Conventional Convergent-beam Electron Diffraction Research Articles

Evaluation of accuracy in the determination of crystal structure factors using large-angle convergent-beam electron diffraction patterns.

The accuracy of electron density distribution analysis using large-angle convergent-beam electron diffraction (LACBED) patterns is evaluated for different convergence angles. An orbital ordered state of FeCr2O4 is used as an example of the analysis. Ideal orbital-ordered and non-ordered states are simulated by using orbital scattering factors. LACBED patterns calculated for the orbital-ordered state were used as hypothetical experimental data sets. Electron density distribution of the Fe 3d orbitals has been successfully reconstructed with a higher accuracy from LACBED patterns with convergence angles larger than 15.2 mrad, which is 4 times as large as that for conventional convergent-beam electron diffraction patterns. Excitation of particular Bloch waves with the aid of LACBED patterns has a key role in the accurate analysis of electron density distributions.

Determination of the polarity of ZnO thin films by electron energy-loss spectroscopy

The polarity of the ZnO film grown on sapphire using an ultra-thin Ga wetting layer has been investigated by electron energy-loss spectroscopy (EELS). The intensity of the oxygen K-edge in electron energy-loss spectrum from the ZnO film shows a prominent difference when the film orientation changes from the (0002) Bragg condition to the (000 2 ̄ ) Bragg condition. The EELS study reveals that the ZnO film with very thin Ga wetting layer has the [0001] polarity, which is further confirmed by the conventional convergent beam electron diffraction method.

Polarity Determination of Wurtzite and Zincblende Structures by TEM

The applicability of transmission electron microscopy (TEM) to determine polarity was examined for GaN, ZnO, AlN and GaAs. The adopted methods are the TEM-EDS (energy dispersive X-ray spectroscopy) and the conventional convergent beam electron diffraction (CBED) methods. TEM observations and dynamical electron diffraction calculations were carried out. The results show that the CBED method is useful for ZnO and GaN, while the TEM-EDS method must be used for AlN and GaAs and can also be used for GaN and ZnO. It is clearly indicated that the TEM-EDS method is of use for the case where the difference in atomic scattering factor between the two constituent elements is small, while the CBED method is useful for the case where the difference is large.

Preliminary studies of the interface strain field in an Al-Al2O3 composite by means of convergent-beam electron diffraction

Various kinds of composite materials have been extensively studied concerning their manufacture processing and applications in recent years. Among them, metallic composite materials have drawn extensive attention, because they can offer selected physical, chemical and mechanical properties which cannot be obtained with traditional alloys, ceramics or organic matrix composites. The interfaces in the composites play an important role in the properties of the composite. Up to now, the studies on the interfaces have been mainly limited in the interface morphologies, interfacial reaction products and orientation relationship between the reinforcing phase, crystalline reaction products and the matrix. However, differences in thermal expansion between the reinforced phase and the matrix, generate residual stresses which may affect the aging characteristics of the matrix and the mechanical properties of such composites. Efforts have been made to measure the local lattice strain in the vicinity of the interface, quantitatively by contrast simulation and by detecting the movements of the higher order Laue zone (HOLZ) lines in conventional convergent-beam electron diffraction (CBED). As the electron probe is very close to the interface, HOLZ lines often become obscure or even split. In such cases, it is very difficult to measure the movements of HOLZ lines. Nevertheless, itmore » is possible to measure the strain by matching a dynamical simulation to the experimental splitting of HOLZ lines. Large-angle CBED (LACBED) combines real space and reciprocal space information. A single LACBED pattern shows directly the spatial variation of the lattice strain. The purpose of the present work is to explore the possibility of studying the interface strain field in composite materials according to the shifts and splittings of HOLZ lines by means of improved LACBED technique.« less

Structure of a metastable Al-Ge phase determined from large angle CBED patterns

Abstract The structure of a new metastable Al-Ge phase recrystallized at 300°C from amorphous films with equiatomic composition deposited on to NaCl substrates cooled with liquid nitrogen was determined by convergent beam electron diffraction (CBED). The micron-sized crystals were tetragonal with space group 14/mcm, lattice parameters a = 6.13 Å and c = 4.92 Å, and composition Al0·9 Ge0·9. The structure was the C16 type with nominal AB 2 composition, although there was no evidence for preferential location of Ge on either A or B sites. The A atoms occupied 4(a) sites, and the B atoms were located on 8(h) sites with x = 0·158. Conditional Patterson transforms parallel to the c axis were calculated from kinematic intensities measured in focused large-angle CBED patterns. The contour maps showed clear peaks from A-B and B-B interatomic vectors. Dynamical perturbations reduced peak visibility but did not affect peak positions. The x parameter for B atoms was refined from the quasi-kinematic intensities of first-order Laue zone reflections diffracted from Bloch states localized on B atom strings in conventional CBED patterns.

Prospects for convergent beam electron diffraction with 200–300 kV cold field emission transmission electron microscopes

Convergent beam electron diffraction (CBED) is commonly used to tackle a wide range of crystallographic problems. As the spatial resolution of the technique is pushed towards its limit there will be a number of advantages. These fall broadly under two headings. First there are the benefits to be obtained from performing conventional CBED at higher spatial resolution. Second, there are interesting prospects for coherent diffraction using probe sizes smaller than that of the projected unit cell. However, one can anticipate several difficulties by working at high spatial resolution, and these are discussed.