RESEARCH NEWS
Nanowires or not?
Upward mobility
Claims by Korean researchers of the successful growth of silver nanowires [Science (2001) 294, 348] have been refuted by a team at the NEC Corporation in Japan [Science (2002) 296, 611]. Byung Hee Hong and colleagues reported the production of arrays of singlecrystal 0.4 nm diameter nanowires using an ambient solution phase inside the pores of self-assembled calix[4]hydroquinone nanotubes last year. Sumio Iijima and Lu-Chang Qin’s interpretation of the data, however, suggests that the material is more likely to be highly defected needlelike silver crystals. But the critics believe that an analysis of electron
The performance of organic semiconductor devices depends on the efficiency of charge transport processes. By using quantum-chemical calculations, researchers at the universities of Arizona and Mons-Hainaut, Belgium can quantify how molecular size, orientation, and packing affects the intrinsic transport properties of π-conjugated oligomers [PNAS (2002) 99 (9) 5804]. The interchain transfer integral, which expresses the ease of charge transfer between two interacting molecules, is a key parameter for understanding electron and hole mobilities in organic semiconductors. Using model π-conjugated systems of oligothiophenes, hexabenzocoronene, oligoacenes, and perylene, the researchers have investigated
micrographs or electron diffraction (ED) patterns of one of the purported nanowires could resolve the issue one way or another. While Hong remains adamant that the ED patterns, backed up by electron energy loss spectroscopy and infrared spectra, do support his claims of the formation of silver nanowires, he agrees that images of a single nanowire would be useful. In a response published in the same issue of Science as Iijima and Qin’s refutation, Hong reports that they may have observed an HREM image of a single nanowire, but that the image is unclear because of radiation damage and background contrast.
Schizophrenic matter A material that simultaneously exhibits bistability in its optical, electrical, and magnetic properties could be the basis for a new generation of electronic devices [Science (2002) 296, 1443-1445]. "We fabricated the material thinking it should have novel conductivity properties," says Robert Haddon of the University of California, Riverside. "But when we measured the properties we discovered the bistability." The 'neutral radical organic conductor' is based on a spirobiphenalenyl neutral radical, which consists of two phenalenyl ring systems spiroconjugated by a boron atom. Alkyl groups modify the crystal packing. In one state the material is paramagnetic,
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insulating, and infrared transparent. In the other, it is diamagnetic, conducting, and infrared opaque. Modifying the basic molecular building block varies the temperature range over which the bistability occurs. This ability to manifest the switch between different states in more than one physical channel could prove extremely useful in transistors, storage, or photonic devices. "Ultimately, to develop very sophisticated devices and new technologies we need materials that combine one or two of three channels: optical, electrical, and magnetic. Our material, as far as we know, is the first organic compound that combines all three," says Haddon.
the influence of various parameters on the transfer integrals. These included intermolecular distance, lateral displacement, chain length, and cluster size. In these conjugated systems, the transfer integrals are sensitive to various aspects of molecular packing. The largest transfer integrals occur with perfectly cofacial configurations of the molecules. In this case, hole mobilities are higher than electron mobilities. Altering the molecular packing modifies the value of the integral, so that in some structures higher mobilities are seen for electrons over holes. The design of organic materials with high electron and hole mobilities for use in field-effect transistors (FETs), light-emitting diodes (LEDs), and solar cells should be aided by these calculations.
Organic discovery Researchers from the Université de Rennes in France have applied a high throughput experiment (HTE) approach to the discovery of fluorescent polymers for the first time [J. Am. Chem. Soc. (2002) 124, 5278-5279]. The HTE approach has been applied, with some success, to fields such as polymers for biomaterials and chiral recognition, inorganic materials, and catalysis. Olivier Lavastre and his colleagues used the HTE approach with the PDcatalyzed carbon-carbon coupling reaction to generate
sets of conjugated polymers and detect fluorescent polymers. Starting with the poly(arylene ethnylene) PAE family of polymers, which are known to have photoluminescent and fluorescent properties, the resulting polymers were irradiated with a hand-held UV lamp to identify the active ones and their color. The technique could enable the fast synthesis of a large variety of conjugated polymers and lead to the discovery of potential new candidates for organic light emitting diodes, say the researchers.