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

Synchrotron radiation photoemission was used to investigate the interfacial electronic structure of calcium and tris(8-hydroxyquinolato) aluminum (Alq(3)). Two phases of interaction are identified, marked out by a critical thickness theta(c) of approximately 7 A. Below theta(c), both N 1s and O 1s core-level spectra exhibit simultaneous charge-transfer components at binding energies lower than that of the original components. The induced O 1s component, which appears at 1.6 eV above the original component, remains fixed in intensity below theta(c). However, the induced N 1s component develops at the expense of the original component, and becomes a sole peak at theta(c). No signs of an ionized state are seen below theta(c), with respect to the Ca 2p core-level spectra, suggesting that the interfaced Ca appears not as ionized atoms at the vicinity of the pyridyl ring, but rather a form of cluster. We propose that all N 1s, O 1s, and Ca 2p core-level results can be attributed to charge hopping between the pyridyl rings. Above theta(c), Ca begins to decompose the Alq(3) molecules, and the core-level spectra of all constituent atoms exhibit decomposed components with binding energies even lower than those shown below theta(c). The decomposed components, accompanied by the appearance of a doubly ionized Ca 2p core level, are associated with direct bonds with Ca. (c) 2006 American Institute of Physics.