The deposit of small amounts of atoms on surfaces may induce the formation of well-organised nanostructures with long-range order. In the case of a vicinal Cu surface [1], the adsorption of Ag induces well-defined, periodic facets of the surface. Moreover, the Ag coverage can be used to tune the periodicity and the orientation of these facets. The Ag/Ni couple was thought to be very similar to Ag/Cu: Ag and Ni are not miscible in the bulk, and their cohesive energies and atomic sizes are very different, leading to Ag segregation at the Ni surface and to an abrupt chemical interface. However, the faceting behaviour of vicinal surfaces under Ag adsorption was found to be different in the two cases, as observed by scanning tunnelling microscopy (STM) and confirmed quantitatively by Grazing incidence X-ray Diffraction (GIXD), performed on the ID03 beamline.

Ni (322) is a surface with a miscut of 11.42° with respect to the (111) planes and can be viewed as a regular succession of (111) terraces of 1.03 nm width separated by {100}-type monatomic steps. When Ag is deposited on this surface in the monolayer range, and after thermal annealing, periodic facets are indeed observed covering the whole surface, but only within a very narrow coverage range of around 0.6 monolayer. Figure 92 (a and b) shows this “ideal” faceted surface observed by STM and the corresponding X-ray reciprocal space map recorded at k = 2 around two nickel Bragg peaks on which the scattering rods originating from the facets are well distinguished. Two orientations are observed for the facets: (111) and (211). The comparison with a corresponding map recorded around the Bragg peaks of the relaxed silver deposit at k = 1.77, Figure 92c, which shows only (211) facet induced diffuse rods arising from the Bragg nodes, enables us to conclude that the surface is made of (111) bare Ni facets and (211) Ag covered facets. The narrow (111) oriented diffuse features in Figure 92c do not stem from silver Bragg peaks and are presumably linked to planar defects in the nickel near surface region.

Fig. 92: Ni(322) surface after Ag deposit followed by annealing at 600 K. The Ag covered and bare Ni facets are clearly recognised. a), b) and c) correspond to a 0.6 monolayer deposit, and d), e) and f) to 0.3 monolayers. a) 3D STM image (50x50) nm2; b) (h, l) reciprocal space map at k = 2;  c) (h, l) reciprocal space map at k = 1.77; d) 3D STM image (250x250) nm2; e) (h, l) reciprocal space map at k = 2; f) (h, l) reciprocal space map at k = 1.77.

For other coverages, in particular lower ones such as 0.3 monolayers, a surface phase separation between faceted regions similar to the case previously described (same facets with similar local periodicity) and bare vicinal nickel as shown in the STM image and the two corresponding maps in Figure 92 (d, e and f) are observed. The energetics of the system would need to be further investigated to understand this surface faceting decomposition.

In addition, as already visible in the STM image, Figure 92a, one of the facets shows a reconstruction. indeed, accurate GIXD data could be collected on the reconstructed Ag covered (211) facets (Figure 93a) corresponding to a (2 x n) surface cell (8 < n < 9). A starting model sketched in Figure 93b involving 6 silver rows on top of two successive (111) terraces of the (211) Ni substrate with (n-1) Ag atoms for n Ni atoms along the [01] direction has been used. When optimised by quenched molecular dynamics simulations involving 60 bulk layers, this model renders the diffraction data well but needs further refinement to quantitatively account for all measured in-plane and out-of plane structure factors. Mixing various coverages of the terraces is being investigated.

Fig. 93: Surface reconstruction induced by silver on the (211) facets. a) GIXD in-plane data which shows the (2 x n) surface periodicity; b) model of the surface structure used for QMD simulations (Ag atoms in orange, Ni atoms in blue).

The knowledge of the system morphology and its surface str ucture are key parameters for its possible use as a nano-organised template for further growth of selected nano objects with magnetic or catalytic properties. This study shows that the coupling between GIXD, STM and atomistic simulations is necessary and fruitful to understand such phenomena.

 

Authors

C. Chambon (a,b), A. Coati (a), M. Sauvage (a) and Y. Garreau (a,b).
(a) Synchrotron SOLEIL (France)
(b) MPQ Paris (France)

References

[1] A. Coati, J. Creuze, and Y. Garreau, Phys. Rev. B 72, 115424 (2005); Y. Garreau, A. Coati, A. Zobelli, J. Creuze, Phys. Rev. Lett. 91, 116101 (2003).