For full article and contact information, see Angew. Chem. Int. Ed. 2002, 41 (10), 1705 - 1709

No. 10/2002

Spiral Semiconductors - Nanowires

Supramolecular organic structures
as templates for inorganic nano-objects

Because of their special optical and electronic properties, new nanostructures of inorganic materials are of interest as building blocks for nanotechnological devices. A promising starting point for the synthesis of such materials seems to be the use of "templates" or molds made of organic molecules that arrange themselves into highly organized nanostructures and guide the shape and size of inorganic compounds. Researchers at Northwestern University in Evanston have now impressively demonstrated the potential of this method: They have succeeded in producing semiconducting nanospirals of cadmium sulfide.

Samuel Stupp, Eli Sone, and Eugene Zubarev synthesized cadmium sulfide, a semiconductor, in the presence of organic structures that form twisted bandlike objects - and obtained cadmium sulfide spirals. Pictures taken with an electron microscope demonstrate that the structures are formed when tiny cadmium sulfide kernels lodge in the organic template and then grow together. "The crystals clearly bind preferentially to one face of the organic band," reports Sam Stupp. This process results in the cadmium sulfide spiral, which is coiled in the fashion of a telephone cord.

The researchers chose to use rod-shaped organic molecules as the components of their template. These rods have a water-loving (hydrophilic) "head", a central section made up of aromatic rings, and a water-hating (hydrophobic) "tail". All of the heads and middle sections of the rods are drawn to each other by attractive forces, which causes the rods to line up parallel to each other. This results in bandlike structures. In some solvents, these bands are flat, but in other solvents, the bands twist into helical structures.

In principle, bands can form two different kinds of helices. They can curl into a corkscrew-shaped form, like a telephone cord. Alternatively, a band can be twisted upon itself. Such a structure can be formed by holding one end of a ribbon in one hand, and then twisting the other end with your finger. In this case the ribbon does not form loops, but rather twists about it's own axis, or center-line. The molecular bands made by the Northwestern researchers fall into the second category. The cadmium sulfide spiral which results by growing along one face of the band has twice as large a distance between turns as the template, as would be expected geometrically.