Changing colours with magnetism

US researchers have discovered a way to change the colour of iron oxide nanoparticles using magnetism, which could be used in high quality display screens and electronic paper.

University of California, Riverside nanotechnologists found that by changing the strength of the magnetic field, they were able to control the colour of tiny particles of iron oxide suspended in water.

When the strength of the magnetic field is changed, it alters the arrangement of the spherical iron oxide particles in solution, modifying how light falling on the particles passes through or is deflected by the 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, an assistant professor of chemistry who led the research.

‘By reflecting light, these photonic crystals show brilliant colours,’ Yin said. ‘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.’

A photonic crystal controls the flow of light and works like a semiconductor for light. 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 the superparamagnetic property of iron oxide particles to tune the spacing between nanoparticles, and therefore the wavelength of the light reflection by changing the strength of the external magnetic field.

‘Other reported photonic crystals can only reflect light with a fixed wavelength,’ Yin said. ‘Our crystals, on the other hand, show a rapid, wide and fully reversible optical response to the external magnetic field.’

Photonic materials such as those used by Yin and his team could be used in optical microelectromechanical systems and reflective colour display units. They also have applications in fibre optics, sensors and lasers.

The new technology could be used to make an inexpensive colour display by forming millions of small pixels using the photonic crystals with a different colour for each pixel assigned using a magnetic field. Images would be formed using reflected light rather than the pixels themselves being lit.