Double-emission mechanism of laser-induced HOPG-exfoliated Graphene Quantum Dots (GQDs)

Abstract

The photoluminescence (PL) mechanism of graphene quantum dots (GQDs) has remained ambiguous, which restricts their simulations and applications. Here, stable and pure GQDs were fabricated by laser ablation of highly oriented pyrolytic graphite along the orientation parallel to the graphite layers. Two intense PL peaks were surprisingly achieved, which experimentally demonstrates that the double emissions coexist in the PL mechanism. This finding contrasts with the conventionally reported cases in which only one PL peak was observed and intensively studied. The peak at shorter wavelengths corresponds to the reported PL peak. This peak is redshifted with excitation wavelengths and is attributed to transitions from π* to π states rather than from π* to surface states as in most conventionally assigned situations. Notably, the peak at longer wavelengths is derived from the emission transition from π* to surface states and exhibits an excitation wavelength-independent feature. Because of the sufficient passivation and the resulting increase in electron density on the GQDs, the bandwidth that is composed of the energy levels of all surface states becomes narrowed and close to that of a single GQD, which causes the peak position to be insensitive to the sizes of the GQDs. Furthermore, an investigation indicates that in quite a few reported PL spectra, with an intense peak, a weak shoulder at longer wavelengths appeared with excitation wavelength-independence although this shoulder was usually ignored. This work is significant not only for true understanding of the PL mechanism but also for multi-colour photoluminescence and cancer cell imaging applications.