NiO growth on Ag(001): A layer-by-layer vibrational study

The vibrational properties of NiO(001) films on Ag(001) with thicknesses up to 50 monolayers (ML) are characterized with high-resolution electron energy loss spectroscopy (HREELS). For NiO growth at 300 K, four different coverage regions are distinguished by HREELS. The film-thickness-dependent Fuchs-Kliewer (FK) phonon frequency shifts and intensity changes are identified from the NiO monolayer to bulklike thick films. Characteristic changes of the vibrational properties are analyzed to resolve restructuring processes during annealing and thermal decomposition of NiO films. A quantitative comparison of the experimental data, including a line shape analysis, with the calculated loss function based on dielectric theory reveals an excellent agreement between the bulk and the NiO(001) thin film phonon properties for film thicknesses above 15 ML. In contrast, a strong FK phonon softening is observed for thin films below 5 ML that cannot be explained by dielectric theory nor phonon standing waves. This softening is attributed to the presence of surface stress, which results from the -2% lattice mismatch between NiO and Ag.