Growth and morphology of metal particles on MgO/Mo(001): A comparative STM and diffraction study

Using scanning tunnelling microscopy and photoelectron diffraction, we have analyzed the nucleation and growth behavior of simple (Mg and Ca), transition (Ni and Fe), and noble metals (Ag and Au) on MgO films grown on Mo(001). With the exception of gold, the particles develop interface planes that are in registry with the surface-oxygen lattice of the MgO(001). To achieve this commensurability, the particles expose facets that differ from the low-energy surfaces of the respective metals and adopt crystallographic structures that are unknown in the bulk materials. This peculiar growth behavior demonstrates the importance of interfacial interactions between the metal deposits and the MgO film, despite the inert nature of the oxide support. The observed equilibrium shapes of the particles have been analyzed with a simple growth model that includes the interfacial interactions, the surface energies, and the elastic distortion of the particle lattice to reach commensurability with the MgO(001). Experimental particle densities have been explained with a scheme that considers formation of metal cations at certain MgO defects to be the initial nucleation step. Although the model is in reasonable agreement with the experimental results for most metals, it fails for gold. We relate this deviation to the large tendency of gold to charge up negatively on MgO thin films, in correspondence with earlier experimental and theoretical work.