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Crystals, Volume 10, Issue 2 (February 2020) – 89 articles

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Cover Story (view full-size image) An XtalController was used to monitor the crystallization of two enzymes, the CCA-adding enzyme of [...] Read more.
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Open AccessArticle
Enhanced Heat Transfer in Moderately Ionized Liquid Due to Hybrid MoS2/SiO2 Nanofluids Exposed by Nonlinear Radiation: Stability Analysis
Crystals 2020, 10(2), 142; https://doi.org/10.3390/cryst10020142 - 24 Feb 2020
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Abstract
This study considers ethylene-glycol as a moderate ionized regular liquid whose rheological behavior can be analyzed through the relations of the Carreau stress–strain tensor. Hybrid nanoliquids are potent liquids that give better performance for heat transfer and the properties of thermo physical than [...] Read more.
This study considers ethylene-glycol as a moderate ionized regular liquid whose rheological behavior can be analyzed through the relations of the Carreau stress–strain tensor. Hybrid nanoliquids are potent liquids that give better performance for heat transfer and the properties of thermo physical than regular heat transfer liquids (water, ethylene glycol, and oil) and nanoliquids by single nanomaterials. Here, a type of hybrid nanoliquid involving silicon oxide (SiO2) and Molybdenum disulfide (MoS2) nanoparticles with ethylene glycol as a base liquid are considered. In addition, the impact of nonlinear radiation along with Lorentz force is invoked. Similarity variables are utilized to acquire the numerical findings and their solutions for transmuting ordinary differential equations (ODEs). Using bvp4c from MATLAB, we can obtain these quantitative and numerical results of the converted nonlinear equations. The impacts of the pertinent constraints on the temperature distribution, velocity, Nusselt number, and skin friction are estimated. The outcomes indicate that the double-edged methods for the results originate from the precise values of the permeable parameters. Further, the critical values (Sc = 1.9699, 2.0700 and 2.2370) are enhanced due to the influence of the local Weissenberg number. This implies that the increasing value of the local Weissenberg number accelerate the boundary layer separation. Furthermore, a stability investigation is performed and confirms that the first solution is a physically reliable solution. Full article
Open AccessArticle
Growth of Freestanding Gallium Nitride (GaN) Through Polyporous Interlayer Formed Directly During Successive Hydride Vapor Phase Epitaxy (HVPE) Process
Crystals 2020, 10(2), 141; https://doi.org/10.3390/cryst10020141 - 24 Feb 2020
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Abstract
The progress of nitride technology is widely limited and hindered by the lack of high-quality gallium nitride (GaN) wafers. Therefore, a large number of GaN epitaxial devices are grown on heterogeneous substrates. Although various additional treatments of substrate have been used to promote [...] Read more.
The progress of nitride technology is widely limited and hindered by the lack of high-quality gallium nitride (GaN) wafers. Therefore, a large number of GaN epitaxial devices are grown on heterogeneous substrates. Although various additional treatments of substrate have been used to promote crystal quality, there is still plenty of room for its improvement, in terms of direct and continuous growth based on the hydride vapor phase epitaxy (HVPE) technique. Here, we report a three-step process that can be used to enhance the quality of GaN crystal by tuning V/III rate during successive HVPE process. In the growth, a metal-organic chemical vapor deposition (MOCVD) grown GaN on sapphire (MOCVD-GaN/Al2O3) was employed as substrate, and a high-quality GaN polyporous interlayer, with successful acquisition, without any additional substrate treatment, caused the growth stress to decrease to 0.06 GPa. Meanwhile the quality of GaN improved, and the freestanding GaN was directly obtained during the growth process. Full article
(This article belongs to the Special Issue Semiconductor Heteroepitaxy)
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Open AccessArticle
Enhanced Thermoelectric Properties of WS2/Single-Walled Carbon Nanohorn Nanocomposites
Crystals 2020, 10(2), 140; https://doi.org/10.3390/cryst10020140 - 24 Feb 2020
Viewed by 146
Abstract
Recently, two-dimensional tungsten disulfide (WS2) has attracted attention as a next generation thermoelectric material due to a favorable Seebeck coefficient. However, its thermoelectric efficiency still needs to be improved due to the intrinsically low electrical conductivity of WS2. In [...] Read more.
Recently, two-dimensional tungsten disulfide (WS2) has attracted attention as a next generation thermoelectric material due to a favorable Seebeck coefficient. However, its thermoelectric efficiency still needs to be improved due to the intrinsically low electrical conductivity of WS2. In the present study, thermoelectric properties of WS2 hybridized with highly conductive single-walled carbon nanohorns (SWCNHs) were investigated. The WS2/SWCNH nanocomposites were fabricated by annealing the mixture of WS2 and SWCNHs using a high-frequency induction heated sintering (HFIHS) system. By adding SWCNHs to WS2, the nanocomposites exhibited increased electrical conductivity and a slightly decreased Seebeck coefficient with the content of SWCNHs. Hence, the maximum power factor of 128.41 μW/mK2 was achieved for WS2/SWCNHs with 0.1 wt.% SWCNHs at 780 K, resulting in a significantly improved thermoelectric figure of merit (zT) value of 0.027 compared to that of pristine WS2 with zT 0.017. Full article
(This article belongs to the Special Issue Carbon Nano-Composite Materials)
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Open AccessArticle
COMF: Comprehensive Model-Fitting Method for Simulating Isothermal and Single-Step Solid-State Reactions
Crystals 2020, 10(2), 139; https://doi.org/10.3390/cryst10020139 - 24 Feb 2020
Viewed by 183
Abstract
It is well known that the implementation of the conventional model-fitting (CMF) method leads to several indistinguishable ‘best’ candidate models (BCMs) for a single-step isothermal solid-state reaction (ISSR), meaning that subjective selection becomes unavoidable. Here, we developed a more robust comprehensive model-fitting method [...] Read more.
It is well known that the implementation of the conventional model-fitting (CMF) method leads to several indistinguishable ‘best’ candidate models (BCMs) for a single-step isothermal solid-state reaction (ISSR), meaning that subjective selection becomes unavoidable. Here, we developed a more robust comprehensive model-fitting method (COMF) which, while maintaining the mathematical simplicity of CMF, utilizes a ranking criterion that enables automatic and unambiguous determination of the BCM. For each model evaluated, COMF, like CMF, fits the integral reaction rate, but, unlike CMF, it also fits the experimental conversion fraction and reaction speed. From this, three different determination coefficients are calculated and combined to rank the considered models. To validate COMF, we used two sets of experimental kinetic data from the literature regarding the isothermal desolvation of pharmaceutical solvates: (i) tetrahydrofuran solvates of sulfameter, and (ii) methanol solvates of ciclesonide. Our results suggest that from an algorithmic perspective, COMF could become the model-fitting method of choice for ISSRs making the selection of BCM easier and more reliable. Full article
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Open AccessArticle
Stable and Metastable Patterns in Chromonic Nematic Liquid Crystal Droplets Forced with Static and Dynamic Magnetic Fields
Crystals 2020, 10(2), 138; https://doi.org/10.3390/cryst10020138 - 24 Feb 2020
Viewed by 176
Abstract
Spherical confinement of nematic liquid crystals leads to the formation of equilibrium director field configurations that include point and line defects. Driving these materials with flows or dynamic fields often results in the formation of alternative metastable states. In this article, we study [...] Read more.
Spherical confinement of nematic liquid crystals leads to the formation of equilibrium director field configurations that include point and line defects. Driving these materials with flows or dynamic fields often results in the formation of alternative metastable states. In this article, we study the effect of magnetic field alignment, both under static and dynamic conditions, of nematic gems (nematic droplets in coexistence with the isotropic phase) and emulsified nematic droplets of a lyotropic chromonic liquid crystal. We use a custom polarizing optical microscopy assembly that incorporates a permanent magnet whose strength and orientation can be dynamically changed. By comparing simulated optical patterns with microscopy images, we measure an equilibrium twisted bipolar pattern within nematic gems that is only marginally different from the one reported for emulsified droplets. Both systems evolve to concentric configurations upon application of a static magnetic field, but behave very differently when the field is rotated. While the concentric texture within the emulsified droplets is preserved and only displays asynchronous oscillations for high rotating speeds, the nematic gems transform into a metastable untwisted bipolar configuration that is memorized by the system when the field is removed. Our results demonstrate the importance of boundary conditions in determining the dynamic behavior of confined liquid crystals even for configurations that share similar equilibrium bulk structures. Full article
(This article belongs to the Special Issue Pattern Formation and Defects in Liquid Crystals)
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Open AccessArticle
Regium Bonds between Silver(I) Pyrazolates Dinuclear Complexes and Lewis Bases (N2, OH2, NCH, SH2, NH3, PH3, CO and CNH)
Crystals 2020, 10(2), 137; https://doi.org/10.3390/cryst10020137 - 24 Feb 2020
Viewed by 149
Abstract
A theoretical study and Cambridge Structural Database (CSD) search of dinuclear Ag(I) pyrazolates interactions with Lewis bases were carried out and the effect of the substituents and ligands on the structure and on the aromaticity were analyzed. A relationship between the intramolecular Ag–Ag [...] Read more.
A theoretical study and Cambridge Structural Database (CSD) search of dinuclear Ag(I) pyrazolates interactions with Lewis bases were carried out and the effect of the substituents and ligands on the structure and on the aromaticity were analyzed. A relationship between the intramolecular Ag–Ag distance and stability was found in the unsubstituted system, which indicates a destabilization at longer distances compensated by ligands upon complexation. It was also observed that the asymmetrical interaction with phosphines as ligands increases the Ag–Ag distance. This increase is dramatically higher when two simultaneous PH3 ligands are taken into account. The calculated 109Ag chemical shielding shows variation up to 1200 ppm due to the complexation. Calculations showed that six-membered rings possessed non-aromatic character while pyrazole rings do not change their aromatic character significantly upon complexation. Full article
(This article belongs to the Special Issue σ- and π-Hole Interactions)
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Open AccessArticle
Comparison of Hafnium Dioxide and Zirconium Dioxide Grown by Plasma-Enhanced Atomic Layer Deposition for the Application of Electronic Materials
Crystals 2020, 10(2), 136; https://doi.org/10.3390/cryst10020136 - 23 Feb 2020
Viewed by 188
Abstract
We report the growth of nanoscale hafnium dioxide (HfO2) and zirconium dioxide (ZrO2) thin films using remote plasma-enhanced atomic layer deposition (PE-ALD), and the fabrication of complementary metal-oxide semiconductor (CMOS) integrated circuits using the HfO2 and ZrO2 [...] Read more.
We report the growth of nanoscale hafnium dioxide (HfO2) and zirconium dioxide (ZrO2) thin films using remote plasma-enhanced atomic layer deposition (PE-ALD), and the fabrication of complementary metal-oxide semiconductor (CMOS) integrated circuits using the HfO2 and ZrO2 thin films as the gate oxide. Tetrakis (dimethylamino) hafnium (Hf[N(CH3)2]4) and tetrakis (dimethylamino) zirconium (IV) (Zr[N(CH3)2]4) were used as the precursors, while O2 gas was used as the reactive gas. The PE-ALD-grown HfO2 and ZrO2 thin films were analyzed using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). The XPS measurements show that the ZrO2 film has the atomic concentrations of 34% Zr, 2% C, and 64% O while the HfO2 film has the atomic concentrations of 29% Hf, 11% C, and 60% O. The HRTEM and XRD measurements show both HfO2 and ZrO2 films have polycrystalline structures. n-channel and p-channel metal-oxide semiconductor field-effect transistors (nFETs and pFETs), CMOS inverters, and CMOS ring oscillators were fabricated to test the quality of the HfO2 and ZrO2 thin films as the gate oxide. Current-voltage (IV) curves, transfer characteristics, and oscillation waveforms were measured from the fabricated transistors, inverters, and oscillators, respectively. The experimental results measured from the HfO2 and ZrO2 thin films were compared. Full article
(This article belongs to the Section Crystalline Materials)
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Open AccessReview
Serial Crystallography: Preface
Crystals 2020, 10(2), 135; https://doi.org/10.3390/cryst10020135 - 23 Feb 2020
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Abstract
The history of serial crystallography (SC) has its origins in the earliest attempts to merge data from several crystals. This preface provides an overview of some recent work, with a survey of the rapid advances made over the past decade in both sample [...] Read more.
The history of serial crystallography (SC) has its origins in the earliest attempts to merge data from several crystals. This preface provides an overview of some recent work, with a survey of the rapid advances made over the past decade in both sample delivery and data analysis. Full article
(This article belongs to the Special Issue Macromolecular Serial Crystallography)
Open AccessArticle
How Nanoscale Dislocation Reactions Govern Low- Temperature and High-Stress Creep of Ni-Base Single Crystal Superalloys
Crystals 2020, 10(2), 134; https://doi.org/10.3390/cryst10020134 - 22 Feb 2020
Viewed by 138
Abstract
The present work investigates γ-channel dislocation reactions, which govern low-temperature (T = 750 °C) and high-stress (resolved shear stress: 300 MPa) creep of Ni-base single crystal superalloys (SX). It is well known that two dislocation families with different b-vectors are required to form [...] Read more.
The present work investigates γ-channel dislocation reactions, which govern low-temperature (T = 750 °C) and high-stress (resolved shear stress: 300 MPa) creep of Ni-base single crystal superalloys (SX). It is well known that two dislocation families with different b-vectors are required to form planar faults, which can shear the ordered γ’-phase. However, so far, no direct mechanical and microstructural evidence has been presented which clearly proves the importance of these reactions. In the mechanical part of the present work, we perform shear creep tests and we compare the deformation behavior of two macroscopic crystallographic shear systems [ 01 1 ¯ ] ( 111 ) and [ 11 2 ¯ ] ( 111 ) . These two shear systems share the same glide plane but differ in loading direction. The [ 11 2 ¯ ] ( 111 ) shear system, where the two dislocation families required to form a planar fault ribbon experience the same resolved shear stresses, deforms significantly faster than the [ 01 1 ¯ ] ( 111 ) shear system, where only one of the two required dislocation families is strongly promoted. Diffraction contrast transmission electron microscopy (TEM) analysis identifies the dislocation reactions, which rationalize this macroscopic behavior. Full article
(This article belongs to the Special Issue Crystal Plasticity)
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Open AccessArticle
Ultrasonic Pulse Velocity—Compressive Strength Relationship for Portland Cement Mortars Cured at Different Conditions
Crystals 2020, 10(2), 133; https://doi.org/10.3390/cryst10020133 - 22 Feb 2020
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Abstract
The purpose of this paper is to establish some correlations between the main technical parameter with regard to the cement-based materials technology, the 28-day compressive strength, and ultrasonic pulse velocity of standard mortar samples cured at three different conditions—(i) under water at 22 [...] Read more.
The purpose of this paper is to establish some correlations between the main technical parameter with regard to the cement-based materials technology, the 28-day compressive strength, and ultrasonic pulse velocity of standard mortar samples cured at three different conditions—(i) under water at 22 °C; (ii) climatic chamber at 95% RH and 22 °C; (iii) lab ambient, 50% RH, and 22 °C—and after five curing periods of 1, 2, 7, 14, and 28 days. Good correlations for each curing conditions were obtained. All the positive linear relationships showed better R2 than exponential ones. These findings may promote the use of ultrasonic pulse velocity for the estimation of the 28-day compressive strength of standard Portland cement samples within the factory internal quality control. Full article
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Open AccessArticle
Structural and Optical Properties of ZnO Thin Films Prepared by Molecular Precursor and Sol–Gel Methods
Crystals 2020, 10(2), 132; https://doi.org/10.3390/cryst10020132 - 22 Feb 2020
Viewed by 214
Abstract
Zinc oxide (ZnO) is a versatile and inexpensive semiconductor with a wide direct band gap that has applicability in several scientific and technological fields. In this work, we report the synthesis of ZnO thin films via two simple and low-cost synthesis routes, i.e., [...] Read more.
Zinc oxide (ZnO) is a versatile and inexpensive semiconductor with a wide direct band gap that has applicability in several scientific and technological fields. In this work, we report the synthesis of ZnO thin films via two simple and low-cost synthesis routes, i.e., the molecular precursor method (MPM) and the sol–gel method, which were deposited successfully on microscope glass substrates. The films were characterized for their structural and optical properties. X-ray diffraction (XRD) characterization showed that the ZnO films were highly c-axis (0 0 2) oriented, which is of interest for piezoelectric applications. The surface roughness derived from atomic force microscopy (AFM) analysis indicates that films prepared via MPM were relatively rough with an average roughness (Ra) of 2.73 nm compared to those prepared via the sol–gel method (Ra = 1.55 nm). Thin films prepared via MPM were more transparent than those prepared via the sol–gel method. The optical band gap of ZnO thin films obtained via the sol–gel method was 3.25 eV, which falls within the range found by other authors. However, there was a broadening of the optical band gap (3.75 eV) in thin films derived from MPM. Full article
(This article belongs to the Special Issue Functional Oxide Based Thin-Film Materials)
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Open AccessArticle
Depth Profiling of Ion-Implanted 4H–SiC Using Confocal Raman Spectroscopy
Crystals 2020, 10(2), 131; https://doi.org/10.3390/cryst10020131 - 21 Feb 2020
Viewed by 160
Abstract
For silicon carbide (SiC) processed by ion-implantation, dedicated test structure fabrication or destructive sample processing on test wafers are usually required to obtain depth profiles of electrical characteristics such as carrier concentration. In this study, a rapid and non-destructive approach for depth profiling [...] Read more.
For silicon carbide (SiC) processed by ion-implantation, dedicated test structure fabrication or destructive sample processing on test wafers are usually required to obtain depth profiles of electrical characteristics such as carrier concentration. In this study, a rapid and non-destructive approach for depth profiling is presented that uses confocal Raman microscopy. As an example, a 4H–SiC substrate with an epitaxial layer of several micrometers thick and top layer in nanoscale that was modified by ion-implantation was characterized. From the Raman depth profiling, longitudinal optical (LO) mode from the epitaxial layer and longitudinal optical phonon-plasmon coupled (LOPC) mode from the substrate layer can be sensitively distinguished at the interface. The position profile of the LOPC peak intensity in the depth direction was found to be effective in estimating the thickness of the epitaxial layer. For three kinds of epitaxial layer with thicknesses of 5.3 μm, 6 μm, and 7.5 μm, the average deviations of the Raman depth analysis were −1.7 μm, −1.2 μm, and −1.4 μm, respectively. Moreover, when moving the focal plane from the heavily doped sample (~1018 cm−3) to the epitaxial layer (~1016 cm−3), the LOPC peak showed a blue shift. The twice travel of the photon (excitation and collection) through the ion-implanted layer with doping concentrations higher than 1 × 1018 cm−3 led to a difference in the LOPC peak position for samples with the same epitaxial layer and substrate layer. Furthermore, the influences of the setup in terms of pinhole size and numerical aperture of objective lens on the depth profiling results were studied. Different from other research on Raman depth profiling, the 50× long working distance objective lens (50L× lens) was found more suitable than the 100× lens for the depth analysis 4H–SiC with a multi-layer structure. Full article
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Open AccessEditorial
Interactions Steering Arrangement of Molecules in Crystals
Crystals 2020, 10(2), 130; https://doi.org/10.3390/cryst10020130 - 20 Feb 2020
Viewed by 195
Abstract
It is well known that the hydrogen bond is an important interaction that influences an arrangement of molecules in numerous organic and metal-organic crystals [1,2]. There are various motifs containing hydrogen bond links in crystal structures; the method based on the graph theory [...] Read more.
It is well known that the hydrogen bond is an important interaction that influences an arrangement of molecules in numerous organic and metal-organic crystals [1,2]. There are various motifs containing hydrogen bond links in crystal structures; the method based on the graph theory was proposed to categorize them [3,4]. For example, the  motif, according to the graph-set assignments, is among the most often occurring ones, it is typical for crystals of carboxylic acids, amides, and numerous other compounds [5]. Figure 1 presents the fragment of the crystal structure - the dimer of the 2-hydroxythiobenzoic acid [6] (RONVAR refcode in the Cambridge Structural Database, CSD [7,8]). One can see here the above-mentioned  motif that corresponds to the eight-member ring formed by two thiocarboxylic groups linked by symmetrically equivalent S-H…O hydrogen bonds. There are also intramolecular O-H…O hydrogen bonds in this structure that close the six-member rings designated as S(6) in the graph set method [3,4]. The latter motifs occur very often in crystal structures and it was stated early on by Etter that the formation of such a type of intramolecular hydrogen bond is preferred in crystal structures to the formation of other interactions [5].[...] Full article
(This article belongs to the Section Crystal Engineering)
Open AccessArticle
MOVPE-Grown Quantum Cascade Laser Structures Studied by Kelvin Probe Force Microscopy
Crystals 2020, 10(2), 129; https://doi.org/10.3390/cryst10020129 - 20 Feb 2020
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Abstract
A technique for direct study of the distribution of the applied voltage within a quantum cascade laser (QCL) has been developed. The detailed profile of the potential in the laser claddings and laser core region has been obtained by gradient scanning Kelvin probe [...] Read more.
A technique for direct study of the distribution of the applied voltage within a quantum cascade laser (QCL) has been developed. The detailed profile of the potential in the laser claddings and laser core region has been obtained by gradient scanning Kelvin probe force microscopy (KPFM) across the cleaved facets for two mid-infrared quantum cascade laser structures. An InGaAs/InAlAs quantum cascade device with InP claddings demonstrates a linear potential distribution across the laser core region with constant voltage drop across the doped claddings. By contrast, a GaAs/AlGaAs device with AlInP claddings has very uneven potential distribution with more than half of the voltage falling across the claddings and interfaces around the laser core, greatly increasing the overall voltage value necessary to achieve the lasing threshold. Thus, KPFM can be used to highlight design and fabrication flaws of QCLs. Full article
(This article belongs to the Special Issue MOVPE Growth of Crystalline Film)
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Open AccessArticle
Side-Chain Liquid Crystal Co-Polymers for Angular Photochromic Anti-Counterfeiting Powder and Fiber
Crystals 2020, 10(2), 128; https://doi.org/10.3390/cryst10020128 - 20 Feb 2020
Viewed by 165
Abstract
Anti-counterfeiting technologies with the features of easy distinguishability, high cost performance, and good processability are needed to meet the demands of a market during the consumption upgrading moment. A series of side-chain liquid crystal co-polymers (SCLCPs) are designed, synthesized, and blended, and the [...] Read more.
Anti-counterfeiting technologies with the features of easy distinguishability, high cost performance, and good processability are needed to meet the demands of a market during the consumption upgrading moment. A series of side-chain liquid crystal co-polymers (SCLCPs) are designed, synthesized, and blended, and the preparation of a series of angular photochromic materials that have different center reflection wavelengths in the visible and near infra-red region is reported in this article. Differential scanning calorimetry and polarized optical microscopy were utilized to characterize the phase transition behaviors and self-assembling structures of the SCLCPs. The selective reflection properties were characterized with a UV/VIS/IR spectrum study and further verified by scanning electron microscopy. The results showed that the SCLCPs had the desired reflection wavelengths and thermal stability. The SCLCPs could easily form a planar texture of cholesteric liquid crystal and, depending on the good processability, anti-counterfeiting powders and fibers with angular photochromic features were prepared and characterized to prove the potential applications of the SCLCPs in anti-counterfeiting labels. Full article
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Open AccessArticle
Spectroscopic and Microscopic Correlation of SRO-HFCVD Films on Quartz and Silicon
Crystals 2020, 10(2), 127; https://doi.org/10.3390/cryst10020127 - 20 Feb 2020
Viewed by 254
Abstract
This work is focused on making a correlation between results obtained by using spectroscopy and microscopy techniques from single and twofold-layer Silicon-Rich Oxide (SRO) films. SRO films single-layer and twofold-layer characterizations were compared considering the conditions as-grown and with thermal treatment at 1100 [...] Read more.
This work is focused on making a correlation between results obtained by using spectroscopy and microscopy techniques from single and twofold-layer Silicon-Rich Oxide (SRO) films. SRO films single-layer and twofold-layer characterizations were compared considering the conditions as-grown and with thermal treatment at 1100 °C for 60 min in a nitrogen atmosphere. The thickness of the single-layer film is 324.7 nm while for the twofold-layer film it is 613.2 nm; after heat-treated, both thicknesses decreased until 28.8 nm. X-ray Photoelectron Spectroscopy shows changes in the excess-silicon in single-layer SRO films, with 10% in as-grown films and decreases to 5% for the heat-treated films. Fourier Transform Infrared Spectroscopy (FTIR) exhibits three characteristic vibrational modes of SiO2, as well as, the vibrating modes associated with the Si-H bonds, which disappear after the heat treatment. With UV–Vis spectroscopy results we obtained the absorbance and the absorption coefficient for the SRO films in order to calculate the optical bandgap energy (Egopt), which increased with heat-treatment. The energy peaks of the photoluminescence spectra were used to calculate the silicon nanocrystal size, obtaining thus an average size of 1.89 ± 0.32 nm for the as-grown layer, decreasing the size to 1.64 ± 0.01 nm with the thermal treatment. On the other hand, scanning electron microscopy and high-resolution transmission electron microscopy images confirm the thickness of the twofold-layer SRO films as 628 nm for the as-grown layer and 540 nm for the layer with heat-treatment, and the silicon nanocrystal size of 2.3 ± 0.6 nm for the films with thermal treatment. Full article
(This article belongs to the Special Issue Defects in Semiconductors)
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Open AccessArticle
A Thermal and Nanomechanical Study of Molecular Crystals as Versatile Mocks for Pentaerythritol Tetranitrate
Crystals 2020, 10(2), 126; https://doi.org/10.3390/cryst10020126 - 19 Feb 2020
Viewed by 163
Abstract
Pentaerythritol tetranitrate (PETN) is a commonly used high explosive (HE) in detonators. Often, surrogate or “mock” materials are used in place of HE for mechanical tests, proofing out equipment, or developing new diagnostics. However, there is no commonly accepted mock for PETN. A [...] Read more.
Pentaerythritol tetranitrate (PETN) is a commonly used high explosive (HE) in detonators. Often, surrogate or “mock” materials are used in place of HE for mechanical tests, proofing out equipment, or developing new diagnostics. However, there is no commonly accepted mock for PETN. A good mock should match at least one physical property of the target material, and ideally mimic multiple thermal and mechanical properties. Here, we investigate several molecular crystals to evaluate their efficacy in mocking PETN density, melting point, elastic modulus, hardness, plastic deformation, and fracture behavior. Materials were tested with a combination of calorimetry and nanoindentation. Two materials, 2,4,6-trifluorobenzoic acid (246 TFBA) and mesoerythritol, were downselected for detailed indentation study after the initial round of screening experiments, both were found to mimic PETN mechanical behavior quite well, 246 TFBA closer to PETN in most properties (hardness, modulus, and density) than erythritol, but erythritol having advantages in relative cost and matching the onset of yield. Depending on the desired implementation of the mock, one material may be preferred over the other, but both have potential as generic mocks for PETN. Nanoindentation is demonstrated as a versatile tool to provide rapid screening of these materials’ mechanical properties. Full article
(This article belongs to the Section Crystalline Materials)
Open AccessArticle
Deep Learning for the Inverse Design of Mid-Infrared Graphene Plasmons
Crystals 2020, 10(2), 125; https://doi.org/10.3390/cryst10020125 (registering DOI) - 19 Feb 2020
Viewed by 161
Abstract
We theoretically investigate the plasmonic properties of mid-infrared graphene-based metamaterials and apply deep learning of a neural network for the inverse design. These artificial structures have square periodic arrays of graphene plasmonic resonators deposited on dielectric thin films. Optical spectra vary significantly with [...] Read more.
We theoretically investigate the plasmonic properties of mid-infrared graphene-based metamaterials and apply deep learning of a neural network for the inverse design. These artificial structures have square periodic arrays of graphene plasmonic resonators deposited on dielectric thin films. Optical spectra vary significantly with changes in structural parameters. To validate our theoretical approach, we carry out finite difference time domain simulations and compare computational results with theoretical calculations. Quantitatively good agreements among theoretical predictions, simulations, and previous experiments allow us to employ this proposed theoretical model to generate reliable data for training and testing deep neural networks. By merging the pre-trained neural network with the inverse network, we implement calculations for inverse design of the graphene-based metameterials. We also discuss the limitation of the data-driven approach. Full article
(This article belongs to the Special Issue Plasmonic Nanostructures)
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Open AccessArticle
Symmetry-Adapted Finite Strain Landau Theory Applied to KMnF3
Crystals 2020, 10(2), 124; https://doi.org/10.3390/cryst10020124 - 17 Feb 2020
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Abstract
In recent years, finite strain Landau theory has been gradually developed as both a conceptual as well as a quantitative framework to study high pressure phase transitions of the group-subgroup type. In the current paper, we introduce a new version of this approach [...] Read more.
In recent years, finite strain Landau theory has been gradually developed as both a conceptual as well as a quantitative framework to study high pressure phase transitions of the group-subgroup type. In the current paper, we introduce a new version of this approach which is based on symmetry-adapted finite strains. This results in a substantial simplification of the original formulation. Moreover, it allows for replacing the clumsy use of truncated Taylor expansions by a convenient functional parametrization. Both the weaknesses of the traditional Landau approach based on infinitesimal strains as well as the major improvements made possible by our new parametrization are illustrated in great detail in an application to the ambient temperature high pressure transition of the perovskite KMnF 3 . Full article
(This article belongs to the Special Issue Pressure-Induced Phase Transformations)
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Open AccessArticle
Dielectric and Electro-Optic Effects in a Nematic Liquid Crystal Doped with h-BN Flakes
Crystals 2020, 10(2), 123; https://doi.org/10.3390/cryst10020123 - 16 Feb 2020
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Abstract
A small quantity of hexagonal boron nitride (h-BN) flakes is doped into a nematic liquid crystal (LC). The epitaxial interaction between the LC molecules and the h-BN flakes rising from the ππ electron stacking between the LC’s benzene [...] Read more.
A small quantity of hexagonal boron nitride (h-BN) flakes is doped into a nematic liquid crystal (LC). The epitaxial interaction between the LC molecules and the h-BN flakes rising from the ππ electron stacking between the LC’s benzene rings and the h-BN’s honeycomb structure stabilizes pseudo-nematic domains surrounding the h-BN flakes. Electric field-dependent dielectric studies reveal that the LC-jacketed h-BN flakes follow the nematic director reorientation upon increasing the applied electric field. These anisotropic pseudo-nematic domains exist in the isotropic phase of the LC+h-BN system as well, and interact with the external electric field, giving rise to a nonzero dielectric anisotropy in the isotropic phase. Further investigations reveal that the presence of the h-BN flakes at a low concentration in the nematic LC enhances the elastic constants, reduces the rotation viscosity, and lowers the pre-tilt angle of the LC. However, the Fréedericksz threshold voltage stays mostly unaffected in the presence of the h-BN flakes. Additional studies show that the presence of the h-BN flakes enhances the effective polar anchoring strength in the cell. The enhanced polar anchoring strength and the reduced rotational viscosity result in faster electro-optic switching in the h-BN-doped LC cell. Full article
(This article belongs to the Special Issue Early Career Stars of the Decade)
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Open AccessArticle
Flexible and Structural Coloured Composite Films from Cellulose Nanocrystals/Hydroxypropyl Cellulose Lyotropic Suspensions
Crystals 2020, 10(2), 122; https://doi.org/10.3390/cryst10020122 - 16 Feb 2020
Viewed by 250
Abstract
Lyotropic colloidal aqueous suspensions of cellulose nanocrystals (CNCs) can, after solvent evaporation, retain their chiral nematic arrangement. As water is removed the pitch value of the suspension decreases and structural colour-generating films, which are mechanically brittle in nature, can be obtained. Increasing their [...] Read more.
Lyotropic colloidal aqueous suspensions of cellulose nanocrystals (CNCs) can, after solvent evaporation, retain their chiral nematic arrangement. As water is removed the pitch value of the suspension decreases and structural colour-generating films, which are mechanically brittle in nature, can be obtained. Increasing their flexibility while keeping the chiral nematic structure and biocompatible nature is a challenging task. However, if achievable, this will promote their use in new and interesting applications. In this study, we report on the addition of different amounts of hydroxypropyl cellulose (HPC) to CNCs suspension within the coexistence of the isotropic-anisotropic phases and infer the influence of this cellulosic derivative on the properties of the obtained solid films. It was possible to add 50 wt.% of HPC to a CNCs aqueous suspension (to obtain a 50/50 solids ratio) without disrupting the LC phase of CNCs and maintaining a left-handed helical structure in the obtained films. When 30 wt.% of HPC was added to the suspension of CNCs, a strong colouration in the film was still observed. This colour shifts to the near-infrared region as the HPC content in the colloidal suspension increases to 40 wt.% or 50 wt.% The all-cellulosic composite films present an increase in the maximum strain as the concentration of HPC increases, as shown by the bending experiments and an improvement in their thermal properties. Full article
(This article belongs to the Special Issue New Trends in Lyotropic Liquid Crystals)
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Open AccessArticle
3D Numerical Analysis of the Asymmetric Three-Phase Line of Floating Zone for Silicon Crystal Growth
Crystals 2020, 10(2), 121; https://doi.org/10.3390/cryst10020121 - 15 Feb 2020
Viewed by 191
Abstract
A numerical simulation has been carried out to study the asymmetric heat transfer, fluid flow, and three-phase line to explain the phenomenon of the spillage of the melt in floating zone (FZ) silicon growth. A three-dimensional high-frequency electromagnetic (EM) field is coupled with [...] Read more.
A numerical simulation has been carried out to study the asymmetric heat transfer, fluid flow, and three-phase line to explain the phenomenon of the spillage of the melt in floating zone (FZ) silicon growth. A three-dimensional high-frequency electromagnetic (EM) field is coupled with the heat transfer in the melt and crystal calculation domains. The current density along the three-phase line is investigated to demonstrate the inhomogeneous heating along the three-phase line. The asymmetric heating is found to affect the flow pattern and temperature distribution of the melt. The three-dimensional solid–liquid interface results show that, below the current supplies, the interface is deflected due to strong heating below the current supplies. The calculated asymmetric three-phase line shows a similar trend as the experimentally observed results. The results indicate that the re-melting and spillage phenomenon could occur below the current supplies. Full article
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Open AccessArticle
Synthesis, X-ray Single Crystal, Conformational Analysis and Cholinesterase Inhibitory Activity of a New Spiropyrrolidine Scaffold Tethered Benzo[b]Thiophene Analogue
Crystals 2020, 10(2), 120; https://doi.org/10.3390/cryst10020120 - 15 Feb 2020
Viewed by 189
Abstract
Described herein is a one-pot protocol for the synthesis of a substituted spiropyrrolidine scaffold tethered benzo[b]thiophene analogue from (E)-3-(benzo[b]thiophen-2-yl)-1-(4-fluoro- phenyl)-prop-2-en-1-one. The described protocol has the advantage of the high purity of the cyclized adduct and high chemical [...] Read more.
Described herein is a one-pot protocol for the synthesis of a substituted spiropyrrolidine scaffold tethered benzo[b]thiophene analogue from (E)-3-(benzo[b]thiophen-2-yl)-1-(4-fluoro- phenyl)-prop-2-en-1-one. The described protocol has the advantage of the high purity of the cyclized adduct and high chemical yield. To assign the chemical structure, different spectrophotometric tools have been applied, including 1H-NMR, 13C-NMR, FTIR, and the X-ray single crystal technique. The X-ray structure showed that the studied compound exist in two disordered parts with equal partial occupancies. The energies of the two conformers were found to be very similar and not exceed 1 kcal/mol, which justifies their coexistence in the crystal with equal percentage. The molecular packing in the crystal was analyzed using Hirshfeld topology analysis. The packing described as two dimensional hydrogen bond network extended along the ac-plane in both conformers but the intermolecular interactions included in each conformer are not similar. The synthesized spiropyrrolidine scaffold tethered benzo[b]thiophene analogue was examined against cholinesterase inhibitory activity and show moderate activity compared to standard drug galantamine. Full article
(This article belongs to the Special Issue Pharmaceutical Crystals and Its Application)
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Open AccessArticle
Uniform Spheres of α-NaYF4:RE3+ (RE=Eu, Tb, Ce, Er, and Tm): Template-Free Synthesis, Multi-Color Photoluminescence, and Their Application in Cellular Imaging
Crystals 2020, 10(2), 119; https://doi.org/10.3390/cryst10020119 - 14 Feb 2020
Viewed by 190
Abstract
Uniformly dispersed luminescent probes with a high brightness and high resolution are desired in bio imaging fields. Here, ~100 nm sized and well-dispersed spheres of RE3+ doped α-NaYF4 (rare earth (RE) = Eu, Tb, Ce, Er, and Tm) have been facile [...] Read more.
Uniformly dispersed luminescent probes with a high brightness and high resolution are desired in bio imaging fields. Here, ~100 nm sized and well-dispersed spheres of RE3+ doped α-NaYF4 (rare earth (RE) = Eu, Tb, Ce, Er, and Tm) have been facile synthesized through hydrothermal processing in the absence of a template, followed by a proper annealing. The processing window of the cubic structured spheres is wide, because the hydrothermal products are independent of the processing conditions, including reaction time and temperature. The original morphology and crystal structure can be well retained with a calcination temperature up to 600 °C. However, calcination gives rise to a reduction of particle sizes, as a result of the crystallite growth and densification. Under ultraviolet radiation, α-NaYF4:RE3+ spheres show characteristic f-f emissions of RE3+ (RE = Eu, Tb, Ce, Er, and Tm), and exhibit orange red, green, ultraviolet (UV), blue green, and blue emissions, respectively. Mainly because of the near-infrared emission at ~697 nm (5D07F4 transitions of Eu3+), the successful imaging of macrophages was achieved by NH2-NaYF4:Eu3+ probes, indicating their excellent imaging capacity for cells in vitro. Full article
Open AccessArticle
Nanosphere Lithography for Structuring Polycrystalline Diamond Films
Crystals 2020, 10(2), 118; https://doi.org/10.3390/cryst10020118 - 14 Feb 2020
Viewed by 190
Abstract
This paper deals with the structuring of polycrystalline diamond thin films using the technique of nanosphere lithography. The presented multistep approaches relied on a spin-coated self-assembled monolayer of polystyrene spheres, which served as a lithographic mask for the further custom nanofabrication steps. Various [...] Read more.
This paper deals with the structuring of polycrystalline diamond thin films using the technique of nanosphere lithography. The presented multistep approaches relied on a spin-coated self-assembled monolayer of polystyrene spheres, which served as a lithographic mask for the further custom nanofabrication steps. Various arrays of diamond nanostructures—close-packed and non-close-packed monolayers over substrates with various levels of surface roughness, noble metal films over nanosphere arrays, ordered arrays of holes, and unordered pores—were created using reactive ion etching, chemical vapour deposition, metallization, and/or lift-off processes. The size and shape of the lithographic mask was altered using oxygen plasma etching. The periodicity of the final structure was defined by the initial diameter of the spheres. The surface morphology of the samples was characterized using scanning electron microscopy. The advantages and limitations of the fabrication technique are discussed. Finally, the potential applications (e.g., photonics, plasmonics) of the obtained nanostructures are reviewed. Full article
(This article belongs to the Special Issue Photonic Crystals)
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Open AccessArticle
Oxidation of Thin Titanium Films: Determination of the Chemical Composition of the Oxide and the Oxygen Diffusion Factor
Crystals 2020, 10(2), 117; https://doi.org/10.3390/cryst10020117 - 14 Feb 2020
Viewed by 209
Abstract
The morphologies and local electronic structures of titanium coatings deposited on the surfaces of highly oriented pyrolytic graphite were determined. Chemical compositions of the oxides formed on the coating surfaces were established. A theoretical model was developed describing the changes in the TiO [...] Read more.
The morphologies and local electronic structures of titanium coatings deposited on the surfaces of highly oriented pyrolytic graphite were determined. Chemical compositions of the oxides formed on the coating surfaces were established. A theoretical model was developed describing the changes in the TiOx oxides (1.75 < x < 2) band gap depending on the duration and temperature of the titanium film annealing procedure in oxygen. The effective activation energy of oxygen diffusion in TiOx (1.75 < x < 2) was determined, and the pre-exponential factor of the diffusion coefficient was estimated. Full article
(This article belongs to the Special Issue Functional Oxide Based Thin-Film Materials (Volume II))
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Open AccessReview
Reaction Initiation in Enzyme Crystals by Diffusion of Substrate
Crystals 2020, 10(2), 116; https://doi.org/10.3390/cryst10020116 - 13 Feb 2020
Viewed by 200
Abstract
Ever since the first structure of an enzyme, lysozyme, was solved, scientists have been eager to explore how these molecules perform their catalytic function. There has been an overwhelmingly large body of publications that report the X-ray structures of enzymes determined after substrate [...] Read more.
Ever since the first structure of an enzyme, lysozyme, was solved, scientists have been eager to explore how these molecules perform their catalytic function. There has been an overwhelmingly large body of publications that report the X-ray structures of enzymes determined after substrate and ligand binding. None of them truly show the structures of an enzyme working freely through a sequence of events that range from the formation of the enzyme–substrate complex to the dissociation of the product. The technical difficulties were too severe. By 1969, Sluyterman and de Graaf had pointed out that there might be a way to start a reaction in an enzyme crystal by diffusion and following its catalytic cycle in its entirety with crystallographic methods. The crystal only has to be thin enough so that the diffusion is not rate limiting. Of course, the key questions are as follows: How thin should the crystal be? Will the existing X-ray sources be able to collect data from a thin enough crystal fast enough? This review shines light on these questions. Full article
(This article belongs to the Special Issue Macromolecular Serial Crystallography)
Open AccessArticle
Effects of Pulsed Magnetic Fields of Different Intensities on Dislocation Density, Residual Stress, and Hardness of Cr4Mo4V Steel
Crystals 2020, 10(2), 115; https://doi.org/10.3390/cryst10020115 - 13 Feb 2020
Viewed by 190
Abstract
To study the effects of pulsed magnetic fields of different intensities on the dislocation density, residual stress, and hardness of Cr4Mo4V steel, magnetic treatment is conducted at 0, 1.0, 1.3, 1.5, 2.0, and 2.5 T. The dislocation density and residual stress are measured [...] Read more.
To study the effects of pulsed magnetic fields of different intensities on the dislocation density, residual stress, and hardness of Cr4Mo4V steel, magnetic treatment is conducted at 0, 1.0, 1.3, 1.5, 2.0, and 2.5 T. The dislocation density and residual stress are measured using Electron Backscatter Diffraction (EBSD) and X-ray technique, respectively. The results reveal the dislocation density and compressive residual stress decrease at lower magnetic fields such as 1.0 T and 1.3 T, while they increase at higher magnetic fields such as 2.0 T and 2.5 T. The average value of kernel averaged misorientation (KAM) and compressive residual stress decrease about 10.4% and 15.8%, respectively, at 1.0 T, while they increase about 5.88% and 18.2%, respectively, at 2.5 T. The average value of hardness decreases about 3.5% at 1.0 T, from 817 HV to 787 HV. With the increments of intensities, the hardness of the treated samples increases. The hardness essentially remains unchanged at 2.0 T and 2.5 T. The reason for the dislocation motion under the action of pulsed magnetic fields is discussed. Full article
(This article belongs to the Special Issue Defects in Crystals)
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Open AccessReview
Biological vs. Crystallographic Protein Interfaces: An Overview of Computational Approaches for Their Classification
Crystals 2020, 10(2), 114; https://doi.org/10.3390/cryst10020114 - 13 Feb 2020
Viewed by 219
Abstract
Complexes between proteins are at the basis of almost every process in cells. Their study, from a structural perspective, has a pivotal role in understanding biological functions and, importantly, in drug development. X-ray crystallography represents the broadest source for the experimental structural characterization [...] Read more.
Complexes between proteins are at the basis of almost every process in cells. Their study, from a structural perspective, has a pivotal role in understanding biological functions and, importantly, in drug development. X-ray crystallography represents the broadest source for the experimental structural characterization of protein-protein complexes. Correctly identifying the biologically relevant interface from the crystallographic ones is, however, not trivial and can be prone to errors. Over the past two decades, computational methodologies have been developed to study the differences of those interfaces and automatically classify them as biological or crystallographic. Overall, protein-protein interfaces show differences in terms of composition, energetics and evolutionary conservation between biological and crystallographic ones. Based on those observations, a number of computational methods have been developed for this classification problem, which can be grouped into three main categories: Energy-, empirical knowledge- and machine learning-based approaches. In this review, we give a comprehensive overview of the training datasets and methods so far implemented, providing useful links and a brief description of each method. Full article
(This article belongs to the Special Issue Protein Crystallography)
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Open AccessArticle
Numerical Analysis of Optical Absorption Effect in Nonhalogen Solution-Processed, Inverted Small Molecule Solar Cell
Crystals 2020, 10(2), 113; https://doi.org/10.3390/cryst10020113 (registering DOI) - 13 Feb 2020
Viewed by 202
Abstract
Inverted solution-processed SMPV1:PC71BM small molecule organic photovoltaic solar cells (SM-OPVs) were fabricated. The power conversion efficiency (PCE) of halogen-free SM-OPVs reached around 5.07%. The absorption spectra at different device thicknesses were simulated by software Fluxim SETFOS 5.0, and compared with the [...] Read more.
Inverted solution-processed SMPV1:PC71BM small molecule organic photovoltaic solar cells (SM-OPVs) were fabricated. The power conversion efficiency (PCE) of halogen-free SM-OPVs reached around 5.07%. The absorption spectra at different device thicknesses were simulated by software Fluxim SETFOS 5.0, and compared with the experimental results. To further enhance the performance of halogen-free SM-OPVs, the interface between the active layer and the electrode of the optimized device was treated with the solvent vapor annealing (SVA) process, improving the PCE of the inverted halogen-free SM-OPV to 7.21%. Full article
(This article belongs to the Special Issue Advances in Thin Film Materials and Devices)
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