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Nanocrystals to harness solar energy

Perovskites were first discovered in Russia in the first half of the 19th century and named after the Russian mineralogist L. A. Perovski. Nowadays, the term has been expanded to include a great variety of synthetic minerals whose structure is similar to the natural mineral, which consists of calcium titanium oxide. Perovskites have the formula ABX3 where A and B represent cations which are linked by an anion X. Perovskites can be generated from many different elements which make them a suitable material for a great variety of applications, such as ultrasound devices, computer chips and also solar cells. Especially the latter have been the focus of increased scientific attention lately, as they form a main pillar of the energy transition.

Now (2021), scientists at ETH Zürich have gained greater insights into perovskite engineering and found that combining nanocubes of perovskites with nanospheres of other materials could bring the nanostructures to form a 3D superlattice that had the same sort of arrangement as the ionic lattice in common perovskites. Also, the high degree of orientational order of the nanocubes in the superlattice caused a phenomenon called superfluorescence, which is a collective emission of photon bursts.

Artificial perovskite-like superlattices can be designed by combining different types of nanocrystals. In this case, the scientists formed binary superlattices out of nanocubes of the perovskite CsPbBr3 (Cs, caesium; Pb, lead; Br, bromine) with spherical nanocrystals of iron(ii,iii) oxide (Fe3O4; Fe, iron) or of NdGdF4 (Nd, neodymium; Gd, gadolinium; F, fluorine). Moreover, when they added truncated-cuboid lead sulfide (PbS) nanoparticles they obtained ternary superlattices. The nanocubes in these superlattices took lattice positions corresponding to the sites of the oxide ions in a common perovskite crystal lattice: the spherical nanocrystals took up the A sites whereas the PbS nanocrystals took up the B sites.

The research highlighted the fact that perovskite-like superlattices can be created with the help of only two types of nanocrystal. The authors also found that in systems consisting of CsPbBr3 nanocubes and Fe3O4 nanospheres, minor changes in the sizes and fractions of the two types of nanocrystal could transform the resulting superlattice. High-resolution transmission electron microscopy and electron diffraction studies of these binary superlattices proved that the orientations of the nanocubes were highly ordered.

The final stage of superlattice engineering included adjusting the relative sizes of the nanocrystals in the ternary system consisting of CsPbBr3 nanocubes, Fe3O4 nanospheres and truncated PbS nanocubes. The authors observed that the nanocrystals in the superlattice could take up approximately 92% of the available space if packed tightly.

Superlattice engineering has a relatively long history of research. In 1995, for example, for the first scientists time facilitated nanocrystallites to organise into three-dimensional semiconductor quantum dot superlattices (colloidal crystals). This achievement was the result of advances in synthetic engineering creating CdSe nanocrystallites with monodisperse properties within the limit of atomic roughness.

Since then, scientists have been able to customise the shapes and sizes of a large variety of nanocrystals and to influence their interactions and environments. In 2018, scientists created caesium lead halide (CsPbX3, X = Cl, Br) perovskite nanocrystals that could form highly ordered three-dimensional superlattices and showed key elements of superfluorescence, taking the shape of dynamically red-shifted emissions with accelerated radiative decay, photon bunching, and delayed emission pulses.

This new approach to perovskite engineering using superlattices might lead to improved energy systems in a great variety of technical fields as engineered solutions enable customising nanocrystal shapes and sizes, and also impacting how they react to their environments. Also, the ordered organisation of nanocrystals can create new or improved properties, including enhanced electron transport, catalytic activity, or light emission and absorption, as opposed to nanocrystals not embedded in superlattices.

In the next step of research, the scientists will focus their attention on expanding the family of superlattices. So far, binary superlattices with a columnar structure as well as such having an arrangement similar to the crystal structure of aluminium diboride – sheets of boron atoms interspersed with layers of aluminium atoms – have been created. In future the knowledge gained by the scientists at ETH Zürich could lead to the production of materials with various functionalities depending on the spatial arrangement and distance between the nanocrystals.