PhD / Masters / Bachelor Projects: click here for details.
Research Interests:
My research interests focus on the development of new methods in quantitative
transmission electron microscopy (TEM) and their application to materials science problems.
Here is a list of some of my current research projects:
- Nonlinear Inline Electron Holography: Mapping electrostatic and magnetic fields in thin TEM samples with
nanometer resolution. I call this technique 'inline holography' instead of 'focal series reconstruction'(FSR)
because it aims at also retrieving low spatial frequency components of the exit wave function.
FSR is already well-established in the field of high-resolution TEM and therefore does not involve
dealing with optical distortions [such as relative rotation, spiral- and pincushion distortions or changes in magnification]
between images [more].
-
Strain mapping
in semiconductors and ceramics by dark-field inline holography and from focal series reconstructions.
I currently run a DFG-funded project to extend this technique to 3D with a target resolution of 1 nm in all 3 dimensions
(no missing wedge).
- The dynamic inversion problem:
The strong interaction of electrons with matter (about 4 orders of magnitude greater than X-rays)
causes electrons to scatter multiple times within the investigated sample. Instead of trying to
avoid multiple scattering events I have developed new methods to extract structure factors from
dynamical electron diffraction patterns. This is not only more precise than applying kinematic
theory, but it also omits the phase problem (i.e. the need to use information about the scattering
sample to recover the phase of the scattered electron beams), since there is no phase problem in
dynamical scattering [more].
- New electron diffraction acquisition methods:
The current approach for solving the dynamic inversion problem requires electron diffraction data
over a fairly large range of incident angles. I have developed a software tool which automatically
acquires LARBED patterns
with a single exposure of the recording medium (CCD, image plate, film, etc)
[see also this presentation given at EMC2008].
- 3D atomic resolution imaging:
Development of methods for reconstructing the 3-dimensional potential from a series of tilt-defocus series of HRTEM images
[more].
- Tomography:
In particular tomography using the signal from reconstructed inline holograms, i.e.
mapping of electrical and magnetic fields in 3D, and energy-filtered transmission electron
microscopy (EFTEM) which allows 3-dimensional chemical mapping at nm resolution, but also
diffraction tomography.
- Simulation of STEM and TEM images:
I developed software for simulating HRTEM and (HAADF-) STEM images as well as CBED patterns of
arbitrarily large structures. This computer code is based on the multislice algorithm and is
aimed at being as quantitative as possible.
The code therefore allows slices to be arbitrarily thin (i.e. < 1A) and includes thermal diffuse scattering (TDS).
- Quantification of HRTEM image contrast:
High-resolution TEM image contrast is still not very well understood. Simulated and experimental images
show severe contrast mismatch, which is often attributed to TDS. I therefore investigate the nature (particularly coherence and energy-loss) of
TDS by theoretical and experimental means.
- Mapping potentials at grain boundaries in ceramics:
The main driving force behind developing the above mentioned inline holography is funding by the EC
for investigating grain boundaries in ceramics
(see more at the INCEMS project website that I am responsible for).
-
Additional collaborative projects include mapping of surface plasmons on metallic nano-particles,
the investigation of grain boundary potentials in solar cell materials, and a more quantitative interpretation
of impedance spectroscopy of ceramic materials.
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