Innovative Electron Microscopy
Nanoscale characterization is essential for the growth of modern nanotechnology, energy science, and biology. The Innovative Electron Microscopy (IEM) group conducts interdisciplinary research at the interface of bio-nanotechnology, materials science, cell biology, physics of electron microscopy (EM), and image processing. The group uses state-of-the-art transmission EM (TEM), and scanning TEM (STEM) for three-dimensional (3D) atom-by-atom structural and chemical analysis of materials, and for the study of biological systems and functional materials at realistic conditions. Various research collaborations exist both with academia, and industry. Students and users obtain high-quality training on modern electron microscopy in our group.
The INM has recently acquired a state-of-the-art aberration corrected STEM/TEM of the type ARM200, JEOL, Japan, with a combined energy filter and an electron energy loss analyzer (Gatan). This microscope has a newly developed cold field emission source (CFEG) with low energy spread. The microscope combines a spot size of 0.08 nm with a probe current of 200 pA and an energy spread of only 0.3 eV. This microscope will be used for three areas of research at the institute and also at the Saarland University.
We have recently developed a novel microscopy methodology to image whole eukaryotic cells in liquid using a microfluidic chamber for scanning transmission electron microscopy (STEM), termed liquid STEM. Eukaryotic cells in liquid are enclosed in a micro-fluidic chamber with a thickness of up to 10 µm contained between two ultra-thin and electron-transparent windows. This chamber is then placed in the vacuum chamber of the electron microscope. The specimen is imaged with STEM. On account of the atomic number (Z) contrast of the STEM, nanoparticles of a high-Z material, such as gold, can be detected within the background signal produced by a low-Z liquid, such as water. Nanoparticles specifically attached to proteins can then be used to study protein distributions in whole cells in liquid, similar as proteins tagged with fluorescent labels can be used to study protein distributions in cells with fluorescence microscopy, but then with a much better spatial resolution.
Our future aim is to study processes by combining liquid STEM with high resolution fluorescence microscopy. With this novel microscopy method we may possibly discover new phenomena that are not visible with existing microscopy methodologies.
The microfluidic device developed for liquid STEM will also be used to study nanomaterials in liquid. The achievable resolution depends on the difference of the atomic number of the materials under investigation compared to the atomic number of the liquid, and on the path length of the electron beam through the liquid. Typically, a resolution of 1 nm can be achieved for the imaging of gold nanoparticles in a water thickness of 3 µm. The microfluidic channel allows the rapid injection of fluids. The capability to image materials in liquid with nanoscale resolution is especially relevant for material science related to energy storage. But, of course the experiments have to be carefully designed and interpreted for effects of Brownian motion, electric charging, and radiation damage.
We are developing a novel methodology to acquire 3D data sets using aberration corrected STEM. The primary method currently used for obtaining nanoscale 3D information of materials is via tilt-series TEM (tomography). A 3D cubic volume is reconstructed from images recorded at several projections obtained by mechanically tilting the sample stage. A novel approach uses aberration-corrected STEM, which is capable of high-resolution 3D imaging without a tilt stage. In a manner similar to confocal light microscopy, the sample is scanned layer-by-layer by changing the objective lens focus so that a focal series is recorded. The technique is possible with high axial (vertical) resolution because of the greatly reduced depth of field in an aberration-corrected STEM.
3D STEM will be developed for both materials science and biology. In biology, the aim is to acquire 3D datasets of whole cells, where STEM provides enhanced contrast on nanoparticles used, for example, as specific protein labels. In materials science 3D STEM is expected to be beneficial, for instance, for 3D mapping nanoparticles in polymer matrices. 3D STEM can be combined with EELS and Z-contrast to characterize the 3D atomic composition of materials, for example, to localize defects in semiconductors. 3D STEM is a wide field technique, where each slice of a 3D dataset contains both the in-focus and out-of-focus information. Deconvolution procedures are being developed to reconstruct 3D models using calculations of the 3D point spread function (PSF).
Additionally, we also apply conventional STEM tomography.
New projects will be developed in the areas of functional nanomaterials, and of energy-related materials, such as solar cells, solid-state lighting elements, and catalytic materials. The properties of functional materials are closely related to the atomic structure and especially dislocations of atoms within the bulk structure, and at interfaces. Aberration corrected STEM is capable of atomic-resolution elemental mapping using EELS and Z-contrast, such that dislocations of single atoms can be studied within the atomic matrix.