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Crystals, Volume 6, Issue 7 (July 2016)

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Research

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Open AccessArticle Optical and X-Ray Topographic Studies of Dislocations, Growth-Sector Boundaries, and Stacking Faults in Synthetic Diamonds
Crystals 2016, 6(7), 71; doi:10.3390/cryst6070071
Received: 2 May 2016 / Revised: 13 June 2016 / Accepted: 17 June 2016 / Published: 24 June 2016
Cited by 3 | PDF Full-text (7908 KB) | HTML Full-text | XML Full-text
Abstract
The characterization of growth features and defects in various high-pressure high-temperature (HPHT) synthetic diamonds has been achieved with optical and X-ray topographic techniques. For the X-ray studies, both characteristic and synchrotron radiation were used. The defects include dislocations, stacking faults, growth banding, growth
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The characterization of growth features and defects in various high-pressure high-temperature (HPHT) synthetic diamonds has been achieved with optical and X-ray topographic techniques. For the X-ray studies, both characteristic and synchrotron radiation were used. The defects include dislocations, stacking faults, growth banding, growth sector boundaries, and metal inclusions. The directions of the Burgers vectors of many dislocations (edge, screw, and mixed 30°, 60°, and 73.2°), and the fault vectors of stacking faults, were determined as <110> and 1/3 <111> respectively. Some dislocations were generated at metallic inclusions; and some dislocations split with the formation of stacking faults. Full article
(This article belongs to the Special Issue Crystal Dislocations)
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Open AccessArticle Electronic and Optical Properties of Dislocations in Silicon
Crystals 2016, 6(7), 74; doi:10.3390/cryst6070074
Received: 10 May 2016 / Revised: 22 June 2016 / Accepted: 24 June 2016 / Published: 30 June 2016
Cited by 2 | PDF Full-text (8340 KB) | HTML Full-text | XML Full-text
Abstract
Dislocations exhibit a number of exceptional electronic properties resulting in a significant increase of the drain current of metal-oxide-semiconductor field-effect transistors (MOSFETs) if defined numbers of these defects are placed in the channel. Measurements on individual dislocations in Si refer to a supermetallic
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Dislocations exhibit a number of exceptional electronic properties resulting in a significant increase of the drain current of metal-oxide-semiconductor field-effect transistors (MOSFETs) if defined numbers of these defects are placed in the channel. Measurements on individual dislocations in Si refer to a supermetallic conductivity. A model of the electronic structure of dislocations is proposed based on experimental measurements and tight binding simulations. It is shown that the high strain level on the dislocation core—exceeding 10% or more—causes locally dramatic changes of the band structure and results in the formation of a quantum well along the dislocation line. This explains experimental findings (two-dimensional electron gas and single-electron transitions). The energy quantization within the quantum well is most important for supermetallic conductivity. Full article
(This article belongs to the Special Issue Crystal Dislocations)
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Open AccessArticle Effect of HNO3 Concentration on the Morphologies and Properties of Bi2WO6 Photocatalyst Synthesized by a Hydrothermal Method
Crystals 2016, 6(7), 75; doi:10.3390/cryst6070075
Received: 10 May 2016 / Revised: 27 June 2016 / Accepted: 28 June 2016 / Published: 20 July 2016
Cited by 4 | PDF Full-text (4644 KB) | HTML Full-text | XML Full-text
Abstract
Bi2WO6 samples were prepared by a hydrothermal method using Bi(NO3)3·5H2O and Na2WO4·2H2O as raw materials. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Raman and Brunauer–Emmett–Teller (BET) were
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Bi2WO6 samples were prepared by a hydrothermal method using Bi(NO3)3·5H2O and Na2WO4·2H2O as raw materials. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Raman and Brunauer–Emmett–Teller (BET) were employed for sample characterization. The photocatalytic activity of the samples was evaluated by the degradation of Rhodamine B under visible-light irradiation. The photocatalytic activity of Bi2WO6, as well as the effect of varying HNO3 concentrations on the morphologies of Bi2WO6, was investigated. The HNO3 concentration significantly affected the structure and morphology of the Bi2WO6. The photocatalytic performance varied with the structure, morphology, and surface area of the Bi2WO6 samples. The results indicated that the H10 sample exhibits uniform morphology and excellent photocatalytic performance; using this sample, the degradation of Rhodamine B reached 96% in 90 min under visible-light irradiation. Full article
(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)
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Open AccessArticle Preparation of Three-Dimensional Photonic Crystals of Zirconia by Electrodeposition in a Colloidal Crystals Template
Crystals 2016, 6(7), 76; doi:10.3390/cryst6070076
Received: 16 June 2016 / Revised: 28 June 2016 / Accepted: 30 June 2016 / Published: 2 July 2016
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Abstract
Three-dimensional photonic crystals of zirconia were prepared by electrodeposition in a colloidal crystals template following calcination at 500 °C. Scanning electron microscopy, thermogravimetric analysis, X-ray diffraction, and reflectance spectroscopy were employed to characterize the photonic crystals of zirconia. It was found that hydrated
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Three-dimensional photonic crystals of zirconia were prepared by electrodeposition in a colloidal crystals template following calcination at 500 °C. Scanning electron microscopy, thermogravimetric analysis, X-ray diffraction, and reflectance spectroscopy were employed to characterize the photonic crystals of zirconia. It was found that hydrated zirconium ions could penetrate the colloidal crystals template and reach the substrate easily by electrodeposition, which resulted in stronger bonding between the substrate and the as-deposited membrane. Moreover, the electrodeposited membrane had low water content, leading to a low amount of shrinkage during calcination. Both these properties could suppress detachment from the substrate upon removal of the colloidal crystals template. Therefore, the three-dimensional photonic crystals of zirconia synthesized in this study exhibited very good preservation of the ordered structures of the colloidal crystals template with a high density. A peak of reflection higher than 70% was formed in the reflectance spectrum because of the strong diffraction of the ordered structures. Full article
(This article belongs to the Special Issue Colloidal Crystals)
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Open AccessArticle Dislocation Nucleation on Grain Boundaries: Low Angle Twist and Asymmetric Tilt Boundaries
Crystals 2016, 6(7), 77; doi:10.3390/cryst6070077
Received: 22 May 2016 / Revised: 21 June 2016 / Accepted: 1 July 2016 / Published: 5 July 2016
Cited by 3 | PDF Full-text (10592 KB) | HTML Full-text | XML Full-text
Abstract
We investigate the mechanisms of incipient plasticity at low angle twist and asymmetric tilt boundaries in fcc metals. To observe plasticity of grain boundaries independently of the bulk plasticity, we simulate nanoindentation of bicrystals. On the low angle twist boundaries, the intrinsic grain
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We investigate the mechanisms of incipient plasticity at low angle twist and asymmetric tilt boundaries in fcc metals. To observe plasticity of grain boundaries independently of the bulk plasticity, we simulate nanoindentation of bicrystals. On the low angle twist boundaries, the intrinsic grain boundary (GB) dislocation network deforms under load until a dislocation segment compatible with glide on a lattice slip plane is created. The half loops are then emitted into the bulk of the crystal. Asymmetric twist boundaries considered here did not produce bulk dislocations under load. Instead, the boundary with a low excess volume nucleated a mobile GB dislocation and additional GB defects. The GB sliding proceeded by motion of the mobile GB dislocation. The boundary with a high excess volume sheared elastically, while bulk-nucleated dislocations produced plastic relaxation. Full article
(This article belongs to the Special Issue Crystal Dislocations)
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Open AccessArticle Magnetic-Sphere-Based Phononic Crystals
Crystals 2016, 6(7), 78; doi:10.3390/cryst6070078
Received: 24 May 2016 / Revised: 28 June 2016 / Accepted: 1 July 2016 / Published: 8 July 2016
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Abstract
Periodic arrays in one, two, and three dimensions, made of magnetic spheres embedded in a fluid matrix, are considered in this study and utilized as phononic structures. The propagation of acoustic waves through these structures is analyzed experimentally, in low- and high-frequency region,
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Periodic arrays in one, two, and three dimensions, made of magnetic spheres embedded in a fluid matrix, are considered in this study and utilized as phononic structures. The propagation of acoustic waves through these structures is analyzed experimentally, in low- and high-frequency region, via laser vibrometry, as well as standard underwater acoustic measurements. A first comparison to theoretical calculations obtained through multiple-scattering techniques and multipole models reveals a distinct behavior depending on the immersion fluid and/or frequency regime. Our results show that the elastodynamic response of these systems can be, under conditions, simply described by classical elastic theory without taking directly (ab initio) into account the magnetic character of the spherical particles. The structures considered above could offer several possibilities including facility of construction and use in filtering applications, but they are also of interest from a theoretical point of view, as a means to investigate the validity of several approximate theoretical descriptions. Full article
(This article belongs to the Special Issue Phononic Crystals)
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Open AccessArticle Adsorption, Desorption, Surface Diffusion, Lattice Defect Formation, and Kink Incorporation Processes of Particles on Growth Interfaces of Colloidal Crystals with Attractive Interactions
Crystals 2016, 6(7), 80; doi:10.3390/cryst6070080
Received: 8 June 2016 / Revised: 4 July 2016 / Accepted: 15 July 2016 / Published: 18 July 2016
PDF Full-text (3067 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Good model systems are required in order to understand crystal growth processes because, in many cases, precise incorporation processes of atoms or molecules cannot be visualized easily at the atomic or molecular level. Using a transmission-type optical microscope, we have successfully observed in
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Good model systems are required in order to understand crystal growth processes because, in many cases, precise incorporation processes of atoms or molecules cannot be visualized easily at the atomic or molecular level. Using a transmission-type optical microscope, we have successfully observed in situ adsorption, desorption, surface diffusion, lattice defect formation, and kink incorporation of particles on growth interfaces of colloidal crystals of polystyrene particles in aqueous sodium polyacrylate solutions. Precise surface transportation and kink incorporation processes of the particles into the colloidal crystals with attractive interactions were observed in situ at the particle level. In particular, contrary to the conventional expectations, the diffusion of particles along steps around a two-dimensional island of the growth interface was not the main route for kink incorporation. This is probably due to the number of bonds between adsorbed particles and particles in a crystal; the number exceeds the limit at which a particle easily exchanges its position to the adjacent one along the step. We also found novel desorption processes of particles from steps to terraces, attributing them to the assistance of attractive forces from additionally adsorbing particles to the particles on the steps. Full article
(This article belongs to the Special Issue Colloidal Crystals)
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Open AccessArticle A Novel, Simple and Green Way to Fabricate BiVO4 with Excellent Photocatalytic Activity and Its Methylene Blue Decomposition Mechanism
Crystals 2016, 6(7), 81; doi:10.3390/cryst6070081
Received: 15 June 2016 / Revised: 5 July 2016 / Accepted: 7 July 2016 / Published: 20 July 2016
Cited by 1 | PDF Full-text (21740 KB) | HTML Full-text | XML Full-text
Abstract
BiVO4 photocatalysts were synthesized via a facile surfactant-free method with heat treatment. The heat treatment temperatures influenced the crystal structures and morphologies. The photocatalytic performance is associated with its crystallinity, Brunauer–Emmett–Teller (BET) specific surface area, and band gap energy. The BiVO4
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BiVO4 photocatalysts were synthesized via a facile surfactant-free method with heat treatment. The heat treatment temperatures influenced the crystal structures and morphologies. The photocatalytic performance is associated with its crystallinity, Brunauer–Emmett–Teller (BET) specific surface area, and band gap energy. The BiVO4 photocatalyst prepared by heat treatment at 700 °C showed the highest photocatalytic activity, promoting 100% degradation of methylene blue (MB) in 60 min under visible-light irradiation. Recycling experiments results indicated that the BiVO4 photocatalysts have excellent photo-stability, and a possible mechanism for the photocatalytic process was proposed by examining the effects of the active species involved in MB degradation. This work could provide new insights into the fabrication of highly efficient and stable BiVO4 photocatalysts for dye degradation. Full article
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Open AccessFeature PaperArticle Soda Cans Metamaterial: A Subwavelength-Scaled Phononic Crystal
Crystals 2016, 6(7), 82; doi:10.3390/cryst6070082
Received: 20 May 2016 / Revised: 6 July 2016 / Accepted: 19 July 2016 / Published: 21 July 2016
Cited by 3 | PDF Full-text (13575 KB) | HTML Full-text | XML Full-text
Abstract
Photonic or phononic crystals and metamaterials, due to their very different typical spatial scales—wavelength and deep subwavelength—and underlying physical mechanisms—Bragg interferences or local resonances—, are often considered to be very different composite media. As such, while the former are commonly used to manipulate
[...] Read more.
Photonic or phononic crystals and metamaterials, due to their very different typical spatial scales—wavelength and deep subwavelength—and underlying physical mechanisms—Bragg interferences or local resonances—, are often considered to be very different composite media. As such, while the former are commonly used to manipulate and control waves at the scale of the unit cell, i.e., wavelength, the latter are usually considered for their effective properties. Yet we have shown in the last few years that under some approximations, metamaterials can be used as photonic or phononic crystals, with the great advantage that they are much more compact. In this review, we will concentrate on metamaterials made out of soda cans, that is, Helmholtz resonators of deep subwavelength dimensions. We will first show that their properties can be understood, likewise phononic crystals, as resulting from interferences only, through multiple scattering effects and Fano interferences. Then, we will demonstrate that below the resonance frequency of its unit cell, a soda can metamaterial supports a band of subwavelength varying modes, which can be excited coherently using time reversal, in order to beat the diffraction limit from the far field. Above this frequency, the metamaterial supports a band gap, which we will use to demonstrate cavities and waveguides, very similar to those obtained in phononic crystals, albeit of deep subwavelength dimensions. We will finally show that multiple scattering can be taken advantage of in these metamaterials, by correctly structuring them. This allows to turn a metamaterial with a single negative effective property into a negative index metamaterial, which refracts waves negatively, hence acting as a superlens. Full article
(This article belongs to the Special Issue Phononic Crystals)
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Review

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Open AccessReview Zirconate Pyrochlore Frustrated Magnets: Crystal Growth by the Floating Zone Technique
Crystals 2016, 6(7), 79; doi:10.3390/cryst6070079
Received: 13 June 2016 / Revised: 29 June 2016 / Accepted: 1 July 2016 / Published: 11 July 2016
Cited by 1 | PDF Full-text (13458 KB) | HTML Full-text | XML Full-text
Abstract
This article reviews recent achievements on the crystal growth of a new series of pyrochlore oxides—lanthanide zirconates, which are frustrated magnets with exotic magnetic properties. Oxides of the type A2B2O7 (where A= Rare Earth, B= Ti,
[...] Read more.
This article reviews recent achievements on the crystal growth of a new series of pyrochlore oxides—lanthanide zirconates, which are frustrated magnets with exotic magnetic properties. Oxides of the type A 2 B 2 O 7 (where A = Rare Earth, B = Ti, Mo) have been successfully synthesised in single crystal form using the floating zone method. The main difficulty of employing this technique for the growth of rare earth zirconium oxides A 2 Zr 2 O 7 arises from the high melting point of these materials. This drawback has been recently overcome by the use of a high power Xenon arc lamp furnace for the growth of single crystals of Pr 2 Zr 2 O 7 . Subsequently, large, high quality single crystals of several members of the zirconate family of pyrochlore oxides A 2 Zr 2 O 7 (with A = La → Gd) have been grown by the floating zone technique. In this work, the authors give an overview of the crystal growth of lanthanide zirconates. The optimum conditions used for the floating zone growth of A 2 Zr 2 O 7 crystals are reported. The characterisation of the crystal boules and their crystal quality is also presented. Full article
(This article belongs to the Special Issue Traveling Solvent Floating Zone (TSFZ) Method in Crystal Growth)
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Open AccessFeature PaperReview Dislocation Motion and the Microphysics of Flash Heating and Weakening of Faults during Earthquakes
Crystals 2016, 6(7), 83; doi:10.3390/cryst6070083
Received: 5 July 2016 / Revised: 14 July 2016 / Accepted: 16 July 2016 / Published: 22 July 2016
Cited by 1 | PDF Full-text (2286 KB) | HTML Full-text | XML Full-text
Abstract
Earthquakes are the result of slip along faults and are due to the decrease of rock frictional strength (dynamic weakening) with increasing slip and slip rate. Friction experiments simulating the abrupt accelerations (>>10 m/s2), slip rates (~1 m/s), and normal stresses
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Earthquakes are the result of slip along faults and are due to the decrease of rock frictional strength (dynamic weakening) with increasing slip and slip rate. Friction experiments simulating the abrupt accelerations (>>10 m/s2), slip rates (~1 m/s), and normal stresses (>>10 MPa) expected at the passage of the earthquake rupture along the front of fault patches, measured large fault dynamic weakening for slip rates larger than a critical velocity of 0.01–0.1 m/s. The dynamic weakening corresponds to a decrease of the friction coefficient (defined as the ratio of shear stress vs. normal stress) up to 40%–50% after few millimetres of slip (flash weakening), almost independently of rock type. The microstructural evolution of the sliding interfaces with slip may yield hints on the microphysical processes responsible for flash weakening. At the microscopic scale, the frictional strength results from the interaction of micro- to nano-scale surface irregularities (asperities) which deform during fault sliding. During flash weakening, the visco-plastic and brittle work on the asperities results in abrupt frictional heating (flash heating) and grain size reduction associated with mechano-chemical reactions (e.g., decarbonation in CO2-bearing minerals such as calcite and dolomite; dehydration in water-bearing minerals such as clays, serpentine, etc.) and phase transitions (e.g., flash melting in silicate-bearing rocks). However, flash weakening is also associated with grain size reduction down to the nanoscale. Using focused ion beam scanning and transmission electron microscopy, we studied the micro-physical mechanisms associated with flash heating and nanograin formation in carbonate-bearing fault rocks. Experiments were conducted on pre-cut Carrara marble (99.9% calcite) cylinders using a rotary shear apparatus at conditions relevant to seismic rupture propagation. Flash heating and weakening in calcite-bearing rocks is associated with a shock-like stress release due to the migration of fast-moving dislocations and the conversion of their kinetic energy into heat. From a review of the current natural and experimental observations we speculate that this mechanism tested for calcite-bearing rocks, is a general mechanism operating during flash weakening (e.g., also precursory to flash melting in the case of silicate-bearing rocks) for all fault rock types undergoing fast slip acceleration due to the passage of the seismic rupture front. Full article
(This article belongs to the Special Issue Crystal Dislocations)
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