"Our results show that the shell of a nanoparticle has an important effect on its orientation even when the shell is thin and takes up only a small portion of the total volume," team leader Wei Lu told nanotechweb.org.
Materials in which nanoparticles are well aligned have superior properties and unique functionalities that are not seen in materials with randomly aligned particles. However, the question is, how can we direct many nanoparticles to orientate and distribute in a designed way?
In their calculations, Lu's team employed an electric field to do this. Nanoparticles are very different from their microscale counterparts: they are often coated with a functional layer to enhance their dispersion in a solution to achieve specific bonding with other particles. This coating may completely change how the particle orientates in an external field. Brownian motion also becomes important at such small sizes and the Michigan scientists considered all of these effects in their model.
"Our study revealed rich behaviours and a significant degree of experimental control over nanoparticle orientation," explained Lu. "For instance, the core-shell showed frequency-dependent behaviour that can be exploited to direct nanoparticles in any designed orientation."
The researchers obtained their results by calculating the electric torque over a closed surface surrounding the particle using a so-called Maxwell stress tensor approach. They also considered torques from viscosity and Brownian motion.
"The work could help construct multifunctional materials by controlling the orientation and distribution of the nanoparticles," said Lu. "Moreover, it could be applied to technologies such as particle light valves and smart glass." The properties of these materials depend on how the nanoparticles are aligned. For example, in the light valves, the amount of light passing through the device changes in response to an applied field and smart glass changes from transparent to opaque when the field is applied and the particles line up.
The team now plans to model how nanoscale structures self-assemble in an electric field. These structures may be composed of nanoparticles with different shapes, material properties and structures. "We will also look into their potential applications in nanoscale devices," added Lu.
The work was reported in Appl. Phys. Lett..