"There are many exciting follow-ups to investigate in these systems," said Ulrich Dahmen of Lawrence Berkeley National Laboratory, "ranging from whether and how the tubes can be filled with liquids or with metal atoms to form wires, to controlling the sizes and patterns of the networks, to understanding the atomic structure of their junctions."
To create the structures, Dahmen and colleagues deposited copper atoms under vacuum onto the cleaved surface of a vanadium selenide (VSe2) crystal. According to the researchers, such a crystal contains "Se-V-Se layer sandwiches". After about 13 minutes of copper deposition the surface of the crystal suddenly formed a nanostructure, taking less than one second to do so.
Scanning and transmission electron microscopy revealed that the nanostructure consisted of a network of nanofolds in the top crystal layer around prism-shaped holes. The angle at the apex of the nanofolds was around 120°. The nanofolds were about 25 nm wide, with a mesh size for the network of around 400–800 nm.
The researchers believe that the copper atoms form an intercalation phase in the uppermost layers of the crystal, resulting in a build-up of compressive stress. The formation of the nanofolds relieves this stress once a critical thickness is reached.
"We believe we are observing a chemical reaction, involving a phase change in the lattice structure of the surface layers," said Erdmann Spiecker of Lawrence Berkeley National Laboratory. "Kinetic energy is much too small for the metal atoms to penetrate into the crystal."
Before this study, many scientists believed that the nanostructure consisted of metal nanowires. The technique could be useful for creating networks of nanotubes, a feat that is difficult to achieve in other ways.
The researchers reported their work in Physical Review Letters.