Evaluation and optimization of the performance of elastic and inelastic scanning transmission electron microscope imaging by correlation analysis.

Scanning Transmission Electron Microscopes (STEM) offer specific characters, such as multidetector configuration, energy loss spectroscopy, external control of the probe raster, which make them quite suited to digital image acquisition and innovative data processing. Considering pixel intensities as random variables, one can apply cross-correlation techniques to pairs of duplicated images. Such methods are used to investigate imaging properties of the instrument, such as the point resolution and the signal to noise ratio, in its different working modes. Concerning the annular dark field, one shows that there exist optimum values for defocus and angle of illumination which minimize the probe size at a value of about 0.5 nm. The same techniques are also used to evaluate the characteristics of energy filtered images recorded in a specific energy window loss around an ionization edge: the degradation in edge resolution with respect to the accompanying ADF images is only of the order of 0.2 to 0.3 nm, which sets the delocalization parameter for an energy loss of 100 eV (i.e. for U-O4-5) well below 1 nm in agreement with theoretical predictions.