Migration of Mg and other interstitial metal dopants in GaN

The minimum energy paths for the migration of interstitial Mg in wurtzite GaN are studied through density functional calculations. The study also comprises Li, Na, and Be dopants to examine the dependence on size and charge of the dopant species. In all cases considered, the impurities diffuse like ions without any tendency of localizing charge. Li, Mg, and to some extent Na, diffuse almost isotropically in GaN, with average diffusion barriers of 1.1, 2.1, and 2.5 eV, respectively. Instead Be shows a marked anisotropy with energy barriers of 0.76 and 1.88 eV for diffusion paths perpendicular and parallel to the c-axis. The diffusion barrier generally increases with ionic charge and ionic radius, but their interplay is not trivial. The calculated migration barrier for Mg is consistent with the values estimated in a recent beta- emission channeling experiment

Metal adatoms on graphene and hexagonal boron nitride: Towards the rational design of self-assembly templates

Periodically corrugated epitaxial graphene and hexagonal boron nitride (h-BN) on metallic substrates are considered as perspective templates for the self-assembly of nanoparticles arrays. By using first-principles calculations, we determine binding energies and diffusion activation barriers of metal adatoms on graphene and h-BN. The observed chemical trends can be understood in terms of the interplay between charge transfer and covalent bonding involving the adatom d electrons. We further investigate the electronic effects of the metallic substrate and find that periodically corrugated templates based on graphene in combination with strong interactions at the metal/graphene interface are the most suitable for the self-assembly of highly regular nanoparticle arrays.Comment: 5 pages, 3 figures, 1 tabl

Magnetoresistive junctions based on epitaxial graphene and hexagonal boron nitride

We propose monolayer epitaxial graphene and hexagonal boron nitride (h-BN) as ultimate thickness covalent spacers for magnetoresistive junctions. Using a first-principles approach, we investigate the structural, magnetic and spin transport properties of such junctions based on structurally well defined interfaces with (111) fcc or (0001) hcp ferromagnetic transition metals. We find low resistance area products, strong exchange couplings across the interface, and magnetoresistance ratios exceeding 100% for certain chemical compositions. These properties can be fine tuned, making the proposed junctions attractive for nanoscale spintronics applications.Comment: 5 page

Liquid Water through Density-Functional Molecular Dynamics: Plane-Wave vs Atomic-Orbital Basis Sets

We determine and compare structural, dynamical, and electronic properties of liquid water at near ambient conditions through density-functional molecular dynamics simulations, when using either plane-wave or atomic-orbital basis sets. In both frameworks, the electronic structure and the atomic forces are self-consistently determined within the same theoretical scheme based on a nonlocal density functional accounting for van der Waals interactions. The overall properties of liquid water achieved within the two frameworks are in excellent agreement with each other. Thus, our study supports that implementations with plane-wave or atomic-orbital basis sets yield equivalent results and can be used indiscriminately in study of liquid water or aqueous solutions

Effect of Metal Element in Catalytic Growth of Carbon Nanotubes

Using first principles calculations, we model the chemical vapor deposition (CVD) growth of carbon nanotubes (CNT) on nanoparticles of late transition (Ni, Pd, Pt) and coinage (Cu, Ag, Au) metals. The process is analyzed in terms of the binding of mono- and diatomic carbon species, their diffusion pathways, and the stability of the growing CNT. We find that the diffusion pathways can be controlled by the choice of the catalyst and the carbon precursor. Binding of the CNT through armchair edges is more favorable than through zigzag ones, but the relative stability varies significantly among the metals. Coinage metals, in particular Cu, are found to favor CVD growth of CNTs at low temperatures and with narrow chirality distributions.Comment: Phys. Rev. Lett., accepte

Band-edge problem in the theoretical determination of defect energy levels: the O vacancy in ZnO as a benchmark case

Calculations of formation energies and charge transition levels of defects routinely rely on density functional theory (DFT) for describing the electronic structure. Since bulk band gaps of semiconductors and insulators are not well described in semilocal approximations to DFT, band-gap correction schemes or advanced theoretical models which properly describe band gaps need to be employed. However, it has become apparent that different methods that reproduce the experimental band gap can yield substantially different results regarding charge transition levels of point defects. We investigate this problem in the case of the (+2/0) charge transition level of the O vacancy in ZnO, which has attracted considerable attention as a benchmark case. For this purpose, we first perform calculations based on non-screened hybrid density functionals, and then compare our results with those of other methods. While our results agree very well with those obtained with screened hybrid functionals, they are strikingly different compared to those obtained with other band-gap corrected schemes. Nevertheless, we show that all the different methods agree well with each other and with our calculations when a suitable alignment procedure is adopted. The proposed procedure consists in aligning the electron band structure through an external potential, such as the vacuum level. When the electron densities are well reproduced, this procedure is equivalent to an alignment through the average electrostatic potential in a calculation subject to periodic boundary conditions. We stress that, in order to give accurate defect levels, a theoretical scheme is required to yield not only band gaps in agreement with experiment, but also band edges correctly positioned with respect to such a reference potential

Charge state of the O2_{2} molecule during silicon oxidation through hybrid functional calculations

We study the charge state of the diffusing O$_2$ molecule during silicon oxidation through hybrid functional calculations. We calculate charge transition levels of O$_2$ in bulk SiO$_2$ and use theoretical band offsets to align these levels with respect to the Si band edges. To overcome the band-gap problem of semilocal density fuctionals, we employ hybrid functionals with both predefined and empirically adjusted mixing coefficients. We find that the charge transition level $\epsilon^{0/-}$ in bulk SiO$_2$ occurs at $\sim$1.1 eV above the silicon conduction band edge, implying that the O$_2$ molecule diffuses through the oxide in the neutral charge state. While interfacial effects concur to lower the charge transition level, our estimates suggest that the neutral charge state persists until silicon oxidation.Comment: 4 pages, 3 figure

Hubbard UU through polaronic defect states

Since the preliminary work of Anisimov and co-workers, the Hubbard corrected DFT+$U$ functional has been used for predicting properties of correlated materials by applying on-site effective Coulomb interactions to specific orbitals. However, the determination of the Hubbard $U$ parameter has remained under intense discussion despite the multitude of approaches proposed. Here, we define a selection criterion based on the use of polaronic defect states for the enforcement of the piecewise linearity of the total energy upon electron occupation. The values of $U$ determined in this way are found to be robust upon variation of the considered state. The corresponding electronic and structural properties are in good agreement with results from piecewise linear hybrid functionals. In particular, defect formation energies are well reproduced, thereby validating the energetics achieved with our selection criterion. It is emphasized that the selection of $U$ should be based on physical properties directly associated with the orbitals to which $U$ is applied, rather than on more global properties such as band gaps. For comparison, we also determine $U$ through a well-established linear response scheme finding noticeably different values of $U$ and consequently different formation energies. Possible origins of these discrepancies are discussed. As case studies, we consider the self-trapped electron in BiVO$_4$, the self-trapped hole in MgO, the Li-trapped hole in MgO, and the Al-trapped hole in $\alpha$-SiO$_2$.Comment: 7 pages, 4 figure

Carbon rehybridization at the graphene/SiC(0001) interface: Effect on stability and atomic-scale corrugation

We address the energetic stability of the graphene/SiC(0001) interface and the associated binding mechanism by studying a series of low-strain commensurate interface structures within a density functional scheme. Among the structures with negligible strain, the 6\surd3\times6\surd3 R30{\deg} SiC periodicity shows the lowest interface energy, providing a rationale for its frequent experimental observation. The interface stability is driven by the enhanced local reactivity of the substrate-bonded graphene atoms undergoing sp2-to-sp3 rehybridization (pyramidalization). By this mechanism, relaxed structures of higher stability exhibit more pronounced graphene corrugations at the atomic scale