Hexagonal [0001] nonpassivated ZnO nanowires are studied with density functional calculations. The band gap and Young's modulus in nanowires which are larger than those in bulk ZnO increase along with the decrease of the radius of nanowires. We find ZnO nanowires have larger effective piezoelectric constant than bulk ZnO due to their free boundary. In addition, the effective piezoelectric constant in small ZnO nanowires doesn't depend monotonously on the radius due to two competitive effects: elongation of the nanowires and increase of the ratio of surface atoms. Although many studies on ZnO nanowires have been conducted, there are some important issues remained to be addressed. First, the mechanical properties, especially the Young's modulus of ZnO nanowires are on debate in the literature [16,17,18,19,20]. For instance, Chen et al. [16] showed that the Young' modulus of ZnO nanowire with diameters smaller than about 120 nm is significantly higher than that of bulk ZnO. However, the elastic modulus of vertically aligned [0001] ZnO nanowires with an average diameter of 45 nm measured by atomic force microscopy was found to be far smaller than that of bulk ZnO[17]. The second issue is about the electromechanical coupling in ZnO nanowires. The effective piezoelectric coefficient of individual (0001) surface dominated ZnO nanobelts measured by piezoresponse force microscopy was found to be much larger than the value for bulk wurtzite ZnO [21]. In contrast, Fan et al. showed that the piezoelectric coefficient for ZnO nanopillar with the diameter about 300 nm is smaller than the bulk values [22]. They suggested that the reduced electromechanical response might be due to structural defects in the pillars [22]. Whether the electromechanical coupling is enhanced or depressed in defect-free ZnO nanowires is not clear. Thirdly, although it is well known that the quantum confinement effect will decrease the band gap of passivated nanowires, the question that how the dangling bond in bare ZnO nanowires affects the band gap remains open. The fundamental study on these issues is crucial for developing future applications of ZnO nanowires.In this letter, we have studied the electronic, mechanical, and piezoelectric properties of [0001] ZnO nanowires using first-principles methods for the first time. We find that the band gap increases along with the decrease of the radius of ZnO nanowires due to the radial confinement. The Young's modulus of nanowires is larger than bulk ZnO, in agreement with the experimental results of Chen et al. [16]. The effective piezoelectric constant in ZnO nanowires is larger than that of bulk ZnO due to the free boundary of nanowires. Moreover, the effective piezoelectric constant in small ZnO nanowires doesn't depend monotonously on the radius due to two competitive effects.Our calculations are performed using the SIESTA package[23], a standard Kohn-Sham density-functional program using norm-conserving pseudopotentials and numerical atomic orbitals as basis sets. The local density approximation (LD...

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We investigated theoretically the hydrogen permeability and selectivity of a porous silicene membrane via first-principles calculations. The subnanometer pores of the silicene membrane are designed as divacancy defects with octagonal and pentagonal rings (585-divacancy). The porous silicene exhibits high selectivity comparable with graphene-based membranes for hydrogen over various gas molecules (N2, CO, CO2, CH4, and H2O). The divacancy defects in silicene are chemically inert to the considered gas molecules. Our results suggest that the porous silicene membrane is expected to find great potential in gas separation and filtering applications.