Crossover from two- to three-dimensional gold particle shapes on CaO films of different thicknesses

The growth of Au particles on Mo(001)-supported CaO films has been investigated as a function of film thickness by means of scanning tunneling microscopy and spectroscopy. Whereas, monolayer Au islands develop in the low-thickness regime [1-25 monolayers (ML)], formation of three-dimensional (3D) deposits is revealed above 40-ML film thickness. The two-dimensional (2D) growth morphology is ascribed to a charge transfer out of the CaO/Mo support into the adparticles, resulting in a reinforcement of the Au-CaO interface interaction and a strong preference for a wetting growth of gold. In the limit of ultrathin films, this charge transfer is governed by electron tunneling from the Mo substrate into the Au islands and is driven by the different Fermi levels of both systems. Although tunneling transport breaks down beyond 5-10-ML film thickness, the signature of charge-mediated Au growth remains detectable for even thicker films. This finding suggests the presence of intrinsic charge centers in the CaO film that are able to donate electrons to the gold. We relate these donor centers to Mo ions that spontaneously have diffused from the Mo support into the CaO lattice during growth. Given the low mobility of Mo in the oxide matrix, the donor concentration diminishes with film thickness and, hence, the ability of the CaO to transfer electrons into the Au deposits. Consequently, the 2D growth behavior of Au changes into the 3D regime that is characteristic for metals on inert oxide surfaces. Spectroscopic characterization of the Au particles revealed pronounced electron quantization and Coulomb charging effects due to the reduced island size.