Polarization Selectivity in Vibrational Electron-Energy-Loss Spectroscopy.

Orientation-dependent aloof-beam vibrational electron-energy-loss spectroscopy is carried out on uniaxial icosahedral B_{12}P_{2} submicron crystals. We demonstrate that the high sensitivity of the signal to the crystal orientation allows for an unambiguous determination of the symmetry of normal modes occurring at the Brillouin zone center of this anisotropic compound. The experimental results are assessed using first-principles quantum mechanical calculations (density functional theory) of the dielectric response of the specimen. The high spatial resolution inherent to this technique when implemented in the transmission electron microscope thus opens the door to nanoscale orientation-dependent vibrational spectroscopy.

Progress in ultrahigh energy resolution EELS.

Electron energy loss spectroscopy (EELS) in the electron microscope has progressed remarkably in the last five years. Advances in monochromator and spectrometer design have improved the energy resolution attainable in a scanning transmission electron microscope (STEM) to 4.2 meV, and new applications of ultrahigh energy resolution EELS have not lagged behind. They include vibrational spectroscopy in the electron microscope, a field that did not exist 5 years ago but has now grown very substantially. Notable examples include vibrational mapping with about 1 nm spatial resolution, analyzing the momentum dependence of vibrational states in very small volumes, determining the local temperature of the sample from the ratio of energy gains to energy losses, detecting hydrogen and analyzing its bonding, probing radiation-sensitive materials with minimized damage by aloof spectroscopy and leap-frog scanning, and identifying biological molecules with different isotopic substitutions. We review the instrumentation advances, provide a summary of key applications, and chart likely future directions.

Electron-Beam Mapping of Vibrational Modes with Nanometer Spatial Resolution.

We demonstrate that a focused beam of high-energy electrons can be used to map the vibrational modes of a material with a spatial resolution of the order of one nanometer. Our demonstration is performed on boron nitride, a polar dielectric which gives rise to both localized and delocalized electron-vibrational scattering, either of which can be selected in our off-axial experimental geometry. Our experimental results are well supported by our calculations, and should reconcile current controversy regarding the spatial resolution achievable in vibrational mapping with focused electron beams.

Aberration-corrected STEM for atomic-resolution imaging and analysis.

Aberration-corrected scanning transmission electron microscopes are able to form electron beams smaller than 100 pm, which is about half the size of an average atom. Probing materials with such beams leads to atomic-resolution images, electron energy loss and energy-dispersive X-ray spectra obtained from single atomic columns and even single atoms, and atomic-resolution elemental maps. We review briefly how such electron beams came about, and show examples of applications. We also summarize recent developments that are propelling aberration-corrected scanning transmission electron microscopes in new directions, such as complete control of geometric aberration up to fifth order, and ultra-high-energy resolution EELS that is allowing vibrational spectroscopy to be carried out in the electron microscope.

Atomic-scale chemical imaging of composition and bonding by aberration-corrected microscopy.

Using a fifth-order aberration-corrected scanning transmission electron microscope, which provides a factor of 100 increase in signal over an uncorrected instrument, we demonstrated two-dimensional elemental and valence-sensitive imaging at atomic resolution by means of electron energy-loss spectroscopy, with acquisition times of well under a minute (for a 4096-pixel image). Applying this method to the study of a La(0.7)Sr(0.3)MnO3/SrTiO3 multilayer, we found an asymmetry between the chemical intermixing on the manganese-titanium and lanthanum-strontium sublattices. The measured changes in the titanium bonding as the local environment changed allowed us to distinguish chemical interdiffusion from imaging artifacts.

An electron microscope for the aberration-corrected era.

Improved resolution made possible by aberration correction has greatly increased the demands on the performance of all parts of high-end electron microscopes. In order to meet these demands, we have designed and built an entirely new scanning transmission electron microscope (STEM). The microscope includes a flexible illumination system that allows the properties of its probe to be changed on-the-fly, a third-generation aberration corrector which corrects all geometric aberrations up to fifth order, an ultra-responsive yet stable five-axis sample stage, and a flexible configuration of optimized detectors. The microscope features many innovations, such as a modular column assembled from building blocks that can be stacked in almost any order, in situ storage and cleaning facilities for up to five samples, computer-controlled loading of samples into the column, and self-diagnosing electronics. The microscope construction is described, and examples of its capabilities are shown.

Improvements in the X-ray analytical capabilities of a scanning transmission electron microscope by spherical-aberration correction.

A Nion spherical-aberration (Cs) corrector was recently installed on Lehigh University's 300-keV cold field-emission gun (FEG) Vacuum Generators HB 603 dedicated scanning transmission electron microscope (STEM), optimized for X-ray analysis of thin specimens. In this article, the impact of the Cs-corrector on X-ray analysis is theoretically evaluated, in terms of expected improvements in spatial resolution and analytical sensitivity, and the calculations are compared with initial experimental results. Finally, the possibilities of atomic-column X-ray analysis in a Cs-corrected STEM are discussed.

Direct sub-angstrom imaging of a crystal lattice.

Despite the use of electrons with wavelengths of just a few picometers, spatial resolution in a transmission electron microscope (TEM) has been limited by spherical aberration to typically around 0.15 nanometer. Individual atomic columns in a crystalline lattice can therefore only be imaged for a few low-order orientations, limiting the range of defects that can be imaged at atomic resolution. The recent development of spherical aberration correctors for transmission electron microscopy allows this limit to be overcome. We present direct images from an aberration-corrected scanning TEM that resolve a lattice in which the atomic columns are separated by less than 0.1 nanometer.

Spectroscopic imaging of single atoms within a bulk solid.

The ability to localize, identify, and measure the electronic environment of individual atoms will provide fundamental insights into many issues in materials science, physics, and nanotechnology. We demonstrate, using an aberration-corrected scanning transmission electron microscope, the spectroscopic imaging of single La atoms inside CaTiO3. Dynamical simulations confirm that the spectroscopic information is spatially confined around the scattering atom. Furthermore, we show how the depth of the atom within the crystal may be estimated.

Towards sub-0.5 A electron beams.

In the 4 years since the previous meeting in the SALSA series, aberration correction has progressed from a promising concept to a powerful research tool. We summarize the factors that have enabled 100-120kV scanning transmission electron microscopes to achieve sub-A resolution, and to increase the current available in an atom-sized probe by a factor of 10 and more. Once C(s) is corrected, fifth-order spherical aberration (C(5)) and chromatic aberration (C(c)) pose new limits on resolution. We describe a quadrupole/octupole corrector of a new design, which will correct all fifth-order aberrations while introducing less than 0.2mm of additional C(c). Coupled to an optimized STEM column, the new corrector promises to lead to routine sub-A electron probes at 100kV, and to sub-0.5A probes at higher operating voltages.

Sub-ångstrom resolution using aberration corrected electron optics.

Following the invention of electron optics during the 1930s, lens aberrations have limited the achievable spatial resolution to about 50 times the wavelength of the imaging electrons. This situation is similar to that faced by Leeuwenhoek in the seventeenth century, whose work to improve the quality of glass lenses led directly to his discovery of the ubiquitous "animalcules" in canal water, the first hints of the cellular basis of life. The electron optical aberration problem was well understood from the start, but more than 60 years elapsed before a practical correction scheme for electron microscopy was demonstrated, and even then the remaining chromatic aberrations still limited the resolution. We report here the implementation of a computer-controlled aberration correction system in a scanning transmission electron microscope, which is less sensitive to chromatic aberration. Using this approach, we achieve an electron probe smaller than 1 A. This performance, about 20 times the electron wavelength at 120 keV energy, allows dynamic imaging of single atoms, clusters of a few atoms, and single atomic layer 'rafts' of atoms coexisting with Au islands on a carbon substrate. This technique should also allow atomic column imaging of semiconductors, for detection of single dopant atoms, using an electron beam with energy below the damage threshold for silicon.

Progress in aberration-corrected scanning transmission electron microscopy.

A new corrector of spherical aberration (C(S)) for a dedicated scanning transmission electron microscope (STEM) is described and its results are presented. The corrector uses strong octupoles and increases C(C) by only 0.2 mm relative to the uncorrected microscope. Its overall stability is greatly improved compared to our previous design. It has achieved a point-to-point resolution of 1.23 A in high-angle annular dark field images at 100 kV. It has also increased the current available in a 1.3 A-sized probe by about a factor of ten compared to existing STEMs. Its operation is greatly assisted by newly developed autotuning software which measures all the aberration coefficients up to fifth order in less than one minute. We conclude by discussing the present limits of aberration-corrected STEM, and likely future developments.

An imaging filter for biological applications.

A recently developed post-column imaging filter optimized for biological electron microscopy is described. The filter uses a single magnetic prism, four strong quadrupole lenses and five sextupole lenses to form images and spectra free of all important aberrations and distortions of first and second order. The performance of the filter is illustrated with practical applications. It is shown that the filter offers several important advantages over currently existing in-column filters, particularly in the areas of imaging at low magnifications, attainable collection efficiency, energy resolution, and general spectroscopic operation.

Applications of a post-column imaging filter in biology and materials science.

We have developed an energy-selecting imaging filter which can be attached to most transmission electron microscopes, and is capable of operating at primary energies of up to 400 keV. We review the filter's design and operation, and illustrate them with several applications including imaging of thick biological materials, energy-filtered diffraction, energy-filtered high-resolution imaging, and elemental mapping. We conclude the paper with a discussion of the areas of transmission electron microscopy in which the filter is likely to make especially significant contributions.