The electronic structures and optical properties of the[110]-oriented Si_(1-x)Ge_x nanowires(NWs) passivated with different functional groups(-H,-F and-OH) are investigated by using first-principles calculations.The results show that surface passivation influences the characteristics of electronic band structures significantly:the band gap widths and types(direct or indirect) of the Si_(1-x)Ge_x NWs with different terminators show complex and robust variations,and the effective masses of the electrons in the NWs can be modulated dramatically by the terminators.The study of optical absorption shows that the main peaks of the parallel polarization component of Si_(1-x)Ge_x NWs passivated with the functional groups exhibit prominent changes both in height and position,and are red-shifted with respect to those of corresponding pure Si NWs,indicating the importance of both the terminators and Ge concentrations.Our results demonstrate that the electronic and optical properties of Si_(1-x)Ge_x NWs can be tuned by utilizing selected functional groups as well as particular Ge concentrations for customizing purposes. The electronic structures and optical properties of the [110] -oriented Si_ (1-x) Ge_x nanowires (NWs) passivated with different functional groups (-H, -F and-OH) are investigated by using first- show that surface passivation influences the characteristics of electronic band structure significantly: the band gap widths and types (direct or indirect) of the Si_ (1-x) Ge_x NWs with different terminators show complex and robust variations, and the effective masses of the electrons in the NWs can be modulated dramatically by the terminators. The study of optical absorption shows that the main peaks of the parallel polarization component of Si - (1-x) Ge_x NWs passivated with the functional groups exhibit highlight changes both in height and position, and are red-shifted with respect to those of corresponding pure Si NWs, indicating the importance of both the terminators and Ge concentrations. Our results demonstrate that the electronic and optical properties of Si_ (1-x) Ge_ x NWs can be tuned by utilizing selected functional groups as well as particular sun Ge for customizing purposes.