Surface-charge polarization effect at an organic and inorganic contact: Case study of tris(8-hydroxyquinolato) aluminum on Si(001)-2×1

We have employed synchrotron radiation photoemission to investigate the tris(8-hydroxyquinolato) aluminum (Alq3) molecules adsorbed on an inorganic Si(001)-2×1 surface at room temperature. The surface-related valence states associated with the dimers and backbonds lying a couple of eV below the Fermi level reduce in strength upon Alq3 adsorption, indicating that the organic molecules not only passivate the dangling bonds but also rupture the dimer bonds. The wave function of the surface dangling bonds and of the pyridyl side of an 8-quinolinol ligand overlaps greatly so that charge is polarized toward the organic adlayer, resulting in the appearance of low-energy components in both N 1s and C 1s core-level spectra, which is similar to the case of Mg-doped Alq3. However, the polarization diminishes at greater coverage, in which case both 1s cores behave as in the nominal states.