High Degree Of Local Order Research Articles

Modelling of high-temperature order-disorder phase transitions of non-stoichiometric Mo2C and Ti2C from first principles.

High-temperature order-disorder phase transitions play an important role in determining the structure and physical and chemical properties of non-stoichiometric transition metal carbides. Due to the large number of possible carbon vacancy arrangements, it is difficult to study these systems with first-principles calculations. Here, we construct a simple atomistic potential capable of accurately reproducing the energetics of the carbon vacancy arrangements in cubic Mo2C and Ti2C obtained from density functional theory calculations. We show that this potential can be applied to correctly predict the transition temperatures between the ordered and disordered states in Monte Carlo simulations on large supercells and reveal the extend of local order in the disordered phases of Mo2C and Ti2C that show interesting physical and chemical properties. We find that even the high-temperature disordered phase exhibit a relatively high degree of local order as indicated by the relatively small change in the root mean square number of C atom neighbours of Mo/Ti compared to the ordered phase (from 3.0 to 3.1-3.2). This atomistic potential enables the study of how the structure of these carbides can be tuned through the synthesis temperature to control the properties of carbide materials that are related to the degree of disorder in the system such as catalytic activity and electrical conductivity and play an important role in applications of these carbides. Fundamentally, the successful modelling of these carbides suggests that despite the presence of metallic, covalent and ionic interactions, bonding in carbides can be modelled by simple and physically intuitive interatomic potentials.

Dynamical coexistence in moderately polydisperse hard-sphere glasses.

We perform extensive numerical simulations of a paradigmatic model glass former, the hard-sphere fluid with 10% polydispersity. We sample from the ensemble of trajectories with fixed observation time, whereby single trajectories are generated by event-driven molecular dynamics. We show that these trajectories can be characterized in terms of the local structure, and we find a dynamical-structural (active-inactive) phase transition between two dynamical phases: one dominated by liquidlike trajectories with a low degree of local order and one dominated by glassylike trajectories with a high degree of local order. We show that both phases coexist and are separated by a spatiotemporal interface. Sampling exceptionally long trajectories allows us to perform a systematic finite-size scaling analysis. We find excellent agreement with Binder's scaling theory for first-order transitions. Interestingly, the coexistence region narrows at higher densities, supporting the idea of a critical point controlling the dynamic arrest. Scaling of the susceptibility suggests that the critical behavior falls into the universality class of directed percolation in 3 + 1 dimensions.

Recent developments in ion beam-induced nanostructures on AIII-BV compound semiconductors

Ion-beam sputtering of two-component substrates constitutes an alternative route for the nanofabrication of 3D (three-dimensional) structures, such as quantum dots or nanowires with unique properties like a high degree of local ordering. To allow for feasibility in precision manufacturing, control and optimization it is necessary to completely understand all the phenomena behind the evolution of nanostructures. The formation and development during the ion irradiation of similar features has been extensively studied for almost a half of century, but only over the last few years have new results appeared, ones stimulating real progress within this field. In this paper we report on the growth of such 3D nanostructures after low energy ion-beam sputtering on specific materials belonging to the group of AIII-BV binary compound crystals. Special emphasis is given to the role of sample temperature (during irradiation or post-annealing) on the evolution of nanostructure patterns and their ordering. The formation of such systems will be explained as seen from a phenomenological perspective.

Emergent pattern formation in an interstitial biofilm.

Collective behavior of bacterial colonies plays critical roles in adaptability, survivability, biofilm expansion and infection. We employ an individual-based model of an interstitial biofilm to study emergent pattern formation based on the assumptions that rod-shaped bacteria furrow through a viscous environment and excrete extracellular polymeric substances which bias their rate of motion. Because the bacteria furrow through their environment, the substratum stiffness is a key control parameter behind the formation of distinct morphological patterns. By systematically varying this property (which we quantify with a stiffness coefficient γ), we show that subtle changes in the substratum stiffness can give rise to a stable state characterized by a high degree of local order and long-range pattern formation. The ordered state exhibits characteristics typically associated with bacterial fitness advantages, even though it is induced by changes in environmental conditions rather than changes in biological parameters. Our findings are applicable to a broad range of biofilms and provide insights into the relationship between bacterial movement and their environment, and basic mechanisms behind self-organization of biophysical systems.

Patterning of papillae on the mouse tongue: A system for the quantitative assessment of planar cell polarity signaling

The dorsal surface of the mouse tongue is covered by ~7000 papillae, asymmetric epithelial protrusions that are precisely oriented to create a stereotyped macroscopic pattern. Within the context of this large-scale pattern, neighboring papillae exhibit a high degree of local order that minimizes the differences in their orientations. We show here that the orientations of lingual papillae are under the control of the core planar cell polarity (PCP) genes Vangl1, Vangl2, and Celsr1. Using K14-Cre and Nkx2.5-Cre to induce conditional knockout of Vangl1 and/or Vangl2 in the tongue epithelium, we observe more severe disruptions to local order among papillae with inactivation of larger numbers of Vangl genes, a greater role for Vangl2 than Vangl1, and a more severe phenotype with the Vangl2 Looptail (Lp) allele than the Vangl2 null allele, consistent with a dominant negative mode of action of the Vangl2Lp allele. Interestingly, Celsr1−/− tongues show disruption of both local and global order, with many papillae in the anterior tongue showing a reversed orientation. To quantify each of these phenotypes, we have developed and applied three procedures for sampling the orientations of papillae and assessing the degree of order on different spatial scales. The experiments reported here establish the dorsal surface of the mouse tongue as a favorable system for studying PCP control of epithelial patterning.

Strategies for solid-state NMR investigations of supramolecular assemblies with large subunit sizes

Solid-state NMR is a versatile tool to study structure and dynamics of insoluble and non-crystalline biopolymers. Supramolecular protein assemblies are formed by self-association of multiple copies of single small-sized proteins. Because of their high degree of local order, solid-state NMR spectra of such systems exhibit an unusually high level of resolution, rendering them an ideal target for solid-state NMR investigations. Recently, our group has solved the structure of one particular supramolecular assembly, the type-iii-secretion-system needle. The needle subunit comprises around 80 residues. Many interesting supramolecular assemblies with unknown structure have subunits larger in size, which requires development of tailored solid-state NMR strategies to address their structures. In this “Perspective” article, we provide a view on different approaches to enhance sensitivity and resolution in biological solid-state NMR with a focus on the possible application to supramolecular assemblies with large subunit sizes.

A functional triazine framework based on N-heterocyclic building blocks

Covalent organic frameworks constitute a subclass of polymeric materials offering enhanced porosity, functionality and stability. In this work a covalent triazine framework based on bipyridine building blocks is presented, along with a comprehensive elucidation of its local structure, porosity, and capacity for metal uptake. A typical synthesis was carried out under ionothermal conditions at 400–700 °C using ZnCl2 as a Lewis acidic trimerization catalyst. A high degree of local order and the presence of triazine and bipyridine moieties are ascertained at a synthesis temperature of 400 °C, along with micropores and specific surface areas of up to 1100 m2 g−1. Mesopores are increasingly formed at synthesis temperatures above 450 °C, yielding highly porous frameworks with hierarchical porosity and exceptionally large surface areas in excess of 3200 m2 g−1 at 700 °C. We demonstrate the capability of the bipyridine unit to provide specific and strong binding sites for a large variety of transition metal ions, including Co, Ni, Pt and Pd. The degree of metal loading (up to 38 wt%) can be tuned by the metal concentration in solution and is dependent on both the type of metal as well as the temperature at which the CTF was synthesized. Evidence for site-specific metal coordination bodes well for the use of metal-loaded CTFs as heterogeneous catalysts carrying homogeneous-type active sites.

When whorls collide: the development of hair patterns in frizzled 6 mutant mice.

Surface appendages such as bristles, feathers and hairs exhibit both long- and short-range order. In the frizzled 6 null (Fz6(-/-)) mouse the orientations of the earliest born hair follicles are uncorrelated, but over time the follicles reorient to create patterns that are characterized by a high degree of local order. By quantifying follicle orientations over time, in both living and fixed tissues, we define the time course of local hair follicle refinement and the resulting evolution of a montage of competing patterns in Fz6(-/-) skin. We observe an apparently local process that within one week can organize a field of many tens of thousands of follicles, generating long-range order that extends over distances of more than one centimeter. Physical systems that undergo an analogous ordering of vector components suggest potential mechanisms that might apply to the patterning of hair follicles and related biological structures.

Direct-current-induced transformation at the interface between platinum anode and holmium-doped ceria electrolyte

Under the influence of a direct current (dc), phase transformation could take place at the interface between platinum anode and holmium-doped ceria (HDC) electrolyte at temperature ≥600 °C. This transformation is insensitive to the doping concentration and the sintering temperature, but strongly depends on the testing temperature. As a result of the dc-induced transformation, products with a size up to hundreds of microns were separated from the anode-side surface and, correspondingly, circular holes with diameters varied from tens to hundreds of microns remained on the surface. Compared with original HDC samples, a high degree of local ordering of oxygen vacancies was found in most of the products. Based on structural and compositional studies, it is suggested that the dc-induced transformation could be attributed to the electrochemical reaction at the anode/electrolyte interface and the mobility of Ho cations promoted by the current.

Structural Environments for Boron and Aluminum in Alumina−Boria Catalysts and Their Precursors from11B and27Al Single- and Double-Resonance MAS NMR Experiments

Alumina−boria catalysts, obtained by calcination (550−950 °C) of pseudo-boehmite precursors impregnated with 0.25−3.4 wt % boron, have been characterized by 11B and 27Al MAS NMR of the central and satellite transitions, MQMAS, 11B{27Al} REDOR, and TRAPDOR NMR, and by homonuclear dipolar recoupling MAS NMR experiments. The 27Al MAS NMR spectrum of the satellite transitions for boehmite (γ-AlOOH) has allowed the first precise determination of the 27Al quadrupole coupling parameters for this important aluminate. The 27Al MAS NMR spectra of the alumina−boria catalysts show that the fraction of tetrahedral AlO4 sites decreases with increasing boron loading. This finding and the results from 11B MAS NMR suggest that a small fraction of tetrahedrally coordinated boron is incorporated within the first molecular layers of the γ-Al2O3 support. The observation of slightly stronger 11B−27Al dipolar couplings for BO4 relative to the trigonal BO3 sites from 11B{27Al} REDOR NMR supports this finding. Information on the dispersion of BO4 and BO3 environments have been obtained from 11B MAS NMR spectra of the satellite transitions and from 11B MQMAS NMR experiments. The presence of BO4 and BO3 species on the surface of the γ-Al2O3 support is demonstrated by 11B{27Al} REDOR and TRAPDOR NMR, while 11B homonuclear correlation experiments indicate BO4−BO3 connectivities and thereby a network on the surface for these units. For the alumina−boria catalyst corresponding to the highest boron content and calcination temperature, an additional BO3 resonance is observed in the 11B MQMAS spectrum, which is assigned to a separate B2O3 phase exhibiting a high degree of local order.

Directing Close-Packing of Midnanosized Gold Nanoparticles at a Water/Hexane Interface

This paper reports a method for assembling midnanosized Au nanoparticles (25 < d < 100 nm) into close-packed 2D arrays with a high degree of local order. An in situ coating of Au nanoparticles with an alkanethiol surfactant can induce close-packed particle arrays when the nanoparticles are trapped at the interface. The film morphology was determined to be either monoparticulate or multiparticulate depending on the 1-dodecanethiol concentration. This paper reports on the optimum conditions for the formation of a close-packed 2D array by suggesting a phase diagram obtained from a plot of the 1-dodecanethiol concentration versus the nanoparticle diameter. When the diameter of Au nanoparticles was larger than ca. 40 nm, the full coverage of 1-dodecanethiols on the surface of nanoparticles was not enough to form close-packed nanoparticle arrays. The larger nanoparticles required the more 1-dodecanethiols to form a close-packed 2D array. This method also allows the nanoparticle surface to be coated asymmetrical...

Reversible thermochromism in self-layered hydrogen-bondedpolydiacetylene assembly

We structurally designed and synthesized a novel class of self-layered supramolecular polymers containing DA derivatives that can be obtained as film-forming materials and processed into a various forms. We found that double H-bonds between the acid hard cores together with a remarkable molecular DA recognition allow for retaining more favorable packing of the reactive groups in spin-coated films or even in dispersed solutions with a high degree of local ordering, suitable for topochemical polymerization. The sample molecularly designed here could be topochemically polymerized by UV irradiation through 1,4 addition of the conjugated triple bonds between adjacent stacking molecules. A reversible thermochromism in the absorption characteristics of spin-coated terephthalic acid/diacetylene polymers was also observed.

Cation Distribution and Interatomic Interactions in Oxides with Heterovalent Isomorphism: IX. Complex Perovskites A2BB′O6 (A = Sr, Ba; B = Fe, Cr; B′ = Nb, Ta)

A high degree of local order in the arrangement of isomorphous cations with a 3D character of ordering was found experimentally in complex stoichiometric oxides crystallizing in a random perovskite structure.

Concentration-dependent short-range order in the relaxor ferroelectric(1−x)Pb(Sc,Ta)O3−xPbTiO3

Using the x-ray diffraction and x-ray absorption fine structure techniques, we have probed $\mathrm{Ti}$, $\mathrm{Ta}$, and $\mathrm{Sc}$ local environments in the solid solution system $(1\ensuremath{-}x{)\mathrm{Pb}(\mathrm{Sc},\mathrm{Ta})\mathrm{O}}_{3}\text{\ensuremath{-}}x{\mathrm{PbTiO}}_{3}$. This system is known to display a variety of ferroelectric behaviors ranging from variable order-disorder, to relaxor, to a mixed phase region, and then finally to normal ferroelectric, as the value of $x$ is increased. We find, in agreement with neutron diffraction studies, no detectable displacements of $\mathrm{Ta}$ or $\mathrm{Sc}$ atoms from their oxygen cage centers in any of these systems. Surprisingly, we find that the $\mathrm{Ti}$ atom is displaced along (111) from its inversion symmetry center for $x=0.05$. However, this average local $\mathrm{Ti}$ displacement gradually changes from (111) to (001) as $x$ increases, whereas the global crystal structure abruptly changes from rhombohedral to tetragonal at $x=0.45$. Our experimental results and theoretical modeling of others together suggest that this system consists of mixed regions, some characterized by a (111) $\mathrm{Ti}$ displacement and others characterized by a (001) $\mathrm{Ti}$ displacement. The displacement averaged over all regions becomes more weighted toward (001) as $x$ increases. Another significant result is that all our samples (with $x$ ranging from 0 to 0.5) have a high degree of local ordering of the $\mathrm{B}$ sites with alternate occupation of $\mathrm{Ta}$ and $\mathrm{Sc}$ atoms.

Cation distribution and interatomic interactions in oxides with heterovalent isomorphism: VII. CrNbO4 and LiFeO2 oxides

A high degree of local ordering of isomorphous cations, determined by a great difference in their effective charges, was found in stoichiometric CrNbO4 and LiFeO2 oxides. The character of the ordering (one-dimensional in CrNbO4, supposedly approaching the AlWO4 type, and two-dimensional in LiFeO2 with a symmetry decreased to tetragonal and approaching the CuAu type) depends on the anisotropy of cation distribution in the cation coordination spheres. Microdomains with an almost complete ordering inside one block of the crystallite mosaic may develop in LiFeO2.

Growth and alloy formation studied by photoelectron spectroscopy and STM

We have grown epitaxial submonolayers of Re on a Pt(111) surface. This system was studied by high-resolution photoelectron spectroscopy and scanning tunneling microscopy (STM). Re was found to grow, at room temperature, as single-layer islands consisting of ∼100 atoms. Core-level spectroscopy showed that these islands contain two different species of Re. We interpret these as Re in the centre and on the edges of the islands, where Pt and Re may be intermixed at the edges. Annealing of the Re covered crystal to 1000 K produces a surface alloy with a high degree of local order and a thickness of approximately three layers. The structure of the alloy depends on the Re coverage prior to annealing. Valence band spectra show a strong depletion of Pt 5d states near the Fermi energy when Re is adsorbed, possibly caused by a Pt d→sp rehybridization.

Dynamics of the spreading of chainlike molecules with asymmetric surface interactions.

In this work we study the spreading dynamics of tiny liquid droplets on solid surfaces in the case where the ends of the molecules feel different interactions with respect to the surface. We consider a simple model of dimers and short chainlike molecules that cannot form chemical bonds with the surface. We use constant temperature Molecular Dynamics techniques to examine in detail the microscopic structure of the time dependent precursor film. We find that in some cases it can exhibit a high degree of local order that can persist even for flexible chains. Our model also reproduces the experimentally observed early and late-time spreading regimes where the radius of the film grows proportional to the square root of time. The ratios of the associated transport coefficients are in good overall agreement with experiments. Our density profiles are also in good agreement with measurements on the spreading of molecules on hydrophobic surfaces.

Molecular ordering of precursor films during spreading of tiny liquid droplets.

In this work we address a feature of spreading dynamics of tiny liquid droplets on solid surfaces, namely, the case where the ends of the molecules have different interactions with the surface. We consider a simple model of dimers and short chainlike molecules which cannot form chemical bonds with the surface. We study the spreading dynamics by molecular dynamics techniques. In particular, we examine the microscopic structure of the time-dependent precursor film and find that in some cases it can exhibit a high degree of local order. This order persists even for flexible chains. Our results suggest the possibility of extracting information about molecular interactions from the structure of the precursor film.

Nucleation and growth of metallic submonolayers on compact metal surfaces.

By use of the molecular-dynamics method we study the high-temperature nonequilibrium and equilibrium properties of a metallic overlayer on a compact metal surface. The system is modeled by the embedded-atom method, and consists of a Pt slab with a large ideal (111) surface (cross section of 930 atoms) on which Ag atoms are randomly distributed with coverages FTHETA=0.10, 0.25, 0.30. We simulate the short-time (\ensuremath{\tau}\ensuremath{\sim}1 ns) process by which the adatoms form dynamically stable aggregates in local equilibrium with a dilute two-dimensional vapor. We analyze the equilibrium properties of the adlayer as a function of T and FTHETA. We compute the cluster size distribution, the adatom radial-distribution function, the diffusion coefficient, and we characterize the cluster shape by the ratio of its principal moments of inertia. At all FTHETA and T the Ag islands grow pseudomorphically on Pt(111). At high temperatures (T\ensuremath{\sim}1000 K) the clusters are fluidlike. There is no evidence of peculiar stability at the sizes corresponding to the filling of two-dimensional close-packing shells (``magic numbers,'' N=7, 10, etc.). At intermediate temperatures (T=600--800 K) the size distribution is shifted toward larger aggregates and, at low coverage, starts to display stability peaks at the magic numbers. The larger clusters develop a solidlike core surrounded by a fluidlike boundary. At the lowest temperature of our study (T=400 K) the clusters display a high degree of local order. The size distribution function is restricted to small N's by the slowing down of the cluster growth beyond the nucleation stage. At FTHETA=0.10 the stability peaks at N=7 and 10 are apparent. We discuss the influence of a step on the behavior of the system.

Local order in liquid and amorphous Cs-Sb.

We have investigated the effects of local order on the density of states and the conductivity of liquid and amorphous Cs-Sb alloys as a function of concentration. By using a numerical Monte Carlo technique that treats the local atomic environment correctly, we show that the very sharp minimum in the conductivity that has been reported in the liquid near the ${\mathrm{Cs}}_{3}$Sb stoichiometry can be explained by a local-order-induced gap in the density of states if a very high degree of local order is assumed. Although we find a substantial degree of charge transfer, \ensuremath{\sim}0.6 electron/Cs atom, this charge transfer is far from complete, in agreement with the analyses of a variety of other types of experimental results. In the concentration region 30--50 at. % Sb, we find that the presence of ${\mathrm{Sb}}^{\mathrm{\ensuremath{-}}}$-${\mathrm{Sb}}^{\mathrm{\ensuremath{-}}}$ covalent bonds, indicated by neutron-diffraction measurements, can accurately account for the measured conductivity which varies from \ensuremath{\sim}50 to \ensuremath{\sim}100 (\ensuremath{\Omega} cm${)}^{\mathrm{\ensuremath{-}}1}$. The strong temperature dependence of the conductivity in this concentration region can then be understood in terms of the dissociation of these bonds. By also applying our model to amorphous Cs-Sb, we explain the differences between the measured conductivities of the liquid and amorphous alloys.