Changing the Deposition Angle
To create the nanoblades, the researchers used oblique angle vapor deposition. This widely used fabrication technique builds nanostructures by vaporizing a material magnesium in this case and allowing the vaporized atoms to deposit on a surface at an angle. As the deposition angle changes, the structure of the material deposited on the surface also changes.
When the researchers deposited the magnesium straight onto a surface at zero degrees, the blades resembled a handful of cornflakes flat, flakey structures overlapping one another. It wasn’t until the deposition angle was increased that the blade-like nature of these new nanomaterials became apparent.
As the magnesium deposition angle was increased, the researchers were surprised that the structures first tilted away from the magnesium vapor source instead of the expected inclination toward the source. The blades then quickly curved upward to form nearly vertical structures resembling nano-scale razorblades.
The blades also become ultra thin. From the side, the nanoblades resemble an overgrown lawn with thin, blade-like spires. At a 75 degree angle, the nanoblades had a thickness of as little as 15 nanometers and a width of a few hundred nanometers. (A nanometer is one billionth of a meter.)
Researchers at Rensselaer are now looking at ways to coat the magnesium nanoblades with metallic catalysts to trap and store hydrogen.
Applying the Surface Pole Figure Technique
The researchers monitored the blades as they were growing using a reflection high-energy electron diffraction (RHEED) technique to create a surface pole figure or image. The new technique, created at Rensselaer, differs from other diffraction techniques such as X-rays because it monitors the surface structure of the material as it grows. X-rays and other technologies measure the entire material, from the tip of the new growth straight through the substrate that the material is growing on.
Tracking the surface evolution of the material provides insight into how the structure evolves over time and helps scientists understand the mechanism of nanostructure formation, allowing engineers to later recreate ideal nanomaterials in the future. The creation of surface pole figures was particularly important in understanding the growth of nanoblades, as the surface morphology changed vastly over time.
The surface pole figure technique was first outline by Fu Tang, a postdoctoral research associate in Wang’s group, in a 2006 issue of Applied Physics Letters. In that paper, surface pole figures were created for nanorod growth. The researchers are now working to analyze nanoblade growth to provide additional insight into the growth patterns of these new nanomaterials.
Other Rensselaer researchers involved with the project are Toh-Ming Lu, the R.P. Baker Distinguished Professor of Physics; GAANN fellow Tom Parker; and postdoctoral research associate Huafang Li.
Photo credits: “Gwo-Ching Wang” by RPI/Kris Qua; Nanoblades by RPI/Tom Parker