Scientific publications

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.

In comparison of a Pt57.5Cu14.7Ni5.3P22.5 metallic glass with a Pt(111) single crystal we find that wearless friction is determined by chemistry through bond formation alloying, while wear is determined by structure through plasticity mechanisms. In the wearless regime, friction is affected by the chemical composition of the counter body and involves the formation of a liquid-like neck and interfacial alloying. The wear behavior of Pt-based metallic surfaces is determined by their structural properties and corresponding mechanisms for plastic deformation. In the case of Pt(111) wear occurs by dislocation-mediated homogeneous plastic deformation. In contrast the wear of Pt57.5Cu14.7Ni5.3P22.5 metallic glass occurs through localized plastic deformation in shear bands that merge together in a single shear zone above a critical load and corresponds to the shear softening of metallic glasses. These results open a new route in the control of friction and wear of metals and are relevant for the development of self-lubricated and wear-resistant mechanical devices.

We present an algorithm for reconstructing a sample surface potential from its Kelvin probe force microscopy (KPFM) image. The measured KPFM image is a weighted average of the surface potential underneath the tip apex due to the long-range electrostatic forces. We model the KPFM measurement by a linear shift-invariant system where the impulse response is the point spread function (PSF). By calculating the PSF of the KPFM probe (tip+cantilever) and using the measured noise statistics, we deconvolve the measured KPFM image to obtain the surface potential of the sample.The reconstruction algorithm is applied to measurements of CdS-PbS nanorods measured in amplitude modulation KPFM (AM-KPFM) and to graphene layers measured in frequency modulation KPFM (FM-KPFM). We show that in the AM-KPFM measurements the averaging effect is substantial, whereas in the FM-KPFM measurements the averaging effect is negligible.

Friction on Au(100) surfaces has been studied by atomic force microscopy under electrochemical control. Atomic-scale stick-slip pattern in the lateral force signal reveal changes in the surface structure upon changing electrochemical potential, in particular between the hexagonal reconstruction and the Au(100)-(1 × 1) structure. Friction on Au(100) is higher on its (1 × 1) structure than on its hexagonal reconstruction. The frictional response after switching between the two structures is delayed due to the necessary surface reorganization. Atomic periodicity in the stick-slip pattern indicates that the increased friction on Au(100)-(1 × 1) is not caused by an ordered anion adlayer, but by the open structure of the (100) surface. Friction is highest on the oxidized surface, and can be switched reversibly between high and low values on the oxidized and the reduced surface.