March 1, 2006 – Researchers from the US and Germany are working to better understand a newly discovered process for forming complex networks of nanotubes in less than a second on a layered crystal, resulting in extensive hexagonal networks, with branches and connections.
Scientists at Lawrence Berkeley National Laboratory’s National Center for Electron Microscopy (NCEM) and the Christian Albrechts U. of Kiel (Germany) observed that when depositing metal atoms on the surfaces of transition metal dichalcogenides (titanium telluride and vanadium selenide), prismatic folds of nanotubes formed.
“Originally it was thought the structures were like cracks in the mud of a dry lake bed, which filled up with the condensed metal to form nanowires,” said Erdmann Spiecker of Berkeley Lab’s materials sciences division (on leave from the U. of Kiev). “We started to question this idea when we failed to find evidence of condensed metal anywhere in the samples.”
Further investigation at Berkeley Lab’s NCEM, utilizing transmission electron microscopy (TEM) of the vanadium selenide crystals, revealed moire patterns, suggesting the network formation was connected to altered spacings between atoms in the crystalline layers near the surface of the material. Then in a custom-built low-energy electron microscope, a vanadium selenide crystal was cleaved to expose a fresh surface, which was monitored as copper atoms were slowly deposited from vapor. After several minutes, the network abruptly formed.
Slicing the sample crossways with a focused ion beam (FIB) to shave unwanted material from the back of the sample, researchers noted the network was formed of hollow nanofolds in the surface layers, which had been pushed together to form roof-like structures.
Rather than building up on the surface, the copper atoms had worked their way into the uppermost layers and lodged between them, forming what’s called an “intercalation compound.” The accumulating atoms create compressive stress, pushing the surface layers sideways in every direction until they abruptly break free, gliding over the underlying layers and crinkling themselves into a hexagonal network.
Researchers still don’t know exactly what conditions trigger the sudden release of strain, and how the several layers of crystal that make up the nanotube roofs compensate for the different stresses experienced by the inner and outer layers where they bend.
Possible applications for surface nanotube networks include networks of pipes to store and transport minute quantities of materials, or templates for the fabrication of nanowire networks. NCEM director Ulrich Dahmen suggested follow-up research would look at whether the tubes can be filled with liquids or with metal atoms to form wires, how to control the sizes and patterns of the networks, and understanding the atomic structure of their junctions.
Full details of the research is published in the March 3 issue of Physical Review Letters.