Electronic structure of strained copper overlayers on Pd(110)

The electronic structure of strained copper thin films on Pd(110) (lattice mismatch: 7.8%) has been probed as a function of film thickness by angle-resolved photoemission spectroscopy (ARPES) using synchrotron radiation. While the copper films grow epitaxially, an extremely slow convergence of the film electronic structure to that typical of a Cu(110) single crystal occurs, with distinct changes occurring in the film electronic structure for films thicknesses up to 60 ML. A new peak at 1.6 eV binding energy, absent in either Pd or Cu(110), which exhibits only weak polarization dependence and little dispersion as a function of photon energy, is shown to be due to emission from an impurity state arising from Pd in a dilute CuPd alloy formed by intermixing during film growth at the Pd–Cu interface. Large changes in the binding energy and peak shape of the lower Cu d and sp bands are observed as a function of film thickness. Self-consistent KKR electronic structure calculations in the ΓK, UX direction have been performed for a range of geometries, including distorted films adopting in-plane lattice parameters of the underlying Pd(110) substrate, and a range of interlayer spacings. The coverage-dependent changes in the film electronic structure are shown to be due to the film initially adopting a highly strained pseudomorphic geometry and a slightly distorted fcc structure (c/a=0.93±0.03). A gradual anisotropic relaxation of the film geometry towards that of bulk fcc copper, initially along the [11̄0] direction and at higher coverages along the [001] azimuth, occurs as the film thickness increases. The binding energy of the lower d and sp bands, which shift up to 0.5 eV per 0.1 Å change in the interlayer separation, is shown to be an accurate monitor of the degree of distortion in pseudomorphic thin Cu films.