Mechanism of Piezoelectric Effects on Electronic Structure and Optical Properties of Two‐Dimensional g‐C ₃ N ₄

Two‐dimensional graphitic carbon nitride (g‐C 3 N 4 ) 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‐C 3 N 4 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‐C 3 N 4 for optoelectronic and photocatalytic applications.