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Even after several years of research, controlled synthesis of photoluminescent carbon nanodots (C-dots) still constitutes a major challenge, and investigation of their photoluminescence (PL) mechanism remains elusive. Various top-down and bottom-up approaches have been reported lately. However, these methods usually suffer from limited control over the major factors that dictate the PL behaviour of these fascinating carbon materials. To this end, we discover a new approach to prepare C-dots from size-tunable single chain polymeric nanoparticles. Taking advantage of the state of the art living radical polymerization technique and unique features of Bergman cyclization, narrowly dispersed C-dots are prepared in a straightforward manner. PL study shows that the optimal emission wavelength of C-dots red-shifts when the size of C-dots decreases, which is different from the trends typically found in semiconductor quantum dots and C-dots prepared from graphitized materials. To clarify the PL mechanism of C-dots prepared from different sources, a theoretical study based on density functional theory is performed. Two series of model compounds, fused aromatic rings and cyclo-1,4-naphthylenes, are chosen for C-dots with different microstructures. The calculation data indicate that PL energy of C-dots is dictated by the size and microstructure of the sp2 carbon core. For a C-dot with a graphitized core, the smaller the size of the core, the higher the PL energy, while for a C-dot with an amorphous core, an inverse trend is revealed. Surface reduction experiments further show that the quantum yield of C-dots is controlled by the surface chemistry.
Zhu et al. (Tue,) studied this question.