Perovskite nanocrystals (green), which are light-emitting diodes, can be made at low cost using earth-abundant metals and still work well under normal conditions. Credit: Los Alamos National Laboratory
The stabilization of nanocrystals has led to a breakthrough in light sources for medical imaging, consumer electronics, and detectors.
The lighting industry's unsung hero is the light-emitting diodes (LEDs). They are efficient, emit little heat and last a long time. Scientists are now looking for new materials to create more efficient LEDs that last longer. These can be used in security, medicine, and consumer electronics.
According to the U.S. Department of Energy (DOE), researchers from Brookhaven National Laboratory in Brookhaven, Los Alamos National Laboratory, Brookhaven National Laboratory and SLAC National Accelerator Laboratory have reported that they have made stable perovskite nanocrystals suitable for LEDs. Academia Sinica from Taiwan also contributed to this effort.
Perovskites, a class of materials that have a specific crystalline structure, are light-emitting and light-absorbing. They are used in various energy-efficient applications such as solar cells and detectors.
Perovskite nanocrystals were considered prime candidates for a new type of LED material, but they proved to be unstable in testing. The nanocrystals were stabilized in a porous structure known as a metal-organic frame (or MOF) by the research team. These LEDs, which were fabricated from earth-abundant material and manufactured at room temperature, could one day be used to make TVs and other consumer electronics. They also have better gamma radiation and self-powered Xray detectors that can be used in medical, security scanning, and scientific research.
"We addressed the stability issue of perovskite material by encapsulating them into MOF structures," stated Xuedan M, scientist at Argonne's Center for Nanoscale Materials. This facility is a DOE Office of Science User Facility. "Our studies have shown that this approach can significantly increase the brightness and stability light-emitting nanocrystals."
Hsinhan Tai, a former J. R. Oppenheimer postdoctoral fellow at Los Alamos added: "The intriguing idea of perovskite nuclestals in MOF had already been demonstrated in powder form but this is our first attempt to integrate it as an emission layer in anLED."
Previous attempts at creating nanocrystal LEDs failed due to their tendency to degrade back to their bulk phase. This causes them to lose their nanocrystal benefits and makes it difficult to use as practical LEDs. Bulk materials are made up of billions upon billions of atoms. Materials like perovskites in nanophase are composed of groups of a few hundred to a few thousand of atoms and behave differently.
The research team created nanocrystals in the MOF matrix, similar to tennis balls trapped in a chain link fence. The framework was used as the metal precursor, and the organic material as the halide salts. The solution of halide sodium salts contains methylammoniumbromide. This reacts with the lead in the framework to form nanocrystals around a lead core that is trapped in the matrix. The matrix separates the nanocrystals so that they don't inter-act and degrade. This solution coating method is far more affordable than the vacuum process used to create inorganic LEDs.
You can make MOF-stabilized LEDs to produce bright red, green, and blue light as well as varying shades.
"In this work we demonstrated for first time that perovskite microcrystals stabilized within a MOF can create bright, stable LEDs with a range of colours," stated Wanyi Nie from the Center for Integrated Nanotechnologies at Los Alamos National Laboratory. "We can create new colors, improve the purity of color, and increase photoluminescence quant yield. This is a measure how a material can produce light.
The Advanced Photon Source (APS), a DOE Office of Science Facility at Argonne, was used by the research team to perform time-resolved Xray absorption spectroscopy. This technique allowed them to see the changes in the perovskite over time. Researchers were able track the electrical charges that moved through the material, and they learned valuable information about how light is affected.
"We could only achieve this with the powerful single X-ray Pulses and unique timing structure APS," stated Xiaoyi Zhang (group leader, Argonne's X-ray Science Division). "We can track the locations of charged particles within the tiny crystals of perovskite."
Durability tests showed that the material was resistant to heat, ultraviolet radiation and electrical fields. The material also performed well in heat, and in an electric field. This is a crucial condition for practical applications like TVs and radiation detectors.
The paper "Bright and stable light-emitting diodes made from perovskite nanocrystals stabilized within metal-organic frameworks" was published in Nature Photonics. Xuedan M, Gary Wiederrecht, and Xiewen wen, all from the CNM, as well as Xiaoyi Zhang, Cunming Lu, from the APS, were Argonne researchers who contributed to this research. Hsinhan Tai, Shreetu Shrestha and Rafael A. Vil are among the other researchers.
Learn more about the compositional dependence of nanocrystal perovskite properties
Hsinhan Tsai and colleagues, Bright and stable light emitting diodes made from perovskite microcrystals stabilized within metalorganic frameworks. Nature Photonics (2021). Information from Nature Photonics Hsinhan Tsai et al. Bright and stable light emitting diodes made using perovskite microcrystals that have been stabilized in metalorganic frames, (2021). DOI: 10.1038/s41566-021-00857-0
