Strain Engineering of Electronic, Elastic, and Optical Properties of a Pentagonal As 2 Ge Monolayer: A Density-Functional Theory Study

Within the framework of density-functional theory based first-principles calculations, the structural, electronic, and optical properties of the As 2 Ge monolayer have been investigated under uniaxial and biaxial strains with varying magnitudes. It has been found that the As 2 Ge monolayer exhibits ultimate strain values of 22% and 16% under uniaxial and biaxial strains, respectively, while the corresponding ideal stresses are 2.1 (2.6) GPa·nm for the uniaxial (biaxial) strain. For both types of strain, as the strain increases, our calculations show that the layer thickness decreases gradually, while an increase in the As–Ge bond lengths along the strained axis is predicted. Applying biaxial strain reveals a decrease in the band gap under both compressive and tensile strains. Furthermore, As2Ge undergoes a semiconductor-to-metal transition at –13.4% and +18.8% under compressive and tensile strains, respectively, indicating that metallization occurs significantly earlier under compressive strain than under tensile strain. The analysis of the optical absorption spectrum of the unstrained As 2 Ge monolayer shows an onset at ~2.8 eV and an appearance of a major absorption peak at ~6 eV, with an absorption coefficient of ~1.2 × 10 6 cm -1 . Under biaxial compressive and tensile strains, the absorption edge has exhibited a slight redshift, and a moderate change in the intensity of the main ultraviolet peak has also been observed. Additionally, as the strain increases, the absorption in the low-energy range gradually increases, leading to metallization at strain values consistent with our band-structure calculations. Therefore, these results are important for highlighting the significance of mechanical strain as an effective approach for tuning the material's structural, electronic, and optical properties.