Interplay between morphology, structure, and electronic properties at diindenoperylene-gold interfaces

We present a study of the morphology, structure, and electronic properties of interfaces formed between Au and the organic semiconductor diindenoperylene (DIP) employing transmission electron microscopy (TEM), atomic-force microscopy (AFM), x-ray diffraction, and ultraviolet photoelectron spectroscopy (UPS). Pronounced islanding of the DIP films deposited on Au is found by AFM as well as by TEM. In addition, TEM images show individual monolayers of DIP with the long molecular axis parallel to the substrate, suggesting a lying-down phase (λ-phase). TEM images also show the formation of Au clusters and a certain degree of Au interdiffusion into the DIP film after Au deposition on DIP. Specular and grazing incidence x-ray diffraction show the coexistence of standing phase (σ-phase) and λ-phase with a preferred growth of the λ-phase. UPS is used to study the evolution of the electronic structure of the DIP-on-Au and Au-on-DIP interfaces. DIP is found to physisorb on Au. The energy difference between substrate Fermi level and the DIP highest occupied molecular orbital at the interface is 1.0 eV. This hole-injection barrier increases to 1.45 eV away from the interface because of decreased screening by the metal and possible changes in molecular conformation. For Au deposition onto DIP, UPS traces the formation of Au clusters as a function of Au coverage. These clusters percolate only for Au coverages higher than 32 Å to give a continuous metal surface coverage and conductivity. The interaction between the Au clusters and DIP is also found to be of physisorptive nature.