MgO/Ag(100): Confined vibrational modes in the limit of ultrathin films

The vibrational modes of clean MgO films are investigated vs film thickness by means of high-resolution electron energy-loss spectroscopy. For thin films (20–30 monolayers) we observe, in accord with the literature, the Fuchs-Kliewer phonon at 677 cm–1 and the Wallis mode at 524 cm–1. For ultrathin films standing wave optical phonons confined in the overlayer are present, whose frequencies depend strongly on film thickness. Comparison with theoretical calculations for MgO slabs on a perfect conductor shows that the experimental frequencies are lower than expected, indicating the presence of compressive stress. At and below one-monolayer thickness an intense loss peak at 427 cm–1 is observed.