The key discovery of LED nanomaterial: a new light source with low cost and energy saving

Researchers from the U.S. Department of Energy (DOE) Argonne National Laboratory, Brookhaven National Laboratory, Los Alamos National Laboratory, and SLAC National Accelerator Laboratory report that they have prepared stable calcium for this type of LED Titanium ore nanocrystals. Research institutes from Taiwan, China also contributed to this research.

Perovskite is a type of material with a special crystal structure, which has the characteristics of light absorption and luminescence, and is very useful in a series of energy-saving applications, including solar cells and various detectors.

Although perovskite nanocrystals are a major candidate for a new type of LED material, they have proven to be unstable in tests. The research team stabilized the nanocrystals in a porous structure called a metal organic framework, or MOF for short. Based on the abundant materials on the earth and manufactured at room temperature, these LEDs may one day enable lower-cost TVs and consumer electronics, as well as better gamma-ray imaging equipment, and even self-defense for medical, security scanning, and scientific research. Power X-ray detector.

"We solved the stability problem by encapsulating the perovskite material in the MOF structure," said Xuedan Ma, a scientist at the Center for Nanomaterials (CNM) in Argonne, DOE User Facilities Office. "Our research shows that this method enables we can greatly improve the brightness and stability of luminescent nanocrystals."

Hsinhan Tsai, a former JR Oppenheimer postdoctoral fellow at Los Alamos University in the United States, added: “The interesting concept of combining perovskite nanocrystals in MOF has been proven in powder form, but this is the first time that we have successfully integrated it into the emitting layer of an LED. ."

Previous attempts to manufacture nanocrystalline LEDs were hindered by the degradation of nanocrystals back to an unwanted volume phase, which made them lose the advantages of nanocrystals and weakened their potential as practical LEDs. Bulk matter is composed of billions of atoms. Materials like perovskites are composed of a few to a few thousand atoms in the nanometer stage, so they behave differently.

In their new method, the research team stabilized the nanocrystals by fabricating them in a matrix of MOF, just like a tennis ball is clamped by a barbed wire fence. They used lead nodes in the framework as metal precursors and halide salts as organic materials. The halide salt solution contains methyl ammonium bromide, which reacts with the lead in the framework and assembles nanocrystals around the lead core in the matrix. Since the matrix keeps the nanocrystals separated, they will not interact and degrade. This method is based on a solution coating method, which is much cheaper than the widely used vacuum process for manufacturing inorganic LEDs.

MOF stabilized LEDs can produce bright red, blue, and green light and different shades of each light.

Wanyi Nie, a scientist at the Center for Integrated Nanotechnology at Los Alamos National Laboratory, said: "In this work, we have demonstrated for the first time that perovskite nanocrystals that are stable in MOF will create bright and stable LEDs of various colors. We can create different colors, improve color purity and increase photoluminescence quantum yield, which is a measure of the luminous ability of materials."

The research team used the Advanced Photon Source (APS)-DOE's Office of Scientific User Facilities in Argonne-to perform time-resolved X-ray absorption spectroscopy, a technique that allowed them to discover changes in perovskite materials over time. Researchers can track the movement of electric charges in the material and understand the important information that occurs when light is emitted.

"We can only achieve this with the powerful single X-ray pulse and unique time structure of APS," said Xiaoyi Zhang, the team leader of the X-ray Science Department of Argonne. "We can track the charged particles in the tiny perovskite crystals position."

In the durability test, the material performs well under ultraviolet radiation, heat and electric field without degradation and loss of its light detection and luminous efficiency, which is a key condition for practical applications such as televisions and radiation detectors.