Band gaps in pseudopotential self-consistent GW calculations

For materials which are incorrectly predicted by density functional theory to be metallic, an iterative procedure must be adopted in order to perform GW calculations. In this paper we test two iterative schemes based on the quasi-particle and pseudopotential approximations for a number of inorganic semiconductors whose electronic structures are well known from experiment. Iterating just the quasi-particle energies yields a systematic, but modest overestimate of the band gaps, confirming conclusions drawn earlier for CaB_6 and YH_3. Iterating the quasi-particle wave functions as well gives rise to an imbalance between the Hartree and Fock potentials and results in bandgaps in far poorer agreement with experiment.Comment: 5 pages, 2 figures, 2 table

Tailoring the performance of ZnO for oxygen evolution by effective transition metal doping

In the quest for active and inexpensive (photo)electrocatalysts, atomistic simulations of the oxygen evolution reaction (OER) are essential for understanding the catalytic process of water splitting at solid surfaces. In this paper, we study the enhancement of the OER by first-row transition-metal (TM) doping of the abundant semiconductor ZnO, using density functional theory (DFT) calculations on a substantial number of possible structures and bonding geometries. The calculated overpotential for undoped ZnO is 1.0 V. For TM dopants in the 3d series from Mn to Ni, the overpotentials decrease from 0.9 V for Mn, and 0.6 V for Fe, down to 0.4 V for Co, and rise again to 0.5 V for Ni and 0.8 eV for Cu. We analyze the overpotentials in terms of the binding to the surface of the species involved in the four reaction steps of the OER. The Gibbs free energies associated with the adsorption of these intermediate species increase down the series from Mn to Zn, but the difference between OH and OOH adsorption (the species involved in the first, respectively the third reaction step) is always in the range 3.0-3.3 eV, despite a considerable variation in possible bonding geometries. The bonding of the O intermediate species (involved in the second reaction step), which is optimal for Co, and to a somewhat lesser extend for Ni, then ultimately determines the overpotential. These results imply that both Co and Ni are promising dopants for increasing the activity of ZnO-based anodes for the OER.</p

Theoretical prediction of perfect spin filtering at interfaces between close-packed surfaces of Ni or Co and graphite or graphene

The in-plane lattice constants of close-packed planes of fcc and hcp Ni and Co match that of graphite almost perfectly so that they share a common two dimensional reciprocal space. Their electronic structures are such that they overlap in this reciprocal space for one spin direction only allowing us to predict perfect spin filtering for interfaces between graphite and (111) fcc or (0001) hcp Ni or Co. First-principles calculations of the scattering matrix show that the spin filtering is quite insensitive to amounts of interface roughness and disorder which drastically influence the spin-filtering properties of conventional magnetic tunnel junctions or interfaces between transition metals and semiconductors. When a single graphene sheet is adsorbed on these open $d$-shell transition metal surfaces, its characteristic electronic structure, with topological singularities at the K points in the two dimensional Brillouin zone, is destroyed by the chemical bonding. Because graphene bonds only weakly to Cu which has no states at the Fermi energy at the K point for either spin, the electronic structure of graphene can be restored by dusting Ni or Co with one or a few monolayers of Cu while still preserving the ideal spin injection property.Comment: 12 pages, 11 figure

Graphite and graphene as perfect spin filters

Based upon the observations (i) that their in-plane lattice constants match almost perfectly and (ii) that their electronic structures overlap in reciprocal space for one spin direction only, we predict perfect spin filtering for interfaces between graphite and (111) fcc or (0001) hcp Ni or Co. The spin filtering is quite insensitive to roughness and disorder. The formation of a chemical bond between graphite and the open $d$-shell transition metals that might complicate or even prevent spin injection into a single graphene sheet can be simply prevented by dusting Ni or Co with one or a few monolayers of Cu while still preserving the ideal spin injection property

Electronic structure and optical properties of lightweight metal hydrides

We study the electronic structures and dielectric functions of the simple hydrides LiH, NaH, MgH2 and AlH3, and the complex hydrides Li3AlH6, Na3AlH6, LiAlH4, NaAlH4 and Mg(AlH4)2, using first principles density functional theory and GW calculations. All these compounds are large gap insulators with GW single particle band gaps varying from 3.5 eV in AlH3 to 6.5 eV in the MAlH4 compounds. The valence bands are dominated by the hydrogen atoms, whereas the conduction bands have mixed contributions from the hydrogens and the metal cations. The electronic structure of the aluminium compounds is determined mainly by aluminium hydride complexes and their mutual interactions. Despite considerable differences between the band structures and the band gaps of the various compounds, their optical responses are qualitatively similar. In most of the spectra the optical absorption rises sharply above 6 eV and has a strong peak around 8 eV. The quantitative differences in the optical spectra are interpreted in terms of the structure and the electronic structure of the compounds.Comment: 13 pages, 10 figure

Doping graphene with metal contacts

Making devices with graphene necessarily involves making contacts with metals. We use density functional theory to study how graphene is doped by adsorption on metal substrates and find that weak bonding on Al, Ag, Cu, Au and Pt, while preserving its unique electronic structure, can still shift the Fermi level with respect to the conical point by $\sim 0.5$ eV. At equilibrium separations, the crossover from $p$-type to $n$-type doping occurs for a metal work function of $\sim 5.4$ eV, a value much larger than the graphene work function of 4.5 eV. The numerical results for the Fermi level shift in graphene are described very well by a simple analytical model which characterizes the metal solely in terms of its work function, greatly extending their applicability.Comment: 4 pages, 5 figure

Identification of a methylase required for 2-methylhopanoid production and implications for the interpretation of sedimentary hopanes

The rise of atmospheric oxygen has driven environmental change and biological evolution throughout much of Earth’s history and was enabled by the evolution of oxygenic photosynthesis in the cyanobacteria. Dating this metabolic innovation using inorganic proxies from sedimentary rocks has been difficult and one important approach has been to study the distributions of fossil lipids, such as steranes and 2-methylhopanes, as biomarkers for this process. 2-methylhopanes arise from degradation of 2-methylbacteriohopanepolyols (2-MeBHPs), lipids thought to be synthesized primarily by cyanobacteria. The discovery that 2-MeBHPs are produced by an anoxygenic phototroph, however, challenged both their taxonomic link with cyanobacteria and their functional link with oxygenic photosynthesis. Here, we identify a radical SAM methylase encoded by the hpnP gene that is required for methylation at the C-2 position in hopanoids. This gene is found in several, but not all, cyanobacteria and also in α -proteobacteria and acidobacteria. Thus, one cannot extrapolate from the presence of 2-methylhopanes alone, in modern environments or ancient sedimentary rocks, to a particular taxonomic group or metabolism. To understand the origin of this gene, we reconstructed the evolutionary history of HpnP. HpnP proteins from cyanobacteria, Methylobacterium species, and other α-proteobacteria form distinct phylogenetic clusters, but the branching order of these clades could not be confidently resolved. Hence,it is unclear whether HpnP, and 2-methylhopanoids, originated first in the cyanobacteria. In summary, existing evidence does not support the use of 2-methylhopanes as biomarkers for oxygenic photosynthesis

Ferromagnetism without flat bands in thin armchair nanoribbons

Describing by a Hubbard type of model a thin armchair graphene ribbon in the armchair hexagon chain limit, one shows in exact terms, that even if the system does not have flat bands at all, at low concentration a mesoscopic sample can have ferromagnetic ground state, being metallic in the same time. The mechanism is connected to a common effect of correlations and confinement.Comment: 37 pages, 12 figures, in press at Eur. Phys. Jour.

STM and RHEED study of the Si(001)-c(8x8) surface

The Si(001) surface deoxidized by short annealing at T~925C in the ultrahigh vacuum molecular beam epitaxy chamber has been in situ investigated by high resolution scanning tunnelling microscopy (STM) and reflected high energy electron diffraction (RHEED). RHEED patterns corresponding to (2x1) and (4x4) structures were observed during sample treatment. The (4x4) reconstruction arose at T<600C after annealing. The reconstruction was observed to be reversible: the (4x4) structure turned into the (2x1) one at T>600C, the (4x4) structure appeared again at recurring cooling. The c(8x8) reconstruction was revealed by STM at room temperature on the same samples. A fraction of the surface area covered by the c(8x8) structure decreased as the sample cooling rate was reduced. The (2x1) structure was observed on the surface free of the c(8x8) one. The c(8x8) structure has been evidenced to manifest itself as the (4x4) one in the RHEED patterns. A model of the c(8x8) structure formation has been built on the basis of the STM data. Origin of the high-order structure on the Si(001) surface and its connection with the epinucleation phenomenon are discussed.Comment: 26 pages, 12 figure

Ketorolak-dekstran konjugati: sinteza, in vitro i in vivo vrednovanje

Ketorolac is a non-steroidal anti-inflammatory drug. Dextran conjugates of ketorolac (KD) were synthesized and characterized to improve ketorolac aqueous solubility and reduce gastrointestinal side effects. An N-acylimidazole derivative of ketorolac (KAI) was condensed with a model carrier polymer, dextran of different molecular masses (40000, 60000, 110000 and 200000). IR spectral data confirmed formation of ester bonding. Ketorolac contents were evaluated by UV-spectrophotometric analysis. The molecular mass was determined by measuring viscosity using the Mark-Howink-Sakurada equation. In vitro hydrolysis studies were performed in aqueous buffers (pH 1.2, 7.4, 9) and in 80% (V/V) human plasma (pH 7.4). At pH 9, a higher rate of ketorolac release from KD was observed as compared to aqueous buffer of pH 7.4 and 80% human plasma (pH 7.4), following first-order kinetics. In vivo biological screening in mice and rats indicated that conjugates retained analgesic and anti-inflammatory activities with significantly reduced ulcerogenicity compared to the parent drug.U radu je opisana sinteza konjugata dektrana i protuupalnog lijeka ketorolaka (KD). Konjugati su pripravljeni da bi se povećala topljivost ketorolaka u vodi i smanjila njegova nusdjelovanja u gastrointestinanom traktu. Ketorak je prvo preveden u N-acilimidazolni derivat (KAI) koji je kondenziran s polimernim nosačem, dekstranom različitih molekulskih masa (40000, 60000, 110000 i 200000). IR-spektri potvrdili su nastajanje esterske veze. Udio ketorolaka u konjugatu određen je UV-spektrofotometrijskom analizom. Molekulske mase određene su mjerenjem viskoznosti koristeći Mark-Howink-Sakurada jednadžbu. Hidroliza in vitro praćena je u puferskim otopinama (pH 1,2, 7,4 i 9) i u 80% V/V humanoj plazmi (pH 7,4). Pri pH 9 primjećeno je značajno brže oslobađanje ketorolaka iz KD nego u puferskoj otopini pH 7,4 i krvnoj plazmi. Oslobađanje je prati kinetiku prvog reda. In vivo biološka ispitivanja na miševima i štakorima ukazuju da konjugati imaju analgetsko i protuupalno djelovanje, a značajno smanjeno ulcerogeno djelovanje