The atomically rough tungsten (111) surface covered with ultra-thin films of certain metals (Pt, Pd, Rh, Ir, Au) and annealed to temperatures higher than 700 K undergoes massive reconstruction to form three-sided pyramids of nanometer scale dimensions with {211} facet sides. The effective control over growth and morphology of nanostructure on surfaces is essential for the design and operation of future generations of devices. A possible promising application is the self-organized growth of nanodots on such a patterned substrate.
The practical importance of the Pt/W(111) system, together with the fundamental questions regarding the faceting, has prompted us to combine in situ Grazing Incidence Small Angle X-Ray Scattering (GISAXS), Grazing Incidence X-Ray Diffraction (GIXD), and ex situ Atomic Force Microscopy (AFM) to probe, during growth, the faceting of the surface. The experiments were carried out at the European Synchrotron Radiation Facility (ESRF) using a newly developed setup allowing performing GISAXS and GIXD in ultra-high vacuum, at different stages. [1] The use of X-rays enables one to follow the faceting in situ and in a non destructive way of a ~1 ML Pt/W(111) surface from very small to very large pyramids from 5 to 65 nm as a function of the annealing temperature from 700 to 1340 K. Before annealing, the Pt deposit was found to be perfectly two-dimensional and lattice-matched to the W(111) surface parallel to it. The surface consists in a short-ranged dense packing of three-sided pyramids having {211} facets and connected with each other. A full characterization of the nanofacetted surface has been obtained through quantitative GISAXS analysis. Moreover, the thermal stability of a Co deposit on a facetted Pt/W(111) surface has been studied. Finally, the thermodynamics conditions for nanofaceting will be discussed, providing new insights on the energetic role of the pyramid edges.
To conclude, the structure and morphology of surface nanofaceting was studied in situ as a function of thermal annealings. We believe that this study is important for understanding dynamic structural rearrangements of surfaces at a nanometer scale.
[1] G. Renaud, R. Lazzari, C. Revenant, et al., Science 300 (2003) 1416.
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