Emergence of long afterglow and room temperature phosphorescence emissions from ultra-small sulfur dots

Abstract

Sulfur dots (S-dots) are one of the most recently developed non-metallic luminescent nanomaterials and possess several advantages over traditional inorganic quantum dots (QDs). Here, we have synthesized highly luminescent ultra-small S-dots using a facile single-step mechanical grinding-assisted technique without consuming additional energy in terms of heat, time, pressure, and harsh chemical treatment. The morphological features are characterized by high-resolution transmission electron microscopy (HR-TEM), and are further correlated with elemental analysis by X-ray photoelectron spectroscopy (XPS) and Fourier transform infra-red (FTIR) spectroscopy. The results suggest the formation of highly homogenous ultra-small S-dots with specific functional groups on the surface. The photophysical properties of the as-synthesized S-dots are characterized by steady-state and time-resolved photoluminescence (PL) spectroscopy. Furthermore, a temperature-dependent PL study has been carried out and it is correlated with the structural features of S-dots. Finally, these ultra-small S-dots show efficient room-temperature phosphorescence and long-term afterglow (up to 5 s) while embedded into B₂O₃ and biuret matrixes. The mechanistic insight behind the phosphorescence emission, the kinetics of phosphorescence, and furthermore their correlation with elemental/structural features has been investigated in depth. Finally, a reversible transformation of room temperature phosphorescence to thermally activated delayed fluorescence has been explored by a detailed temperature-dependent study. Furthermore, we have also presented the applicability of these smart materials in anti-counterfeiting applications and security devices.