Application of Zach Phase Plates for Phase-Contrast Transmission Electron Microscopy: Status and Future Experiments

Abstract

Achieving phase contrast for weak-phase objects has been the driving force for the development of physical phase plates (PP) for transmission electron microscopy (TEM). Thin-film PPs have meanwhile reached a well-developed state [1]. However, carbon-based PPs suffer from fast material degradation under intense electron irradiation which has initiated the evaluation of alternative materials [2] and other PP approaches as reviewed in [3]. In contrast to thin-film PPs, electrostatic PPs allow the variation of the phase shift which opens the possibility to optimize phase contrast for different types of specimens and facilitate, e.g., wave-function reconstruction not only of weak-phase objects [4]. Among the different electrostatic PP approaches, the Zach PP [5] appears to be most promising because it minimizes obstructing structures in the back focal plane (BFP) and, hence, artifacts induced by blockade of spatial frequencies. Substantial improvements have been made with respect to contamination and charging which yields phase-contrast TEM images with well-controlled phase shift. This was demonstrated by Frindt et al. [6] where imaging with a Zach PP yielded for the first time in-focus phase-contrast images of weak-phase objects (filamentous actin embedded in vitreous ice) with negligible artifacts.