A comparison of HREM and weak beam transmission electron microscopy for the quantitative measurement of the thickness of ferroelectric domain walls

In this paper we present two methods for the quantitative measurement of the thickness of ferroelectric domain walls, one using high-resolution electron microscopy (HREM) and the other weak beam transmission electron microscopy (WBTEM). These techniques can be used to determine the thickness of domain walls at room temperature as well as close to the ferroelectric to paraelectric phase transition. The first method allows a direct visualization of the lattice distortion across the domain wall, by measuring the continuous deviation of a set of planes with respect to the undistorted lattice. The second method consists in a quantitative analysis of the thickness fringes that appear on weak beam images of inclined domain walls. By fitting simulated fringe profiles to experimental ones, we can extract the thickness of the domain walls in a quantitative way. These two complementary techniques lead to a complete characterization of the thickness of ferroelectric domain walls over a wide range of specimen thicknesses at different magnifications. As an example we apply these methods to ferroelectric domain walls in PbTiO3 The domain wall thickness at room temperature is found to be 1.5 ± 0.3 nm using HREM (in very thin samples≈10 nm) and 2.1 ± 0.7 nm using WBTEM (in samples thicker than 30 nm

Formation of an Icosahedral Structure during the Freezing of Gold Nanoclusters: Surface-Induced Mechanism

The freezing behavior of gold nanoclusters was studied by employing molecular dynamics simulations based on a semi-empirical embedded-atom method. Investigations of the gold nanoclusters revealed that, just after freezing, ordered nano-surfaces with a fivefold symmetry were formed with interior atoms remaining in the disordered state. Further lowering of temperatures induced nano-crystallization of the interior atoms that proceeded from the surface towards the core region, finally leading to an icosahedral structure. These dynamic processes explain why the icosahedral cluster structure is dominantly formed in spite of its energetic metastability.Comment: 9 pages, 4 figures(including 14 eps-files

Neonatal high protein intake enhances neonatal growth without significant adverse renal effects in spontaneous IUGR piglets.

In humans, early high protein (HP) intake has been recommended to prevent postnatal growth restriction and complications of intrauterine growth restriction (IUGR). However, the impact of such a strategy on the kidneys remains unknown, while significant renal hypertrophy, proteinuria, and glomerular sclerosis have been demonstrated in few experimental studies. The objective of this study was to evaluate the effects of a neonatal HP formula on renal structure in IUGR piglets. Spontaneous IUGR piglets were randomly allocated to normal protein (NP, javax.xml.bind.JAXBElement@68d5845e  = 10) formula or to HP formula (+50% protein content, javax.xml.bind.JAXBElement@3e768c15  = 10) up to day 28 after birth. Body weight, body composition, renal functions, and structure were assessed at the end of the neonatal period. While birth weights were similar, 28-day-old HP piglets were 18% heavier than NP piglets ( javax.xml.bind.JAXBElement@206b72ec < javax.xml.bind.JAXBElement@7f241a6d 0.01). Carcass protein content was 22% higher in HP than in NP offspring ( javax.xml.bind.JAXBElement@3b9786a3 < javax.xml.bind.JAXBElement@318ba3e0 0.01). Despite a HP intake, kidney weight and glomerular fibrosis were unaltered in HP piglets. Only a 20% increase in glomerular volume was noted in HP piglets ( javax.xml.bind.JAXBElement@4a0c5b2f  < 0.05) and restricted to the inner cortical area nephrons ( javax.xml.bind.JAXBElement@1524c771 = javax.xml.bind.JAXBElement@1281f9e8 0.03). Plasma urea/creatinine ratio and proteinuria were unchanged in HP piglets. In conclusion, neonatal HP feeding in IUGR piglets significantly enhanced neonatal growth and tissue protein deposition but mildly affected glomerular volume. It can be speculated that a sustained tissue protein anabolism in response to HP intake have limited single nephron glomerular hyperfiltration

Melting Point and Lattice Parameter Shifts in Supported Metal Nanoclusters

The dependencies of the melting point and the lattice parameter of supported metal nanoclusters as functions of clusters height are theoretically investigated in the framework of the uniform approach. The vacancy mechanism describing the melting point and the lattice parameter shifts in nanoclusters with decrease of their size is proposed. It is shown that under the high vacuum conditions (p<10^-7 torr) the essential role in clusters melting point and lattice parameter shifts is played by the van der Waals forces of cluster-substrate interation. The proposed model satisfactorily accounts for the experimental data.Comment: 6 pages, 3 figures, 1 tabl

Structured Au/FeOx/C catalysts for low temperature CO oxidation

Innovative structured catalysts based on nanoparticles of gold supported on activated carbon fibers (ACF) in the form of woven fabrics are presented for low-temperature CO oxidation. Gold was deposited by adsorption from aqueous solution of ethylenediamine complex [Au(en)2]Cl3 followed by reduction in hydrogen. The catalysts were studied under transient reaction conditions and characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray energy dispersive analysis (EDS). HRTEM-EDS shows that gold is present on the surface of Au/ACF catalyst in the form of metallic particles with sizes of ~2.5–5 and ~30–50 nm. A predeposition of iron oxide on the ACF was beneficial for the Au dispersion and catalytic activity in CO oxidation. Gold particles in the Au/FeOx/ACF samples were not in direct contact with the Fe2O3 phase and their size was smaller than without doping by iron oxide. The mechanism of catalyst formation, its morphology, and the influence of preparative conditions are discussed. The activity of Au/FeOx/ACF was substantially higher as compared to Au/Al2O3 and Au/FeOx/Al2O3 catalysts. A reductive pretreatment with H2 was necessary to activate the catalyst, but the activity decreased rapidly in CO/O2 atmosphere. Addition of hydrogen or water vapors to the reaction mixture increases the catalyst activity

Diffusion of gold nanoclusters on graphite

We present a detailed molecular-dynamics study of the diffusion and coalescence of large (249-atom) gold clusters on graphite surfaces. The diffusivity of monoclusters is found to be comparable to that for single adatoms. Likewise, and even more important, cluster dimers are also found to diffuse at a rate which is comparable to that for adatoms and monoclusters. As a consequence, large islands formed by cluster aggregation are also expected to be mobile. Using kinetic Monte Carlo simulations, and assuming a proper scaling law for the dependence on size of the diffusivity of large clusters, we find that islands consisting of as many as 100 monoclusters should exhibit significant mobility. This result has profound implications for the morphology of cluster-assembled materials

Spontaneous alloying in binary metal microclusters - A molecular dynamics study -

Microcanonical molecular dynamics study of the spontaneous alloying(SA), which is a manifestation of fast atomic diffusion in a nano-sized metal cluster, is done in terms of a simple two dimensional binary Morse model. Important features observed by Yasuda and Mori are well reproduced in our simulation. The temperature dependence and size dependence of the SA phenomena are extensively explored by examining long time dynamics. The dominant role of negative heat of solution in completing the SA is also discussed. We point out that a presence of melting surface induces the diffusion of core atoms even if they are solid-like. In other words, the {\it surface melting} at substantially low temperature plays a key role in attaining the SA.Comment: 15 pages, 12 fgures, Submitted to Phys.Rev.

Molecular dynamics simulations of lead clusters

Molecular dynamics simulations of nanometer-sized lead clusters have been performed using the Lim, Ong and Ercolessi glue potential (Surf. Sci. {\bf 269/270}, 1109 (1992)). The binding energies of clusters forming crystalline (fcc), decahedron and icosahedron structures are compared, showing that fcc cuboctahedra are the most energetically favoured of these polyhedral model structures. However, simulations of the freezing of liquid droplets produced a characteristic form of ``shaved'' icosahedron, in which atoms are absent at the edges and apexes of the polyhedron. This arrangement is energetically favoured for 600-4000 atom clusters. Larger clusters favour crystalline structures. Indeed, simulated freezing of a 6525-atom liquid droplet produced an imperfect fcc Wulff particle, containing a number of parallel stacking faults. The effects of temperature on the preferred structure of crystalline clusters below the melting point have been considered. The implications of these results for the interpretation of experimental data is discussed.Comment: 11 pages, 18 figues, new section added and one figure added, other minor changes for publicatio

Thermodynamics of Na_8 and Na_{20} clusters studied with ab-initio electronic structure methods

We study the thermodynamics of Na_8 and Na_{20} clusters using multiple-histogram methods and an ab initio treatment of the valence electrons within density functional theory. We consider the influence of various electron kinetic-energy functionals and pseudopotentials on the canonical ionic specific heats. The results for all models we consider show qualitative similarities, but also significant temperature shifts from model to model of peaks and other features in the specific-heat curves. The use of phenomenological pseudopotentials shifts the melting peak substantially (~ 50--100 K) when compared to ab-initio results. It is argued that the choice of a good pseudopotential and use of better electronic kinetic-energy functionals has the potential for performing large time scale and large sized thermodynamical simulations on clusters.Comment: LaTeX file and EPS figures. 24 pages, 13 figures. Submitted to Phys. Rev.

Superconductivity in the YIr2Si2 and LaIr2Si2 Polymorphs

We report on existence of superconductivity in YIr2Si2 and LaIr2Si2 compounds in relation to crystal structure. The two compounds crystallize in two structural polymorphs, both tetragonal. The high temperature polymorph (HTP) adopts the CaBe2Ge2-structure type (space group P4/nmm) while the low temperature polymorph (LTP) is of the ThCr2Si2 type (I4/mmm). By studying polycrystals prepared by arc melting we have observed that the rapidly cooled samples retain the HTP even at room temperature (RT) and below. Annealing such samples at 900C followed by slow cooling to RT provides the LTP. Both, the HTP and LTP were subsequently studied with respect to magnetism and superconductivity by electrical resistivity, magnetization, AC susceptibility and specific heat measurements. The HTP and LTP of both compounds respectively, behave as Pauli paramagnets. Superconductivity has been found exclusively in the HTP of both compounds below Tsc (= 2.52 K in YIr2Si2 and 1.24 K in LaIr2Si2). The relations of magnetism and superconductivity with the electronic and crystal structure are discussed with comparing experimental data with the results of first principles electronic structure calculations