Shape is turning out to be a particularly important feature of some commercially
important nanoparticles-but in subtle ways. New studies* by scientists at the
National Institute for Standards
and Technology (NIST) show that changing the shape of cobalt nanoparticles
from spherical to cubic can fundamentally change their behavior.
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. In the absence of
a magnetic field, both the spherical and cubic nanoparticles spontaneously form
chains—lining up as a string of microscopic magnets. Then, when placed
in an external magnetic field, the individual 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 until the particles rearrange their magnetic moments from linear chains
into small circular groups, resulting in a sudden drop in their magnetization.
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. The effect, not observed with the spheres,
is surprising because the cubes average 50 nm across, much larger than the spheres’
10 nm diameters. “You might expect the smaller objects to have a lower
melting point,” Hight Walker says. “However, the sharp edges and
corners in the nanocubes could be the locations to initiate melting.”
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,”
* G. Cheng, R.D. Shull and A.R. Hight Walker. Dipolar chains formed by chemically
synthesized cobalt nanocubes. Journal of Magnetism and Magnetic Materials, May
11, 2009, Vol. 321, issue 10, pp. 1351—1355.
** G. Cheng, D. Romero, G.T. Fraser and A.R. Hight Walker. Magnetic-field-induced
assemblies of cobalt nanoparticles. Langmuir, December 2005. See Oct. 20, 2007,
Tech Beat article, “Magnetic Nanoparticles Assembled into Long Chains”.