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Short-range ordering as the formation of couples and pairs between solutes is affected by their formation energies. This is not reflected in the standard regular solution model. Here, we present a ...

In BiFeO

The response of single SnO2 nanowire gas sensors with different diameters between 20 and 140 nm are evaluated by calculating the nanowire conductivity as a function of the surface charge density. The procedure involves the numerical solution of the Poisson-Boltzmann equation for the electrostatic potential in cylindrical geometry in order to model the depletion region and band bending at the SnO2 nanowire surface. In the model we take into account varying surface charge densities σ and bulk electron concentrations n0 to calculate the electrical conductivity. Considering the fact that the surface charge density depends on the nanowire surface interactions with ambient gas, the model allows us to simulate the sensor response when the nanowire is employed as gas sensing component. We report a saturation in depletion length λ versus surface charge density σ which is the principal reason for limiting the sensor responses. The results also show that the conductivity is decreasing by increasing surface charge density, the smaller the nanowire diameter the steeper the decrease. As a result the nanowire response is proportional to 1/d where d is the nanowire diameter. Furthermore, we argue about the validity of the modeling results and their relevance to experimental findings on SnO2 nanowire based gas sensors reported in literature.

The search of new designs for communication technologies is driven by the required high data transfer rates. Piezoelectric single crystals such as LiTaO3 and LiNbO3 have qualified as materials for efficient and precise frequency filters also considered for the new 5G standards. To ensure optimal high frequency functionality, single crystal wafers are grown and cut in particular directions. However, due to the high anisotropic physical properties of these brittle materials, the structural integrity and reliability of devices are affected by the crystal orientation with respect to the occurring thermo-mechanical stresses. In this work, the anisotropic fracture response of LiTaO3 and LiNbO3 single crystals is investigated through combination of (i) toughness measurements in notched micro-cantilevers along weak cleavage planes and (ii) atomistic modelling of cleavage fracture energies using density functional theory. It is demonstrated that differences in fracture behaviour between LiTaO3 and LiNbO3 can be explained by the stronger chemical bonding in LiTaO3 as compared to LiNbO3 within the loaded crystallographic planes. The knowledge on the alignment of tough as well as weak planes (i.e. cleavage planes) can be used to tailor the design of single crystal based functional components, aiming to exhibit enhanced mechanical reliability without compromising the functionality.

Diffusion of impurities along grain boundaries of compound materials is of high relevance for multiple practical applications. In this work, we perform a case study of Cu-impurity diffusion along the special coincident site lattice model Σ 5 (012)[100] GB in stoichiometric and off-stoichiometric TiN. The study includes a thorough investigation of the GB structure, formation of point defects and their interaction with a Cu-impurity at the interface as well as the evaluation of energy barriers for a few plausible impurity migration pathways along TiN grain boundaries. The results show that one of the most probable diffusion mechanisms (with an activation energy of 0.77 eV) is Cu-interstitial migration along a grain boundary channel. For a non-stoichiometric case when nitrogen vacancies can be present at GB, the results show that they can act as trapping centers for Cu atoms and increase the migration energy barrier to 1.2 eV. However, our results also indicate that at equilibrium conditions the effect of trapping may be reduced due to slightly unfavored segregation of N vacancies at TiN grain boundaries.

Silica-based resistive random access memory devices have become an active research area due to complementary metal-oxide-semiconductor compatibility and recent dramatic increases in their performance and endurance. In spite of both experimental and theoretical insights gained into the electroforming process, many atomistic aspects of the set and reset operation of these devices are still poorly understood. Recently a mechanism of electroforming process based on the formation of neutral oxygen vacancies (V

Point defects in the group IVb transition metal carbides and nitrides are investigated by means of density functional theory calculations. We focus on a description of a complex vacancy behavior of the sub-stoichiometric carbides and nitrides and find a strong tendency toward vacancy clustering in the carbides. Our results demonstrate that a special type of a stable point defect, a metal vacancy “dressed” in a shell of six carbon vacancies can be a dominant type of metal-vacancy-containing defect in the carbon-deficient sub-stoichiometric carbides, whereas the simplest point defects appear to be dominant in the nitrogen-deficient sub-stoichiometric nitrides. We also show that such clusters are strongly bound in carbides and that temperature has a relatively small effect on the overall defect stability of group IVb transition metal carbides and nitrides.

Ab initio calculations employing density-functional theory have become a state-of-the-art tool of knowledge-based material science. We present a series of research topics from the Materials Center Leoben and demonstrate how calculations on the atomic scale can be used to understand and predict materials properties.

Carbon nanotubes decorated with transition metal, in particular, scandium, titanium, and vanadium, atoms offer promise for use in various applied science fields. We report the results of quantum-chemical calculations of the structure of the metallic layer of atoms of these metals coating the surface of (9, 0) and (10, 0) carbon nanotubes. It was shown that uniform one-layer coating by scandium and titanium could form on nanotubes with diameters no less than the diameter of (10, 0) nanotubes. Vanadium atoms could not uniformly cover nanotubes irrespective of their diameters.

Complexes of zigzag-type carbon nanotubes (CNTs) with transition metal atoms, scandium and titanium, were studied. It was demonstrated that the energy of binding of both atoms with a carbon surface decreases whereas the rate of diffusion along the surface increases with increasing nanotube diameter. The rate constant of migration of scandium atoms over a CNT surface are several orders of magnitude higher than that for titanium atoms, because the CNT surface—Sc atom binding energy is substantially lower.

This work is concerned with modeling the diffusion of atomic hydrogen on the surface of differently sized carbon nanotubes with different chiralities. The calculations were based on the estimation of activation energy by the density functional theory method and the use of transition state theory.

Potential profiles were obtained for the chemisorption of hydrogen on (n, n) and (n, 0) carbon nanotubes. The energy barriers and rate constants for hydrogen molecule sorption on and desorption from various nanotubes were determined. The constants for sorption and desorption were used to calculate sorption-desorption equilibrium constants. Sorption on outside nanotube surfaces was found to be more favorable energetically than sorption on inside surfaces.

Dense single-crystal and polycrystalline TiN/Cu stacks were prepared by unbalanced DC magnetron sputter deposition at a substrate temperature of 700 °C and a pulsed bias potential of −100 V. The microstructural variation was achieved by using two different substrate materials, MgO(001) and thermally oxidized Si(001), respectively. Subsequently, the stacks were subjected to isothermal annealing treatments at 900 °C for 1 h in high vacuum to induce the diffusion of Cu into the TiN. The performance of the TiN diffusion barrier layers was evaluated by cross-sectional transmission electron microscopy in combination with energy-dispersive X-ray spectrometry mapping and atom probe tomography. No Cu penetration was evident in the single-crystal stack up to annealing temperatures of 900 °C, due to the low density of line and planar defects in single-crystal TiN. However, at higher annealing temperatures when diffusion becomes more prominent, density-functional theory calculations predict a stoichiometry-dependent ...

Segregation is a crucial phenomenon, which has to be considered in functional material design. Segregation processes in perovskite oxides have been the subject of ongoing scientific interest, since they can lead to a modification of properties and a loss of functionality. Many studies in oxide thin films have focused on segregation toward the surface using a variety of surface-sensitive analysis techniques. In contrast, here we report a Ca segregation toward an in-plane compressively strained heterostructure interface in a Ca- and Mn-codoped bismuth ferrite film. We are using advanced transmission electron microscopy techniques, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. Ca segregation is found to trigger atomic and electronic structure changes at the interface. This includes the reduction of the interface strain according to the Ca concentration gradient, interplanar spacing variations, and oxygen vacancies at the interface. The experimental results are supported by DFT calculations, which explore two segregation scenarios, i.e., one without oxygen vacancies and Fe oxidation from 3+ to 4+ and one with vacancies for charge compensation. Comparison with electron energy loss spectroscopy (EELS) measurements confirms the second segregation scenario with vacancy formation. The findings contribute to the understanding of segregation and indicate promising effects of a Ca-rich buffer layer in this heterostructure system.