Vibrational properties of Na nanoclusters self-assembled on Si(1 1 1) surface

We have investigated structural and vibrational properties of Na nanoclusters self-assembled on the Si(1 1 1)-7×7 surface at room temperature mainly using high-resolution electron-energy-loss spectroscopy. We observe three characteristic loss peaks L1, L2, and L3 ascribed to an interband transition, a local atomic vibration, and another interband transition from an Na nanocluster-induced state, respectively. The spectral change of L1 with Na coverage θ suggests that the S1 dangling bond band is gradually filled up to open a band gap with increasing θ up to 1.1 eV when all three loss peaks completely disappeared. The relatively high loss energy El=243meV of L2 with a narrow linewidth of 32 meV indicates the only Na-Si atomic vibrational mode with Na atoms occupying the tilted on-top sites above Si rest atoms. Furthermore the extremely weak loss peak L3 visible at a coverage range unique only to the Na nanoclusters proves the presence of a Na nanoclusters-induced electronic state N2. These observations provide explanation to most unresolved spectral behavior of earlier photoemission study and evidence for the atomic structure of the Na nanocluster.