Tracing Different Emissive Species in Nitrogen-Doped Sulfur Dots during the Growth Process and the Intriguing Photophysics Thereof

The growth process of N-doped luminescent sulfur dots (NSDs) was investigated by varying reaction time and precursor concentration. Initially, an excitation wavelength-dependent emission phenomenon is observed, indicating the presence of heterogeneous emissive species. Later, a dominant excitation wavelength-independent emission gradually evolves, reflecting the transformation of multiple emissive species into almost a single emitting species. This is accompanied by enhanced crystallinity and photoluminescence quantum yield (PLQY). Column chromatography confirms the appearance of two main emissive moieties during the growth process, i.e ., (a) strong blue emission with a high PLQY (∼80%) originating from defined molecular polysulfides and (b) greenish blue emission with relatively lower PLQY (∼9%) consisting of more defect states. Femtosecond transient absorption spectroscopy further shows the respective excited-state relaxation dynamics for these two different types of emitting centers and the direct effect of defined molecular polysulfides as well as surface defect states.