Conventional and scanning transmission electron microscopy: image contrast and radiation damage

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Abstract
A theoretical comparison is made of the image contrast that can be obtained in conventional transmission electron microscopy (CTEM), both bright-field and dark-field, and scanning transmission electron microscopy (STEM) from unstained biological specimens and single heavy atoms. The contrast in bright-field CTEM is generally lower than in dark-field CTEM and dark-field STEM, but the image contrast is significantly decreased in dark-field CTEM when the effect of a specimen substrate is included. The deciding factor in evaluating the structure of biological materials by electron microscopy is radiation damage. An evaluation is made of the signal-to-noise ratio in both CTEM and STEM images for a low (subminimal) electron dose (8 C m-2) and a minimal electron dose (250 C m-2). Generally the signal-to-noise ratio of the image is about a factor of two greater in bright-field CTEM than in dark-field STEM and a factor of five greater than in dark-field CTEM. This advantage of bright-field CTEM decreases as the specimen substrate thickness increases.