scanning transmission electron microscopy

A conventional TEM floods the whole specimen with a broad beam at once and forms an image the way a slide projector throws a picture on a wall. Scanning transmission electron microscopy (STEM) works the opposite way, more like a flashlight explored in the dark: it focuses the electrons into an extremely fine probe — down to less than one angstrom in an aberration-corrected instrument — and scans that probe across the thin specimen point by point, building the image one pixel at a time from what each point transmits. It is essentially the scanning electron microscope's raster idea, but carried out in transmission through a thin foil.

Because the image is built pixel by pixel, you choose what to measure at each point by placing detectors below the specimen at different angles. A small on-axis detector catches the un-deviated beam for a bright-field image. An annular (ring-shaped) detector catches electrons scattered to moderate angles (annular dark field). A larger ring catching only the electrons flung out to high angles gives high-angle annular dark field (HAADF), the basis of Z-contrast imaging. The great bonus is that while the probe sits on one atomic column you can simultaneously collect its energy-loss spectrum (EELS) or emitted X-rays (EDS), so structure and chemistry are mapped together, column by column.

STEM's strength is that its images can be more directly interpretable than the interference fringes of HRTEM: high-angle scattering is largely incoherent, so a HAADF image behaves like an honest map of where the heavy atoms are, without the focus-dependent contrast reversals of phase-contrast imaging. The price is demanding hardware and stability. You need a bright, coherent field-emission source and, for atomic resolution, aberration correction; and because the picture is assembled slowly point by point, specimen drift and scan distortions during the scan are the constant enemies, smearing or warping the atomic lattice if the sample moves even slightly.