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>i.e</i>., (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.