Thin MgO films with a nominal thickness ranging between 1 to 60 ML have been gr own on a Mo(001) surface. The film morphology was studied by LEED and STM, revealing the presence of a coincidence pattern with the Mo support in the low coverage regime, a dislocation network at medium thickness and a rather flat and defect-poor MgO surface for thicker layers. The MgO optical properties were investigated as a function of film thickness by analyzing electroluminescence spectra obtained via electron injection from the STM tip into well-defined surface areas. The spectra are characterized by two distinct emission bands at 3.1 and 4.4 eV. Their origin is discussed in the light of earlier photo-luminescence measurements on MgO nanocubes and smokes.

Using a combination of reciprocal and real-space techniques, the structural evolution and its effect on the surface morphology is investigated for MgO films of 1-30 ML thickness epitaxially grown on Mo͑001͒. The strain induced by the mismatch with the substrate is relieved between 1 and 7 ML MgO due to the formation of an ordered network of interfacial misfit dislocations aligned along the MgO ͗110͘ directions, particularly evident after annealing the film at 1070 K. A dislocation periodicity of about 60 Å has been determined by means of grazing incidence x-ray diffraction. The dislocations induce a tilting of the surface that appears in electron diffraction along the ͗100͘ MgO directions for thin films and changes to ͗110͘ directions when the oxide thickness increases. Scanning tunneling microscopy ͑STM͒ shows the presence of a regular pattern on the surface below 7 ML thickness associated to the dislocation network. With increasing thickness, screw dislocations connected by nonpolar steps appear on the oxide surface. Thanks to the combination of different diffraction techniques and STM measurements, a comprehensive picture of the relaxation mechanisms in MgO films on Mo͑001͒ can be drawn.

Monolayer films of MgO(001) have been prepared on an Au(111) surface and explored by means of scanning tunneling microscopy (STM) and spectroscopy. The symmetry mismatch between the hexagonal substrate and the squared overlayer results in the formation of a (6 × 1) superlattice, as revealed from the distinct stripe pattern observed in the STM images. The presence of the oxide film also modifies the potential situation at the interface, which induces a substantial upshift of the Shockley-type surface band on Au(111). The resulting MgO/Au interface band is characterized by a pseudogap at around 500 mV that opens at the position of the new Brillouin zone of the enlarged (6 × 1) unit cell. In addition the oxide layer gives rise to a drastic decrease of the Au(111) work function, as deduced from the energy position of the first field-emission resonance on the bare and MgO-covered surface. The work-function drop is explained by an interfacial charge transfer from the oxide film into the electro-negative gold surface.

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.

We report a growth study of MgO thin films on an Au(111) support, performed with scanning tunneling microscopy, X-ray photoelectron spectroscopy, and low-energy-electron and X-ray-diffraction techniques. Depending on the deposition temperature, the O 2 partial pressure, and the availability of water during oxide formation, two growth regimes can be distinguished. At high oxygen pressure, the MgO mainly adopts a square-lattice configuration and exposes the nonpolar (001) surface, whereas at low O 2 pressure a hexagonal lattice develops that resembles the (111) surface of rocksalt MgO. For films beyond the monolayer limit, the emerging electrostatic dipole along the MgO(111) direction becomes important for the film morphology. Depending on the preparation conditions, the system takes either structural or adsorption-mediated routes to remove the polarity. Whereas surface roughening is identified as main polarity-compensation mechanism at perfect vacuum conditions, hydroxylation becomes important if water is present during oxide growth.