Investigations of nanocrystalline ceramics and powders by TEM, AFM and plasticity measurements

Nanocrystalline zirconia powders prepared by laser evaporation were analyzed by electron microscopy and X-ray diffraction. A very high volume fraction of tetragonal particles was found, although the majority of particles is significantly larger than the equilibrium size of the tetragonal → monoclinic transformation. Nanopowder of yttria stabilized (2.4 mol% Y₂O₃) zirconia was used to prepare nanocrystalline ceramics by pressureless sintering at T = 1400?°C. At T ≥ 1200?°C the samples show superplastic behavior with an activation energy of 585 kJ mol^–1 and a stress exponent of about 1.8.     

聚硅氧烷聚脲嵌段共聚物的TEM及SAXS研究

采用透射电子显微镜与小角X光散射分别研究了不同软、硬段分子量及软段极性对聚硅氧烷聚脲多嵌段共聚体系微相结构的影响。结果表明,增加软段分子量及硬段含量有利于聚硅氧烷与聚脲的相分离。将极性氰丙基引入聚硅氧烷后两相混合度明显改善,同时聚脲硬段粒径减小并趋于均一。在聚氰丙基甲基硅氧烷基体中增加聚脲含量及其分子量,则两相界面厚度也随之增大。     

Fractal structure in base-catalyzed silica aerogels examined by TEM, SAXS and porosimetry

A fractal analysis of three base catalyzed silica aerogels was performed using different experimental techniques: image analysis of electron micrographs, SAXS and study of pore size distribution determined from nitrogen adsorption isotherms. The aerogels appeared to exhibit self-similar properties over the range from 3–10 to 50–130 nm. The values of mass fractal dimension varied from 1.75 to 2.05 depending on the reactants concentration (TEOS, H₂O) and were found to be similar irrespective of the method applied.     

Study of metal containing hydrogenated carbon films by STM/AFM and SAXS

Metal-containing amorphous hydrogenated carbon films are of high interest for industrial applications because of their excellent frictional properties, their high abrasive wear resistance and their electrical conductivity, which can be adjusted in a range of 10–12 orders of magnitude. In order to get insight into the mechanical and electrical properties it is necessary to study the nanostructure of the films. The structure consists of small nanometer sized metallic or carbidic particles, which are embedded in a three dimensional amorphous hydrogen-carbon matrix. Anomalous small angle X-ray scattering (ASAXS) and scanning tunneling microscopy (STM) have been used to determine size- and distance-distributions of the particles as a function of metal content. Problems and restrictions of both methods will be discussed. Furthermore the capabilities of scanning probe techniques to distinguish different materials on a nanometer scale (material contrast) have been studied employing barrier height imaging (dI/dz) and friction force microscopy.Dedicated to Professor Dr. rer. nat. Dr. h. c. Hubertus Nickel on the occasion of his 65th birthday     

Correlated AFM and SERS imaging of the transition from nanotriangle to nanohole arrays

Mapping the transition of Ag nanotriangle to nanohole arrays revealed that the optimal SERS response was obtained near the transition. Correlated AFM and Raman imaging provided novel experimental proof that hot spots are located on Ag islands for nanotriangle arrays and in the core area of the hole for nanohole arrays, which is in agreement with previous theoretical predictions.     

Behavior of pure and mixed DPPC liposomes spread or adsorbed at the air-water interface

Liposomes from pure dipalmitoylphosphatidylcholine (DPPC) and mixed DPPC: distearoylphosphatidylcholine (DSPC): soybean lecithin (SL) prepared by the Bangham method with sonication were dispersed into solution or spread at the interface and the kinetics of the surface film formation was studied by measuring and recording the evolution of superficial tension, surface potential, and superficial (¹⁴C labeled) DPPC density.A simple theoretical approach can describe these kinetics by two processes: irreversible diffusion of closed vesicles into or from the bulk phase, and irrevers ible transformation of closed spherical vesicles into destroyed ones which form the surface film. Diffusion controls the phenomenon for small initial amounts of liposomes.Transformation controls the phenomenon for important initial amounts of liposomes. The kinetic constant of the transformation,K, does not depend on the technique used to form the surface film (spreading or adsorption).The equilibrium and rheological properties of surface films formed after liposome spreading are compared to those of monolayers     

Structural investigation on bulk poly(3-hexylthiophene): Combined SAXS, WAXD, and AFM studies

A detailed structural and morphological study on bulk poly(3-hexylthiophene), having high regioregularity (about 98%) and molecular weight of 16 kDa, has been performed. The combination of wide- and small-angle X-ray diffraction techniques, for the first time on bulk semicrystalline conjugated polymer, allowed to get insight on the structural features of poly(3-hexylthiophene), subjected to different thermal treatments. The evolution or the periodicity, the crystallite dimension and defects, as a function of the annealing were determined. Fibrillar organization, evidenced atomic force microscopy, results in the coexistence of disordered areas where fibrils are randomly oriented and ordered regions where fibrils are organized into stacks, with periodic pattern matching the periodicity determined by X-ray scattering.     

Structures of Fibrous Supramolecular Assemblies Constructed by Amino Acid Surfactants: Investigation by AFM, SANS, and SAXS

Aqueous gel-like solutions of N-acyl-L-aspartic acids (C(n)Asp, n=14, 16, 18) and N-dodecanoyl-beta-alanine (C(12)Ala) were prepared at pH 5-6 at room temperature. Structures of supramolecular assemblies in the solutions were investigated by atomic force microscopy (AFM), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS). The cross-sectional radii, 22-30 ?, of helical, fibrous assemblies were obtained from analysis of SANS for 1% gel-like C(n)Asp solutions. Three Bragg spacings were observed in a SANS spectrum for a 6% C(16)Asp solution. C(n)Asp molecules are associated into the unit chain of a helical bilayer strand with a diameter of 50-60 ?. Unit chains where linear bilayers twist form a double strand with helical sense of approximately 650-? pitch. It was confirmed from AFM images that cylindrical fibers in a gel-like C(12)Ala solution had a circular cross-section. The SAXS spectrum showed characteristic Bragg spacings. Cylindrical C(12)Ala fibers consist of multilamellar layers of period approximately 34-?. The fibers are laterally organized with period 365-380 ?. Copyright 2000 Academic Press.     

Absolute integrated SAXS measurements during the crystallization of linear polyethylene

Absolute intensity measurements of a dynamic small-angle x-ray scattering from a linear polyethylene were carried out during polymer crystallization from melt in a temperature range of 113.5° to 124.5°C. The mean-square modulation of the electron density over the irradiated volume was evaluated and the feasibility of dynamic experimentation for crystallization kinetic analysis was established. The results provide an absolute value of mass density of the amorphous phase of a semicrystalline polymer at the crystallization temperature.     

Investigation of the solution structure of hyaluronic acid by light scattering,SAXS, and viscosity measurements

Summary Hyaluronic acid in water exhibits a non-monotonous scattering function, which is explained in terms of multimerization leading to gel-like supermolecular particles. This tendency is highest in water, lower in buffer, and lowest in isotonic NaCl-solution + NaN₃. Viscosity measurements show that with increasing concentration a network solution is formed. SAXS-measurements show that nonaggregated Hyaluronic acid is present either in form of double strands, or as supercoils with helical portions.

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Zusammenfassung Hyaluronsäure zeigt in Wasser eine nicht-monotone Streufunktion, die als Ausdruck einer Multimerisation gedeutet wird, welche zu gelartigen übermolekularen Teilchen führt. Diese Tendenz ist in Wasser am stärksten, schwächer in Puffern, und am schwächsten in isotoner NaCl-Lösung + NaN₃. Viskositätsmessungen zeigen, daß bei höheren Konzentrationen eine Netzwerk-Lösung gebildet wird. Röntgenkleinwinkel-Messungen ergeben, daß die nicht aggregierte Hyaluronsäure entweder in Form von Doppelsträngen oder als Superknäuel mit helikalen Anteilen vorliegt.

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With 14 figures and 4 tables     

Comparative study of nanoscale surface structures of calcite microcrystals using FE-SEM, AFM, and TEM.

The bulk morphology and surface features that developed upon precipitation on micrometer-size calcite powders and millimeter-size cleavage fragments were imaged by three different microscopic techniques: field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) of Pt-C replicas, and atomic force microscopy (AFM). Each technique can resolve some nanoscale surface features, but they offer different ranges of magnification and dimensional resolutions. Because sample preparation and imaging is not constrained by crystal orientation, FE-SEM and TEM of Pt-C replicas are best suited to image the overall morphology of microcrystals. However, owing to the decoration effect of Pt-C on the crystal faces, TEM of Pt-C replicas is superior at resolving nanoscale surface structures, including the development of new faces and the different microtopography among nonequivalent faces in microcrystals, which cannot be revealed by FE-SEM. In conjunction with SEM, Pt-C replica provides the evidence that crystals grow in diverse and face-specific modes. The TEM imaging of Pt-C replicas has nanoscale resolution comparable to AFM. AFM yielded quantitative information (e.g., crystallographic orientation and height of steps) of microtopographic features. In contrast to Pt-C replicas and SEM providing three-dimensional images of the crystals, AFM can only image one individual cleavage or flat surface at a time.     

Investigation of surface topography, morphology and structure of amorphous carbon films by AFM and TEM

Morphology and structure of amorphous carbon films deposited with a pulsed arc source (LASER-ARC) have been studied using microscopical methods (SEM, TEM and AFM), electron diffraction and spectroscopical investigation (EELS). The parameters of the arc source and the deposition conditions (substrate temperature) influence morphology and structure of deposited amorphous carbon films. Especially the incorporation and growth of particles, embedded in the film have been investigated. By particle analysis using an optical microscope a majority of particles that is smaller than 500 nm has been determined. The morphology has been also demonstrated similar by AFM and TEM images. Their number and size of particles is strongly influenced by the deposition temperature. The structure of amorphous film is characterized by the EELS-spectra, but the particle structure was not detectable.     

Examination of the influence of F6H10 fluorinated diblocks on DPPC liposomes

The interactions of unilamellar vesicles obtained by the incorporation of (1,2,3,4,5,6)-tridecafluoro-hexadecane (F6H10 diblock) to dipalmitophosphatidyl-choline (DPPC), with Gd³⁺, Ca²⁺, Na⁺ ions were studied by electrophoretic measurements, dynamic light scattering and differential scanning calorimetry (DSC). Electrophoretic mobility measurements on unilamellar vesicles as a function of ion concentrations show that the vesicles adsorb the different ions employed. DSC has been used to determine the effect of diblock on the transition temperature (T _c) and on the change of enthalpy (ΔH _c) associated with the process.     

Influence of temperature on the colloidal stability of the F-DPPC and DPPC liposomes induced by lanthanum ions

The influence of La(3+) on the colloidal stability of liposomes made up by two zwitterionic phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-[16-fluoropalmitoyl-phosphatidylcholine (F-DPPC), in aqueous media has been investigated by dynamic light scattering and electrophoretic mobility. The critical aggregation concentration (c.a.c.) of La(3+) for F-DPPC and DPPC liposomes were experimentally obtained, and the results were compared with theoretical predictions using the Derjaguin-Landau-Verwey-Overbeek theory. In order to evaluate the influence of the state of the bilayer on the stability of liposomes, all experiments were performed at temperatures below and above the chain-melting phase-transition temperature of lipids (transition temperature of lipids). Changes in the size of both types of liposomes and high values of polydispersity in the presence of La(3+) showed that these ions induce aggregation of liposomes at 25 °C and at 60 °C. At 25 °C, when the bilayer of F-DPPC liposomes is interdigited, DPPC liposomes are more resistant to aggregation than the liposomes formed with F-DPPC. However, this difference disappears at 60 °C, when both bilayers have the same conformation. The experimental results also indicate that the c.a.c. is higher at 60 °C than at 25 °C for both types of liposomes. In fact, it has been observed by dynamic light scattering measurements that aggregation of liposomes at 25 °C can be prevented by increasing the solution temperature for La(3+) concentrations near to the c.a.c. Moreover, the behavior of these liposomes in the presence of the ion was studied at temperatures above and below the transition temperature of the phospholipids.     

Complexes of anionic liposomes and a cationic polymer: Composition,structure, and characteristics

Adsorption of the synthetic polycation poly-N-ethyl-4-vinylpyridinium bromide (PEP) on the surface of bilayered lipid vesicles (liposomes) is studied. Two types of liposomes are used: (i) traditional two-component liposomes formed from neutral phosphatidylcholine (PC) and anionic diphosphatidylglycerol (cardiolipin, CL²⁻) and (ii) PC/CL²⁻ anionic liposomes with the built-in nonionogenic surfactant poly(ethyleneglycol) cetyl ether with a degree of polymerization of 20 (Brij-58). PEP is quantitatively linked with both types of liposomes; this process is electrostatic in character and fully reversible. The formation of a poly(ethylene glycol) layer on liposomal membrane decreases the stability of polycation-liposome complexes in aqueous salt solutions. Adsorption of PEP on the surface of PC/CL²⁻ liposomes is accompanied by their aggregation; PC/CL²⁻/Brij liposomes do not aggregates, even during complete neutralization of their charge by the adsorbed PEP. DSC measurements showed that the adsorption of the polycation is accompanied by microphase separation in the liposomal membrane: formation of domains, which are composed primarily of CL²⁻ molecules and linked to the complex with PEP, and regions, where electroneutral lipids are primarily concentrated. With the use of a spin probe, the packing density of bilayers (their microviscosity) is estimated, and a preferential localization of the probe at the boundaries of lipid domains in the membrane based on PC/CL²⁻/Brij liposomes is proposed. The causes of the aggregative stability of three-component PC/CL²⁻/Brij liposomes are described, and the structure of the prepared polymer-liposome complexes is discussed.     

Effects of nonionic sugar surfactants on the phase transition of DPPC membranes

The influence of newly synthesized N-alkanoyl-N-methyllactitolamines (decanoyl [C₁₀MELA], lauoroyl [C₁₂MELA] and miristoyl [C₁₄MELA]) on the thermotropic phase transition of phosphatidylcholine bilayer was compared with common sugar-based surfactants N-dodecyl-β-d-glucopyranoside [C12G1] and decanoyl-N-methyl glucamide [MEGA-10]. The results indicate that C_nMELA are very active at the membrane surface and disturb the phospholipid bilayer structure less than commercially used MEGA-10 and C12G1.     

Determination of nanocrystal sizes: a comparison of TEM, SAXS, and XRD studies of highly monodisperse CoPt3 particles

One of the most fundamental tasks in nanoscience is the accurate determination of particle sizes. Various methods have been developed to elucidate the mean particle diameter and the standard deviation for an ensemble of nanocrystals. However, good agreement between the results from different methods is not always encountered in the literature. In this study, we investigate colloidally prepared, highly monodisperse CoPt3 nanoparticles by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and powder X-ray diffraction (XRD). The results are compared in order to examine to which extent agreement is obtained by the different techniques when applied to small nanocrystals in the size range below 10 nm. In particular, the applicability of the simple Scherrer formula for size determination from the broadening of XRD reflections is checked. When the different techniques are correctly applied, the results from all methods are in good agreement.     

Calorimetric study of the effects of 2,4-dichlorophenol on the thermotropic phase behavior of DPPC liposomes

The effect of 2,4-dichlorophenol (DCP) on the main transition and pretransition of fully hydrated (20 mass%) dipalmitoylphosphatidylcholine (DPPC) multilamellar liposomes has been studied by differential scanning calorimetry (DSC). It was observed that an increase in the molar ratio of DCP/DPPC (from 4·10^-5 up to 2·10^-2) causes progressive reductions in the temperature and enthalpy of the pretransition. The higher concentration of DCP eliminates the pretransition. The influence of DCP on the main transition in this molar ratio range is not drastic, but a decrease in temperature and in the enthalpy values was observed. In the molar ratio range (from 2·10^-1 up to 4·10^-1) the DSC scans show multiple main transition peaks instead of the characteristic single peak of the main transition. Above a DCP/DPPC molar ratio of 0.6 a new peak appears at 25°C having about the same transition enthalpy as the main transition of the pure system.This revised version was published online in November 2005 with corrections to the Cover Date.     

Characterization of the B4 phase by dielectric and AFM measurements

Dielectric measurements on a sample consisting of banana-shaped molecules were carried out in a frequency range between 0.1 Hz and 10 MHz. The sample exhibited B₂ and B₄ phases. As usual, two ranges of relaxation were detected in the B₂ phase, the fast reorientation about the long axes of the molecules and a slow collective process. In the B₄ phase, only one dielectric active process at low frequencies was found; this does not differ from the low frequency relaxation of the B₂ modification. This relaxation is probably related to the dynamics of superstructures. Crystallization could be observed after keeping the sample for a longer time at higher temperature. Thus, it was possible to differentiate clearly between the crystalline and the B₄ phases. AFM investigations prove the existence of focal-conic domains and periodic superstructures in the B₄ phase; then do not appear in the crystalline state. For this reason the B₄ phase is regarded as different from a classical crystalline phase.