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A little zap turns this glass from clear to black

09 March 2017

New research from Rice University could expand the colour palette for glass windows that change colour at the flick of an electric switch.

Rice University researchers demonstrated a new type of glass that turns from clear to black when a low voltage is applied. (Credit: Jeff Fitlow/Rice University)

In a new paper in the journal ACS Nano, researchers report using a readily available, inexpensive hydrocarbon molecule called perylene to create glass that can turn two different colours at low voltages.

“When we put charges on the molecules or remove charges from them, they go from clear to a vivid colour,” says Naomi Halas, director of the Laboratory for Nanophotonics at Rice University and lead scientist on the new study. “We sandwiched these molecules between glass, and we’re able to make something that looks like a window, but the window changes to different types of colour depending on how we apply a very low voltage.”

Adam Lauchner, an applied physics graduate student at Rice and co-lead author of the study, says the colour-changing glass has polarity-dependent colours, which means that a positive voltage produces one colour and a negative voltage produces a different colour.

“That’s pretty novel,” Lauchner says. “Most colour-changing glass has just one colour, and the multicolour varieties we’re aware of require significant voltage.”

Glass that changes colour with an applied voltage is known as “electrochromic,” and there’s a growing demand for the light- and heat-blocking properties of such glass. The projected annual market for electrochromic glass in 2020 has been estimated at more $2.5 billion.

Lauchner says the glass project took almost two years to complete, and he credits co-lead author Grant Stec, an undergraduate researcher, with designing the perylene-containing nonwater-based conductive gel that’s sandwiched between glass layers.

“Perylene is part of a family of molecules known as polycyclic aromatic hydrocarbons,” Stec says. “They’re a fairly common byproduct of the petrochemical industry, and for the most part they are low-value byproducts, which means they’re inexpensive.”

There are dozens of polycyclic aromatic hydrocarbons (PAHs), but each contains rings of carbon atoms that are decorated with hydrogen atoms. In many PAHs, carbon rings have six sides, just like the rings in graphene, the much-celebrated subject of the 2010 Nobel Prize in physics.

Sloshing plasmons

“This is a really cool application of what started as fundamental science in plasmonics,” Lauchner says.

A plasmon is a wave of energy, a rhythmic sloshing in the sea of electrons that constantly flow across the surface of conductive nanoparticles. Depending upon the frequency of a plasmon’s sloshing, it can interact with and harvest the energy from passing light. In dozens of studies over the past two decades, Halas, Rice physicist Peter Nordlander, and colleagues have explored both the basic physics of plasmons and potential applications as diverse as cancer treatment, solar-energy collection, electronic displays and optical computing.

The quintessential plasmonic nanoparticle is metallic, often made of gold or silver, and precisely shaped. For example, gold nanoshells, which Halas invented in the 1990s, consist of a nonconducting core that’s covered by a thin shell of gold.

“Our group studies many kinds of metallic nanoparticles, but graphene is also conductive, and we’ve explored its plasmonic properties for several years,” says Halas, professor of electrical and computer engineering and professor of chemistry, bioengineering, physics, and astronomy, and materials science and nanoengineering.

Smaller and smaller

She notes that large sheets of atomically thin graphene have been found to support plasmons, but they emit infrared light that’s invisible to the human eye.

“Studies have shown that if you make graphene smaller and smaller, as you go down to nanoribbons, nanodots, and these little things called nanoislands, you can actually get graphene’s plasmon closer and closer to the edge of the visible regime,” Lauchner says.

In 2013, then-Rice physicist Alejandro Manjavacas, a postdoctoral researcher in Nordlander’s lab, showed that the smallest versions of graphene—PAHs with just a few carbon rings—should produce visible plasmons. Moreover, Manjavacas calculated the exact colours that would be emitted by different types of PAHs.

“One of the most interesting things was that unlike plasmons in metals, the plasmons in these PAH molecules were very sensitive to charge, which suggested that a very small electrical charge would produce dramatic colours,” Halas says.

Lauchner says the project really took off after Stec joined the research team in 2015 and created a perylene formulation that could be sandwiched between sheets of conductive glass.

Just 4 volts

In their experiments, the researchers found that applying just 4V was enough to turn the clear window greenish-yellow and applying negative 3.5V turned it blue. It took several minutes for the windows to fully change colour, but Halas says the transition time could easily be improved with additional engineering.

Stec says the team’s other window, which turns from clear to black, was produced later in the project.

“Dr. Halas learned that one of the major hurdles in the electrochromic device industry was making a window that could be clear in one state and completely black in another,” Stec says. “We set out to do that and found a combination of PAHs that captured no visible light at zero volts and almost all visible light at low voltage.”

Video courtesy of Rice University.

Materials sourced from Futurity

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