Tuning Optoelectronic Properties of Hydrogen-Terminated Silicon Nanowires via Functionalized Alkenyl Moieties: A First Principles Study

First-principles calculations are used to investigate how terminal functionalization of conjugated alkenyl C8 chains, an eight-carbon backbone with alternating single and double bonds, grafted onto hydrogen-terminated silicon nanowires can be used to engineer band alignment and low-energy optical absorption. We focus on functionalization with carboxyl (–COOH), amino (–NH2), or phenyl (–C6H5) groups. All terminations preserve good passivation and comparable charge transfer from the nanowire while shifting the HOMO from slightly below the valence-band maximum to either deeper below (–COOH), within the gap (–NH2), or nearly resonant with the valence band maximum (–C6H5). Calculated longitudinal optical conductivities show that these shifts translate into distinct enhancements of optical absorption over complementary windows in the visible range, driven by transitions below energy gaps from the HOMO energy level to hybridized conduction states in the nanowire that are absent in pristine silicon nanowires and in the isolated molecules. Our results establish terminally functionalized alkenyl chains as an effective method to tailor silicon-based hybrid nanostructures for optoelectronic and photovoltaic applications.