Off-axis STEM or TEM holography combined with four-dimensional diffraction imaging.

Ultrahigh-resolution imaging may be achieved using modifications of the off-axis holography scheme in a scanning transmission electron microscopy (STEM) instrument equipped with one or more electrostatic biprisms in the illuminating system. The resolution is governed by the diameter of a reference beam, reduced by channeling through a line of atoms in an atomic-focuser crystal. Alternatively, the off-axis holography may be combined with the Rodenburg method in which a four-dimensional data set is obtained by recording a nanodiffraction pattern from each point of the specimen as the incident beams are scanned. An ultrahigh-resolution image is derived by computer processing to give a particular two-dimensional section of this data set. The large amount of data recording and data processing involved with this method may be avoided if the two-dimensional section is derived by recording the hologram while the four beams produced by two perpendicular biprisms are scanned in opposing directions across the specimen by varying the voltages on the biprisms. An equivalent scheme for conventional TEM is also possible. In each case, the complex transmission function of the specimen may be derived and resolutions of about 0.05 nm may be expected.

Electron nanodiffraction and high-resolution electron microscopy studies of the structure and composition of physiological and pathological ferritin.

Structures of core nanocrystals of physiological (horse spleen, human liver, and brain) and pathological human brain of patients with progressive supranuclear palsy (PSP) and Alzheimer's disease (AD) ferritin molecules were determined using electron nanodiffraction and high-resolution transmission electron microscopy. The poly-phasic structure of the ferritin cores is confirmed. There are significant differences in the mineral composition between the physiological and pathological ferritins. The physiological ferritin cores mainly consist of single nanocrystals containing hexagonal ferrihydrite (Fh) and hematite (Hm) and some cubic magnetite/maghemite phase. In the pathological cores, Fh is present but only as a minor phase and Hm is absent. The major phases are a face-centered-cubic (fcc) structure with a = 0.43 nm and a high degree of disorder, related to wustite, and a cubic magnetite-like structure. These two cubic phases are also present in human aged normal brain. Evidence for the presence of hemosiderin together with ferritin in the pathological brains is deduced from the similarities of the diffraction patterns with those from patients with primary hemochromatosis, and differences in the shapes and protein composition of the protein shell. These findings suggest a disfunction of the ferritin associated with PSP and AD, associated with an increase in the concentration of brain ferrous toxic iron.

Applications of electron nanodifraction.

Diffraction patterns from regions 1 nm or less in diameter may be recorded in scanning transmission electron microscopy instruments, and have been applied to the investigation of the structures of various nanoparticles, including catalysts, ferrihydrite and ferritins. Applications to nanotubes and related materials and near-amorphous thin films are reported. The coherence of the incident beams may be exploited in studies of crystals and their defects. Several schemes are outlined whereby the information from sequences of nanodiffraction patterns may be combined to provide ultra-high resolution in electron microscope imaging.

Ultra-high resolution with off-axis STEM holography.

It has previously been demonstrated that the scheme of Rodenburg and colleagues for deriving ultra-high resolution images from arrays of nanodiffraction patterns recorded in a scanning transmission electron microscopy instrument is capable of producing reconstructed images with an improvement in resolution by a factor greater than two, but the method is limited to weak-phase objects and the desired image is accompanied by an unwanted background. It is now shown that these limitations of the method can be avoided if the technique is combined with an off-axis electron holography scheme.

Ultra-high-resolution electron microscopy of carbon nanotube walls.

The resolution in scanning transmission electron microscopy may be enhanced by taking advantage of the information contained in the nanodiffraction patterns recorded for each position of the scanning incident beam. We have demonstrated the first production of ultrahigh resolution, better than 0.1 nm, by this method, in the imaging of an essentially one-dimensional object, the wall of a multiwalled carbon nanotube, using an instrument for which the resolution for normal imaging is about 0.3 nm.

STEM imaging with a thin annular detector.

In scanning transmission electron microscopy, the use of a thin annular detector (TAD), with a ratio of outer to inner radii of about 1.1, allows the selective imaging of various components of a specimen so that different crystalline or amorphous phases may be distinguished. The range of scattering angles to be selected can be varied by using post-specimen lenses to magnify the diffraction pattern of the object. Marginal image modes, giving differentiated images, and ultra-high resolution bright-field images may be obtained when the central spot of the diffraction pattern is magnified so that its radius approximates the inner radius of the TAD. Consideration of the relationship of the TAD imaging to the form of the nanodiffraction pattern obtainable from any small area of the specimen shows that TAD imaging is equivalent to 'variable coherence imaging' and may be used in the determination of medium-range ordering in amorphous or disordered materials.

Comments on ultra-high-resolution STEM.

The resolution of an image in a scanning transmission electron microscope may be improved if an image of the specimen is recorded for each point in the nanodiffraction pattern. It is shown that the method suggested by Rodenburg et al. (Ultramicroscopy 48 (1993) 304) may serve as the basis for an experimentally feasible scheme in which a resolution of better than 0.1 nm is achieved for regions of 1 nm diameter chosen from normal STEM images.

Electron nanodiffraction methods for measuring medium-range order.

Electron nanodiffraction in a scanning transmission electron microscopy (STEM) instrument with a beam diameter of the order of 1 nm can be used to assess the medium-range ordering, or the correlation of atom positions over distances of 1-3 nm, in thin films of disordered materials. Proposals are made for measurements of medium-range order by use of a thin annular detector in STEM, to give the equivalent of the variable-coherence microscopy of Treacy and Gibson (Acta Cryst. A 52 (1996) 212) and by measuring the correlation of diffraction intensities from neighboring nanometer-diameter regions. Two simpler methods for measuring the average dimensions of regions of correlated structure, by observing the persistence of diffraction spots as the beam is translated over the specimen, and by observing the dimensions of spots in greatly defocused diffraction patterns, have been proposed and applied to the study of thin films of amorphous carbon, silica and silicon nitride.

The structure of ferritin cores determined by electron nanodiffraction.

Electron nanodiffraction, with a 100-keV electron beam less than 1 nm in diameter, has been used to obtain single-crystal diffraction patterns from individual iron-containing cores of ferritin molecules. We show that, while a majority of the cores have a hexagonal structure somewhat similar to the major phase in the mineral ferrihydrite, as previously assumed, several minor phases are present including some that are similar in structure to the iron oxides magnetite and hematite and also some composed of highly disordered material. In general, each core consists of one single crystal of one phase.

Electron nanodiffraction.

Electron nanodiffraction, particularly as performed in a dedicated STEM instrument with coherent illumination, provides, in principle, a means for obtaining information on structural detail in the range between that of STEM image resolution, about 0.2 nm, and the limits of information possible from elastic scattering from atoms, about 0. 03 nm. In practice, most nanodiffraction work has been concerned with finding the crystal structure, crystal defects, and sometimes crystal shapes for nanoparticles in the size range of 1-2 nm or for regions of thin crystalline films of about these dimensions. Available equipment allows for the recording of nanodiffraction patterns at the rate of 30 per second, or faster, providing means for detailed study of extended areas or of dynamical processes.

Shadow images for in-line holography in a STEM instrument.

In a dedicated STEM instrument equipped with a field emission gun, shadow images are easily obtained and have many uses. They are very sensitive to misalignment of the instrument and astigmatism, and therefore can be used for rapid and accurate alignment of the microscope. For crystalline materials, the shadow image contains both the bright-field and dark-field images. It is a summation of the transmitted and diffracted beams, and is basically a kind of Gabor's in-line hologram. Under small or medium defocus, shadow images of a thin, well-orientated crystalline specimen take the characteristic form of Ronchigrams, which offer a unique means to calibrate the microscope operation parameters, such as the spherical aberration coefficient Cs and defocus settings of the objective lens, with high accuracy. With the calibrated values of Cs and delta, a transfer function of the objective lens may be generated. In the stage of numerical reconstruction, by adapting this transfer function to the experimentally recorded hologram the lens aberration introduced in forming the hologram may be corrected and an improved resolution may be achieved for electron microscope images.

A simplified mode of differential phase contrast Lorentz microscopy.

A simplified mode of differential phase contrast Lorentz microscopy for the study of magnetic domain structures in thin films is proposed and demonstrated. This mode employs a single annular detector in a scanning transmission electron microscope rather than the specialized split detectors that have been previously used. The resulting signal is sufficiently linear with magnetic field strength to allow quantitative data to be obtained on the domain configurations and the natures of the domain walls.

Stepped surfaces of sapphire (alpha-Al2O3) with low Miller indices.

Oxygen-annealed surfaces of sapphire with low Miller indices ((0001), [1010], [1120], [1011]) have been studied in both transmission electron microscopy (TEM) and reflection electron microscopy (REM) configurations. The significance of REM diffraction conditions for the determination of the nature of the step heights is discussed. The relationship between the TEM and REM images is explained. The structural features are those that might be expected from considerations of the atom arrangement in the low Miller index planes. The structural features on the surfaces varied with respect to annealing temperature and surface condition. Thermally stable structures that might appear from consideration of the equilibrium-annealing temperature are proposed.

Observation of double line contrast in surface imaging.

The double line contrast of a single-atom height step observed in surface imaging for a single crystal in reflection electron microscopy is studied under a variety of experimental conditions. It is suggested that this abnormal contrast is directly associated with the dynamical electron diffraction process. The behavior of the double line contrast is closely related to the order of the Bragg reflected beam, and can be observed mostly under one of the two commonly cited resonance conditions. This phenomenon clearly reveals the differences in the surface imaging for various resonance conditions.

Preparation and characterization of MgO surfaces by reflection electron microscopy.

We have employed several different methods to prepare (100) and (111) surfaces of MgO crystals. (100) surfaces prepared by simple cleaving give good reflection high energy electron diffraction (RHEED) patterns and surfaces with a high density of coarse steps. Chemical polishing of this surface results in a roughening of the topography whilst annealing in oxygen considerably smoothens the surfaces although they appear to be contaminated. Under certain conditions we find that the MgO crystals will cleave along the (111) plane. Both cleaved and mechanically polished (111) surfaces are atomically flat and reconstructed after oxygen annealing.

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.

Observation of microdiffraction patterns with a dedicated STEM instrument.

A two-dimensional detector system, designed for the observation and recording of microdiffraction patterns formed in an HB 5 scanning transmission electron microscopy (STEM) is described and discussed. Possibilities are described and demonstrated for the simultaneous or successive recording of microdiffraction patterns from regions of diameter 3 A or more, bright- or dark-field STEM images, EELS spectra, secondary electron images, and in-line holograms. Applications of the system have been made to studies of catalyst particles, reflection-mode imaging of bulk surfaces, and image reconstruction from microdiffraction patterns obtained from each point of a STEM image.

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.