Achieving phase contrast of weak-phase objects in transmission electron microscopy (TEM) relies on the transmission characteristics of the objective lens. Suitable defocusing conditions have to be chosen to achieve a phase shift of approximately 90 degrees between the scattered and unscattered electrons. Due to the complex transmission characteristics of the objective lens (variation of the phase shift as a function of the spatial frequency), the interpretation of the images is not straightforward. In addition, imposing a phase shift on electrons scattered into relatively small angles is limited by the properties of the contrast transfer function. These effects hamper in particular high-resolution studies of biological objects where information at spatial frequencies around 1 nm-1 is interesting. Besides defocusing, phase contrast can be also achieved by a phase plate, which is commonly used in light microscopy (Zernicke “λ/4 Plättchen”). We fabricate and study different types of phase plates on the basis of thin amorphous films and electrostatic devices. Aim of all investigated PPs is to induce a relative phase shift between the scattered and unscattered part of the electron wave leading to a phase-contrast enhancement.
An electrostatic design for a phase plate put forward by Boersch already in 1947 could be realized for the first time in our group. In this concept, the phase of the unscattered electrons is shifted by a micro-scaled electrostatic lens, which is placed around the unscattered beam in the back focal plane of the objective lens [1,2]. If a suitable voltage is applied to the inner electrode, a 90° phase shift of the unscattered electrons is achieved. However, charging and contamination effects are strong with this type of PP due to the intense electron-beam illumination of the PP material. Artifacts were strongly reduced with the proposed design by Zach [6,8] which then allowed the beneficial application of the so-called Zach PP in different fields such as biology  or material science  as well as the study of the role of inelastic scattering in phase contrast images . The work on Zach PPs is ongoing and is especially focused on high-resolution and cryo TEM.
The research on thin-film PPs started with Hilbert and Zernike-type PPs which are realized in a transmission electron microscope by placing a microstructured thin amorphous carbon film in the back focal plane of the objective lens. If a carbon film with a suitable thickness is used, the phase of the scattered electrons is shifted by 90 degrees. However, it turned out that PPs made from amorphous carbon tend to charge and degenerate during electron-beam illumination in the microscope. Therefore the search for alternative materials is an important part of our research [10,12]. Another approach is to exploit charging of the amorphous carbon thin film by using an unstructured and thus hole-free PP where the phase shift is created by illumination of the PP with the intense beam of unscattered electrons. Therefore we studied both contamination and charging of amorphous thin films under intense electron-beam illumination and proposed a model for the working principle of hole-free PPs [14-15].
The work on PPs is carried out in collaboration with the groups of R.R. Schröder (CellNetworks, Bioquant, University of Heidelberg), M. Malac (National Institute for Nanotechnology and Department of Physics, University of Alberta) and M. Marko (New York State Department of Health, Wadsworth Center).
Selected conference poster presentations:
 K. Schultheiß, F. Pérez-Willard, B. Barton, D. Gerthsen, R.R. Schröder,
Fabrication of a Boersch phase plate for phase contrast imaging in a transmission electron microscope,
(Also selected for publication in Virtual Journal of Nanoscale Science & Technology 13, 11, 2006)
Rev. Sci. Instr. 77, 033701 (2006)
 E. Majorovits, B. Barton, K. Schultheiß, F. Pérez-Willard, D. Gerthsen, R. Schröder,
Achieving optimized phase contrast electron microscopy with an electrostatic Boersch phase plate,
Ultramicroscopy 107, 2, 213 (2007)
 B. Gamm, K. Schultheiß, D. Gerthsen and R.R. Schröder,
Effect of a physical phase plate on contrast transfer in an aberration-corrected transmission electron microscope,
Ultramicroscopy 108, 9, 878 (2008)
 B. Gamm, M. Dries, K. Schultheiss, H. Blank, A. Rosenauer, R.R. Schröder and D. Gerthsen,
Object Wave Reconstruction by Phase-Plate Transmission Electron Microscopy,
Ultramicroscopy, in press (2010)
B.Gamm, M. Dries, K. Schultheiss, H. Blank, A. Rosenauer, R.R. Schröder, D.Gerthsen
Object wave reconstruction by phase plate transmission electron microscopy
Ultramicroscopy 110, 807 (2010)
 K. Schultheiss, J. Zach, B. Gamm, M. Dries, N. Frindt, R.R. Schröder, D. Gerthsen
New electrostatic phase plate for phase-contrast transmission electron microscopy and its application
for wave-function reconstruction
Microsc. Microanal. 16, 785 (2010)
 M. Dries, K. Schultheiss, B. Gamm, A. Rosenauer, R.R. Schröder, D. Gerthsen
Object-wave reconstruction by carbon film-based Zernike- and Hilbert-phase plates
Ultramicroscopy 111, 159 (2011)
 S. Hettler, B. Gamm, M. Dries, N., R.R. Schröder, D. Gerthsen
Improving fabrication and application of Zach phase plates for phase-contrast transmission electron microscopy
Microsc. Microanal. 18, 1010 (2012)
 N. Frindt, M. Oster, S. Hettler, B. Gamm, L. Dieterle, D. Gerthsen, R.R. Schröder
In-focus electrostatic Zach phase plate imaging for Transmission Electron Microscopy with tunable phase
contrast of frozen-hydrated biological samples
Microsc. Microanal (2014).
 M. Dries, S. Hettler, B. Gamm, E. Müller, W. Send, K. Müller, A. Rosenauer, D. Gerthsen
A nanocrystalline Hilbert phase-plate for phase-contrast transmission electron microscopy
Ultramicroscopy 139, 29 (2014)
 S. Hettler, J. Wagner, M. Dries, M. Oster, C. Wacker, R. R. Schröder, D. Gerthsen.
On the role of inelastic scattering in phase-plate transmission electron microscopy.
Ultramicroscopy 155, 27-41 (2015).
 M. Dries, S. Hettler, T. Schulze, W. Send, E. Müller, R. Schneider, D. Gerthsen, Y. Luo, K. Samwer.
Thin-film phase plates for transmission electron microscopy fabricated from metallic glasses.
Microsc Microanal 22, 5, 955-963 (2016).
 S. Hettler, M. Dries, J. Zeelen, M. Oster, R. R. Schröder, D. Gerthsen.
High-resolution transmission electron microscopy with an electrostatic Zach phase plate.
New J Phys 18, 053005 (2016). OPEN ACCESS!
 S. Hettler, M. Dries, P. Hermann, M. Obermair, D. Gerthsen, M. Malac.
Carbon contamination in scanning transmission electron microscopy and its impact on phase-plate applications.
Micron 96, 38-47 (2017).
 S. Hettler, E. Kano, M. Dries, D. Gerthsen, L. Pfaffmann, M. Bruns, M. Beleggia, M. Malac.
Charging of carbon thin films in scanning and phase-plate transmission electron microscopy.
Ultramicroscopy 184 A, 252-266 (2018).