Femtosecond Laser Selects Nanoparticle Size
Scientists with Italy's National Institute for the Physics of Matter have found that, when it comes to nanoparticle films, size matters. Because of that, laser attributes are also important. The investigators, who are based in the institute's Naples research unit, used femtosecond laser ablation to produce nickel nanoparticle films.
They found that the deposition process favored a particular shape and orientation of nanoparticles. As a result, the films had unique magnetic properties, such as requiring relatively weak outside fields to be magnetized.
In another unusual characteristic, the films retained a relatively strong remnant magnetization. Consequently, they could be useful in magnetic nanodevices.
Nickel nanoparticle films produced by femtosecond pulsed-laser deposition displayed different morphologies depending on the energy per pulse and the pulse repetition rate. The film on the far left was created under exposure to 240-µJ pulses at 100 Hz; the one at left was created under exposure to 80-µJ pulses at 1 kHz.
The researchers produced the nickel nanoparticle films by firing a Spectra-Physics Ti:sapphire laser at a solid nickel target in a vacuum. They focused the beam to approximately 100 µm and hit the target with 120-fs pulses of 780-nm radiation. As a result, the nickel ablated and was deposited as nanoparticles on a nearby glass slide. Using atomic force microscopy, they found that the nanoparticles were oblate ellipsoids with the major axis parallel to the deposition substrate.
In the study, the researchers adjusted the laser pulse energies from 80 µJ at a 1-kHz repetition rate to 240 µJ at a 100-Hz repetition rate. By doing so, they kept the total energy the same while producing films with a thickness of approximately 1 µm. Either condition produced films that were a dense array of nanoparticles. However, the lower laser energy yielded nanoparticles that averaged 40 µm in size, and the higher energy produced 92-µm nanoparticles.
The latter film had a very high value of remnant magnetization, almost twice that of the film with smaller nanoparticles and 140 times that of ordinary nickel film. According to the scientists, this is due in part to the reduction in film stress from the larger nanoparticles.
As for the future, they plan to produce films of cobalt and copper with nickel, as well as films of silver and silicon with iron. These films could be fine-tuned to a particular application based on the composition and type of magnetic and conductive nanoparticles.
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