Direct Evidence of the Temperature-Induced Molecular Reorientation in Tetracene Thin Films on AlOx/Ni3Al(111)

Abstract

Control over the optical properties of the fluorescent organic layer plays a key role in the development of organic light-emitting diodes. A combination of near-edge X-ray absorption fine structure spectroscopy and X-ray photoelectron spectroscopy was used to study structural changes in thin films of tetracene on AlOx/Ni3Al(111). It is shown that upon deposition onto the cold (100 K) substrate, a monolayer of tetracene molecules adopts a disordered adsorption configuration with the molecular planes orientated almost parallel to the surface. Upon annealing at 280 K, the molecular packing changes and the tetracene units adopt a more upright orientation. The consequences of this orientational change for the luminescent properties of the molecular adlayer are discussed, in particular with regard to a quenching of the optical excitation by an electronic coupling of occupied and unoccupied molecular states to those of the metal substrate.