Nanotechnologists at the University of California have shown that they can control the colour of small particles of iron oxide suspended in water by applying an external magnetic field to the solution.
The discovery has potential to improve the quality and size of electronic display screens and to enable the manufacture of products such as erasable and rewritable electronic paper and ink that can change colour electromagnetically.
Researchers found that changing the strength of the magnetic field altered the arrangement of the spherical iron oxide particles in solution. In doing so they modified how light falling on the particles passes through or is deflected by the solution and changed the colour of the iron oxide solution.
‘The key is to design the structure of iron oxide nanoparticles through chemical synthesis so that these nanoparticles self-assemble into three-dimensionally ordered colloidal crystals in a magnetic field,’ said Yadong Yin, who led the research. ‘By reflecting light, these photonic crystals show brilliant colours.’
A photonic crystal controls the flow of light and works like a semiconductor. The nanoparticles’ spacing dictates the wavelength of light that a photonic crystal reflects.
Iron oxide nanoparticles are ‘superparamagnetic,’ meaning that they turn magnetic only in the presence of an external magnetic field. The researchers used this property of iron oxide particles to tune the spacing between nanoparticles, therefore changing the colour of the colloidal crystals in the solution by changing the strength of the external magnetic field.
‘Ours is the first report of a photonic crystal that is fully tuneable in the visible range of the electromagnetic spectrum, from violet light to red light,’ Yin said. ‘Other reported photonic crystals can only reflect light with a fixed wavelength. Our crystals show a rapid, wide and fully reversible optical response to the external magnetic field.’
Photonic materials such as these could be used in the fabrication of new optical microelectromechanical systems and reflective colour display units. They also have applications in telecommunication, fibre optics, sensors and lasers.