Tuesday, May 01, 2012

Molecular electronics: Flipping a single proton switch

Molecular electronics: Flipping a single proton switch
By: Omar Gómez Rojas ID:139236

The basic idea of molecular electronics — using molecules as components in electrical circuitry — has been verified experimentally by using molecules as diodes, switches and transistors1, 2, 3. The use of molecules in electronics is conceptually appealing because the required function can be encoded into the molecule through chemical synthesis, making molecules an ideal tunable nanoscale building block. However, the molecular components have to be interfaced with the macroscopic world and this places a number of constraints on their design.

In molecular switches, for example, the switching action should not change the overall size of the molecule because this will disturb the external contacts to the switch. Ideally, small changes of the atomic structure of the molecule should lead to changes in the electronic properties of the whole molecule that are large enough to allow the different states to be distinguished from each other by measuring the electric current through the molecule. The switching unit should also be protected in some way so that the switch is not affected by changes in its immediate environment. In addition to these strict requirements, it might also be beneficial to have access to more than two levels (states) in a switch. Furthermore, if the switching unit is part of a molecule that already has a well-developed chemistry, this could help broaden the range of different molecular architectures that the switch can be incorporated into. Writing in Nature Nanotechnology, Willi Auwärter and colleagues at the Technical University, Munich, now report using a porphyrin molecule to create a four-level molecular switch that fulfils all of the above criteria4.

Porphyrins are flat organic molecules composed of four modified pyrrole (C4H4NH) subunits connected by CH bridges. The molecules have a central cavity that can incorporate a metal ion or two hydrogen atoms. When the porphyrin has two central hydrogen atoms — a free-base porphyrin — the two protons can transfer between the two pairs of opposing nitrogens in the cavity of the molecule, a reaction know as tautomerization. It has also been shown previously that a related molecule (a phthalocyanine) can function as a tautomerization-based two-level molecular switch5. To upgrade such two-state systems, Auwärter and colleagues used voltage pulses from the tip of a low-temperature scanning tunnelling microscope (STM) to controllably remove one of the hydrogens from the cavity of a porphyrin molecule that was deposited on a silver surface (Fig. 1a). This allowed the remaining central hydrogen to then hop between the four pyrrole rings giving rise to a four-level molecular switch based on the transfer of a single proton.

Find more information on this work in:

Molecular electronics: Flipping a single proton switch
Peter Liljeroth is at the Department of Applied Physics, Aalto University School of Science, PO Box 15100, 00076 Aalto, Finland
Published online 28 December 2011
DOI: doi:10.1038/nnano.2011.242

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