Negative spherical aberration imaging (NCSI)

Employing an aberration corrector in a high-resolution transmission electron microscope, the spherical aberration CS can be tuned to negative values, resulting in a novel imaging technique, which is called the negative CS imaging (NCSI) technique. The image contrast obtained with the NCSI technique is compared quantitatively with the image contrast formed with the traditional positive CS imaging (PCSI) technique. For the case of thin objects negative CS images are superior to positive CS images concerning the magnitude of the obtained contrast, which is due to constructive rather than destructive superposition of fundamental contrast contributions.

As a consequence, the image signal obtained with a negative spherical aberration is significantly more robust against noise caused by amorphous surface layers, resulting in a measurement precision of atomic positions which is by a factor of 2–3 better at an identical noise level. The quantitative comparison of the two alternative CS-corrected imaging modes shows that the NCSI mode yields significantly more precise results in quantitative high-resolution transmission electron microscopy of thin objects than the traditional PCSI mode.

In particular, the application of combined chromatic and spherical aberration correction in high-resolution transmission electron microscopy enables a significant improvement of the spatial resolution down to 50 pm. We demonstrate that such a resolution can be achieved in practice at 200 kV. Diffractograms of images of gold nanoparticles on amorphous carbon demonstrate corresponding information transfer. The Y-atom pairs in [010] oriented yttrium orthoaluminate (YAP) are successfully imaged together with the Al and the O atoms with high contrast. Although the 57 pm pair separation is well demonstrated, separations between 55 pm and 80 pm are measured. This observation is tentatively attributed to structural relaxations and surface reconstruction in the very thin samples used. Quantification of the resolution limiting effective image spread is achieved based on an absolute match between experimental and simulated image intensity distributions. To the best of our knowledge this demonstrates the highest direct resolution in coherent TEM atomic imaging in materials at 200 kV to date.

By matching not only the atomic positions, but also the absolute image contrast between the simulated and experimental images, we are also able to determine the 3-dimensional shape of a nanoscale crystal with atomic resolution from a single NCSI image. The sensitivity of the reconstruction procedure is not only sucient to reveal the surface morphology of the crystal with atomic resolution, but also to detect the presence of adsorbed impurity atoms. The single-image approach that we introduce offers important advantages for threedimensional studies of radiation-sensitive crystals.

Contact:

Dr. Chun-Lin Jia
Phone: +49 2461 61-2408
E-Mail: c.jia@fz-juelich.de