Efficient linear phase contrast in scanning transmission electron microscopy with matched illumination and detector interferometry.

The ability to image light elements in soft matter at atomic resolution enables unprecedented insight into the structure and properties of molecular heterostructures and beam-sensitive nanomaterials. In this study, we introduce a scanning transmission electron microscopy technique combining a pre-specimen phase plate designed to produce a probe with structured phase with a high-speed direct electron detector to generate nearly linear contrast images with high efficiency. We demonstrate this method by using both experiment and simulation to simultaneously image the atomic-scale structure of weakly scattering amorphous carbon and strongly scattering gold nanoparticles. Our method demonstrates strong contrast for both materials, making it a promising candidate for structural determination of heterogeneous soft/hard matter samples even at low electron doses comparable to traditional phase-contrast transmission electron microscopy. Simulated images demonstrate the extension of this technique to the challenging problem of structural determination of biological material at the surface of inorganic crystals.

Future trends in aberration-corrected electron microscopy.

The attainable specimen resolution is determined by the instrumental resolution limit d(i) and by radiation damage. Solid objects such as metals are primarily damaged by atom displacement resulting from knock-on collisions of the incident electrons with the atomic nuclei. The instrumental resolution improves appreciably by means of aberration correction. To achieve atomic resolution at voltages below approximately 100 kV and a large number of equally resolved image points, we propose an achromatic electron-optical aplanat, which is free of chromatic aberration, spherical aberration and total off-axial coma. Its anisotropic component is eliminated either by a dual objective lens consisting of two separate windings with opposite directions of their currents or by skew octopoles employed in the TEAM corrector. We obtain optimum imaging conditions by operating the aberration-corrected electron microscope at voltages below the knock-on threshold for atom displacement and by shifting the phase of the non-scattered wave by pi/2 or that of the scattered wave by -pi/2. In this negative contrast mode, the phase contrast and the scattering contrast add up with the same sign. The realization of a low-voltage aberration-corrected phase transmission electron microscope for the visualization of radiation-sensitive objects is the aim of the proposed SALVE (Sub-A Low-Voltage Electron microscope) project. This microscope will employ a coma-free objective lens, an obstruction-free phase plate and a novel corrector compensating for the spherical and chromatic aberrations.

Historical aspects of aberration correction.

A brief history of the development of direct aberration correction in electron microscopy is outlined starting from the famous Scherzer theorem established in 1936. Aberration correction is the long story of many seemingly fruitless efforts to improve the resolution of electron microscopes by compensating for the unavoidable resolution-limiting aberrations of round electron lenses over a period of 50 years. The successful breakthrough, in 1997, can be considered as a quantum step in electron microscopy because it provides genuine atomic resolution approaching the size of the radius of the hydrogen atom. The additional realization of monochromators, aberration-free imaging energy filters and spectrometers has been leading to a new generation of analytical electron microscopes providing elemental and electronic information about the object on an atomic scale.