Structural and electric-optical properties of zinc phthalocyanine evaporated thin films: Temperature and thickness effects

It is known that the molecular architecture plays a fundamental role in the electrical and optical properties of materials processed in the form of thin films. Here, zinc phthalocyanine (ZnPc) thin films were fabricated through the vacuum thermal evaporation technique (PVD, physical vapor deposition) up to 50 nm thickness with the objective of determining their molecular architecture and some electrical and optical properties. Structurally, the results showed a uniform growth of the films depending on how the evaporation is performed (step-by-step or straightforward). The uniform films present a molecular organization dominated by the ZnPc macrocycle ring forming almost 90° in relation to the substrate surface. These films are crystalline (α-form) and possess molecular aggregates in the form of dimers (or higher order of aggregates) and monomers. Such aggregates are seen at the nanometer scale; however, at the micrometer scale, the films are morphologically homogeneous. In relation to the optical properties, it was observed that these films, besides absorbing in the ultraviolet-visible region, present a photoluminescence when irradiated with the 785 nm laser line. In terms of electrical properties, it was determined an electrical conductivity of ca. 10^-10 S/m and a significant photoconducting activity. Finally, a dependence of the molecular organization, crystallinity, and optical properties on the film annealing (and thickness) was investigated, and the sensitivity of the ZnPc PVD films against gasoline vapor was tested as proof-of-principle.