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Crystals, Volume 7, Issue 7 (July 2017) – 46 articles

Cover Story (view full-size image): In this review, we briefly summarize the history of mesocrystal research. We introduce the current structural definition of mesocrystals and discuss the appropriate base for the classification of mesocrystals and their relations with other classes of solid state materials in terms of their structure. Building up on this, we comment on the problems in mesocrystal research both fundamental and methodological. Additionally, we make the short overview of the mesocrystal formation principles and synthetic routes used for their fabrications. As an outlook into the future, we highlight the most notable trends in mesocrystal research and developments. View this paper
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5285 KiB  
Article
Synthesis, Crystal Structures, and Photoluminescent Properties of Two Supramolecular Architectures Based on Difunctional Ligands Containing Imidazolyl and Carboxyl Groups
by Mei-An Zhu, Xing-Zhe Guo, Shan-Shan Shi and Shui-Sheng Chen
Crystals 2017, 7(7), 228; https://doi.org/10.3390/cryst7070228 - 23 Jul 2017
Cited by 1 | Viewed by 3342
Abstract
Two new supramolecular architectures, namely, [Cd(L1)2(H2O)]n (1) and [Ni(L2)2(H2O)]n (2), were synthesized by the reaction of corresponding metal salts of CdCl2·2.5H2O [...] Read more.
Two new supramolecular architectures, namely, [Cd(L1)2(H2O)]n (1) and [Ni(L2)2(H2O)]n (2), were synthesized by the reaction of corresponding metal salts of CdCl2·2.5H2O and NiCl2·6H2O with 2-(1H-imidazol-4-yl)benzoic acid (HL1) and 3-(1H-imidazol-4-yl)benzoic acid (HL2) respectively, and characterized by single-crystal X-ray diffraction, IR spectroscopy, elemental analysis and powder X-ray diffraction (PXRD). Both HL1 and HL2 ligands are deprotonated to be L1- and L2- anions that coordinate with Cd(II) and Ni(II) atoms to form two-dimensional (2D) layer structure. Topologically, complex 1 is a 2D network with (4, 4) sql topology, while 2 is a typical 63-hcb topology net. Complex 1 exhibits intense light blue emission in the solid state at room temperature. Full article
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12404 KiB  
Communication
Electrohydrodynamics-Induced Abnormal Electro-Optic Characteristics in a Polymer-Dispersed Liquid Crystal Film
by Sheng-Kuang Wu, Ting-Shan Mo, Jia-De Lin, Shuan-Yu Huang, Chia-Yi Huang, Hui-Chen Yeh, Lin-Jer Chen and Chia-Rong Lee
Crystals 2017, 7(7), 227; https://doi.org/10.3390/cryst7070227 - 21 Jul 2017
Cited by 28 | Viewed by 4493
Abstract
This study demonstrates for the first time abnormal electro-optic (EO) characteristics induced by electrohydrodynamics (EHD) in a polymer-dispersed liquid crystal (PDLC) film in the presence of a low-frequency (1 kHz) AC voltage. Large LC droplets (20−40 µm) buried in the film can be [...] Read more.
This study demonstrates for the first time abnormal electro-optic (EO) characteristics induced by electrohydrodynamics (EHD) in a polymer-dispersed liquid crystal (PDLC) film in the presence of a low-frequency (1 kHz) AC voltage. Large LC droplets (20−40 µm) buried in the film can be obtained after the illumination of one UV light with a weak intensity (~0.96 mW/cm2) for 12 h. This film exhibits abnormal EO features, including the transmittance’s decay at a high voltage regime at normal incidence and the conversion between polarization independence and polarization dependence for the transmittance-voltage curve at normal and oblique incidences, respectively, of which properties are different from those shown in traditional PDLC films with small droplets. The abnormal EO characteristics of the large-droplet PDLC at the high voltage regime are attributed to a strong scattering effect associated with the formation of the foggy LC droplets in the cell. This effect is induced by a vortex-like LC director field with a rotational axis normal to the cell substrates in each dome-like droplet of the cell at the high voltage regime. The vortex-like director field is induced by a vortex-like turbulence of charged impurity generated by the EHD effect under the action of the AC electric field along the cell normal and the confinement of the dome-like boundary of the droplet on the charged impurities in each droplet. The scattering is decided by the degrees of mismatch between the refractive indices of the LC droplet and polymer, and the local fluctuation of the vortex-like director field in the droplet, resulting in the abnormal EO behaviors of the large-droplet PDLC. This investigation provides novel insight into the EHD effect in three dimensional (3D) microdroplets with anisotropic fluid. Such a large-droplet PDLC has potential in photonic applications, such as electrically controlled polarization-based optical components or optical converters between polarization independence and polarization dependence. Full article
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11531 KiB  
Article
The Role of Halogen Bonding in Controlling Assembly and Organization of Cu(II)-Acac Based Coordination Complexes
by Janaka C. Gamekkanda, Abhijeet S. Sinha, John Desper, Marijana Ðaković and Christer B. Aakeröy
Crystals 2017, 7(7), 226; https://doi.org/10.3390/cryst7070226 - 20 Jul 2017
Cited by 24 | Viewed by 6552
Abstract
In order to explore the use of non-covalent interactions in the deliberate assembly of metal-supramolecular architectures, a series of β-diketone based ligands capable of simultaneously acting as halogen-bond donors and chelating ligands were synthesized. The three ligands, L1, L2, and L3 [...] Read more.
In order to explore the use of non-covalent interactions in the deliberate assembly of metal-supramolecular architectures, a series of β-diketone based ligands capable of simultaneously acting as halogen-bond donors and chelating ligands were synthesized. The three ligands, L1, L2, and L3, carry ethynyl-activated chlorine, bromine, and iodine atoms, respectively and copper(II) complexes of all three ligands were crystallized from different solvents, acetonitrile, ethyl acetate, and nitromethane in order to study specific ligand-solvent interaction. The free ligands L2 and L3, with more polarizable halogen atoms, display C-X⋯O halogen bonds in the solid state, whereas the chloro-analogue (L1) does not engage in halogen bonding. Both acetonitrile and ethyl acetate act as halogen-bond acceptors in Cu(II)-complexes of L2 and L3 whereas nitromethane is present as a ‘space-filling’ guest without participating in any significant intermolecular interactions in Cu(II)-complexes of L2. L3, which is decorated with an iodoethynyl moiety and consistently engages in halogen-bonds with suitable acceptors. This systematic structural analysis allows us to rank the relative importance of a variety of electron-pair donors in these metal complexes. Full article
(This article belongs to the Special Issue Analysis of Halogen and Other σ-Hole Bonds in Crystals)
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2627 KiB  
Article
The Interplay between Various σ- and π-Hole Interactions of Trigonal Boron and Trigonal Pyramidal Arsenic Triiodides
by Jindřich Fanfrlík, Petr Švec, Zdeňka Růžičková, Drahomír Hnyk, Aleš Růžička and Pavel Hobza
Crystals 2017, 7(7), 225; https://doi.org/10.3390/cryst7070225 - 19 Jul 2017
Cited by 6 | Viewed by 4421
Abstract
Boron and arsenic triiodides (BI3 and AsI3, respectively) are similar molecules that differ mainly in their geometries. BI3 is a planar trigonal molecule with D3h symmetry, while AsI3 exhibits a trigonal pyramidal shape with C3v symmetry. [...] Read more.
Boron and arsenic triiodides (BI3 and AsI3, respectively) are similar molecules that differ mainly in their geometries. BI3 is a planar trigonal molecule with D3h symmetry, while AsI3 exhibits a trigonal pyramidal shape with C3v symmetry. Consequently, the As atom of the AsI3 molecule has three σ-holes, whereas the B atom of the BI3 molecule has two symmetrical π-holes. Additionally, there are σ-holes on the iodine atoms in the molecules studied. In the first step, we have studied σ-hole and π-hole interactions in the known monocrystals of BI3 and AsI3. Quantum mechanical calculations have revealed that the crystal packing of BI3 is dominated by π-hole interactions. In the case of AsI3, the overall contribution of dihalogen bonding is comparable to that of pnictogen bonding. Additionally, we have prepared the [Na(THF)6]+[I(AsI3)6](AsI3)2 complex, which can be described as the inverse coordination compound where the iodine anion is the center of the aggregate surrounded by six AsI3 molecules in the close octahedral environment and adjacent two molecules in remote distances. This complex is, besides expected dihalogen and pnictogen bonds, also stabilized by systematically attractive dispersion interactions. Full article
(This article belongs to the Special Issue Analysis of Halogen and Other σ-Hole Bonds in Crystals)
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4609 KiB  
Article
Inorganic Anions Regulate the Phase Transition in Two Organic Cation Salts Containing [(4-Nitroanilinium)(18-crown-6)]+ Supramolecules
by Yuan Chen, Yang Liu, Binzu Gao, Chuli Zhu and Zunqi Liu
Crystals 2017, 7(7), 224; https://doi.org/10.3390/cryst7070224 - 15 Jul 2017
Cited by 13 | Viewed by 3567
Abstract
Inorganic Anions Regulate the Phase Transition in Two Organic Cation Salts Containing [(4-Nitroanilinium)(18-crown-6)]+ Supramolecules Full article
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6145 KiB  
Review
Structural Aspects of Porphyrins for Functional Materials Applications
by Lawrence P. Cook, Greg Brewer and Winnie Wong-Ng
Crystals 2017, 7(7), 223; https://doi.org/10.3390/cryst7070223 - 15 Jul 2017
Cited by 63 | Viewed by 9255
Abstract
Porphyrinic compounds comprise a diverse group of materials which have in common the presence of one or more cyclic tetrapyrroles known as porphyrins in their molecular structures. The resulting aromaticity gives rise to the semiconducting properties that make these compounds of interest for [...] Read more.
Porphyrinic compounds comprise a diverse group of materials which have in common the presence of one or more cyclic tetrapyrroles known as porphyrins in their molecular structures. The resulting aromaticity gives rise to the semiconducting properties that make these compounds of interest for a broad range of applications, including artificial photosynthesis, catalysis, molecular electronics, sensors, non-linear optics, and solar cells. In this brief review, the crystallographic attributes of porphyrins are emphasized. Examples are given showing how the structural orientations of the porphyrin macrocycle, and the inter-porphyrin covalent bonding present in multiporphyrins influence the semiconducting properties. Beginning with porphine, the simplest porphyrin, we discuss how the more complex structures that have been reported are described by adding peripheral substituents and internal metalation to the macrocycles. We illustrate how the conjugation of the π-bonding, and the presence of electron donor/acceptor pairs, which are the basis for the semiconducting properties, are affected by the crystallographic topology. Full article
(This article belongs to the Special Issue Crystallography of Functional Materials)
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6211 KiB  
Article
σ-Holes on Transition Metal Nanoclusters and Their Influence on the Local Lewis Acidity
by Joakim H. Stenlid, Adam Johannes Johansson and Tore Brinck
Crystals 2017, 7(7), 222; https://doi.org/10.3390/cryst7070222 - 14 Jul 2017
Cited by 15 | Viewed by 6072
Abstract
Understanding the molecular interaction behavior of transition metal nanoclusters lies at the heart of their efficient use in, e.g., heterogeneous catalysis, medical therapy and solar energy harvesting. For this purpose, we have evaluated the applicability of the surface electrostatic potential [VS [...] Read more.
Understanding the molecular interaction behavior of transition metal nanoclusters lies at the heart of their efficient use in, e.g., heterogeneous catalysis, medical therapy and solar energy harvesting. For this purpose, we have evaluated the applicability of the surface electrostatic potential [VS(r)] and the local surface electron attachment energy [ES(r)] properties for characterizing the local Lewis acidity of a series of low-energy TM13 transition metal nanoclusters (TM = Au, Cu, Ru, Rh, Pd, Ir, Pt, Co), including also Pt7Cu6. The clusters have been studied using hybrid Kohn–Sham density functional theory (DFT) calculations. The VS(r) and ES(r), evaluated at 0.001 a.u. isodensity contours, are used to analyze the interactions with H2O. We find that the maxima of VS(r), σ-holes, are either localized or diffuse. This is rationalized in terms of the nanocluster geometry and occupation of the clusters’s, p and d valence orbitals. Our findings motivate a new scheme for characterizing σ-holes as σs (diffuse), σp (localized) or σd (localized) depending on their electronic origin. The positions of the maxima in VS(r) (and minima in ES(r)) are found to coincide with O-down adsorption sites of H2O, whereas minima in VS(r) leads to H-down adsorption. Linear relationships between VS,max (and ES,min) and H2O interaction energies are further discussed. Full article
(This article belongs to the Special Issue Analysis of Halogen and Other σ-Hole Bonds in Crystals)
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1009 KiB  
Article
Analysis of Diffracted Intensities from Finite Protein Crystals with Incomplete Unit Cells
by Sophie R. Williams, Ruben A. Dilanian, Harry M. Quiney and Andrew V. Martin
Crystals 2017, 7(7), 220; https://doi.org/10.3390/cryst7070220 - 14 Jul 2017
Cited by 3 | Viewed by 3940
Abstract
Developments in experimental techniques in micro electron diffraction and serial X-ray crystallography provide the opportunity to collect diffraction data from protein nanocrystals. Incomplete unit cells on the surfaces of protein crystals can affect the distribution of diffracted intensities for crystals with very high [...] Read more.
Developments in experimental techniques in micro electron diffraction and serial X-ray crystallography provide the opportunity to collect diffraction data from protein nanocrystals. Incomplete unit cells on the surfaces of protein crystals can affect the distribution of diffracted intensities for crystals with very high surface-to-volume ratios. The extraction of structure factors from diffraction data for such finite protein crystals sizes is considered here. A theoretical model for the continuous diffracted intensity distribution for data merged from finite crystals with two symmetry-related sub-units of the conventional unit cell is presented. This is used to extend a whole-pattern fitting technique to account for incomplete unit cells in the extraction of structure factor amplitudes. The accuracy of structure factor amplitudes found from this whole-pattern fitting technique and from an integration approach are evaluated. Full article
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2569 KiB  
Review
Understanding Mn-Based Intercalation Cathodes from Thermodynamics and Kinetics
by Yin Xie, Yongcheng Jin and Lan Xiang
Crystals 2017, 7(7), 221; https://doi.org/10.3390/cryst7070221 - 13 Jul 2017
Cited by 12 | Viewed by 6893
Abstract
A series of Mn-based intercalation compounds have been applied as the cathode materials of Li-ion batteries, such as LiMn2O4, LiNi1xyCoxMnyO2, etc. With open structures, intercalation compounds exhibit [...] Read more.
A series of Mn-based intercalation compounds have been applied as the cathode materials of Li-ion batteries, such as LiMn2O4, LiNi1xyCoxMnyO2, etc. With open structures, intercalation compounds exhibit a wide variety of thermodynamic and kinetic properties depending on their crystal structures, host chemistries, etc. Understanding these materials from thermodynamic and kinetic points of view can facilitate the exploration of cathodes with better electrochemical performances. This article reviews the current available thermodynamic and kinetic knowledge on Mn-based intercalation compounds, including the thermal stability, structural intrinsic features, involved redox couples, phase transformations as well as the electrical and ionic conductivity. Full article
(This article belongs to the Special Issue Solution-Processed Inorganic Functional Crystals)
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17726 KiB  
Review
A Review on Metal Nanoparticles Nucleation and Growth on/in Graphene
by Francesco Ruffino and Filippo Giannazzo
Crystals 2017, 7(7), 219; https://doi.org/10.3390/cryst7070219 - 13 Jul 2017
Cited by 34 | Viewed by 8649
Abstract
In this review, the fundamental aspects (with particular focus to the microscopic thermodynamics and kinetics mechanisms) concerning the fabrication of graphene-metal nanoparticles composites are discussed. In particular, the attention is devoted to those fabrication methods involving vapor-phase depositions of metals on/in graphene-based materials. [...] Read more.
In this review, the fundamental aspects (with particular focus to the microscopic thermodynamics and kinetics mechanisms) concerning the fabrication of graphene-metal nanoparticles composites are discussed. In particular, the attention is devoted to those fabrication methods involving vapor-phase depositions of metals on/in graphene-based materials. Graphene-metal nanoparticles composites are, nowadays, widely investigated both from a basic scientific and from several technological point of views. In fact, these graphene-based systems present wide-range tunable and functional electrical, optical, and mechanical properties which can be exploited for the design and production of innovative and high-efficiency devices. This research field is, so, a wide and multidisciplinary section in the nanotechnology field of study. So, this review aims to discuss, in a synthetic and systematic framework, the basic microscopic mechanisms and processes involved in metal nanoparticles formation on graphene sheets by physical vapor deposition methods and on their evolution by post-deposition processes. This is made by putting at the basis of the discussions some specific examples to draw insights on the common general physical and chemical properties and parameters involved in the synergistic interaction processes between graphene and metals. Full article
(This article belongs to the Special Issue Integration of 2D Materials for Electronics Applications)
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4677 KiB  
Article
Crystal Structure of 17α-Dihydroequilin, C18H22O2, from Synchrotron Powder Diffraction Data and Density Functional Theory
by James A. Kaduk, Amy M. Gindhart and Thomas N. Blanton
Crystals 2017, 7(7), 218; https://doi.org/10.3390/cryst7070218 - 13 Jul 2017
Cited by 11 | Viewed by 4043
Abstract
The crystal structure of 17α-dihydroequilin has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional techniques. 17α-dihydroequilin crystallizes in space group P212121 (#19) with a = 6.76849(1) Å, b = 8.96849(1) Å, [...] Read more.
The crystal structure of 17α-dihydroequilin has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional techniques. 17α-dihydroequilin crystallizes in space group P212121 (#19) with a = 6.76849(1) Å, b = 8.96849(1) Å, c = 23.39031(5) Å, V = 1419.915(3) Å3, and Z = 4. Both hydroxyl groups form hydrogen bonds to each other, resulting in zig-zag chains along the b-axis. The powder diffraction pattern has been submitted to ICDD for inclusion in the Powder Diffraction File™ as the entry 00-066-1608. Full article
(This article belongs to the Special Issue Structural Analysis of Crystalline Materials from Powders)
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6526 KiB  
Article
Synthesis, Crystal Structures, and Properties of a New Supramolecular Polymer Based on Mixed Imidazole and Carboxylate Ligands
by Mei-An Zhu, Shan-Shan Shi, Shuai-Shuai Han, Jin Mei and Shui-Sheng Chen
Crystals 2017, 7(7), 210; https://doi.org/10.3390/cryst7070210 - 13 Jul 2017
Cited by 4 | Viewed by 3986
Abstract
One new coordination polymer, namely, [Cd3(H2L)3(Pza)2(H2O)2]n (1) was synthesized by the reaction of Cd(NO3)2·4H2O with 1,4-di(1H-imidazol-4-yl)benzene (H2L) and [...] Read more.
One new coordination polymer, namely, [Cd3(H2L)3(Pza)2(H2O)2]n (1) was synthesized by the reaction of Cd(NO3)2·4H2O with 1,4-di(1H-imidazol-4-yl)benzene (H2L) and 3,5-pyrazoledicarboxylic acid (H3pza) and characterized by single-crystal X-ray diffraction, IR spectroscopy, elemental analysis, and powder X-ray diffraction (PXRD). The H3pza ligand was completely deprotonated to pza3−, which bridged the Cd2+ to form one-dimensional (1D) chain. The adjacent 1D chains were further linked into the two-dimensional (2D) layer by the linear H2L ligands. The weak interaction, including hydrogen bonds and π−π stacking interactions, extends the 2D layers into three-dimensional (3D) supramolecular polymer. Complex 1 shows intense light blue emission in the solid state at room temperature. Full article
(This article belongs to the Section Crystalline Materials)
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2429 KiB  
Article
Syntheses, Crystal Structures and Thermal Behaviors of Two Supramolecular Salamo-Type Cobalt(II) and Zinc(II) Complexes
by Gang Li, Jing Hao, Ling-Zhi Liu, Wei-Min Zhou and Wen-Kui Dong
Crystals 2017, 7(7), 217; https://doi.org/10.3390/cryst7070217 - 12 Jul 2017
Cited by 29 | Viewed by 4854
Abstract
This paper reports the syntheses of two new complexes, [Co(L1)(H2O)2] (1) and [{Zn(L2)(μ-OAc)Zn(n-PrOH)}2] (2), from asymmetric halogen-substituted Salamo-type ligands H2L1 and H3L [...] Read more.
This paper reports the syntheses of two new complexes, [Co(L1)(H2O)2] (1) and [{Zn(L2)(μ-OAc)Zn(n-PrOH)}2] (2), from asymmetric halogen-substituted Salamo-type ligands H2L1 and H3L2, respectively. Investigation of the crystal structure of complex 1 reveals that the complex includes one Co(II) ion, one (L1)2− unit and two coordinated water molecules. Complex 1 shows slightly distorted octahedral coordination geometry, forming an infinite 2D supramolecular structure by intermolecular hydrogen bond and π–π stacking interactions. Complex 2 contains four Zn(II)ions, two completely deprotonated (L2)3− moieties, two coordinated μ-OAc ions and n-propanol molecules. The Zn(II) ions in complex 2 display slightly distorted trigonal bipyramidal or square pyramidal geometries. Full article
(This article belongs to the Section Crystal Engineering)
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6169 KiB  
Article
σ-Hole Interactions: Perspectives and Misconceptions
by Peter Politzer and Jane S. Murray
Crystals 2017, 7(7), 212; https://doi.org/10.3390/cryst7070212 - 12 Jul 2017
Cited by 139 | Viewed by 8235
Abstract
After a brief discussion of the σ-hole concept and the significance of molecular electrostatic potentials in noncovalent interactions, we draw attention to some common misconceptions that are encountered in that context: (1) Since the electrostatic potential reflects the contributions of both the nuclei [...] Read more.
After a brief discussion of the σ-hole concept and the significance of molecular electrostatic potentials in noncovalent interactions, we draw attention to some common misconceptions that are encountered in that context: (1) Since the electrostatic potential reflects the contributions of both the nuclei and the electrons, it cannot be assumed that negative potentials correspond to “electron-rich” regions and positive potentials to “electron-poor” ones; (2) The electrostatic potential in a given region is determined not only by the electrons and nuclei in that region, but also by those in other portions of the molecule, especially neighboring ones; (3) A σ-hole is a region of lower electronic density on the extension of a covalent bond, not an electrostatic potential; (4) Noncovalent interactions are between positive and negative regions, which are not necessarily associated with specific atoms, so that “close contacts” between atoms do not always indicate the actual interactions. Full article
(This article belongs to the Special Issue Analysis of Halogen and Other σ-Hole Bonds in Crystals)
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1669 KiB  
Review
Advanced Scanning Probe Microscopy of Graphene and Other 2D Materials
by Chiara Musumeci
Crystals 2017, 7(7), 216; https://doi.org/10.3390/cryst7070216 - 11 Jul 2017
Cited by 26 | Viewed by 14076
Abstract
Two-dimensional (2D) materials, such as graphene and metal dichalcogenides, are an emerging class of materials, which hold the promise to enable next-generation electronics. Features such as average flake size, shape, concentration, and density of defects are among the most significant properties affecting these [...] Read more.
Two-dimensional (2D) materials, such as graphene and metal dichalcogenides, are an emerging class of materials, which hold the promise to enable next-generation electronics. Features such as average flake size, shape, concentration, and density of defects are among the most significant properties affecting these materials’ functions. Because of the nanoscopic nature of these features, a tool performing morphological and functional characterization on this scale is required. Scanning Probe Microscopy (SPM) techniques offer the possibility to correlate morphology and structure with other significant properties, such as opto-electronic and mechanical properties, in a multilevel characterization at atomic- and nanoscale. This review gives an overview of the different SPM techniques used for the characterization of 2D materials. A basic introduction of the working principles of these methods is provided along with some of the most significant examples reported in the literature. Particular attention is given to those techniques where the scanning probe is not used as a simple imaging tool, but rather as a force sensor with very high sensitivity and resolution. Full article
(This article belongs to the Special Issue Integration of 2D Materials for Electronics Applications)
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9527 KiB  
Article
Fabrication of Nano-Micro Hybrid Structures by Replication and Surface Treatment of Nanowires
by Yeonho Jeong, Seunghang Shin, Hyunmin Choi, Seonjun Kim, Jihoon Kim, Sin Kwon, Kwang-Young Kim, Seung-Hyun Lee, Yoon-Gyo Jung and Young Tae Cho
Crystals 2017, 7(7), 215; https://doi.org/10.3390/cryst7070215 - 11 Jul 2017
Cited by 5 | Viewed by 5955
Abstract
Nanowire structures have attracted attention in various fields, since new characteristics could be acquired in minute regions. Especially, Anodic Aluminum Oxide (AAO) is widely used in the fabrication of nanostructures, which has many nanosized pores and well-organized nano pattern. Using AAO as a [...] Read more.
Nanowire structures have attracted attention in various fields, since new characteristics could be acquired in minute regions. Especially, Anodic Aluminum Oxide (AAO) is widely used in the fabrication of nanostructures, which has many nanosized pores and well-organized nano pattern. Using AAO as a template for replication, nanowires with a very high aspect ratio can be fabricated. Herein, we propose a facile method to fabricate a nano-micro hybrid structure using nanowires replicated from AAO, and surface treatment. A polymer resin was coated between Polyethylene terephthalate (PET) and the AAO filter, roller pressed, and UV-cured. After the removal of aluminum by using NaOH solution, the nanowires aggregated to form a micropattern. The resulting structure was subjected to various surface treatments to investigate the surface behavior and wettability. As opposed to reported data, UV-ozone treatment can enhance surface hydrophobicity because the UV energy affects the nanowire surface, thus altering the shape of the aggregated nanowires. The hydrophobicity of the surface could be further improved by octadecyltrichlorosilane (OTS) coating immediately after UV-ozone treatment. We thus demonstrated that the nano-micro hybrid structure could be formed in the middle of nanowire replication, and then, the shape and surface characteristics could be controlled by surface treatment. Full article
(This article belongs to the Special Issue Advance in Crystalline Thin Wires)
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4591 KiB  
Article
Halogen-Bonded Co-Crystals of Aromatic N-oxides: Polydentate Acceptors for Halogen and Hydrogen Bonds
by Rakesh Puttreddy, Filip Topić, Arto Valkonen and Kari Rissanen
Crystals 2017, 7(7), 214; https://doi.org/10.3390/cryst7070214 - 11 Jul 2017
Cited by 21 | Viewed by 6493
Abstract
Seventeen new halogen-bonded co-crystals characterized by single crystal X-ray analysis are presented from 8 × 4 combinations using methyl-substituted pyridine N-oxides and 1,ω-diiodoperfluoroalkanes. The N−O group in six of 17 co-crystals is monodentate and 11 have μ-O,O bidentate halogen [...] Read more.
Seventeen new halogen-bonded co-crystals characterized by single crystal X-ray analysis are presented from 8 × 4 combinations using methyl-substituted pyridine N-oxides and 1,ω-diiodoperfluoroalkanes. The N−O group in six of 17 co-crystals is monodentate and 11 have μ-O,O bidentate halogen bond acceptor modes. Remarkably, the N−O group in co-crystals of 3-methyl-, 4-methyl- and 3,4-dimethylpyridineN-oxides with octafluoro-1,4-diiodobutane acted as a μ-O,O,O,O halogen and hydrogen bond acceptor, while acting as a μ-O,O,O acceptor in the co-crystal of 2,5-dimethylpyridineN-oxide and tetrafluoro-1,2-diiodoethane. The C−H···O−N hydrogen bonds demonstrated the polydentate cooperativity of the N−O group as a mixed halogen-hydrogen bond acceptor. The co-crystal of 2,4,6-trimethylpyridineN-oxide and dodecafluoro-1,6-diiodohexane exhibited C−I···O−N+ halogen bonds with RXB value 0.76, the shortest of its kind compared to previously reported structures. The RXB values between 0.76 and 0.83 suggested that the C−I···O−N halogen bonds are moderately strong compared to our previously studied N−I···O−N system, with RXB in the order 0.66. Full article
(This article belongs to the Special Issue Analysis of Halogen and Other σ-Hole Bonds in Crystals)
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3903 KiB  
Article
Element Strategy Using Ru-Mn Substitution in CuO-CaCu3Ru4O12 Composite Ceramics with High Electrical Conductivity
by Akihiro Tsuruta, Masashi Mikami, Yoshiaki Kinemuchi, Ichiro Terasaki, Norimitsu Murayama and Woosuck Shin
Crystals 2017, 7(7), 213; https://doi.org/10.3390/cryst7070213 - 10 Jul 2017
Cited by 4 | Viewed by 4411
Abstract
CaCu3Ru4xMnxO12 bulks with various substitution amounts x and sintering additive CuO (20 vol.%) were prepared, and the influence of x on the electrical conductivity in a wide temperature range (8–900 K) was investigated. Microstructural [...] Read more.
CaCu3Ru4xMnxO12 bulks with various substitution amounts x and sintering additive CuO (20 vol.%) were prepared, and the influence of x on the electrical conductivity in a wide temperature range (8–900 K) was investigated. Microstructural observations showed an enhancement of bulk densification upon Mn substitution. Although the resistivity increased with increasing x, the resistivity was as low as a few mΩcm even in the sample with x = 2.00, where half of Ru is substituted by Mn. This high conductivity despite the loss of Ru 4d conduction following the substitution is explained by the A-site (Cu2+) conduction in CaCu3Ru4xMnxO12. The thermopower of CaCu3Ru4xMnxO12 was found to be influenced by the substitution, and a sign inversion was observed in the substituted samples at low temperature. The partial substitution of Ru by Mn in CaCu3Ru4O12 enables the reduction of the materials cost while maintaining good electrical conductivity for applications as a conducting device component. Full article
(This article belongs to the Special Issue Crystal Structure of Electroceramics)
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3915 KiB  
Communication
Dual Inhibition of AChE and BChE with the C-5 Substituted Derivative of Meldrum’s Acid: Synthesis, Structure Elucidation, and Molecular Docking Studies
by Haroon Mehfooz, Aamer Saeed, Anamika Sharma, Fernando Albericio, Fayaz Ali Larik, Farukh Jabeen, Pervaiz Ali Channar and Ulrich Flörke
Crystals 2017, 7(7), 211; https://doi.org/10.3390/cryst7070211 - 09 Jul 2017
Cited by 21 | Viewed by 5157
Abstract
Alzheimer’s disease (AD) lies in the category of those diseases which are still posing challenges to medicinal chemists, and the search for super-effective drugs for the treatment of AD is a work in progress. The inhibition of cholinesterase is considered a viable strategy [...] Read more.
Alzheimer’s disease (AD) lies in the category of those diseases which are still posing challenges to medicinal chemists, and the search for super-effective drugs for the treatment of AD is a work in progress. The inhibition of cholinesterase is considered a viable strategy to enhance the level of acetylcholine in the brain. The C-5 substituted derivative of Meldrum’s acid was synthesized and screened against acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) enzyme inhibition activity. The simple and unique structure of synthesized derivative 3 was found to be good for the dual inhibition of both enzymes (AChE and BChE). 2,2-Dimethyl-5-(([2-(trifluoromethyl) phenyl]amino)methylidene)-1,3-dioxane-4,6-dione (3) showed significant inhibition against AChE, with an IC50 value of 1.13 ± 0.03 µ M (Standard Neostigmine 22.2 ± 3.2 µM), and moderate inhibition against BChE, with an IC50 value of 2.12 ± 1.22 µM (Standard Neostigmine 49.6 ± 6.11 µM). The structural insights reveal that compound 3 possesses intriguing reactive groups, which can potentially evoke the non-covalent interactions and possibly assist by binding in the active site of the target protein. Docking simulations revealed that the compound 3 showed binding inside the active site gorges of both AChE and BChE. An excellent agreement was obtained, as the best docked poses showed important binding features mostly based on interactions due to oxygen atoms and the aromatic moieties of the compound. The docking computations coupled with the experimental findings ascertained that the compound 3 can serve as a scaffold for the dual inhibitors of the human acetylcholine esterases. Full article
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5201 KiB  
Review
Mesocrystals: Past, Presence, Future
by Elena V. Sturm (née Rosseeva) and Helmut Cölfen
Crystals 2017, 7(7), 207; https://doi.org/10.3390/cryst7070207 - 09 Jul 2017
Cited by 72 | Viewed by 9880
Abstract
In this review, we briefly summarize the history of mesocrystal research. We introduce the current structural definition of mesocrystals and discuss the appropriate base for the classification of mesocrystals and their relations with other classes of solid state materials in terms of their [...] Read more.
In this review, we briefly summarize the history of mesocrystal research. We introduce the current structural definition of mesocrystals and discuss the appropriate base for the classification of mesocrystals and their relations with other classes of solid state materials in terms of their structure. Building up on this, we comment on the problems in mesocrystal research both fundamental and methodological. Additionally, we make the short overview of the mesocrystal formation principles and synthetic routes used for their fabrications. As an outlook into the future, we highlight the most notable trends in mesocrystal research and developments. Full article
(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)
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17842 KiB  
Article
An Electrically Tunable Liquid Crystal Lens with Coaxial Bi-Focus and Single Focus Switching Modes
by Chun-Yu Chien, Cheng-Hau Li and Chia-Rong Sheu
Crystals 2017, 7(7), 209; https://doi.org/10.3390/cryst7070209 - 07 Jul 2017
Cited by 13 | Viewed by 7302
Abstract
A hole-patterned electrode liquid crystal (LC) lens with electrically switching coaxial bi-focus and single focus modes of tuning is demonstrated. The proposed LC lens mainly consists of a two LC layer (TLCL) structure with different thicknesses to achieve higher focusing power than the [...] Read more.
A hole-patterned electrode liquid crystal (LC) lens with electrically switching coaxial bi-focus and single focus modes of tuning is demonstrated. The proposed LC lens mainly consists of a two LC layer (TLCL) structure with different thicknesses to achieve higher focusing power than the conventional hole-patterned electrode LC lens with the same aperture size. In the TLCL structure, one LC layer, doped with 3 wt % RM257, was photopolymerized to achieve a fixed focusing power of 18.5 Diopter. Due to polarization dependence in TLCL lenses, an additional 90° twisted nematic (TN) cell was used to change the incident polarization in order to switch lens functions on or off. As a result, a fixed focusing power of 18.5 Diopter was achieved when voltages of 10 Vrms were applied to the 90° TN cell. In addition, the switching capabilities of the bi-focus and single focus modes were achieved when operating individually with applied voltages from 20 Vrms to 90 Vrms, and higher voltages of over 90 Vrms, respectively. The maximum focusing power in the fabricated TLCL lens is 30.9 Diopter. Full article
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6289 KiB  
Article
Doping Liquid Crystal Cells with Photocurable Monomer via Holographic Exposure to Realize Optical-Scattering-Free Infrared Phase Modulators with Fast Response Time
by Chun-Yu Chien and Chia-Rong Sheu
Crystals 2017, 7(7), 208; https://doi.org/10.3390/cryst7070208 - 07 Jul 2017
Cited by 6 | Viewed by 4779
Abstract
Photocurable monomer-doped liquid crystal (LC) cells were processed via holographic exposure using a low-power He–Ne laser to generate holographic polymer networks. The polymer network LC (PNLC) cells are used to fabricate infrared phase modulators at 1550 nm wavelength possessing favorable electro-optical performance. Compared [...] Read more.
Photocurable monomer-doped liquid crystal (LC) cells were processed via holographic exposure using a low-power He–Ne laser to generate holographic polymer networks. The polymer network LC (PNLC) cells are used to fabricate infrared phase modulators at 1550 nm wavelength possessing favorable electro-optical performance. Compared with our previous work, the percentages of ingredients in the LC mixture filled in PNLC cells underwent a slight change. The 2 wt% concentration of anisotropic monomer RM257 were in place of isotropic monomer N–vinyl–2–pyrrolidinone (NVP). As a result, the fabricated phase modulators also maintained well homogeneous LC alignments and optical-scattering-free characteristics. Furthermore, NVP dopant successfully reduced the operating voltages from 95 Vrms to 79 Vrms to prevent polymer network deformation when electrically operating with higher voltages. The fabricated infrared phase modulators had a good average response time (i.e., rising time of 0.88 ms and falling time of 0.40 ms). Full article
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3179 KiB  
Article
Crystallization under an External Electric Field: A Case Study of Glucose Isomerase
by Evgeniya Rubin, Christopher Owen and Vivian Stojanoff
Crystals 2017, 7(7), 206; https://doi.org/10.3390/cryst7070206 - 06 Jul 2017
Cited by 18 | Viewed by 6510
Abstract
Electric fields have been employed to promote macromolecular crystallization for several decades. Although crystals grown in electric fields seem to present higher diffraction quality, these methods are not widespread. For most configurations, electrodes are in direct contact with the protein solution. Here, we [...] Read more.
Electric fields have been employed to promote macromolecular crystallization for several decades. Although crystals grown in electric fields seem to present higher diffraction quality, these methods are not widespread. For most configurations, electrodes are in direct contact with the protein solution. Here, we propose a configuration that can be easily extended to standard crystallization methods for which the electrodes are not in direct contact with the protein solution. Furthermore, the proposed electrode configuration supplies an external DC electric field. Glucose Isomerase from Streptomyces rubiginosus crystals were grown at room temperature using the microbatch method in the presence of 1, 2, 4, and 6 kV. Several crystallization trials were carried out for reproducibility and statistical analysis purposes. The comparison with crystals grown in the absence of electric fields showed that crystallization in the presence of electric fields increases the size of crystals, while decreasing the number of nucleations. X-ray diffraction analysis of the crystals showed that those grown in the presence of electric fields are of higher crystal quality. Full article
(This article belongs to the Special Issue Protein Crystallization under the Presence of an Electric Field)
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5160 KiB  
Article
Shape-Controlled TiCx Particles Fabricated by Combustion Synthesis in the Cu-Ti-C System
by Dongdong Zhang, Haolong Liu, Liping Sun, Fang Bai, Yong Wang and Jinguo Wang
Crystals 2017, 7(7), 205; https://doi.org/10.3390/cryst7070205 - 06 Jul 2017
Cited by 12 | Viewed by 3749
Abstract
TiCx particle-reinforced Cu-matrix composites were prepared in the Cu-Ti-C system by thermal explosion and hot press. Extracted TiCx particles with various shapes of in situ TiCx particles in the Cu-Ti-C system were observed through the Field Emission Scanning Electron Microscope [...] Read more.
TiCx particle-reinforced Cu-matrix composites were prepared in the Cu-Ti-C system by thermal explosion and hot press. Extracted TiCx particles with various shapes of in situ TiCx particles in the Cu-Ti-C system were observed through the Field Emission Scanning Electron Microscope (FESEM). It was found that octahedral and close-to-spherical, spherical or cubic TiCx could be fabricated by changing the C/Ti molar ratio and Cu content. Then, the effect of the C/Ti molar ratio and constituent element concentrations on the shape of in situ TiCx particles was determined: the shape of TiCx particles is octahedral at a C/Ti ratio of 0.4–0.6 with the presence of 70 vol% Cu; or spherical and close-to-spherical at 0.8–1.0 with the presence of 70 vol% Cu; or cubic at C/Ti ratios ≥1.0 with the presence of Cu from 80 vol%–90 vol% and even at C/Ti ratios >1.0 with the presence of 70 vol% Cu. The shape-controlled synthesis of TiCx particles in the Cu-Ti-C system is realized. Full article
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11379 KiB  
Article
Morphological and Crystallographic Characterization of Primary Zinc-Rich Crystals in a Ternary Sn-Zn-Bi Alloy under a High Magnetic Field
by Lei Li, Chunyan Ban, Ruixue Zhang, Haitao Zhang, Minghui Cai, Yubo Zuo, Qingfeng Zhu, Xiangjie Wang and Jianzhong Cui
Crystals 2017, 7(7), 204; https://doi.org/10.3390/cryst7070204 - 06 Jul 2017
Cited by 6 | Viewed by 4002
Abstract
Due to the unique capacity for structural control, high magnetic fields (HMFs) have been widely applied to the solidification process of alloys. In zinc-based alloys, the primary zinc-rich crystals can be dendritic or needle-like in two dimensions. For the dendritic crystals, their growth [...] Read more.
Due to the unique capacity for structural control, high magnetic fields (HMFs) have been widely applied to the solidification process of alloys. In zinc-based alloys, the primary zinc-rich crystals can be dendritic or needle-like in two dimensions. For the dendritic crystals, their growth pattern and orientation behaviors under HMFs have been investigated. However, the three-dimensional crystallographic growth pattern and the orientation behaviors of the needle-like primary zinc-rich crystals under a high magnetic field have not been studied. In this work, a ternary Sn-Zn-Bi alloy was solidified under different HMFs. The above-mentioned two aspects of the needle-like primary zinc-rich crystals were characterized using the Electron Backscattered Diffraction (EBSD) technique. The results show that the primary zinc-rich crystals are characterized by the plate-shaped faceted growth in three dimensions. They grow in the following manner: spreading rapidly in the {0001} basal plane with a gradual decrease in thickness at the edges. The application of HMFs has no effect on the growth form of the primary zinc-rich crystals, but induces their vertical alignment. Crystallographic analysis indicates that the vertically aligned primary zinc-rich crystals orient preferentially with the c-axis perpendicular to the direction of the magnetic field. Full article
(This article belongs to the Special Issue Crystal Chemistry of Zinc, Cadmium and Mercury)
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5256 KiB  
Article
Lanthanide Coordination Polymers as Luminescent Sensors for the Selective and Recyclable Detection of Acetone
by Kaimin Wang, Yulu Ma and Huaijun Tang
Crystals 2017, 7(7), 199; https://doi.org/10.3390/cryst7070199 - 05 Jul 2017
Cited by 12 | Viewed by 3963
Abstract
Three new isostructural lanthanide coordination polymers {[Ln(L)2·2H2O]·Cl·4H2O}, Ln = La (LaL 1), Tb (TbL 2), Eu (EuL 3), L = 4-carboxy-1-(4-carboxybenzyl)pyridinium, have been synthesized under hydrothermal conditions and characterized by single [...] Read more.
Three new isostructural lanthanide coordination polymers {[Ln(L)2·2H2O]·Cl·4H2O}, Ln = La (LaL 1), Tb (TbL 2), Eu (EuL 3), L = 4-carboxy-1-(4-carboxybenzyl)pyridinium, have been synthesized under hydrothermal conditions and characterized by single crystal X-ray diffraction, IR, TG, PXRD, and luminescence. The solid-state luminescence properties of those Ln-CPs were investigated, realizing the zwitterionic ligand (L) is an excellent antenna chromophore for sensitizing both Tb3+ and Eu3+ ions. We utilized TbL 2 as a representative chemosensor to consider the potential luminescence sensing properties in different solvent suspension, which has the potential to serve as the first case of a luminescent Ln-CP material based on the zwitterionic type of organic ligand for selective and recyclable sensing of acetone in methanol solution. Full article
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2046 KiB  
Article
Incompatibility Stresses and Lattice Rotations Due to Grain Boundary Sliding in Heterogeneous Anisotropic Elasticity
by Thiebaud Richeton
Crystals 2017, 7(7), 203; https://doi.org/10.3390/cryst7070203 - 04 Jul 2017
Cited by 5 | Viewed by 3757
Abstract
Non-uniform grain boundary sliding can induce strain and rotation incompatibilities at perfectly planar interfaces. Explicit analytic expressions of stress and lattice rotation jumps are thus derived at a planar interface in the general framework of heterogeneous anisotropic thermo-elasticity with plasticity and grain boundary [...] Read more.
Non-uniform grain boundary sliding can induce strain and rotation incompatibilities at perfectly planar interfaces. Explicit analytic expressions of stress and lattice rotation jumps are thus derived at a planar interface in the general framework of heterogeneous anisotropic thermo-elasticity with plasticity and grain boundary sliding. Both elastic fields are directly dependent on in-plane gradients of grain boundary sliding. It is also shown that grain boundary sliding is a mechanism that may relax incompatibility stresses of elastic, plastic and thermal origin although the latter are not resolved on the grain boundary plane. This relaxation may be a driving force for grain boundary sliding in addition to the traditionally considered local shears on the grain boundary plane. Moreover, the obtained analytic expressions are checked by different kinds of bicrystal shearing finite element simulations allowing grain boundary sliding and where a pinned line in the interface plane aims at representing the effect of a triple junction. A very good agreement is found between the analytic solutions and the finite element results. The performed simulations particularly emphasize the role of grain boundary sliding as a possible strong stress generator around the grain boundary close to the triple line because of the presence of pronounced gradients of sliding. Full article
(This article belongs to the Special Issue Plasticity of Crystals and Interfaces)
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3050 KiB  
Communication
External-Voltage-Free Dielectrophoresis of Liquid Crystal Droplets
by Sheng-Kuang Wu, Ting-Shan Mo, Jia-De Lin, Shuan-Yu Huang, Hui-Chen Yeh, Lin-Jer Chen and Chia-Rong Lee
Crystals 2017, 7(7), 202; https://doi.org/10.3390/cryst7070202 - 03 Jul 2017
Cited by 11 | Viewed by 3664
Abstract
This work reports, for the first time, a dielectrophoresis (DEP) effect-induced motion of liquid crystal (LC) droplets in an LC/monomer mixture sample with a poly-(N-vinyl carbazole) PVK-coated substrate without an external voltage. With the UV pre-irradiation of the PVK layer through [...] Read more.
This work reports, for the first time, a dielectrophoresis (DEP) effect-induced motion of liquid crystal (LC) droplets in an LC/monomer mixture sample with a poly-(N-vinyl carbazole) PVK-coated substrate without an external voltage. With the UV pre-irradiation of the PVK layer through a binary mask, a laterally non-uniform electric field can be induced between the pre-illuminated regions and the neighboring non-pre-illuminated PVK regions near the borders of the two regions. The phase separation occurs once the temperature is lower than 50 °C and the LC droplets can form in the sample. The pre-formed non-uniform field provides a DEP-like force to manipulate the small LC microdroplets in the pre-illuminated regions to effectively migrate to the adjacent non-pre-illuminated regions. The continuous supply of the LC from the pre-illuminated regions to the adjacent non-pre-illuminated regions significantly increases the diffraction efficiency of the grating sample. This study provides an insight into developing new external-voltage-free DEP-based devices that can be applied on various fields, such as photonics, displays, and biomedicines. Full article
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2801 KiB  
Article
Fast Turn-Off Switching of Vertically-Aligned Negative Liquid Crystals by Fine Patterning of Pixel Electrodes
by Yeongyu Choi, Tae-Hoon Choi, Jae-Hyeon Woo, Byoung-Gyu Jeon and Tae-Hoon Yoon
Crystals 2017, 7(7), 201; https://doi.org/10.3390/cryst7070201 - 03 Jul 2017
Cited by 13 | Viewed by 4437
Abstract
We investigated the two-dimensional (2D) confinement effect on the switching of vertically-aligned negative liquid crystals (LCs) by an electric field applied between the top and bottom patterned electrodes. When an electric field is applied to a patterned vertical alignment (PVA) cell, virtual walls [...] Read more.
We investigated the two-dimensional (2D) confinement effect on the switching of vertically-aligned negative liquid crystals (LCs) by an electric field applied between the top and bottom patterned electrodes. When an electric field is applied to a patterned vertical alignment (PVA) cell, virtual walls form in the middle of the gaps between and at the center of the patterned electrodes. These virtual walls formed in a PVA cell results in the turn-off time being dependent on the pitch of the patterned electrodes as well as the cell gap. We found that a short response time can be achieved by the fine patterning of pixel electrodes with little decrease in the transmittance. The obtained numerical results agree well with the model based on the 2D confinement effect of LCs. Full article
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696 KiB  
Article
Growth by the μ-PD Method and Visible Laser Operation of a Single-Crystal Fiber of Pr3+:KY3F10
by Jun Shu, Eugenio Damiano, Alberto Sottile, Zhonghan Zhang and Mauro Tonelli
Crystals 2017, 7(7), 200; https://doi.org/10.3390/cryst7070200 - 02 Jul 2017
Cited by 8 | Viewed by 4073
Abstract
We report on the first growth, spectroscopy, and visible laser operation of a single-crystal fiber (SCF) of KY3F10 (KYF) grown by the micro-pulling-down (μ-PD) method, doped with Pr3+ ions. This material has a cubic lattice, which makes it appealing [...] Read more.
We report on the first growth, spectroscopy, and visible laser operation of a single-crystal fiber (SCF) of KY3F10 (KYF) grown by the micro-pulling-down (μ-PD) method, doped with Pr3+ ions. This material has a cubic lattice, which makes it appealing for use in the industry. However, KYF crystals are considered difficult to grow with high optical quality, even with well-established methods. Nevertheless, we grew a 50-mm-long SCF of Pr:KYF, which was transparent in its inner part. We studied the spectroscopic features of it in comparison with existing literature and with samples of the same crystal grown by the Czochralski method, and we did not notice any large differences. These characterizations confirmed that is indeed possible to grow high-quality crystals of Pr:KYF by the μ-PD method. Unfortunately, the crystal proved to be more brittle than typical KYF and especially difficult to polish, leading to rough and irregular facets, as evidenced by transmission measurements. Despite these issues, we obtained continuous-wave laser operation in the orange, red, and deep red regions, using a sample carved from the SCF as active medium and an InGaN-based laser diode as pump source, though with lower performances than in existing reports on this crystal. Full article
(This article belongs to the Special Issue Advances in Optical Crystal Fibers)
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