Exploration of highly photoluminescent first-row transition metals (manganese, iron, cobalt, nickel, copper and zinc) co-doped nano carbon dots as energy storage materials

[Display omitted] •Highly luminescent first-row transition elements (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) co-doped with nitrogen and nitrogen-sulphur carbon dots were synthesized by a greener microwave-assisted pyrolysis pathway.•The photo-emissive properties of the doped and co-doped carbon dots can be fine-tuned by the preferential/ selective doping of heteroatoms.•The highly fluorescent transition metal nanocarbon dots exhibited significant super capacitance properties and will be a promising material for its energy storage.•The capacitance values were found to be increased with the effect of co-doping and the capacitance values lie in the range of 2.6 µF/cm2 to 0.5 µF/cm2. Carbon nanodots have currently emerged as a unique and promising material focusing on advanced applications especially in the field of optoelectronic and energy storage devices. The preferential doping of nanocarbon dots with first row transition elements will enhance the photoluminescence characteristics and thereby, we can fine-tune its photophysical properties. In the present work, we have synthesized a series of first-row transition metal atoms (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) co-doped with nitrogen and nitrogen- sulfur carbon dots by a greener synthetic strategy. The formation of doped carbon dots were confirmed by its characteristic surface plasmon resonance absorptions using the Ultraviolet–visible spectroscopy. The presence of major elements and the functional group identification was done with the aid of EDS and FT-IR spectral analysis, the morphology and size were analyzed by high-resolution TEM analysis and photophysical characteristics were investigated from the fluorescence measurements. The photoluminescence studies reveal that by co-doping the transition metals with nitrogen or nitrogen- sulfur carbon dots, the emissions maxima of the nanodots can be tuned. The doped carbon dots were explored for their energy storage application and their super capacitance characteristics. The capacitance values of the carbon dots were found to be increased with the effect of co-doping with the first-row transition elements and the capacitance values were in the range of 2.6 µF/cm2 to 0.5 µF/cm2 and are in tune with the capacitance values for carbon-based materials including graphene and C-dots reported in the literature. Also the cyclic voltammetric loops were found to be sufficiently rectangular in shape, desirable for a supercapacitor, and the rectangular areas were found