Small, But Super

Gold comes in many colors. Since ancient times, glass artists and alchemists alike have known how to grind the metal into fine particles that would take on hues such as red or mauve. At scales even smaller, clusters of just a few dozen atoms display even more outlandish behavior. Gold and certain other atoms often tend to aggregate in specific numbers and highly symmetrical geometries, and sometimes these clusters can mimic the chemistry of single atoms of a completely different element. They become, as some researchers say, superatoms.

Recently researchers have reported successes in creating new superatoms and deciphering their structures. In certain conditions, even familiar molecules such as buckyballs — the soccer-ball–shaped cages made of 60 carbon atoms — unexpectedly turn into superatoms.

Scientists are already studying how superatoms bind to each other and to organic molecules. Tracking superatoms can help researchers learn how biological molecules move inside cells and tissues, or determine the structure of those molecules precisely using electron microscopes.

And by assembling superatoms of elements such as gold, carbon or aluminum, researchers may soon be able to create entirely new materials. Such materials could store hydrogen fuel in solid form at room temperature, make more powerful rocket fuels or lead to computer chips with molecule-sized features.

“Designer” materials made of superatoms could have combinations of physical properties that don’t exist in nature. As Kit Bowen, a chemical physicist at JohnsHopkinsUniversity in Baltimore, puts it, it’s as if you felt like eating something hot and something cold at the same time, and could have it both ways. “Like a hot-fudge sundae.”

Small numbers of atoms often form structures as symmetrical, and almost as intricate, as those of snowflakes. But while no two snowflakes, even if they have the same number of water molecules, are identical, a small, specific number of atoms of the same element typically will assemble into the same, specific shape. The quintessential example is how 60 carbon atoms form buckyballs. Metal atoms such as gold, aluminum or tin also like symmetry. For example, 20 atoms of gold will assemble into a solid pyramid, but 16 will form cage-like structures, as Lai-Sheng Wang, a physical chemist at WashingtonStateUniversity and at the Pacific Northwest National Laboratory in Richland, Wash., and his collaborators have discovered in recent years (SN: 5/20/06, p. 308).

The strange behavior of atoms in small groupings has been known for a long time, though only recently have scientists begun to understand it in detail.

“The whole idea is that small is different,” says Bowen, quoting what he says is a motto of Uzi Landman of the Georgia Institute of Technology in Atlanta. The physical properties of a material, such as hardness and color, are the same for a 1-pound lump of the stuff as they are for a 100-ton chunk. But when you get to specks made of a few million atoms or less, properties usually begin to change.

A material such as silicon, which is usually brittle, can become as ductile as gold, researchers from the National Institute of Standards and Technology reported last November in Applied Physics Letters. Another example is particles called quantum dots, which fluoresce in a rainbow of different colors depending on their size (SN 2/15/03, p. 107).

But with even fewer atoms — a few hundred or less — the changes become more dramatic. “If you keep going smaller, then you enter a region where properties are erratic,” Bowen says. “Often, one atom counts.” For example, Wang and his collaborators have shown that tin clusters behave like conductors or semiconductors, depending on their size; Bowen found something similar with magnesium.

A job for superatom

For larger clusters, it’s not always clear when atoms will aggregate into regular structures or into shapeless blobs with any number of atoms. “What’s to stop the cluster from adding a few more atoms?” asks Roger Kornberg, a structural biologist at StanfordUniversity’s MedicalSchool.

Last fall in Science, Kornberg and colleagues described an intricate cluster they created with exactly 102 gold atoms. He and his team synthesized their gold superatoms in a liquid. To control the clusters’ growth, the team added sulfur-based organic molecules called thiols, which bind easily to gold. Forty-four thiols bound to each gold cluster’s surface, preventing the 102-atom clusters from coalescing to form larger clusters.

What resulted was a superatom (or maybe a “supermolecule”) with a core of 79 gold atoms arranged into a truncated decahedron: two pyramids with pentagonal bases joined together into a diamond shape, but with the pyramids’ tips chopped off. Around the core, more gold atoms formed an unusual pattern, joining the thiols in shapes looking like handles. “The thing that surprises me the most,” says Kornberg’s collaborator Pablo Jadzinsky, “is that the geometry of the cluster cannot be described in simple words.”

The team determined the cluster’s structure using X-rays, which required first coaxing the clusters into forming a crystalline solid. Jadzinsky says that the very fact that the clusters could form a crystalline solid means that they are all identical and that their shapes are fixed. At 1.5 nanometers, the clusters’ shapes may have fluctuated, as other nanometer-scale shapes often do (SN: 3/15/03, p. 174).

But the numerology seems sort of random: Why 102 gold atoms? Why 44 thiols? As it happens, superatom theory has a good explanation.

Each of the gold atoms donates an electron to the cluster, just as inside larger chunks of metal, where mobile electrons can conduct electricity. Forty-four of those electrons get immobilized in bonds between gold atoms and thiols, leaving 58 electrons free to roam. These 58 electrons then orbit the cluster’s core — made of positive gold ions — just as they would orbit the nucleus of a stand-alone atom. And 58 happens to be a “magic number.” It’s the number of electrons needed to fill a shell around the superatom, so that it won’t feel a desire to add or shed electrons, which would destabilize its structure

This process is similar to what happens in noble gases, which are chemically inert because they have just the right number of electrons to fill a shell around the atom.

Chemist Royce Murray of the University of North Carolina at Chapel Hill and his collaborators describe the structure of a similar, though smaller, gold-thiol cluster in the March 26 Journal of the American Chemical Society.

Kornberg says that by tweaking the conditions in their lab’s vials, he and his colleagues can obtain clusters of different numbers of gold atoms and thiol molecules, although they haven’t determined the precise structure in those cases yet.

Superbucky

Thiol-gold clusters could have medical uses because they easily hook onto organic molecules. For example, clusters could help deliver drugs through cellular membranes, or, once inside cells and tissues, act as dyes for biomedical imaging.

Kornberg has another application in mind. His specialty is figuring out the exact structure of proteins and other complex biomolecules. “We have devoted decades to solving one structure,” Kornberg says, referring to his work on the crucial enzyme RNA polymerase, for which he earned the 2006 Nobel Prize in chemistry. Superatoms could speed up the process dramatically. The idea is to attach superatoms at specific sites on biomolecules in solution, then flash freeze the superatoms and put them under the electron microscope. Because their shapes are precisely known, the superatoms would act as signposts, forming reference frames around each biomolecule. Computer processing of the electron-microscope data could then pinpoint the exact position of each atom in the biomolecule, producing an image of its structure, Kornberg says.

“This would open up a whole new vista for structural biology,” potentially revealing the structure of molecules that can’t be imaged by standard methods.

In other recent work, Hrvoje Petek of the University of Pittsburgh and his collaborators found that buckyballs can also turn into superatoms in some situations. Normally, superatoms are made of metal atoms, which pitch electrons into a common pot. In buckyballs, carbon atoms share electrons only with their neighbors, the way they would in ordinary graphite. But researchers were surprised when they placed buckyballs on a surface and made electrons flow through them to the tip of a scanning tunneling microscope. The data showed that the electrons briefly orbited the buckyballs, rather than just the individual carbon atoms, the way they would orbit an atom. The electrons occupied high energy orbits, from which they essentially couldn’t see the single carbon atoms.

“Above some energy, the structure of the molecule disappears,” Petek says, and looks virtually like a smooth, hollow sphere. He and his colleagues report in the April 18 Science that a buckyball pair with added electrons might even form molecular bonds, similar to those in hydrogen molecules.

Petek says rows of buckyballs aligned on a surface might form circuits and be the basis of molecular-scale electronic chips. At less than a nanometer thick, the circuits in buckychips would be tens of times thinner than those in state-of-the-art silicon chips, allowing engineers to pack more power into a chip.

Petek’s buckyballs are not the first superatoms to be discovered that can behave like single atoms. Sometimes, clusters with a particular number of atoms can even mimic the chemistry of a single atom of a different element. The first hints of this surfaced more than two decades ago.

Spreading jellium

The story of the superatom begins when two physicists walk into a barber shop. Marvin Cohen of the University of California, Berkeley recalls how he and a colleague, the late Walter Knight, ran into each other at their favorite barber’s one afternoon in 1984.

While waiting for his haircut, Knight talked about some surprising data from an experiment in which he had baked a block of sodium and then measured the masses, and thus the sizes, of vaporized particles that came out.

Knight’s particles came in a range of sizes. But those made of eight, 20, 40, 58 (remember 58?) or 92 atoms were a lot more abundant. Cohen guessed what might be happening, and he started scribbling some back-of-the-envelope calculations. “Tony, the barber, thought we were figuring out a way to beat the stock market,” Cohen recalls.

Sodium is a metal, with a propensity to shed one of its 11 electrons. In a cluster, atoms share these electrons in a “socialistic” way, Cohen says. For simplicity, in his calculation he imagined the positive electric charge of a cluster’s sodium ions (each of them an atom minus one electron) as being spread uniformly like jelly, rather than concentrated at the ions. Nuclear physicists use a similar model for atomic nuclei; they call it the “jellium” model.

Jellium gave the right answer. The shared electrons orbiting the cluster do so in different energy levels, or shells, just as they would in an atom, Cohen figured. Computer calculations confirmed his guess. Like ordinary atoms, clusters with unfilled electron shells are chemically reactive. Full shells, with “magic numbers” of electrons, are not. Sodium clusters with eight, 20 or 40 atoms are the analog of helium, neon, and the other noble gases, which rarely form molecules. Clusters with non-magic numbers of atoms tend to lose or gain electrons, making them more likely to also lose or gain atoms (to get a magic number) through collisions with other clusters.

A year later, Exxon Corporate Research Lab chemist Robert Whetten, now at Georgia Tech, and his collaborators noticed that clusters of six aluminum atoms could split hydrogen molecules at room temperature, something smaller clusters couldn’t do. “Only aluminum-6 jumped up and shouted ‘Here I am, I can do this!’” says Whetten. And in the late 1980s, Welford Castleman of PennsylvaniaStateUniversity in University Park and his colleagues discovered that clusters of 13, 23 or 37 aluminum atoms, plus an extra electron, become chemically inert, even though pure aluminum usually reacts violently with oxygen.

The researchers realized that Cohen and Knight’s magic numbers could explain the perplexing phenomenon. In an aluminum cluster, each atom donates three electrons to the cause. The 13-atom cluster, or Al13, for example, ended up with 39 common electrons (3 x 13), and the extra electron in the ion Al13^– was just what the cluster needed to reach the magic number 40.

Researchers say that chemically stable forms of aluminum, which can be destabilized and burned when needed, could someday yield a powerful yet safe-to-handle additive for rocket fuel.

But the team went further. It showed that the neutral clusters Al13, Al23 and Al37 get into similar chemical reactions as do elements that crave one extra electron. Those are the elements such as chlorine or fluorine, which in the periodic table are the halogens, the column directly to the left of the noble gases.

Then in 1995, Shiv Khanna and Purusottam Jena of VirginiaCommonwealthUniversity in Richmond found a theoretical explanation for Castleman’s discovery. While Cohen’s calculation could predict which clusters would be stable, understanding chlorinelike behavior required calculating the energetics of adding or removing an electron from the cluster, which is what Khanna and Jena did. They proposed the term “super atom” (two words, originally) for such clusters.

Hot-fudge sundae

Jena and Khanna then predicted that Castleman’s aluminum clusters should form tightly bound ion pairs with elements such as sodium or potassium, which like to donate one electron. Ionic bonds are what occur in sodium chloride (Na⁺Cl^–), also known as table salt. Aluminum clusters, Jena and Khanna proposed, could become part of the first all-metal salts, clusters that include superatoms of one metal (which act like halogen atoms) and atoms of the same or a different metal.

Two years ago, a team led by Bowen indeed produced K⁺Al13^– (potassium-aluminum) molecules and showed that their chemical properties resemble those of molecules like K⁺Cl^– or Na⁺Cl^–.

In the last few years, researchers have also begun looking for ways to use super-
atoms to store hydrogen in solid form. The difficulty of transporting and storing hydrogen at room temperature poses a formidable obstacle to the much-touted hydrogen economy.

Several teams are now trying to create superatom-based salt crystals — something that’s proving trickier than expected, since once the molecules start aggregating, the superatoms tend to merge with each other, forming clumps more than crystals. “When you put them together, they slag themselves,” Bowen says. One approach is to coat superatoms with other kinds of stuff, as Kornberg did. On the other hand, Castleman hopes that replacing potassium ions with larger molecules might prevent superatoms from coalescing. “You have a chance of keeping them away from each other,” he says.

The interest in making crystals out of superatoms goes beyond pure curiosity. By adjusting the types, shapes and sizes of a material’s ingredients, scientists and engineers could tune physical properties to their likes. “You would have a way of making materials with tailored properties,” Bowen says.

For example, a material that can be transparent typically won’t conduct electricity, and vice versa. But a suitable all-metal salt, say, might be able to do both. And with a stretch of imagination, all-aluminum salts could make airplanes with see-through fuselages possible. Almost as cool as a hot-fudge sundae.