31.05.2021
An international team led by Empa and ETH researchers is playing with 3D building blocks in the nano range that are up to 100 times larger than atoms and ions. And although completely different, much weaker forces act between these nano “Lego bricks” than those that hold atoms and ions together, they form crystals of their own accord, the structures of which resemble natural minerals.
These new megacrystals or superlattices, as seen on the cover of the latest issue of the science magazine “Nature”, show unique properties like superfluorescence – and could herald a new era in materials science.
In order to understand exactly what the research team led by Maksym Kovalenko and Maryna Bodnarchuk succeeded in, it is best to start with something everyday: anyone who has ever had to spice up an overly bland lunch knows table salt crystals. Sodium chloride – chemically NaCl – is the name of the helpful chemical; it consists of positively charged sodium ions and negatively charged chloride ions. The ions can be thought of as small spheres that attract each other strongly and form tightly packed, hard crystals, as we can see them in the salt shaker.
Many naturally occurring minerals consist of ions – positive metal ions and negative counterions, which, depending on their size, are arranged in different crystal lattice structures. There are also structures such as diamond and silicon: These crystals consist of only one type of atom – in the case of diamond, carbon – but here too the individual atoms are held together by strong bonding forces, similar to the ions in minerals.
What if you could just turn off all these strong bonding forces between the atoms? In the realm of atoms, with all the quantum mechanics involved, this would not result in a molecule or solid, at least not in ambient conditions. “But modern chemistry can produce alternative building blocks that can actually have very different interactions than those between atoms,” says Maksym Kovalenko, Empa researcher and professor of chemistry at ETH Zurich.
“They can be as hard as billiard balls, in the sense that they only perceive each other when they collide. Or they can be softer on the surface, like tennis balls. In addition, they can be made in many different shapes, not just as balls, but also as cubes or other polyhedra, or even in the form of anisotropic, irregular shapes. “
Such building blocks consist of hundreds or thousands of atoms and are known as inorganic nanocrystals. Kovalenko’s team of chemists from Empa and ETH Zurich is able to synthesize them in large quantities and very homogeneously. Kovalenko, Bodnarchuk and some of their colleagues around the world have been working with such building blocks for around 20 years. The scientists call them “Lego materials” because they form dense, spacious lattice structures, so-called super lattices.
It has long been speculated that completely new supramolecular structures can be created by mixing different types of nanocrystals. It was expected that the electronic, optical, or magnetic properties of such multicomponent assemblies would be a mixture of the properties of the individual components.
Initially, the researchers concentrated on mixing spheres of different sizes, which led to dozens of different superlattices with structures that are similar to common crystal structures, such as common salt – but with crystal unit cells ten to 100 times larger.
The team around Kovalenko and Bodnarchuk have now succeeded in significantly expanding their knowledge with their latest work in “Nature”: They set out to examine a mixture of different shapes – initially spheres and cubes. This apparently simple deviation from the previous experimental arrangements immediately led to completely different observations. The cube-shaped nanocrystals, colloidal cesium lead halide perovskite nanocrystals, have been considered to be some of the brightest light emitters ever since they were first produced by the same research team around six years ago.
The superlattices now produced are not only unique in terms of their structure, but also in terms of some of their properties. In particular, they show superfluorescence – that is, they emit light collectively and much faster than the same nanocrystals can do in their conventional state, as a liquid or as a powder.
Entropy as an ordering force?
When balls and cubes are mixed, miraculous things happen: the nanocrystals arrange themselves into structures that are familiar from the world of minerals such as perovskite or rock salt. However, the new structures are 100 times larger than their counterparts in conventional crystals. And what’s more: a perovskite-like structure had never been observed before in the arrangement of such non-interacting nanocrystals.
Particularly curious: These highly ordered structures are created solely by the power of entropy – that is, the eternal endeavor of nature to cause maximum disorder. What irony of the laws of nature! This paradoxical behavior arises because the particles tend to use the space around them as efficiently as possible during crystal formation in order to maximize their freedom of movement in the later phases of solvent evaporation, shortly before they are “fixed” in their later crystal lattice position.
In this regard, the shape of the individual nanocrystals plays a decisive role – soft perovskite cubes allow a much closer packing than that which can be achieved in mixtures only from spheres. The power of entropy ensures that the nanocrystals are always arranged in the closest possible packing – provided that the surface of the crystallites is designed in such a way that they do not attract or repel one another, for example through electrostatic forces.
Departure into a new science
“Our experiments have shown that we can produce new structures with high reliability,” says Maksym Kovalenko. “And that raises many more questions, we are still at the very beginning: What physical properties do such weakly bound superlattices have, and how is their structure related to the observed properties?
Can we use them for certain technical applications, for example for optical quantum computers or in quantum imaging? According to which mathematical laws are they formed? Are they really thermodynamically stable or just trapped in a kinetic barrier? “Kovalenko is now looking for theorists who might be able to predict what else could happen.
“At some point we will discover completely new classes of crystals,” he suspects, “those for which there are no natural models. They then have to be measured, classified and described.” Now that he has opened the first chapter in the textbook for a new type of chemistry, Kovalenko and his team want to ensure that things move forward quickly: “We are currently experimenting with disk-shaped and cylindrical nanocrystallites. And we are very excited about the new structures that we’ll soon see, “he smiles.
Those: Federal Materials Testing and Research Institute (EMPA)
