photonic crystal displays its true colors: optical materials
TRANSCRIPT
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