Coherent low-energy electron diffraction on individual nanometer sized objects

Today's structural biology techniques require averaging over millions of molecules to obtain detailed structural information. Derivation of the molecular structure from a scattering experiment with just one single 3D-molecule imposes major challenges. Coherent and damage-free radiation is needed to ensure sufficient elastic scattering events before destroying the molecule and a means to solve the phase problem is wanted. We have devised such a scheme using coherent low-energy electrons shaped into a collimated beam by an electrostatic microlens. Initial experiments using a carbon nanotube sample demonstrate the feasibility of coherent low-energy electron diffraction on an individual nanometer-sized object.     

Energy-filtered electron backscatter diffraction

The electron backscatter diffraction (EBSD) analytical technique is invaluable for determining the crystallography of bulk alloys, thin films, and nanoparticles. However, our physical understanding of EBSD pattern generation is incomplete, which hinders our ability to push the limits of EBSD analysis. Here, using an energy filter with better than 10 eV resolution, we experimentally demonstrate the energy dependence of EBSD patterns from elements over a large atomic number range. We verify that low-loss electrons are the major contributors to EBSD patterns, but that there is still a diffraction contribution from electrons with only 80% of the incident beam energy. Additionally, the bands in filtered EBSD patterns have contrast that is more than twice the contrast of their unfiltered counterparts. The band contrast reaches a maximum for a cutoff energy in the filter of about 3% below the energy of the incident beam. Different mechanisms are used to explain the drop in contrast on each side of the maximum. With the cutoff set very close to the energy of the incident beam, the patterns become more blurred. We used a Monte Carlo simulation in the analysis of these experiments.     

Coherent electron emission from point sources

We study the electron emission and propagation in ultrasharp tips field-emission experiments. The intensity-voltage characteristics as well as the angular spreading of the electron beam is analysed. We show that non-coherent emission cannot explain the very narrow experimental electron beams. The properties of coherent electron sources and the beam spatial coherence width are discussed. The quantum mechanical current distribution of field-emitted electrons from tubes seems to explain the experimental beam widths.     

Microscopy in electron diffraction apparatus

A simple method was devised for observing electron micrographs as well as diffraction figures from a given object. A dielectric emulsion composed of paraffin and barium titanate acted here as an electrostatic lens when charged up with the incident electrons. Electron diffraction figures could immediately be observed from the same object as for microscopy when the lens was mechanically removed out of the electron path.     

Bragg's Law diffraction simulations for electron backscatter diffraction analysis

In 2006, Angus Wilkinson introduced a cross-correlation-based electron backscatter diffraction (EBSD) texture analysis system capable of measuring lattice rotations and elastic strains to high resolution. A variation of the cross-correlation method is introduced using Bragg's Law-based simulated EBSD patterns as strain free reference patterns that facilitates the use of the cross-correlation method with polycrystalline materials. The lattice state is found by comparing simulated patterns to collected patterns at a number of regions on the pattern using the cross-correlation function and calculating the deformation from the measured shifts of each region. A new pattern can be simulated at the deformed state, and the process can be iterated a number of times to converge on the absolute lattice state. By analyzing an iteratively rotated single crystal silicon sample and recovering the rotation, this method is shown to have an angular resolution of ∼0.04° and an elastic strain resolution of ∼7e−4. As an example of applications, elastic strain and curvature measurements are used to estimate the dislocation density in a single grain of a compressed polycrystalline Mg-based AZ91 alloy.     

Aberration-compensated large-angle rocking-beam electron diffraction

The application of convergent beam electron diffraction (CBED) to determine symmetry, refine structure factors, and measure specimen thickness requires rather thick specimen and is very difficult or even impossible in the case of large unit cell materials. The large-angle rocking-beam electron diffraction (LARBED) technique introduced in this paper gives access to the kind of experimental data contained in CBED patterns but over a much larger angular range. In addition to symmetry determination and thickness measurement even for thin samples this technique also allows, in principle, very accurate measurements of structure factors. Similar to precession electron diffraction (PED), LARBED uses the illumination tilt coils to sequentially change the angle of incidence of the electron beam over a very large range. I will present results obtained by a recently developed self-calibrating acquisition software which compensates for aberration-induced probe shifts during the acquisition of LARBED patterns and keeps the probe within a few nm, while covering a tilt range from 0 to 100 mrad. This paper is dedicated to Prof. John C. H. Spence on the occasion of his 65th birthday.     

Techniques of convergent beam electron diffraction

The techniques required to record standard convergent beam electron diffraction patterns in an analytical electron microscope are discussed in detail, with emphasis on the design of electron optics in commercial instruments. Practical comments are included on specimen preparation, the influence of crystal defects, tilting to major zone axes, and alignment of the instrument. The influence of parameters under experimental control such as probe size, accelerating voltage, temperature, specimen tickness, and convergence angle is discussed in detail. Some comments are included on the alignment and limitations of large angle patterns formed by a defocused probe.     

Translation symmetries in convergent-beam electron diffraction

The interpretation of the CBED pattern symmetries derived from the specimen symmetry has been given by Goodman and by Buxton, Eades, Steeds and Rackham. The former paper deals with the effect of the translation components of symmetries, which are normal to the incident surface, while the latter paper ignores the presence of these translation components. The graphical method proposed by Buxton et al. cannot be applied to the symmetry with translation. The Ewald construction method used by Goodman is also not applicable to the symmetry with translation. A simple interpretation of the CBED pattern symmetries can be obtained based on the multiple-scattering formula, where the translation component is also taken into account.     

Quantitative metallography by electron backscattered diffraction

Although electron backscattered diffraction (EBSD) in the scanning electron microscope is used mainly to investigate the relationship between local textures and microstructures, the technique has now developed to the stage where it requires serious consideration as a tool for routine quantitative characterization of microstructures. This paper examines the application of EBSD to the characterization of phase distributions, grain and subgrain structures and also textures. Comparisons are made with the standard methods of quantitative metallography and it is shown that in many cases EBSD can produce more accurate and detailed measurements than the standard methods and that the data may sometimes be obtained more rapidly. The factors which currently limit the use of EBSD for quantitative microstructural characterization, including the speed of data acquisition and the angular and spatial resolutions, are discussed, and future developments are considered.     

A pulsed electron gun for ultrafast electron diffraction at surfaces

The construction of a pulsed electron gun for ultrafast reflection high-energy electron diffraction experiments at surfaces is reported. Special emphasis is placed on the characterization of the electron source: a photocathode, consisting of a 10 nm thin Au film deposited onto a sapphire substrate. Electron pulses are generated by the illumination of the film with ultraviolet laser pulses of femtosecond duration. The photoelectrons are emitted homogeneously across the photocathode with an energy distribution of 0.1 eV width. After leaving the Au film, the electrons are accelerated to kinetic energies of up to 15 keV. Focusing is accomplished by an electrostatic lens. The temporal resolution of the experiment is determined by the probing time of the electrons traveling across the surface which is about 30 ps. However, the duration of the electron pulses can be reduced to less than 6 ps.     

Orientation averaging of electron backscattered diffraction data

The use of data averaging to improve the angular precision of electron backscattered diffraction (EBSD) maps is discussed. It is shown that orientations may be conveniently and rapidly averaged using the four Euler-symmetric parameters which are coefficients of a quaternion representation. The processing of EBSD data requires the use of an edge preserving filter and a modified Kuwahara filter has been successfully implemented and tested. Three passes of such a filter have been shown to reduce orientation noise by a factor of ∼10. Application of the method to deformed and recovered aluminium alloys has shown that such data processing enables small subgrain misorientation (< 0.5°) to be detected reliably.     

The possibilities for electron diffraction in biology

The techniques whereby an electron microscope may be used to observe the low angle electron diffraction pattern from crystallized macromolecules are described. A discussion is then presented of the advantages and disadvantages of the technique relative to X-ray diffraction, high resolution electron microscopy and optical diffraction or computer analysis of electron micrographs.     

Multivariate statistical approach to electron backscattered diffraction

This paper assesses the potential of multivariate statistical analysis (MSA) applied to electron backscattered diffraction (EBSD) data. Instead of directly indexing EBSD patterns on an individual basis, this multivariate approach reduces a large (thousands) set of individual EBSD patterns into a core set of statistically derived component EBSD patterns which can be subsequently indexed. The following hypotheses are considered in this paper: (1) experimental EBSD patterns from a microstructure can be analytically treated as linear combinations of spatially simple components, (2) MSA has an angular resolution on par with standard EBSD, (3) MSA can discriminate between similar and dissimilar phases, and (4) the MSA approach can improve the effective spatial resolution of automated EBSD.     

Assessment of electron irradiation damage to biomolecules by electron diffraction and electron energy-loss spectroscopy

Electron radiation damage is one of the most severe problems in high resolution electron microscopy of biomolecules. The techniques of electron diffraction and electron energy-loss spectroscopy were applied to gain a better understanding of radiation damage in amino acids and nucleic acid bases. The results when compared with G-values for the release of ammonia and hydrogen sulphide from amino acids seem to indicate that bond scission is an important cause of radiation damage at moderate doses of irradiation. High resolution structural disorder in nucleic acid bases was found to involve loss of atoms peripheral to the main ring structure.