Michael A. O'Keefe

Early results from an aberration-corrected JEOL 2200FS STEM/TEM at Oak Ridge National Laboratory.

The resolution-limiting aberrations of round electromagnetic lenses can now be successfully overcome via the use of multipole element aberration correctors. The installation and performance of a hexapole-based corrector (CEOS GmbH) integrated on the probe-forming side of a JEOL 2200FS FEG STEM/TEM is described. For the resolution of the microscope not to be severely compromised by its environment, a new, specially designed building at Oak Ridge National Laboratory has been built. The Advanced Microscopy Laboratory was designed with the goal of providing a suitable location for aberration-corrected electron microscopes. Construction methods and performance of the building are discussed in the context of the performance of the microscope. Initial performance of the microscope on relevant specimens and modifications made to eliminate resolution-limiting conditions are also discussed.

Screw dislocations in GaN grown by different methods

A study of screw dislocations in hydride-vapor-phase-epitaxy (HVPE) template and molecular-beam-epitaxy (MBE) overlayers was performed using transmission electron microscopy (TEM) in plan view and in cross section. It was observed that screw dislocations in the HVPE layers were decorated by small voids arranged along the screw axis. However, no voids were observed along screw dislocations in MBE overlayers. This was true both for MBE samples grown under Ga-lean and Ga-rich conditions. Dislocation core structures have been studied in these samples in the plan-view configuration. These experiments were supported by image simulation using the most recent models. A direct reconstruction of the phase and amplitude of the scattered electron wave from a focal series of high-resolution images was applied. It was shown that the core structures of screw dislocations in the studied materials were filled. The filed dislocation cores in an MBE samples were stoichiometric. However, in HVPE materials, single atomic columns show substantial differences in intensities and might indicate the possibility of higher Ga concentration in the core than in the matrix. A much lower intensity of the atomic column at the tip of the void was observed. This might suggest presence of lighter elements, such as oxygen, responsible for their formation.

Sub-angstrom atomic-resolution imaging from heavy atoms to light atoms.

John Cowley and his group at Arizona State University pioneered the use of transmission electron microscopy (TEM) for high-resolution imaging. Three decades ago they achieved images showing the crystal unit cell content at better than 4 angstroms resolution. Over the years, this achievement has inspired improvements in resolution that have enabled researchers to pinpoint the positions of heavy atom columns within the cell. More recently, this ability has been extended to light atoms as resolution has improved. Sub-angstrom resolution has enabled researchers to image the columns of light atoms (carbon, oxygen, and nitrogen) that are present in many complex structures. By using sub-angstrom focal-series reconstruction of the specimen exit surface wave to image columns of cobalt, oxygen, and lithium atoms in a transition metal oxide structure commonly used as positive electrodes in lithium rechargeable batteries, we show that the range of detectable light atoms extends to lithium. HRTEM at sub-angstrom resolution will provide the essential role of experimental verification for the emergent nanotech revolution. Our results foreshadow those to be expected from next-generation TEMs with CS-corrected lenses and monochromated electron beams.

Atomic-Resolution 3D Electron Microscopy with Dynamic Diffraction

Achievement of atomic-resolution electron-beam tomography will allow determination of the three-dimensional structure of nanoparticles (and other suitable specimens) at atomic resolution. Three-dimensional reconstructions will yield section images that resolve atoms overlapped in normal electron microscope images (projections), resolving lighter atoms such as oxygen in the presence of heavier atoms, and atoms that lie on non-lattice sites such as those in non-periodic defect structures. Lower-resolution electron microscope tomography has been used to produce reconstructed 3D images of nanoparticles [1] but extension to atomic resolution is considered not to be straightforward. Accurate three-dimensional reconstruction from two-dimensional projections generally requires that intensity in the series of 2-D images be a monotonic function of the specimen structure (often specimen density, but in our case atomic potential). This condition is not satisfied in electron microscopy when specimens with strong periodicity are tilted close to zone-axis orientation and produce anomalous image contrast because of strong dynamic diffraction components. Atomic-resolution reconstructions from tilt series containing zone-axis images (with their contrast enhanced by strong dynamical scattering) can be distorted when the stronger zone-axis images overwhelm images obtained in other random orientations in which atoms do not line up in neat columns. The first demonstrations of 3-D reconstruction to atomic resolution used five zone-axis images from test specimens of staurolite consisting of a mix of light and heavy atoms [2,3]. Initial resolution was to the 1.6{angstrom} Scherzer limit of a JEOL-ARM1000. Later experiments used focal-series reconstruction from 5 to 10 images to produce staurolite images from the ARM1000 with resolution extended beyond the Scherzer limit to 1.38{angstrom} [4,5]. To obtain a representation of the three-dimensional structure, images were obtained in zone-axis projections , , , , , and combined to produce a three-dimensional map of Coulomb potential. Images of specimen sections are much more easily interpreted than projection images such as electron micrographs, reducing the need for techniques such as imaging at sub-Rayleigh resolution [6]. Sections through the 3D staurolite potential show atom positions as density peaks that display streaking from insufficient sampling in direction [1]. Three different specimens of perfect crystal were required to achieve the five projection directions; this makes the technique atomic-resolution electron crystallography rather than atomic-resolution tomography. Nevertheless, our results illustrate that dynamic diffraction need not be a limiting factor in atomic-resolution tomographic reconstruction. We have proposed combining ultra-high (sub-Angstrom) resolution zone-axis images with off-zone images by first using linear reconstruction of the off-zone images while excluding images obtained within a small range of tilts (of the order of 60 milliradian) of any zone-axis orientation [7], since it has been shown that dynamical effects can be mitigated by slight off-axis tilt of the specimen [8]. The (partial) reconstruction would then be used as a model for forward calculation by image simulation [9] in zone-axis directions and the structure refined iteratively to achieve satisfactory fits with the experimental zone-axis data. Another path to atomic-resolution tomography would combine zone-axis tomography with high-resolution dark-field hollow-cone (DFHC) imaging. Electron diffraction theory indicates that dynamic (multiple) scattering is much reduced under highly-convergent illumination. DFHC TEM is the analog of HAADF STEM, and imaging theory shows that image resolution can be enhanced under these conditions [10]. Images obtained in this mode could provide the initial reconstruction, with zone-axis images used for refinement [11].

First Results from the Aberration-Corrected JEOL 2200FS-AC STEM/TEM

Author(s): Allard, L.F.; Blom, D.A.; OKeefe, M.A.; Kiely, C.; Ackland, D.; Watanabe, M.; Kawasaki, M.; Kaneyama, T.; Sawada, H.

Design and Performance Characteristics of the ORNL Advanced Microscopy Laboratory and JEOL 2200FS-AC Aberration-Corrected STEM/TEM

At ORNL, the new Advanced Microscopy Laboratory (AML) has recently been completed, with two aberration-corrected instruments installed, and two more planned in the near future to fill the 4-laboratory building. The installed JEOL 2200FS-AC has demonstrated aTEM information limit of 0.9A. This limit is expected given the measured instrument parameters (HT and OL power supply stabilities, beam energy spread, etc.), and illustrates that the environmental influences are not adversely affecting the instrument performance. In STEM high-angle annular dark-field (HA-ADF) mode, images of a thin Si crystal in zone axis orientation, after primary aberrations in the illuminating beam were optimally corrected, showed a significant vibration effect. The microscope is fitted with three magnetically levitated turbo pumps (one on the column at about the specimen position,and two near floor level) that pump the Omega energy filter and detector chamber. These pumps run at 48,000 rpm, precisely equivalent to 800Hz. It was determined that the upper turbo pump was contributing essentially all of the 800Hz signal to the image, and in fact that the pump was defective. After replacing the pump with one significantly quieter than the original, the Si atomic column image and associated diffractogram(Fig. 4b) show a much-reduced effect of the 800Hzmorexa0» signal, but still some residual effect from the turbo pump. The upper pump will be removed from the main column to an adjacent frame on the floor, and will have a large-diameter, well-damped, pump line to the original connection to the column to effectively isolate the pump from the column. If the 800Hz signal results from mechanical vibrations, they will be damped, and if the signal results from acoustic coupling to the column, it can be damped by appropriate acoustic materials.«xa0less

Seeing atoms at sub-Angstrom resolution with aberration-corrected TEM

Author(s): OKeefe, Michael A. | Abstract: Hardware and software correction of spherical aberration have each produced sub-Angstrom images, and allowed the imaging of light atoms such as oxygen. The LBNL One-Angstrom Microscope (O Angstrom M) combines a modified CM300FEG/UT TEM with FEI focal-series reconstruction software to achieve sub-Angstrom resolution to 0.78 Angstrom. Modifications include hardware correction of 3-fold astigmatism to 0.68 Angstrom and information limit extension to 0.78 Angstrom. The O Angstrom M can image atoms as light (small) as nitrogen, carbon, and lithium. Focal-series reconstruction (FSR) compensates for imperfect objective lens transfer, and provides improvement over any single image. Reconstructed O Angstrom M images, assembled from 20-member focal series, are cleaner than single-shot images, due to lack of second-order contributions. However, second-order components can be removed from single images by subtracting a minimum-contrast image, thus extending the interpretable specimen thickness. In general, TEM images are able to depict atom positions just as well as do FSR images, provided both have the same resolution.

3-D Imaging of Crystals at Atomic Resolution

Electron crystallography has now been used to investigate the structures of inorganic materials in three dimensions. As a test of the method, amplitudes and phases of structure factors were obtained experimentally from high resolution images of staurolite taken in a number of different projections. From images in five orientations, a three-dimensional Coulomb potential map was constructed with a resolution of better than 1.4A. The map clearly resolves all the cations (Al,Si,Fe) in the structure, and all of the oxygen atoms. This method promises great potential for structure determinations of small domains in heterogeneous crystals which are inaccessible to x-ray analysis. Three-dimensional structure determinations should be possible on small domains only approximately 10 unit cells wide, and may resolve site occupancies in addition to atom positions. Given a microscope stage with a suitable range of tilt and enough mechanical stability, the method could also be applied to small crystalline particles larger than about 50A to 100A. In addition, it may be possible to apply the method to derive the two-dimensional structure of periodic defects.

Resolution Quality and Atom Positions in Sub-Angstrom Electron Microscopy

Ability to determine whether an image peak represents one single atom or several depends on resolution of the HR-(S)TEM. Rayleighs resolution criterion, an accepted standard in optics, was derived as a means for judging when two image intensity peaks from two sources of light (stars) are distinguishable from a single source. Atom spacings closer than the Rayleigh limit have been resolved in HR-TEM, suggesting that it may be useful to consider other limits, such as the Sparrow resolution criterion. From the viewpoint of the materials scientist, it is important to be able to use the image to deter-mine whether an image feature represents one or more atoms (resolution), and where the atoms (or atom columns) are positioned relative to one another (resolution quality). When atoms and the corresponding image peaks are separated by more than the Rayleigh limit of the HR-(S)TEM, it is possible to adjust imaging parameters so that relative peak positions in the image correspond to relative atom positions in the specimen. When atoms are closer than the Rayleigh limit, we must find the relationship of the peak position to the atom position by peak fitting or, if we have a suitable model, by image simulation. Our Rayleigh-Sparrow parameter QRS reveals the resolution quality of a microscope image. QRS values greater than 1 indicate a clearly resolved twin peak, while values between 1 and 0 mean a lower-quality resolution and an image with peaks displaced from the relative atom positions. The depth of the twin-peak minimum can be used to determine the value of QRS and the true separation of the atom peaks that sum to produce the twin peak in the image. The Rayleigh-Sparrow parameter can be used to refine relative atom positions in defect images where atoms are closer than the Rayleigh limit of the HR-(S)TEM, reducing the necessity for full image simulations from large defect models.

Accurate Objective Lens Defocus Calibration for Focal-Series Aberration-Corrected HRTEM at Sub-Ångström Resolution

Aberration correction via software or hardware [1] allows an HRTEM to reach much higher values of resolution. For aberration correction using focal-series reconstruction of the electron wave at the specimen exit surface [2,3] to be able to extend resolution to the microscope information limit [4], we need accurate values for all known microscope parameters, including Cs and defocus. For best possible resolution in a reconstruction of the exit-surface electron wave from focal-series of images, we need images containing the highest possible spatial frequencies, preferably to the microscope information limit, d∆ = √{πλ∆/2}. Unlike the “fixed” parameters such as Cs and higher-order aberrations (such as three-fold astigmatism), defocus varies from image to image. Since defocus must be accurate for each image (at least to within the correction limits of the reconstruction code), both the focal series step size and starting defocus must be known accurately.