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-Khewer 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.
MgO/Ag(100): confined vibrational modes in the limit of ultrathin films
SAVIO, LETIZIA;CELASCO, EDVIGE;VATTUONE, LUCA;ROCCA, MARIO AGOSTINO;
2003-01-01
Abstract
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-Khewer 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.
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