating that Peaks I-III decay faster than the nanosecond window. Ultrafast optical Kerr-gate measurements further resolve distinct lifetimes for hydroxyl (<5 ps), carboxyl (5-10 ps), amine (20-30 ps), and carbonyl (40-80 ps) groups. The transient evolution displays cascade relaxation from deep to shallow traps, evidenced by a progressive blue-shift of Peak II. Cryogenic PL shows stable emission of Peak I, whereas Peak III red-shifts and broadens with temperature, revealing strong electron-phonon coupling and deep-level trapping. These results clarify the multichannel emission mechanisms of GQDs and provide design principles for tuning their optical properties.