Achieving phase contrast of weak-phase objects in transmission electron microscopy relies today 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. Do 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. 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 carbon films and electrostatic devices. Our research on phase plates is carried out in close collaboration with the group of R.R. Schröder (CellNetworks, Bioquant, University of Heidelberg).
Hilbert half-plane phase plates were already realized in a transmission electron microscope by placing a thin amorphous carbon film in the back focal plane of the objective lens, e.g. by Nagayama et al. (J. Phys. Soc. Jpn., Vol. 73, 2718 (2004)). If a carbon film with a suitable thickness is used, the phase of the scattered electrons is shifted by 180 degrees. The amorphous carbon film is positioned in such a way in the back focal plane, that the scattered electron in one half of the diffraction pattern experience a phase shift while the unscattered electrons and the second half of the scattered electrons propagate through vacuum. Examples for phase contrast obtained by a Hilbert phase plate are shown in the poster “Application of a Hilbert phase plate in transmission electron microscopy of materials science samples”.
The picture on the left shows a Hilbert phase plate, a thin Carbon film with an opening cut by focused-ion-beam milling. Half of the diffraction pattern is covered by the amorphous carbon film while the unscattered electron and scattered electron in the other half of the diffraction pattern propagate through the rectangular opening without a phase shift
An alternative suggestion 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 transmitted beam in the back focal plane of the objective lens. A Boersch phase plate with a five-layered electrode structure held by three bars is shown on the right. If a suitable voltage is applied to the inner electrode a 90° phase shift of the unscattered electrons is achieved.
Theoretical studies of the contrast-transfer characteristics of a transmission electron microscope equipped with a spherical-aberration corrector (Cs-corrector) in the imaging lens system and a phase plate shows that almost ideal contrast transfer can be achieved. Phase contrast is significantly enhanced by the use of a phase plate. Moreover, negligible image delocalization is achieved for enlarged intervals of the spherical aberration coefficient Cs and objective lens defocus. This will be of particular interest for future high resolution microscopes where parameter intervals for optimal imaging conditions will be far smaller.
The understanding of image formation using phase plates is significantly supported by image simulations. While image formation for weak-phase object is readily understood, it is much more complex for most real (crystalline) objects. Especially non-linear image formation in combination with a phase plate is studied to understand imaging of material science specimen. The simulation program STEMSIM by A. Rosenauer (Institute for Solid State Physics, University of Bremen) was extended to account for phase shifts introduced by phase plates with variable phase shifts and geometries. This is illustrated by the two simulated high-resolution TEM images below which show in both cases silicon along the <100> zone axis containing vacancies. The image on the left-hand side corresponds to a conventional high-resolution TEM image. The image on the right-hand side was simulated assuming a 90 degree phase shift for the unscattered electrons which enhances the contrast in regions containing vacancies.
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)