The optimum imaging of an object structure at the sub-angstrom length scale requires precise adjustment of the lens aberrations of a high-resolution instrument up to the fifth order. A least-squares ...optimization of defocus aberration C1, third-order spherical aberration C3, and fifth-order spherical aberration C5 yields two sets of aberration coefficients for strong phase contrast up to the information limit: one for variable C1 and C3, at fixed C5, another for variable C1, C3, and C5. An additional correction to the defocus aberration, dependent on object thickness, is described, which becomes important for the use of image simulation programs in predicting optimum high-resolution contrast from thin objects at the sub-angstrom scale. For instruments with a sub-angstrom information limit the ultimate structure resolution, the power to resolve adjacent atom columns in a crystalline object, depends on both the instrumental pointspread and an object pointspread due to finite width of the atomic column potentials. A simulation study on a simple double-column model yields a range for structure resolutions, dependent on the atomic scattering power, from 0.070 nm down to 0.059 nm, for a hypothetical 300-kV instrument with an information limit of 0.050 nm.
The design and construction of a double-hexapole aberration corrector
has made it possible to build the prototype of a spherical-aberration
corrected transmission electron microscope dedicated to ...high-resolution
imaging on the atomic scale. The corrected instrument, a Philips CM200 FEG
ST, has an information limit of better than 0.13 nm, and the spherical
aberration can be varied within wide limits, even to negative values. The
aberration measurement and the corrector control provide instrument
alignments stable enough for materials science investigations. Analysis of
the contrast transfer with the possibility of tunable spherical aberration
has revealed new imaging modes: high-resolution amplitude contrast,
extension of the point resolution to the information limit, and enhanced
image intensity modulation for negative phase contrast. In particular,
through the combination of small negative spherical aberration and small
overfocus, the latter mode provides the high-resolution imaging of weakly
scattering atom columns, such as oxygen, in the vicinity of strongly
scattering atom columns. This article reviews further lens aberration
theory, the principle of aberration correction through multipole lenses,
aspects for practical work, and materials science applications.
In this study, the spin-dependent nonlinear contrast transfer in transmission electron microscopy is derived from the eikonal expansion of the Dirac equation. The transmission cross-coefficient of ...the standard imaging theory is amended by a spin-dependent factor, whose effect is investigated for single scattering in the object by an electrical field under polarized and unpolarized illumination, and it is illustrated with numerical results and plots for a kinetic energy of 80 keV. The resulting image displacement and image convolution increase with decreasing kinetic energy but are always smaller than a wavelength. General features of the cross-coefficient are discussed to identify favorable conditions for the measurement of the small spin effects, possibly in an unmodified instrument.
Relativistic electron diffraction depends on linear and quadratic terms in the electric potential, the latter being neglected in the frequently used relativistically corrected Schrödinger equation. ...The quadratic electric potential term modifies atomic scattering amplitudes in particular for large‐angle scattering and backscattering. The respective correction increases with increasing scattering angle, increasing atomic number and increasing kinetic energy. Conventional tabulations for electron scattering and its large‐angle extrapolations can be amended in closed form by a universal correction based on the screened Coulomb potential squared.
Relativistic electron diffraction depends on linear and quadratic terms in the electric potential, the latter being neglected in the frequently used relativistically corrected Schrödinger equation. Conventional tabulations for electron scattering and its large‐angle extrapolations can be amended in closed form by a universal correction based on the screened Coulomb potential squared.
Aberration-corrected transmission electron microscopy allows us to image the structure of matter at genuine atomic resolution. A prominent role for the imaging of crystalline samples is played by the ...negative spherical aberration imaging (NCSI) technique. The physical background of this technique is reviewed. The especially high contrast observed under these conditions owes its origin to an enhancing combination of amplitude contrast due to electron diffraction channelling and phase contrast. A number of examples of the application of NCSI are reviewed in order to illustrate the applicability and the state-of-the-art of this technique.
A novel imaging mode for high-resolution transmission electron
microscopy is described. It is based on the adjustment of a negative
value of the spherical aberration CS of the
objective lens of a ...transmission electron microscope equipped with a
multipole aberration corrector system. Negative spherical aberration
applied together with an overfocus yields high-resolution images with
bright-atom contrast. Compared to all kinds of images taken in
conventional transmission electron microscopes, where the then
unavoidable positive spherical aberration is combined with an
underfocus, the contrast is dramatically increased. This effect can
only be understood on the basis of a full nonlinear imaging theory.
Calculations show that the nonlinear contrast contributions diminish
the image contrast relative to the linear image for a
positive-CS setting whereas they reinforce the image
contrast relative to the linear image for a negative-CS
setting. The application of the new mode to the imaging of oxygen in
SrTiO3 and YBa2Cu3O7
demonstrates the benefit to materials science investigations. It allows
us to image directly, without further image processing, strongly
scattering heavy-atom columns together with weakly scattering
light-atom columns.
A single layer of LaAlO3 with a nominal thickness of one unit cell, which is sandwiched between a SrTiO3 substrate and a SrTiO3 capping layer, is quantitatively investigated by high-resolution ...transmission electron microscopy. By the use of an aberration-corrected electron microscope and by employing sophisticated numerical image simulation procedures, significant progress is made in two aspects. First, the structural as well as the chemical features of the interface are determined simultaneously on an atomic scale from the same specimen area. Second, the evaluation of the structural and chemical data is carried out in a fully quantitative way on the basis of the absolute image contrast, which has not been achieved so far in materials science investigations using high-resolution electron microscopy. Considering the strong influence of even subtle structural details on the electronic properties of interfaces in oxide materials, a fully quantitative interface analysis, which makes positional data available with picometer precision together with the related chemical information, can contribute to a better understanding of the functionality of such interfaces.
Transmission electron microscopy is an indispensable tool in modern materials science. It enables the structure of materials to be studied with high spatial resolution, and thus makes a decisive ...contribution to the fact that it is now possible to understand the microstructure-related physical and chemical characteristics and to correlate these with the macroscopic materials properties. It was tantamount to a paradigm shift when electron microscopy reached atomic resolution in the late 1990s due to the invention of aberration-corrected electron optics. It is now generally accepted practice to perform picometer-scale measurements and chemical analyses with reference to single atomic units. This review has three objectives. Microscopy in atomic dimensions is applied quantum physics. The consequences of this for practical work and for the understanding and application of the results shall be worked out. Typical applications in materials science will be used to show what can be done with this kind of microscopy and where its limitations lie. In the absence of relevant monographs, the aim is to provide an introduction to this new type of electron microscopy and to enable the reader to access the literature in which special issues are addressed. The paper begins with a brief presentation of the principles of optical aberration correction. It then discusses the fundamentals of atomic imaging and covers typical examples of practical applications to problems in modern materials science. It is emphasized that in atomic-resolution electron microscopy the quantitative interpretation of the images must always be based on the solution of the quantum physical and optical problem on a computer.
We investigate a possible dependence between the applied electron dose-rate and the magnitude of the resulting image contrast in HRTEM of inorganic crystalline objects. The present study is focussed ...on the question whether electron irradiation can induce excessively strong atom vibrations or displacements, which in turn could significantly reduce the resulting image contrast. For this purpose, high-resolution images of MgO, Ge, and Au samples were acquired with varying dose rates using a CS-corrected FEI Titan 80–300 microscope operated at 300kV accelerating voltage. This investigation shows that the magnitude of the signal contrast is independent from the dose rates occurring in conventional HRTEM experiments and that excessively strong vibrations or displacements of bulk atoms are not induced by the applied electron irradiation.
•No dependence between electron dose rate and HRTEM image contrast is found.•This finding is in full accordance with established solid-state physics theory.•Object-related causes for the previous Stobbs-factor phenomenon are ruled out.
•No evidence for a dose-rate driven contrast reduction in HRTEM was found.•Experiments at high and medium dose rates do not show any dose-rate dependence.•Robust results for very low dose rates are ...missing.
In a recent article 1 we examined the influence of the applied electron dose rate on the magnitude of the image contrast in high-resolution transmission electron microscopy (HRTEM). We concluded that the magnitude of the image contrast is not substantially affected by the applied electron dose rate. This result is in obvious contradiction to numerous earlier publications by Kisielowski and coworkers 2–7, who commented our recent article due to this contradiction. The present short communication is a response to the comment of Kisielowski and coworkers on our recent article, where we provide additional arguments supporting our initial findings and conclusions on the magnitude of the image contrast in HRTEM.
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