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Vapor-phase mechanisms [e.g., vapor-liquid-solid (VLS), vapor-solid-solid, oxide-assisted growth,and the self-catalytic growth mechanisms] for the unidirectional nanowire (NW) growth are not yetwell understood. For this understand...
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Vapor-phase mechanisms [e.g., vapor-liquid-solid (VLS), vapor-solid-solid, oxide-assisted growth,and the self-catalytic growth mechanisms] for the unidirectional nanowire (NW) growth are not yetwell understood. For this understanding, growths of GaN and InN NWs in our laboratory, withoutand with the assistance of foreign element catalytic agents (FECAs), such as Au and In, wereperformed. GaN NW growth, in the presence of FECA Ni, was possible at temperatures below theNi/Ga eutectic temperature. InN NWs were grown, in the presence of Au, and at temperatures in thevicinity of Au/In eutectic temperature. They were found to have Au at the NW tip, NW base, andNW sidewalls. Extensive investigation of the fundamentals underlying these anomalies has beencarried out. The temperature dependence of the VLS mechanism has also been elucidated. A largenumber of available elemental and compound semiconductor NWs exhibiting similar characteristicshave been considered for the investigation. Based on this investigation, a chemicophysicalmechanism called the vapor-quasisolid-solid (VQS) (or vapor-semisolid-solid, or vapor-quasiliquid-solid, or vapor-semiliquid-solid) mechanism has been proposed. The cause oftemperature dependence of the VLS growth under different growth conditions and growthenvironments, and the possible relationship between the VLS and the VQS mechanisms has beenpresented. To better describe the vapor-phase mechanisms, including the VQS mechanism, a unifieddefinition of droplets has been proposed. A series of experimental evidences has been set forth tosubstantiate the validity of the proposed mechanism, and to justify the definition of the unifieddroplet model. They together appear to explain the fundamental basis of the NW growth by variousmechanisms, including the VQS mechanisms. They also provide solutions of many knownproblems, conflicts, confusions, and controversies involving NW growth.
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Photodynamic therapy as an antibiotic-free method could effectively solve the bacterial infection problem and avoid the emergence of multidrug-resistant bacteria. In this paper, Ti3C2@gold nanorods (GNRs)/PDA nanoparticles with sy...
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Photodynamic therapy as an antibiotic-free method could effectively solve the bacterial infection problem and avoid the emergence of multidrug-resistant bacteria. In this paper, Ti3C2@gold nanorods (GNRs)/PDA nanoparticles with synergistic photodynamic antibacterial properties were obtained by coating PDA on the surface of in situ growth Ti3C2@GNRs. GNRs growing on the surface of Ti3C2 MXene nanosheets effectively prevented the aggregation of Ti3C2 nanosheets, which ensured the distinct photodynamic antibacterial performance of Ti3C2. In addition, the introduction of SH-PEG-CH3 to Ti3C2@GNRs nanosheets endowed Ti3C2@GNRs/PDA nanoparticles with good biocompatibility. Furthermore, Ti3C2@GNRs/PDA nanoparticles possessed synergistically enhanced photodynamic properties, caused by the remarkable photodynamic properties of Ti3C2 MXene and PDA. In addition, with the increase in PDA content, Ti3C2@GNRs/PDA nanoparticles could produce more O-1(2) under 660 nm laser irradiation and the minimum inhibitory concentrations of Ti3C2@GNRs/PDA50 nanoparticles against E. coli and S. aureus decreased to about 0.07 mg/mL. Thus, this work utilizes the O-1(2)-generating performance of Ti3C2 MXene and PDA to fabricate Ti3C2@GNRs/PDA nanoparticles with synergistically enhanced photodynamic antibacterial activities, which is expected to be applied in antibiotic-free antibacterial field. [GRAPHICS] .
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We present results underlining the conjecture [J. Patera, R. Twarock, J. Phys. A, in press] that the affine extensions for non-crystallographic Coxeter groups introduced in this reference are suitable mathematical objects for the ...
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We present results underlining the conjecture [J. Patera, R. Twarock, J. Phys. A, in press] that the affine extensions for non-crystallographic Coxeter groups introduced in this reference are suitable mathematical objects for the description of the symmetries of Carbon onions and Carbon nanotubes. It is the hope that these considerations will shed new light on open questions concerning structure, stability and formation of these fullerenes. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 16]
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The widespread occurrence of siderite at the Earth's surface has motivated investigations into the thermodynamic factors controlling its stability for most of the last century. However, despite a new appreciation for multiple Fe(I...
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The widespread occurrence of siderite at the Earth's surface has motivated investigations into the thermodynamic factors controlling its stability for most of the last century. However, despite a new appreciation for multiple Fe(II)-carbonate mineralisation pathways, the rates of siderite growth have not been accurately predicted as a function of solution chemistry. This has impeded a quantitative understanding of myriad geochemical systems, both modern and ancient. To address this issue, we investigated the growth kinetics of synthetic siderite seeds in anoxic closed-system conditions at 298.15 K and 1 bar. On the basis of monitoring the chemical evolution of the bulk solutions over the course of 46 days, our results demonstrate two distinct relationships between kinetic behaviour and solution saturation (Omega) with respect to siderite. These relationships are best explained by chemical affinity-based rate laws that have been extensively applied to the growth kinetics of calcite (a mineral isostructural with siderite). More specifically, the surface area-normalised siderite growth rates (mol.m(-2).s(-1)) display a linear correlation with supersaturation (Omega - 1) when Omega greater than or similar to 5, suggesting a growth rate controlled by the transport of ions to the mineral surface (r(tr)):
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Epitaxial growth of SiC on hexagonal (or a)-SiC(0001) has been performed by means of solid-source molecular beam epitaxy (MBE). The solid-source MBE growth conditions have been analyzed concerning the supersaturation and the exces...
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Epitaxial growth of SiC on hexagonal (or a)-SiC(0001) has been performed by means of solid-source molecular beam epitaxy (MBE). The solid-source MBE growth conditions have been analyzed concerning the supersaturation and the excess phase formation of silicon and carbon. In general, our results demonstrate that control of the SVC ratio and the supersaturation (S) is essential for the growth mode and the kind of polytype grown. Low temperature (T<1450K) deposition on on-axis iC substrates always results in the growth of 3C-SiC, which is significantly improved by an alternating supply of Si and C. On vicinal substrates, a step flow growth mode has been realized at T down to 1300K. In experiments performed at T>1450K under near surface equilibrium conditions, different growth modes, and conditions stabilizing the growth of certain polytypes have been found. With a step decrease of S, a step-flow growth mode of both 4H- and 6H-SiC was Obtained and, for the first time in case of epitaxal SiC growth, a homogeneous nucleation of α-SiC at more C-rich conditions has been realized. Conditions stabilizing the growth of certain polytypes have been estimated by thermodynamic calculations considering the influence of polytype structure on the supersaturation and the surface energy. Based on these results, we have ; demonstrated the growth of a double-heterostructure by firstly growing a 3C-SiC layer on 4H-SiC(0001) at low temperature and a subsequent growth of 4H-SiC under near surface equilibrium conditions on a C-stabilized surface on top of this layer.
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Material formation mechanisms and their selective realization must be well understood for the development of new materials for advanced technologies. Since nanomaterials demonstrate higher specific surface energies compared to the...
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Material formation mechanisms and their selective realization must be well understood for the development of new materials for advanced technologies. Since nanomaterials demonstrate higher specific surface energies compared to their corresponding bulk materials, the homoepitaxial growth of nanomaterials on bulk materials is not thermodynamically favorable. We observed the homoepitaxial growth of nanowires with constant outer diameters on bulk materials in two different, solution-based growth systems. We also suggested potential mechanisms of the spontaneous and homoepitaxial growth of the ZnO nanostructures based on the characterization results. The first key factor for favorable growth was the crystal facet stabilization effect of capping agents during the early stages of growth. The second factor was the change in the dominant growth mode during the reaction in a closed system. The spontaneous, homoepitaxial growth of nanomaterials enables the realization of unprecedented, complex, hierarchical, single-crystalline structures required for future technologies. (C) 2020 Elsevier Inc. All rights reserved.
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A model has been developed successfully to study the nucleation and growth mechanisms of the Ga_xIn_(1-x)P vapour phase epitaxial (VPE) growth in the Ga-In-HCl-PH_3-H_2 system. Using classical heterogeneous nucleation theory the c...
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A model has been developed successfully to study the nucleation and growth mechanisms of the Ga_xIn_(1-x)P vapour phase epitaxial (VPE) growth in the Ga-In-HCl-PH_3-H_2 system. Using classical heterogeneous nucleation theory the critical nucleation parameters have been derived and the effect of substrate orientations has been studied. By applying physico-chemical concepts, an expression for the composition has been derived as a function of experimental input parameters by considering the number of intermediate reactions. The effect of experimental input parameters on the nucleation and growth mechanism has been analysed in detail and it was found that the (100) orientation has the highest nucleation barrier, and the composition was strongly affected by deposition temperature and input gas mole ratio. It was also found that Ga is preferentially deposited rather than In. The predicted results are compared with the experimental data and found to be in good agreement.
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The definition of shape in multicellular organisms is a major issue of developmental biology. It is well established that morphogenesis relies on genetic regulation. However, cells, tissues, and organism behaviors are also bound b...
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The definition of shape in multicellular organisms is a major issue of developmental biology. It is well established that morphogenesis relies on genetic regulation. However, cells, tissues, and organism behaviors are also bound by the laws of physics, which limit the range of possible deformations organisms can undergo but also define what organisms must do to achieve specific shapes. Besides experiments, theoretical models and numerical simulations of growing tissues are powerful tools to investigate the link between genetic regulation and mechanics. Here, we provide an overview of the main mechanical models of plant morphogenesis developed so far, from subcellular scales to whole tissues. The common concepts and discrepancies between the various models are discussed.
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Effective computational procedures are established and used to characterize dynamic crack propagation along material interfaces between isotropic and orthotropic materials. We first simulate a dynamic fracture experiment in which ...
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Effective computational procedures are established and used to characterize dynamic crack propagation along material interfaces between isotropic and orthotropic materials. We first simulate a dynamic fracture experiment in which crack propagation occurs along an interface between PMMA and steel in a dynamically loaded bend specimen. In the analysis, the dynamic energy release rate and mixed-mode stress intensity factors are extracted from finite element field solutions using suitably formulated conservation integrals. These variables form the basis for defining an interface fracture criterion under dynamic conditions. In order to propagate the crack according to such a criterion, an iterative procedure is utilized to determine the correct crack tip velocity history. An important computed result, which is consistent with experimental observation, is that the energy release rate decreases as the crack propagation accelerates. The physical interpretation of this result is that less energy is absorbed by the moving crack as its velocity increases. In the second part of our study, the computational procedures are modified For the dynamic fracture analysis of a thin composite panel consisting of differently oriented orthotropic laminae. Here we investigate the interplay between delamination and buckling of the more complex structure. It is assumed that the panel contains an initial finite interlaminar crack and is subjected to a uniaxial compressive load. Without any crack extension, two buckling modes are observed under quasistatic conditions. One is characterized by an overall panel buckling and the other is dominated by a local ligament buckling near the crack. Coupling of the two modes produces not only a lower critical buckling load but also an unstable post-buckling behavior. Therefore, an embedded delamination is seen to create imperfection sensitivity, leading to limit load type behavior. Such a condition is potentially troublesome for compression loaded thin composite panels. In addition, the large compressive load may also trigger a dynamic propagation of the embedded delamination. We have simulated this unstable dynamic crack growth using the iterative method and a simplified delamination criterion. In the simulation, the energy release rate, the mixed-mode stress intensity factors and the resulting crack tip speed are obtained. These results predict that an existing embedded delamination can weaken and significantly alter the post-buckling behavior of composite panel. [References: 13]
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Pressure solution seams (PSSs) can be idealized as localized volume reduction structures (LVRSs) in terms of their mechanics. Previous mechanical analyses of LVRSs, including compaction bands, showed that the normal stresses at th...
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Pressure solution seams (PSSs) can be idealized as localized volume reduction structures (LVRSs) in terms of their mechanics. Previous mechanical analyses of LVRSs, including compaction bands, showed that the normal stresses at the tips of LVRSs are compressive and significantly amplified with respect to the remote stresses, whereas on the flanks they are slightly reduced. These results can be used to rationalize the in-plane growth of PSSs for a certain distance, however, based on these stress conditions alone, it is not possible to explain the widening and transverse coalescence of PSSs. In this study, based on laboratory and field observations that the PSS surfaces are extremely rough, we introduced asperities with triangular, semicircular, and rectangular geometries on the flanks of the LVRSs into our mechanical model to see if these asperities can raise stresses in these locations, which may rationalize the transverse growth of PSSs. It is found that these asperities can produce strong stress perturbations on the LVRS flanks thereby inducing a significant increase in the compressive normal stresses. In addition, to account for the rate factor of the pressure solution process, a creep law was adopted to simulate growth and coalescence of the LVRSs. Using the calculated normal compressive stresses and volumetric plastic strains as proxy for the growth, we show that (1) a single LVRS is able to grow both laterally and transversely for a short distance and (2) two in-plane aligned neighboring LVRSs with a short distance between the adjacent tips and two parallel echelon and overlapped LVRSs with a small spacing may be able to link and coalesce to form a longer and wider LVRS, respectively. The influences of the LVRS geometric configurations, the material properties within the LVRSs, and the distance and spacing of the aligned in-plane, echelon, and overlapping neighboring parallel LVRSs on the growth and coalescence of the LVRSs are investigated and their implications specifically for the PSSs are also discussed.
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