Contrast Enhancement of Long-Range Periodic Structures using Hole-Free Phase Plate

Phase contrast transmission electron microscopy is a powerful tool to enhance the image contrast of transparent materials such as ice-embedded biological specimens and polymer materials. In this method, a phase plate, which is placed at the back-focal plane of the objective lens, gives a phase shift for scattered electron waves, resulting in a significant enhancement contrast of specimens in images. Zernike phase plate (ZPP), consisting of a thin carbon film with a small central hole, is first tested practical phase plate [1]. However, ZPP has disadvantages for image quality. Phase contrast of specimens in low spatial frequency does not improve, since the scattered electron passing through a central hole of ZPP does not change their phases. The threshold frequency at the center hole edge is called cut-on frequency. The additional disadvantageous effect of the abrupt cut-on frequency reveals that strong fringes appear.