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Scientists explain visible light from 2-D lead halide perovskites



  Scientists explain visible light from 2D lead halide perovskites
Jiming Bao, associate professor of electrical engineering and computer technology at the University of Houston, led an international group of researchers investigating how a two-dimensional perovskite consisting of cesium, lead and bromine could provide a strong green light. Credit: University of Houston

Three years ago, researchers were noted when they reported that a two-dimensional perovskite material with a specific crystal structure consisting of cesium, lead and bromine triggered a strong green light. Crystals that produce light on the green spectrum are desirable, since green light, although valuable in itself, can also be converted relatively easily to other forms that emit blue or red light, making it particularly important for optical applications ranging from light-emitting devices for sensitive diagnostic tools.
However, there was no agreement on how the crystal, CsPB produced the green photoluminescence. Several theories emerged, but a definite answer.

Now, scientists from the United States, Mexico and China, led by an electrician engineer from the University of Houston, have reported in the journal Advanced Materials They have used sophisticated optical and high-pressure diamond mutant cell techniques to determine not only the mechanism of light emission but also how to replicate it. They originally synthesized CsPB from a related material called CsPbBr and found that the cause of the light emission is a slight overgrowth of nanocrystals that existed. of the original material and growing along the edge of CsPB 2 [crystals]. While the CsPbBr 3 base crystal is three-dimensional and acts green under ultraviolet light, the new material CsPB2Br5 has a layered structure and is optically inactive.

"Now that the mechanism of broadcasting This light is understood, it can be replicated," says Jiming Bao, associate professor of electrical engineering and computer engineering at the University of Applied Sciences and the corresponding author on the paper. "Both crystals have the same chemical composition as diamond versus graphite, but they has 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 201

6, a project that ultimately involved 19 researchers from UH and institutions in China and Mexico At that time, there were two schools of scientific thought about cesium crystal light emission: that it emitted green light due to a defect, mainly the lack of bromine, rather than the material itself or that a variation had inadvertently His group began with the synthesis of a pure sample by dripping CsPbBr 3 powder into water which resulted in sharper edges.The sharper edges gave off a stronger green light, Bao said.

The researchers then used an optical microscope to study the individual crystals of the compound, which Bao said that they decided that even if the compound is transparent, something happened at the edge, resulting in photoluminescence. They trusted Raman's spectroscopy – an optical technique that uses information on how light interacts with a material to determine the material's grating characteristics – to identify nanocrystals of the original source material, CsPbBr 3 along the edges of the crystal as The source of light.

Bao said CsPbBr 3 is too unstable to use on its own, but the stability of the converted form is not hindered by the small amount of the original crystal. The new understanding of light emission will provide new opportunities for designing and manufacturing new 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 says.


Provera semiconductor crystals with a sphere of light


More information:
Chong Wang et al., Extrinsic Green Photoluminescence from the Edges of 2D Cesium Lead Halides, Advanced Materials (2019). DOI: 10.1002 / adma.201902492

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University of Houston




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Scientists explain visible light from 2-D lead halide perovskites (2019, June 24)
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