WP3: In situ x-ray diffraction

  1. Preliminary tests of the x-ray diffraction experiment using the LMCat set-up
  2. In situ investigation of the atomic structure of LMCat surface under 2DM formation conditions
  3. Performing simultaneous x-ray diffraction and Raman spectroscopy measurements during 2DM formation

Obtaining knowledge about the atomic structure of the growing 2DM flakes and of the LMCat surface during the 2DM growth is of critical importance. It is known that differences in surface tension of melt constituents can cause segregation and formation of surface phases that would have a radically different composition compared to the liquid bulk. The extent of atomic ordering on the molten phase, both perpendicular and parallel to the surface, is also an important, presently unknown factor. Growing 2DM flakes is expected to influence the underlying LMCat by dictating their order onto it. The order is expected to extend outside of the flakes, possibly influencing the lateral growth of the flakes and even giving rise to interactions between neighboring flakes. LMCat-mediated interaction is thus one of the potential mechanisms that can explain the observed self-ordering patterns between separated graphene flakes grown on molten copper. Thermal capillary waves are induced by thermal fluctuation at the molecular scale on the phase boundary of a fluid, whose dynamics are dominated by the surface tension. The existence of a 2DM on the surface is expected to change or suppress the capillary wave spectrum on the melt. The capillary waves, on the other hand, induce a certain effective roughness on the liquid surface, which in turn can influence the catalytic activity of the molten phase. The extent of mutual influence between 2DM, thermal capillary waves, and catalytic activity of the LMCats and its consequences for the 2DM growth is presently completely unknown.

We will investigate these effects using surface-sensitive techniques including x-ray reflectivity (XRR), grazing- incidence x-ray diffraction/scattering (GIXD/GIXS), and surface x-ray diffraction (SXRD). We will use XRR to probe the liquid density profile below the surface, with potential evidence of density oscillations, i.e. layering, which might be strongly correlated to the growing 2DM flakes. GIXS, on the other hand, can provide information on short-range order in the liquid, both parallel and perpendicular to the surface. GIXD will be used to search for 2DM diffraction rods, whose quantitative measurements using SXRD can provide the detailed atomic structures of the 2DM in the flakes. The experiments will be performed under grazing incidence, possibly well below the critical angle for total external reflection of x-rays, in order to minimize the background arising from thermal diffuse scattering from the liquid substrate. x-ray scattering techniques have already been used to investigate adsorbed organic molecules on the surface of water or mercury at room temperature, as well as on the surface structure of the binary liquid Au-Si alloy system at temperatures up to 650 K. These techniques have also been successfully used to study the structure of graphene grown on solid catalytic substrates.

The x-ray scattering experiments for this project will be performed at the EH1 diffractometer of the ESRF-ID10 beamline, which is customized for measurements on liquids. The specially designed diffractometer keeps the sample stage horizontal and fixed in space during the measurements (to avoid surface wave formation on the liquid). The sample stage is equipped with vibration isolation facilities to further minimize the risk of vibrations.