Rice University’s Nanophotonics team creates low-voltage, multicolor, electrochromic glass

nanophotonicAdding 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 …more
Rice University’s latest nanophotonics research could expand the color palette for companies in the fast-growing market for glass windows that change color at the flick of an electric switch.

In a new paper in the American Chemical Society journal ACS Nano, researchers from the laboratory of Rice plasmonics pioneer Naomi Halas report using a readily available, inexpensive hydrocarbon molecule called perylene to create that can turn two different colors at low voltages.

“When we put charges on the molecules or remove charges from them, they go from clear to a vivid color,” said Halas, director of the Laboratory for Nanophotonics (LANP), lead scientist on the new study and the director of Rice’s Smalley-Curl Institute. “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.”

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

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

 Credit: Rice University

Glass that changes color 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 said the glass project took almost two years to complete, and he credited co-lead author Grant Stec, a Rice 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 said. “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 (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.

Nanophotonics team creates low-voltage, multicolor, electrochromic glass
Grant Stec and Adam Lauchner of Rice University’s Laboratory for Nanophotonics have used an inexpensive hydrocarbon molecule called perylene to create a low-voltage, multicolor, electrochromic glass. Credit: Jeff Fitlow/Rice University

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

A plasmon is 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 at Rice 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,” Halas said.

Nanophotonics team creates low-voltage, multicolor, electrochromic glass
Rice University researchers demonstrated a new type of glass that turns from clear to black when a low voltage is applied. The glass uses a combination of molecules that block almost all visible light when they each gain a single electron. Credit: Jeff Fitlow/Rice University

She noted 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 said.

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 colors 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 colors,” Halas said.

Nanophotonics team creates low-voltage, multicolor, electrochromic glass
Student researchers Grant Stec (left) and Adam Lauchner (right) with Rice plasmonics pioneer Naomi Halas, director of Rice University’s Laboratory for Nanophotonics. Credit: Jeff Fitlow/Rice University

Lauchner said 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.

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

Stec said 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 said. “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.”

Explore further: Nanoscale drawbridges open path to color displays

More information: Grant J. Stec et al. Multicolor Electrochromic Devices Based on Molecular Plasmonics, ACS Nano (2017). DOI: 10.1021/acsnano.7b00364

NIST Method for Spotting Quantum Dots Could Help Make High-Performance Nanophotonic Devices

NIST 081115 15CNST009_quantum_dot_finder_LRLife may be as unpredictable as a box of chocolates, but ideally, you always know what you’re going to get from a quantum dot. A quantum dot should produce one, and only one, photon—the smallest constituent of light—each time it is energized. This characteristic makes it attractive for use in various quantum technologies such as secure communications. Oftentimes, however, the trick is in finding the dots.

NIST 081115 15CNST009_quantum_dot_finder_LR[Clockwise from top left] Circular grating for extracting single photons from a quantum dot. For optimal performance, the quantum dot must be located at the center of the grating. Image taken with the camera-based optical location technique. A single quantum dot appears as a bright spot within an area defined by four alignment marks. Electron-beam lithography is used to define a circular grating at the quantum dot’s location. Image of the emission of the quantum dot within the grating. The bright spot appears in the center of the device, as desired.

Credit: NIST
View hi-resolution image

“Self-assembled, epitaxially grown” quantum dots have the highest optical quality. They randomly emerge (self-assemble) at the interface between two layers of a semiconductor crystal as it is built up layer-by-layer (epitaxially grown).

They grow randomly, but in order for the dots to be useful, they need to be located in a precise relation to some other photonic structure, be it a grating, resonator or waveguide, that can control the photons that the quantum dot generates. However, finding the dots—they’re just about 10 nanometers across—is no small feat.

Always up for a challenge, researchers working at the National Institute of Standards and Technology (NIST) have developed a simple new technique for locating them, and used it to create high-performance single photon sources.

This new development, which appeared in Nature Communications,* may make the manufacture of high-performance photonic devices using quantum dots much more efficient. Such devices are usually made in regular arrays using standard nanofabrication techniques for the control structures. However because of the random distribution of the dots, only a small percentage of them will line up correctly with the control structures. This process produces very few working devices.

“This is a first step towards providing accurate location information for the manufacture of high performance quantum dot devices,” says NIST physicist Kartik Srinivasan. “So far, the general approach has been statistical—make a lot of devices and end up with a small fraction that work. Our camera-based imaging technique maps the location of the quantum dots first, and then uses that knowledge to build optimized light-control devices in the right place.”

According to co-lead researcher Luca Sapienza of the University of Southampton in the United Kingdom, the new technique is sort of a twist on a red-eye reducing camera flash, where the first flash causes the subject’s pupils to close and the second illuminates the scene. Instead of a xenon-powered flash, the NIST team uses two LEDs.

In their setup, one LED activates the quantum dots when it flashes (so the LED gives the quantum dots red-eye). At the same time, a second, different color LED flash illuminates metallic orientation marks placed on the surface of the semiconductor wafer the dots are embedded in. Then a sensitive camera snaps a 100-micrometer by 100-micrometer picture.

By cross-referencing the glowing dots with the orientation marks, the researchers can determine the dots’ locations with an uncertainty of less than 30 nanometers. The coordinates in hand, scientists can then tell the computer-controlled electron beam lithography tool to place the control structures in the correct places, with the result being many more usable devices.

Using this technique, the researchers demonstrated grating-based single photon sources in which they were able to collect 50 percent of the quantum dot’s emitted photons, the theoretical limit for this type of structure.

They also demonstrated that more than 99 percent of the light produced from their source came out as single photons. Such high purity is partly due to the fact that the location technique helps the researchers to quickly survey the wafer (10,000 square micrometers at a time) to find regions where the quantum dot density is especially low, only about one per 1,000 square micrometers. This makes it far more likely that each grating device contains one—and only one—quantum dot.

This work was performed in part at NIST’s Center for Nanoscale Science and Technology (CNST), a national user facility available to researchers from industry, academia and government. In addition to NIST and the University of Southampton, researchers from the University of Rochester contributed to this work.

* L. Sapienza, M. Davanço, A. Badolato and K. Srinivasan. Nanoscale optical positioning of single quantum dots for bright and pure
single-photon emission.
Nature Communications, 6, 7833 doi:10.1038/ncomms8833. Published 27 July 2015.