Anisotropic and tunable properties of hydrogen/halogen-terminated germanene nanoribbons for advanced optoelectronics

This study explores the structural, electronic, and optical characteristics of X-terminated germanene nanoribbons (GeNRs) featuring armchair (7-AGeNR) and zigzag (5-ZGeNR) arrangements through first-principles calculations. The findings illustrate how halogen edge functionalization (H, F, Cl, Br, I) significantly modifies the geometric, electronic, and optical traits of GeNRs. Specifically, the Ge-Ge bond length (2.44 Å) and the buckling height (0.69 Å) of the 7-AGeNRs are affected by the size of the X-terminated atoms, with larger halogens leading to increased bond lengths and changes in electronic properties. The analysis of the electronic band structure indicates that halogen passivation introduces a bandgap in the nanoribbons, with 7-AGeNRs-F displaying the largest bandgap of 0.60 eV, as opposed to 0.37 eV for 7-AGeNRs-I. We provide new insights into the tunable anisotropy of the dielectric constant and distinct optical transitions induced by halogen edge functionalization. The optical properties exhibit notable anisotropy, with 7-AGeNR-H showing a dielectric constant of ${\epsilon }_1^{xx}\left(0\right)=12.984$ and ${\epsilon }_1^{yy}\left(0\right)=4.127$, while I-termination leads to reduced values of 7.762 and 5.031, respectively. Furthermore, a redshift in absorption is observed for 7-AGeNRs with heavier halogens, while 5-ZGeNRs demonstrate a blueshift. Reflectivity and plasma frequency analyses point to an optical anisotropy, where H-terminated 7-AGeNR shows a high reflectivity of $R^{xx}\left(0\right)=0.320$, which diminishes with heavier halogens. These adjustable properties indicate the potential application of GeNRs in optoelectronic devices, including infrared (IR) detectors, ultraviolet (UV) sensors, and photovoltaic systems.