Single nanowires shown emitting different colors. The top panel shows a cesium lead bromide (CsPbBr3)-cesium lead chloride (CsPbCl3) heterojunction simultaneously emitting green and blue lights, respectively, under UV excitation. The bottom panel shows a cesium lead iodide (CsPbI3)-cesium lead bromide-cesium lead chloride configuration emitting red, green, and blue lights, respectively. Credit: Letian Dou/Berkeley Lab and Connor G. Bischak/UC Berkeley
A new type of semiconductor may be coming to a high-definition display near you. Scientists at Berkeley Lab have shown that a class of semiconductor called halide perovskites is capable of emitting multiple, bright colors from a single nanowire at resolutions as small as 500 nanometers. The findings represent a clear challenge to quantum dot displays that rely upon traditional semiconductor nanocrystals to emit light. It could also influence the development of new applications in optoelectronics, photovoltaics, nanoscopic lasers, and ultrasensitive photodetectors, among others.
They used electron beam lithography to fabricate halide perovskite nanowire heterojunctions, the junction of two different semiconductors. In device applications, heterojunctions determine the energy level and bandgap characteristics, and are therefore considered a key building block of modern electronics and photovoltaics. The researchers pointed out that the lattice in halide perovskites is held together by ionic instead of covalent bonds.
“With inorganic halide perovskite, we can easily swap the anions in the ionic bonds while maintaining the single crystalline nature of the materials,” said Peidong Yang, senior faculty scientist at Berkeley Lab’s Materials Sciences Division. “This allows us to easily reconfigure the structure and composition of the material. That’s why halide perovskites are considered soft lattice semiconductors. Covalent bonds, in contrast, are relatively robust and require more energy to change. Our study basically showed that we can pretty much change the composition of any segment of this soft semiconductor.”
In this case, they tested cesium lead halide perovskite, and then they used a common nanofabrication technique combined with anion exchange chemistry to swap out the halide ions to create cesium lead iodide, bromide, and chloride perovskites. Each variation resulted in a different color emitted. Moreover, they showed that multiple heterojunctions could be engineered on a single nanowire. They were able to achieve a pixel size down to 500 nm, and they determined that the color of the material was tunable throughout the entire range of visible light.
They said that the chemical solution-processing technique used to treat this class of soft, ionic-bonded semiconductors is far simpler than methods used to manufacture traditional colloidal semiconductors. “For conventional semiconductors, fabricating the junction is quite complicated and expensive,” said Letian Dou, postdoctoral fellow in Yang’s lab. “High temperatures and vacuum conditions are usually involved to control the materials’ growth and doping. Precisely controlling the materials composition and property is also challenging because conventional semiconductors are ‘hard’ due to strong covalent bonding.”
To swap the anions in a soft semiconductor, the material is soaked in a special chemical solution at room temperature. “It’s a simple process, and it is very easy to scale up,” said Yang, who is also a professor of chemistry at UC Berkeley. “You don’t need to spend long hours in a clean room, and you don’t need high temperatures.” The researchers are continuing to improve the resolution of these soft semiconductors, and are working to integrate them into an electric circuit. https://newscenter.lbl.gov/2017/06/26/halide-perovskites-soft-semiconductors-hd-displays/
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