At B3LYP level of theory, we predict that the half-metallicity in zigzag edge graphene nanoribbon (ZGNR) can be realized when an external electric field is applied across the ribbon. The critical electric field decreases with the increase of the ribbon width to induce the half-metallicity. Both the spin polarization and half-metallicity are removed when the edge state electrons fully transferred from one side to the other under very strong electric field. The electric field range under which ZGNR remains half-metallic increases with the ribbon width. Our study demonstrates a rich field-induced spin polarization behavior, which may lead to some important applications in spinstronics.

Electronic structures of SiC nanoribbons have been studied by spin-polarized first-principles calculations. The armchair nanoribbons are nonmagnetic semiconductors, while the zigzag nanoribbons are magnetic metals. The spin polarization in the zigzag SiC nanoribbons is originated from the unpaired electrons localized on the ribbon edges. Interestingly, the zigzag nanoribbons narrower than ∼4 nm present half-metallic behavior. Without the aid of external field or chemical modification, the metal-free half-metallicity predicted for narrow SiC zigzag nanoribbons opens a facile way for nanomaterial-based spintronics applications.

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...

Negative differential-resistance (NDR) molecular device is realized involving two C60 molecules, one is adsorbed on the tip of a scanning tunneling microscope and the other is on the surface of the hexanethiol self-assembled monolayer. The narrow local density of states features near the Fermi energy of the C60 molecules lead to the obvious NDR effect. Such controllable tunneling structure and the associated known electronic states ensure the stability and reproducibility of the NDR device.