Physicists Tap Properties of Plasmons to Expand the Color Range of Color-Changing Glass

Adding and removing an electron from neutral perylene (center column) produces an anion (left) and cation (right), respectively, with different electronic structures (middle row). Upon excitation with visible light, the anion and cation give rise to two unique molecular plasmon resonances, each with their own distinct color (bottom row). CREDIT: Image courtesy of Grant Stec/Rice University.

A recent nanophotonics study carried out at Rice University can broaden the color palette available for companies in the rapidly growing market for glass windows with the ability to change color by just the click of an electric switch.

A research team at the laboratory of Rice plasmonics pioneer Naomi Halas has developed a glass that can change two different colors at lower voltages by using an inexpensive, readily accessible hydrocarbon molecule known as perylene. They have reported the outcomes of their research in a paper published in ACS Nano, an American Chemical Society journal.

When we put charges on the molecules or remove charges from them, they go from clear to a vivid color. 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 color depending on how we apply a very low voltage.

Naomi Halas, Director of the Laboratory for Nanophotonics (LANP)

Adam Lauchner, co-lead author of the study and an applied physics graduate student at Rice, stated that LANP’s color-changing glass has polarity-dependent colors, that is, a negative voltage produces one color and a positive voltage produces a different color.

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

Glass capable of changing color with respect to an applied voltage is termed as “electrochromic.” Additionally, the heat- and light-blocking characteristics of such glass are in great demand. The estimated annual market for electrochromic glass by 2020 is more than $2.5 billion.

Lauchner stated that the study took nearly two years to complete. He accredited the design of the perylene-containing nonwater-based conductive gel sandwiched between the glass layers to Grant Stec, co-lead author of the study and a Rice undergraduate researcher.

Perylene is part of a family of molecules known as polycyclic aromatic hydrocarbons. 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.

Grant Stec, Undergraduate Researcher, Rice University

Although a number of polycyclic aromatic hydrocarbons (PAHs) exist, each type includes carbon atom rings decorated with hydrogen atoms. In most of the PAH, the carbon rings have six sides, similar to the rings in graphene, the widely acknowledged topic of the Nobel Prize in physics in the year 2010.

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

A plasmon can be defined as wave of energy, that is, a rhythmic sloshing in the electron sea that constantly flows across the surface in conductive nanoparticles. A plasmon’s sloshing can interact with as well as tap the energy of passing light based on its frequency.

Halas, Rice physicist Peter Nordlander and other collaborators have carried out a number of studies in the past 20 years to investigate not only the basic physics behind plasmons but also their diverse potential applications such as optical computing, cancer treatment, electronic displays, and solar-energy collection.

The quintessential plasmonic nanoparticle is metallic and normally formed of silver or gold, as well as accurately shaped. For instance, gold nanoshells invented by Halas at Rice in the 1990s comprise of a nonconducting core covered by a thin gold shell.

Our group studies many kinds of metallic nanoparticles, but graphene is also conductive, and we’ve explored its plasmonic properties for several years.

Naomi Halas, Director of the Laboratory for Nanophotonics (LANP)

Halas pointed out that plasmons are supported by large sheets of atomically thin graphene. However, these sheets emit infrared light that cannot be viewed by 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,” stated Lauchner.

In the year 2013, Alejandro Manjavacas, then-Rice physicist and a postdoctoral researcher in Nordlander’s lab, demonstrated that smallest graphene particles, that is, PAH including only a few carbon rings, must be able to produce visible plasmons. In addition, Manjavacas computed the accurate colors 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 colors.

Naomi Halas, Director of the Laboratory for Nanophotonics (LANP)

According to Lauchner, the research actually gained momentum when Stec joined the researchers in 2015 and developed a perylene formulation that could be sandwiched between conductive glass sheets.

The experiments performed by the researchers revealed that the application of a mere 4 V of electrical charge was adequate to change the color of the clear window to greenish-yellow, and when a negative 3.5 V charge was applied, the color of the glass changed into blue.

Although the complete change of color took several minutes, Halas stated that the transition time can be easily reduced by performing additional engineering.

According to Stec, another window whose color changed from clear to black was produced at later stages of the research.

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. 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.

Grant Stec, Undergraduate Researcher, Rice University

Robert A. Welch Foundation supported the study. Yao Cui, former Rice graduate student and a data scientist at KUKA North America in Austin who earned a doctorate in computational chemistry from Rice in 2016, co-authored the ACS Nano paper.

Halas is Stanley C. Moore Professor of Electrical and Computer Engineering and professor of chemistry, bioengineering, physics and astronomy, and materials science and nanoengineering at Rice University. Nordlander is professor of physics and astronomy, electrical and computer engineering, and materials science and nanoengineering. Manjavacas is assistant professor of astronomy and physics from the University of New Mexico.

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