J. G. Hou

Will zigzag graphene nanoribbon turn to half metal under electric field

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

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 approximately 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.

Synthesis and optical properties of well-aligned ZnO nanorod array on an undoped ZnO film

High-density well-aligned ZnO nanorod array was successfully synthesized on a PLD prepared undoped ZnO film by a catalyst-free method. X-ray diffraction and scanning electron microscopy show that the nanorods are well-oriented perpendicular to the substrate. The photoluminescence from free excitons and Raman spectra of the ZnO nanorods reflect the high purity and nearly defect free structure of nanorods. The well-aligned feature of the nanorod array is attributed to the nanorods’ epitaxial growth from the ZnO film.

Adsorption energies of molecular oxygen on Au clusters

The adsorption properties of O(2) molecules on anionic, cationic, and neutral Au(n) clusters (n=1-6) are studied using the density functional theory (DFT) with the generalized gradient approximation (GGA), and with the hybrid functional. The results show that the GGA calculations with the PW91 functional systemically overestimate the adsorption energy by 0.2-0.4 eV than the DFT ones with the hybrid functional, resulting in the failure of GGA with the PW91 functional for predicting the adsorption behavior of molecular oxygen on Au clusters. Our DFT calculations with the hybrid functional give the same adsorption behavior of molecular oxygen on Au cluster anions and cations as the experimental measurements. For the neutral Au clusters, the hybrid DFT predicts that only Au(3) and Au(5) clusters can adsorb one O(2) molecule.

The electronic structure of oxygen atom vacancy and hydroxyl impurity defects on titanium dioxide (110) surface

Introducing a charge into a solid such as a metal oxide through chemical, electrical, or optical means can dramatically change its chemical or physical properties. To minimize its free energy, a lattice will distort in a material specific way to accommodate (screen) the Coulomb and exchange interactions presented by the excess charge. The carrier-lattice correlation in response to these interactions defines the spatial extent of the perturbing charge and can impart extraordinary physical and chemical properties such as superconductivity and catalytic activity. Here we investigate by experiment and theory the atomically resolved distribution of the excess charge created by a single oxygen atom vacancy and a hydroxyl (OH) impurity defects on rutile TiO(2)(110) surface. Contrary to the conventional model where the charge remains localized at the defect, scanning tunneling microscopy and density functional theory show it to be delocalized over multiple surrounding titanium atoms. The characteristic charge distribution controls the chemical, photocatalytic, and electronic properties of TiO(2) surfaces.

Piezoelectricity in ZnO nanowires: A first-principles study

Hexagonal [0001] nonpassivated ZnO nanowires with diameters up to 2.8nm are studied with density functional calculations. The authors find that ZnO nanowires have larger effective piezoelectric constant than bulk ZnO due to their free boundary. For ZnO nanowires with diameters larger than 2.8nm, the effective piezoelectric constant is almost a constant. Surprisingly, the effective piezoelectric constant in small ZnO nanowires does not depend monotonically on the radius due to two competitive effects. Moreover, the quantum confinement effect results in larger band gaps of bare ZnO nanowires compared to that of bulk ZnO.

Half-metallicity in hybrid BCN nanoribbons

The established chemical synthetic strategy toward graphene nanoribbons has greatly prompted and justified the research of theoretical designs of novel materials based on graphene. In this paper, we report the novel half-metallicity in C and BN hybrid zigzag nanoribbons even though stand-alone C or BN nanoribbon possesses a finite band gap. By performing first-principles electronic-structure calculations, we find this unexpected half-metallicity in the hybrid nanostructures stems from a competition between the charge and spin polarizations, as well as from the pi orbital hybridization between C and BN. Molecular dynamics simulations indicate that the hybrid nanoribbons are stable. Our results point out a possibility of making spintronic devices solely based on nanoribbons and a new way of fabricating metal-free half metals.

Negative differential-resistance device involving two C60 molecules

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.

Electron-phonon coupling in a boron-doped diamond superconductor

The electronic structure, lattice dynamics, and electron-phonon coupling of the boron-doped diamond are investigated using the density-functional supercell method. Our results indicate the boron-doped diamond is a phonon mediated superconductor, confirming previous theoretical conclusions deduced from the calculations employing the virtual-crystal approximation. We show that the optical-phonon modes involving B vibrations play an important role in the electron-phonon coupling. Different from previous theoretical results, our calculated electron-phonon coupling constant is 0.39 and the estimated superconducting transition temperature

Chiral selective tunneling induced negative differential resistance in zigzag graphene nanoribbon: A theoretical study

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