Photocurrent of Single Molecule Measured

Single small molecules can be used as functional components in optoelectronic circuits, but until now, controlling and probing such molecules for photovoltaic and photoelectrochemical applications has proved difficult.

Scientists at the Technical University of Munich, the Munich Center for Advanced Photonics, Nanosystems Initiative Munich and Tel Aviv University, however, have developed a method sensitive enough to provide these measurements using a scanning near-field optical microscope setup.

Photosystem-I (green) is optically excited by an electrode (top). An electron is then transferred step by step in only 16 ns. Courtesy of Christoph Hohmann, Nanosystems Initiative Munich.

The team investigated the photocurrent generated by a single photosynthetic protein — photosystem I — that exhibits outstanding optoelectronic properties found only in photosynthetic systems.

The photocurrent was measured with a gold-covered glass tip employed in a scanning near-field optical microscopy setup. The photosynthetic proteins were optically excited by a photon flux guided through the tetrahedral tip — the microscope’s probe — that at the same time provided the electrical contact. With this technique, the physicists were able to monitor the photocurrent generated in single proteins.

They demonstrated that such a system can be integrated and selectively addressed in an artificial photovoltaic device while still retaining its biomolecular functional properties.

“They act as light-driven, highly efficient single-molecule electron pumps that can function as current generators in nanoscale electric circuits,” the researchers said in Nature Nanotechnology (doi: 10.1038/nnano.2012.165).

The research was supported by the German Research Foundation, the Clusters of Excellence Munich Center for Advanced Photonics and Nanosystems Initiative Munich, and the ERC Advanced Grant MolArt.

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