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Shape Matters in the Case of Cobalt
Nanocubes melt and fuse when
illuminated by a transmission
electron microscope; © NIST
Building on a previous paper that examined the properties of cobalt formed into spheres just a few nanometers in diameter, the new work explores what happens when the cobalt is synthesized instead as nanocubes. Nanoparticles of cobalt possess large magnetic moments - a measure of magnetic strength - and unique catalytic properties, and have potential applications in information storage, energy and medicine.
One striking difference is the behavior of the two different particle types when external magnetic fields are applied and then removed. When placed in an external magnetic field, the individual particle chains bundle together in parallel lines to form thick columns aligned with the field. These induced columns, says NIST physicist Angela Hight Walker, imply that the external magnetic fields have a strong impact on the magnetic behavior of both nanoparticle shapes.
But their group interactions are somewhat different. As the strength of the external field is gradually reduced to zero, the magnetization of the spherical nanoparticles in the columns also decreases gradually. On the other hand, the magnetization of the cubic particles in the columns decreases in a much slower fashion.
The team also showed that the cubes can be altered merely by observing with one of nanotechnology’s microscopes of choice. After a few minutes’ exposure to the illuminating beam of a transmission electron microscope, the nanocubes melt together, forming “nanowires” that are no longer separable as individual nanoparticles.
While Walker says that the melting effect could be a potential method for fabricating nanostructures, it also demands further attention. “This newfound effect demonstrates the need to characterize the physico-chemical properties of nanoparticles extremely well in order to pursue their applications in biology and medicine,” she says.
MEDICA.de; Source: National Institute for Standards and Technology