Appearance Of Arrays Research Articles

Moiré band structures of twisted phosphorene bilayers

We report on the theoretical electronic spectra of twisted phosphorene bilayers exhibiting moir\'e patterns, as computed by means of a continuous approximation to the moir\'e superlattice Hamiltonian. Our model is constructed by interpolating between effective $\Gamma$-point conduction- and valence-band Hamiltonians for the different stacking configurations approximately realized across the moir\'e supercell, formulated on symmetry grounds. We predict the realization of three distinct regimes for $\Gamma$-point electrons and holes at different twist angle ranges: a Hubbard regime for small twist angles $\theta < 2^\circ$, where the electronic states form arrays of quantum-dot-like states, one per moir\'e supercell; a Tomonaga-Luttinger regime at intermediate twist angles $2^\circ < \theta \lesssim 10^\circ$, characterized by the appearance of arrays of quasi-1D states; and finally, a ballistic regime at large twist angles $\theta \gtrsim 10^\circ$, where the band-edge states are delocalized, with dispersion anisotropies modulated by the twist angle. Our method correctly reproduces recent results based on large-scale ab initio calculations at a much lower computational cost, and with fewer restrictions on the twist angles considered.

Simulation-validated generative design of acoustical arrays with diffusion and absorption

This paper presents several new results for on-going efforts in combining computational design methods and acoustical needs. Past investigations employed rule-based design strategies that expanded upon current monolithic deployment of quadratic residue diffusers (QRD) in the field. The algorithm generated novel, and visually diverse arrays of N = 7 QRD panels. The arrays generated visually strayed from the typical appearance of QRD arrays; appearing more expressive as rules were applied. Furthermore, evaluation metrics including a visual complexity coefficient and 3-D polar plots from PML-FEM simulations provided visual evidence that a relationship exists between the appearance of an array and its corresponding acoustical response. Building on this previous work, representative arrays generated by the output of the algorithms with expanded parameters will be presented alongside updated evaluation metrics. The algorithm uses an expanded set of simulation-validated rules that include non-square grid configurations, multiple design frequencies, a varying number of wells, and incorporate zones of absorption. These new arrays, along with results and discussion of acoustical response to these rules, will be presented. Using these expanded parameters absorption, field of room acoustics design is opened up to wide-ranging visual design possibilities that are fully cooperative and incorporated with the acoustic response.

Optical anisotropy in micromechanically rolled carbon nanotube forest

The bulk appearance of arrays of vertically aligned carbon nanotubes (VACNT arrays or CNT forests) is dark as they absorb most of the incident light. In this paper, two postprocessing techniques have been described where the CNT forest can be patterned by selective bending of the tips of the nanotubes using a rigid cylindrical tool. A tungsten tool was used to bend the vertical structure of CNTs with predefined parameters in two different ways as stated above: bending using the bottom surface of the tool (micromechanical bending (M2B)) and rolling using the side of the tool (micromechanical rolling (M2R)). The processed zone was investigated using a Field Emission Scanning Electron Microscope (FESEM) and optical setup to reveal the surface morphology and optical characteristics of the patterned CNTs on the substrate. Interestingly, the polarized optical reflection from the micromechanical rolled (M2R) sample was found to be significantly influenced by the rotation of the sample. It was observed that, if the polarization of the light is parallel to the alignment of the CNTs, the reflectance is at least 2 x higher than for the perpendicular direction. Furthermore, the reflectance varied almost linearly with good repeatability (~10%) as the processed CNT forest sample was rotated from 0° to 90°.[Figure not available: see fulltext.].

Magnetization due to localized states on graphene grain boundary.

Magnetism in graphene has been found to originate from various defects, e.g., vacancy, edge formation, add-atoms etc. Here, we discuss about an alternate route of achieving magnetism in graphene via grain boundary. During chemical vapor deposition of graphene, several graphene nucleation centers grow independently and face themselves with unusual bonding environment, giving rise to the formation of grain boundaries. We investigate the origin of magnetism in such grain boundaries within first-principles calculations, by letting two nucleation centers interact with each other at their interface. We observe formation of unprecedented point defect, consisting of fused three-membered and larger carbon rings, which induces net magnetization to graphene quantum dots. In case of periodic lattices, the appearance of array of point defects leads to the formation of magnetic grain boundaries. The net magnetization on these defects arises due to the deviation from bipartite characteristics of pristine graphene. We observe magnetic grain boundary induced dispersion less flat bands near Fermi energy, showing higher localization of electrons. These flat bands can be accessed via small doping, leading to enhanced magnetism. Moreover, the grain boundaries can induce asymmetric spin conduction behavior along the cross boundary direction. These properties can be exploited for sensor and spin-filtering applications.

Coexistence of Fluid and Crystalline Phases of Proteins in Photosynthetic Membranes

Photosystem II (PSII) and its associated light-harvesting complex II (LHCII) are highly concentrated in the stacked grana regions of photosynthetic thylakoid membranes. PSII-LHCII supercomplexes can be arranged in disordered packings, ordered arrays, or mixtures thereof. The physical driving forces underlying array formation are unknown, complicating attempts to determine a possible functional role for arrays in regulating light harvesting or energy conversion efficiency. Here, we introduce a coarse-grained model of protein interactions in coupled photosynthetic membranes, focusing on just two particle types that feature simple shapes and potential energies motivated by structural studies. Reporting on computer simulations of the model’s equilibrium fluctuations, we demonstrate its success in reproducing diverse structural features observed in experiments, including extended PSII-LHCII arrays. Free energy calculations reveal that the appearance of arrays marks a phase transition from the disordered fluid state to a system-spanning crystal. The predicted region of fluid-crystal coexistence is broad, encompassing much of the physiologically relevant parameter regime; we propose experiments that could test this prediction. Our results suggest that grana membranes lie at or near phase coexistence, conferring significant structural and functional flexibility to this densely packed membrane protein system.

Morphologic Characterization of Organized Extracellular Matrix Deposition by Ascorbic Acid–Stimulated Human Corneal Fibroblasts

To characterize the structure and morphology of extracellular matrix (ECM) synthesized by untransformed, cultured human corneal fibroblasts in long-term cultures. Human corneal stromal keratocytes were expanded in transwell culture in the presence of fetal bovine serum and a stable derivative of vitamin C. The cells were allowed to synthesize a fibrillar ECM for up to 5 weeks. Constructs were assessed by light (phase-contrast and differential interference-contrast) and transmission (standard and quick freeze/deep etch) microscopy. Electron micrographs revealed stratified constructs with multiple parallel layers of cells and an extracellular matrix comprising parallel arrays of small, polydisperse fibrils (27-51 nm) that often alternate in direction. Differential interference contrast images demonstrated oriented ECM fibril arrays parallel to the plane of the construct, whereas quick-freeze, deep-etch micrographs showed the details of the matrix interaction with fibroblasts through arrays of membrane surface structures. Human keratocytes, cultured in a stable vitamin C derivative, are capable of assembling extracellular matrix, which comprises parallel arrays of ECM fibrils. The resultant constructs, which are highly cellular, are morphologically similar to the developing mammalian stroma, where organized matrix is derived. The appearance of arrays of structures on the cell membranes suggests a role in the local organization of synthesized ECM. This model could provide critical insight into the fundamental processes that govern the genesis of organized connective tissues such as the cornea and may provide a scaffolding suitable for tissue engineering a biomimetic stroma.

Electrochemical adsorbate-induced substrate restructuring: gold(110) in aqueous bromide electrolytes

The electrode potential-dependent atomic structure of ordered Au(110) in bromide electrolytes is examined by in situ scanning tunneling microscopy, focusing on the ordered adlayer arrangements formed on the (1×1) substrate terraces, and especially the adsorbate-induced nanoscale restructuring which is observed to occur at higher potentials. Comparison is made with the previously examined behavior of Au(110) in iodide electrolytes. Several ordered bromine structures were identified, all featuring ‘close-packed’ Br rows along the (001) direction, i.e. perpendicular to the (110) ‘rails’. At lower coverages, θ Br<0.75, rectangular packing was commonly observed, featuring also Br rows parallel to the (110) direction. The Br binding sites were determined unambiguously from ‘potentiodynamic’ STM data, featuring juxtaposed substrate and adlayer domains created by potential-step perturbations during image acquisition. The rectangular unit cells contain Br atoms lying within the (110) troughs, and exhibit periodic variations from ‘long twofold’ to fourfold-hollow binding sites. These include (3×1) and (4×1) unit cells, corresponding to θ Br=0.66 and 0.75, respectively. At similar potentials to the latter, an alternative (4×1) structure can be formed having the same coverage, yet with quasi-hexagonal Br packing. Increasing the potential further yields erosion of terrace edges, especially along the (001) direction, and the appearance of arrays of single atomic-layer gold ‘nanorods’, 1.8 nm wide, also oriented along (001). These strings eventually self-assemble into ordered arrays, with rod separations of 0.9–1.2 nm. The restructuring can be reversed rapidly by returning the potential to lower values. The ordered restructuring is markedly different from that observed previously in the Au(110)–I system, which features ‘miniterrace stripes’ largely paralleling the (1 1 ̄ 0) direction. These structural differences can be understood in terms of the known dissimilarities in the close-packed Br and I adlattices; the strict (001) orientation of the nanorods in the Au(110)–Br system is apparently triggered by the similarly oriented Br strings. The widths of the restructured gold miniterraces in both electrolytes are surmised to be determined by adsorbate-induced dipole repulsion forces.

Isolation and characterization of ferritin from soyabeans (Glycine max).

Ferritin from the soyabean Glycine max was isolated and characterized. The protein has many features in common with ferritin from mammalian systems, including extensive sequence homology, as determined by two-dimensional peptide mapping. No immunocross-reactivity between the plant and animal proteins was detected. The ferritin isolated by MgCl2 precipitation has a single subunit of 28 kDa, whereas the ferritin remaining in the supernatant exhibits marked heterogeneity, with a main subunit of 22 kDa. This form of the protein appears to be the result of specific proteolytic processing that is not affected by serine protease inhibitors, and appears only after the seeds have been soaked long enough to induce germination. The appearance of the 22-kDa form corresponds to the appearance of "crystalline arrays" of ferritin in the amyloplasts of the plant cotyledons and may represent a plant form of hemosiderin. In support of this hypothesis, the 22-kDa protein appears to be incompletely assembled, as determined by sucrose gradient centrifugation and iron uptake studies. Although ferritin is normally quite resistant to proteolysis, the 22-kDa protein is easily generated from the 28-kDa form by treatment with subtilisin, suggesting the presence of a specific, protease-sensitive sequence on the protein's surface, possibly used to mark the phytoferritin for conversion to hemosiderin and construction of ferritin crystalline arrays.

Interaction of plasma membrane fibronectin receptor with talin--a transmembrane linkage.

Many observations suggest the presence of transmembrane linkages between the cytoskeleton and the extracellular matrix. In fibroblasts both light and electron microscopic observations reveal a co-alignment between actin filaments at the cell surface and extracellular fibronectin. These associations are seen at sites of cell matrix interaction, frequently along stress fibres and sometimes where these bundles of microfilaments terminate at adhesion plaques (focal contacts). Non-morphological evidence also indicates a functional linkage between the cytoskeleton and extracellular matrix. Addition of fibronectin to transformed cells induces flattening of the cells and a reorganization of the actin cytoskeleton, with the concomitant appearance of arrays of stress fibres. Conversely, disruption of the actin cytoskeleton by treatment with cytochalasin B leads to release of fibronectin from the cell surface. As yet, there is no detailed knowledge of the molecules involved in this transmembrane linkage, although several proteins have been suggested as candidates in the chain of attachment between bundles of actin filaments and the cytoplasmic face of the plasma membrane: these include vinculin, alpha-actinin and talin, each one having been identified at regions where bundles of actin filaments interact with the plasma membrane and underlying cell-surface fibronectin. Recently, the cell-substrate attachment (CSAT) antigen has been identified as a plasma membrane receptor for fibronectin, raising the possibility that this glycoprotein complex may serve as a bridge between fibronectin and one or more of the underlying cytoskeletal components mentioned. Here we have investigated the interaction of the purified CSAT antigen with these cytoskeletal components, and we demonstrate an interaction specifically between the CSAT antigen and talin.