Applicability of classical Lifshitz-Slyozov theory of Ostwald ripening is analyzed and found limited by relatively large cluster sizes due to restrictions imposed by theoretical assumptions. An assumption about the steady state ripening regime poses
an upper limit, while another, implicit assumption of continuous description poses a cluster size-dependent lower limit on the supersaturation level. These two limits mismatch for the clusters under certain size in the nanometer scale making the theory inapplicable. We present a more generic, molecular theory of Ostwald ripening, which reproduces classical Lifshitz-Slyozov and Wagner theories in appropriate extreme cases. This theory has a wider applicability than classical theories, especially at lower supersaturation levels, and is more suitable for nanoscale systems.
We investigate the topology of the spin-polarized charge density in bcc and fcc iron. While the total spin-density is found to possess the topology of the non-magnetic prototypical structures, in some cases the spin-polarized densities are characteri
zed by unique topologies; for example, the spin-polarized charge densities of bcc and high-spin fcc iron are atypical of any known for non-magnetic materials. In these cases, the two spin-densities are correlated: the spin-minority electrons have directional bond paths with deep minima in the minority density, while the spin-majority electrons fill these holes, reducing bond directionality. The presence of two distinct spin topologies suggests that a well-known magnetic phase transition in iron can be fruitfully reexamined in light of these topological changes. We show that the two phase changes seen in fcc iron (paramagnetic to low-spin and low-spin to high-spin) are different. The former follows the Landau symmetry-breaking paradigm and proceeds without a topological transformation, while the latter also involves a topological catastrophe.
Investigations of the Fermi surface via the electron momentum distribution reconstructed from either angular correlation of annihilation radiation (or Compton scattering) experimental spectra are presented. The basis of these experiments and mathemat
ical methods applied in reconstructing three-dimensional densities from line (or plane) projections measured in these experiments are described. The review of papers where such techniques have been applied to study the Fermi surface of metallic materials with showing their main results is also done.
Potassium intercalation in graphite is investigated by first-principles theory. The bonding in the potassium-graphite compound is reasonably well accounted for by traditional semilocal density functional theory (DFT) calculations. However, to investi
gate the intercalate formation energy from pure potassium atoms and graphite requires use of a description of the graphite interlayer binding and thus a consistent account of the nonlocal dispersive interactions. This is included seamlessly with ordinary DFT by a van der Waals density functional (vdW-DF) approach [Phys. Rev. Lett. 92, 246401 (2004)]. The use of the vdW-DF is found to stabilize the graphite crystal, with crystal parameters in fair agreement with experiments. For graphite and potassium-intercalated graphite structural parameters such as binding separation, layer binding energy, formation energy, and bulk modulus are reported. Also the adsorption and sub-surface potassium absorption energies are reported. The vdW-DF description, compared with the traditional semilocal approach, is found to weakly soften the elastic response.
We consider single particle Schrodinger operators with a gap in the en ergy spectrum. We construct a complete, orthonormal basis function set for the inv ariant space corresponding to the spectrum below the spectral gap, which are exponentially local
ized a round a set of closed surfaces of monotonically increasing sizes. Estimates on the exponential dec ay rate and a discussion of the geometry of these surfaces is included.
GaAs nanowires were grown by metalorganic vapor phase epitaxy on evaporated metal films (Au, Au / Pd, Ag, Ni, Ga, Cu, Al, Ti). The samples were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). SEM images
reveal that nanowires grow directly on the metals. TEM characterization shows crystalline nanowire (nw) structure originating from Au. Article presents state of the art about nanowire-metal interface growth and enumerates nanowire contacting methods with metals.
The magnetic properties of materials based on two-dimensional transition-metal dichalcogenides (TMDC) have been investigated by means of first-principles DFT calculations, namely Fe-intercalated bulk Fe$_{1/4}$TaS$_2$ compounds as well as TMDC monola
yers with deposited Fe films. Changing the structure and the composition of systems consisting of Fe overlayers on top of a TMDC monolayers resulted in considerable variations of their physical properties. For the considered systems the Dzyaloshinskii-Moriya (DM) interaction has been determined and used for the subsequent investigation of their magnetic structure using Monte Carlo simulations. Rather strong DM interactions as well as large $D/J$ ratios have been obtained in some of these materials, which can lead to the formation of skyrmionic structures varying with the strength of the applied external magnetic field.
The electrical properties of a set of seven-helix transmembrane proteins, whose space arrangement (3D structure) is known, are investigated by using regular arrays of the amino acids. These structures, specifically cubes, have topological features si
milar to those shown by the chosen proteins. The theoretical results show a good agreement between the predicted current-voltage characteristics obtained from a cubic array and those obtained from a detailed 3D structure. The agreement is confirmed by available experiments on bacteriorhodopsin. Furthermore, all the analyzed proteins are found to share the same critical behaviour of the voltage-dependent conductance and of its variance. In particular, the cubic arrangement evidences a short plateau of the excess conductance and its variance at high voltages. The results of the present investigation show the possibility to predict the I-V characteristics of multiple-protein sample even in the absence of a detailed knowledge of their 3D structure.
In this paper synchrotron microtomography on Plasma Sprayed Tungsten (PS-W) is presented and discussed. PS-W is a challenging material for microtomography since it exhibits a random porous network at different length-scales (from nanometers to microm
eters) and is hardly penetrable by X-rays. Furthermore, inner porosity causes strong internal scattering. The key challenges were, firstly, to optimize the beam parameters considering the inherent trade-off between photon energy (penetration depth) and spatial resolution and, secondly, to develop effective signal filtering algorithms. Despite the limited signal-to-noise ratio detected, large volumes of PS-W could be reconstructed with good image quality and micrometric resolution (voxel size = 1.4 {mu}m). As an important result, we report excellent image quality and higher penetration depth by applying the same setup on a ferrous microstructure, namely a 10%W/Steel MMC used as interlayer between PS-W and a ferritic/martensitic steel substrate. The paper reports a detailed 3D morphological analysis of all inclusion types in PS-W and W/Steel, which led to disclosure of a complex connected porous network in both media. The analysis is presented in terms of multiphase volume fraction, ratio of percolation and 3D shape descriptors. 3D percolation patterns are analyzed in detail and sensitivity towards segmentation threshold for the noise-affected PS-W region is discussed. Remarkably, percolation of the porous phase was found throughout the entire coating thickness of PS-W. In W-Steel MMC percolation was found in the perpendicular plane and interpreted as onset of delamination caused by thermomechanical stress.
Nanofibers functionalized by metal nanostructures and particles are exploited as effective flexible substrates for SERS analysis. Their complex three-dimensional structure may provide Raman signals enhanced by orders of magnitude compared to untextur
ed surfaces. Understanding the origin of such improved performances is therefore very important for pushing nanofiber-based analytical technologies to their upper limit. Here we report on polymer nanofiber mats which can be exploited as substrates for enhancing the Raman spectra of adsorbed probe molecules. The increased surface area and the scattering of light in the nanofibrous system are individually analyzed as mechanisms to enhance Raman scattering. The deposition of gold nanorods on the fibers further amplifies Raman signals due to SERS. This study suggests that Raman signals can be finely tuned in intensity and effectively enhanced in nanofiber mats and arrays by properly tailoring the architecture, composition, and light-scattering properties of the complex networks of filaments.
