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چکیده
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The presented study tries to propose high quality carbon dots (CDs) prepared via facile and efficient method, in large scale. The optical properties demonstrate that both urea concentration and CaCl₂ play complementary roles in determining the emissive characteristics of the CDs. Nitrogen introduced by urea progressively passivates surface defects and creates new nitrogen-containing emissive states, resulting in narrower emission bands, reduced excitation dependence, and smaller Stokes shifts. An intermediate urea content (1 g) yields the most homogeneous emissive centers, producing intense green fluorescence with emission quantum yield of %86.5 with relatively narrow FWHM and nearly excitation-independent emission. It showed relatively long emission lifetime of 12.4 ns with a binary recombination pathway. Increasing the urea content beyond this level introduces additional defect states that broaden the emission, whereas eliminating CaCl₂ disrupts controlled nucleation and surface passivation, leading to heterogeneous emissive sites, broader spectra, larger Stokes shifts, and pronounced excitation-dependent fluorescence. The presence of CaCl₂ was further determinative to reach high-yield product. Collectively, these results indicate that the optimal balance between nitrogen doping and Ca-assisted carbonization is achieved at 1 g urea in the presence of CaCl₂, producing CDs with the most uniform electronic structure and the highest-quality green photoluminescence, suitable for optoelectronic purposes.
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