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"... The field of self-repairing materials is rapidly expanding, and what used to be science fiction might soon become reality ... scientists who developed eco-friendly nanocrystal semiconductors capable of self-healing. Their findings, recently published in Advanced Functional Materials, describe the process in which a group of materials called double perovskites display self-healing properties after being damaged by the radiation of an electron beam. ...
They saw that the holes moved freely within the nanocrystal but avoided its edges. The researchers developed a code that analyzed dozens of videos made using the electron microscope to understand the movement dynamics within the crystal. They found that holes formed on the surface of the nanoparticles, and then moved to energetically stable areas inside. The reason for the holes’ movement inwards was hypothesized to be organic molecules coating the nanocrystals’ surface. Once these organic molecules were removed, the group discovered the crystal spontaneously ejected the holes to the surface and out, returning to its original pristine structure – in other words, the crustal repaired itself."
Nanocrystals are the smallest material particles that remain naturally stable. Their size makes certain properties more pronounced and enables research approaches that would be impossible on larger crystals, such as imaging using electron microscopy to see how atoms in the materials move. This was, in fact, the method that enabled the discovery of self-repair in the lead-free perovskites.
The perovskite nanoparticles [researchers] examined the particles using a transmission electron microscope, they discovered the exciting phenomenon. The high-voltage electron beam used by this type of microscope caused faults and holes in the nanocrystals. The researchers were then able to explore how these holes interact with the material surrounding them and how they move and transform within it.
From the abstract:
"... Void trajectories and velocities are calculated for TEM [transmission electron microscopy] videos. An inaccessible, protected volume for migration near the nanocrystal outer surface is discovered, confining the migration of voids to inner crystal parts. Once surface passivation in the form of organic ligands is removed, void dynamics changes, to enable annealing of the voids and self-healing of the crystal. It is determined that surface ligand protection against void migration is extending several atomic layers below the crystal surface. Modeling based on these results predict equilibrium positions for the voids, which are discovered in the data. The study suggests that tuning of organic ligand density influences structural stability and crystal defect tolerance in double perovskites. ..."
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