Mechanism of Piezoelectric Effects on Electronic Structure and Optical Properties of Two-Dimensional g-C₃N₄

Two-dimensional graphitic carbon nitride (g-C₃N₄) is a promising photocatalyst, though its efficiency is hindered by fast charge recombination and limited visible-light absorption. In this work, we investigated the influence of stress applied along the armchair (X-axis) and zigzag (Y-axis) directions on the electronic structure and optical properties of monolayer g-C₃N₄ using first-principles calculations. The results show that applied stress effectively tunes the C─N bond lengths and significantly modulates the bandgap: compressive stress substantially reduces the bandgap from 2.64 eV (pristine) to 2.04 eV (-9 GPa, Y-axis) and 2.13 eV (-9 GPa, X-axis), while tensile stress leads to a slight widening. Moreover, a technologically critical indirect-to-direct bandgap transition occurs at specific critical strains-under Y-axis tensile stress exceeding +4 GPa or X-axis compressive stress beyond -6 GPa. This transition promotes direct electron excitation. Density of states analysis reveals the underlying mechanism: compressive stress lowers the conduction band minimum by enhancing the hybridization of C/N-2p orbitals, while tensile stress increases the contribution of s-orbitals. Modifications in optical properties under specific stress conditions significantly extend the visible-light absorption range (380-760 nm). This work establishes stress engineering as an effective strategy for optimizing g-C₃N₄ for optoelectronic and photocatalytic applications.