Reconstruction from In-plane X-ray Diffraction Data


In recent years, much attention has been paid to the study of thin films of C60, generally named fullerene, deposited on different substrates. C60 molecules interact via Van der Waals forces, forming a bulk fcc crystal structure at room temperature. Recent studies have also shown that the bond character of C60 with different substrates may vary widely from Van der Waals to strong ionic. As the electronic properties of C60, including superconductivity, are strictly related to their charge state, the understanding of early film growth stages on different surfaces has both scientific and technological importance. The quantitative structural investigation of well ordered C60 monolayers (MLs) can enlighten the interaction mechanism of the fullerene molecules with the environment. Ordered C60 MLs can be grown on various surfaces and the charge state strongly depends on the type of substrate. In general, a C60 single layer deposited on an uncorrugated substrate shows a tendency to be hexagonal close packed.

The Au(110) surface exhibits the (1x2) missing row reconstruction: one out of every two close packed atomic rows along the [1 -1 0] direction is missing. As a consequence it is a very anisotropic surface. C60 interacts with it via ionic bonding as indicated by electron spectroscopy studies. The strong electronic interaction is associated with a strong structural re-ordering as indicated by STM [1]. In that work, evidence was shown of an hexagonal close packed and corrugated C60 layer resulting in a (6x5) surface periodicity.

In order to obtain further information on the atomic arrangement of the C60/Au(110) interface, X-ray diffraction experiments were performed on the (6x5) C60/Au(110) system. The measurements were carried out at ID3 (Surface Diffraction Beamline). A total of 300 in-plane fractional order reflections were measured which reduced to 130 symmetrically independent ones.

The application of standard procedures routinely employed in Surface Crystallography to solve the structural details of the C60/Au(110) p(6x5) surface reconstruction proved to be useless due to the complexity of the structure. Instead 'direct methods' were used to attempt to solve the problem. The phases of the largest structure factors which are necessary for computing the projected -map (map of the atomic distortions in the unit cell), were determined by maximising the 'direct methods' sum function [2,3]. In a preliminary low resolution study a hexagonal packing model as well as (1 x 2) and (1 x 3) symmetry of the surface rows compatible with the model obtained by STM were shown [4].

Although the analysis of the data is still not complete, good agreement was achieved with the experiments using the model depicted in Figure 41. The surprising result is that the interaction of the fullerene molecules with the substrate is so strong that several Au atoms are displaced from their original configurations in order to form a sort of calyx-shaped arrangement to better accommodate the fullerene balls indicated in the figure as dashed circles.

[1] S. Modesti, S. Cerasari, P. Rudolf, Phys. Rev. Lett., 71, 2469 (1993).
[2] C.A.M. Carvalho, H. Hashizume, A.W. Stevenson, I.K. Robinson, Physica (Utrecht), B221, 469 (1996).
[3] X. Torrelles, J. Rius, F. Boscherini, S. Heun, B.H. Mueller, S. Ferrer, J. Alvarez, C. Miravitlles, Phys. Rev. B57, R4281, (1998).
[4] X. Torrelles, J. Rius, M. Pedio, R. Felici, P. Rudolf, J. Alvarez, S. Ferrer, C. Miravitlles, Phys. Stat. Sol., 215, 773 (1999).

M. Pedio (a), R. Felici (b), X. Torrelles (c), P. Rudolf (d), M. Capozi (a), J. Rius (c), S. Ferrer (e).

(a) ISM-CNR, Area Science Park Trieste (Italy)
(b) INFM-OGG, c/o ESRF
(c) I.C.M.A.B. (CSIC), Barcelona (Spain)
(d) L.I.S.E. Facultes Universitaires Notre Dame de la Paix, Namur (Belgium)
(e) ESRF