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Aberration-Corrected Transmission Electron Microscopy (AC-TEM) is the general term for microscopes where electro optical components are introduced to reduce the aberrations that would otherwise reduce the resolution of images. Historically electron microscopes had quite severe aberrations, for until about the start of the 21st century the resolution was quite limited, at best able to image the atomic structure of materials so long as the atoms were far enough apart. Methods of correcting the aberrations existed for some time, but could not be implemented in practice. Around the turn of the century the electron optical components were coupled with computer control of the lenses and their alignment; this was the breakthrough which led to significant improvements both in resolution and the clarity of the images. As of 2024 both correction of optical as well as chromatic aberrations is standard in many commercial electron microscopes.

There have been many milestones in this evolution. The first was the prototype demonstration by two groups, one Ondrej Krivanek and the other by Maximilian Haider and Harald Rose. Another major was the first steps towards chromatic aberration correction in the TEAM project. Beyond this...

As the electron optic resolution improved, it became apparent that there also needed to be improvements to the mechanical stability of the microscopes to keep pace.

As of 2024 resolutions of about 0.1nm are fairly routine in microscopes around the world. This is true for both standard higher-voltage electron microscopes as well as a few ones specially designed to operate at lower electron energies. Exploiting the improvements significantly better identification of chemical contents of materials has become possible, as well as local electronic structure. This has had a major impact on our understanding across multiple fields of study.

History

Early Theoretical Work

Scherzer's theorem is a theorem in the field of electron microscopy. It states that there is a limit of resolution for electronic lenses because of unavoidable aberrations.

German physicist Otto Scherzer found in 1936 ^[1] that the electromagnetic lenses, which are used in electron microscopes to focus the electron beam, entail unavoidable imaging errors. These aberrations are of spherical and chromatic nature, that is, the spherical aberration coefficient C_s and the chromatic aberration coefficient C_c are always positive. ^[2]

Scherzer solved the system of Laplace equations for electromagnetic potentials assuming the following conditions:

1. electromagnetic fields are rotationally symmetric,
2. electromagnetic fields are static,
3. there are no space charges. ^[3]

He showed that under these conditions the aberrations that emerge degrade the resolution of an electron microscope up to one hundred times the wavelength of the electron. ^[4] He concluded that the aberrations cannot be fixed with a combination of rotationally symmetrical lenses. ^[1]

In his original paper, Scherzer summarized: "Chromatic and spherical aberration are unavoidable errors of the space charge-free electron lens. In principle, distortion (strain and twist) and (all types of) coma can be eliminated. Due to the inevitability of spherical aberration, there is a practical, but not a fundamental, limit to the resolving power of the electron microscope." ^[1]

The resolution limit provided by Scherzer's theorem can be overcome by breaking one of the above mentioned three conditions. Giving up rotational symmetry in electronic lenses helps in correcting spherical aberrations. ^{[5] [6]} A correction of the chromatic aberration can be achieved with time-dependent, ie non-static, electromagnetic fields (for example in particle accelerators). ^[7]

Scherzer himself experimented with space charges (eg with charged foils), dynamic lenses, and combinations of lenses and mirrors to minimize aberrations in electron microscopes. ^[8]

Prototypes

Effort of Albert Crewe

Low voltage electrostatic correctors

Phase plate and similar ideas

First demonstrations

Early Commercial Products

TEAM Project

The Transmission Electron Aberration-Corrected Microscope (TEAM) project was a collaborative effort between Lawrence Berkeley National Laboratory, Argonne National Laboratory, Brookhaven National Laboratory, Oak Ridge National Laboratory, and the University of Illinois, Urbana-Chamaign ^[9] with the technical goal of reaching spatial resolution 0.05 nanometers, smooth sample translation and tilt, while allowing for a variety of in-situ experiments. ^[10]

The TEAM project resulted in two microscopes, TEAM 0.5 and TEAM I. Both microscopes are S/TEMs (they can be used in both TEM mode and STEM mode) that correct for both spherical aberration and chromatic aberration. ^{[11] [12]}

Present State

Applications

References