PAMELA ZURER

Fullerenes are notoriously difficult to dissolve in water; their C₆₀ core is just too hydrophobic. One way fullerenes can be enticed into aqueous solution is by attaching charged functional groups. At the University of Tokyo, chemistry professor Eiichi Nakamura has created charged fullerenes that incorporate stable cyclopentadienide anions surrounded by phenyl groups.

"In taking a closer look, these molecules have amphiphilic characteristics like surfactants," notes chemistry professor Benjamin Chu at the State University of New York, Stony Brook, who for years has studied the self-assembly behaviors of colloids and polymers. "They have a hydrophobic portion attached to a charged portion. But the hydrophobic C₆₀ part is shaped like a ball, different from the long alkyl chains of many surfactants."

Chu, Nakamura, and coworkers now report that the potassium salt of pentaphenylfullerene can assemble into vesicles--hollow bilayer shells [Science, 291, 1944 (2001)]. The fullerene vesicles are all roughly the same size, with an average diameter of about 34 nm. They are composed of about 13,000 of the modified C₆₀ derivatives.

The vesicles' size, Chu notes, is largely controlled by the size of the groups that surround the anionic portion of the fullerene molecule. This charged hydrophilic part must face the water on the inside and outside of the vesicle. "Because the phenyl groups are so large, the inside curvature of the vesicle should be large enough to accommodate them," he says. Replacing the phenyl groups with other substituents and mixing the molecules with other surfactants could lead to new bilayer systems, particularly membranes, with interesting and well-defined properties, he notes.

At the Max Planck Institute of Solid-State Research, meanwhile, physicists Oliver G. Schmidt and Karl Eberl have devised a method that can create nanotubes out of a wide range of materials, positioning the tubes precisely on a solid surface [Nature, 410, 168 (2001)].

To roll their own nanotubes, the researchers first deposit on a solid surface a layer of a material that can be etched away. They then lay down a thin film of the material that will become the tube. As the sacrificial layer is etched away, the top film wraps back onto itself, forming a nanotube at the edge of the back-etched layer.

The size of the nanotube walls is controlled by the thickness of the thin films. Starting with a 16-nm-thick film of silicon-germanium, for example, the team made a nanotube 230 nm in diameter and 12 mm long. A 6-nm-thick layer of SiGe resulted in a nanotube with a diameter of 50 nm.

The nanotubes' position on the surface is determined by how long the etching continues. The sacrificial layer is pure Ge, which is selectively removed with a H₂O₂/H₂O solution. The flow of the solution during the etching procedure is likely what causes the top film to wrap back upon itself to form the tube, Schmidt tells C&EN. In a variation of the method, two different, inherently strained thin films are laid down on top of the sacrificial layer. As that layer is removed, the bilayer above it curls upward, eventually forming a nanotube after a complete revolution.

"Very recently, we have accomplished spectacular nanotube formation with InGaAs on GaAs substrates," Schmidt says.

Among other applications, the exactly placed tubes could be used as nanopipelines for fluid transport, the researchers suggest.

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