Optoelectronic Properties of Hydrogen-Terminated Silicon Nanowires via Aliphatic C₈ Moieties: Impact of C–C Bond Order from First Principles

In the present work we investigate by first principles calculations the structural, electronic, and optical properties of alkyl, 1-alkenyl and 1-alkynyl C₈ moieties chemisorbed on hydrogen-terminated silicon nanowire oriented along the ⟨112⟩ direction. Our results disclose how the nature of the carbon–carbon bond contiguous to the Si surface influences the behavior of the system. While 1-alkynyl groups exhibit the strongest Si–C bonding, it is 1-alkenyl functionalization that induces the most significant enhancement in optical absorption within the visible range due to charge transfer. The charge transferred from the nanowire to the moiety confirms the electronic coupling of the two systems. We found that the highest occupied molecular orbital of the 1-alkenyl moiety lies only 0.3 eV below the valence band edge of the hydrogen-terminated silicon nanowire, enabling new low-energy optical transitions which are absent in both the unmodified silicon nanowire and the isolated molecule. These findings demonstrate a synergistic effect of functionalization. Our study provides valuable insights into the design of functionalized silicon nanostructures with tailored optical properties, with potential implications for applications in sensing, photonics, and energy conversion.