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Posted: Aug 08, 2013
Molecules form 2-D patterns never before observed
(Nanowerk News) Tessellation patterns that have fascinated mathematicians since Johannes Kepler worked out their systematics 400 years ago – and that more recently have caught the eye of both artists and crystallographers – can now be seen in the laboratory. They first took shape on a surface more perfectly two-dimensional than any sheet of writing paper, a single layer of atoms and molecules atop an atomically smooth substrate. Physicists coaxed these so-called Kepler tilings "onto the page" through guided self-assembly of nanostructures.
Organic molecules equipped with functional groups to express distinct linkages to metal atoms were deposited onto a smooth silver substrate under vacuum conditions. Subsequently the organic layer on this platform was exposed to an atomic flux of the lanthanide cerium. At a certain ratio of cerium atoms to molecules, self-assembly produced a symmetrical complex 2-D pattern described originally by Kepler and known today as the snub square tiling. Clearly identifiable through scanning tunneling microscopy was a recurring, five-vertex connecting element less than one nanometer across, a cerium-ligand coordination unit.
That the snub square tiling pattern had never been fabricated and seen at the molecular level by exploiting self-assembly protocols was interesting in itself. Beyond that, the physicists explain, every new surface architecture could potentially open the way to novel physics and chemistry, and until now five-vertex structures have proven elusive. In particular, the fact that the lanthanide element cerium played such a key role marks this as the beginning of a new line of research.
This is the first time the TUM researchers – members of Prof. Johannes Barth's Institute for Molecular Nanoscience and Chemical Physics of Interfaces – have coordinated molecules with a lanthanide, and the first time anyone has done this in 2-D. "And lanthanides are special," David Ecija explains. "They have very intriguing optical, magnetic, and chemical properties that could be interesting for nanoscience, and possibly also for nanotechnology. Now we have a new playground for research with the lanthanides, and beyond."