Focus: Stones from Sticks
Natural diamonds are forged in the high temperatures and crushing pressures of the earth’s interior. But to make nanoscale diamond crystals, researchers have used their own tricks, including recipes involving carbon nanotubes. Now a team explains at the atomic level how nanotubes can convert to diamonds. Their computational studies in the October Physical Review B show that it is possible for carbon atoms from adjacent walls of multi-walled nanotubes to bond to each other to form both the cubic and hexagonal structures of diamond. While such nano-carat diamonds won’t appear at the jeweler’s anytime soon, researchers think that their strength and hardness may make them useful components of nanoscale machines.
Carbon forms the familiar cubic diamond crystals under geological conditions and hexagonal diamond crystals (also called lonsdaleite) in meteorite impacts. Under normal conditions, though, graphite is the most stable configuration of pure carbon. It consists of stacked, honeycomb-like sheets, where each hexagon of the honeycomb contains six carbon atoms. One of these sheets rolled into a tube is a single-walled nanotube, and if you roll several sheets together, you get a multi-walled nanotube. The angle of the roll across the honeycomb pattern determines the tube’s “chirality,” which can be different for each layer of a multi-walled tube.
Researchers have exposed multi-walled carbon nanotubes to a hydrogen plasma to make nanodiamonds, but the mechanism was not clear. To investigate the process, Dimitrios Maroudas of the University of Massachusetts, Amherst, and his colleagues first used computer models to analyze the structures of stacks of graphite sheets. The goal was to categorize the kinds of crystals that would result if chemical bonds could form between carbon atoms on adjacent sheets. They found that if two graphite sheets were perfectly aligned, the atoms would be arranged at just the right distances and angles to produce hexagonal diamond. Offsetting the two sheets slightly would cause the carbon bonds to make cubic diamond crystals instead.
Next, they looked for the same types of alignment regions in multi-walled nanotubes. For a nanotube with any possible set of chiralities and a wide range of diameters, they found that the right alignments always existed in some regions to form both types of diamonds. If the chirality of neighboring walls closely matched one another, the size of these aligned regions increased. If they matched perfectly, the nanotube could form a long, hollow, diamond wire.
To predict where chemical bonds might actually form between carbon layers in these regions, Maroudas and his colleagues used a standard technique to solve the quantum mechanical equations for the likely locations of electrons. The results showed an increased electron probability between the carbon atoms that the geometrical study had indicated would bond, meaning that chemical bonds were predicted to form between adjacent walls.
However, the calculations also showed that the original multi-walled nanotube is a lower energy configuration of the carbon atoms than the nanotube with embedded diamonds. To move to the higher energy state, an external source of energy was needed–the hydrogen gas that experiments had found was an essential ingredient. The hydrogen atoms can release energy by forming bonds with carbon atoms in the walls. Additional thermal energy from hydrogen in the form of an atomic beam or high-energy plasma can accelerate the bond formation reactions. The detailed mechanisms by which the hydrogen facilitates the nanodiamond bonding will be the subject of an upcoming paper by the team and is important for optimizing any nanodiamond fabrication process.
Such nanodiamonds could be used as hard protective coatings in small scale systems or sensors. Already researchers have used nanodiamonds to deliver molecules and small snippets of DNA into cells. “These are nice results,” says Jerzy Bernholc of North Carolina State University in Raleigh. But he cautions that controlled growth of nanotubes of a given chirality remains “one of the most important holy grails of nanotube research.” Maroudas responds that, according to the results, “chirality control is not required for diamonds to form,” although chirality does determine the size of the crystals.
Michelangelo D’Agostino is a physicist and freelance science writer in Berkeley, California.