Early stages of formation of the Ag-Ni(111) interface studied by grazing incidence x-ray diffraction and x-ray photoelectron diffraction

Ultra-thin Ag/Ni(111) reconstructed interfaces have been revisited by a combination of in-situ grazing incidence x-ray diffraction (GIXD) and x-ray photoelectron diffraction (XPD) in order to determine the growth mode and to evaluate the interface spacing. Evidence for predominance of single-layer growth in the early stages was obtained through the analysis of the x-ray diffraction rods from the Ag/Ni(111) $(\sqrt{52}\times \sqrt{52})\mathrm{R}13.9^{\circ }$ reconstructed interface, whereas photoelectron diffraction patterns could reveal traces of second-layer Ag scatterers before full wetting of the substrate. Refinement of the atomic coordinates provided by quenched molecular dynamics simulation on the basis of the new x-ray data set enabled us to assess the Ag/Ni average interplanar distance, which was found unexpanded at 2.44 ± 0.07 Å, in contrast with recent determination by low-energy electron diffraction and microscopy. For increasing deposited amounts, both GIXD and XPD showed the expected features of two- and three-layer silver epitaxial overgrowths.

Figure 1

Ag covered fraction as a function of the Ag deposited amount for the two series of experiments. Straight lines correspond, respectively, to monolayer growth (black full line) and bilayer growth (red dashed line). The error bars correspond to an accuracy of 10$\%$ for both measurements methods.

Figure 2

Main forward scattering directions for a three-layer Ag deposit (large red dots) on Ni(111) (small blue dots) in a twinned fcc stacking relationship, viewed in the (0 1 −1) plane. Red arrows and labels refer to the Ag lattice.

Figure 3

(a) Experimental and (b) simulated XPD patterns of the clean Ni substrate (Ni 3$p$ core-level electrons).

Figure 4

(a) Experimental XPD patterns of the 2.9-ML Ag deposit; (b) simulated pattern for 3-ML Ag (Ag 3$d$5/2 core-level electrons).

Figure 5

Azimuthal scans at two polar angles for bulk Ni (top panels) and three Ag epilayers (bottom panels). Twin relationship between the 2.9-ML Ag deposit and the substrate is evident. Simulations are displayed by smooth thin lines. The arrows point to features unaccounted for by epilayer scattering. Ag curves are shifted for the sake of clarity.

Figure 6

(a) Experimental XPD pattern for a 0.58-ML silver deposit on Ni(111) (Ag 3$d$5/2). (b) Simulated XPD pattern from a 1-ML silver, $(\sqrt{52}\times \sqrt{52})\mathrm{R}13.9^{\circ }$ reconstructed on Ni(111). Well-fitted areas are outlined, and a threefold symmetry is visible in the experimental pattern.

Figure 7

Comparison between experimental (0.58-ML sample: square dots) and fitted (continuous curve) structure factor amplitudes for the 1-ML Ag/Ni(111), $(\sqrt{52}\times \sqrt{52})\mathrm{R}13{.9}^{\circ }$ reconstructed surface.

Figure 8

Comparison between experimental data (square dots) and fit (continuous line) for the 1.5-ML sample.