New Way of Modulating Color Emissions from Transparent Films Found

Researchers at the Tokyo University of Science have described a method for easily tuning the luminescence of a newly modified light-emitting solid transparent material by modulating its proton concentration through the application of voltage.

The material was developed in the lab of Makoto Tadokoro. He and his team, including Hajime Kamebuchi of Nihon University and Taiho Yoshioka of Tokyo University of Science, began with a transparent polymeric film called Nafion. Nafion films are well-known proton conductors (materials in which electricity is conducted via the movement of protons) and cation exchangers (materials that readily attract positively charged particles).

Another property of Nafion that proved useful is its molecular structure. The structure of Nafion allowed “complexes” of two metals, terbium (Tb) and europium (Eu), both light emitters, to be embedded in it when dipped in a solution containing the metal complexes.

When the final product — a metal-complex-containing polymeric film — was immersed in an acidic solution, it turned green. When soaked in an alkaline solution, it turned red. In a neutral solution it turned yellow.

The researchers turned to spectroscopic analysis to understand why. In acidic solutions, the protons taken up by Nafion were “turning on” the Tb metal ions, though not the Eu metal ions. In alkaline solutions, Eu metal ions took the spotlight, and emissions from the Tb ions were quenched. In neutral solutions, both emitted light. That analysis confirmed for the researchers that the proton concentration gradient within the material determined its luminescence.

The scientists were then able to easily tune the luminescence by hooking up the material to a battery after dipping it into an acidic solution, turning it green. But upon application of a voltage, as protons moved toward the negatively charged side of the material, the proton-deficient positively charged side began to turn red. The central portion of the material became yellow.

“We think that this was the most challenging part of our study — and incidentally also our biggest success. The finding that the flow of protons in a solid medium under an electric field can be controlled, which in turn allows us to control the ‘color’ of emitted light, is unprecedented,” Tadokoro said. “In biological systems, ion flows are responsible for many essential biochemical activities. The ‘solid-state ionics’ demonstrated by us can find applications in a lot of different fields.

“Our findings show that it is possible to fabricate inexpensive multicolor-emitting glass or film materials whose emissions can be tuned by simply applying a voltage to control the proton flow, and therefore proton gradient, within the material. In other words, not only electron conduction, but proton conduction can be a way in which the luminescence of materials is controlled.”

The researchers now aim to add a blue-light-emitting complex into the system to create a material that can emit light over the full visible spectrum.

The research was published in Materials Advances (www.doi.org/10.1039/d0ma00237b).