AuNi alloy monolayer films electrodeposited on Au(111): An in situ STM study

In this work, we study the electrochemical deposition of NiAu monolayers on Au(111) surfaces using in situ STM. This system which is characterized by a miscibility gap at room temperature in the case of a bulk alloy is important for connectors in electronic circuits. The microstructure of the alloy is investigated as a function of the composition and the deposition rate. At intermediate Au contents and using a relatively slow alloy deposition rate, a complete phase separation on a 10 nm length scale is observed. At larger deposition rates, the alloy structure presents different phases with several characteristic length scales. For a Au content smaller than 10%, Au and Ni phase separation yields Au clusters of 1-2 nm size embedded in a Ni layer at a specific position with respect to the Ni moiré pattern. This phenomenon is interpreted in light of preliminary atomic-scale simulations of the equilibrium structure of the pure Ni monolayer pattern and site-dependent total energy calculations of exchanging Ni atoms by Au.