Search

We present a real-time X-ray scattering study of the growth modes in organic molecular-beam epitaxy. We have studied the model system 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) on Ag(111) and find a temperature-dependent transition from layer-by-layer growth to islanding. The transition smears out for low substrate temperatures, T, implying that the degree of the layer-by-layer growth of the wetting layer decreases with decreasing T. This behavior has been analyzed quantitatively and reproduced by kinetic Monte Carlo simulations. The implications and consequences of our findings for the understanding of the organic molecular-beam epitaxy are discussed.

Liquid octamethylcyclotetrasiloxane (OMCTS) films have been prepared between two flat silicon substrates with varying nanometer-sized gaps which correspond to two to three molecular diameters of OMCTS. These extremely confined liquid films have been investigated using X-ray scattering methods as a function of applied pressure, in particular by observing the specular reflectivity and the diffuse scattering. For the first time, scattering methods show that the gap size cannot be changed continuously with changing pressure. Instead, the gap is quantized due to ordering effects of the OMCTS molecules. The use of scattering methods has enormous future implications as they allow very detailed investigations of confined liquids at a molecular level.

We have employed a new experimental configuration to measure ferromagnetic X-ray resonance exchange scattering (XRES) from a EuS single crystal. Using polarisation analysis we could achieve a magnetic-scattering intensity stronger than charge scattering resulting in an asymmetry ratio as large as Ra = 0.67 at the EuLII edge. By a combined refinement of the dependencies of the scattered intensity on the energy and the applied magnetic field we could uniquely determine spectroscopic information such as the 5d conduction band exchange splitting = 0.27(1) eV.

X-ray diffraction measurements within the region of total external reflection from a thin wetting film of liquid CCl4 on a Si/SiO2 substrate are presented. The in-plane structure of the liquid surface yields a peak in the diffuse scattering at the specular position. Since there is no way to separate specular and off-specular contributions, the data were analysed simultaneously, which leads to a consistent set of interface parameters. Although the obtained power spectral density function of the liquid surface is very similar to that used in the case of free capillary waves, a modified cut-off has to be introduced. The difference results from the constrained thin-film geometry and is therefore caused by the interaction with the underlying substrate.