RESEARCH NEWS

SEPTEMBER 2007 | VOLUME 10 | NUMBER 9 13

Colloidal nanocrystal clusters (CNCs) of iron

oxide that change color in response to a magnetic

field could lead to improved technology for

making flat panel displays.

Researchers from the University of California,

Riverside (UCR) claim theirs is one of the first

reports of a photonic crystal that is fully tunable

in the visible range of the electromagnetic

spectrum, from violet to red light [Ge et al., Angew. Chem. Int. Ed. (2007), doi: 10.1002/

anie.200701992].

“The key is to design the structure of the

nanoparticles through chemical synthesis so they

self-assemble in a magnetic field,” explains lead

researcher Yadong Yin. The nanoparticles need

to be superparamagnetic and have high surface

charge, high magnetic moment, and optimal size.

Yin’s team synthesized CNCs by hydrolyzing FeCl3

with NaOH at 220°C in a solution containing

polyacrylic acid as a surfactant in diethylene

glycol. This produces clusters 30-180 nm in size

that contain nanocrystals ~10 nm in diameter.

Each nanocrystal supports a single magnetic

domain. But the domain is so small that when

the magnetic field is removed, the domains

become thermally randomized across the sample,

leaving no net magnetic moment – a property

called superparamagnetism. Single nanocrystals

of the same size as the cluster would become

permanently magnetized and clump together.

Self-assembly into an ordered structure – a

photonic crystal that strongly reflects certain

wavelengths of light – occurs thanks to a

trade off between electrostatic repulsion of

the highly charged polyacrylate nanoparticle

coating and magnetic attraction of the magnetite

(Fe3O4) nanocrystals in the core. Altering the

magnetic field changes the spacing between

particles, so a different wavelength of light is

reflected.

In similar work, Sanford Asher’s group at the

University of Pittsburgh has explored the

properties of magnetically tunable colloidal

crystals assembled from polystyrene beads

that are doped with superparamagnetic iron

oxide particles. “Their particles had a much

smaller magnetic moment because of the low

loading of magnetic material,” says Yin. “In our

case the magnetic interaction is much stronger”.

Future applications could be in chemical and

biological sensing, integrated optical switches,

and products such as large area or flexible color

displays. Since the color of the photonic crystals

is based on reflection, displays based on this

technology should be brighter, especially in direct

sunlight where conventional flat-panel displays,

which are back-lit, tend to perform poorly.

“What should make the technology commercially

attractive is that iron oxide is cheap, nontoxic,

and available in plenty,” Yin notes.

Pauline Rigby

Colloidal crystals display a rainbow of colorsOPTICAL MATERIALS

Photographs of colloidal photonic crystals formed in a magnetic field. Magnet-sample distance decreases

from left to right. (Courtesy of Yadong Yin.)

A new method for producing flexible polymer opals

of high quality shows great promise for industrial

scale-up, say researchers from the UK and Germany

[Pursiainen et al., Opt. Express (2007) 15, 9553].

The strong color arising from the photonic crystal

structure of these films could replace the use of toxic

and carcinogenic dyes in everything from clothes to

building materials.

Intriguingly, the color of the polymer opals appears

to arise from a new mechanism distinct from that of

conventional photonic crystals. One outcome may be

the need to re-examine the origin of color in natural

photonic materials, such as butterfly wings, beetles,

and peacock feathers, suggest the researchers from

the University of Southampton, UK, and Merck and

the Deutches Kunststoff-Institut in Darmstadt,

Germany.

The researchers make the opaline films from melts

of submicron polymer particles consisting of a soft

polyethylacrylate (PEA) shell anchored to a hard

polystyrene (PS) sphere. Under shear flow, the

particles assemble into a regular lattice with the soft

PEA filling the voids between the PS spheres. The

resulting flexible films are single domain in nature with

excellent lattice ordering across the whole film. The

team, led by Jeremy J. Baumberg of the University of

Southampton, has produced >100 m lengths of the

polymer opals to show the potential scalability for

industrial manufacture.

Standard, manmade photonic crystals make use of a

large contrast in the refractive index of the periodically

arranged components to produce a complete photonic

bandgap. Very specific enhanced reflected light from

the ordered layers gives the photonic crystals their

color, but it depends strongly on viewing angle and

getting the right orientation of the crystal.

Similarly in the polymer opals, the constant spacing

between the spheres means certain colors of light

are trapped inside the films, explains Baumberg. But

the small contrast in refractive index between the

components means Bragg diffraction is weak and they

appear a milky white color.

A dramatic change is seen, however, on doping the

polymer opals with small amounts of nanoparticles

during fabrication. The films now show an intense

green color that is relatively insensitive to viewing

angle.

“We put tiny black carbon nanoparticles into the films

(without disrupting their order) and these strongly

scatter light of only the particular trapped color,” says

Baumberg. Furthermore, because the films are elastic,

stretching them changes the PS sphere spacing, and

hence the color.

As natural materials often have a low contrast in

refractive index, it is possible they also use this

mechanism of resonant scattering events within the

photonic crystal environment to produce color.

“Currently we are trying to develop good theoretical

models to predict the strength of the effect,” says

Baumberg. “Applications are in any smart packaging,

high-value coatings (e.g. in cars, mobile phones,

planes), and security.”

Jonathan Wood

Polymer opals suggest a new source of color in photonic crystalsOPTICAL MATERIALS

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