New study gives green light to 2D perovskite

Three years ago, researchers reported that a two-dimensional (2D) 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 blue or red light, making it especially important for optical applications that range from light-emitting devices to sensitive diagnostic tools.

But there was no agreement about how the crystal, CsPb₂Br₅, produced the green photoluminescence. Several theories emerged, but there was no definitive answer.

Now, in a paper in Advanced Materials, researchers from the US, Mexico and China, led by an electrical engineer from the University of Houston (UH), have reported using sophisticated optical and high-pressure diamond anvil cell techniques to determine not only the mechanism for the light emission but also how to replicate it.

The researchers initially synthesized CsPb₂Br₅ from a related crystalline material known as CsPbBr₃, and found that the root cause of the light emission is a small overgrowth of nanocrystals composed of that original material, which grow along the edge of the CsPb₂Br₅ crystals. While CsPbBr₃, the base crystal, is three-dimensional and appears green under ultraviolet light, the new material, CsPb₂Br₅, has a layered structure and is optically inactive.

"Now that the mechanism for emitting this light is understood, it can be replicated," said Jiming Bao, associate professor of electrical and computer engineering at UH and corresponding author of the paper. "Both crystals have the same chemical composition, much like diamond versus graphite, but they have very different optical and electronic properties. People will be able to integrate the two materials to make better devices." Potential applications range from solar cells to LED lighting and other electronic devices.

Bao began working on the problem in 2016, and the project grew until it ultimately involved 19 researchers from UH and institutions in China and Mexico. At the time, there were two schools of scientific thought on the light emission from the cesium crystal: that it emitted green light due to a defect, mainly a lack of bromine; or that a variation had unintentionally been introduced, resulting in the emission.

Bao’s group started by synthesizing a clean sample, by dropping CsPbBr₃ powder in water, resulting in sharper-edged crystals; the sharper edges emitted a stronger green light. The researchers then used an optical microscope to study the individual crystals of the compound. This allowed them to determine that although the compound is transparent, "something was going on at the edge, resulting in the photoluminescence," Bao said.

They relied on Raman spectroscopy – an optical technique that uses information about how light interacts with a material to determine the material's lattice properties – to identify nanocrystals of the original source material, CsPbBr₃, along the edges of the crystal as the source of the light.

Bao said CsPbBr₃ is too unstable to use on its own, but the stability of the converted form isn't hampered by the small amount of the original crystal along its edges.

According to the researchers, this new understanding of the light emission will yield new opportunities to design and fabricate novel optoelectronic devices. The techniques used to understand the cesium-lead-halide compound can also be applied to other optical materials to learn more about how they emit light, Bao said.

This story is adapted from material from the University of Houston, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.