Hans J. Hug

Switching the chirality of single adsorbate complexes.

Pumped up: Propene molecules form chiral complexes when adsorbed on a copper surface. Inelastically scattered tunneling electrons from the tip of a scanning tunneling microscope induce rotation or diffusion of the adsorbate on the surface. Higher tunneling currents can lead to conversion of the adsorbate into the opposite enantiomer.

Nanostructure characterization by a combined x-ray absorption/scanning force microscopy system

A combined x-ray transmission and scanning force microscope setup (NanoXAS) recently installed at a dedicated beamline of the Swiss Light Source combines complementary experimental techniques to access chemical and physical sample properties with nanometer scale resolution. While scanning force microscopy probes physical properties such as sample topography, local mechanical properties, adhesion, electric and magnetic properties on lateral scales even down to atomic resolution, scanning transmission x-ray microscopy offers direct access to the local chemical composition, electronic structure and magnetization. Here we present three studies which underline the advantages of complementary access to nanoscale properties in prototype thin film samples.

Single-molecule chemistry and analysis: mode-specific dehydrogenation of adsorbed propene by inelastic electron tunneling.

A single propene molecule, located in the junction between the tip of a scanning tunneling microscope (STM) and a Cu(211) surface can be dehydrogenated by inelastic electron tunneling. This reaction requires excitation of the asymmetric C-H stretching vibration of the ═CH(2) group. The product is then identified by inelastic electron tunneling action spectroscopy (IETAS).

Complex phase compositions in nanostructured coatings as evidenced by photoelectron spectroscopy: The case of Al-Si-N hard coatings

The chemical state evolution of the Al–Si–N thin films at various Si contents is investigated by x-ray photoelectron spectroscopy (XPS). The detailed evolution of the Al 2p, Si 2p, and N 1s photoelectrons line positions and widths are used to identify different chemical environments as the Si content is changed. The results are compared to x-ray diffraction (XRD) data that indicate the formation of a two-phase Al1−xSixN/SiNy composite when the solubility limit of 6 at. % of Si in AlN is exceeded. In contrast to XRD data, no particular effect is observed in the XPS data at the solubility limit of Si. Instead, two compositional regions can be identified that are separated by a distinct change in the evolution of core level binding energy differences and chemical shifts at about 10–15 at. % of Si. This silicon concentration is identified as the onset of the formation of a SiNy intergranular phase that is a few monolayers thick, having a chemical bonding similar to that in bulk silicon nitride. The observed c...

High resolution magnetic force microscopy study of e-beam lithography patterned Co∕Pt nanodots

E-beam lithography has been used to pattern a continuous Co∕Pt multilayer film with perpendicular anisotropy into circular islands of various sizes down to 25nm diameter on a 60nm pitch. High resolution magnetic force microscopy with in situ applied field has been used to directly determine the switching field distribution (SFD) and hysteresis loop of the islands. For the smallest islands, the coercivity is reduced and the width of the SFD is increased, indicating that they would not be well suited to data storage applications.

Role of negatively charged defects in the lattice contraction of Al–Si–N

Experiments reveal that incorporation of substitutional Si in wurtzite AlN up to 6 at. % results in a lattice contraction in the [0001] direction. The contraction is linear and, for higher silicon contents, the lattice parameters remain constant. We investigate the geometric and electronic properties of Al–Si–N compounds with Si content varying from 0 to 9 at. % by means of ab initio simulations based on density functional theory. We demonstrate that charged defects are necessary to support the experimental evidence of a shrinking cell parameter: an ideal Al–Si–N wurtzite structure with delocalized charges would undergo lattice expansion due to Coulomb repulsion upon Si incorporation. Charged defects that act as acceptors and compensate the excess charge coming from Si overcompensate the lattice expansion and therefore promote a lattice contraction.

Microstructural and magnetic properties of thermally mixed Ni/Si bilayers

Polycrystalline nickel layers, deposited on Si(1 1 0) wafers via electron beam evaporation to a thickness of 29 or 68–70 nm, were thermally annealed in vacuo at 493 or 530 K. The elemental interdiffusion across the Ni/Si interface was measured by means of Rutherford backscattering spectroscopy, and the relaxation of stress and grain growth by means of x-ray diffraction. At 530 K, a slight logarithmic increase in the interface variance with the annealing time, but no crystalline silicide formation was observed. The in-plane magneto-optical Kerr effect and magnetic force microscopy were used to investigate the changes in the magnetic properties. With increasing annealing time, the decrease in coercivity and gain in magnetic remanence were correlated with the relaxation of stress. Similarities with ion-irradiated Ni/Si couples will be discussed.

NanoXAS, a novel concept for high resolution microscopy

We currently develop a novel type of scanning x-ray microscope. This instrument will combine the chemical specificity of x-ray absorption spectroscopy with the very high spatial resolution of scanning probe microscopy . In a fundamentally new instrumental approach, the instrument can be used as a conventional scanning transmission x-ray microscope (STXM), as a conventional scanning probe microscope (SPM) or in a mode combining these two techniques. In the latter case, the sample is placed in the focus generated by the fresnel zone plate of the STXM. The SPM-tip placed downstream acts as as a local detector of the emitted photoelectrons. Simulations and experiments have shown that the use of shielded SPM-tips is crucial to obtain a strongly increased chemical resolution. In contrast to similar projects underway at other synchrotrons we use a coaxial geometry. This should greatly enhance the flux density and reduce background signals caused by straylight illuminating the tip.

Engineering the ferromagnetic domain size for optimized imaging of the pinned uncompensated spins in exchange-biased samples by magnetic force microscopy

Magnetic force microscopy (MFM) is able to image and quantify patterns of pinned uncompensated spins (UCS) in exchange-biased samples with high spatial resolution and submonolayer spin sensitivity. However, MFM can only detect magnetic moment distributions with spatial wavelengths within a certain range. Samples with large domains, homogeneous, or divergence-free magnetization fields are not accessible to MFM analysis. In this work we discuss the sample structure constraints placed by the requirement to measure UCS at high spatial resolution, and point out a method to engineer the size of the ferromagnetic domains accordingly.

NanoXAS—The in situ Combination of Scanning Transmission X‐ray and Scanning Probe Microscopy

NanoXAS is a novel x‐ray microscope installed at the Swiss Light Source combining laterally resolved soft x‐ray spectroscopy with scanning probe microscopy. We report on first in situ studies from thin polymer blend films and magnetic materials where topographic and spectroscopic contrast are used and show how complementary imaging modes provide new insight into many materials. In the future the scanning probe tip will be used to collect photoelectrons. By this we expect a spatial resolution in the few‐nm range and unique information on surface and bulk properties of nano‐materials.