Side-Surface Structure of a Commercial β-Silicon Carbide Whisker

The side surfaces of a commercial β-SiC whisker were analyzed by calculating the surface energy and observing the microstructure of the whiskers. The results indicated that the side surfaces displayed a type of zigzag structure and were composed of {111}, {110}, and {100} crystal planes.     

Formation Energies of Antiphase Boundaries in GaAs and GaP: An ab Initio Study

Electronic and structural properties of antiphase boundaries in group III-V semiconductor compounds have been receiving increased attention due to the potential to integration of optically-active III-V heterostructures on silicon or germanium substrates. The formation energies of {110}, {111}, {112}, and {113} antiphase boundaries in GaAs and GaP were studied theoretically using a full-potential linearized augmented plane-wave density-functional approach. Results of the study reveal that the stoichiometric {110} boundaries are the most energetically favorable in both compounds. The specific formation energy γ of the remaining antiphase boundaries increases in the order of γ_{113} ≈ γ_{112} < γ_{111}, which suggests {113} and {112} as possible planes for faceting and annihilation of antiphase boundaries in GaAs and GaP.     

Seed-mediated growth of palladium nanocrystals: the effect of pseudo-halide thiocyanate ions

In synthesis in a solution phase, adsorbates such as halides can interact selectively with different metal crystal facets and affect the final morphology of nanocrystals. Pseudo-halide thiocyanate ions (SCN-) can also adsorb on the metal surface, but they have never been used for the synthesis of shape-controlled colloidal metal nanocrystals. In this study, we first investigated the effect of SCN- on the morphology of palladium nanocrystals through a seed-mediated growth method. The presence of 1 μM SCN- in the growth solutions could lead to the formation of palladium polyhedra: truncated rhombic dodecahedra enclosed by twelve {110}, eight {111} and six {100} facets. The products were nanocubes enclosed with six {100} facets if cetyltrimethylammonium bromide (CTAB) was the only capping agent. Meanwhile, the mechanism of the effect of SCN- on the morphology of Pd nanocrystals is discussed.     

Arrowhead-like diamond crystals formed by hot-filament chemical vapor deposition

Arrowhead-like diamond crystals have been formed by using a simple method of hot-filament-assisted chemical vapor deposition. These are contact twins with {111} as the twin plane, of which each individual is composed of {100}, {110} and {111} faces. These twins flatten along {110} face and elongate parallel to {111} contact plane. The flattened {110} face consists of many {110} terraces sided by 〈110〉 and 〈112〉 steps. So the twinned crystal looks like an arrowhead. These twins are formed just underneath the uppermost substrate temperature for diamond growth.     

Homoepitaxial growth on fine columns of single crystal diamond for a field emitter

We have fabricated a pyramidal emitter array on single crystal diamond using an etching technique and a homoepitaxial growth technique. We carried out homoepitaxial growth on fine columns of a single crystal diamond in an NIRIM-type microwave-plasma-assisted chemical vapor deposition reactor in the undoped condition. It was found that temperature and methane concentration had a great influence on the oriented growth. We have found that there is a substantial tendency for a lower methane concentration to result in 〈111〉-oriented growth and for a higher methane concentration to give 〈100〉-oriented growth for a polycrystalline diamond film on Si. By controlling the growth conditions, we have obtained various shapes of diamond particle (cubic, cuboctahedral, octahedral) and have also fabricated a pyramidal sharp tip on single crystal diamond (100) and (110) surfaces. For a (100) substrate, we found that a fine pyramid tip was surrounded by not only four {111} planes but also four slightly slopes faces at the base. We surmise that the crystalline face agreed with the slightly sloped plane and grew selectively because it has the highest lateral growth rate among the other oriented crystalline faces. For a (110) substrate, a fine pyramidal tip was surrounded by two {100} planes and two {111} planes and the top of the tip had a ridge between two {111} planes because of the less than optimum conditions. The field emission from the (100) and (110) substrates was also measured. The emission current of the (110) substrate was comparatively large.     

Crystallographic Facetting in Sintered Barium Titanate

A commercial TiO₂-excess BaTiO₃ powder has been sintered and its microstructure analyzed for crystallographic facetting via both scanning and transmission electron microscopy (SEM and TEM). Facetted grain surfaces are developed initially from {111} at a low temperature of 1215°C, which are then altered to {111} and {100} at 1290°C in the presence of a grain-boundary liquid phase. The grain shape is also modified correspondingly from platelike to polygonal. Facetting of the intragranularly located residual pores in BaTiO₃ along the {141} planes further develops on the (quasi-)equilibrium shape after annealing at 1400°C for 100 h from the initially well-characterized {111}, {110}, and {100} in as-sintered samples sintered at the same temperature for 10 h. The Wulff plots derived from the residual pores in as-sintered and annealed samples are constructed for the 〈011〉 zone. Microstructural analysis also suggests that the shape of grains and intragranular residual pores is modified progressively upon annealing. The initial solid–vapor surface energy has become less anisotropic crystallographically. Abnormal grain growth in relation to the surface energy anisotropy is discussed.     

Surface Energy Anisotropy of SrTiO3 at 1400°C in Air

Geometric and crystallographic measurements of grain-boundary thermal grooves and surface faceting behavior as a function of orientation have been used to determine the surface energy anisotropy of SrTiO₃ at 1400°C in air. Under these conditions, thermal grooves are formed by surface diffusion. The surface energy anisotropy was determined using the capillarity vector reconstruction method under the assumption that Herring's local equilibrium condition holds at the groove root. The results indicate that the (100) surface has the minimum energy. For surfaces inclined between 0° and 30° from (100), the energy increases with the inclination angle. Orientations inclined by more than 30° from (100) are all about 10% higher in energy and, within experimental uncertainty, energetically equivalent. A procedure for estimating the uncertainties in the reconstructed energies is also introduced. Taken together, the orientation dependence of the surface-facet formation and the measured energy anisotropy lead to the conclusion that the equilibrium crystal shape is dominated by {100}, but also includes {110} and {111} facets. Complex planes within about 15° of {100} and 5° of {110} are also part of the equilibrium shape.     

Growth of homoepitaxial diamond doped with nitrogen for electron emitter

Although we had reported the remarkable low threshold emission from polycrystalline diamond heavily doped with nitrogen (N) [Nature 381 (1996) 140], the problems caused by polycrystallinity still remain for understanding the electron emission mechanism. This paper describes the growth of N-doped homoepitaxial diamond film {100}, {111} and {110}, and their electron emission properties. N-doped homoepitaxial diamond is grown on synthetic diamond by hot filament chemical vapor deposition. Urea [(NH₂)₂CO] is used as a dopant for N. Atomic force microscope (AFM) observations indicate that the relatively smooth surface morphologies are obtained for all the films. The epitaxial growth of all the film is confirmed using reflective high energy electron diffraction (RHEED) patterns. Reflective electron energy loss spectra (REELS) indicate that the very surfaces of {100} and {111} are diamond while {110} is graphite rather than diamond. Raman spectra suggest that the bulk of the obtained films are diamond. The resistivities of the films are found to be much higher than the detection limit of the system. The relatively low threshold emission was observed even from the smooth surface and the threshold voltage is confirmed to depend on the crystal orientation. It is speculated from the film characterizations and the electron emission properties that the low threshold emission is due to high resistance rather than rough surface and/or grain boundaries.     

Atomistic Simulation of the Surface Energy of Spinel MgAl2O4

Atomistic simulations with atomic potentials including anion polarizibility have been performed for the low-index surfaces of spinel MgAl₂O₄ with various terminations. The calculations show that for the most stable surface the surface energy is 2.27 J/m² for the {100}, about 2.85 J/m² for the {110}, and 3.07 J/m² for the {111} orientation. The ratio between the experimental values to the calculated relaxed surface energies is about 1.5. Strong surface relaxation was found for the {110} and {111} orientation but only moderate surface relaxation for the {100} surface.     

Electrocrystallization of copper on single crystal spheres—I. Morphology of the deposit

The electrodeposition of copper from an acid copper bath on single crystal copper spheres in the current density range of 1 to 20 mA/cm² results in characteristic growth structures which depend on the underlying base crystal. These growth structures are dominantly determined by a nearby principal pole representing one of the densest planes of copper, {100}, {110} and {111}. In the region of a {100} pole, the structure is one of layers with faces parallel to the {100} plane. The {110} poles are surrounded by a ridged structure and the {111} poles by triangular pyramids. Between these are transition regions of little distinguishable structure. The areas of the sphere occupied by the various structures depend upon the current density.     

On the Structure Sensitivity of Direct NO Decomposition over Low-Index Transition Metal Facets

We present a study of the dissociative chemisorption of NO, O₂, and N₂ over close-packed, stepped, kinked, and open (fcc {111}, {211}, {311}, {532}, {100}, and {110}) transition metal facets using density functional theory (DFT). The offset of the Brønsted–Evans–Polanyi (BEP) relations suggest that the {111} surface is the least reactive, and that the {110} surface is the most reactive. This observation is verified by establishing volcano-relations based on mean-field microkinetic models for each facet, showing that the maximum rate over {110} is 4 orders of magnitude larger than the maximum {111} rate. The ordering of the maximum activity over the facets is: {110} > {100} ~ {532} > {311} ~ {211} > {111}, which is in general agreement with the offset in the BEP relations. We show that the top-point location and shape of the volcano relations are approximately independent of facet. This observation lends credibility to the approach of analyzing trends in catalytic reactivity over a single low-index facet, and assuming the experimentally observed activity trends are qualitatively well-described by such a single-facet analysis. Our study suggests that a key element for generally obtaining quantitative agreement between theory and experiments is for the simulations to address in detail the propensities of the various types of active sites. Finally, we show that the ordering of NO decomposition rates among metals and facets is essentially unaltered when using BEP- and scaling relations in the microkinetics instead of explicit DFT calculations for each elementary reaction step, and that using a “universal” BEP relation introduces no significant additional qualitative error in trend prediction.     

Computer Simulation of Dissociative Adsorption of Water on the Surfaces of Spinel MgAl2O4

Atomistic simulation techniques have been used to model the dissociative adsorption of water onto the low-index {100}, {110}, and {111} surfaces of spinel MgAl₂O₄. The Born model of solids and the shell model for oxygen polarization have been used. The resulting structures and chemical bonding on the clean and hydrated surfaces are described. The calculations show that the dissociative adsorption of water on the low-index surfaces is generally energetically favorable. For the {110} and {111} orientations, the surfaces cleaved between oxygen layers show high absorption and stability. The calculations also show that, for the {111} orientation, the surfaces may absorb chemically water molecules up to ∼90% coverage and have the highest stability. It is suggested that, during fracture, only partial hydration occurs, leading to cleavage preferentially along the {100} orientation.     

Bi12SiO20晶体的生长习性

分别沿[001],[110]及[111]3种方向用提拉法生长Bi_(12)SiO_(20)晶体,研究了生长条件对晶体形态的影响。应用PBC理论,分析了各晶面的特性:{100}和{110}为F面,{211}为S面,{111}属于K面。并依据连接能的计算,得到晶面的重要性顺序。PBC解析形态与在特定条件下生长的晶体形态相当一致。     

High resolution mapping of surface reduction in ceria nanoparticles

Surface reduction of ceria nano octahedra with predominant {111} and {100} type surfaces is studied using a combination of aberration-corrected Transmission Electron Microscopy (TEM) and spatially resolved electron energy-loss spectroscopy (EELS) at high energy resolution and atomic spatial resolution. The valency of cerium ions at the surface of the nanoparticles is mapped using the fine structure of the Ce M(4,5) edge as a fingerprint. The valency of the surface cerium ions is found to change from 4+ to 3+ owing to oxygen deficiency (vacancies) close to the surface. The thickness of this Ce(3+) shell is measured using atomic-resolution Scanning Transmission Electron Microscopy (STEM)-EELS mapping over a {111} surface (the predominant facet for this ceria morphology), {111} type surface island steps and {100} terminating planes. For the {111} facets and for {111} surface islands, the reduction shell is found to extend over a single fully reduced surface plane and 1-2 underlying mixed valency planes. For the {100} facets the reduction shell extends over a larger area of 5-6 oxygen vacancy-rich planes. This finding provides a plausible explanation for the higher catalytic activity of the {100} surface facets in ceria.     

Plasticity and Creep in Single Crystals of Zirconium Carbide

High-temperature deformation in ZrC single crystals was studied. Seeded crystals were grown by a direct rf-coupling floating-zone process. Yield stresses were measured from 1080° to 2000°C as a function of stress axis orientation. The Burgers vector was shown to be parallel to the 〈110〉 axes by transmission electron microscopy. Slip was observed on {100}, {110}, or {111} planes, depending on the orientation of the stress axis; it always occurred on the most favorably oriented slip system. The dependence of steady-state creep rate on the applied stress indicated that recovery occurred by a dislocation climb mechanism. Examination of the dislocation structure in deformed crystals by transmission electron microscopy supported this conclusion.     

Influence of surface orientation on the photochemical reactivity of CaTiO3

The orientations and surface structures of ferroelastic CaTiO₃ crystals within a polycrystalline ceramic were measured by electron backscatter diffraction and atomic force microscopy (AFM) respectively. Surfaces annealed at 1250°C in air are made up of a combination of facets with orientations near {100}, {110}, and {111} (indexed with reference to the cubic perovskite cell). The samples were immersed in an aqueous AgNO₃ solution and illuminated to photochemically reduce silver. Based on the amount of silver reduced on different surfaces, it was determined that surfaces with a {111} orientation had the greatest photocathodic reactivity, those with a {110} orientation had the least photocathodic reactivity, and surfaces with the {100} orientation had intermediate reactivity. While the {110} surfaces were essentially inert for photocathodic reduction, they were active for the photoanodic oxidation of Mn²⁺. Ferroelastic domains were observed to enhance and retard the photochemical reduction of silver on spatially alternating domains on 20% of the grains. However, domains had no influence on the reactivity of the other 80% and are therefore secondary to the surface orientation in determining the reactivity.     

Controlled Epitaxial Nucleation of Nickel Oxide on Microfabricated Magnesium Oxide Substrates in a CVD Process

It is shown that the nucleation of nickel oxide crystals grown by CVD on MgO depends on the surface structure of the substrates. On freshly cleaved {100} surfaces of MgO, nucleation occurs readily on cleavage steps. The crystallographic orientation of the surfaces of these steps is {110}, and their height ranges from 1 to 8 μm. In contrast, nucleation is sparse on highly polished {100} surfaces having a step height of less than 0.02 μm. Nucleation on polished {111} surfaces is also poor. However, highly polished {110} surfaces show profuse nucleation under similar growing conditions. This preference for nucleation on {110} surfaces and on the step height was used to obtain periodically spaced nuclei of NiO crystals by etching and microlithography of the (100) MgO surface which exposed the {110} surfaces at periodic locations. In this way the spacing of the nuclei could be controlled from 10 μm up to 100 μm.     

Structural study of β‐SiC(001) films on Si(001) by laser chemical vapor deposition

β‐SiC thin films have been epitaxially grown on Si(001) substrates by laser chemical vapor deposition. The epitaxial relationship was β‐SiC(001){111}//Si(001){111}, and multiple twins {111} planes were identified. The maximum deposition rate was 23.6 μm/h, which is 5‐200 times higher than that of conventional chemical vapor deposition methods. The density of twins increased with increasing β‐SiC thickness. The cross section of the films exhibited a columnar structure, containing twins at {111} planes that were tilted 15.8° to the surface of substrate. The growth mechanism of the films was discussed.     

亚微米LiFe0.05Mn1.95O4正极材料的制备及电化学性能

采用固相燃烧法合成了亚微米单晶多面体LiFe_0.05Mn_1.95O₄正极材料。借助XRD、FE-SEM、TEM、XPS和恒电流充放电等手段对样品的结构、形貌、物相组成和电化学性能进行测试。结果表明,Fe掺杂未改变尖晶石型LiMn₂O₄的立方晶系结构,其{400}和{440}衍射峰相对应的晶面出现显著的择优生长,形成了形貌为{111}、{110}和{100}晶面的单晶去顶角八面体晶粒。LiFe_0.05Mn_1.95O₄正极材料表现出比纯LiMn₂O₄材料更为优异的电化学性能,在1C和5C时有着114.7mA·h/g、104.7mA·h/g首次放电比容量,10C倍率下经1000次循环后,容量保持率为83.9%。循环伏安与阻抗分析得出掺杂后的样品有着较大的锂离子扩散系数与较小的活化能。对5C倍率循环1000次后Fe掺杂样品的极片分析发现,其晶体结构基本无变化,适量的Fe掺杂能够有效抑制尖晶石型LiMn₂O₄在充放电循环过程中的Jahn-Teller效应以及Mn的溶解,提升材料的结构稳定性与容量保持率。     

The Wulff Shape of Alumina: III, Undoped Alumina

Controlled-geometry cavities were introduced into the m{10     0} plane of undoped sapphire substrates using photolithographic methods, and subsequently internalized by diffusion bonding the etched sapphire to an undoped high-purity polycrystalline alumina. Pore-boundary separation during growth of the sapphire seed into the polycrystal entrapped the pores within the single crystal. Pores with an equivalent spherical radius of ≈1 μm reached a quasi-equilibrium shape after prolonged anneals at 1600° and 1800°C. The introduction of mechanically induced surface defects accelerated pore shape equilibration. The Wulff shape of undoped alumina was determined by characterizing the shape and facet structure of these equilibrated internal pores using optical microscopy, scanning electron microscopy, and atomic force microscopy. The observed planes in the Wulff shape of undoped alumina, c(0001), r{     012}, s{1     01}, a{11     0}, and p{11     3} planes, were consistent with those reported by Choi et al .; however, a different energy sequence is inferred. The absence of the m-plane in the Wulff shape is consistent with other experimental studies, but inconsistent with those lattice simulations that predict the m-plane to be one of the lowest energy planes in pure alumina. A comparison of Wulff shapes at 1600° and 1800°C suggests that the surface energy of undoped alumina becomes more isotropic as temperature increases.