Nanopillars of the Earth

by Michael Mullaney on October 21, 2010

Professor Daniel Gall is committed to pushing the nano envelope. In addition to being a visual treat, his research is yielding potential solutions to a diverse group of challenges, from hydrogen storage to coatings for ball bearings in jet engines.

To talk about Gall’s work requires a quick primer on PVD – or physical vapor deposition. When making nanorods – which, remember, can measure only billionths of a meter diameter – researchers introduce atoms of different metals into a vacuum tube at low temperatures. The vacuum tube resembles a basic overhead fluorescent light. Inside the vacuum is a substrate, which can be made of different materials and is not unlike a very fancy microscope slide.

The careful calibration and perfect storm of metal atoms, chemicals, heat, and pressure result in the growth of nanorods. In most cases, the metal atoms are introduced into the vacuum from the top, or from an angle of 12’o’clock. The atoms accumulate on the tops of the rods, which grow taller and thicker. Here’s an image of conventionally-gown copper nanorods made at Rensselaer:

Gall, however, is interested in introducing the different metal atoms into the vacuum at glancing angles – maybe 3’o’clock or 9’o’clock. The results, as you can see with the “nanosprings” below or the “half moons” at the top of ths post, are fascinating. Rather than depositing on the very top of the nanorod, the atoms disperse in intricate patterns. It makes me think of a nanoscale snow drift. In many cases, the result is a forest of nanorods that have interesting properties as well as a unique visual identity.

Gall presented on his work yesterday at the annual meeting of the American Vacuum Society. His talk was titled “Nanorods by Extreme Shadowing: New Pictures and New Physics.”

Below are more images, accompanied by captions, all courtesy of Gall:

Square pattern of tantalum nanorods | Scanning electron micrograph of tantalum pillars grown from a pattern of square-perimeter pyramidal holes. Such wide channels may be useful in fuel cell membranes and structures where both high flow and surface area are desired.

Dual-element nanopillars | This transmission electron micrograph shows pillars that are half silicon (left/light) and half tantalum (right/dark). They are grown atop silicon oxide microspheres by simultaneously depositing both elements at glancing angles from opposite directions. The image shows how seamlessly two compatible elements can be co-deposited at the same time.

Zigzag structure make good nanosprings | This scanning electron micrograph shows “nanosprings“ grown in four sequential steps with alternating glancing-angle depositions of chromium from the right and silicon from the left. Rafts of this type of structure may become useful as nanopressure sensors or electrical contacts that can buffer variations in thermal expansion.

Unusual half-moon columns created atop spheres | This scanning electron micrograph shows unusual-looking half-moon-shaped chromium nanorods grown atop self-assembled silicon oxide nanospheres.  The spherical substrate resulted in rods that first grew away from the back-left source of atomic chromium vapor, then gradually reverted back to the usual toward-the-source growth pattern.