Chameleon for Optoelectronics

Jul 13, 2007

A liquid that changes its color “on demand” and can take on any color of the rainbow one desires?

A research team headed by Yadong Yin at the University of California, Riverside (USA) has now shared the secret of their wonderful liquid with the journal Angewandte Chemie: Nanoscopic particles made of tiny magnetic crystals coated with a plastic shell self-assemble in solution to form photonic crystals—semiconductors for light. When a magnetic field is applied, the optical properties of the crystals change, allowing their color to be very precisely adjusted through variation of the strength of the field.

The crystals involved here are no “conventional” lattices of ions or molecules like the one we are familiar with for salt; instead they are colloidal crystals, periodic structures that form from uniform solid particles that are finely dispersed in a liquid. Colloidal crystals can be produced at little cost and on a large scale—and can be used as photonic crystals. Photonic crystals are the optical analogue of electronic semiconductor materials. Like their electronic counterparts, they have photonic band gaps, forbidden energy levels, or wavelengths, at which the photonic crystal does not transmit light. These optical properties depend on the spatial relationships within the crystal.

Current research is concerned with photonic crystals whose forbidden bands are variable and can be adjusted quickly and precisely in response to an external stimulus. These requirements have been impossible to meet until now.

One stimulus that could be used is a magnetic field, if the crystals are made of magnetic materials, such as iron oxide. The problem with this is that the magnetization is maintained when the particles grow into larger domains (ferromagnetism). Yin and his team have found a solution: They coated nanoscopic iron oxide particles with a plastic called polyacrylate.

This results in separate clusters of nanocrystals, which self-assemble in solution to form colloidal photonic crystals. The forces of the magnetic field affect every individual cluster, changing the cluster-to-cluster distances within the crystal lattice. Depending on the distance from the magnet and thus the field strength, the color of the colloidal crystal changes right across the whole visible spectrum.

This response is rapid and fully reversible because the nanocrystals in clusters are so small that they lose their magnetism when the magnetic field is shut off (superparamagnetism). Potential applications for these switchable “optical semiconductors” include novel optoelectronic components for telecommunications, displays, and sensors.

Citation: Yadong Yin, Highly Tunable Superparamagnetic Colloidal Photonic Crystals, Angewandte Chemie International Edition, doi: 10.1002/anie.200701992

Source: Angewandte Chemie

Explore further: Graphene reinvents the future

add to favorites email to friend print save as pdf

Related Stories

A mirror with a peephole

Aug 13, 2014

When light shines through air onto water, some of the light usually will be reflected back into the air. But at one specific angle, called the Brewster angle, all of the p-polarized light travels into the ...

Earth-crushing pressure? This electron spin doesn't care

Jul 09, 2014

(Phys.org) —To fully understand something, it is often instructive to view it at its extremes. How do materials behave when their bits are forced much closer together than is comfortable? How do electrons ...

From pencil marks to quantum computers

Jul 03, 2014

Pick up a pencil. Make a mark on a piece of paper. Congratulations: you are doing cutting-edge condensed matter physics. You might even be making the first mark on the road to quantum computers, according ...

Recommended for you

Graphene reinvents the future

Aug 27, 2014

For many scientists, the discovery of one-atom-thick sheets of graphene is hugely significant, something with the potential to affect just about every aspect of human activity and endeavour.

Catalytic gold nanoclusters promise rich chemical yields

Aug 25, 2014

(Phys.org) —Old thinking was that gold, while good for jewelry, was not of much use for chemists because it is relatively nonreactive. That changed a decade ago when scientists hit a rich vein of discoveries ...

Copper shines as flexible conductor

Aug 22, 2014

Bend them, stretch them, twist them, fold them: modern materials that are light, flexible and highly conductive have extraordinary technological potential, whether as artificial skin or electronic paper.

User comments : 0