Luminescence quenching of tetracene films adsorbed on an ultrathin alumina AlOx layer on Ni3Al(111)

We have investigated the luminescence quenching of the fluorescent aromatic molecule tetracene (Tc) adsorbed on an ultrathin film of alumina on a Ni3Al(111) substrate. The alumina film (thickness similar to 5 A) was grown by heating a Ni3Al(111) crystal under an oxygen atmosphere. Directly after deposition of Tc molecules at low temperatures (45 K), the Tc films are disordered and no luminescence can be detected. We explain this by charge transfer to the Ni3Al(111) substrate from mainly flat-lying Tc molecules across the alumina film. After an additional annealing cycle at 240 K, we observe a significant luminescence signal for films with nominal film thickness down to submonolayers. Low energy electron diffraction measurements reveal that the annealing cycle leads to formation of (001) oriented Tc crystallites with Tc molecules standing nearly perpendicular to the surface. We hence propose that the luminescence quenching by charge transfer is inhibited for the annealed Tc films due to the different molecule orientation and film morphology. In control experiments performed on the sapphire (0001) surface, we find a very similar behavior, except that we can detect a very low luminescence signal already for the as grown Tc films. However, the luminescence yield is very small, likely due to structural disorder.