Photoemission of Alq_3 and C_(60) films on Al and LiF/Al substrates

摘要

Photoemission studies of thin films of Alq_3 and C_(60) deposited on Al and LiF/Al substrates have been performed in order to deduce the interactions of the organic films with the substrates. For all cases there is evidence of strong interaction resulting in the formation of interfacial dipoles. Attempts to explain the origin of these interfacial dipoles and the type of interface formed in each case have been done through analysis of the valence electronic structure and core levels of the materials. The origin of the interfacial dipoles is mainly covalent interaction when the organic films are deposited on Al substrates, and charge transfer between the organic molecules and the metal through the LiF sandwich layer when the organic films are deposited on LiF/Al substrates. For thick-enough LiF films, however, there is no interaction between the organic films and the substrates. In no case does the LiF dissociate, unlike what is found for the reverse order of deposition. Two charge-transfer-induced gap states are found for (sub)monolayer films of Alq_3 deposited on LiF/Al. We propose that the formation of two gap states corresponds to negatively charged fac-Alq_3.