Technology: Molecule's one-way route to electronics

By DANIEL CLERY An organic molecule, nicknamed George, has provided a key step towards making electronic circuits from organic molecules rather than inorganic semiconductors. British researchers have demonstrated that a layer of the material only a few molecules thick can behave as a rectifier, in other words it will allow electric current to flow in one direction but blocks current flowing the other way. The most obvious application for molecular circuits would be in sensors for organic compounds and chemicals, says Roy Sambles of the physics department at the University of Exeter, leader of the team that developed the rectifier. Reactions between the compounds and sensing circuit generate an electrical current that passes to a conventional circuit to display the result. To make circuits, Sambles says, you need an insulating material – fatty acids in the case of organic circuits – and switches so that you can turn currents on and off or store charge. Rectifiers are the key to switches: in semiconductor electronics, basic transistors are made by fabricating two rectifiers back-to-back. The ideal of molecular electronics is to produce single chain-like molecules that behave as circuits, with different structures along the chain acting as electronic components. These could be made in a laboratory using chemical synthesis without the complex and expensive wafer fabrication plants needed for semiconductors. Layers of organic molecules have been shown before to behave like rectifiers. But researchers were never sure that it was the molecule causing the results or simply an interaction between the active groups of the molecule and the metal contacts. Sambles along with Scott Martin, also from Exeter, and Geoff Ashwell from the Cranfield Institute of Technology have shown conclusively that it is the molecule itself (Physical Review Letters, 11 January, p 218). Ashwell, a synthetic chemist, prepared a molecule with the formula C16H33-&ggr;Q3CNQ, dubbed George for convenience. It is a type of double ion, known as a zwitterion, that has both a positive and a negative ion group attached. Also, one end of the molecule is water soluble and the other end, the paraffin group, is not. So when the molecules are spread on the surface of water they form a layer one molecule thick, known as a Langmuir-Blodgett film, with all the molecules the same way up. The film is lifted off the water surface with a silver-coated slide, the silver later acting as one electrode of the rectifier. This process is repeated seven times to produce a layer seven molecules thick which is then dried for two weeks. A number of small magnesium electrodes are then fabricated on top of the layers of molecules. The team applied a voltage across the film which swept smoothly from around -1 volts up to +1 volts and back again repeatedly. The resulting current flowing through the film, from a single sweep, is shown in the diagram. At positive voltages the current rises sharply, but negative voltages produce virtually no current. To show that the molecule was responsible for this effect, the researchers made identical films using molecules that had been bleached to inactivate the molecule’s active ion groups. This film simply acted as an insulator, blocking current in both directions. The team also made a layer of normal unbleached molecules sandwiched between layers of w-tricosenoic acid, a material related to soap, to isolate the molecule’s active groups from the metal contacts. This film sandwich also behaved like a rectifier. The team are now making films on transparent conducting electrodes so that they can shine light at them. The conductivity of the brightly coloured films is influenced by light and the researchers hope this will give clues about how they work. The next stage, says Sambles, is to make a double,
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