A comparative DFT evaluation of the photocatalytic activity and NLO properties of monolayer two-dimensional M3X4 (M = C, Si, Ge; X = N, P, As) quantum dots

Abstract

Density functional theory (DFT) calculations were carried out to examine the electronic structure of a new class of two-dimensional quantum dots (QDs), including C₃P₄, C₃As₄, Ge₃N₄, Ge₃P₄, Ge₃As₄, Si₃N₄, Si₃P₄, and Si₃As₄. These QDs were derived from a monolayer of two-dimensional C₃N₄ QD, which consists of three tri-s-triazine units. Except for Si₃N₄, Ge₃N₄, and C₃N₄, all optimized QD structures displayed a non-planar geometry. Compared to the C₃N₄ QD (which has a band gap energy of 3.93 eV), all selected QDs demonstrated a reduction in band gap energy, with the C₃P₄ QD showing the lowest value at 1.24 eV. The absorption spectra of the QDs were also analyzed and compared with that of the C₃N₄ QD. Most of the QDs absorbed light in the visible spectrum, whereas Si₃N₄ and Ge₃N₄ QDs absorbed light in the UV region, similar to the C₃N₄ QD. Furthermore, some QDs namely Si₃As₄, C₃As₄, Ge₃As₄, and C₃P₄ exhibited absorption extending into the near-infrared region. The energy levels of the HOMO and LUMO of the QDs were compared with the redox potentials of standard photocatalytic reactions at pH = 7, indicating that all QDs except C₃P₄ and C₃As₄ QDs are promising for photocatalytic applications. The polarizability (α₀) and hyperpolarizability (β₀) of the QDs were also evaluated, with C₃N₄ and Ge₃As₄ showing the lowest and highest α₀ values, respectively and Ge₃P₄ had the highest β₀ value. Electron–hole analysis revealed that Ge₃P₄ QD demonstrated effective photogenerated charge carrier separation, which is essential for photocatalytic efficiency.