Phase contrast in TEM has been accomplished in several ways, most usefully by a so-called Zernike phase plate (ZPP). The ZPP is a thin membrane with a small aperture (~1 µm) centered on the beam near the back focal plane of the objective. Unfortunately, a ZPP is difficult to align, is subject to contamination and charging, and causes a certain amount of ringing in the image due to the sharp phase shift at the edge of the aperture.
The Baumeister group at the Max Planck Institute of Biochemistry along with FEI, Eindhoven, have discovered that a thin membrane of amorphous carbon without aperture gives superior results. They hypothesize that in the area where the beam intensity is highest, the film develops a negative surface charge. The mechanism is not yet understood, but after aging, the phase shift is reproducible for months. The resulting device has been dubbed the Volta phase plate.
Since the charge apparently develops where the beam is most intense, it is insensitive to changes in column alignment. Furthermore, the phase edge is soft. Therefore, images show no phase ringing.