Relative stability of nanosized β-C3N4 and graphitic C3N4 from first principles calculations

β-C3N4 Nanocrystals: Carbon Dots with Extraordinary Morphological, Structural, and Optical Homogeneity

“…Notably, β-C 3 N 4 nanocrystals are characterized by a high band gap (∼4 eV) . We speculate that this may be advantageous for the optical performance to avoid various possible types of losses.…”

Solvatochromism Unravels the Emission Mechanism of Carbon Nanodots

“…These CDs are carbon nitride nanocrystals with an average size of 3 nm, surface-functionalized with amide, carboxylic and hydroxyl groups, similar to other N-doped CDs in the literature. ,, They exhibit an absorption band at 3.05 eV; the corresponding luminescence undergoes a redshift with decreasing excitation energy (see Figure ), reflecting the typical tunability of CDs. Notably, the observation of a well-defined absorption band at energies below band gap transitions of the crystalline core (above ∼3.5–4.0 eV, see Figure S1 and , ) is strongly suggestive of midgap electronic states.…”

“…Although the solvation dependence directly evidence the central role of surface groups in the emissive transitions, the amide and carboxylic surface moieties are not expected in themselves to undergo transitions at these low energies (2–3 eV). Instead, we propose that the transitions involve a coupling between surface-localized states and those of the crystalline carbon-nitride core, which has a bandgap >3.50 eV. , Because the dots appear much more solvent-sensitive in the excited state (Figure a) than in ground-state (Figure S2), this means that the HB accepting capability of CDs is strongly enhanced by photoexcitation, likely due to an increase of the negative charge localized on surface HB-accepting groups. Indeed, similar situations where an increase the HB propensity of a certain site is caused by the charge transfer character of the electronic transition are common for optically active systems. , Therefore, our results lead to attribute the lowest-energy absorption at 3.05 eV of these CDs to a transition of an electron from the valence band of the core to the manifold of midgap (π*) empty states localized on surface carboxylic or amide groups, whereas the fluorescence arises from the radiative recombination of the photoexcited electron with the hole left in the core.…”

“…6,8,9 They exhibit an absorption band at 3.05 eV; the corresponding luminescence undergoes a redshift with decreasing excitation energy (see Figure 1), reflecting the typical tunability of CDs. Notably, the observation of a well-defined absorption band at energies below band gap transitions of the crystalline core (above ∼3.5−4.0 eV, see Figure S1 and 12,33 ) is strongly suggestive of midgap electronic states.…”

“…Instead, we propose that the transitions involve a coupling between surface-localized states and those of the crystalline carbon-nitride core, which has a bandgap >3.50 eV. 12,33 Because the dots appear much more solvent-sensitive in the excited state (Figure 2a) than in ground-state (Figure S2), this means that the HB accepting capability of CDs is strongly enhanced by photoexcitation, likely due to an increase of the negative charge localized on surface HB-accepting groups. Indeed, similar situations where an increase the HB propensity of a certain site is caused by the charge transfer character of the electronic transition are common for optically active systems.…”