Conductivity tuning of charged triazine and heptazine graphitic carbon nitride (g-C3N4) quantum dots via nonmetal (B, O, S, P) doping: DFT calculations

Chemical doping of graphitic carbon nitride (g-C3N4) quantum dots with nontoxic heteroatoms has proven to be an effective means for tuning the electrical properties of this two-dimensional (2D) nanomaterial. In this investigation, triazine (tg-CN) and heptazine (hg-CN) clusters were doped with the p-block (B, O, S, and P) elements, and were further compared in terms of siting and conductance using density functional theory (DFT) at the HSE06/6-311+G* level. The calculation results predicted that B doping in both types of g-C3N4 was favored in place of carbon atoms while the O, S, and P dopants preferred nitrogen atoms, where P siting was dependent on the type of material. Both the initial HOMO-LUMO gap and global hardness were decreased after the substitution, with the most substantial changes after the O and S doping in the hg-CN and tg-CN structures, respectively. The HOMO-LUMO gap changed most significantly (by up to 3.79 eV) with the [+/-] charge switching for the O-doped nanocluster. In contrast to hg-CN, tg-CN turned from an insulator into metallic or half-metallic material upon electron charging. Finally, both hg-CN and tg-CN became better electrophiles after modification, particularly with B doping.