A team from Israel's Technion Institute of Technology has announced the development of self-healing perovskite nanocrystals.
Having to frequently replace electronics due to malfunctioning of materials is unavoidable today, since every device suffers from degradation as a result of defects that accumulate during use over time. This generates, in addition to customer frustration and costs, a heavy environmental footprint.
“They still don’t know what to do with them, because the materials that are used in making the devices have already become defective,” says Technion's Dr. Yehonadav Bekenstein. “Of the items we throw away, 90 percent are sent to a landfill, even if we put them in recycling bins.” It’s no wonder, then, that manufacture of devices that can spontaneously repair their own defects is considered one of the most important goals in the world of materials engineering".
In recent years there has been progress in the field of developing soft materials that repair themselves and polymers that do that have already been produced. Now, scientists are hoping to devise a way to get semiconductors to repair themselves too.
The Technion’s new study was carried out on halide perovskites. About five years ago researchers discovered that if a perovskite solar cell damaged due to use is left in the dark, under certain circumstances its operation returns to a level that’s almost identical to the way it functioned immediately after leaving the assembly line. This discovery led to the theory that these may be materials that can repair themselves.
Prof. Dan Oron, a researcher in the Department of Molecular Chemistry and Materials Sciences at the Weizmann Institute of Science in Rehovot, explains that halide perovskites are solid crystals, inside of which there is a constant movement of atoms that carry an electric charge (ions) – a characteristic not usually found in solid materials. “That’s the basis for the process of self-healing,” he says. “Defects are created in the crystal, but over time the ions find their way back and in that way the defects disappear.”
The process of repairing the crystal therefore takes place naturally, since it “wants” to rearrange itself into its initial, basic form, when the atoms inside the crystal are at their lowest energy state. In their new study, Bekenstein and doctoral students Sasha Khalfin and Noam Veber engineered nanoparticles of perovskites. In past studies, nanometric layers were created by abrading an ordinary-sized crystal, but the team discovered the right conditions under which a chemical reaction creates nucleation and the growth of nanocrystals. “We grew these crystals, which are on the verge of being the smallest crystals nature can create, in the lab, one to ten thousand of a human hair width” explains Bekenstein.
Nanocrystals are very convenient to use in research because an electron microscope dispatching a high-energy, focused beam can penetrate them and the transmitted electrons form an image, which is instructive when it comes to learning about the structure of the material. With large crystals that is impossible. The researchers sat for hundreds of hours in front of the electron microscope, trying to identify the processes that affect the structure of the crystal, in order to examine its self-healing abilities. “We observe the crystals day after day,” says Khalfin. “Once I sat for six straight hours.”
The team also filmed dozens of video clips and analyzed them with a computer, thus creating a picture of the dynamic that takes place in the nanocrystal. They discovered that the focused electron beam creates holes on the surface of the crystal, because the beam removes atoms from the material. After the holes are created, they migrate toward the center of the crystal. “We saw the holes being created and then pushed away from the edges, so that the margins always remained complete,” Bekenstein says.
As a result of this surprising observation, the researchers re-engineered the crystal. While during the first stage of the experiment they added organic molecules to the crystal while it was still growing, which attached onto its surface and stopped the growth so that it remained the size of a nanoparticle – during the second stage the team removed the organic layer on top in order to understand why the margins remained intact. Then Khalfin also identified an opposite phenomenon: Instead of the hole that was created moving to the center of the crystal, it seemed to be gradually pushed out to the sides, until in the end it was expelled from the crystal, which then returned to its original, whole state.
Bekenstein says that this marks the first time scientists have been able to observe the healing process of a substance at the atomic level. “We managed to cause it, to understand what it comes from and then to control it, in other words, to understand what we have to put on the surface in order for it to happen,” he explains. “There are all kinds of defects that affect materials, and in this case, it was a most serious defect – a hole, in other words, part of the material was missing. We discovered that the perovskite crystal can return to its complete state if we rearrange things so the hole moves outward.”
This discovery constitutes an important step in understanding the mechanisms that enable perovskite nanoparticles to heal themselves, and is paving the way to the inclusion of such materials in electronic and solar devices. “We realized that the organic molecules are important when it comes to shaping the material, but they interrupt its process of self-repair,” Bekestein notes. “Now we can engineer devices that will exploit that situation.”
What is still unknown is the maximum distance to which a hole in a crystal can migrate and as a result, reach its edge and disappear from it. “In this case, it was a very short distance since we’re talking about a nanocrystal, but it’s not clear if that will happen with a big crystal. The science of self-healing has been understood, and now we have to engineer larger materials in which such a process will also take place,” he adds.
The Technion researchers point to two additional advantages of the perovskite materials they have developed. As opposed to materials from the previous generation, the new perovskites do not contain toxic lead and should be recyclable.
“Today it’s hard to recycle used electronic devices, because the material itself is damaged over time,” Bekenstein notes. “Due to our invention we’ll be able to recycle material so that it will assume its complete shape, the same as on the day it was manufactured, and then reuse it.” These materials may be of great importance in places where it is impossible to replace defective materials – for example, in the solar panels that help to power satellites in space.