Publikationen

Abstract Reversible adhesion is the key functionality to grip, place, and release objects nondestructively. Inspired by nature, micropatterned dry adhesives are promising candidates for this purpose and have attracted the attention of research groups worldwide. Their enhanced adhesion compared to nonpatterned surfaces is frequently demonstrated. An important conclusion is that the contact mechanics involved is at least as important as the surface energy and chemistry. In this paper, the roles of the contact geometry and mechanical properties are reviewed. With a focus on applications, the effects of substrate roughness and of temperature variations, and the long‐term performance of micropatterned adhesives are discussed. The paper provides a link between the current, detailed understanding of micropatterned adhesives and emerging applications.

The present work investigates the effect of crystal orientation and temperature on the size effect using micro-sized single-crystal bending beams of copper with thicknesses between 1 and 5 μm. Two crystal orientations were studied: {111}<153> for single slip and {111}<001> for multiple slip. The bending tests were carried out with a nanoindenter installed in a scanning electron microscope at temperatures between room temperature and 200 °C. The results show clearly that, besides the phenomenon smaller is stronger, the size effect is influenced by temperature particularly in beams oriented for single slip. We demonstrated that, in the presence of strain gradients, the mechanical response is governed by dislocation pile-ups below a critical beam size, being more pronounced in single slip conditions. The pile-ups, however, collapse by “temperature-induced” cross-slip with increasing temperature, while in bigger samples, dislocation networks govern the strength.

We show that various quantities of relevance to the contact mechanics of randomly rough surfaces can be directly estimated from the easy-to-acquire height-difference-autocorrelation function Gδh(Δr). These include the areal elastic energy density and the stress autocorrelation function, for which we derive expressions that are exact for full contact (within linear elasticity) and approximate for partial contact (within Persson theory). Our approach makes it possible to estimate scale–dependent contact areas, stresses, stress gradients, or to make well-informed corrections to the Dahlquist criterion for adhesion with elementary mathematical operations that do not necessitate the Fourier transform to be taken.

Receptor membrane proteins in the plasma membranes of cells respond to extracellular chemical signals by conformational changes, spatial redistribution, and (re-)assembly into protein complexes, for example, into homodimers (pairs of the same protein type). The functional state of the proteins can be determined from information about how subunits are assembled into protein complexes. Stoichiometric information about the protein complex subunits, however, is generally not obtained from intact cells but from pooled material extracted from many cells, resulting in a lack of fundamental knowledge about the functioning of membrane proteins. First, functional states may dramatically differ from cell to cell on account of cell heterogeneity. Second, extracting the membrane proteins from the plasma membrane may lead to many artefacts. Liquid-phase scanning transmission electron microscopy (STEM), in short liquid STEM, is a new technique capable of determining the locations of individual membrane proteins within the intact plasma membranes of cells in liquid. Many tens of whole cells can readily be imaged. It is possible to analyse the stoichiometry of membrane proteins in single cells while accounting for heterogenic cell populations. Liquid STEM was used to image epidermal growth factor receptors in whole COS7 cells. A study of the dimerisation of the HER2 protein in breast cancer cells revealed the presence of rare cancer cells in which HER2 was in a different functional state than in the bulk cells. Stoichiometric information about receptors is essential not only for basic science but also for biomedical application because they present many important pharmaceutical targets.
Membrane proteins in the plasma membranes of cells are responsible for important cellular functions such as the response to external signals. Their functional state is often assessed from the way they are assembled into protein complexes. However, state-of-the-art methods to examine protein assembly involve the extraction of the proteins from cells, and analysing pooled protein material from many cells, at least for naturally expressed proteins. We describe a new electron microscopy method capable of examining protein complexes in whole cells at the single molecule and single cell level. This technique is liquid-phase scanning transmission electron microscopy, in short, liquid STEM. The method was used to image epidermal growth factor receptors in whole mammalian cells. Its application to study the functional state of so-called HER2 protein in breast cancer cells revealed the presence of rare cancer cells in which HER2 was in a different functional state than in the bulk cells, and these rare cells are possibly important as pharmaceutical targets.

The sample dependent spatial resolution was calculated for transmission electron microscopy (TEM) and scanning TEM (STEM) of objects (e.g., nanoparticles, proteins) embedded in a layer of liquid water or amorphous ice. The theoretical model includes elastic- and inelastic scattering, beam broadening, and chromatic aberration. Different contrast mechanisms were evaluated as function of the electron dose, the detection angle, and the sample configuration. It was found that the spatial resolution scales with the electron dose to the −1/4th power. Gold- and carbon nanoparticles were examined in the middle of water layers ranging from 0.01–-10 µm thickness representing relevant classes of experiments in both materials science and biology. The optimal microscope settings differ between experimental configurations. STEM performs the best for gold nanoparticles for all layer thicknesses, while carbon is best imaged with phase-contrast TEM for thin layers but bright field STEM is preferred for thicker layers. The resolution was also calculated for a water layer enclosed between thin membranes. The influence of chromatic aberration correction for TEM was examined as well. The theory is broadly applicable to other types of materials and sample configurations.

The spatial resolution of aberration-corrected annular dark field scanning transmission electron microscopy was studied as function of the vertical position z within a sample. The samples consisted of gold nanoparticles (AuNPs) positioned in different horizontal layers within aluminum matrices of 0.6 and 1.0 µm thickness. The highest resolution was achieved in the top layer, whereas the resolution was reduced by beam broadening for AuNPs deeper in the sample. To examine the influence of the beam broadening, the intensity profiles of line scans over nanoparticles at a certain vertical location were analyzed. The experimental data were compared with Monte Carlo simulations that accurately matched the data. The spatial resolution was also calculated using three different theoretical models of the beam blurring as function of the vertical position within the sample. One model considered beam blurring to occur as a single scattering event but was found to be inaccurate for larger depths of the AuNPs in the sample. Two models were adapted and evaluated that include estimates for multiple scattering, and these described the data with sufficient accuracy to be able to predict the resolution. The beam broadening depended on z 1.5 in all three models.

The controlled gas-phase loading of a metal–organic framework (MOF) to generate iridium nanoparticles is described. The key is the use of an Ir complex, which was small enough to fit through the pore apertures of the MOF, stable enough to be vaporized and labile enough to be reduced inside the pores of the MOF. Homogenously distributed, nanometer-sized and highly crystalline iridium nanoparticles, located inside the pores of the still intact host matrix of MIL-101 (Cr) could be generated. The material was characterized using electron microscopy, X-ray diffraction, elemental analysis and nitrogen physisorption techniques. Initial catalytic tests showed a high activity in olefin hydrogenation.

The observation and control of interweaving spin, charge, orbital, and structural degrees of freedom in materials on ultrafast time scales reveal exotic quantum phenomena and enable new active forms of nanotechnology. Bonding is the prime example of the relation between electronic and nuclear degrees of freedom. We report direct evidence illustrating that photoexcitation can be used for ultrafast control of the breaking and recovery of bonds in solids on unprecedented time scales, near the limit for nuclear motions. We describe experimental and theoretical studies of IrTe2 using femtosecond electron diffraction and density functional theory to investigate bonding instability. Ir-Ir dimerization shows an unexpected fast dissociation and recovery due to the filling of the antibonding dxy orbital. Bond length changes of 20% in IrTe2 are achieved by effectively addressing the bonds directly through this relaxation process. These results could pave the way to ultrafast switching between metastable structures by photoinduced manipulation of the relative degree of bonding in this manner.