Tuning carbon quantum dot optics through nitrogen surface chemistry

Nitrogen-doped carbon quantum dots (N-CQDs) were synthesized via rapid microwave carbonization of citric acid monohydrate and urea at three CA : Urea mass ratios-2 : 1 (B1-CQD), 1 : 1 (B2-CQD), and 1 : 2 (B3-CQD) to systematically investigate how precursor stoichiometry influences their structural, compositional, and optical properties. XRD and Raman spectroscopy indicated semi-graphitic carbon structures with increasing structural disorder at higher urea content, together with additional crystalline reflections in B3-CQD attributed to reaction by-products. TEM revealed a decrease in particle size with increasing urea content, with B3-CQD exhibiting the smallest (2.95 ± 0.89 nm) and most monodisperse dots. EDX and XPS analyses showed a systematic increase in nitrogen content from 20.5% (B1-CQD) to 31.9% (B3-CQD) by EDX, while total oxygen remained nearly invariant (∼28-30%). The surface-to-bulk nitrogen ratio (SBNR), defined as the ratio of the XPS- to the EDX-derived nitrogen content, increased progressively from 34.1% to 82.4% with increasing urea content, approaching parity between the surface and bulk nitrogen contents; because the XPS sampling depth is comparable to the particle size, the SBNR is presented as a descriptive composition ratio. High-resolution N 1s XPS indicated a redistribution among nitrogen environments, from predominantly pyridinic/amine nitrogen in B1-CQD toward pyrrolic/amide nitrogen in B3-CQD, which XPS alone cannot fully resolve. UV-Vis absorption showed principal maxima at 342.7 nm (B1-CQD), 328.7 nm (B2-CQD), and 328.0 nm (B3-CQD), with an additional shoulder at 273-274 nm in B2-CQD and B3-CQD. Tauc analysis revealed a core-state (π → π*) gap of ∼5.4-5.5 eV together with sub-gap levels between 2.61 and 4.24 eV associated with nitrogen- and oxygen-containing groups. As particle size and nitrogen content vary together in this series, their individual contributions to the optical properties cannot be separated. Excitation-dependent photoluminescence was observed in all samples; under 360 nm excitation, the emission maxima were 438-442 nm, and B3-CQD showed a maximum at 370 nm under 320 nm excitation. Collectively, these findings show that precursor stoichiometry tunes the nitrogen content, surface chemistry, and optical properties of the filtered N-CQD dispersions, which may also contain co-produced molecular species, providing a structure-chemistry-property framework for rational CQD design.