|Spherical Aberration Cs:|
|Chromatic Aberration Cc:|
|Philips double-tilt low background holder|
Images recorded at different defocus settings contain differently-phased spatial frequencies. Generating one image with sub-Å resolution requires extraction and combination of the correct band of frequencies from members of a focal series. For focal series reconstruction, the OÅM uses the Philips/Brite-Euram software, by Coene and Thust, to process focal series acquired automatically from the microscope under computer control, thereby establishing the exit-surface wave. This process can correct residual lens aberrations due to 2- and 3- fold astigmatism, coma, and spherical aberration.
Electron holography can produce high-resolution images by recording images (holograms) that have been multiplexed with a suitable high-frequency carrier. The carrier frequency is then removed with image processing, yielding a complex image (amplitudes and phases). The complex image is corrected for the phase changes imposed by the microscope imaging system to produce the corrected image at enhanced resolution. In a lower-resolution mode electron holograms can be used to detect electric and magnetic fields, as well as the sample’s mean inner potential.
The 2k Gatan imaging filter (GIF), installed after the microscope column, can be used to map chemical elements at near-atomic resolution (<10Å). Images are acquired with the multiscan CCD camera at sizes up to 2048×2048 pixels and can be read out at speeds up to 2MHz under computer control. Local chemical concentrations can be quantified using the GIF in EELS (electron energy-loss spectroscopy) mode to access the spectroscopic signatures of light elements such as O, N, C, and even He.
Annular Dark Field TEM Imaging
Annular dark field transmission electron microscopy (ADF-TEM) imaging enables Z-contrast imaging without scanning noise and with reduced sample contamination. ADF-TEM uses an objective aperture that acts as a central beam stop in the back focal plane of the objective lens. This causes the central beam and all electrons scattered up to a certain semiangle to be excluded from imaging, and the image formation is entirely nonlinear.
Customized sample holders can be used to produce a tilt series of images in steps of 1 to 2 degrees over an annular range that is limited (to ~70 degrees) by shadow effects from the utilized sample grids. Electron tomograms can be reconstructed from such tilt series. Different imaging modes (Bright Field, Annular Dark field, EFTEM) are available to highlight specific aspects of the investigated sample.