Long, vertical and conductive molecular wires have attracted great interest due to their promising potential to single molecular devices.[1] Usually, long synthesized molecules are used as molecular wires but their synthesizes are complicate and expensive.[2] Stepwise methods based on self assembly properties of molecules could be a good and reproducible solution to produce supramolecular structures with molecular control. The scanning tunneling microscope is a versatile tool to fabricate and to control the molecular assemblies at molecular scale. Mainly, when it is operated at solid/liquid interface where trough a drop, positioned between STM tip and substrate, it is possible to add molecules in order to create organized monolayers.[3]
We have been applying a stepwise method to built long molecular wires composed by zinc-octaethylporphyrins separated by 4,4’-bipyridines, as is illustrated in Figure 1.[4,5] STM at solid/liquid interface was used to assembly the molecules on Highly Oriented Pyrolitc Graphite (HOPG). Each molecule was deposited individually where each step was controlled in real time using molecular resolution images obtained by STM. In figure 2 it is shown a STM image of 4th monolayer composed by bypiridines whit a darker region that show the porphyrins of previous monolayer. With this method we fabricated wires composed by 25 individual molecules arranged in a well-defined sequence and assembled via bonding the central metal of porphyrin and nitrogen atom of bypiridine. The final structure is a molecular wire with 14.29 nm in length (determined by density functional theory using a SPARTAN software).
In addition, we have also measured the conductivity of each molecular wire using scanning tunneling spectroscopy (STS) were hundreds of I/V curves were recorded at each wire growth step. The electrical resistance was represented in function of molecular length in order to study the contribution of tunneling and hopping transport. The results showed that these molecular wires are highly conductive with ultralow attenuation factor[5] and it is in agreement with previous reported results.[6]

References

[1] S. H. Choi, C. D. Frisbie, “Enhanced hopping conductivity in low band gap donor-acceptor molecular wires Up to 20 nm in length”. Journal of the American Chemical Society, 132(45), 16191–201, 2010.
[2] G. A. Ashwell, B. Urasinska, C. Wang, M. R. Bryce, I. Grace, C. J. Lambert, “Single-molecule electrical studies on a 7 nm long molecular wire”, Chem. Commun. 4706-4708 (2006).
[3] Q. Ferreira, A. M. Bragança, N. M. M. Moura, M. A. F. Faustino, L. Alcácer, J. Morgado, “Dynamics of porphyrin adsorption on highly oriented pyrolytic graphite monitored by scanning tunnelling microscopy at the liquid/solid interface”, Applied Surface Science, 273(2013), 220.
[4] Q. Ferreira, L. Alcácer, J. Morgado, “Stepwise Preparation and Characterization of Molecular Wires made of Zinc octaethylporphyrin complexes bridged by 4,4’-bipyridine on HOPG, Nanotechnology, 22 (2011), 435604,
[5] Q. Ferreira, Ana Margarida Bragança, L. Alcácer, J. Morgado, “Conductance of well-defined porphyrin self-assembled molecular wires up to 14 nm in length”, submitted
[6] G. Sedghi, L. J. Esdaile, H. L. Anderson, S. Martin, D. Bethell, S. J. Higgins, R. J. Nichols, “Comparison of the Conductance of Three Types of Porphyrin-Based Molecular Wires: β, meso, β-Fused Tapes, meso-Butadiene-Linked and Twisted meso-meso Linked Oligomers”, Advanced Materials, 24, 653-657, 2012