Annular Dark Field STEM Imaging Research Articles

Visibility of single heavy atoms on thin crystalline silicon in simulated annular dark-field STEM images

The multislice method, pioneered by Cowley and Moodie, has recently been adapted to simulate annular dark-field scanning transmission electron-microscope (ADF STEM) images. This paper presents a series of calculations using this new approach with experimental parameters appropriate for a VG-HB501 STEM to investigate the visibility of single heavy adatoms on thin crystalline silicon membranes. The tendency for electrons to channel along columns of atoms in crystals can greatly increase the intensity incident on an adatom on the exit surface, thereby increasing the adatom visibility. The simulations indicate that an adatom on the exit surface on a column of crystal atoms is up to three times as visible as an adatom on the entrance surface, and that the adatom remains highly visible as the crystal thickness is increased. Tilting the specimen or displacing the adatom from the column appears to lower the visibility of the adatom dramatically. These calculations suggest that, with the appropriate imaging conditions, a single gold adatom may be visible on at least 235 of (111) silicon.

Digital restoration of ADF-STEM images

Frieden and Wells have derived a maximum likelihood (ML) image restoration algorithm that accurately models the noise in each image element using Poisson counting statistics. This method also leads naturally to “maximum entropy” like ideas. They have reported a rather dramatic resolution enhancement when this method was applied to low light level astronomical images in which the image noise was essentially due to photon counting statistics. This low light level situation also accurately models the ADF (annular dark field) STEM (scanning transmission microscope) image if the image is aquired digitally by electronically counting the individual scattered electrons for each position of the focused probe.The simplest ADF-STEM image model assumes that the electron intensity distribution in the focused probe is the incoherent point spread function (psf) of the image.where g(x)=recorded image, f(x)=ideal image, n(x)=random noise, * represents convolution and

Improved visualization of single- and double-stranded nucleic acids by STEM

Annular dark field STEM images have been used to visualize nucleic acid molecules positively stained with uranyl acetate. The regular distribution of uranium clusters makes clearly visible the existence of segments of double and single strands on partially denatured RNA molecules. Unpaired regions, as short as about 8 nm, are detectable by this combination of a highly efficient imaging mode and a well adapted preparation technique.