The researchers drew attention three years ago when they reported that a two-dimensional perovskite – a material with a specific crystalline 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, although valuable in itself, can also be converted relatively easily into other forms that emit blue or red light, making it particularly important for optical applications ranging from emitting devices. light to sensitive Diagnostic tools.
But there was no agreement on how the crystal, CsPB2Br5, produced the green photoluminescence. Several theories emerged, without a definitive answer.
Now, however, researchers from the United States, Mexico and China, led by an electronic engineer from the University of Houston, reported in the newspaper Advanced material they used sophisticated optical and high pressure diamond anvil cell techniques to determine not only the mechanism for emitting light, but also how to replicate it.
Initially they synthesized CsPB2Br5 from a related material known as CsPbBr3 and found that the main cause of light emission is a small overgrowth of nanocrystals composed of that original material, which grows 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 on the paper. "Both crystals have the same chemical composition, very similar to diamond compared to graphite, but 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 started working on the problem in 2016, a project that eventually involved 19 UH researchers and institutions in China and Mexico. At that time, there were two schools of scientific thought on the emission of light from the cesium crystal: which emitted green light due to a defect, mainly a lack of bromine, rather than the material itself, or which a variation had been unintentionally introduced, resulting in the issue.
His group started with the synthesis of a clean sample dropping CsPbBr3 dust in water, resulting in sharp-edged crystals. "The sharper edges emitted a stronger green light," said Bao.
The researchers then used an optical microscope to study the individual crystals of the compound, which according to Bao allowed them to determine that, although the compound was transparent, "something was happening on the edge, resulting in photoluminescence".
They were based on Raman spectroscopy – an optical technique that uses information on how light interacts with a material to determine the properties of the lattice of the material – to identify the nanocrystals of the original material, CsPbBr3, along the edges of the crystal as a source of light.
Bao said CsPbBr3 it's too unstable to use alone, but the stability of the converted form is not hampered by the small amount of the original crystal.
The researchers said the new understanding of light output will offer new opportunities to design and manufacture 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 said.
Sound the semiconductor crystals with a light sphere
Chong Wang et al., Green Photoluminescence extrinsic from the edges of 2D cesium lead halides, Advanced material (2019). DOI: 10.1002 / adma.201902492
Researchers explain visible light from 2-D perovskite lead halide (2019, June 24)
recovered on 24 June 2019
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