RESEARCH NEWS

OCTOBER 2007 | VOLUME 10 | NUMBER 10 9

Flat panel displays based on photonic crystal

technology have taken a significant step closer to the

market thanks to research carried out at the

University of Toronto in Canada, and the University

of Bristol in the UK. The team have fabricated a

proof-of-principle device that shows bright reflective

colors in red, green, and blue [Arsenault et al., Nat.

Photonics (2007) 1, 468].

Reflective displays should outperform backlit or

emissive displays in a range of environments,

especially bright sunlight. Several technologies

could fit the bill, but photonic crystals are attractive

because they can be tuned to different wavelengths.

Unlike every other display on the market today, this

technology would not require expensive color filters

nor would suffer the loss of light from filters or

spatially modulated color schemes.

“The most important result is that we now have

a material that on its own can produce the whole

spectrum of colors. The potential to simplify

manufacturing is tremendous,” claims André C.

Arsenault of the University of Toronto and Opalux Inc.,

a spin-out company founded to commercialize the

technology. The material is dubbed ‘photonic ink’.

The active material in the display is a nanocomposite

comprising an ordered array of silica microspheres in

a cross-linked matrix of polyferrocenylsilane (PFS), a

metallopolymer. When a solvent is absorbed into the

polymer matrix, it swells and increases the spacing

between the silica microspheres, thus changing the

reflected color. This swelling is controlled by applying

a voltage to the silica-PFS composite inside an

electrochemical cell.

“This is a beautiful piece of work,” agrees Yadong

Yin of the University of California, Riverside. “The

system has a significantly improved tuning range and

long-term bistability, and promises easy integration

on flexible substrates.” But Yin points out that the

slow switching speed of a few seconds will mean that

photonic crystals will not be replacing liquid crystal

displays in your computer or TV any time soon.

More likely applications include outdoor signage,

portable electronics, and full-color electronic paper.

There is still plenty of work to do, such as overcoming

the viewing angle dependence, scaling the process for

production, and, for a full-color display, how to control

the brightness and saturation of each pixel.

Pauline Rigby

What do you get when carbon nanotubes are

added to paper? Not just strong paper, but a

flexible material that can be made into ultrathin,

completely flexible batteries and supercapacitors

[Pushparaj et al., Proc. Nat. Acad. Sci. USA (2007)

104, 13574]. Scientists at Rensselaer Polytechnic

Institute chanced upon this technology while

investigating a way to strengthen membranes for

kidney dialysis.

The paper batteries are remarkable because they

can be rolled or folded just like paper without

any loss of efficiency. “The ability to flex is much

greater than flexible batteries on the market

today, which are more like bending a CD,”

comments Robert J. Linhardt. “Being able to

mold the battery to any shape that corresponds

to the space available is a real advantage,” he

adds.

Like all charge-storage devices, the paper

version comprises electrodes, electrolyte, and a

separator. First, vertically aligned multiwalled

carbon nanotubes, which form the first electrode,

are deposited on Si substrates using a vapor

deposition method. The nanotubes give the paper

its black color. Plant cellulose is cast on top of the

layer, solidified, and dried to form the separator.

This paper layer is then impregnated with an

ionic liquid – an organic salt that is liquid at room

temperature – that provides the electrolyte.

Since the ionic liquid contains no water, there is

nothing in the batteries to freeze or evaporate,

which enables them to withstand extreme

temperatures from 195 K to 450 K.

To make a supercapacitor, the paper is simply

folded in half so that there is a carbon electrode

at both top and bottom. To make a battery,

the paper side is coated with lithium oxide as a

second electrode. The team also fabricated dual-

storage devices containing three electrodes that

act as both supercapacitors and batteries.

According to Linhardt, the paper supercapacitor

has comparable performance to commercial

devices, and could reach the market in as little as

two years.

A postage-stamp-sized supercapacitor has a

voltage of almost 2.5 V. Stacking sheets of the

paper increases the voltage; and increasing its size

increases the power stored. The paper battery

has enough power to light a small light-emitting

diode, but will need optimization to improve its

power density. Paper is extremely biocompatible,

so the new devices could be used as power

supplies for devices implanted in the body. The

researchers printed paper batteries without

adding electrolyte and demonstrated that

naturally occurring electrolytes such as human

sweat, blood, and urine could be used to activate

the battery.

Pauline Rigby

The flexible nanocomposite battery. (Courtesy of

Rensselaer/Victor Pushparaj.)

Bendy battery made from paperENERGY GENERATION MATERIALS

‘Photonic ink’ in blue, green, red, and off states. (Courtesy of André Arsenault.)

Photonic crystal displays its true colorsOPTICAL MATERIALS