Raising the IQ of Smart Windows: Embedded Nanocrystals Provide Selective Control Over Visible Light and Heat-Producing Near-Infrared Light

Raising the IQ of Smart Windows:
Embedded Nanocrystals Provide
Selective Control Over Visible Light
and Heat-Producing Near-Infrared
Light
Aug. 14, 2013 — Researchers at the
U.S. Department of Energy's
Lawrence Berkeley National
Laboratory (Berkeley Lab) have
designed a new material to make
smart windows even smarter. The
material is a thin coating of
nanocrystals embedded in glass that
can dynamically modify sunlight as it
passes through a window. Unlike
existing technologies, the coating
provides selective control over visible
light and heat-producing near-
infrared (NIR) light, so windows can
maximize both energy savings and
occupant comfort in a wide range of
climates.
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"In the US, we spend about a quarter
of our total energy on lighting,
heating and cooling our buildings,"
says Delia Milliron, a chemist at
Berkeley Lab's Molecular Foundry
who led this research. "When used
as a window coating, our new
material can have a major impact on
building energy efficiency."
Milliron is corresponding author on a
paper describing the results the
journal Nature . The paper is titled,
"Tunable near-infrared and visible
light transmittance in nanocrystal-in-
glass composites," co-authored by
Anna Llordés, Guillermo Garcia, and
Jaume Gazquez.
Milliron's research group is already
well known for their smart-window
technology that blocks NIR without
blocking visible light. The technology
hinges on an electrochromic effect,
where a small jolt of electricity
switches the material between NIR-
transmitting and NIR-blocking states.
This new work takes their approach
to the next level by providing
independent control over both visible
and NIR light. The innovation was
recently recognized with a 2013 R&D
100 Award and the researchers are in
the early stages of commercializing
their technology.
Independent control over NIR light
means that occupants can have
natural lighting indoors without
unwanted thermal gain, reducing the
need for both air-conditioning and
artificial lighting. The same window
can also be switched to a dark mode,
blocking both light and heat, or to a
bright, fully transparent mode.
"We're very excited about the
combination of unique optical
function with the low-cost and
environmentally friendly processing
technique," said Llordés, a project
scientist working with Milliron.
"That's what turns this 'universal
smart window' concept into a
promising competitive technology."
At the heart of their technology is a
new "designer" electrochromic
material, made from nanocrystals of
indium tin oxide embedded in a
glassy matrix of niobium oxide. The
resulting composite material
combines two distinct functionalities
-- one providing control over visible
light and the other, control over NIR
-- but it is more than the sum of its
parts. The researchers found a
synergistic interaction in the tiny
region where glassy matrix meets
nanocrystal that increases the
potency of the electrochromic effect,
which means they can use thinner
coatings without compromising
performance. The key is that the way
atoms connect across the
nanocrystal-glass interface causes a
structural rearrangement in the glass
matrix.
The interaction opens up space inside
the glass, allowing charge to move in
and out more readily. Beyond
electrochromic windows, this
discovery suggests new opportunities
for battery materials where transport
of ions through electrodes can be a
challenge.
"From a materials-design
perspective, we've shown that you
can combine very dissimilar materials
to create new properties that are not
accessible in a homogeneous single
phase material, either amorphous or
crystalline, by taking nanocrystals
and putting them in glass," says
Milliron.
But for Milliron, the research journey
is even more satisfying than either
basic scientific discovery or
technological advances alone.
"The most exciting part has been
taking this project all the way from
synthesizing a new material, to
understanding it in great detail, and
finally to realizing a completely new
functionality that can have a big
impact on technology," says Milliron.
"Taking a materials development
project all the way through that
process is really quite remarkable. It
really speaks to what we can do at
Berkeley Lab, where you have access
to not just the scientific facilities but
also to people who can inform your
perspective."