Ultrathin Films of Co on Pt(111): an STM View

Abstract

The growth, structure and morphology of ultrathin Co layers with a thickness up to 15 layers deposited at room temperature on Pt(111) have been studied by using scanning tunnelling microscopy (STM) with atomic resolution and chemical discrimination between Co and Pt. This chemical contrast has been confirmed by simulations with an FLAPW (Full Potential Linearized Augmented Plane Waves) ab-initio computer code based on density functional theory. By the help of this contrast between Pt and Co atoms in STM constant current images it is shown that in the early stages of submonolayer growth Co is incorporated into the Pt surface, thereby forming dislocation lines. We were also able to demonstrate that Co atoms descend from the upper terrace to the lower one by an exchange diffusion process with the Pt atoms at the step edges. It is shown that this interlayer diffusion does not take place at straight steps, but rather at corners or kinks. The first completed Co monolayer (ML) is almost pseudomorphic (Co in the Pt fcc lattice sites) with a high density of defects due to the lattice mismatch. The second layer exhibits a moire structure, with the Co in-plane lattice distance close to that of bulk Co. The step edges which are very rough at a coverage of two monolayers become smoother with increasing Co deposition. The growth mode is two-dimensional (layer-by-layer) around two to three monolayers and changes afterward into three-dimensional growth (island growth). We observe that the change of the step edge morphology is also correlated to this change from 2D to 3D growth mode. The reason for the 2D growth at the beginning is attributed to the strained interface between the Co overlayer and the Pt(111) surface which hinders the formation of straight steps. Therefore, many kinks and corners are formed, increasing the probability for interlayer diffusion by the above mentioned exchange process. With increasing number of layers the strain decreases, steps become smoother, interlayer diffusion decreases and therefore island growth develops. Up to the highest coverage (15 ML) studied the growth is characterised by a mainly twinned fcc-like stacking. Only a small amount of hcp stacking has been observed. Further experiments showed that preadsorption of carbon monoxide acts as a surfactant which extends the layer-by-layer growth up to higher Co coverages.