Interface gap states of 8-hydroxyquinoline aluminum induced by cesium metal

Ultra-violet photoemission spectroscopy (UPS) showed no Cs-induced gap states at the interface when tris-(8-hydroxyquinoline) aluminum (Alq3) was deposited on sub-monolayer Cs metals coated on Ag substrate. When Cs coating approached one monolayer, UPS detected a new state in the gap at 1.7 eV above the HOMO of Alq3, similar to the case of direct Cs deposition on Alq3. The observation is consistent with a strong and localized interaction between Cs and Ag. At low Cs coverage, all Cs interacted and were tied up with the Ag, leaving no free Cs to interact with Alq3, whereas at higher coverage some free Cs interacted with Alq3, giving rise to the Cs-induced gap states.