Europium and terbium are two rare earth elements that share a colorful similarity: they emit bright red and green light, respectively, when exposed to ultraviolet light. In the image above, there are five thin polymer films embedded with different concentrations of europium and terbium. The far-left film contains primarily terbium, hence the bright green light, while the far-right film contains primarily the red light-emitting europium. The three middle films demonstrate how, by varying the amount of europium and terbium, we can tune the color emission to also include shades of yellow and orange. By using a third rare earth element that emits blue light, we could make films with all the colors of the rainbow as well as white light. Not only are these light-emitting films stunning to look at, they can have applications in imaging and sensing. For example, they could be used as anti-counterfeiting markers on money, as tags to see different components of a biological system, or as sensors that change color when exposed to different environmental conditions such as temperature or acidity changes.

Contributed by Andy Greenspon, a Ph.D. candidate in Applied Physics at the Harvard John A. Paulson School of Engineering and Applied Sciences.