We have investigated the geometry, electronic structure and mechanical properties of halogenated silicene XSi (X = F, Cl, Br and I) in various conformers from first principles systematically. The energetics, structure, band structure, elastic constants and the corresponding modulus are given in detail. Our results indicate that halogenated silicene shows enhanced stability compared with silicene and presents a moderate and tunable direct gap with small carrier effective mass. The element- and conformer-dependence of the energy gap can be understood well by the variance of buckling and a bond energy perturbation theory based on orbital hybridization. Moreover, the direction-dependent mechanical properties are also calculated and discussed. Interestingly, a negative Poisson ratio is predicted in fluorinated silicene with boat2 structure. The enhanced stability, moderate energy gap, small carrier effective masses and improved mechanical properties predicted here make halogenated silicene a promising candidate for future electronic devices.
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