Publikationen

Transmission electron microscopy (TEM) in combination with electron tomography is widely used to obtain nanometer scale three-dimensional (3D) structural information about biological samples. However, studies of whole eukaryotic cells are limited in resolution and/or contrast on account of the effect of chromatic aberration of the TEM objective lens on electrons that have been scattered inelastically in the specimen. As a result, 3D information is usually obtained from sections and not from whole cells. Here, we use chromatic aberration-corrected TEM to record bright-field TEM images of nanoparticles in a whole mount macrophage cell. Tilt series of images are used to generate electron tomograms, which are analyzed to assess the spatial resolution that can be achieved for different vertical positions in the specimen. The uptake of gold nanoparticles coated with low-density lipoprotein (LDL) is studied. The LDL is found to assemble in clusters. The clusters contain nanoparticles taken up on different days, which are joined without mixing their nanoparticle cargo.

Imaging single epidermal growth factor receptors (EGFR) in intact cells is presently limited by the available microscopy methods. Environmental scanning electron microscopy (ESEM) of whole cells in hydrated state in combination with specific labeling with gold nanoparticles was used to localize activated EGFRs in the plasma membranes of COS7 and A549 cells. The use of a scanning transmission electron microscopy (STEM) detector yielded a spatial resolution of 3 nm, sufficient to identify the locations of individual EGFR dimer subunits. The sizes and distribution of dimers and higher order clusters of EGFRs were determined. The distance between labels bound to dimers amounted to 19 nm, consistent with a molecular model. A fraction of the EGFRs was found in higher order clusters with sizes ranging from 32-56 nm. ESEM can be used for quantitative whole cell screening studies of membrane receptors, and for the study of nanoparticle-cell interactions in general.

The lateral and axial resolution of three-dimensional (3D) focal series aberration-corrected scanning transmission electron microscopy was studied for samples of different thicknesses. The samples consisted of gold nanoparticles placed on the top and at the bottom of silicon nitride membranes of thickness between 50 and 500 nm. Atomic resolution was obtained for nanoparticles on top of 50-, 100-, and 200-nm-thick membranes with respect to the electron beam traveling downward. Atomic resolution was also achieved for nanoparticles placed below 50-, 100-, and 200-nm-thick membranes but with a lower contrast at the larger thicknesses. Beam broadening led to a reduced resolution for a 500-nm-thick membrane. The influence of the beam broadening on the axial resolution was also studied using Monte Carlo simulations with a 3D sample geometry.

The formation of characteristic inversion domain structures in zinc oxide (ZnO) is triggered by the addition of trivalent Fe3+ or In3+ dopants. As-grown and inverted ZnO domains are separated by two types of inversion domain boundaries (IDBs): basal b-IDBs parallel to {0001}, and pyramidal p-IDBs parallel to {2 over(1, -) over(1, -) 5} lattice planes in three equivalent variants. Cs-corrected analytical TEM/STEM is the method of choice for a comprehensive structural and compositional characterization of these materials. It is shown by electron and X-ray spectroscopic imaging in STEM that dopant species are essentially localized within both types of IDBs, whereas solid solubility of trivalent dopants within ZnO domains is rather low (<0.5 at%). Under the assumption of one monolayer per IDB the relation between inversion domain structure and integral dopant concentration correlates well with integral EDS and EELS measurements in STEM over well defined sample regions. The presence of one close-packed monolayer of trivalent dopant ions within a b-IDB is unambiguously confirmed by atomic resolution STEM imaging. Columns of cations are clearly imaged in high-angle annular dark-field (HAADF) STEM imaging, whereas annular bright-field (ABF) STEM is capable of imaging both light and heavy atom columns simultaneously. It is shown that structural details in ABF images are directly interpretable even in specimen regions with thickness >50 nm. The structural inversion associated with a stacking fault as a consequence of the presence of octahedrally coordinated In3+ in the b-IDB is directly revealed by atomic resolution imaging. Column positions in atomic resolution ABF imaging in In2O3-ZnO nanorods show that the oxygen sub-lattice continues across the b-IDB with only marginal distortions, whereas the cation sub-lattice suffers a rigid shift relative to the oxygen lattice as a result of the coordination geometry of ZnO4 tetrahedrons sharing common oxygen ions with the InO6 coordination octahedrons.

Straight, axial InAs nanowire with multiple segments of GaxIn1-xAs was grown. High resolution X-ray energy-dispersive spectroscopy (EDS) mapping reveals the distribution of group III atoms at the axial interfaces and at the sidewalls. Significant Ga enrichment, accompanied by a structural change is observed at the GaxIn1−xAs/InAs interfaces and a higher Ga concentration for the early grown GaxIn1−xAs segments. The elemental map and EDS line profile infer Ga enrichment at the facet junctions between the sidewalls. The relative chemical potentials of ternary alloys and the thermodynamic driving force for liquid to solid transition explains the growth mechanisms behind the enrichment.

Gold surfaces exhibit most interesting frictional properties on the nanometer scale. They can be studied in detail by means of friction force microscopy. Atomic-scale variations of the lateral force allow investigation of microscopic mechanisms of sliding. Friction force microscopy even reveals surface reconstruction of the gold surface as a modulation of the lateral force signal. Experiments indicate that the mobility of surface atoms at room temperature and plastic deformation mechanisms give rise to neck formation between gold and microscopic asperities in sliding contact. The frictional properties of gold surfaces change dramatically at temperatures below 150 K, where the surface diffusion is greatly reduced. Insight into the lubrication properties of self-assembled monolayers is provided by molecular-scale modulations of frictional forces. Molecular-scale maps of the friction force also allow identification of the relevant surface structure in experiments on electrochemically modified gold surfaces. Variation of the electrochemical potential is a means to reversibly switch between low and high friction states on gold surfaces.

The friction of microstructured polydimethylsiloxane samples against a glass surface is studied through force measurements and simultaneous optical microscopy. Both average friction forces and the amplitude of stick-slip oscillations are greatly reduced by the structuring. Optical microscopy reveals waves propagating through the contact in connection which stick-slip events. The experimental observations are interpreted with the help of simulations of a spring-block model for which parameters are directly derived from the experiment. Stress gradients across the contact area are found to play an important role for the frictional behavior.