R Vincent

Double conical beam-rocking system for measurement of integrated electron diffraction intensities

Abstract The performance of a scanning system for a transmission electron microscope is described, where the focused beam is scanned at a constant angle around the optic axis. Below the specimen, the beam is de-scanned, the net effect being equivalent to precessing the specimen around a stationary beam focused to a probe size of 0.1 μm for typical precession and convergence angles. Unlike standard convergent beam electron diffraction (CBED) patterns aligned on a zone axis, precessed patterns include many reflections intercepted by the Ewald sphere, where the diffracted intensities are determined by integration through the Bragg condition. The double conical scanning system was designed to collect a large data set of integrated intensities that are more suitable for structure determination by electron diffraction, both by removing excitation errors due to curvature of the Ewald sphere and also by reducing non-systematic dynamical effects. Experimental patterns obtained from the Si〈110〉 axis are discussed, followed by analysis of the intensities diffracted into the higher order Laue zones of a rare earth pyrogermanate ( Er 2 Ge 2 O 7 ) with a large unit cell. The scanning system is equally suitable for collection of precessed spot patterns from radiation-sensitive specimens such as organic crystals.

Electron spin resonance and Raman studies of Mn-doped ZnO ceramics

In the present work, the influence of annealing on structure, electron spin resonance (ESR), and Raman scattering (RS) spectra of x at. % Mn-doped ZnO (x=4 and 8) ceramic compounds has been systematically investigated. The samples were annealed at temperatures (Tan) between 400 and 1000°C for 12h. The obtained results revealed the strong dependence of x-ray diffraction, ESR, and RS spectra on the annealing temperature Tan. Mn2+ ions did not substitute into Zn2+ sites in samples annealed in the range of 400–600°C but started to substitute into Zn2+ sites for annealing temperatures Tan>600°C. The results of this investigation provide further insights into the physical processes occurring in Mn-doped ZnO materials due to annealing.

Raman scattering in Me-doped ZnO nanorods (Me = Mn, Co, Cu and Ni) prepared by thermal diffusion

We have investigated normal and resonant Raman scattering in Me-doped ZnO nanorods (Me = Mn, Co, Cu and Ni) prepared by thermal diffusion. Experimental results show that the normal Raman spectra consist of the conventional modes associated with wurtzite ZnO and impurity-related additional modes. Under resonant conditions, only longitudinal optical (LO) phonon scattering and its overtones are observed. The number of LO phonon lines and their relative intensity depend on the doping element and level. For the nanorods doped with Cu and Ni, we have observed LO phonon overtones up to eleventh order. This situation does not happen for the Mn-doped nanorods, which show only five LO phonon modes. By co-doping Mn and Co into the ZnO host lattice, however, the LO phonon overtones up to eleventh order are observed again. The nature of this phenomenon is explained by means of the study of XRD, TEM and photoluminescence.

Enhancement of multiple-phonon resonant Raman scattering in Co-doped ZnO nanorods

We have studied Raman scattering in Co-doped ZnO nanorods prepared by thermal diffusion. Experimental results show that the features of their non-resonant spectra are similar to Raman spectra from Co-doped ZnO materials investigated previously. Under resonant conditions, however, there is a strong enhancement of multiple-phonon Raman scattering processes. Longitudinal optical (LO)-phonon overtones up to eleventh order are observed. The modes become more obvious when the Co concentration diffused into ZnO nanorods goes to an appropriate value. This phenomenon is explained due to the shift of the band-gap energy and also due to the decrease in the intensity of near-band-edge luminescence. Our observation is in agreement with the prediction [J. F. Scott, Phys. Rev. B 2, 1209 (1970)] that the number of LO-phonon lines in ZnO is higher than that observed for CdS.

Observation of phase contrast in covergent-beam electron diffraction patterns

Abstract A conventional transmission electron microscope equipped with a cold field emission source (Hitachi HF-2000) was used to record coherent phase contrast from the overlap regions of convergent-beam electron diffraction (CBED) patterns in 6H SiC. The focused probe size (∼ nm) was smaller than the planar spacing allong the c axis (1.5 nm). When a probe was slightly defocused to illuminate several units cells, the overlaps between adjacent CBED discs showed parallel sets of fringes, with relative shifts that were consistent with the phasing of the SiC structure factors.

Influence of Mn doping on structural, optical, and magnetic properties of Zn1-xMnxO nanorods

We prepared Zn1−xMnxO nanorods by thermal diffusion. These samples were then studied the structural, optical, and magnetic properties. The structural analyses basing on x-ray diffraction and transmission electron microscope revealed the absence of Mn-related secondary phases. The study of photoluminescence spectra revealed the blueshift in the UV emission when the Mn doping concentration was increased, as a consequence of the extension of the band gap energy. Besides this situation, the increase in emission intensity associated with extrinsic defects at about 680 nm also took place. Concerning the Raman scattering spectra, apart from conventional phonon modes related to the ZnO wurtize-type structure, there were some additional modes introduced by the doping. Their origin was assessed carefully. Particularly, the shift in peak position of E2(high) toward low frequencies due to the increase in the Mn doping concentration could be explained well by means of the spatial correlation model. Magnetic measuremen...

Measurement of Debye-Waller factors by electron precession

Abstract The electron precession technique was developed for the ab initio structure determination of unknown phases. However, in this paper we have explored the possibility of using precession to determine accurate values of the Debye–Waller factor. As a test of the technique, silicon, diamond and chromium were studied and the results obtained compared with known values determined from other techniques. The agreement was found to be excellent. The accuracy and simplicity of the technique makes it suited to many situations where the Debye–Waller factor needs to be known accurately and with sub-micron spatial resolution.

Energy-filtered convergent-beam diffraction: examples and future prospects

Abstract The addition of an imaging filter to a cold field-emission TEM has led to the development of new techniques based on the analysis of the zero-loss or a specific energy-loss signal. For example, the removal of inelastically scattered electrons can improve the fringe contrast in coherent CBED patterns from relatively thick crystals. Similarly, this energy-filtering is essential for the accurate measurement of low-order structure factors by CBED pattern matching. Studies of convergent-beam patterns recorded at specific core-loss energies show that they contain chemically sensitive features. In addition, hybrid modes of operation have been developed that allow spectrum-images and spectrum-diffraction patterns to be obtained. New results are presented that show the use of these techniques. Finally, it is argued that energy-filtering should lead to improvements in critical-voltage measurements, latticeparameter determination and in the resolution of branch cluster information.

Crystal structure of an orthorhombic phase in β-(Al-Fe-Si) precipitates determined by convergent-beam electron diffraction

Abstract It has been found that the β-(Al-Fe-Si) precipitate with approximate composition Al5FeSi and lamellar microstructure is not a single-phase compound but a multiphase composite. An orthorhombic phase has been identified from the precipitate and its crystal structure determined by means of convergent-beam electron diffraction. This phase has an orthorhombic A-centred lattice with parameters a = 6.18 Å, b = 6.20 Å and c = 20.8 Å, point group mmm and space group Amam, equivalent to Cmcm (No. 63) in standard notation.

An ordered Ga2Te3 phase in the ZnTe/GaSb interface

A Ga2Te3 interfacial phase has been observed in a ZnTe/(001)GaSb heterostructure by high resolution electron microscopy under special imaging conditions. This phase exists in domains 5–10 nm in size on the ZnTe side of, and usually 2–4 nm away from, the interface. A structural model has been proposed for this phase that is derived from the sphalerite cell with cation sites occupied either fully (occupancy 1) or partially (occupancy 5/9) by Ga atoms. The fully occupied Ga sites form a regular array of uninterrupted chains along the [110] direction of the sphalerite unit cell. The partially occupied Ga sites can also be considered as forming chains containing both Ga atoms and vacancies along the [110] direction. Within these chains vacancies are highly mobile, resulting in an average Ga occupancy of 5/9. The unit cell of Ga2Te3 is orthorhombic with the space group Amm2. The lattice parameters of the unit cell have been derived from electron diffraction data.