Nanodots: No Assembly Required
At North Carolina State University in Raleigh, two scientists have demonstrated a laser-assisted nanostructure self-assembly method that yields arrays of uniformly sized nickel nanodots in matrices of aluminum oxide and titanium nitride. They created the nanodots ranging in diameter from 2 to 50 nm, smaller than is possible with other techniques. The new method could be used on other materials or to create other shapes, with potential applications in solid-state lighting and dense data storage.
Figure 1. A laser-assisted process produces uniformly sized nickel nanodots in aluminum oxide and titanium nitride matrices. A transmission electron microscope image reveals nanodots embedded in aluminum oxide. Courtesy of Jagdish Narayan and Ashutosh Tiwari, North Carolina State University/NSF Center for Advanced Materials and Smart Structures.
The patented technique uses a pulsed KrF laser operating at 248 nm. There are two variations of the process, one involving sequential growth of the matrix and nanodots, and the other, simultaneous growth of both. In either case, the laser strikes a target in a vacuum, ablating the material and allowing it to be deposited on a nearby substrate.
"The laser energy, substrate heating and interfacial strains provide the driving force for self-assembly," explained Jagdish Narayan, a professor of materials science and engineering at the university and director of the NSF Center for Advanced Materials and Smart Structures. Narayan teamed with Ashutosh Tiwari in the work.
Figure 2. A high-resolution scanning transmission electron microscope image reveals a single nanodot. Each "bump" is a nickel atom.
Studies by the researchers showed less than a 10 percent variation in the size of the nanodots both in aluminum oxide and in titanium nitride matrices. Depositing the nickel nanodots within a crystalline matrix enables the optimization of magnetic and optical properties. Varying the nanodot size in different layers should yield red, green and blue layers, which could be used to produce a solid-state white-light source.
Narayan estimated that it would be at least five years before the method finds use in products, and it is expected that lighting will be the first application. Data storage requires nanostructures with better magnetic properties.
Among the candidates are alloys of platinum with nickel, iron or cobalt. This may require ablation by other means, but Narayan noted that the method might be extended to other processing approaches, such as magnetron sputtering.
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