Researchers drew attention three years ago when they reported that a two-dimensional perovskite—a material with a specific crystal structure—composed of cesium, lead and bromine emitted a strong green light. Crystals that produce light on the green spectrum are desirable because green light, while valuable in itself, can also be relatively easily converted to other forms that emit blue or red light, making it especially important for optical applications ranging from light-emitting devices to sensitive diagnostic tools. But there was no agreement about how the crystal, CsPB2Br5, produced the green photoluminescence. Now, however, researchers from the United States, Mexico and China,...