Structure and energetics of ultrathin Cu adlayers on Ru(1 0 1¯ 0)

Adsorption of ultrathin epitaxial layers of Cu on Ru(1 0 1¯ 0) was investigated by ab initio density functional theory (DFT) simulations combined with directional elastic peak electron spectroscopy (DEPES) measurements. In the low-coverage regime, energetics of the adsorption process indicates that the deposited Cu atoms form instantly a bilayer (BL) and leave some substrate uncovered rather than wet the substrate with a single monolayer. The bilayer-by-bilayer growth is energetically favorable up to 3 BLs, but the third BL already occurs thermodynamically metastable. The DFT-predicted optimal arrangement of Cu adatoms in substrate-lattice-continuation sites was validated by DEPES measurements. This confirms the formation of a stable ultrathin film of hcp-like copper on the Ru(1 0 1¯ 0) substrate.