Thin film properties and surface morphology of metal free phthalocyanine films grown by organic molecular beam deposition

The structure, optical properties and surface morphology of thin films of metal free phthalocyanine (H₂Pc) deposited in an ultra-high vacuum environment by organic molecular beam deposition have been studied using a variety of ex-situ techniques. The growth conditions have a strong influence on the properties of the films. H₂Pc undergoes a phase transition (α → β) at a deposition temperature of ~ 330 °C, or upon post annealing a film grown at room temperature. Both the structure and optical properties of the films change and powder X-ray diffraction, electronic absorption spectroscopy, Raman and photoluminescence spectroscopies are used to characterise the differences between the two phases. Atomic force microscopy and Nomarski interference microscopy show that the lower temperature α- phase is characterised by a smooth morphology with spherical islands that show no apparent long-range order. By contrast, the β-phase has a much greater root mean square roughness and long thin needle-like crystals are observed on the surface of the films. The morphology of the β-phase depends on the method of preparation and there are two distinct types, β₁ and β₂. The crystallites show a preferential orientation and alignment with respect to each other for growth at room temperature followed by annealing (β₁), but are randomly oriented for films grown at elevated substrate temperatures (β₂).