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Laser Yields Tiny Bubbles of Boron

Photonics Spectra
Oct 2001
Kevin Robinson

Don Ho is known for his "Tiny Bubbles in the Wine," but a research group is staking a claim to bubbles of boron. By synchronizing a laser-induced plasma with a radio-frequency-modulated plasma, the scientists have created tiny hollow balls of amorphous boron that are coated with a crystal shell of boron nitride. The balls, dubbed nanoballoons, could lead to applications in materials science, electronics and medicine.

Researchers have created tiny balloonlike structures by coupling laser ablation of boron nitride with exposure to a radio-frequency-modulated plasma

The discovery of carbon structures such as fullerenes and nanotubes opened the door to investigations into and applications of other nanometer-size materials, explained Shojiro Komatsu, a researcher on the project from the National Institute for Materials Science. "We believe there are a lot of nanomaterials still undiscovered and that we first need to invent new synthesis methods to find them," he said. Such methods could be variants of laser ablation and laser vapor deposition.

The group, which also included researchers from Hosei University in Tokyo, selected boron nitride because it has a crystalline structure similar to carbon. To create the nanoballoons, the scientists place a disc of boron nitride inside a flow chamber at a 45° angle to both an incident laser beam and to an ammonia plasma. The 193-nm ArF laser beam ablates the surface of the disc and interacts with the radio-frequency-modulated plasma striking the sample.

The technique yields nanoballoons of boron coated with boron nitride that are 20 to 300 nm in diameter, seen in this transmission electron microscope image. Courtesy of Shojiro Komatsu.

The key is the synchronization of the two plasmas. Without synchronization, the setup yields particles no larger than 40 nm. Synchronizing the laser and plasma with a function generator produces a wider range of particle sizes, from 20 to approximately 300 nm. The researchers described their work in the July 9 issue of Applied Physics Letters.

Electronics and medicine

Komatsu said that boron nitride, which coats the internal and external surfaces of the nanoballoons, is physically and chemically stable. This chemical inertness makes boron nitride a safe material for use in the human body and may lead to medical applications for the nanoballoons.

He added that the synthesis technique should be compatible with other materials, such as carbon and silicon. The nanoballoons may display interesting physical properties that could have applications in electronics.

The group will focus on developing ways to control the size and shape of the nanoballoons, and hopes to collaborate with other researchers to explore the potential applications of the structures. Komatsu has begun to measure the plasma flow field to better understand the nanoballoon growth process.

"I expected that some new materials would be formed by this new method," he said. "This sort of hollow nanoparticles was beyond my imagination and expectation."

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