Single crystals of both pure and doped buckyballs have been successfully synthesized and characterized by a team of LBL and UC Berkeley researchers led by physicist Alex Zettl of LBL's Materials Sciences Division (MSD). Members of Zettl's team included post-doctoral associates Xiao-Dong Xiang, Jian Guo Hou, and Li Lu, and graduate students Gabriel Briceno, Brian Burk, Nasreen Chopra, Michael Fuhrer, and William Vareka.
Buckyball is the popular name for buckminster-fullerene (fullerene for short), the soccerball-shaped 60-atom cluster that has joined diamond and graphite as the third known form of pure carbon. Depending on whether they are pure or doped with other atoms or molecules, fullerenes (C60) can be insulators, conductors, semiconductors, or even superconductors. So enormous is its potential that fullerene was judged the "molecule of the year" in 1991 by Science magazine.
"Buckyballs are like a playground for chemists and materials scientists," Zettl says. "They are easily modified, using organic chemistry techniques, into other molecular structures, which makes them remarkably versatile. But even if there weren't any practical applications, fullerenes would be interesting to study because they have such a beautiful geometric shape and a very unique electronic structure."
To learn more about the properties of fullerene molecules and fullerene-based materials, it is advantageous to make buckyballs in a crystalline form. However, until the breakthrough achieved by Zettl and his group, most characterizations of fullerene properties were based on granular thin films and pressed powders containing an abundance of fullerene clusters.
"Thin films and powders are very useful for a number of experiments," Zettl says. "However, reliable measurements on single crystals are essential for establishing intrinsic properties."
The flat shiny bits of crystalline material produced by his group have already helped establish intrinsic parameters that should enable scientists to determine whether the mechanism behind the superconductivity of fullerenes is as exotic as some proposals indicate.
"Earlier electronic transport studies had shown anomalous fluctuations in thin films of fullerenes doped with potassium just above the critical temperature," Zettl says. "Based on studies with single crystals, we confirmed that these fluctuations do exist and are consistent with pure three-dimensional fluctuation conductivity."
The experimental transport results of Zettls group appear to support the theoretical work of MSD theorist Marvin Cohen, who maintains that the electronic nature of fullerenes can be explained by the standard model of superconductivity.
Researchers in Zettl's lab make their fullerene crystals by first creating an arc between two carbon rods in the presence of helium gas. The arcs heat vaporizes the rods into carbon atoms that coalesce into sheets. The inert helium holds the carbon sheets near the arc long enough for them to close in on themselves, forming millions and millions of fullerene spheres, most of which are C60 and C70. For the creation of pure crystals, the C60 is separated from the C70 through chromatography and condensed into powder. This powder of buckyballs is then vaporized at a high temperature inside a quartz tube.
"If the combination of temperature, gradient, and the amount of gas inside the tube are exactly right," he says, "fullerene crystals will grow on the walls of the tube. We are continually changing the details of our technique in order to optimize it."
It takes about 10 minutes to make fullerene powders, but about 10 days to grow crystals. Once the pure crystals have been obtained, they can be doped through intercalation, a process in which a desired type of atom, i.e., potassium or rubidium, is inserted into the lattice in between individual buckyballs.
Zettl and his group have refined their technique for making single crystals of fullerenes to the point where they can now experiment with other fullerene-based structures. Perhaps the most intriguing of these experiments involves the production of fullerene tubescylindrical crystals that are essentially one dimensional.
"Fullerene tubes, first discovered in Japan, have an even wider range of potential applications than buckyballs," Zettl says. "For example, you could make the world's strongest wire or fiber, which could be either an insulator, a conductor or a semiconductor depending on how you closed up the tubes."
The flexibility of tubes stems from being able to attach other atoms to their ends, which are highly reactive because of their curvature. It is believed that reactions could also be used to temporarily open tube ends for the insertion of atoms or molecules to produce a bonanza of exciting new materials.
Zettl and his group are currently exploring several ways of putting atoms inside fullerenes, including the growing of a carbon cage around another atom and the use of a scanning tunneling microscope to forcibly inject the atom through the carbon walls. In collaboration with the research group of LBL Director Charles Shank, Zettl has also launched a series of experiments on the ultrafast dynamics of fullerenes that will literally shed more light on the optical properties of these materials.