Fig. 4

Time-dependent DFT calculation results. The triangular structure model CQDs consisting of 4, 10, and 19 fused benzene rings: (1) functionalized with pure electron-donating hydroxyl groups (T-CQDs-OH-1 (a), T-CQDs-OH-2 (e), T-CQDs-OH-3 (i)), (2) without functionalization (T-CQDs-1 (b), T-CQDs-2 (f), T-CQDs-3 (j)), (3) functionalized with pure electron-withdrawing carboxyl groups (T-CQDs-COOH-1 (c), T-CQDs-COOH-2 (g), T-CQDs-COOH-3 (k)). The square-like structure model CQDs without functionalization consisting of 4 (S-CQDs-1 (d)), 10 (S-CQDs-2 (h)) and 20 (S-CQDs-3 (l)) fused benzene rings. The calculated HOMO (m, p, s, v), LUMO (n, q, t, w), and PL spectra (o, r, u, x) of T-CQDs-OH-3, T-CQDs-3, T-CQDs-COOH-3, and S-CQDs-3, respectively