Buckyballs: Carbon Goes 3-D

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Buckyballs: Carbon Goes 3-D

Overview

In 1985 Harold Kroto (1939- ), Robert Curl (1933- ), and Richard Smalley (1943- ) discovered a novel form of pure carbon, called fullerenes, and opened up a new field of chemistry and materials science. Buckminsterfullerene, named after American engineer Richard Buckminster Fuller (1895-1983), consists of 60 carbon atoms joined together into the shape of a soccer ball. It and related fullerenes are hollow, highly stable, and have unusual physical and chemical properties, including optical activity and superconductivity. Nanotubes, which are "buckyballs" rolled into cylindrical forms, are being tested for a variety of applications and are considered to be the most promising material for nanotechnology, the building of new materials at the molecular scale.

Background

Fullerenes represent a new form of the element carbon in which the atoms are arranged to create closed shells. Much of the potential of these new compounds is rooted in the vast experience scientists and engineers have in carbon chemistry, but the first fullerenes, clusters of 60 carbon atoms (C₆₀) and 70 carbon atoms (C₇₀), have their origins in radioastronomy and cluster chemistry.

In 1985 Kroto, Curl, and Smalley came together at Rice University to perform an experiment. Kroto was an expert in microwave spectroscopy at the University of Sussex. He studied the formation of carbon in stars and had already established the existence of long carbon chains in the atmospheres of red giant stars and in interstellar space. He was interested in understanding the structure of these chains.

Curl, also a microwave spectroscopist, had told Kroto that his colleague, Smalley had a device that could provide data he needed. Smalley was using a laser-supersonic cluster beam apparatus to study the design and distribution of chemical clusters. Clusters are aggregations of molecules at a scale between the macroscopic and microscopic worlds. In the gas phase, atoms tend to condense to form clusters of various sizes. Smalley was trying to understand why cluster components of certain sizes were much more common that those others. His apparatus, which could vaporize materials into plasmas of atoms, seemed ideal to simulate the conditions in the atmospheres of red giant stars.

The scientists put a sample of graphite into Smalley's apparatus. An intense pulse of laser light quickly vaporized the carbon, and its atoms were drawn into a stream of helium, where they combined to form clusters. These were then captured in a vacuum chamber maintained at only a few degrees above absolute zero.

The scientists analyzed the carbon clusters using mass spectrometry. As they fine-tuned the experiment, two strong peaks, representing compounds C₆₀ and C₇₀, showed up. It was soon clear that these compounds were highly stable because their spectrographic peaks persisted even when a half dozen different gases were added to the mix. A new, highly symmetrical, cage-like form of carbon fit the data. Before the scientists finished the second week of experiments, they had mailed off their report to Nature. Because the C₆₀ cluster resembled Buckminster Fuller's geodetic dome, it was name buckminsterfullerene. It and related compounds are now popularly known as "buckyballs."

Within a month, the three scientists' results were in print. The scientific community reacted both with excitement and skepticism. While applications suggested themselves from the beginning, the new compounds needed to undergo extensive chemical testing to confirm the surprising structure. The analytic community jumped on buckyballs almost immediately, building evidence that the cage-like structures postulated were correct, but final proof required larger (gram) quantities. This came in 1990, when Donald Huffman and Wolfgang Kraetschmer used carbon arc vaporization between two graphite electrodes to produce macroscopic quantities.

The result was a black powder that was highly stable to both pressure and temperature. (Breaking a fullerene requires temperatures over 1,000°C [1,832°F]). As a cubic crystal, the material is extremely hard and electrically insulating. Buckyballs have strong optical effects, linking together into chains upon irradiation with intense ultraviolet laser light. With large quantities to test, C₆₀ passed every test to prove its soccer-ball shape (20 hexagons and 12 pentagons in a spherical configuration). In fact, Kroto himself verified the structure with nuclear magnetic resonance.

Impact

In 1996 Kroto, Curl, and Smalley were awarded the Nobel Prize for Chemistry. Since then, fullerene chemistry has become one of the most dynamic and fastest growing areas of investigation in chemistry and materials science. Thanks to the work of Huffman and Kraetschmer, fullerenes can be worked on with the full tool-box of organic chemistry, and thousands of scientific papers on fullerenes and their derivatives have been published. Specialty chemical companies even sell a variety of buckyballs for a few hundred dollars a gram.

For most commercial applications of fullerenes, lower prices and simple methods of mass production are required. However, their properties—and the variety of additional properties that are achievable with known organic chemical reactions—promise a number of important uses that are actively being investigated.

The shapes of fullerenes immediately suggests ball bearings, so these compounds may find a use as lubricants. Their cage-like structure offers the possibility of using them to enclose drugs or radioactive tracers for medical imaging. The electrical and optical properties of fullerenes also make them good candidates for transistors, memory bits, photoconductors, tunnel diodes, and sensors.

Organic chemistry—the chemistry of manipulating carbon compounds—is a well-established field and has supported the development of chemical methods to manipulate the structure and properties of fullerenes. The stability and the aromaticity (alternating single and double bonds) of fullerenes make them available to chemical reactions that do not destroy their hollow, cage-like structure. Chemists can make specific modifications, such as adding chemically polar groups to make the compounds water soluble, and can even begin to work in three dimensions to create catalysts and other useful materials.

Because of the relatively large size of the individual fullerene molecules, big, empty spaces exist between them when they are in crystalline form, much like the space between gum balls in a gum ball machine. It's possible to put small molecules and atoms into these spaces (intercalating them). With certain metal compounds this has an interesting result: the materials become superconductors. While the practical value of this effect may be limited since the presence of air destroys these materials, fullerene-based superconductors do provide a new and flexible model for understanding the phenomenon of high temperature superconductivity.

Astrochemistry has also benefited from fullerene research, with better explanations of the galactic carbon cycle. Nature has been producing buckyballs for eons, and spectroscopy confirms their presence in space.

Buckyballs have an intellectual appeal because the shape, a truncated icosahedron, is one of the Archimedian polyhedra (which, like Platonic solids, were once thought to have magical properties). This, along with their aromatic bond structure, makes them an object of theoretical interest. The simplicity and symmetry of fullerenes make them nearly ideal for testing chemical theories in the lab. And since fullerenes can be fabricated as both thin films and crystals, they are available for a wide variety of experimental and analytical techniques.

Perhaps the most intriguing work that has come from the discovery of buckyballs is the creation of thin tubes with closed ends, nanotubes, that arrange carbon atoms in the same cage-like way as fullerenes. These molecular drinking straws can be made into an abundant variety of sizes, lengths, and diameters. They can be twisted into helices, with their conductivity determined by their shapes. They can be empty or filled with other materials, and they offer the possibility of insulating metallic and semiconducting wires. They could provide the molecular electronics for nanomachines and are being looked into as a basis for creating artificial muscles. Nanotubes also promise applications in energy, including fuel cell technology, lithium ion batteries, and hydrogen storage.

Already, nanotubes are used to strengthen polymers and as nearly indestructible tips and cantilevers for atomic force microscopes. A string of metal atoms down the center could provide a nanowire, which could be extremely strong. A cable made up of nanowires would have 100 times the weight-bearing strength of steel. The strength and stability of nanotube fibers could make them a potential replacement for graphite fibers in body armor, helmets, airframes and rocket nozzles.

The comparisons of buckyballs to soccer balls and the structures of architect Buckminster Fuller have created a public awareness that few other chemical compounds enjoy. Less directly, buckyballs have provided justification for speculations in science fiction, particularly in those stories that exploit the possibilities of nanotechnology. Star Trek has referenced buckyballs as the focus of a character's chemistry lab and as a key component of communicators. The combination of the material properties of fullerenes is inspiring engineers. For instance, Arthur C. Clarke's novel The Fountains of Paradise proposes a skyhook, a cable connecting an orbiting satellite to the ground that allows an elevator into space. Such a structure would greatly reduce the costs of traveling and living in space. On paper, nanotubes seem to provide the material properties for constructing such a skyhook, but they will first need to get beyond the current length limit of one millimeter.

While thousands of patents have already been filed for the production and application of fullerenes and nanotubes, there are no products that have any serious commercial or social impact. There are no obviously successful uses on the horizon, but these materials are still early in their development and show great potential for providing future benefits.

PETER J. ANDREWS