W0214

The Structure and Phase Behavior of Electrodeposited Monolayers and Thin Films. B.M. Ocko, Department of Physics, Brookhaven National Laboratory, Upton, NY, 11973

The structure and phase behavior of electrodeposited monolayers and thin films have been investigated using surface x-ray scattering techniques under in-situ electrochemical conditions. The results of two systems, bromide on Ag(100) and copper on Au(100), will be compared with previous electrochemical and scanning tunneling microscopy (STM) results.

In NaBr solutions, the initial bromide which deposits on Ag(100) forms a lattice-gas phase (disordered) and at a critical potential an ordered c(2x2) phase forms with a half-monolayer coverage. An analysis of the potential dependent scattering intensity, modeled as an interference between the scattering from the bromide and silver, is used to determine the bromide isotherm over the entire coverage regime. Information on the interatomic interactions is obtained from an analysis of the isotherm using simple free energy models. A comparison with electrochemical results provides a measure of the potential dependent lateral displacement amplitude.

The bulk electrodeposition of copper on Au(100) proceeds in several stages. Whereas the first ten copper layers are psuedomorphic with the Au(100) surface, additional deposition leads to a highly modulated copper structure. The initial psuedomorphic structure (no new in-plane reflections) is surprising since the bulk copper lattice constant is 12% smaller than that of the underlying gold. The strained copper unit cell for these thin layers is tetragonal, 2 atoms per unit cell, and is very close to the bcc unit cell. Additional copper deposition leads to additional in-plane reflections characteristic of a one dimensional modulated phase with a periodicity of 70 Å in agreement with previous STM measurements (R.J Randler and D.M. Kolb). The x-ray measurements conclusively show that the entire copper film restructures and that the resulting strain field propagates into the underlying gold lattice. The transformation which occurs with thickness is similar to the temperature dependent Martensitic transition observed in some bcc metals.

Work at Brookhaven National Laboratory is supported by the Division of Materials Sciences, U.S. Department of Energy, under contract DE-AC02-98CH10886. The bromide electrodeposition study was carried out in collaboration with Jia Wang and Thomas Wandlowski. The copper electrodeposition study was carried out in collaboration with Rudi Randler, Dieter Kolb, and Ian Robinson.