Put together diamonds, copper, and sulfur, and you can make the thinnest wires humanly possible. These nanometer-scale wires could help shrink electronic circuits, cramming more computing power into ever-smaller devices, and allow researchers to explore exotic material physics.
A team of scientists from Stanford University and the US Department of Energy’s SLAC National Accelerator Laboratory recently took molecule-sized diamond fragments, attached them to atoms of sulfur, and dropped them into a solution with copper atoms. The result: a wire three atoms across sheathed in diamond. In addition, the wire assembled itself, like a set of Lego bricks spontaneously coming together to make a structure. The team published the results of their work Dec. 26 in Nature Materials.
Self-assembly has been done before with organic—i.e., carbon-based—molecules. Many are variations on DNA, and there’s been a lot of other work with other organic chemicals as well. What’s new here is using inorganic chemicals, which are the kinds of materials you’d need to make wires—or electronics.
As important, this kind of work shows that you can reliably make atom-scale structures to begin with. Nicholas Melosh, associate professor of materials science and engineering at Stanford and SLAC, who led the research, says a big challenge to date has been getting the conditions exactly right, so that the attractive forces that bind molecules together balance with the repulsive forces that keep them apart. Further, you want your molecules to assemble in the right shape. This is a sort of proof-of-concept that you can make all this work. Melosh’s team wasn’t setting out to just make efficient wires, though the copper and sulfur make a pretty good semiconductor. The point was to show you could make a wire and minimize the defects—breaks between the copper and sulfur molecules. It was also to demonstrate control of the growth process. The result was nanowires without defects, as much as a few millimeters long. That might not sound like much but in materials science it’s a lot—a millimeter is seven or eight orders of magnitude larger than a molecule. “We think we could maybe make a whole sheet of this kind of material,” Melosh says.
That’s been an ongoing challenge when it comes to manipulating atomic-scale materials. Graphene, for example, is just a form of ordinary graphite. But the odd and useful properties of the substance only happen because it’s in sheets a single-atom thick, a chicken-wire pattern of single carbon atoms. As a result, it conducts heat far more efficiently than bulk graphite does: electrons move through graphene easily, and that makes it a good candidate for new kinds of electronics. Yet so far making sheets of graphene bigger than a few square inches has proven difficult and expensive, so the potential of the material hasn’t been realized. (The growth process in this latest experiment wouldn’t apply to graphene.)
To make their nanowire the team used a singularly commonplace chemical: petroleum from Arkansas. Petroleum has a lot of impurities in it when it’s pulled from the ground, and what they are depends on the local geology. In this case the oil had molecule-sized fragments of diamond in it. In diamonds, the carbon atoms are arranged in box-like shapes. These “diamondoids” can then be chemically altered in the lab so that one of the carbon atoms is attached to a sulfur atom. Each one of these structures can only attach to others like it in a certain orientation because of their shape. This isn’t uncommon in nature; DNA molecules and receptors only link to certain molecules in a particular way. In addition, diamondoids form strong bonds with each other, making them perfect for the outside of the wire, and acting like a glue to link the building blocks together.
When the research team dropped the sulfur-linked diamondoid in a solution full of copper atoms, the copper would link to the sulfur atom. The sulfur atom, now linked to a diamondoid and a copper atom, would bump into another structure like it. Since the diamondoid-sulfur-copper structures only fit together one way, they form the wire, a string of copper and sulfur atoms surrounded by the diamond.
As the team does more research they’ll try to see if they can make bulk amounts of the wire, turning it into flat sheets. They’ll also explore what happens when the use materials other than copper and sulfur—like gold, iron, zinc, or cadmium. What the nanowire can do depends on what’s in it. The copper and sulfur aren’t all that exciting, says Melosh. But a diamond wire built with silicon, for example, might generate energy like a solar cell.