Applications of modern microdiffraction to materials science

Modern experimental techniques for static convergent beam electron microdiffraction in transmission electron microscopes are discussed and compared to the better known selected area electron microdiffraction methods. The effects of probe coherence are qualitatively discussed. Some recent applications of convergent beam microdiffraction to microstructural characterization of small particles, various lattice defects and amorphous solids are described.     

Electron diffraction from micro- and nanoparticles of hydroxyapatite

Hydroxyapatite (HAP) obtained from aqueous solutions under different conditions has been examined by high-resolution transmission electron microscopy (HRTEM) and electron diffraction, including selected-area electron diffraction (SAED) and microdiffraction. A Philips CM300 field-emission gun electron microscope with a Schottky W/ZrO field-emission tip and a spherical aberration constant of 0.65 mm was used at 300 kV. The HAP crystals had different sizes, ranging from a few nanometres to a few micrometres. Single-crystal diffraction patterns have been obtained from the largest microcrystals using the conventional SAED technique. Assemblies of nanoparticles gave only broad diffuse rings. Nevertheless, microdiffraction with electron microprobes 3.5–10 nm in diameter clearly indicated the crystalline character of the nanoparticles in these assemblies. Experimental HRTEM images, Fourier transforms and calculated images exhibited the fine structure of the HAP crystals.     

The application of scanning transmission electron microscopy (STEM) to the study of thin anodic films on nickel and nickel-molybdenum alloys

Anodized thin-film samples of nickel, molybedenum and a nickel-13 w/o molybdenum alloy have been analysed by scanning transmission electron microscopy, using selected area diffraction, microdiffraction and X-ray micro-analysis. Thin-film samples were obtained by ion thinning with argon and by electropolishing in acetic acid-perchloric acid. Electropolishing is the preferred technique for these studies, producing a surface with roughness on a scale of 1 nm. The anodized films on nickel and nickel 13 w/o molybdenum alloy exhibit a rugosity with a ‘particle’ size of 3 nm. The crystal structure of the films is similar to f.c.c. nickel oxide. Molybdenum enrichment was detected in the anodized alloy surfaces but there is no electron diffraction evidence for a crystalline molybdenum oxide. The electron diffraction pattern from anodized pure molybdenum suggests that the surface film is amorphous.     

Effects of the coherence of illumination on electron microdiffraction pattern intensities

Microdiffraction is capable of revealing the local structure within an area of the specimen consisting of only a few, or a few tens of, unit cells. However, the extent to which the diffraction pattern intensities can show the local structure depends strongly on the coherence of the illumination. If the coherence width of the illumination is smaller than the diameter of the electron probe at the specimen level, the details within the diffraction spots, which indicate deviations of the local structure from the periodicity of the crystal, will be lost. The differences in the amount of spot splitting observed in microdiffraction patterns from out-of-phase domain boundaries, observed with two instruments, are attributed to differences in the effective source sizes.     

High-resolution electron microscopy of disordered lithium ferrites

The disordered phase of lithium ferrite (α-LiFeO₂) shows characteristic circular short-range order (SRO) diffuse scattering around the 110 and the equivalent reciprocal lattice points in the diffraction pattern with the [001] incidence. The origin of the diffuse scattering has been explained by the SRO arrangement of lithium and iron ions inside an octahedron as the unit of the sodium chloride derivative structure (cluster model). In the present study, the local structure of the specimen is directly observed by high-resolution electron microscopy and microdiffraction to clarify the origin of the SRO diffuse scattering. The results obtained from the micrographs show that the SRO state of the specimen consists of regular arrangements of iron-rich regions with total size of 1.5 to 2.0 nm in the [001] and [0$\text{1}$1] projections. It is confirmed from the optical diffraction patterns of the micrographs and the microdiffraction patterns that the arrangements, as the results by the projections, explain the loci in the 001 and 0$\text{1}$1 sections of the characteristic SRO diffuse scattering. These results show that a part of the origin of the SRO diffuse scattering arises from ordering in the regions larger than the octahedral unit.     

The initial stages of ordering in alloys studied by electron diffraction and high resolution microscopy

The combined use of selected area electron diffraction and high resolution electron microscopy can provide important information about the microstructure of materials. Both techniques have been applied to study the ordering behaviour in a number of alloys. The ordering process is not always straightforward and mostly intermediate long period modulated structures are formed. Using dark field high resolution electron microscopy the ordering sequence as well as the structure of the intermediate phases have been established for Au₄Mn and Au₅Mn₂. For Ni₄Mo the high resolution observations provide new information about the ordering mechanism in its very earliest stages.     

电化学方法制备Cu_2O微晶及其形貌控制

以简单的电化学方法在碱性NaCl溶液中以金属铜片为阳极,石墨片为阴极,Na_2Cr_2O_2为添加剂制备Cu_2O微晶,对制备的产品用X射线衍射(XRD)、扫描电子显微镜(FESEM)、透射电子显微镜(TEM)、选区电子衍射(SAED)和紫外吸收光谱(UV-vis)进行表征。结果表明除Cu_2O本身的属性以外,溶液的pH值对它的形貌也产生很大的影响,并讨论Cu_2O微晶的形成机理。     

Interactions of electron beams with surfaces of MgO crystals

The strong interaction of electrons with the flat surfaces of small crystals has been investigated by high resolution CTEM and STEM instruments. When cubic crystals of MgO smoke with edges 20–300 nm are oriented so that the ?001? or ?011? zone axis is parallel to the optical axis, then two kinds of external fringes are observed at (100) surfaces. One kind is parallel to the surface, having spacings up to 0.4 nm. These are caused by interference among the electron channelled along the surface. Fresnel-diffracted ones and the remnant of the incident beam. Fringes of the other kind, which appear as fine structure in the first kind of fringes, are perpendicular to the crystal edge. When an electron beam is parallel to the ?011? axis, the second kind of fringe, whose spacing is 0.3 nm corresponding to d₀₁₁, shows the difference of the surface potential between magnesium atoms and oxygen atoms. Selected area diffraction patterns and microdiffraction patterns also show the same periodicities as in the two kinds of fringes. Simulated images, using the scattering amplitudes for ions, are compared with observations.     

A mirror electron microscope for surface analysis

The principle of mirror microscopy has been adapted to provide a relatively low resolution surface microscope (<1000 ×), a large transfer width low energy electron diffractometer and a photoelectron analyser in k_|| space. A focused electron beam of ? 10 kV is decelerated through a Johansson lens, reflected in front of the sample and reaccelerated back through the lens to produce an electron image over a field of view of a few microns. The image can be interpreted as a micrograph of work function variations on the surface if other effects (geometry, magnetic field) are uniform. In the LEED mode, diffracted beams virtually retain their positions on the screen over the whole impact energy range used (0.160 V). Secondary electrons are preferentially focused around the lens-gun electro-optic axis, thus effectively filtering them out from the diffraction pattern. The design has an inherently large coherence length, of up to 10⁴ Å. Photoelectrons can similarly be imaged in k_|| space on the detector plane. The addition of energy filtering at the screen allows the two-dimensional Fermi surface to be imaged.     

Characterization of the annealed (0001) surface of sapphire (alpha-Al2O3) and interaction with silver by reflection electron microscopy and scanning reflection electron microscopy.

Annealed (0001) surfaces of single-crystal sapphire (alpha-Al2O3) rod have been studied in the electron microscope using reflection electron microscopy (REM), scanning reflection electron microscopy (SREM), and reflection high energy electron diffraction (RHEED). Annealed surfaces of (0001) sapphire are vicinal and characterized by close-packed (0001)-oriented terraces separated by faceted multiple-height steps, with edges parallel to energetically preferred low-index directions (less than 1010 greater than and less than 1120 greater than). These structural features are not seen on cleaved surfaces or polished surfaces treated at temperatures less than 1,250 degrees C. Oxygen-annealing produces clean surfaces which prove useful for investigating the interaction of deposited metals with the (0001) sapphire. Both REM and SREM (with microdiffraction spots) techniques have been used to observe fine structure of flat Ag islands on the scale of 1-100 nm on the (0001)-oriented terraces as well as aggregations at the steps. A preliminary result on interaction with Cu is also included.     

Interpretation of electron diffraction patterns from amorphous and fullerene-like carbon allotropes

The short range order in amorphous and fullerene-like carbon compounds has been characterized by selected area electron diffraction (SAED) patterns and compared with simulations of model nanoclusters. Broad rings in SAED pattern from fullerene-like CN_x at ∼1.2, ∼2, and ∼3.5 Å indicate short-range order similar to that in graphite, but peak shifts indicate sheet curvature in agreement with high-resolution transmission electron microscopy images. Fullerene-like CP_x exhibits rings at ∼1.6 and 2.6 Å, which can be explained if it consists of fragments with short-range order and high curvature similar to that of C₂₀.     

Epitaxial nucleation of crystallization at polymer-filler interfaces

In polymer composites the interaction between polymer matrix and filler particles often results in nucleation of spherulites. The principles of polymer crystal nucleation and spherulite growth are investigated using scanning transmission electron microscopy (STEM) and microdiffraction techniques in combination with polarized light microscopy. Simultaneous diffraction patterns from the interface of the filler and the polymer were obtained. Special precautions for successful recording of the diffraction patterns were used to overcome the rapid loss of polymer crystallinity, resulting from electron beam damage. Analysis of the diffraction patterns has shown that partial epitaxial correlation between the atomic periodicity of the particle surface and the molecular periodicity of polymer chains is always present when spherulites are nucleated. STEM images show that only large particles, with well developed facets (cleavage planes), are nucleating. The nucleating efficiency of the filler is therefore dependent on the size as well as on the crystallographic orientation of the facet. Small particles, or those with no suitable facets, do not affect the crystalline structure of the polymer. It is also shown that anisotropic polymer structures can be formed by inhomogeneous dispersion of nucleating filler particles.     

7050铝合金的表面增压喷丸纳米化

在普通喷丸设备上加增压罐,采用增压喷丸方法对7050铝合金表面进行喷丸处理;用X射线衍射仪、光学显微镜、透射电镜、选区电子衍射仪等对合金表层结构进行了观察与分析,并用显微硬度计测试了表层的硬度。结果表明:经增压喷丸处理后7050铝合金表层的衍射峰明显宽化,随着处理时间的增加,塑性变形层厚度增加,表层晶粒细化至纳米级,平均晶粒尺寸约为40 nm;纳米化后的合金表层硬度比心部硬度提高了1.5倍,其原因可归结为细晶强化及加工硬化。     

Diffraction contrast STEM of dislocations: imaging and simulations

The application of scanning transmission electron microscopy (STEM) to crystalline defect analysis has been extended to dislocations. The present contribution highlights the use of STEM on two oppositely signed sets of near-screw dislocations in hcp α-Ti with 6wt% Al in solid solution. In addition to common systematic row diffraction conditions, other configurations such as zone axis and 3g imaging are explored, and appear to be very useful not only for defect analysis, but for general defect observation. It is demonstrated that conventional TEM rules for diffraction contrast such as g·b and g·R are applicable in STEM. Experimental and computational micrographs of dislocations imaged in the aforementioned modes are presented.     

An efficient approach to characterize pseudo-merohedral twins by precession electron diffraction: Application to the LaGaO3 perovskite

Pseudo-merohedral twins are frequently observed in crystals displaying pseudo-symmetry. In these crystals, many [u v w] zone axis electron diffraction patterns are very close and can only be distinguished from intensity considerations. On conventional diffraction patterns (selected-area electron diffraction or microdiffraction), a strong dynamical behaviour averages the diffracted intensities so that only the positions of the reflections on a pattern can be considered. On precession electron diffraction patterns, the diffracted beams display an integrated intensity and a “few-beam” or “systematic row” behaviour prevails which strongly reduces the dynamical interactions. Therefore the diffracted intensity can be taken into account. A procedure based on observation of the weak extra-reflections connected with the pseudo-symmetry is given to identify without ambiguity any zone axis. It is successfully applied to the identification and characterization of {1 2 1} reflection twins present in the LaGaO₃ perovskite.     

Contribution of electron precession to the identification of the space group from microdiffraction patterns

In a previous study, it was reported that the possible space groups of a crystal can be identified at a microscopic or nanoscopic scale, thanks to microdiffraction patterns obtained with a nearly parallel electron incident beam focused on a very small area of the specimen. A systematic method was proposed, which consists of the observation of a few microdiffraction patterns displaying at least two Laue zones. These microdiffraction patterns can also be obtained by using an electron precession equipment. In this case, the patterns display a very large number of reflections in the Laue zones whose intensity is the integrated intensity. These original features greatly facilitate the space group identification method and are particularly useful when the high-order Laue zones (HOLZ) are not visible on microdiffraction patterns or when very thin specimens are not available.     

Electron microdiffraction from very small gold particles

By use of a scanning transmission electron microscope equipped with a two-dimensional detection system, microdiffraction patterns have been obtained from gold crystals of diameter 15–20 Å dispersed in a thin polyester film. Comparison with diffraction patterns calculated for various models suggests that only a small proportion of the particles have the multiply twinned form which has been proposed as the equilibrium state from very small particles, although some twinning is common. It is shown to be difficult to determine, on the basis of the form of the diffraction pattern, whether multiply twinned particles have localized or distributed strains to accommodate the misfit of assemblies of tetrahedral twinned component crystal regions.