The importance of surface states in N-doped carbon quantum dots

Nitrogen-doped carbon quantum dots are synthesized by a one-step atmospheric pressure microplasma process. The origin of the observed photoluminescence emission and its relationship with nitrogen doping is studied using a range of optical and chemical measurements along with verification by theoretical calculations. Nitrogen doping into the core and functionalization of surface states with nitrogen and oxygen groups gives rise to a hybrid structure which is responsible for the luminescence with quantum yields up to 33%. Carrier multiplication is observed as a step-like enhancement in the quantum yield. The analysis of visible-light emission suggests that the emission originates for the most part from surface states and not due to recombination within the quantum dot core. The role of surface functional groups is dominant over quantum confinement in determining the optical properties.