Atomic resolution elemental mapping using energy-filtered imaging scanning transmission electron microscopy with chromatic aberration correction
Introduction
The technique of energy-filtered transmission electron microscopy (EFTEM) uses inelastically scattered electrons that have undergone a specific range of energy losses within a specimen to form an image. Selecting energy windows that cover a range of energies above inner-shell edges pertinent to elements present in the sample, it is possible, in principle, to obtain elemental maps locating different atomic species in the specimen. Until recently, obtaining atomic resolution experimental EFTEM images has been difficult due to chromatic aberration and low signal to noise ratios. Chromatic aberration degrades the image formed, as electrons that have lost different amounts of energy within an energy window will be focused in different planes by the imaging lens. This effect can be reduced by decreasing the width of the energy window used in image formation, however this also leads to a reduction of the signal to noise ratio. Due to these competing effects, the resolution of energy-filtered images has been limited. Recently chromatic aberration correction has been implemented to supplement the now ubiquitous spherical aberration correctors [1]. The FEI Titan 60–300 PICO at the Ernst Ruska-Centre in Jülich is an example of such a system. This allows wide energy windows to be used, improving signal to noise ratios and sub-Ångstrom resolution can be realized [2], [3].
However, despite the improved quality of the elemental maps which can be obtained on such an achromatic system, the interpretation of these images is problematic due to the so-called phenomenon of the preservation of coherent elastic contrast [2], [3], [4], [5]. As discussed in detail in Ref. [5], for thicker specimens we find situations where the combination of the delocalized nature of ionization transition potentials and the multiple elastic scattering through the specimen produces features in the EFTEM image, for which plane wave (parallel) illumination is used, that do not allow for reliable interpretation by visual analysis alone. In Ref. [3] it was found that EFTEM images of strontium titanate (STO) down the [001] zone axis based on the titanium L2,3-edge and the oxygen K-edge were not directly interpretable as elemental maps of titanium and oxygen. Due to the delocalized nature of the transition potentials associated with these core-loss edges, a particular column containing the element of interest makes a delocalized contribution to the EFTEM image contrast. In particular intensity is seen around columns as far as 0.3 nm away from the column in which the inelastic scattering event occurred. Localization of elemental information in EFTEM maps for these edges is only possible for very thin specimens (<3 nm).
Section snippets
Experimental setup and relationship to other imaging modes
The imaging scanning transmission electron microscopy (ISTEM) mode introduced by Rosenauer, Krause et al. [6] could be a way to address the effects of preservation of elastic contrast in EFTEM. ISTEM effectively implements transmission electron microscopy (TEM) imaging with spatially incoherent illumination (filling a cone with angle analogous to that of the probe-forming semi-angle used in the ISTEM experiment). This is implemented by using, for image formation in TEM, a strongly focused probe
Experiment
We have taken both ISTEM and EFISTEM images of a specimen of STO, approximately 15 nm thick, down the [001] zone axis using the FEI Titan 60–300 PICO at the ER-C Jülich [12], currently one of only a handful of microscopes worldwide with chromatic aberration correction. The STO specimen was prepared using focused ion beam and low-energy Ar-ion milling to obtain a lamella with an edge thickness between 10 and 20 nm. The microscope was operated at 200 kV, with a probe forming aperture of
Theory and simulation
We calculated the ISTEM, EFISTEM and EFTEM images using the quantum excitation of phonons (QEP) model [20]. In the context of the QEP model, the key equation used to calculate the intensity in the recording plane for the EFISTEM (for each probe position) and EFTEM simulations is given by [2] where with perpendicular to the optical axis, is the transfer function of the imaging lens and is convolved with the auxiliary functions ϕα, n(r, t, τ) that
Summary and conclusions
We have demonstrated the acquisition of elemental maps using an incoherent approach to EFTEM, namely energy-filtered imaging STEM (EFISTEM) that substantially reduces the effects of the preservation of elastic contrast and provides more directly interpretable images than those obtained in conventional EFTEM. This more incoherent approach is equivalent to tilting the probe through all angles inside a cone and incoherently adding the results but has the advantage that it is easier to implement.
Acknowledgements
This research was supported by the Deutsche Forschungsgemeinschaft (Contract No. RO2057/4-2) and by the Discovery Projects funding scheme of the Australian Research Council (Project No. DP110102228).
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