Electronic structure of K-intercalated 8-tris-hydroxyquinoline aluminum studied by photoemission spectroscopy

We have studied the electronic structure of potassium-intercalated 8-tris-hydroxyquinoline aluminum using valence-band and core-level photoemission spectroscopy. It is shown that low K intercalation leads to a rigid shift of the chemical potential to higher energies. This shift allows the determination of a lower limit of the transport energy gap of pristine 8-tris-hydroxyquinoline aluminum of 3.05 eV. This number is significantly larger than the gap measured using absorption techniques that shows that the optical excitation is excitonic. The energy levels of 8-tris-hydroxyquinoline aluminum are considerably modified upon further intercalation that is assigned to a combination of structural relaxation and the impact of the alkali counter ions.