On the benefit of the negative-spherical-aberration imaging technique for quantitative HRTEM

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.