Theoretical Prediction and Anisotropic Optoelectronic Properties of the Two-Dimensional Carbon Material Sq-Biphenylene

This paper theoretically proposes and systematically studies a novel two-dimensional carbon material, namely, Sq-biphenylene, for the first time using first-principles calculations. The research reveals that this structure exhibits excellent mechanical, thermal, and dynamic stability, demonstrating its potential as a stable carbon-based material. Analysis of the electronic structure indicates that it is a direct band gap semiconductor featuring significant anisotropy in electronic properties. The material also exhibits high in-plane stiffness, an adjustable Poisson’s ratio, and effective mass. The linear and nonlinear optical properties of the material are comprehensively studied by introducing the scissors correction based on the HSE06 band gap and the volume correction for two-dimensional systems. The calculations demonstrate that it exhibits strong in-plane and out-of-plane optical anisotropy. Notably, the material exhibits an exceptional second-order nonlinear optical response (χ zyy (2) = χ zxx (2) ) near 2.3 eV, reaching a maximum value of 769 pm/V. This value significantly surpasses those of traditional nonlinear crystals, such as LiNbO 3 (50 pm/V) and GaAs (340 pm/V), and the material exhibits distinct polarization selectivity. This work theoretically expands the family of two-dimensional carbon materials and systematically reveals the unique advantages of Sq-biphenylene in terms of structural stability, electronic and mechanical properties, and linear and nonlinear optical responses. These findings provide solid theoretical guidance for subsequent experimental synthesis and functional device development.