Controlling Nanoparticle Crystallinity

The ability to control crystallinity could help researchers develop nanoparticles optimized for specific applications.

AsianScientist (Jan. 9, 2015) – Researchers at the Nanoparticles by Design Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) have patented a method to create semiconductor nanoparticles of varying crystallinity, allowing them to control the way that atoms align within the crystal structure.

“Most scientists and even companies nowadays are using nanoparticles not optimized for their applications or devices,” says Professor Mukhles Sowwan who conducted the research with team members Dr. Cathal Cassidy and Vidyadhar Singh. “We hope, at a certain time, we will optimize the nanoparticles for specific applications.”

To start though, they had to figure out how to control a few basic characteristics of nanoparticles, such as crystallinity. A crystalline nanoparticle will have all of its atoms aligned in neat rows, while an amorphous nanoparticle will have more disordered atoms. A polycrystalline structure has atoms aligned in groups, which are also known as grains.

A nanoparticle’s crystallinity impacts its optical, magnetic and electrical properties, and is responsible for profound differences between products made of the same material. For example, soot is amorphous carbon, or carbon without any crystal grains, while diamonds are crystalline carbon.

Professor Sowwan sits in front of a nanoparticle deposition system. Credit: OIST.
Professor Sowwan sits in front of a nanoparticle deposition system. Credit: OIST.

“It’s the first time [people have been able] to control the crystallinity and the number of crystallites of very small semiconductor nanoparticle,” Sowwan says, explaining that people have long known how to induce crystallinity in bulk semiconductor materials.

Part of the reason why Sowwan can control certain characteristics is due to the experimental method he and his researchers use, a modified nanoparticle deposition system. One of the most important features of this system is the possibility of interacting with or modifying freshly formed semiconductor nanoparticles in flight before reaching a substrate.

“That substrate is problematic,” explains Sowwan, “because it is always impacting the properties of the nanoparticle.”

To work around the substrate problem, the team exposed the nanoparticles to a beam of metal atoms. The metal atoms diffuse onto the surface of the nanoparticles and form metal nanoclusters just a few nanometers wide, inducing crystallization in the product. The researchers can then selectively remove the metal nanoclusters with plasma cleaning, a fairly simple physical procedure, retaining only the intact semiconductor nanoparticles of desired crystallinity.

Their patented method has added a new set of directions in the rulebook of how to customize a nanoparticle. Looking forward, crystallinity is one of many characteristics Sowwan and his team would like to control in order to produce more specialized nanoparticles.


Source: OIST.
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