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Alloy Monolayers by Electrochemical co-deposition

par Rosso Michel - publié le , mis à jour le

Participants : P. Allongue and F. Maroun

PhD students : F. Le Cadre (2013), A. Damian (2009)

In this work we investigate the microstructure of alloy monolayers Ni-M obtained by co-electrodeposition on a unreconstructed AuPd(111) surface. Element M = Ag, Au and Pd to study the influence of the enthalpy of alloy formation. The monolayers are observed using in situ STM. The alloy composition is determined after selective dissolution of Ni.

The STM observations in Fig. 1 suggest the following points :

 NiAg (a-b) : there is a clear phase separation at any composition. The relative coverage of Ag domains (light colored domains) is in fact equal to that given by the film composition. One also notices that the moiré pattern of the Ni(111) ML is preserved.

 NiAu (c-d) : The phase separation is again complete (image d), with a short length scale than for Ni-Ag. At very low Au content, nm gold inclusions are observed due to place exchange with Ni atoms located close to the maxima of the moiré pattern (image c, red circle).

 NiPd (e-f) : This system presents a phase transition around 30% Pd content. Below 30%, a solid solution forms as evidenced by the regular corrugation (e) which periodicity is consistent with an average Ni – Pd distance following by Vegard’s law. Above 30%, a phase separation occurs although Ni and Pd are still miscible. This phenomenon is resulting from a strain relaxation in Ni and the strong interface energy between Pd and Au.

Figure 1 : In situ STM observations of Ni-M / Au(111) monolayers of different compositions : (a-b) AgNi ; (c-d) AuNi ; (e-f) PdNi. The ML composition is indicated in each image. It was determined after Ni selective dissolution.

Publications :

 F. Le Cadre, F. Maroun, I. Braems, F. Berthier, C. Goyhenex, and P. Allongue, "AuNi alloy monolayer films electrodeposited on Au(111) : An in situ STM study," Surf. Sci. 607, 25-32 (2013).

 A. Damian, I. Braems, F. Maroun, and P. Allongue, Electrodeposition of NiPd monolayer on Au(111) : An in situ scanning tunneling microscopy study, Electrochim. Acta 112, 824 (2013)