Ultra-Thin Perovskites Reveal Surprising Atomic Behavior in New Study
Scientists at Uppsala University have uncovered how extreme thinness alters the atomic vibrations in two-dimensional halide perovskites. Their work focused on synthesising ultra-thin crystals—just a few atomic layers thick—and analysing their unique properties. The findings reveal unexpected shifts in material behavior when confined to such tiny scales.
The research team, including Long Zhang, Weihua Lin, and Feng Liu, produced CsPbBr₃ nanoplatelets with precise thicknesses of 2 to 5 monolayers. These atomically thin structures allowed them to study how confinement affects both optical and vibrational characteristics. Polarisation-resolved Raman spectroscopy, combined with first-principles calculations, became key tools in their investigation.
In the thinnest samples—just two monolayers—a mix of cubic and orthorhombic crystal structures appeared. But as soon as a third layer was added, the structure stabilised into a purely orthorhombic form. This shift highlighted how delicate changes in thickness can dictate material symmetry.
The experiments also exposed a surprising trend: the intensity of B1g Raman modes grew relative to Ag modes as the nanoplatelets thickened. This behavior defied predictions from simple confinement theories, pointing to more complex underlying physics. By tracking the symmetric stretching of Pb-Br bonds, the team established a reliable way to measure thickness—using the B1g/Ag intensity ratio as a non-destructive indicator.
The study clarifies how confinement reorganises phonons, the atomic vibrations that shape a material’s thermal and electronic properties. Until now, the exact role of symmetry in these vibrations had remained unclear. The researchers’ combination of advanced spectroscopy and theoretical modelling has now filled that gap.
The work provides a practical method for determining nanoplatelet thickness without damaging the samples. It also deepens the understanding of how confinement reshapes vibrations in low-dimensional semiconductors. These insights could guide the design of future perovskite-based devices, from solar cells to optoelectronics.
