Paper NS-MoM1
On-Surface Synthesis of Metallo-Porphyrin Nano-tubes by Vacuum Sublimation onto Noble Metals Substrates

Monday, October 15, 2007, 8:00 am, Room 616

Molecular electronics is a promising alternative for the continuing miniaturization of electronic devices. The capability of synthesizing and depositing molecular wires on solid surfaces is a key step within this context. The majority of the most promising current candidates to molecular wires, such as carbon nanotubes, DNA strands or individual polymer molecules, are too large and fragile for depositing them on solid surfaces by in-vacuum sublimation. Other kinds of wet deposition methods, such as drop-casting, are not so well controlled, which, in turn, hinders the understanding and design of their conductivity. Since the molecular mass of the chemical species well-suited for deposition on solid surfaces by in-vacuum sublimation is limited, one could envisage a mechanism to grow the polymeric molecular wires by depositing smaller subunits that would subsequently assemble into the polymer on the surface. This approach has successfully been used to steer the assembly of organic molecules into 1D structures. The 1D molecular wires so created are mostly like 'ribbons', due to the fact the molecules tend to lie flat on the surface to maximize substrate-adsorbate interaction. The on-surface synthesis of molecular wires with more complex geometrical shapes, such as tubular or cylindrical, has only been described for the case of carbon nanotubes, and only on surfaces covered with catalytic nanoparticles or on silicon carbide surfaces, where the constituent carbon for the nanotube growth arises from bulk segregation. The aforementioned ideal situation in which molecular subunits assemble into 1D structures with cylindrical symmetry has not been described so far in the literature. In this work we describe the growth of 1D tube or cylinder-looking structures obtained after deposition of a metallo-porphyrin (Zn-Tmp) on the noble-metal surfaces Au(111) and Cu(100) by vacuum sublimation with the substrate held at room temperature (RT). Our experiments indicate that the 'nanotubes' are only a meta-stable structure, since they disassemble into flat-lying Zn-Tmp monomers upon annealing to 500 K. The disassembled Zn-Tmp adsorbates are tightly bound to the surface, as evidenced by the fact that they can be easily imaged by Scanning Tunnelling Microscopy even at room temperature, and the difficulty to manipulate them even under the harsher tunnelling conditions. Two mechanisms for such a phenomenon will be discussed.