Carbon Buckyballs Have a New Silicon Rival

Could silicon fullarenes eventually outperform buckyballs in molecular electronics?

A roughly spherical arrangement of atoms
Illustration: Goethe University
The silicon buckyball (left) contains a chlorine atom. It is then stabilized by additional silicon atoms (middle) that join up with more chlorine atoms to form a molecular exoskeleton (right)

Three years ago researchers created a 2-D silicon analogue to graphene, whereby silicon atoms form a similar honeycomb monolayer of atoms, a material that could combine the extraordinary properties of graphene with silicon’s semiconducting abilities. Researchers quickly moved to try to replace the carbon atoms of buckyballs with silicon atoms as well, but ran into difficulties because silicon’s chemical behavior is very different from that of carbon.

Now researchers have reported in Angewandte Chemie, International Editionthe discovery of a 3-D silicon analogue to carbon buckyballs. It contains a core of 20 silicon atoms stabilized by chlorine atoms (the researchers are therefore suggesting the term “fullerane” instead of “fullerene” for their compound.) The discovery was serendipitous, says Matthias Wagner, who with Max Holthausen, led the research at the Goethe University in Frankfurt, Germany. “One of my Ph.D. students, Jan Tillmann, discovered a small amount of crystals produced in a reaction involving hexachlorodisilane (Si2Cl6). He X-rayed them and found that they contained cages made up of 20 silicon atoms. It took him a year to optimize the reaction and now he gets a yield of about 30 percent,” says Wagner.

While carbon atoms happily link to one, two, three, or four other atoms, silicon atoms prefer to form bonds with four other atoms, which precludes a buckyball consisting of only silicon.  Consequently, the compound Tillmann discovered had a more complex structure. It contains a buckyball structure in the shape of a dodecahedron formed by 20 silicon atoms, with a chloride ion sitting at its center. Forming an exoskeleton of sorts around the silicon dodecahedron, are 12 silyl groups of atoms consisting of one silicon atom linked to three chlorine atoms (SiCl3). Another eight individual chlorine atoms are bound to the remaining vertices.

The 12 silyl groups make the silicon fullarene vulnerable to moisture. On the other hand, their chlorine atoms can easily be substituted by hydrogen atoms, explains Wagner: “This tells us that we can use the SiCl3 groups as functionalization sites” he adds. And this is where the silicon buckyballs could have a clear advantage over their carbon relatives, which are not very amenable to linking up with each other.

“What we are dreaming about is that we can use these SiCl3 substituents as anchor groups to interlink these fullerenes to form two or three dimensional networks—this is our long-term goal,” says Wagner.

Will this allow the introduction of new strategies for the further miniaturization of nanoscale silicon circuits? Finding out will be the focus of much of their future work. One project will be to determine the electrical and optical properties of these fullerenes. “We have just prepared the building blocks, and we hope for something like semiconductive properties. Compounds like this did not exist up to now, and now we have to see what we can do with them,” says Wagner.

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