Band gap engineering in armchair germanene nanoribbons under vertical and horizontal electromagnetic irradiation

A finite and controllable band gap is essential for enabling on/off switching in nanoelectronics devices, such as field-effect transistors. Moreover, such a gap permits optical transitions to occur within specific spectral ranges, enabling light emission or photodetection at the nanoscale. Therefore, a comprehensive analysis of the electronic and transport properties of armchair germanene nanoribbons (AGeNRs) with widths N = 5, 6, and 7 under electromagnetic irradiation with orientations θ = 90° (vertical) and θ = 0° (horizontal)is presented. Band structure calculations reveal distinct width-dependent behavior: the pristine N = 5 ribbon is metallic ( E g ≈ 0.00 eV ), while N = 6 and N = 7 exhibit semiconducting gaps of 0.64 eV and 0.69 eV , respectively. Vertical irradiation opens a modest gap in the metallic ribbon ( E g ≈ 0.14 eV at N = 5) and slightly modulates the semiconducting gaps ( E g ≈ 0.48 eV at N = 6 and 0.72 eV at N = 7), preserving the overall electronic phase. In contrast, horizontal irradiation induces strong field–orbital coupling and strain effects, driving N = 6 into a narrow-gap semiconductor regime ( E g ≈ 0.20 eV ) and partial suppression in N = 7 ( E g ≈ 0.58 eV ), while significantly enhancing the gap in N = 5 ( E g ≈ 0.34 eV ). Density of states, transmission spectra, and current–voltage characteristics confirm these trends. These findings underscore the dual role of geometric confinement and field orientation in tailoring the electronic phase and conductivity of AGeNRs, offering a tunable platform for field-responsive nanoelectronic devices.