Semiconductor nanoparticles can emit light or act as sensors, and their properties can be tailored to specific purposes. One particle alone is often not enough, but in an ordered material composed of many nanoparticles, the properties are amplified. A research team has used MAX IV to show that such ordered materials can be formed from low-cost, versatile perovskite semiconductor nanoparticles.
Cesium lead tribromide is a semiconductor that belongs to the so-called perovskite class of materials. Perovskites are grouped together due to their similar crystal structure and composition. They currently receive a lot of scientific attention as a cheap, abundant, and easy-to-make alternative to more conventional semiconductors. One particular scientific focus is perovskite materials in nanoparticle form. A well-known form of semiconductor nanoparticles is quantum dots, and a Nobel Prize for the discovery of quantum dots was awarded in 2023.
“Perovskite nanoparticles are attractive as semiconductor building blocks with highly tunable optical and electronic properties. For example, they are very bright light emitters, which makes them promising for LEDs and lasers, scintillators, and sensors, among other applications,” says Dmitry Baranov from Lund University.
The nanoparticles can also be ordered like the atoms in a solid crystal into a so-called artificial solid.
“Artificial solids made of perovskite nanoparticles are interesting because they resemble atomic crystals, but on a much larger and more designable length scale. This gives chemists a way to engineer the properties arising from many ordered particles working together. We can tune the properties by nanoparticle size, surface chemistry, and packing,” says Dmitry.
The research team have used MAX IV to study cesium lead tribromide nanoparticles with different molecules, so-called ligands, covering their surfaces to aid ordering. The team shows that variations in size and ligands change the degree of ordering and that even when the particles are not perfectly ordered, the disorder follows a certain model. The model predicts how an out-of-place particle will affect its neighbours. When developing the model, the researchers were inspired by wave-like deformations in other crystalline systems, such as liquid crystals, steel, and other metal alloys.
“We found that disorder in perovskite nanocrystal superlattices is not the same in all directions and random, as previously assumed, but instead follows directional patterns that depend on interactions with other particles. A simple model based on wavelike, so-called sinusoidal, interactions captured these structural distortions remarkably well”, says Dmitry.
The study is not only an important step towards harnessing the properties of perovskite artificial solids, but also contributes important knowledge about disorder in materials in general.
“Disorder strongly shapes how materials function. For instance, window glass is much more disordered than quartz, which leads to very different properties. Understanding disorder, especially in future materials such as artificial solids, is essential for designing them with predictable optical, electronic, and thermal properties,” says Dmitry.
