Hanging on the nanowires

Tiny spirals of the semiconductor cadmium sulfide could form the basis of new nanodevices, according to US researchers.

Nanostructures are being actively investigated by materials scientists for potential applications as building blocks for all kinds of technologies from quantum computing to opto-electronics. Using a "template" or mold composed of self-assembling organic molecules is one fruitful approach to creating materials that are highly organized on the nanometre scale.

	Samuel Stupp

Now Samuel Stupp, Eli Sone, and Eugene Zubarev of Northwestern University in Evanston, have used a template approach to make nanospirals of the semiconductor cadmium sulfide. The twisted, band-like structures reveal themselves through electron microscopy to resemble tiny coils like a twisted length of silk ribbon. The structures are formed when cadmium sulfide kernels lodge in the rod-shaped amphiphilic organic template and then grow together. "The crystals clearly bind preferentially to one face of the organic band," explains Stupp.

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The organic templates - so-called dendron rodcoils, DRCs - are water-loving (hydrophilic) at one end, have a central aromatic region and a hydrophobic tail. They can also hydrogen bond in a head-to-head manner through dendron segments and self-assemble into nanoribbons. This property allows them to act as gelating agents for various organic solvents. The organic structures line up parallel to each other giving flat band-like structures in some solvents, but adding a crucial twist in others. The resulting nanospirals of cadmium sulfide mirror the helical pitch of their organic templates at 40-60 nanometers.

	Click image to magnify

"The semiconductor helical structures reported possess a unique morphology for CdS that may have interesting electronic or photonic properties," explains Stupp. He adds that the research may also represent a step towards the goal of harnessing the power of supramolecular organic chemistry to build inorganic structures for nanotechnology beyond what has been accomplished with liquid crystals. "Our results suggest that by using extremely uniform, stable, nonaggregated supramolecular objects as templates one can achieve good control over the morphology of the templated product," he adds.

Angew. Chem. Int. Ed. 2001, 41 (10), 1705-1709; DOI:
10.1002/1521-3773(20020517)41:10<1705::AID-ANIE1705>3.0.CO;2-M.