Surface electronic structure of Mg-doped tris(8-hydroxyquinolato) aluminum studied by synchrotron radiation photoemission

摘要

Synchrotron radiation photoemission was used to investigate the electronic structure of magnesium deposited on tris(8-hydroxyquinolato) aluminum (Alq_3) with the film thickness from one molecular layer to a thick solid. In a monolayer-thick Alq_3 on the Si(001)-2 X 1 surface, the organic molecules accept from the charge at the surface dimers. However, as Mg is deposited on top of this thin Alq_3 film, the surface dimers underneath the film start to show reacted magnesium silicide, indicating clearly a prompt diffusion of Mg through Alq_3. In regard to the Alq_3 solid, the highest occupied molecular orbital of Alq_3 shifts monotonically towards high binding energy, and the lowest unoccupied molecular orbital remains fixed in energy with increasing Mg concentration. The N 1s core appears an Mg-induced charge-transfer component with a binding energy lower than the original component. This new component grows gradually in intensity with increasing concentration of the dopant. Its energy separation from the original component is -1.51 eV at small exposure, but widened up to -1.85 eV at large coverages. Moreover, Mg also affects the O 1s core as manifested by a components lying at +1.09 eV higher binding energy than the original component. The Mg 2p core-level spectra, although rather broad, exhibit a shift towards lower binding energy with increasing Mg vapor. All these experimental evidences suggest that magnesium organometallic complex residing in the surface exhibits an electronic structure differently from the bulk. We propose that Mg in the surface Alq_3 molecules forms actually clusters. It attaches to a pyridyl ring, affecting not only the nitrogen atom at that ring, but the oxygen atom in the adjacent phenoxide ring. The depleted charge in the affected oxygen flows then about its adherent ligand and resides on the pyridyl ring at that ligand, resulting in a high Alq_3 anion state.