Results 1 - 10 of 19886
Results 1 - 10 of 19886. Search took: 0.04 seconds
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[en] The atomic structures of clean and hydrogen-saturated Mo(111) surfaces were investigated by quantitative structure analyses combining low-energy electron diffraction and density functional theory (DFT) calculations. Both methods corroborate, in good agreement, the pronounced contraction of the top two interlayer spacings for the clean surface predicted theoretically earlier. Upon hydrogen saturation with three adatoms per surface unit cell, the drastic contraction of the uppermost interlayer distance is not reduced as usually observed for other surfaces, but remains practically unchanged. In contrast, the second interlayer spacing de-relaxes completely to the bulk value. Hydrogen is found to adsorb at sites with twofold coordination, bonding to atoms of the top two substrate layers, shifted off the ideal bridge position. (author)
[en] Highlights: • Mechanical properties of novel analogous of α-graphyne named α2-graphyne were studied. • The α-graphyne and α2-graphyne sheets are mechanically stable with stiffness. • These structures are proper materials for use in nanomechanical applications. The mechanical properties of two forms of graphyne sheets named α-graphyne and α2-graphyne under uniaxial and biaxial strains were studied. In-plane stiffness, bulk modulus, and shear modulus were calculated based on density functional theory. The in-plane stiffness, bulk modulus, and shear modulus of α2-graphyne were found to be larger than that of α-graphyne. The maximum values of supported uniaxial and biaxial strains before failure were determined. The α-graphyne was entered into the plastic region with the higher magnitude of tension in comparison to α2-graphyne. The mechanical properties of α-graphyne family revealed that these forms of graphyne are proper materials for use in nanomechanical applications.
[en] A simple method for non-empirical ligand field multiplet calculations for transition metal L-edge spectra is presented. Ligand field splittings and anisotropic scaling factors for Coulomb integrals are obtained from density functional theory. The method is applied to transition metal monoxide solids and nickel and cobalt phthalocyanines molecules and good agreement with experiment is obtained.
[en] Complete text of publication follows: The 18-electron principle goes back to Langmuir . For its history and interpretation, see the recent paper . Formally it would correspond to fully occupying at a central atom its ns, np and (n-1)d orbitals. For early 5f-elements the f-shell becomes chemically available and remains so until about Am. Theoretically it could be filled with 14 further electrons, bringing the total to 32, a theoretical possibility already evoked by Langmuir . How far towards that limit can one go? Thorocene, Th(C8H8)2, was classified as a '20e' case . In the 'metalloactinyl' compounds, like the linear IrThIr2-, one could potentially reach '24e' . We now find that the 6p valence band of the recently discovered icosahedral [Pb12]2- shells forms a perfect partner for the 5f shell of an enclosed actinide atom, like plutonium. Detailed DFT calculations suggest that the system is viable. It could be on good grounds characterised as a '32e' system. The calculated molecular geometries, an orbital analysis and a bonding energy analysis in term of Morokuma-type decomposition will be presented for [Pb12]2- and [M*Pb12]x- with M=Yb, Th, U, Np, Pu, Am, Cm. The orbital-energy spectra and the densities of states for [Pb12]2-, [An*Pb12]x- (An=Pu, Am, Cm) will be given. Finally we will show for [Pu*Pb12] an ELF distribution, clearly demonstrating the radial bonds. References:  J.-P. Dognon, C. Clavaguera, P. Pyykko, Angew. Chem. Int. Ed. 2007, 46, 1- 5  I. Langmuir, Science 1921, 54, 59-67, this paper mentions the 8, 18 and 32-electron closed shells and uses on pp. 65-66 Fe(CO)5, Ni(CO)4 and Mo(CO)6 as examples on 18e.  P. Pyykko, J. Organomet. Chem. 2006, 691, 4336-4340  P. Hrobarik, M. Straka, P. Pyykko, Chem. Phys. Lett. 2006, 431, 6-12
[en] The electronic band structures of Si and Ge low-dimensional nanostructure such as nanofilms and nanowires have been calculated using first principles based on density functional theory (DFT) with the generalized gradient approximation (GGA). The calculation results show that a direct band gap can be obtained from Si orientation  or in Ge orientation  confined low dimensional nanostructure. However, an indirect band gap is still kept in the Si orientation  or in the Ge orientation  confined low dimensional nanostructure. The calculation results are interesting and the transition mechanism from indirect to direct band gap in low dimensional nanostructures is given in the physical structures model.
[en] The high-throughput search paradigm adopted by the newly established caloric materials consortium—CaloriCool®—with the goal to substantially accelerate discovery and design of novel caloric materials is briefly discussed. Here, we begin with describing material selection criteria based on known properties, which are then followed by heuristic fast estimates, ab initio calculations, all of which has been implemented in a set of automated computational tools and measurements. We also demonstrate how theoretical and computational methods serve as a guide for experimental efforts by considering a representative example from the field of magnetocaloric materials.
[en] Current-voltage characteristics of armchair and zigzag γ-graphyne nanotubes with three different diameters under uniaxial strain are investigated by using first-principles quantum transport calculations through density functional theory (DFT) and non-equilibrium Green’s function (NEGF) method. It is shown that for a given value of bias voltage, the resulting current depends strongly on the applied load so that tensile and compressive strain can generate Negative Differential Resistance (NDR) mostly into the armchair nanotubes. Our study reveals that the rectification behavior of the systems is mainly due to the asymmetric electron transmission function distribution in the conduction and valence bands. (paper)
[en] In this work, electrophilicity indices are calculated for 50 peroxyl radicals with density-functional theory (B3LYP hybrid functional and the 6-311+G(d,p) basis set). There is a relationship between general and local electrophilicity indices of peroxyl radicals and Taft inductive constants (σ*) of the substituents at the–OO• group.
[en] Molecular electronics aims at integrating controllable molecular devices into circuits or machines to realize certain functions. According to device configuration, molecular field-effect transistors with top-gate electrodes have great advantages for integration. Nevertheless, from technical aspects, it is difficult to control lateral scale and position of a top-gate electrode precisely. Therefore, one problem arises in how lateral scaling and positioning effects of a top-gate electrode affect device performance. To solve this problem, the electronic transport properties of single-molecule field-effect transistor configurations modulated by a series of partial-scale top-gate electrodes with different lateral scales and positions are studied by using non-equilibrium Green’s function in combination with density functional theory, and compared with those of the full gate electrode (can be considered as a bottom gate electrode). The results show that lateral scaling and positioning effects indeed have a great impact on electronic transport properties of single-molecule field-effect transistor configurations. For -saturated 1,12-dodecanedithiol devices, larger lateral scale of a partial-scale top-gate electrode obtains larger amplification coefficient (ratio of device conductances with/without a gate electrode), and even larger than that of the full gate electrode. While lateral positioning effect has little influence on this device. For -conjugated 1,3,5,7,9,11-dodehexaene-1,12-dithiol devices, performance of a partial-scale top-gate electrode mainly depends on locations of its two edges, i.e. the number of bonds that it breaks. These results will provide theoretical directions in device designing and manufacturing in the future. (paper)
[en] We have investigated site preference and bonding properties of alloying elements including Nb, Mo, Ni and Ag in TiAl with O impurity using a first-principles method based on the density functional theory. We found that the preferable sites for O are the Ti-rich octahedral interstitial ones, while those for the alloying elements are the substitutional ones. Among these elements which are beneficial to improve the mechanical properties of TiAl, Ni and Ag occupy the Al sites, while Nb and Mo occupy the Ti sites. We demonstrate that the presence of O alters the site preference of these alloying elements in TiAl, making these elements prefer to substitute Al that is the first nearest neighbor of O, because O prefers to bond with Ti rather than Al. We suggest that, according to the local density of states results, O can be deleterious to the ductility of TiAl with Nb and Mo, but has little effect on that of TiAl with Ni and Ag