Molecular self-assembly of nanowires

Materials scientists from the Empa institute in Zürich, Switzerland, have succeeded in getting two organic molecules to self-assemble into long chains on a surface.

While others have managed to create chains of molecules, until now the chains have not been very long, the problem being that all surfaces show unevenness at the atomic scale. A step edge on a surface, even if it is only a few atomic layers high, will block the growth of a molecular chain across it.

But Roman Fasel and his colleagues asked themselves what would happen if all the steps on a surface were arranged parallel to each other, somewhat like a miniature staircase or terrace. Molecular chains should assemble preferentially along step edges, organising themselves into a long, parallel lattice.

Doctoral student Marta Cañas-Ventura from EPF Lausanne, together with Fasel and others at Empa, prepared the surface of a gold single-crystal, leaving a terrace with 0.24 nm-high steps, with each step separated from the next by a constant distance of 5.8 nm. Two matching molecular species – 1,4-bis-(2,4-diamino-1,3,5,-triazine) and 3,4,9,10-perylenetetracarboxylic diimide – were then evaporated onto the surface.

Looking at the result with a scanning tunnelling microscope, the researchers found that when low concentrations of the two organic molecules were used, a single chain was formed at each step edge. With higher concentrations, a double chain was deposited.

Unfortunately, the self-assembled nanowires cannot be used as conductors in molecular electronics as they are in contact with a metallic substrate, and possess poor conductivity.

But, says Fasel: “One way to electronically decouple supramolecular systems from the metallic substrate is to insert a very thin insulating layer such as sodium chloride. This is done by depositing 1 or 2 monolayers – i.e., less than 1 nm deep – of NaCl onto the clean metal surface.” To address the issue of poor conductivity, the researchers are exploring the possibility of using covalently bonded supramolecular wires.

The study, which is reported in the journal Angewandte Chemie, was funded primarily through the European Union’s Sixth Framework programme.


Further reading

Self-Assembly of Periodic Bicomponent Wires and Ribbons, Cañas-Ventura et al., Angewandte Chemie Int. Ed. 46, 1814 (2007) (subscription required)

Figure

Scanning tunnelling microscope image of regularly ordered steps on a gold surface, each 0.24 nm high and 5.8 nm wide (image area 100 x 100 nm). Image credit: Empa.

Article first published in Nanomaterials News.