Quantitative Sub-Angstrom Imaging Through ADF STEM

ORNL/CP-100163 Annular dark-field (ADF) imaging in a scanning transmission electron microscope (STEM) at atomic resolution provides an incoherent image that can be described as the convolution between the intensity of the illuminating STEM probe and an object function consisting of localised sources at the atomic-column positions. It has been shown that the resolution limit of the microscope limits the accuracy to which the object function can be reconstructed [1]. Here we demonstrate how a number of images recorded at various degrees of underfocus can be reconstructed to give sub- angstrom information, and discuss how quantitative physical measurements may be deduced from these images. The conventionally used optimum probe intensity profile for the VG Microscopes HB603U STEM (300 kV, C{sub 5}=l mm), shown in Fig. la, requires an objective aperture radius of 9 mrad and 40 nm of underfocus. However, using a larger aperture and a greater degree of underfocus can give a probe with a much narrower central maximum (Fig 1b), but at the expense of creating side-lobes with increased intensity. Since this probe contains sharper features than the conventionally optimum probe, information at much higher spatial frequencies can be recorded. Figure 2 shows information transfer down to a resolution of 0.78 A. Although the images recorded using such a probe are not as intuitively interpretable as those recorded using the optimum probe, the lack of a phase problem in incoherent imaging means that the probe maybe immediately deconvolved from the image intensity data, and phase retrieval techniques are not required [1]. The geometry of ADF imaging makes this method robust to chromatic defocus spread, unlike focal-series reconstruction methods in conventional, coherent transmission electron microscopy