Transformation of the habitus of electrolytic copper microcrystals with inhibition of the growth of low-energy facets

Experimental evidence of the possibility of controllable transformation of the habitus of microcrystals formed under electrodeposition of copper has been presented. A premeditated change in the chemical composition of the standard sulfuric electrolyte results in the prevalent evolution of the microcrystal facets with an alternative crystallographic orientation. The analysis of the experimental results has revealed the formation of needle-like pentagonal microcrystals with a self-similar structure.     

Nucleation of Polypropylene Crystallization with Gold Nanoparticles. Part 2: Relation between Particle Morphology and Nucleation Activity

The influence of gold nanoparticle morphology on nucleation of isotactic polypropylene (PP) crystallization was investigated. Previous experiments indicated certain nucleation activity of gold nanoparticles, varying with their size. In this work, eight types of gold micro/nanoparticles were used: vacuum-sputtered nanostructures (nanoparticles, nanoislands, and nanolayers), chemically prepared isometric gold nanocrystals (5, 20, and 100 nm diameters), and two types of gold microcrystals with well-developed crystal facets [with (100) and (111) facets, respectively]. To minimize the effect of particle agglomeration, we used our recently introduced sandwich method, in which the nucleating agent was deposited between thin PP films and the nucleation was evaluated by polarized light microscopy (PLM), X-ray scattering (WAXS), and differential scanning calorimetry (DSC). The nucleation activity of Au particles in PP was lower than it might be expected from the previous studies and depended on their morphology. The nucleation activity of Au microcrystals with well-developed facets was higher than the activity of non-faceted Au nanocrystals.     

On the Reasons for the Formation and Stability of Single-Component Microcrystals of Electrolytic Origin with Fullerene-Like Truncated Icosahedral Habit

JETP Letters - The formation of microcrystals with fullerene-like truncated icosahedral habit observed in experiments on the electrodeposition of copper has been explained. The analysis has been...     

Evolution of the morphology of small pentagonal particles and microcrystals during the electrodeposition of silver in a potentiostatic regime

A procedure for obtaining icosahedral and decahedral small particles and microcrystals of silver is presented. The evolution of the morphology during growth is studied in detail. Mechanisms of the relaxation of elastic stress fields in small silver particles and microcrystals with pentagonal symmetry are found experimentally.     

Voids in icosahedral small particles of an electrolytic metal

Results of an experiment aimed at the detection of voids in small icosahedral particles of electrolytic copper are presented. The chosen experimental technique is theoretically justified. The presence of voids revealed in the experiment gives strong evidence that the formation of small particles and microcrystals with pentagonal symmetry is governed by the disclination mechanism.     

Broadband laser emission from color centers inside MgO microcrystals

We report room-temperature broadband laser emission in the near-ultraviolet to the blue-green spectral range using color centers in MgO microcrystals. The lasing MgO microcrystals are obtained through a solid phase reaction between SiO and Mg at 450 degrees C in an argon atmosphere and are mostly composed of an accumulation of microcubes enclosed by [100] facets. The laser action was observed in the wavelength region from 350 to 600 nm without using cavity mirrors. In the present MgO microcrystals, some of the color centers will be stabilized at the interfaces and/or boundaries of the microcrystalline domains, probably explaining the stable laser action even at room temperature.     

微晶的磁性

在穆斯堡尔谱学的实验结果基础上讨论了微晶的磁性。微晶的尺寸小于10um时,穆斯堡尔谱将受到磁化方向起伏,即超顺磁弛豫与集体磁激发的影响。这些效应可用来确定颗粒体积与磁各向异性常数二者之积。测量铁磁、亚铁磁颗粒穆斯堡尔谱随外磁场的变化可以确定颗粒的体积。当铁、钴、镍以及Fe_3O_4颗粒表面化学吸附不同的分子时,微晶的磁晶各向异性常数将随之改变。细颗粒的穆斯堡尔谱亦给出了表面层原子磁性的信息。α-Fe颗粒表面层原子的超精细场大于块状样品的值。FeCo合金颗粒的表面是富铁层。α-Fe_2O_3的Morin转变温度与Fe_3O_4的Verwey温度均发现随颗粒尺寸减小而降低。α-FeOOH微晶密堆积体的研究表明,这些微晶间的磁耦合显著地影响穆斯堡尔谱。     

Optical properties of a polymer host matrix containing mercuric iodide microcrystals

Microcrystals of mercuric iodide have been grown in a polymer matrix with pores 200 nm in diameter. The absorption and luminescence spectra indicate the formation of microcrystals of two (red and orange) modifications in the host matrix. The temperature dependence of the emission spectra and the evolution of absorption and emission spectra in the course of sample ageing have been studied. The evolution of the spectra is governed by the transition from the orange to red modification and the increase in size of microcrystals.     

Amorphous-like Raman spectra of semiconductor microcrystals

The size dependence of Raman scattering from gas-evaporated Si, Ge and GaP microcrystals, for which a free boundary condition can be assumed, has been investigated. As the microcrystals become sufficiently small (smaller than about 100A), spectra very similar to those of amorphous materials are observed, even though electron microscopy proves that they are crystalline. The amorphous-like Raman signals are believed to come from the surface layers of microcrystals.     

Evolution with time of the dielectric properties of dispersed ice microcrystals

Summary The evolution with time of the characteristics of the dielectric relaxation of water molecules in dispersions of ice microcrystals has been studied by means of the thermally stimulated depolarization current (TSDC) method. With advancing stage of evolution,i.e. with increasing preservation time at constant preservation temperature and with increasing preservation temperature at constant preservation time, the mean values of relaxation time and activation energy have been found to decrease. The extent and the rate of the evolution have been compared to those in other forms of ice: ice samples with high content of disturbance, polycrystalline ice samples and single ice crystals. The observed similarities and differences in the behaviour of the different ice forms have been discussed in terms of the influence of extrinsic physical and chemical defects on the generation and motion of orientational Bjerrum defects.     

Aggregates of chemisorbed copper on the (110) and (100) surfaces of tungsten: electronic structure

In a certain range of temperature and coverage, copper atoms chemisorbed on low-index facets of a thermally annealed tungsten field emitter aggregate to form stable islands surrounded by regions of extremely low adsorbate density, yielding a spatially inhomogeneous overlayer. Spatially resolved field emission and photofield emission measurements of the surface density of electronic states have been carried out for copper aggregates on the (110) and (100) facets. Features detected in the surface density of states near the centre of an aggregate yield clues to its structure. On the (110) facet, the measurements reveal striking differences between the surface density of states near the edge of an aggregate and that near the centre. It is argued that the adsorbate configuration in the region between the aggregates might approximate single-atom adsorption.     

Ultrasonically controlled growth of monodispersed octahedral BiVO4 microcrystals for improved photoelectrochemical water oxidation

The synthesis of facet-controlled structures with precise morphology and exposed reactive surface is one of the key research challenges. We effectively endeavoured to obtain the monodisperse octahedral bismuth vanadate microcrystals with exposed {1 0 1},{2 0 0},{3 1 2} and {0 2 1} dominant facets through an optimized sonochemical assisted hydrothermal process. A pulse sonication (5-s ON and 2-s OFF cycle, 21 W ultrasonic power and 20 kHz ultrasonic frequency) for 30 mins followed by 1 h hydrothermal treatment was found to yield the preferred octahedral morphology. The microscopic and X-ray analysis suggested a potent role of ultrasonic waves for the initial seed growth and its evolution into a well-defined monodisperse microcrystals. The density functional theory (DFT) calculations revealed strongly localized bandgap states with a bandgap of ~2.47 eV. The PEC measurements for water oxidation demonstrated the efficacy of these microcrystals as photoanode. Notably, the optimized octahedral BiVO₄ microstructure exhibited a superior performance as evident from photocurrent density ~0.9 mAcm⁻² at 1.23 V vs. RHE and %IPCE value of ~22% compared to analogous photoanodes under visible light irradiation.     

Equilibrium Shape of Copper Crystals Grown on Sapphire

The equilibrium crystal shape (ECS) of copper has been studied by scanning electron microscopy on m-sized copper crystallites supported on single-crystals of -alumina. In addition, the orientation relationships between copper crystals and the sapphire substrate were investigated by X-ray techniques. A detailed discussion of the kinetic factors that can inhibit equilibration is provided, and it is shown that only crystals ranging in radius from 3 to 4.5 m can achieve equilibrium shapes under the conditions of the experiment. The maximum anisotropy of surface energy was found to be about 1.02, which is significantly lower than that of the other two fcc metals (lead and gold) for which reliable data are available. Another distinction between copper and those other fcc metals is that its ECS displays {110} facets, and possibly {311} facets, in addition to the commonly observed {111} and {100} facets, at temperatures where equilibration is possible. The observed facets connect tangentially to the curved parts of the ECS, so that all possible surface orientations are present on the copper ECS.     

Recombination instability of dust plasma of a non-self-sustained discharge in the collision regime

Recombination instability of a dust plasma of a non-self-sustained discharge maintained by an ionizing radiation beam has been studied for the conditions when the collisions of ions with atoms in the charged layer of a dust particle considerably affect the ion current to the particle. It has been shown that the collisions of ions elevate the stability of the plasma. The dependence of the conditions for the evolution of instability and its increment on the discharge parameters has been analyzed.     

低维Bi2Fe4O9晶体的控制生长及其光催化性能研究

Low-dimensional Bi₂Fe₄O₉ nanosheets and microrods have been selectively prepared by a solvothermal method, from which the growth of the Bi₂Fe₄O₉ crystals can be controlled by the variation of reaction conditions. Structure determination showed that the nanosheets are mainly exposed by {001} facets while the microrods are exposed by {110} facets. Ab- sorption spectra revealed that there are two bandgaps observed for both nanosheets (at 1.9 and 1.55 eV) and microrods (1.7 and 1.45 eV), and they both would be available for the sunlight photocatalysis e ciently due to the intensive absorption ability in a wide region. Photocatalytic investigation demonstrated that the overall photocatalytic performance of the microrods is prior to that of the nanosheets due to the variation of bandgaps and exposed facets. The present report provides a useful alternative strategy for the controlling growth of nanostructures and/or microcrystals besides the present demonstration of the Bi₂Fe₄O₉ crystals with diflerent bandgaps and facets that would be able to tune the corresponding photocatalytic ability selectively.     

Size and temperature dependence of the mössbauer debye waller factor of iron microcrystals

Iron microcrystal ranging in size from 70 to 450 Å have been investigated by the Mössbauer effect. There is, compared to the bulk value, a small enhancement of the magnetic field at the site of the nucleus of about 1% which can be interpreted as due to the demagnetisation field of the microcrystals and their geometric arrangement. The observed drastic size and temperature dependent lowering of the Debye Waller factor cannot be explained by a change of the Debye temperature of the microcrystals. Two different models are proposed to explain the effect.     

Self-aligned Ge and SiGe three-dimensional epitaxy on dense Si pillar arrays

In this report we present a novel strategy in selective epitaxial growth on top of Si pillars, which results in a tessellated Ge film, composed by self-aligned micron-sized crystals in a maskless process. Modelling by rate equations the morphology evolution of fully facetted crystal profiles is extensively outlined, showing an excellent prediction of the peculiar role played by flux shielding among microcrystals, in the case of dense array configuration. Crack formation and substrate bending, caused by the mismatch in thermal expansion coefficients, are eliminated by the mechanical decoupling among individual microcrystals, which are also shown to be dislocation- and strain-free. The method has been also tested for Si_1?xGe_x alloys, with compositions ranging from pure silicon to pure germanium. There are ample reasons to believe that this approach could be extended to other material combinations and substrate orientations, actually providing a technology platform for several device applications.     

Magnetic hyperfine splitting in mössbauer spectra of microcrystals

Below the superparamagnetic blocking temperature of a microcrystal the magnetization direction is in general not fixed, but fluctuates in directions close to an easy direction of magnetization. Such fluctuations (collective magnetic excitations) result in a reduction in the magnetic splitting of the Mössbauer spectrum. A low-temperature approximation for this reduction is derived for microcrystals with arbitrary magnetic energy. Moreover, explicit expressions are presented for particles with special types of magnetic anisotropy and for particles exposed to external magnetic fields. The reduction in the magnetic hyperfine field has its maximum value just below the blocking temperature but does not exceed 5–15% in isolated particles with uniaxial anisotropy in zero applied magnetic field. However, ferro- and ferrimagnetic particles, exposed to external magnetic fields, and particles for which exchange anisotropy is predominant, may exhibit any magnetic hyperfine splitting between zero and the saturation value. It is shown that in special cases an assembly of microcrystals in close contact with each other may behave like a spin-glass. We discuss how studies of the magnetic hyperfine splitting ofMössbauer spectra of microcrystals give information of the particle size and the prevailing magnetic anisotropies.     

Observation of higher order facets on 3He crystals

Faceting has been observed on 3He crystals investigated with a low-temperature Fabry-Pérot interferometer. Nine types of facets were clearly identified during growth of a bcc- 3He single crystal at a temperature of 0.55 mK, while previously only three types of facets have been seen. Because of the weak coupling between the liquid-solid interface and the solid lattice in 3He the facets are apparently too small to be observed in equilibrium. The number of facets observed in our experimental conditions is consistent with the theory of dynamic roughening.     

Surface of the BaFe12O19 crystal and its influence on the interparticle magnetic interaction

Physics of the Solid State - The influence of the physicochemical state of the particle surface on the interparticle magnetic interaction in a close-packed system of single-domain microcrystals of...