news of the week
WINDOW SHADES AND BLINDS could be- come light-blocking relics, thanks to a nano- composite material that dramatically darkens
or simply filters out warm near-infrared light with the
flip of a switch. When incorporated in a window, the
new material could boost a building's energy efficiency
by keeping out some of the heat from sunlight without
blocking visible light.
The new material is electrochromic, meaning that
it switches among different light-blocking states when
a voltage is applied. Other smart windows can change
from light to dark, such as thermochromics and photo-chromics, but these are passive and therefore uncontrollable by the user. Electrochromic windows, which
can be actively controlled, aren't new either. But until
now no one has been able to block near-IR light without
also blocking visible light.
Delia L. Milliron of Lawrence Berkeley National
Laboratory's Molecular Foundry and colleagues accomplished that feat by making an electrochromic material out of tin-doped indium oxide nanocrystals covalently bound to niobium oxide glass (Nature 2013, DOI:
10.1038/nature12398). ª Both of those materials are
electrochromically active, º Milliron explains. When a
voltage is first applied, the nanocrystals charge to produce the near-IR blocking effect. Increasing the voltage
makes the niobium oxide glass block visible light.
People had looked at niobium oxide as an electro-
chromic material before
and discarded it partly
because it didn't darken
notes. But by covalently
linking the niobium oxide
glass to the nanocrystals,
Milliron's team was able
to increase its optical con-
trast to a level five times
its original value.
From studying the structure, Milliron and colleagues
discovered that the covalent bonds between the nanocrystals and the niobium oxide glass distort the mate-rial's overall structure, creating more open space on the
atomic scale and more free volume in the glass. That
allows ions to travel more easily through the material,
which is why there's a higher optical contrast when the
voltage is increased. ª Now that we understand this behavior a little bit, it's something we're going to be able to
leverage in many other kinds of material systems for different applications,º she says.
The approach offers novel ways to create energy-ef-
ficient windows for buildings, says Claes-Gorän Granq-
vist, an electrochromics expert at Sweden's Uppsala
University and founder of electrochromic window mak-
er ChromoGenics. ª However, the technique obviously
is a new one, and it may yet be difficult to assess the pos-
sibilities for implementation on practical windows.º
Milliron agrees.ª This is just an initial demonstra-
tion, º she says. The material still needs to be optimized
and incorporated into an active windo w. Still, she's
confident that her team is at a good starting point.
Costs should be relatively low, she points out, because
the composite is made via a solution-based method.
Low production costs are important, she adds, because
ªt his is not an application that can tolerate a high price
point.º Ð BETHANYHAL FORD
NANOTECHNOLOGY: New material
filters out warm, near-infrared light
while letting visible light through
material can block
visible light (left) or
AUGUST 19, 2013 EDITED BY WILLIAM G. SCHULZ & CRAIG BETTENHAUSEN
In the new
5 to 15 nm wide) are
to niobium oxide
glass (green). Inset
depicts the details
of the binding.