Na-doped carbon nanodots: shed light on the concentration modulated photoluminescence and two-photon absorption performance

Abstract Metal-doped carbon nanodots (m-CNDs) have recently received attention due to their tunable optical properties useful in biophotonics and optoelectronics. Here, we present a green, rapid, and facile synthesis of m-CNDs via a one-pot solid phase (powder) flux technique in a solvent-free environment, with Na as a metal doping agent adsorbed to their surface. The average radius of the m-CNDs in colloids of different concentration varied from 1.5 up to 5 nm due to aggregation effects. Concentration-dependent optical properties from these colloids were determined by studying thoroughly one- and two-photon excitations. The results show that the Na-doping in m-CNDs not only can enhance 100% the photoluminescence quantum yield (PLQY), but also increased considerably the two-photon absorption cross sections (σTPA) compared with pristine carbon nanodots. m-CNDs exhibited multifarious emissive centers with PLQY from 10 to 22%, while a maximum σTPA of 19.7 × 103 GM was observed for m-CNDs with radius of 5 nm at 730 nm, a nonlinearity that is comparable or higher than in other heteroatom (metal and nonmetal) doped carbon nanodots. These large values of σTPA are discussed in the context of previous reports for a variety of carbon nanodots, pointing out that Na had not been used previously as doping agent to enhance optical nonlinearities in m-CNDs. This work offers a novel avenue for tuning σTPA in m-CNDs and endows them with potential for various applications.