Contrast enhancement of nanomaterials using phase plate STEM.

Visualizing materials composed of light elements is difficult, and the development of an imaging method that enhances the phase contrast of such materials has been of much interest. In this study, we demonstrate phase-plate scanning transmission electron microscopy (P-STEM), which we developed recently, and its application to nanomaterials. An amorphous carbon film with a small hole in its center was used to control the phase of incident electron waves, and the phase-contrast transfer function (PCTF) was modified from sine-type to cosine-type. The modification of the PCTF enhances image contrast with a spatial frequency below 1 nm. The PCTF for P-STEM with a spatial frequency below 1 nm is about three times stronger than that of bright field STEM. The ratio obtained using power spectra is consistent with the result obtained from images of quantum dots. The image contrast of biological materials was also enhanced by P-STEM.