Laser Ablation Generates Nanoparticles
Daniel S. Burgess
AUSTIN, Texas -- Nanoparticles are finding diverse application as devices such as biological sensors, and they show potential as the basis of novel LEDs. Typically, nanometer-size particles are grown through vapor condensation or flame or spray techniques, but all suffer from inefficiency, impurities or complexity. A laser-based system currently under investigation may lead to the industrial production of nanoparticles without any of these limitations.
Developed by researchers at the University of Texas and DuPont Electronic Materials in Wilmington, Del., the nanoparticle generator ablates a low-velocity aerosol stream of 2-µm-diameter particles with 10-ns pulses from a 249-nm, KrF laser. The energy-efficient process yields crystalline particles of high compositional purity, and the apparatus can produce nanoparticles of metals, semiconductors, ceramics, and magnetic and nonmagnetic alloys without noxious waste.
A system under development at the University of Texas may produce nanoparticles for industrial applications by illuminating a stream of microparticles with a pulsed excimer laser. The setup as shown utilizes an impaction scheme for the collection of the nanoparticles; a stagnant gas serves as the virtual impactor.
The researchers spent several brainstorming sessions looking for techniques that could be scaled to high production rates, recalled John W. Keto, a professor at the university and a member of the research team. Based on their colleague and fellow researcher James R. Brock's theoretical study of the laser ablation of water droplets, they predicted that nanoparticles would nucleate in the rarefactions behind the shock waves in a laser-ablated material. "We also realized that by using an aerosol of the starting powder, that we would have a scalable, continuous process," he said.
In initial experiments, the team determined that the generated nanoparticles were both unagglomerated and relatively uniform in size, depending on the type and pressure of carrier gas used with the microparticles. The next step was to develop a collection system.
The researchers investigated both impaction through a supersonic jet and electrostatic collection with a negatively charged electrode. They found the latter to be generally superior. "The biggest advantage of the electrostatic collection is that it operates at atmospheric pressure; hence, no large and expensive vacuum pumps are required," Keto said. The electrostatic scheme, however, loses this advantage when collecting semiconductor nanoparticles because they form chains if not collected at a lower pressure and near the point of ablation, he noted.
A significant limitation of the laser process is that it lacks the yield of flame techniques. Microparticles of silver, for example, absorb only about 5 percent of the laser energy at the current production rate of 20 g/h, Keto said. But the team has constructed a cell that will increase the focal volume of the particles. This should enable the absorption of up to 70 percent of the light and should boost yields by an order of magnitude. More powerful lasers (the demonstration system incorporated a 100-W laser) also may enable higher generation rates.
Currently, the team is investigating the production of nanocomposites with the process, and DuPont is examining the use of silver nanoparticles as thick-film pastes for electronic devices. The researchers also hope to write the nanoparticles onto substrates under ultrahigh-vacuum conditions to produce patterned nanostructures for optical devices.
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