Molecular Sudoku

Sudoku puzzles represent a popular exercise recommended to improve logical and creative thinking.

A team of scientists from the Catalan Institute of Nanotechnology, ICREA, and Universitat Autonoma de Barcelona investigated the properties of a special kind of sudoku, made by assembling tiny molecules into a 3x3 square array (Figure 1). The result is not a mind-boggling game, but a detailed picture of how each molecule interacts with its neighbors and conducts electricity when squeezed between two metallic electrodes.

As reported this week in Nature Communications, the researchers used the atomically-sharp tip of a scanning tunneling microscope to move 1-nanometer sized molecules on top of a silver substrate. The tip is controlled with such great accuracy that it is possible to precisely choose the position of each molecule and build tiny molecular squares, crosses, and chains of controlled size and orientation. The same tip is then used as a mobile electrode to probe the electrical conductivity of the molecules as a function of their position in the array. Figures a-d show an example of such measurements: a represent the topography of a “sudoku” molecular cluster, whereas b-d show regions of high conductivity at different voltages. At low voltage, electrons prefer to pass through the corner molecules, whereas at high voltage, only the central molecule is conducting. This is so because the conductivity depends strongly on a small set of electronic states, which conduct electricity to the substrate, and these are modified by the presence of side-to-side neighbors.

The molecular conductance was found to vary strongly not only from one molecule to another, but also within each molecule, due to the possibility of exploiting different electron transport channels at different positions. Such conduction channels arise from the excitation of internal degrees of freedom of the molecules, such as atomic vibrations and magnetic coupling of the electronic spins. All together, these results demonstrate the intricacy and beauty of molecular electronics, providing a glimpse of its advantages, such as the fabrication of versatile miniaturized circuits, and challenges, which may prove harder to solve than a sudoku game.

Molecular Sudoku: a Topography of a 3x3 cluster of copper phthalocyanines artificially created by manipulating individual molecules. The single phthalocyanine at the top-left corner of the image serves as a reference of a non-interacting molecule. b-d Electronic properties inside the cluster. Yellow-colored areas indicate regions of high current at a given voltage. Corner molecules conduct best at lowest voltage (b), side molecules at intermediate voltage (c), and the center molecule at the highest voltage (d). e Maps of the spin density inside the cluster. Corner molecules exhibit a magnetic state similar to the isolated reference molecule. Molecules with a larger number of bonds do not present such a magnetic state.

Spin coupling and relaxation inside molecule-metal contacts

Aitor Mugarza^1,2*, Cornelius Krull^1,2, Roberto Robles², Sebastian Stepanow^1,2, Gustavo Ceballos^1,2, Pietro Gambardella^1,2,3,4

¹Catalan Institute of Nanotechnology (ICN), UAB Campus, E-08193 Barcelona, Spain

² Centre d’Investigacions en Nanociència i Nanotecnologia (CIN2), UAB Campus, E-08193 Barcelona, Spain

³ Institució Catalana de Recerca i Estudis Avançats (ICREA)

⁴ Departament de Física, Universitat Autonoma de Barcelona, E-08193 Barcelona, Spain

DOI: 10.1038/ncomms1497

For further information:

Catalan Institute of Nanotechnology (ICN) www.icn.cat