Fullerene-like molecules without carbon.

To date, all known hollow, cage-like molecules have contained at least some carbon. The widely studied fullerenes consist of nothing but carbon atoms, while the metallo-carbohedrenes (SN: 4/18/92, p.250) mixed in a few titanium atoms to help bend the structure into a puckered ball.

Now materials scientists have discovered a molecular cage with no carbon whatsoever -- tungsten disulfide. This inorganic semiconductor will also curl up to form cylindrical and closed polyhedral structures, says Reshef Tenne at the Weizmann Institute of Science in Rehovot, Israel. He and his colleagues have made microscopic tubules ranging from less than 10 nanometers to more than 100 nanometers long, as well as cages of various sizes, they report in the Dec. 3 NATURE.

Because it is an inorganic cage, the tungsten disulfide crystal will likely have properties very different from those of fullernes. [The discovery] opens up a whole new area; it will stimulate research on nanotubes in new materials," says Thomas W. Ebbesen, materials scientist at NEC Corp. in Ibabaki, Japan.

For the past two years, Tenne and his colleagues have been designing better photovoltaic cells by making thin films of tungsten sulfide. In one experiment, they deposited tungsten in thin layers onto quartz, then exposed it to hydrogen sulfide in an oven heated to 1,000 [degrees] C. They examined the resulting films with an electron microscope.

Only after seeing electron micrographs of the onion-like fullernes that form when fullerene films are subjected to high-energy electron beams (SN: 10/24/92, p.277) did the Israeli scientists realize that the unusual shapes in their micrographs of the tungsten disulfide warranted a closer look, Tenne says.

When Tenne and his colleagues tilted their samples in the electron microscope, they could distinguish closed three-dimensional structures from open curved sheets. Also, the electron diffraction patterns and a technique called lattice imaging further verified the closed nature of these molecules, says Tenne. However, they have yet to develop a way to make large quantities of these new molecular cages.

Like fullerene tubules (SN: 7/18/92, p.36), the tungsten disulfide tubules consist of concentric layers. They seem to sprout from the tungsten film and are sealed at the top. The smallest, with four layers, has an internal diameter of 4 nanometers. The polyhedrons exist singly or in linked chains of three or more, Tenne's team reports.

Like graphite atoms, tungsten disulfide atoms arrange in layers of parallel honeycomb sheets. Hexagons of tungsten are sandwiched between hexagons of sulfur. A seventh atom lies in the center of each hexagon. Weak forces link the sulfur sheets.

High temperature may cause the sheet to curl or convert the hexagons to pentagons or other formations that can stabilize the rounded shape, the researchers suggest. Or, oxygen or some other contaminant may escape from the quartz substrate during heating and help cause the sheets to curve, they add.