Imaging of light-atom nanocrystals with a thin annular detector in STEM

Dark-field imaging in a scanning transmission electron microscope using a thin annular detector, with a ratio of detector width to detector radius of only about 10%, rather than the commonly-used broad detector, is shown to be especially valuable for the detection of crystallites of diameter 1-3 mn supported on relatively thick amorphous films, even when the crystallites are of light-atom material. Nanocrystals of carbon of average diameter 1.0 nm have been clearly imaged on films of amorphous silica 6 nm thick by using the thin detector to select the diffraction maximum corresponding to the inter-layer graphitic spacing of 0.34 nm. Also, nanoparticles in this size range, whether crystalline or amorphous, may be imaged with the thin annular detector set to detect small-angle scattering. A simple theoretical treatment suggests how the image contrast may vary with the particle size, image resolution and support thickness. Observations on the nanoparticles of carbon on amorphous silica confirm the theoretical predictions.