Graphene quantum dots (GQDs) are promising for application in bioimaging due to their non-toxic nature. But the application is still limited because of the low quantum yield with a non-emissive feature. Despite higher quantum yield is reported in N-doped graphene and graphene oxide, the role of the N atoms and the mechanism to enhance quantum yield is not fully investigated yet. In this work, we explore the effects of the N-doped graphene and graphene oxide using the linear response time-dependent DFT method. N-doping on GODs shows two red-shifted absorption peaks but the lowest energy transition is not optically active. On the other hand, N-doping on the graphene oxides show a similar absorption feature but it increases the brightness of the lowest energy transitions. The O-atom modification on the graphene edge increases the brightness up to an optimal C/O ratio. The electron transition density map and change transfer length analysis demonstrate that the two red-shifted absorption bands are resulting mostly from the charge transfer dominated excitations. Our study suggested that the optimal C/O ratio and N doping can be used to tune the charge transfer nature, excitation energy, and brightness of the lowest energy absorption band, which is essential for enhancing quantum yield for the bioimaging application.

Photophysical properties of the N-doped graphene oxide: an ab initio study

ACS Spring Meeting 2021

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