Atomic studies on ferroelectric oxides by aberration corrected transmission electron microscopy

Abstract

The advent of aberration-corrected transmission electron microscopy in 1998 [1] has provided materials science with entirely new tools for quantitative investigations. Four key innovations have to be mentioned: (1) The possibility to operate the electron microscope as a variable-sphericalaberration instrument allows to derive a new phase contrast theory optimizing both resolution and point spread [2]. In classical Scherzer phase contrast theory the radius of the point spread disc amounts to three times the Scherzer resolution limit. Besides the fact that information is lost by placing an aperture in the diffraction plane to keep the contrast oscillations in the contrast transfer function from affecting the images this point spread is a second disadvantage of the classical Scherzer approach to phase contrast. Both limitations can be substantially reduced in a new theory in which by both the objective lens defocus Z as well as C S the spherical aberration parameter adopt specific values. As a result the resolution limit coincides with the information limit and the point spread gets reduced to about one half of the latter making it an uncritical parameter in practice. (2) The negative spherical aberration imaging (NCSI) technique leads to enhanced contrast of atoms with low nuclear charge number [3]. It relies on two advantages compared to the classical Zernike technique. The shift of the phase of the diffracted waves is, in contrast to the classical Scherzer technique, in clockwise direction leading to white atom contrast. Furthermore the contrast is enhanced by a dynamic non-linear effect. Oxygen, nitrogen and even boron can be imaged directly even when these atomic species occur in close distance to heavy cations. (3) Essentially point-spread-free atomic images allow to measure occupancies of atomic columns, i.e. local concentrations, with lateral atomic resolution evaluating atomically resolved intensity measurements [4]. This means that high-resolution is not only a structural technique. From now on also local composition maps can be derived which are forming an excellent starting point for ab-initio calculations of interface-, boundary- and defect structures. (4) Measurements of atomic distances can be carried out at an accuracy of a few picometers