Growth and self-assembly of MnN overlayers on Cu(001)

The growth of MnN overlayers on Cu(001) has been investigated by low energy electron diffraction and scanning tunneling microscopy with emphasis on the morphology evolution with coverage. A striking feature in the growth is the formation of the self-assembled two-dimensional superstructure at the coverage of 0.9 ML At this coverage, the MnN nano-islands show a square shape and a well-defined size. They are regularly arrayed with a periodicity of (3.5 +/- 0.1) nanometer and form a two-dimensional square superstructure. The MnN island superstructure is stabilized by an unconventional short-range mechanism. A structural model has been proposed to explain the self-assembly and the high-quality of the superstructure. To explore possible wide applications of the new self-assembly mechanism, the growths of MnN on Pd(001) and calcium nitride on Cu(001) have also been investigated. MnN overlayers on Pd(001) present a layer-by-layer-like growth behavior, and highly ordered calcium nitride nanostructure is formed on Cu(001). Simple discussion is made on the conditions under which the novel mechanism leading to the self-assembly of MnN islands can be applied to other systems. We propose that a large lattice misfit between the overlayer and the substrate, low vacancy formation energy for the substrate, moderate interfacial bonding strength, and a large elastic constant for the overlayer compared with the substrate could be important factors for the split growth and self-assembly. (C) 2008 Elsevier B.V. All rights reserved.