Revealing the tunable photoluminescence properties of graphene quantum dots

Graphene quantum dots (GQDs) are a new class of fluorescent reporters promising various novel applications such as bio-imaging, optical sensing and photovoltaics. They have recently attracted enormous interest because of their extraordinary and tunable optical, electrical, chemical and structural properties. However, the widespread use of GQDs is hindered by the poor understanding of their photoluminescence (PL) mechanisms. Using density-functional theory (DFT) and time-dependent DFT calculations, we reveal that the PL of a GQD can be sensitively tuned by its size, edge configuration, shape, attached chemical functionalities, heteroatom doping and defects. In addition, it is discovered that the PL of a large GQD consisting of heterogeneously hybridized carbon network is essentially determined by the embedded small sp² clusters isolated by sp³ carbons. This study not only provides an explanation to the previous experimental observations but also provides insightful guidance to develop methods for the controllable synthesis and engineering of GQDs.