Dissecting the Large Stokes Shift Fluorescence of Graphene-Sheet-Based Carbon Dots by Excitation-Dependent Ultrafast Spectroscopy

Carbon dots (CDs) represent a new class of nontoxic and sustainable nanomaterials with increasing applications. Among them, bright and large Stokes-shift CDs are highly desirable for display and imaging, yet the emission mechanisms remain unclear. We obtained structural signatures for the recently engineered green and red CDs by ground-state femtosecond stimulated Raman spectroscopy (FSRS), then synthesized orange CDs with similar size but much higher nitrogen dopants than red CDs. We implemented femtosecond transient absorption (fs-TA) spectroscopy to capture charge transfer (CT) from the core/edge to surface states on the ∼450 fs time scale via an ultrafast excited-state absorption (ESA) band shift, confirmed by solvent-dependent studies. Subsequently, optical and acoustic phonons contribute to edge-state dynamics after 400 nm excitation, whereas optical phonons become prominent after 500 and 267 nm excitations, unveiling an interplay among the excitation region, exciton-phonon coupling, and energy dissipation. Global analysis and probe-dependent fits corroborate key excited-state dynamics for large-Stokes-shift emissions, revealing a characteristic relaxation pathway toward an excitation-independent single emissive state at surface. These insights enable a bottom-up approach to rationally design and optimize nitrogen-doped CDs for redder and brighter emissions.