Island Structure Research Articles

Structure and Optical Properties of AlN Crystals Grown by Metal Nitride Vapor Phase Epitaxy with Different V/III Ratios.

The epitaxial aluminum nitride (AlN) crystals were grown on c-plane sapphire using high-temperature metal nitride vapor phase epitaxy at the source materials’ different molar flow ratios (V/III ratios). The effects of various V/III ratios on the surface morphology, crystalline quality, material straining, and optical properties of heteroepitaxial AlN thin films were studied using X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and photoluminescence (PL). With the increase in the V/III ratio from 1473 to 7367, the substrate surface underwent changes that vary from whiskers to three-dimensional island structures, two-dimensional layered stack structures, and stacked sheet structures. Additionally, due to the presence of nanoscale pits on the substrate surface, almost all samples were tensile stressers. The PL spectra demonstrated the defect luminescence of the epitaxial films, indicating that nitrogen vacancies and oxygen impurities were the samples’ main defects.

Effect of Graphene/Spherical Graphite Ratio on the Properties of PLA/TPU Composites.

Wave-absorbing materials are developing in the direction of “light weight, wide frequency band, thin layer and high strength”, and it is difficult to achieve the synergy between wave-absorbing performance and mechanical properties when graphene absorbent is compounded with a single resin matrix. In this paper, based on the preparation of a new composite absorbing wire with a graphene (GR)/spherical graphite (SG) double absorbent and polylactic acid (PLA)/thermoplastic polyurethane (TPU) double matrix, we proposed a new method to prepare samples for testing the electromagnetic parameters and tensile strength by fused deposition modeling (FDM). Furthermore, the effect of SG/GR ratio on the microwave absorbing properties and mechanical properties of PLA/TPU composites was specifically studied. It was found that when the ratio of SG/GR was small (0:5, 1:4), the dielectric loss (interfacial polarization loss, dipole polarization loss, conductivity loss) and attenuation ability of the composites were stronger, and the impedance matching was better. When the SG/GR ratio was large (5:0, 4:1), the composites had high strength and toughness. When the ratio of SG/GR was moderate (2:3, 3:2), it could retain the absorbing and mechanical properties of the absorbing materials. On the one hand, the SG and PLA/TPU matrix formed an “island structure”, which improves the dispersion of GR; on the other hand, the GR and PLA/TPU matrix formed a “core-shell structure”, which promotes polarization and multiple scattering.

A mechanism explaining the metamorphoses of KAM islands in nonhyperbolic chaotic scattering

In the context of nonhyperbolic chaotic scattering, it has been shown that the evolution of the KAM islands exhibits four abrupt metamorphoses that strongly affect the predictability of Hamiltonian systems. It has been suggested that these metamorphoses are related to significant changes in the structure of the KAM islands. However, previous research has not provided an explanation of the mechanisms underlying the metamorphoses. Here, we show that they occur due to the formation of a homoclinic or heteroclinic tangle that breaks the internal structure of the main KAM island. We obtain similar qualitative results in a two-dimensional Hamiltonian system and a two-dimensional area-preserving map. The equivalence of the results obtained in both systems suggests that the same four metamorphoses play an important role in conservative systems.

Fractal structure of magnetic island in tokamak plasma

Abstract The fractal structure of magnetic island is studied qualitatively and quantitatively. The fractal dimension of magnetic island is obtained, and the formation of fractal structure of magnetic island is explained.

Investigation on HfO2 properties grown by ALD using TDMAH as precursor

HfO2 films have increasingly extensive applications in optical and electronic industry. In order to obtain controllable properties of HfO2 film, its growth behavior needs to be studied. HfO2 thin films prepared by Atomic Layer Deposition (ALD) were studied in this work. Growth behavior was elucidated by controlling the cycles of ALD and characterized by both structural and optical measurements. As the increase of thickness of the film, HfO2 converts from amorphous to monoclinic and the preferred orientation of (200) and (020) lattice plane is strengthened in monoclinic phase. Oriented arrangement of atoms is relevant to the gradient structure of HfO2 film and abundant protuberant islands on the surface of HfO2 film. Annealing experiment proved the orientation of atomic arrangement in amorphous HfO2 which promotes the continuous variation of HfO2 films in refractive index (n). And the orientation arrangement eventually contributes to crystallization of the film which will significantly reduce the laser induced damage threshold (LIDT) of the film. Analysis of optical properties in this work can provide useful information for the preparation of optical coating containing HfO2.

Island-like random scattering layer for light extraction in organic light-emitting diodes with enhanced luminous efficiencies and angular spectral stability

Optical outcoupling efficiency of organic light-emitting diode (OLED) can be improved using nanoscale or sub-micrometer quasi-periodic structures. An embossed nano/micro-structure inserted between the substrate and anode is known to be effective in extracting the waveguide mode. However, the techniques to fabricate these nanoscale or sub-micrometer structures are usually complicated, and the light extraction is wavelength-oriented. Herein, we develop a simple process to enhance the performance of OLED. We texture the glass surface with shrinking Ag droplets inthe form of island structures and then deposit SiNx planarization film to form an island-like random scattering layer (IRSL), which changes the light path of emitting light in a way to reduce light attenuation and extract emission outward. Furthermore, IRSL has a low haze and a high optical transparency, both of which are beneficial to light output. The OLEDs constructed with IRSL as an internal light extraction structure exhibit a significant improvement of current and power efficiencies by 52% and 62%, respectively, compared to the planar OLED device. In addition, compared to the conventional pillar-structured RSL, the IRSL not only facilitates the extraction of light for a broad range of visible light region, but also favors better angular spectral stability and Lambertian emission.

Nanodiamond islands confined between two graphene sheets as perspective 2D quantum materials

Based on direct space DFT PBE0/6-31G* electronic structure calculations, the structure and properties of nanodiamond islands confined between two finite graphene fragments (NDI-c2G) was proposed and theoretically explored. DFT simulations revealed that fusion of planar aromatic molecules with two parent graphene fragments may form either cubic or hexagonal allotropes of NDI-c2Gs lattices accompanied with formation of local corrugated sp3 sites with substituted dangling bonds embedded into graphene sublattices. It was shown that at the DFT level of theory, low distortion energies of NDI-c2Gs lattices are comparable or smaller than the energy of van-der-Waals interactions, which allows the NDI-c2G lattices to be stabilized by a support and finally synthesized. The Nuclear Independent Chemical Shift calculations, shows that in the vicinity of NDI regions, graphene lattices are estimated to be either low-, or anti-aromatic. Furthermore, the formation of NDI-c2Gs leads to localization of HOMO and LUMO states at different sites of the NDI-c2Gs lattices. The NDI-c2G regions with confined frontier orbitals can be considered as arrays of quantum dots isolated from each other by NDI scattering centres of 3.82 Å dimension. The results demonstrates that NDI-c2G should be considered as strongly-correlated entangled hybrid quantum dots which may form extended quantum ensembles with great potential for advanced quantum applications.

Femtosecond X-ray Laser Reveals Intact Sea-Island Structures of Metastable Solid-State Electrolytes for Batteries.

Experimental characterization of the nanostructure of metastable functional materials has attracted significant attention with recent advances in computational materials discovery. However, since metastable glass-ceramics are easily damaged by irradiation, damage-free nanoimaging has not been realized thus far. Herein, we propose novel high-contrast coherent diffractive imaging that quantitatively analyzes the intact internal nanostructure of metastable glass-ceramics using femtosecond X-ray pulses. The immersion of sample particles in a solvent helps enhance the reconstructed image contrast and allows us to distinguish an ∼7% electron density difference between an amorphous form and crystals. Furthermore, morphological operations with a band-pass filter quantitatively elucidate the depth information. The evaluated volume ratio of the amorphous to crystalline phases is ∼2.5:1 for the measured metastable (Li2S)70-(P2S5)30 glass-ceramic particle. Sulfide glass-ceramics are used as electrolytes for all-solid-state batteries, which are indispensable for reducing the carbon footprint. Our results will facilitate structural studies on fragile metastable materials with important scientific and industrial implications.

Nanoarchitectonics for Ultrathin Gold Films Deposited on Collagen Fabric by High-Power Impulse Magnetron Sputtering.

Gold nanoparticles conjugated with collagen molecules and fibers have been proven to improve structure strength, water and enzyme degradation resistance, cell attachment, cell proliferation, and skin wound healing. In this study, high-power impulse magnetron sputtering (HiPIMS) was used to deposit ultrathin gold films (UTGF) and discontinuous island structures on type I collagen substrates. A long turn-off time of duty cycle and low chamber temperature of HiPIMS maintained substrate morphology. Increasing the deposition time from 6 s to 30 s elevated the substrate surface coverage by UTGF up to 91.79%, as observed by a field emission scanning electron microscope. X-ray diffractometry analysis revealed signature low and wide peaks for Au (111). The important surface functional groups and signature peaks of collagen substrate remained unchanged according to Fourier transform infrared spectroscopy results. Multi-peak curve fitting of the Amide I spectrum revealed the non-changed protein secondary structure of type I collagen, which mainly consists of α-helix. Atomic force microscopy observation showed that the roughness average value shifted from 1.74 to 4.17 nm by increasing the deposition time from 13 s to 77 s. The uneven surface of collagen substrate made quantification of thin film thickness by AFM difficult. Instead, UTGF thickness was measured using simultaneously deposited glass specimens placed in an HiPIMS chamber with collagen substrates. Film thickness was 3.99 and 10.37 nm at deposition times of 13 and 77 s, respectively. X-ray photoelectron spectroscopy showed preserved substrate elements on the surface. Surface water contact angle measurement revealed the same temporary hydrophobic behavior before water absorption via exposed collagen substrates, regardless of deposition time. In conclusion, HiPIMS is an effective method to deposit UTGF on biomedical materials such as collagen without damaging valuable substrates. The composition of two materials could be further used for biomedical purposes with preserved functions of UTGF and collagen.

Visualization of Macrophase Separation and Transformation in Immiscible Polymer Blends

Visualization of Macrophase Separation and Transformation in Immiscible Polymer Blends

Exploration of coal tar asphaltene molecules based on high resolution mass spectrometry and advanced extraction separation method

The structure of coal tar asphaltenes has been recognized as one of the most challenging research topics in coal chemistry. In this work, a stepwise extraction subfraction separation method is developed for coal tar asphaltenes to reveal the polar molecule distribution and structure information using (+) ESI FT-ICR MS coupled with collision induced dissociation (CID). It is found that the most easily ionized molecules are preferentially observed in unfractionated asphaltenes, while the strongly polar molecules are suffered with ionization suppression. The heteroatoms are more likely to be present in aromatic cores instead of substituent chains, and this phenomenon grows more significantly with the increase of solvent polarity. The O/C increases with the increase in solvent polarity, and O plays an important role in the adsorption and desorption of asphaltenes on silica gel surface. The “atypical” asphaltenes molecules are rich in N, S and particularly O containing groups with low aromaticity and small aromatic structures, and tending to be island structures. Moreover, the coal tar asphaltenes has a significantly higher island/archipelago ratio than petroleum asphaltenes. The polar driving force has a great impact on archipelago molecules, the proportion of which is high in polar fractions.

Multidrug Resistance Genes Carried by a Novel Transposon Tn7376 and a Genomic Island Named MMGI-4 in a Pathogenic Morganella morganii Isolate.

ABSTRACTAntimicrobial resistance in Morganella morganii is increasing in recent years, which is mainly introduced via extra genetic and mobile elements. The aim of our study is to analyze the multidrug resistance (MDR) and characterize the mobile genetic elements (MGEs) in M. morganii isolates. Here, we report the characteristic of a pathogenic M. morganii isolate containing multidrug resistance genes that are mainly carried by a novel transposon Tn7376 and a genomic island. Sequence analysis suggested that the Tn7376 could be generated through homologous recombination between two different IS26-bounded translocatable units (TUs), namely, module A (IS26-Hp-IS26-mph(A)-mrx(A)-mphR-IS6100-chrA-sul1-qacEΔ1) and module B (ISCR1-sul1-qacEΔ1-cmlA1-aadA1-aadB-intI1-IS26), and the genomic island named MMGI-4 might derive from a partial structure of different original genomic islands that also carried IS26-mediated TUs. Notably, a 2,518-bp sequence linked to the module A and B contains a 570-bp dfrA24 gene. To the best of our knowledge, this is the first report of the novel Tn7376 possessing a complex class 1 integron that carried an infrequent gene dfrA24 in M. morganii.IMPORTANCE Mobile genetic elements (MGEs), especially for IS26-bounded translocatable units, may act as a reservoir for a variety of antimicrobial resistance genes in clinically important pathogenic bacteria. We expounded this significant genetic characteristic by investigating a representative M. morganii isolate containing multidrug resistance genes, including the infrequent dfrA24. Our study suggested that these acquired resistance genes were mainly driven by IS26-flanked important MGEs, such as the novel Tn7376 and the MMGI-4. We demonstrated that IS26-related MGEs contributed to the emergence of the extra gene dfrA24 in M. morganii through some potential genetic events like recombination, transposition, and integration. Therefore, it is of importance to investigate persistently the prevalence these MEGs in the clinical pathogens to provide risk assessment of emergence and development of novel resistance genes.

From island biogeography to landscape and metacommunity ecology: A macroecological perspective of bat communities

The equilibrium theory of island biogeography and its quantitative consideration of origination and extinction dynamics as they relate to island area and distance from source populations have evolved over time and enriched theory related to many disciplines in spatial ecology. Indeed, the island focus was catalytic to the emergence of landscape ecology and macroecology in the late 20th century. We integrate concepts and perspectives of island biogeography, landscape ecology, macroecology, and metacommunity ecology, and show how these disciplines have advanced the understanding of variation in abundance, biodiversity, and composition of bat communities. We leverage the well-studied bat fauna of the islands in the Caribbean to illustrate the complex interplay of ecological, biogeographical, and evolutionary processes in molding local biodiversity and system-wide structure. Thereafter, we highlight the role of habitat loss and fragmentation, which is increasing at an accelerating rate during the Anthropocene, on the structure of local bat communities and regional metacommunities across landscapes. Bat species richness increases with the amount of available habitat, often forming nested subsets along gradients of patch or island area. Similarly, the distance to and identity of sources of colonization influence the richness, composition, and metacommunity structure of islands and landscape networks.

Laser desorption ionization and collision induced dissociastion as powerful tools for FT-ICR mass spectrometric characterization of asphaltene fractions enriched in island and archipelago motifs

Due to advances in mass spectrometry, it is widely accepted that asphaltenes molecules from the island and archipelago motifs coexist in the samples. However, analyzing asphaltenes remains a challenging task mostly due to these species' high complexity and self-association tendencies. Herein we introduce laser laser desorption ionization (LDI) as an adequate tool to ionize even the most unstable samples. In addition, we demonstrate collision-induced dissociation (CID) experiments as a powerful tool to perform structural characterization of asphaltenes. Using these tools, it was possible to characterize asphaltene samples enriched in island and archipelago-type structures in a much easier way without any capillary obstructions and damages to the equipment. Also, it was possible to perform an in-depth analysis of asphaltene and propose structures to asphaltene detected in the most abundance in the selected samples. Finally, the relative percentages of product ions derived from the cleavage of alkyl bridges between aromatic nuclei in archipelago-type molecules were calculated, and we tried to establish correlations with the asphaltene structures with their stability.

Adsorption of molecular iodine on the Ag(111) surface: Phase transitions, silver reconstruction, and iodide growth.

Room temperature adsorption of molecular iodine on Ag(111) has been studied by scanning tunneling microscopy (STM), low energy electron diffraction, Auger electron spectroscopy with factor analysis, and density functional theory (DFT). At the chemisorption stage, iodine first forms a (3×3)R30° structure. Further iodine dosing leads to continuous commensurate-incommensurate phase transition, taking place via the formation of striped superheavy domain walls. As a result, the uniaxially compressed (13 ×3-R30°) phase is formed at an iodine coverage (θ) of 0.38 ML. At θ > 0.38 ML, first-order phase transition begins, leading to the formation of hexagonal moiré-like phases, which exhibit an anomalously large corrugation in STM (0.8-2.3 Å). In the range of 0.40-0.43 ML, the compression of hexagonal phases occurs, which ends at the formation of the (7 × 7)R21.8° structure at saturation. The DFT calculations allow us to explain the anomalous atomic corrugation of the hexagonal phases by the strong violation of the atomic structure of the substrate including up to ten layers of silver. Iodine dosing above 0.43 ML leads to the growth of 2D islands of silver iodide. The STM images of the silver iodide surface demonstrate a clear visible hexagonal superstructure with a periodicity of 25 Å superimposed with a quasi-hexagonal atomic modulation. DFT calculations of the atomic structure of AgI islands point to the formation of a sandwich-like double layer honeycomb structure similar to the case of I/Ag(100).

Radial coordinate maps, radial vectors, and binormal vectors for 5/6, 5/5 and 5/4 edge island domains in W7-X

The edge island domain in Wendelstein 7-X consists of divertor islands whose position coincide with the location of rational values of the rotational transform and surround the main confinement volume (the ‘main plasma’), providing the magnetic topology for an island divertor. The 5/5 edge consists of five individual islands that are unconnected. In contrast, a single island connects onto itself after 6 or 4 toroidal transits in the 5/6 or 5/4 edge, respectively. Many interesting phenomena are related to these islands and diagnostic analyses require a mapping from ‘laboratory’ or real space coordinates to the island coordinate system (ICS). A procedure is described here to calculate several scalar and vector quantities for closed island structures which can be utilized in fast interpolation schemes for inverse maps. For the 5/5 edge, a fixed-boundary vacuum (zero beta) magneto-hydrodynamic solution of the 5/5 island is found with the Variational Moments Equilibrium Code, VMEC. The solution is compatible with already existing routines which determine the radial and binormal vectors of VMEC solutions at arbitrary laboratory coordinates. VMEC does not support solutions for the 5/4 and 5/6 islands, but the radial and binormal vectors are available in a local 2D Fourier ICS.

Strain relief by controlled cracking in highly stretchable multi-layer composites

Stretchable electronics exploit the characteristics of soft biocompatible polymers and utilize microstructural designs to push the boundaries of brittle functional electronic materials. Extreme stretchability is required in novel applications such as in situ health monitors or sensors integrated into, e.g., wound dressing. A novel approach to stretchable electronics aims to increase macroscopic stretchability by introducing controlled cracks in the polymer substrate’s surface (Polywka et al., 2016). The cracks are deliberately induced by a micropattern of soft island crack starters in the hardened surface. Here, we introduce the first numerical model of fracture and deformation behavior of soft islands structures. We study crack evolution and material strain in dependence of the microstructure’s design. The fracture behavior in the polymer is modeled with a cohesive zone model. We examine the microstructured composite’s behavior with regard to its material and geometry properties, e.g. ratio of Young’s moduli, hardened layer thickness, or distance between cracks. The results show that cracks and 3D soft islands accommodate the majority of the applied macroscopic strain while the hardened surface remains almost strain-free. The soft islands microstructure design, thus, demonstrates outstanding strain relief capabilities and can accommodate rigid functional parts while remaining highly stretchable. The simulations reveal critical parameters and allow to identify design principles for large usable surfaces on polymer substrates utilizing controlled cracking.

Effect of Mechanical Activation of Al-Ti-B Powder Mixture on Phase Composition and Structure of Al-TiB2 Composite Materials Obtained by Self-Propagating High-Temperature Synthesis (SHS).

In this study, we successfully obtained Al-TiB2 composite materials using self-propagating high-temperature synthesis and preliminary mechanical activation of the initial Al-(Ti + 2B) powder mixture with a high aluminum content (70 wt.%). We investigated the possibility of controlling the structure of synthesis products, in particular, the size and shape of ceramic particles. We examined the effects of the mechanical activation of the initial powder mixture on the structure and particle size of titanium diboride in the synthesis products. We proposed a mechanism of structure formation in the synthesis products obtained by SHS using the method of preliminary mechanical activation of the initial mixture. We found that mechanical activation for 60-180 s led to the formation of isolated TiB2 particles of prolate and irregular shape. The average particle size of TiB2 in the synthesis products was 0.77 (after 60 s of mechanical activation) and 1.5 µm (after 180 s of mechanical activation), respectively. An increase in the duration of mechanical activation to 900 s led to the formation of an island (skeletal) structure, in which there were interconnected aggregates and isolated particles of titanium diboride. The average size of these particles was 4.3 µm.

Advances in physics and applications of 3D magnetic perturbations on the J-TEXT tokamak

In the last two years, three major technical improvements have been made on J-TEXT in supporting of the expanded operation regions and diagnostic capabilities. (1) The successful commission of the 105 GHz/500 kW/1 s electron cyclotron resonance heating (ECRH) system increasing the core electron temperature from 0.9 keV up to around 1.5 keV. (2) The poloidal divertor configuration with an X-point in the high-field side has been achieved. In particular, the 400 kW electron cyclotron wave has also been successfully injected into the diverted plasma. (3) A 256-channel electron cyclotron emission imaging diagnostic system and two sets of four-channel Doppler backscattering diagnostics have been successfully developed on J-TEXT, allowing detailed measurement of the electron temperature and density fluctuations for turbulence and MHD research. The locked mode (LM), especially the 2/1 LM, is one of the biggest threats to the plasma operation. Both the thresholds of 2/1 and 3/1 LM are observed to vary non-monotonically on electron density. The electrode biasing was applied successfully to unlock the LM from either a rotating or static resonant magnetic perturbation (RMP) field. In the presence of 2/1 LM, three kinds of standing wave (SW) structures have been observed to share a similar connection to the island structure, i.e. the nodes of the SWs locate around the O- or X-points of the 2/1 island. The control and mitigation of disruption is essential to the safe operation of ITER, and it has been systematically studied by applying a RMP field, massive gas injection (MGI) and shattered pellet injection on J-TEXT. When the RMP-induced 2/1 LM is larger than a critical width, the MGI shutdown process can be significantly influenced. If the phase difference between the O-point of LM and the MGI valve is +90° (or −90°), the penetration depth and the assimilation of impurities can be enhanced (or suppressed) during the pre-thermal quench (TQ) phase and result in a faster (or slower) TQ. A secondary MGI can also suppress the runaway electron (RE) generation, if the additional high-Z impurity gas arrives at the plasma edge before TQ. When the secondary MGI has been applied after the formation of the RE current plateau, the RE current can be dissipated, and the dissipation rate increases with the injected impurity quantity but saturates with a maximum of 28 MA s−1. The non-local transport is experimentally observed in the ion transport channel. The electron thermal diffusivity significantly increases with the ECRH power. Theoretical work shows that significant intrinsic current can be driven by electromagnetic turbulence, and the robust formation mechanism of the E × B staircase is identified from the Hasegawa–Wakatani system.

Experimental and computational investigation of airwake aerodynamics of the generic aircraft carrier with ski-jump

A comprehensive experimental and computational investigation of flow past over a generic aircraft carrier with ski-jump is presented in this paper. The study has been performed to gain a deeper understanding of the airwake aerodynamics past such an aircraft carrier for different crosswind conditions. An experimental investigation is carried out in a wind tunnel on a prototype model of having a length of 1.1 m (1:250 scale). The prototype model was conceived from the various aircraft carriers existing in the world. A five-hole cobra probe is used to measure all the three velocity components along the 3° aircraft's landing slope for validation purposes. Reynolds-Averaged Navier-Stokes (RANS) based computational investigations are conducted to predict the overall airwake aerodynamics over the flight deck. Subsequently, a detailed parametric study has been conducted to examine the effect of relative wind conditions on the aircraft's landing slope. Detailed comparisons of velocity variations along the aircraft glide slope are discussed along with the experimental data. Results show that the top deck island structure and the oblique wind conditions are the two main contributors for adverse airwake aerodynamics over the flight deck.