Morphology and local transport characteristics of metalloporphyrin thin films

Abstract

Organic systems such as porphyrin-based molecules stand as a promising alternative for molecular devices. These systems have important photophysical and electrochemical properties and can be easily prepared by well-established synthesis and deposition techniques. Nevertheless, not very much is known about their electrical properties and charge transport mechanisms. In this work, the local electrical properties of [5,10,15,20-tetra(p-methoxyphenyl)porphyrin]copper(II) (Cu-TMPP) thin films deposited on nickel substrates are investigated. The implementation of current-sensing atomic force microscopy (cs-AFM) allows the detection of local variations of the topographic and transport properties of Cu-TMPP thin films with nanometric spatial resolution. In particular, the electric current mapping approach adopted here allows the quantification of the local charge transfer through the organic structures in a reliable and controlled manner. The formation of organic dendrites with heights between 2 and 5nm and lengths in the micrometric scale is observed. On thicker organic films, a layered filamentary-like growth is observed. These dendrites behave as a semiconducting matrix structure over a conducting metallic substrate and could be used for tuning transport properties on a device scale by reducing the contact area at the organic film-metal electrode interface. In addition, a detailed investigation of the electrical evolution of the conducting sites in the organic thin films is presented as a function of the thickness and applied electric field. For the majority of the conducting sites (>70–80%), a field dependent transition from a linear-like to an exponential transport regime is identified. We relate the non-homogeneous electrical response to the formation of molecular dendrites and interface defects. This electrical analysis and the understanding of the underlying transport mechanism become important for future implementation of porphyrin-based devices.