Tailoring the energy gap to promote long wavelength emission of nitrogen-doped sulfur quantum dots via dual functional ethylenediamine

The direct conversion of abundant sulfur powder to stable long-wavelength emission sulfur quantum dots (SQDs) is now in the early stage due to the non-soluble property of bulk sulfur powder. To solve this issue, ethylenediamine (EDA) was employed as an effective solvent to disperse sulfur while simultaneously acts as a nitrogen doping agent to construct green-emitting nitrogen doped sulfur quantum dots (N-SQDs). The pronounced N-SQDs shows a maximum emission wavelength at 526 nm with satisfactory quantum yield reaching up to 25.3%. The N-SQDs even exhibited a red-shifted photoluminescence from blue to green as the concentration increased. Moreover, we proposed a tentative mechanism for the formation of N-SQDs through the cleavage of the sulfur ring in the presence of EDA by sulfur-amine reaction, which then rapidly converted zero-valent sulfur into polysulfide to offer N-SQDs. Comprehensive theoretical calculations suggest that the nitrogen groups contribute to the upshift of the highest occupied molecular orbital energy, reducing the energy gaps of N-SQDs and yield red-shifted wavelength. The EDA even offers an electron donation to the skeleton of SQDs, resulting in an enhanced fluorescence quantum yield.