Dissected spinach produces molecular diode

Word out of Oak Ridge National Laboratory (Oak Ridge, TN) is that today`s brain food is tomorrow`s biomolecular, nanoscale photodiode or solar battery, which should be of interest to both spinach haters and nanofabrication engineers. Instead of ingesting the green plant, they could use its photosynthetic membrane to fuel nanomachinery.

Dissected spinach produces molecular diode

Paula M. Noaker

Word out of Oak Ridge National Laboratory (Oak Ridge, TN) is that today`s brain food is tomorrow`s biomolecular, nanoscale photodiode or solar battery, which should be of interest to both spinach haters and nanofabrication engineers. Instead of ingesting the green plant, they could use its photosynthetic membrane to fuel nanomachinery.

According to Elias Greenbaum, Ida Lee, and James Lee at the Oak Ridge Chemical Technology Div ision, the keys to harnessing plant-based energy are the specialized photosynthetic reaction centers embedded in plant photosynthetic membranes. Higher-order plants, such as spinach, have two such components to convert light energy into chemical energy--photosystem I and photosystem II. The Oak Ridge scientists have isolated, extracted, and purified the first center, which absorbs photons and generates a voltage across a membrane.

Photosystem I is of interest for nanofabrication applications because of its good optical and electrical properties combined with a nanoscale size (about 5 nm). Response time, for example, is quite fast. In 5 to 10 ps the primary event of the reaction center generates about 1 V across the polar regions of the membrane with both high quantum efficiency and minimal side reactions. By extracting reaction centers from the green plants and integrating them into self-assembled arrays that mimic membrane properties--capturing a photon, triggering a charge separation, and generating a voltage--it is possible to produce a molecular photovoltaic device.

Greenbaum`s group has recently demonstrated that photosystem I has stable current-rectification properties and is a molecular diode.1 The scientists used molecular self-assembly to anchor two-dimensional photosystem-I arrays to an electrode composed of atomically flat gold, which was produced by growing gold epitaxially on freshly cleaved mica--an atomically flat mineral. The process also chemically modified the surface of the gold with organosulfur compounds. Different compounds produced different preferential orientations of the reaction centers and thus different electrical properties (see figure).

The researchers imaged the reaction centers with a scanning tunneling microscope and varied the bias voltage between the tip and the base. Measuring the resulting (I-V) current-voltage curve allowed determination of the orientation of individual reaction centers. With the reaction centers primarily anchored perpendicular to the gold surface, diode-like I-V curves were observed.

Molecular diodes produced by the Oak Ridge technique are fairly stable, with test samples, so far, maintaining current-rectification stability for as long as four months. Greenbaum believes that it will be possible to construct elementary molecular circuits based on the arrangement and properties of these molecules rather than the bulk properties of silicon.

Nanofabrication is not designed to replace traditional semiconductor-fabrication techniques, however, but to supplement and enhance them. "The ability to etch finer and finer lines on silicon surfaces runs into technical problems related to the required wavelength of light," Greenbaum said. "While it is possible to work with shorter wavelengths or use electron-beam lithography to get very fine structures, this technique could provide another option. Epitaxial gold growth and chemical surface modification may allow placing preferentially oriented reaction centers onto defined silicon substrates that have been prepared by conventional silicon techniques. This may allow producing a finer patterning or texturing of the reaction centers on the surfaces."

REFERENCE

1. I. Lee, J. W. Lee, and E. Greenbaum, Phys. Rev. Lett. 79(17), 3294 (1997).

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