Spectral DQE of the Volta phase plate

•Signal-loss associated with the VPP increases linearly as a function of spatial frequency.•This loss represents a decrease in the spectral detective quantum efficiency, DQE(s), which is attributable solely to the VPP.•At 2 Å resolution the DQE has been reduced to ~40% for both 200 kV and 300 kV beam energy.•Experiments have narrowed down the root cause of this reduction, pointing at a surface effect of the VPP material that influences both the unscattered and the scattered part of the electron beam. The Volta Phase Plate (VPP) consists of a heated, thin film that is placed in the same plane as the focused diffraction pattern of an electron microscope. A change in surface potential develops at the point irradiated by the intense, unscattered electron beam, and this altered surface potential produces, in turn, a phase shift between the unscattered and scattered parts of the electron wave. While the VPP thus increases the image contrast for weak-phase objects at low spatial frequencies, we report here that it also leads to the loss of an increasing fraction of the signal at higher resolution. The approximately linear dependence (with increasing resolution) of this loss has been quantified at 200 kV and 300 kV, using evaporated-carbon films of different thicknesses as Volta phase plates. In all cases, the loss of signal remains almost independent of variation of the conditions and parameters that were tested. In spite of having done a number of additional, discovery-based experiments, the cause of this loss of signal remains unexplained at this point.