Crystals, Volume 6, Issue 11 (November 2016) – 20 articles

Starting from Ni-Co-Mn carbonate nanoparticles prepared by microreaction technology, uniform spherical particles of Ni_1/3Co_1/3Mn_1/3CO₃ with a size of 3–4 μm were obtained by a controllable hydrothermal conversion with the addition of (NH₄)₂CO₃. Based on characterizations on the evolution of morphology and composition with hydrothermal treatment time, we clarified the mechanism of this novel method as a dissolution-recrystallization process, as well as the effects of (NH₄)₂CO₃ concentration on the morphology and composition of particles. By changing concentrations and the ratio of the starting materials for nano-precipitation preparation, we achieved monotonic regulation on the size of the spherical particles, and the synthesis of Ni_0.4Co_0.2Mn_0.4CO₃ and Ni_0.5Co_0.2Mn_0.3CO₃, respectively. In addition, the spherical particles with a core-shell structure were preliminarily verified to be available by introducing nano-precipitates with different compositions in the hydrothermal treatment in sequence. Full article

A hierarchical zeolite CaA with microporous, mesoporous and macroporous structure was hydrothermally synthesized by a ”Bond-Blocking” method using organo-functionalized mesoporous silica (MS) as a silica source. The characterization by XRD, SEM/TEM and N₂ adsorption/desorption techniques showed that the prepared material had well-crystalline zeolite Linde Type A (LTA) topological structure, microspherical particle morphologies, and hierarchically intracrystalline micro-meso-macropores structure. With the Bond-Blocking principle, the external surface area and macro-mesoporosity of the hierarchical zeolite CaA can be adjusted by varying the organo-functionalized degree of the mesoporous silica surface. Similarly, the distribution of the micro-meso-macroporous structure in the zeolite CaA can be controlled purposely. Compared with the conventional microporous zeolite CaA, the hierarchical zeolite CaA as a catalyst in the conversion of methanol to dimethyl ether (DME), exhibited complete DME selectivity and stable catalytic activity with high methanol conversion. The catalytic performances of the hierarchical zeolite CaA results clearly from the micro-meso-macroporous structure, improving diffusion properties, favoring the access to the active surface and avoiding secondary reactions (no hydrocarbon products were detected after 3 h of reaction). Full article

In this article, we will review recent progress in the growth of topological insulator (TI) thin films by molecular beam epitaxy (MBE). The materials we focus on are the V₂-VI₃ family of TIs. These materials are ideally bulk insulating with surface states housing Dirac excitations which are spin-momentum locked. These surface states are interesting for fundamental physics studies (such as the search for Majorana fermions) as well as applications in spintronics and other fields. However, the majority of TI films and bulk crystals exhibit significant bulk conductivity, which obscures these states. In addition, many TI films have a high defect density. This review will discuss progress in reducing the bulk conductivity while increasing the crystal quality. We will describe in detail how growth parameters, substrate choice, and growth technique influence the resulting TI film properties for binary and ternary TIs. We then give an overview of progress in the growth of TI heterostructures. We close by discussing the bright future for TI film growth by MBE. Full article

A review on elasto-dynamics of quasicrystals (QCs) and their applications based on mathematical elasticity is given. In this study, recent studies on elasto-dynamics of QCs are reviewed, in which the focus of the problem lies in the role of phason variables and the coupling effect between phonons and phasons in the dynamic deformation process. On summarizing and describing the development of the elastic dynamics of QCs, this review mainly presents theelasto-dynamics of QCs and their application in a variety of research areas, ranging from problems with different QCs, including one-, two-, and three-dimensional QCs to various coupling problems. The plane elasticity and anti-plane elasticity of quasicrystals are included in this review. Full article

While mechanosynthesis of the target compound, 1-[2-(1H-indol-3-yl)-ethyl]-pyrrole-2,5-dione, C₁₄ H₁₂ N₂ O₂, did not yield the desired product, it instead resulted in an open intermediate. On the other hand, synthesis starting from the activated maleic anhydride yielded the final maleimide compound. The outcome of the mechanosynthesis has been evaluated by powder X-ray diffraction, and structures of both the final product and open intermediate have been confirmed using single-crystal crystallography. Full article

Molybdenum disulfide (MoS₂) has recently attracted considerable interests due to its unique properties and potential applications. Chemical vapor deposition (CVD) method is used widely to grow large-area and high-quality MoS₂ single crystals. Here, we report our investigation on thermal strain-fractured (SF) single crystalline MoS₂, oxidation-fractured MoS₂, and normal MoS₂ by atomic force microscopy (AFM), Raman and photoluminescence (PL) measurements. Several new Raman modes are observed for SF-MoS₂. The band gap of SF-MoS₂ is enlarged by 150 meV and the PL intensity is reduced substantially. These results imply that a structural transformation occurs in SF-MoS₂. Our findings here are useful for the design of MoS₂-based nanocatalysts with relative high catalytic activity. Full article

Strained-layer superlattices (SLSs) are an active research topic in the molecular beam epitaxy (MBE) and infrared focal plane array communities. These structures undergo a >500 K temperature change between deposition and operation. As a result, the lattice constants of the substrate and superlattice are expected to change by approximately 0.3%, and at approximately the same rate. However, we present the first temperature-dependent X-ray diffraction (XRD) measurements of SLS material on GaSb and show that the superlattice does not contract in the same manner as the substrate. In both InAs/InAs_0.65Sb_0.35 and In_0.8Ga_0.2As/InAs_0.65Sb_0.35 SLS structures, the apparent out-of-plane strain states of the superlattices switch from tensile at deposition to compressive at operation. These changes have ramifications for material characterization, defect generation, carrier lifetime, and overall device performance of superlattices grown by MBE. Full article

In recent years, hybrid organic–inorganic perovskite light absorbers have attracted much attention in the field of solar cells due to their optoelectronic characteristics that enable high power conversion efficiencies. Perovskite-based solar cells’ efficiency has increased dramatically from 3.8% to more than 20% in just a few years, making them a promising low-cost alternative for photovoltaic applications. The deposition of perovskite into a mesoporous metal oxide is an influential factor affecting solar cell performance. Full coverage and pore filling into the porous metal oxide are important issues in the fabrication of highly-efficient mesoporous perovskite solar cells. In this work, we carry out a structural and quantitative investigation of CH₃NH₃PbI₃ pore filling deposited via sequential two-step deposition into two different mesoporous metal oxides—TiO₂ and Al₂O₃. We avoid using a hole conductor in the perovskite solar cells studied in this work to eliminate undesirable end results. Filling oxide pores with perovskite was characterized by Energy Dispersive X-ray Spectroscopy (EDS) in Transmission Electron Microscopy (TEM) on cross-sectional focused ion beam (FIB) lamellae. Complete pore filling of CH₃NH₃PbI₃ perovskite into the metal oxide pores was observed down to X-depth, showing the presence of Pb and I inside the pores. The observations reported in this work are particularly important for mesoporous Al₂O₃ perovskite solar cells, as pore filling is essential for the operation of this solar cell structure. This work presents structural and quantitative proof of complete pore filling into mesoporous perovskite-based solar cells, substantiating their high power conversion efficiency. Full article

Semiconductor photocatalysis provides potential solutions for many energy and environmental-related issues. Recently, various semiconductors with hierarchical nanostructures have been fabricated to achieve efficient photocatalysts owing to their multiple advantages, such as high surface area, porous structures, as well as enhanced light harvesting. ZnO has been widely investigated and considered as the most promising alternative photocatalyst to TiO₂. Herein, we present a review on the fabrication methods, growth mechanisms and photocatalytic applications of hierarchical ZnO nanostructures. Various synthetic strategies and growth mechanisms, including multistep sequential growth routes, template-based synthesis, template-free self-organization and precursor or self-templating strategies, are highlighted. In addition, the fabrication of multicomponent ZnO-based nanocomposites with hierarchical structures is also included. Finally, the application of hierarchical ZnO nanostructures and nanocomposites in typical photocatalytic reactions, such as pollutant degradation and H₂ evolution, is reviewed. Full article

In order to match the growth of the decentered square and the evolution of the interface cell in a two-dimensional cellular automaton-finite volume method (CA-FVM) model with decentered square algorithm, the present work first alters the determination of the half length of the square diagonal according to the preferential growth orientation, and then modifies the interface evolution considering the contribution of neighboring solid cells. Accordingly, the sharp interface (physical basis of the model), the growth orientation, and the growth consistence are reasonably guaranteed. The CA-FVM model presents some capabilities in predicting the free growth of equiaxed dendrites. With the increase of the cooling rate, the solidification structure gradually changes from cell to dendrite, and the solute segregation becomes more severe. Meanwhile, the predicted solute segregation under the intensive cooling condition is consistent with the calculation by Ueshima model at the initial solidification stage. The predicted competition behavior of columnar dendrites is qualitatively consistent with the observation in the continuously cast steel billet. The predicted dendrite arm spacings are close to the measurements. Full article

High quality stoichiometric brownmillerite-type oxide single crystals have been successfully grown by the floating zone method using a mirror furnace. We report here on the growth conditions and structural characterization of two model compounds: Ca₂Fe₂O₅ and Sr₂ScGaO₅. Both show oxygen deficiency with respect to the average perovskite structure, and are promising candidates for oxygen ion conductivity at moderate temperatures. While Sr₂ScGaO₅ single crystals were obtained in the cubic oxygen-deficient perovskite structure, Ca₂Fe₂O₅ crystallizes in the brownmillerite framework. Having no cubic parent high temperature counterpart, Ca₂Fe₂O₅ crystals were found to be not twinned. We report on structural characterization of the as-grown single crystals by neutron and X-ray diffraction, as well as scanning electron microscopy (SEM) coupled with EDX (Energy Dispersive X-Ray Spectroscopy) analysis and isotope exchange experiments. Full article

This work investigated changes in the physical properties of nematic liquid crystals (NLCs) upon doping with nanoparticles. Shear viscosity measurements demonstrated the shear-thinning of typical NLCs following the addition of small amounts of nanoparticles at approximately 1 wt. %. However, neither the birefringence nor the dielectric anisotropy was significantly affected at these levels of doping. The shear-thinning appears to result from the locally ordered alignment of the NLCs in the vicinity of the nanoparticles rather than from reductions in the bulk order parameters. Full article

Carrier transfer in vertically-coupled InAs/GaAs quantum dot (QD) pairs is investigated. Photoluminescence (PL) and PL excitation spectra measured at low temperature indicate that the PL peak intensity ratio between the emission from the two sets of QDs—i.e., the relative population of carriers between the two layers of QDs—changes with increasing excitation intensity. Temperature-dependent PL reveals unexpected non-monotonic variations in the peak wavelength and linewidth of the seed layer of QDs with temperature. The PL intensity ratio exhibits a “W” behavior with respect to the temperature due to the interplay between temperature and excitation intensity on the inter-layer carrier transfer. Full article

The 2D outlook of graphene and similar layers has initiated a number of theoretical considerations of electronic structure that are both interesting and exciting, but applying these ideas to real layered systems, in terms of a model 2D system, must be done with extreme care. In the present review, we will discuss the applicability of the 2D concept with examples of peculiarities of electronic structures and interactions in particular layered systems: (i) Dirac points and cones in graphene; (ii) van der Waals interaction between MoS₂ monolayers; and (iii) the issue of a 2D screening in estimates of the band gap for MoS₂ monolayers. Full article

A sequence of structural/magnetic transitions on cooling is reported in the literature for hole-doped iron-based superconductor (Ba_{1 − x}K_x)Fe₂As₂ with x = 0.24. By using polarized light microscopy, we directly observe the formation of orthorhombic domains in (Ba_{1 − x}K_x)Fe₂As₂ (x = 0.24) single crystal below a temperature of simultaneous structural/magnetic transition T_N ~ 80 K. The structural domains vanish below ~30 K, but reappear below T = 15 K. Our results are consistent with reentrance transformation sequence from high-temperature tetragonal (HTT) to low temperature orthorhombic (LTO1) structure at T_N ~ 80 K, LTO1 to low temperature tetragonal (LTT) structure at T_c ~ 25 K, and LTT to low temperature orthorhombic (LTO2) structure at T ~ 15 K. Full article

We present a novel method to determine the projected atomic potential of a specimen directly from transmission electron microscopy coherent electron nano-diffraction patterns, overcoming common limitations encountered so far due to the dynamical nature of electron-matter interaction. The projected potential is obtained by deconvolution of the inverse Fourier transform of experimental diffraction patterns rescaled in intensity by using theoretical values of the kinematical atomic scattering factors. This novelty enables the compensation of dynamical effects typical of transmission electron microscopy (TEM) experiments on standard specimens with thicknesses up to a few tens of nm. The projected atomic potentials so obtained are averaged on sample regions illuminated by nano-sized electron probes and are in good quantitative agreement with theoretical expectations. Contrary to lens-based microscopy, here the spatial resolution in the retrieved projected atomic potential profiles is related to the finer lattice spacing measured in the electron diffraction pattern. The method has been successfully applied to experimental nano-diffraction data of crystalline centrosymmetric and non-centrosymmetric specimens achieving a resolution of 65 pm. Full article

3D hierarchical Bi₂S₃ architectures have been successfully synthesized via a simple and effective hydrothermal process. The as-prepared Bi₂S₃ samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption- desorption isotherms, and UV-vis diffuse reflectance spectrum (DRS). The observation of field emission scanning electron microscope (FESEM) images showed that numerous nanoplatelets are randomly arranged and interconnected with each other, which are assembled into 3D hierarchical Bi₂S₃ architectures. The photocatalytic activity of the as-prepared Bi₂S₃ samples was evaluated by the degradation of Rhodamine B (RhB) under visible light irradiation. The effect of hydrothermal temperature, reaction time, pH value and shape on the photocatalytic efficiency of the as-prepared Bi₂S₃ samples was investigated. The results showed that 3D hierarchical Bi₂S₃ architectures prepared at 165 °C for 12 h at a pH of 2.4 exhibits high photocatalytic efficiency, which could be ascribed to the synergetic effect of the shape, surface area, crystallinity, band gap and crystalline size. Full article

The hydronium salt (H₃O)₂[Cu(N7–acv)₂(H₂O)₂(SO₄)₂]·2H₂O (1, acv = acyclovir) has been synthesized and characterized by single-crystal X-ray diffraction and spectral methods. Solvated Cu(OH)₂ is a by-product of the synthesis. In the all-trans centrosymmetric complex anion, (a) the Cu(II) atom exhibits an elongated octahedral coordination; (b) the metal-binding pattern of acyclovir (acv) consists of a Cu–N7(acv) bond plus an (aqua)O–H···O6(acv) interligand interaction; and (c) trans-apical/distal sites are occupied by monodentate O-sulfate donor anions. Neutral acyclovir and aqua-proximal ligands occupy the basal positions, stabilizing the metal binding pattern of acv. Each hydronium(1+) ion builds three H-bonds with O–sulfate, O6(acv), and O–alcohol(acv) from three neighboring complex anions. No O atoms of solvent water molecules are involved as acceptors. Theoretical calculations of molecular electrostatic potential surfaces and atomic charges also support that the O-alcohol of the N9(acv) side chain is a better H-acceptor than the N3 or the O-ether atoms of acv. Full article

Two new linear silver(I) nitrate coordination polymers with bitopic ligand 1,1,2,2-tetra(pyrazol-1-yl)ethane were synthesized. Synthesized compounds were characterized by IR spectroscopy, elemental analysis, powder X-ray diffraction and thermal analysis. Silver coordination polymers demonstrated a yellow emission near 500 nm upon excitation at 360 nm. Crystal structures of coordination polymers were determined and structural peculiarities are discussed. In both of the structures, silver ions are connected via bridging ligand molecules to form polymeric chains with a five-atomic environment. The coordination environment of the central atom corresponds to a distorted trigonal bipyramid with two N atoms of different ligands in apical positions. The Ag–N bond distances vary in a wide range of 2.31–2.62 Å, giving strongly distorted metallacycles. Thermolysis of coordination polymers in reductive atmosphere (H₂/He) leads to the formation of silver nanoparticles with a narrow size distribution. Full article

In this article, we first present and discuss eighteenth-century descriptions of minerals that contributed decisively to the development of crystallography. Remarkably, these old crystallographic descriptions included morphologies with symmetries incompatible with an internal periodic order of atoms, which, however, have been recognised to be characteristics of quasicrystals. Moreover, we also review a number of studies of minerals with aperiodic crystal structures, including recently reported natural quasicrystals of extra-terrestrial origin. Finally, we discuss the current investigations addressing the search for new quasicrystalline minerals in nature. Full article