Size-dependent radiative decay processes in graphene quantum dots

Radiative decay processes have been studied in graphene quantum dots (GQDs) by varying their size. The photoluminescence (PL) decay traces are well fitted to a biexponential function with lifetimes of τ ₁ and τ ₂, indicating their fast and slow components, respectively. The τ ₁ is almost constant, irrespective of the average GQD size (d _a) for two excitation wavelengths of 305 and 356 nm. In contrast, the τ ₂ decreases as d _a increases for d _a ∼17 nm, but d _a > ∼17 nm, it increases with increasing d _a for both the excitation wavelengths, similar to the size-dependent behaviors of the time-integrated PL peak energy. We propose that the τ ₁ and τ ₂ originate from size-independent fast band-to-band transition and size-dependent slow transition resulting from the edge-state variation at the periphery of GQDs, respectively.