Photoemission study of aluminum/tris-(8-hydroxyquinoline) aluminum and aluminum/LiF/tris-(8-hydroxyquinoline) aluminum interfaces

Abstract

We have investigated the interfaces of aluminum on tris-(8-hydroxyquinoline) aluminum $({\mathrm{Alq}}_{\mathrm{3}})$ and aluminum on ${\mathrm{LiF/Alq}}_{\mathrm{3}},$ using x-ray and ultraviolet photoemission spectroscopy (UPS). Aluminum appears to react destructively with ${\mathrm{Alq}}_{\mathrm{3}}$ causing significant modification of the oxygen, nitrogen, and aluminum spectra. The well-defined UPS spectrum of ${\mathrm{Alq}}_{\mathrm{3}}$ is quickly destroyed by very low coverages of aluminum. With only a 5 Å layer of LiF on the ${\mathrm{Alq}}_{\mathrm{3}},$ the reaction with aluminum is significantly suppressed. The ${\mathrm{Alq}}_{\mathrm{3}}$ molecular orbital features in the UPS shift to higher binding energy but remain easily recognizable. In addition, a well-defined gap-state forms which is significantly different from that produced without LiF. Both the core-level spectra and the gap-state suggest that the ${\mathrm{Alq}}_{\mathrm{3}}$ anion is formed in the presence of aluminum and LiF.