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Light-scattering nanoparticles open door to colour displays

09 Dec 2015  | Jade Boyd

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Rice's novel drawbridge method for colour switching incorporates metal nanoparticles that absorb light energy and convert it into plasmons, waves of electrons that flow like a fluid across a particle's surface. Each plasmon scatters and absorbs a characteristic frequency of light, and even minor changes in the wave-like sloshing of a plasmon shift that frequency. The greater the change in plasmonic frequency, the greater the difference between the colours observed.

"Engineers hoping to make a display from optically active nanoparticles need to be able to switch the colour," Landes said. "That type of switching has proven very difficult to achieve with nanoparticles. People have achieved moderate success using various plasmon-coupling schemes in particle assemblies. What we've shown though is variation of the coupling mechanism itself, which can be used to produce huge colour changes both rapidly and reversibly."

Gold nanoparticles

This electron microscope image shows a dimer of silver plated gold nanoparticles. A layer of silver connects the particles. Credit: C. Byers/Rice University

To demonstrate the method, Landes and study lead author Chad Byers, a graduate student in her lab, anchored pairs of gold nanoparticles to a glass surface covered with indium tin oxide (ITO), the same conductor that's used in many smartphone screens. By sealing the particles in a chamber filled with a saltwater electrolyte and a silver electrode, Byers and Landes were able form a device with a complete circuit. They then showed they could apply a small voltage to the ITO to electroplate silver onto the surface of the gold particles. In that process, the particles were first coated with a thin layer of silver chloride. By later applying a negative voltage, the researchers caused a conductive silver 'drawbridge' to form. Reversing the voltage caused the bridge to withdraw.

"The great thing about these chemical bridges is that we can create and eliminate them simply by applying or reversing a voltage," Landes said. "This is the first method yet demonstrated to produce dramatic, reversible colour changes for devices built from light-activated nanoparticles."

Byers said his research into the plasmonic behaviour of gold dimers began about two years ago.

"We were pursuing the idea that we could make significant changes in optical properties of individual particles simply by altering charge density," he said. "Theory predicts that colours can be changed just by adding or removing electrons, and we wanted to see if we could do that reversibly, simply by turning a voltage on or off."

Plasmonic shifts

This image illustrates the green and orange hues of light that are scattered thanks to plasmonic shifts that occur when metal bridges are present (bottom) and when they are not (top). Credit: C. Byers/Rice University

The experiments worked. The colour shift was observed and reversible, but the change in the colour was minute.

"It wasn't going to get anybody excited about any sort of switchable display applications," Landes said.

But she and Byers also noticed that their results differed from the theoretical predictions.

Landes said that was because the predictions were based upon using an inert electrode made of a metal such as palladium that isn't subject to oxidation. But silver is not inert. It reacts easily with oxygen in air or water to form a coat of unsightly silver oxide. This oxidizing layer can also form from silver chloride, and Landes said that is what was occurring when the silver counter electrode was used in Byers' first experiments.

"It was an imperfection that was throwing off our results, but rather than run away from it, we decided to use it to our advantage," Landes said.

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