Single-chain single crystals

Single-chain single crystals of isotactic polystyrene and poly(ethylene oxide) were studied by using transmission electron microscopy, high resolution electron microscopy, electron diffraction. Single-chain single crystals were prepared by spreading a dilute solution of polymers on a water surface and collecting the resulting single-chain particles on copper grids, followed by isothermal crystallization. A statistical analysis of the sizes of single-chain crystals was found to match with the known molecular weight distribution of original sample, indicating the particles to be composed of single chain. Observation of the morphology and electron diffraction gave evidence of the single crystal nature. Regular-shaped single-chain crystals were obtained after isothermal crystallization for a longer time. By close observation, several types of morphologies were found for single-chain crystals of isotactic polystyrene and poly(ethylene oxide); in addition to the conventional morphologies observed for multi-chain crystals, new morphologies were observed in both cases. The morphologies of poly(ethylene oxide) were explained according to the crystal structure and twin modes. Tent-like single-chain crystals were often observed. Because of the small size of the crystals, they can avoid collapse on the substrate. The crystalline c-axis of single-chain crystals were found to orient preferably in the direction normal to the substrate. The investigation of electron diffraction and high resolution electron microscopy revealed that the structure of the single-chain crystals of isotactic polystyrene is the same as for multi-chain crystals. A reasonable explanation is given for the unusual resistance to electron irradiation and the missing of lower-index reflections. Regular periodic stripes were found on the top surface of single-chain crystal of isotactic polystyrene with an average periodic length in accordance with (220) spacing. In addition, a statistical thermodynamics theory was developed for single-chain crystal. It is found that the equilibrium dimensions are related to molecular weight and annealing temperature, while the equilibrium melting temperature depends on molecular weight.     

碳化硅单晶材料的溶剂热合成与表征

SiC single crystals have been prepared by the method of solvothermal synthesis with a system of SiCl4, CCl4 and metal K in an auto clave. X-ray diffraction (XRD), Raman spectra and transmission electron microscopy(TEM) were used to characterize the products. XRD reveals that the products are SiC crystals and TEM exhibits that SiC single crystal sofwires and platelets are obtained under different usages of metal K. The SiC wires have diameters of 10～20 nm and length up to 1.5μm; the platelets have lateral dimensions of 0.1～3 μm, exhibiting regular polygonal shapes and step-bunched side surface. Furthermore, the growth mechanism of the SiC single crystals is discussed and the effect of super saturation on the crystal growth and morphology is also investigated.     

A Chiral Self‐Assembled Monolayer Derived from a Resolving Agent and its Performance as a Crystallization Template for an Organic Compound from Organic Solvents

A new chiral nonracemic thiol derived from a popular acidic resolving agent that incorporates a cyclic disubstituted phosphate group (phencyphos) has been prepared in enantiomerically pure form. The stereochemistry and absolute configuration were established by performing a single‐crystal X‐ray structural analysis of a synthetic intermediate. The thiol compound was used for the preparation of self‐assembled monolayers (SAMs) on both monocrystalline and polycrystalline metallic gold, which have very different surface roughness. The monolayers were used to promote the nucleation and growth of crystals from nonaqueous solutions of an organic molecule (the parent phencyphos) of similar structure to the compound present in the monolayer. The template layers influence the nucleation and growth of the phencyphos crystals despite the lack of two‐dimensional order in the surfaces. Heterogeneous nucleation of phencyphos takes place upon evaporation of either CHCl₃ or isopropanol solutions of the compound on the SAM surfaces, where the evaporation rate merely influences the size and homogeneity of the crystals. The roughness of the surface also plays an important role; the polycrystalline gold produces more homogeneous samples because of the greater number of nucleation sites. Clear evidence for nucleation and growth on the surfaces is shown by scanning electron microscopy. The variation in crystal form achieved by using different surfaces and solvents suggests that the layers are applicable for the preparation of organic crystals from organic solutions.     

Crystallization of isotactic polystyrene from solution

Difficulties previously encountered in the growth of chain-folded single crystals of isotactic polystyrene suitable for study by electron microscopy and electron diffraction have been overcome using very poor solvents (including atactic polystyrene of low molecular weight). The hexagonal lamellar crystals produced are relatively stable under electron bombardment and, as a consequence, dark-field moiré patterns produced by double diffraction from overlapping layers are easy to study. These patterns show no evidence of differences in lattice spacing between fold and nonfold planes such as have been reported in single crystals of several other polymers. Such differences were attributed to congestion at fold surfaces and their absence in polystyrene, for which the surface energy of fold surfaces is small, supports this interpretation. A comparison of crystallization kinetics of polystyrene crystals grown from good and from poor solvents reveals differences in growth rates of three or more orders of magnitude at comparable supercoolings. This disparity cannot be accounted for by acceptable adjustments of thermodynamic parameters in current theories of crystallization with chain folding. The role of molecular conformation in solution appears to exert an unexpectedly large influence on crystallization rate.     

Controlled‐Atmosphere Flame Fusion Single‐Crystal Growth of Non‐Noble fcc,hcp, and bcc Metals Using Copper,Cobalt, and Iron

The growth of noble‐metal single crystals via the flame fusion method was developed in the 1980s. Since then, there have been no major advancements to the technique until the recent development of the controlled‐atmosphere flame fusion (CAFF) method to grow non‐noble Ni single crystals. Herein, we demonstrate the generality of this method with the first preparation of fcc Cu as well as the first hcp and bcc single crystals of Co and Fe, respectively. The high quality of the single crystals was verified using scanning electron microscopy and Laue X‐ray backscattering. Based on Wulff constructions, the equilibrium shapes of the single‐crystal particles were studied, confirming the symmetry of the fcc, hcp, and bcc single‐crystal lattices. The low cost of the CAFF method makes all kinds of high‐quality non‐noble single crystals independent of their lattice accessible for use in electrocatalysis, electrochemistry, surface science, and materials science.     

Study of the Interface Microstructures of CVD Diamond Films by TEM

 The characteristics of the interface microstructures between a CVD diamond film and the silicon substrate have been studied by transmission electron microscopy and electron energy loss spectroscopy. The investigations are performed on plan-view TEM specimens which were intentionally thinned only from the film surface side allowing the overall microstructural features of the interface to be studied. A prominent interfacial layer with amorphous-like features has been directly observed for CVD diamond films that shows a highly twinned defective diamond surface morphology. Similar interfacial layers have also been observed on films with a <100> growth texture but having the {100} crystal faces randomly oriented on the silicon substrate. These interfacial layers have been unambiguously identified as diamond phase carbon by both electron diffraction and electron energy loss spectroscopy. For the CVD diamond films that exhibit heteroepitaxial growth features, with the {100} crystal faces aligned crystallographically on the silicon substrate, such an interfacial layer was not observed. This is consistent with the expectation that the epitaxial growth of CVD diamond films requires diamond crystals to directly nucleate and grow on the substrate surface or on an epitaxial interface layer that has a small lattice misfit to both the substrate and the thin film material.     

Assisted desolvation as a key kinetic step for crystal growth

The crystallization of materials from a supersaturated solution is a fundamental chemical process. Although several very successful models that provide a qualitative understanding of the crystal growth process exist, in most cases the atomistic detail of crystal growth is not fully understood. In this work, molecular dynamics simulations of the morphologically most important surfaces of barite in contact with a supersaturated solution have been performed. The simulations show that an ordered and tightly bound layer of water molecules is present on the crystal surface. The approach of an ion to the surface requires desolvation of both the surface and the ion itself leading to an activated process that is rate limiting for two-dimensional nucleation to occur. However, desolvation on specific surfaces can be assisted by anions adsorbed on the crystal surface. This hypothesis, corroborated by crystallization and scanning electron microscopy studies, allows the rationalization of the morphology of barite crystals grown at different supersaturations.     

In situ study of colloid crystallization in constrained geometry

We visualized in real time electrodeposition-driven colloid crystal growth on patterned conductive surfaces. The electrode was patterned with dielectric ribs and conductive grooves; the groove width was commensurate or incommensurate with a two-dimensional colloid crystal lattice. Electrodeposition was carried out against gravity to decouple sedimentation and electrodeposition of colloid particles. Our experiments reveal the following: (i) Colloid crystal growth occurs under the action of electrohydrodynamic forces, in contrast with colloid assembly under the action of capillary forces. (ii) Confinement of the colloid arrays reduces the size of particle clusters. Small clusters easily undergo structural rearrangements to produce close-packed crystals when the groove width is commensurate or nearly commensurate with the 2D lattice. (iii) Incommensurability between the two-dimensional crystalline lattice and the groove width exceeding ca. 15% leads to the formation of non-close-packed structures and the distortion of colloid arrays.     

Vapour-growth of Sn on stearic acid molecular layers

Tin metal was vacuum deposited at room temperature on to Langmuir–Blodgett (LB) films with surfaces of either hydrophilic head groups or hydrophobic tail groups. Different growth modes on different surfaces of the LB films were observed with an atomic force microscope. Fine Sn particles deposited on the hydrophobic surface were uniform in size and similar in shape, but on the hydrophilic surface large Sn particles were observed. Chemical interactions between organic functional groups and deposited metal seems critical for the manner of crystal growth. The possibility of control over the crystallization of metals using two-dimensionally assembled organic molecules is demonstrated.     

Structures and Structural Evolution of Sublayer Surfaces of Metal–Organic Frameworks

The structural characterization of sublayer surfaces of MIL-101 is reported by low-dose spherical aberration-corrected high-resolution transmission electron microscopy (HRTEM). The state-of-the-art microscopy directly images atomic/molecular configurations in thin crystals from charge density projections, and uncovers the structures of sublayer surfaces and their evolution to stable surfaces regulated by inorganic Cr₃(μ₃-O) trimers. This study provides compelling evidence of metal–organic frameworks (MOFs) crystal growth via the assembly of sublayer surfaces and has important implications in understanding the crystal growth and surface-related properties of MOFs.     

Production and structural characterization of crystalline silver nanoparticles from Bacillus cereus isolate

To date, biosynthesis of metal nanoparticles has been intensively studied using bacteria and fungi. We have isolated and identified metal resistant bacterial strains from electroplating industries, they produce silver nanoparticles. The reduction reaction of aqueous silver nitrate with bacterial biomass was carried out for 120 h. Bacteria produced metallic nanoparticles showed a strong absorbance at surface plasmon resonance wavelength around 420 nm. The size and morphology of these nanoparticles were typically imaged using high resolution transmission electron microscopy, the particles size ranges between 4 and 5 nm and are spherical in shape. The crystal structure of the particles was characterized by X-ray diffraction pattern. The full width half maxima from X-ray diffraction measurements indicated that the particles exhibited face-centered cubic phase.     

Enzymatic degradation of poly(octamethylene suberate) lamellar crystals

Enzymatic degradation of poly(octamethylene suberate) single crystals was investigated by electron microscopy. Different lamellar morphologies were obtained using 2.5-hexanediol as a solvent and at a temperature between 42 and 51 °C. Crystals with a different degree of truncation and with monolayer or bilayer organization were analyzed. Lipases from Rhizopus oryzae were found to be highly effective in degrading crystalline domains and showed different attack mechanisms. Thus, enzymes preferentially attacked the lateral crystal growth faces or the lamellar fold surfaces depending on the crystallization conditions. Temperature and indeed its fluctuation during the crystallization process were crucial to determine how degradation started and progressed. The most interesting results were obtained for single crystals characterized by a low degree of truncation and formed in crystallization baths with a small temperature oscillation. In this case, it was shown that degradation started on the folding surface of specific sectors and progressed along a preferred crystal direction.     

Microstructural investigations of tin-molybdenum oxides by high-resolution electron microscopy

Tin-molybdenum oxides, formed by the calcination of precipitates in air, have been examined by high-resolution electron microscopy. Low-temperature calcination gives rise to the formation of small tin(IV) oxide-type crystals amidst an amorphous material whereas higher-temperature treatment results in the development of a highly crystalline rutile-related phase composed of larger particles. High concentrations of molybdenum in the initial precipitates inhibits the thermally induced crystal growth. The common occurrence of superficial disorder in the larger particles is associated with surface damage resulting from the volatilization of excess molybdenum as molybdenum(VI) oxide. Planar faults were frequently observed within the particles and, in some cases, these defects were identified as twin boundaries enriched with molybdenum. The formation of these planar faults is discussed in terms of the preparative procedure.     

Selective graphene formation on copper twin crystals

Selective graphene growth on copper twin crystals by chemical vapor deposition has been achieved. Graphene ribbons can be formed only on narrow twin crystal regions with a (001) or high-index surface sandwiched between Cu crystals having (111) surfaces by tuning the growth conditions, especially by controlling the partial pressure of CH(4) in Ar/H(2) carrier gas. At a relatively low CH(4) pressure, graphene nucleation at steps on Cu (111) surfaces is suppressed, and graphene is preferentially nucleated and formed on twin crystal regions. Graphene ribbons as narrow as ～100 nm have been obtained in experiments. The preferential graphene nucleation and formation seem to be caused primarily by a difference in surface-dependent adsorption energies of reactants, which has been estimated by first principles calculations. Concentrations of reactants on a Cu surface have also been analyzed by solving a diffusion equation that qualitatively explains our experimental observations of the preferential graphene nucleation. Our findings may lead to self-organizing formation of graphene nanoribbons without reliance on top-down approaches in the future.     

Polymers designed to control nucleation and growth of inorganic crystals from aqueous media

Water soluble graft polymers prepared by copolymerization of either methacrylic acid (MAA) or vinylsulfonic acid (VS) with α‐methoxy‐ω‐methacroyl‐oligo(oxyethylene)s (PEO_n‐MA) serve to control nucleation and crystal growth during precipitation of inorganic crystals from aqueous media. Precipitation of zinc oxide crystals (‘zincite’) is used as example for such mineralization processes. Homogeneous and narrow crystal size distributions are obtained in presence of ppm‐amounts of graft copolymers. Copolymer is incorporated into the crystals demonstrated by using latex particles with ‐CO₂H‐group rich surfaces as controlling additives. Incorporation of these particles leads to single crystals with pores of the size of the particles (‘Swiss cheese’ morphologies).     

Microcontact click printing for templating ultrathin films of metal-organic frameworks

The controlled growth of metal-organic frameworks (MOFs) over surfaces has been investigated using a variety of surface analytical techniques. The use of microcontact printing to prepare surfaces, patterned with regions capable of nucleating the growth of MOFs, has been explored by employing copper-catalyzed alkyne-azide cycloaddition (CuAAC) to pattern silicon wafers with carboxylic acids, a functional group that has been shown to nucleate the growth of MOFs on surfaces. Upon subjecting the patterned silicon surfaces to solvothermal conditions, MOF thin films were obtained and characterized subsequently by AFM, SEM, and grazing-incidence XRD (GIXRD). Large crystals (～0.5 mm) have also been nucleated, as indicated by the presence of a bas-relief of the original pattern on one surface of the crystal, suggesting that it is possible to transfer the template surface pattern onto a single crystal of a MOF.     

Shape dependent properties of CdSe nanostructures

We investigate the impact of surface character, shape and size of CdSe particles on their electrostatic and structural properties and discuss consequences for their role as sensitizers. Other than previous theoretical studies which have focussed on a fixed particle shape and a small set of sizes we systematically investigate a large set of more than a thousand nanocrystals with different shapes, sizes, and aspect ratios. This novel approach allows us to draw conclusions on significant correlations of the relevant properties of these crystals with shape, surface character and size. In particular, we find significant differences between the properties of approximately spherically shaped particles, and particles with well defined surfaces along crystal planes but also between particles of the latter class. A surprisingly strong impact of the specific shape and surface configuration is found for the internal structure well inside the crystal as measured by bond lengths correlated with coordination numbers.     

Dynamic studies of surface reactions with microscopic techniques

Work function changes which are combined with surface processes can be investigated and used to characterize adsorption states during heterogeneous catalytic reactions. The oscillatory behaviour of the CO-oxidation was studied on palladium and platinum single crystal surfaces as well as on platinum field emitter tips. On macroscopic crystals the reaction was simultaneously characterized by mass spectrometric analysis and by work function measurements via a Kelvin probe with an area of ≈ 30 mm² being probed. In a second series of experiments photoelectron emission microscopy (PEEM) was applied to follow surface processes with a spatial resolution of ≈ 1 μm. Finally, Pt surfaces were investigated with a lateral resolution of ≈ 20 Å by using field emission microscopy (FEM).

Gas phase coupling of oscillations as demonstrated by the simultaneous oscillations of two separated macroscopic crystals, was missing at small areas of a field emitter. The PEEM registers local oscillations under conditions where the Kelvin probe fails to register dynamic processes. FEM-data showed that critical radii for nucleation could be as small as few Å.     

The equilibrium shapes of crystals and of cavities in crystals

Surface free energies are assumed to be the sum of the excess free energies of bonding of molecules in or near the surface, and the stable form of a crystal or cavity is assumed to be the form that makes the sum of these excess free energies a minimum. When only plane surfaces are allowed, this model predicts the same shapes for crystals as an equation of Wulff (2. Kristallogr. 34, 449 (1901)), which is based on the macroscopic thermodynamic relation of Gibbs (“The Collected Works, Vol. 1.: Thermodynamics,” Longmans, Green, New York (1931)). The model predicts rounding of edges and corners of kinds which are not allowed by the Wulff relation and predicts that spherical forms of particles and cavities can be stable despite anisotropic surface free energies. The model provides a useful framework for analysis of whether unstable crystal or cavity shapes will evolve into stable or metastable forms. Some crystals and cavities that have been assumed to have equilibrium shapes instead have metastable shapes.     

Microtubule particle dispersion in liquid crystal hosts

Abstract

Microtubule particles and metal-coated microtubules were dispersed in various host liquid crystal mixtures. Dispersion effects were evaluated as a function of liquid crystal type, viscosity, dielectric anisotropy and surface interaction. Experimental results indicated that all the types of liquid crystals studied were aligned perpendicular to the microtubule surfaces, regardless of liquid crystal composition or various surface coatings used on the metal-coated microtubules. Low concentrations of the metal-coated microtubules in nematic liquid crystal hosts were aligned by flow or cell surface alignment conditions, and could be modulated by electric or magnetic fields. We observed better microtubules dispersion uniformity in high viscosity liquid crystal host mixtures and in liquid crystal-monomers than in isotropic fluids. Microtubules particles dispersed in ROTN-404 liquid crystal mixture had a much higher birefringence in the microwave region than dispersion in a paraffin oil.