Journal of Electron Microscopy Advance Access originally published online on April 30, 2009
Journal of Electron Microscopy 2009 58(3):73-75; doi:10.1093/jmicro/dfp014
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This article appears in the following Journal of Electron Microscopy issue: Special number: Advanced electron microscopy in materials physics [View the issue table of contents]
Advanced electron microscopy in materials physics
Regional Editor, Journal of Electron Microscopy, Brookhaven National Laboratory
Guest Editor, Journal of Electron Microscopy, Formerly with Hitachi High Technology
Aberration correction has opened a new frontier in electron microscopy by overcoming the limitations of conventional round lenses, providing sub-angstrom-sized probes and extending information limits. The imaging and analytical performance of these corrector-equipped microscopes affords an unprecedented opportunity to study structure–property relationships of matter at the atomic scale. This new generation of microscopes is able to retrieve high-quality structural information comparable to neutron and synchrotron x-ray experiments, but with local atomic resolution. These advances in instrumentation are accelerating the research and development of various functional materials ranging from those for energy generation, conversion, transportation and storage to those for catalysis and nano-device applications. The dramatic improvements in electron-beam illumination and detection also present a host of new challenges for the interpretation and optimization of experiments. During 7– 9 November 2007, a workshop, entitled ‘Aberration Corrected Electron Microscopy in Material Physics’, was convened at the Center for Functional Nanomaterials, Brookhaven National Laboratories (BNL) to address these opportunities and challenges. The workshop was co-sponsored by Hitachi High Technologies, a leader in electron microscopy instrumentation, and BNL's Institute of Advanced Electron Microscopy, a leader in materials physics research using electron microscopy. The workshop featured presentations by internationally prominent scientists working at the frontiers of electron microscopy, both on developing instrumentation and applying it in materials physics. The meeting, structured to stimulate scientific exchanges and explore new capabilities, brought together 
100 people from over 10 countries. This special issue complies many of the advances in instrument performance and materials physics reported by the invited speakers and attendees at the workshop.
The basic theory of aberration correction for electron microscopy was first introduced by Scherzer in the 1940s, but it was not until the 1990s that the technology and computer control matured to the point at which implementing aberration correctors became practical. Crew, Rose and Krivanek pioneered the basic design of spherical aberration (Cs) correctors and refined the associated theories of image formation. Haider and Krivanek then were able to complete the elaborate design and electron-optical engineering to realize this novel aberration correction technology. The new electron-optical elements reduce Cs that affects the probe formation in a scanning transmission electron microscope (STEM) and the image formation in a transmission electron microscope (TEM). Incorporating Cs correctors has driven parallel improvements in the performance and stability of STEMs and TEMs by the designers, engineers and suppliers of these microscopes. Research is underway to correct higher order Cs as well as chromatic aberration (Cc) which will further improve both the spatial and energy resolution of these instruments.
The history of aberration correction and latest advances in electron microscopy were discussed at this workshop, and are featured in this special issue. Harald Rose (p. 77) reviews the development of direct aberration correction in electron microscopy, starting from Scherzer's famous theorem of 1936. He describes the successful 1997 breakthroughs as ‘a quantum step in electron microscopy because it provides genuine atomic resolution’. Pennycook et al.'s review (p. 87) examines the evolution of atomically resolved electron energy-loss spectroscopy (EELS) from the first demonstration of plane-by-plane compositional profiling, through column-by-column spectroscopy, to full two-dimensional and potentially, three-dimensional spectroscopic imaging. The authors highlight one of the themes of the workshop by emphasizing that the interpretation of local electronic structure at atomic resolution requires that non-local and dynamical effects be simulated or modeled. Advances in signal detection are occurring in parallel with illumination improvements, and Maigné and Twesten (p. 99) present a review which covers some recent developments in multi-dimensional spectrum imaging. They describe how advances in instrumentation allow the acquisition of STEM–EELS data with other complementary data, such as x-ray spectroscopy, cathodoluminescence and a new method called diffraction imaging. Inada and Zhu (p. 111) detail how the advances in illumination and detection come together in the first aberration-corrected STEM manufactured by Hitachi, now installed at BNL. Their work highlights the increased complexity of contrast interpretation and quantification due to the large convergence angles of the aberration-corrected electron probe. These papers focus on the critical milestones and current state of the art in aberration-corrected STEMs.
Advances in electron-beam illumination and detection for STEMs and TEMs continue unabated. In this special issue, Batson (p. 123) discusses improving the performance of existing third-order Cs correctors for STEMs by controlling the fourth-order aberrations of a third-order quadrupole–octupole corrector. He presents a methodology for systematically exploring and optimizing resolution-limiting parasitic aberrations that does not require an a priori knowledge of the electron-optical systems. To extend the quantitative capabilities of STEMs, Kimoto et al. (p. 131) demonstrate a technique for improving annular dark-field (ADF) imaging and local crystal structure analysis. In their work, the accuracy and signal-to-noise of STEM ADF imaging is improved in order to detect the sub-angstrom cation displacements in crystals associated with various materials properties such as ferroelectricity or colossal magneto-resistivity. These manuscripts, along with those found in this special issue, illustrate the unprecedented electron microscope control and stability required to realize resolution gains.
Aberration correction in a TEM improves the interpretation of high-resolution images by removing the oscillations of the contrast-transfer function and reducing the image delocalization. Chen and Kai (p. 137) describe how a Boersch electrostatic phase plate can be used in a TEM to provide tunable phase shifts and overcome the problem of low contrast for biological imaging. The authors describe the current limitations of phase-plate imaging and propose a new gener- ation of high phase-efficiency TEMs optimized for biological imaging. Looking further ahead, contrast-transfer calculations indicate that Cc-correction should be highly beneficial for high-resolution TEM and energy-filtered TEM. Kabius et al. (p. 147) describe the latest TEAM and CEOS efforts to develop an electron-optical system that can correct both spherical- and chromatic-aberrations. Based on initial experiments, they report a strong improvement in resolution, confirming for the first time that incorporating an aberration correction system improves the information limit in the TEM mode. As these papers suggest, aberration correctors furnish additional opportunities to improve image contrast and information limits for TEM.
Several electron microscopy techniques are established for resolving three-dimensional (3D) structures from a series of 2D projections. Xin and Muller (p. 157) provide a theoretical and experimental treatment of how the short depth of focus of an aberration-corrected STEM could potentially enable 3D reconstruction of nanomaterials through the acquisition of a through-focal series. The authors investigate the elongation factor which causes distortions, similar to the missing cone or wedge of conventional tilt-series tomography techniques. Dunin-Borkowski et al. (p. 167) describe using high-angle ADF (HAADF) STEM and conventional tilt-series tomography techniques to explore the 3D structure of twinned nanoparticles which are used as heterogeneous catalysts. They examine the relationship of 3D structures with their 2D atomic resolution projections. In a variation in the traditional tilt-series approach, Jarausch and Leonard (p. 175) report a technique enabling the STEM or TEM characterization of individual nanoparticles with complex 3D geometries. A protocol is described for customizing the geometry, surface and chemistry of a novel sample holder to facilitate 360° STEM imaging and analysis of individual nanoparticles. These papers illustrate the latest techniques available for investigating 3D structures.
Aberration-corrected electron microscopy is demonstrating great promise as a tool for measuring materials properties at the atomic scale. Klie et al. (p. 185) use aberration-corrected STEM and EELS to explore the atomic-scale influence of the atomic and electronic structure on the properties of grain boundaries and interfaces of perovskite oxides such as SrTiO3. They demonstrate that aberration correction allows the direct imaging of the structure of a dissociated dislocation core with a sub-angstrom spatial resolution. Blom et al. (p. 193) use an aberration-corrected STEM to investigate the local atomic structure of MoVNbTeO oxidation/ammoxidation catalysts, which cannot be resolved by synchrotron x-ray and neutron diffraction. They compare HAADF images to Bloch-wave HAADF image simulations to reveal subtle changes in the site occupancies for both Te and the Mo/V ratio. Allard et al. (p. 199) use high-resolution aberration-corrected electron microscopy to explore Au and Au/FeOx-based catalysts at different temperatures. A novel MEMS-based sample heater allows the structural evolution of gold as a function of thermal treatment to be explored in situ, distinguishing structural effects produced ex situ via redox, and catalytic treatments. These papers emphasize only a few of the many invaluable applications of aberration-corrected electron microscopy.
The improved illumination and detection conditions afforded by aberration-corrected microscopes open up a range of new challenges for optimizing and interpreting atomic-scale microscopic experiments. A common theme of the workshop and this special issue is that contrast interpretation increasingly depends upon careful optimizations of experiments and simulations of the interaction of the electron beam and sample. Herring (p. 213) presents a review of how diffracted-beam interferometry or holography can be used to explore the Stobbs factor, that is, the contrast mismatch between experimental lattice images and simulated ones. Microscopes equipped with field-emission sources and aberration correctors which are outfitted with biprisms and energy-filtered imaging systems present a unique opportunity to investigate the fundamental mechanisms of image formation. Morgan and Browning (p. 223) provide a tutorial on how to employ low-dose image processing techniques developed for the electron microscopic analysis of 2D crystals of biological macromolecules, to improve the analysis of aberration-corrected STEM-ADF images. These methods enhance the signal-to-noise ratio of the original images, remove distortions in them arising from the instrument, the specimen, or other artifacts and thereby improve the quantification of the structure that determines the material's properties. Each of the papers in this special issue stresses the need to carefully optimize the imaging and sample conditions, as well as the need to carry out simulations to validate the analysis.
The articles included in this special issue reflect the advances and remaining challenges resulting from the development of aberration correction. By opening up the beam convergence angle, aberration-corrected instruments substantially increase the probe current while simultaneously reducing the probe size below 1 Å. These benefits translate into improved spatial resolution and superior analytical capability. As described in this special issue, these improvements enable the exploration of structure, bonding and oxidation state in various functional materials, including multilayered oxides, catalysts and energy-related materials. A local probe of sub-angstrom size and spatial resolution propels modern electron microscopy into a new realm of structural characterization, especially for studying interfaces, defects and electronic inhomogeneity. Nevertheless, challenges remain. For example, to achieve the expected performance of an aberration-corrected instrument, the laboratory housing the instrument must be specially designed and built which is not a trivial task. The high electron-beam current and dose, one of the benefits of aberration correction, dramatically increases the risk of electron-beam damage, even for materials previously thought to be insensitive to beam irradiation. Furthermore, while aberration correction provides superior resolution in microscopy and ample opportunities for materials research, it does not automatically ease the interpretation of experimental data, as many of the invited authors illustrated in their articles. It is our hope that this special issue will stimulate further discussions on the promise and limitations of aberration-corrected microscopy which are captured in this volume, and will prove particularly valuable to those who were unable to attend the workshop.
BNL—Hitachi Workshop Organizers
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