Symmetry-driven Phonon Confinement in 2D CsPbBr3 Nanoplatelets, 2-5 Monolayers Thick, Reshapes Vibrational Landscapes

The behaviour of atomic vibrations, known as phonons, fundamentally influences the properties of materials, yet their relationship to structural confinement in layered semiconductors remains poorly understood. Mustafa Mahmoud Aboulsaad, Olivier Donzel-Gargand, Rafael B. Araujo, and Tomas Edvinsson, all from Uppsala University, now demonstrate how confinement dramatically reshapes these vibrations in two-dimensional halide perovskites. The team synthesised atomically thin crystals and used advanced spectroscopic techniques combined with theoretical modelling to reveal a striking connection between crystal symmetry and phonon behaviour. Their work establishes a framework for understanding how phonons become localised within these layered materials, and importantly, provides a non-destructive method for determining crystal thickness using Raman spectroscopy, paving the way for improved design of low-dimensional optoelectronic devices.

Confinement significantly reshapes electronic states and reorganizes the vibrational landscape of low-dimensional semiconductors. In halide perovskites, however, the role of confinement in governing symmetry effects on vibrational modes remained unresolved. Researchers synthesized 2D CsPbBr3 nanoplatelets with atomically defined thicknesses ranging from 2 to 5 monolayers and performed detailed analysis of their optical and vibrational properties. The lowest dimensional structure, consisting of 2 monolayers, revealed a co-existence of cubic and orthorhombic structures, which energetically converged to orthorhombic for 3 monolayers and beyond. Through polarization-resolved Raman spectroscopy and first-principles calculations, scientists investigated the structural and vibrational properties of these nanoplatelets.

Cesium Perovskite Nanocrystal Structure and Morphology

This research centers on the investigation of Cesium Lead Halide Perovskite Nanocrystals, specifically CsPbBr3, and explores their structural properties, optical characteristics, and stability. Scientists employed a range of techniques including transmission electron microscopy to visualize the nanocrystals’ structure, X-ray diffraction to determine their crystal structure, and Raman spectroscopy to characterize vibrational modes and understand the relationship between structure and optical properties. Alongside experimental work, scientists utilized ab initio calculations, including Density Functional Theory, to complement their results, predict structural properties, calculate vibrational modes, and understand the electronic structure of the materials. This comprehensive approach allowed for a detailed understanding of the nanocrystals’ behavior.

Thickness Drives Raman Mode Symmetry Changes

Scientists synthesized CsPbBr3 nanoplatelets with thicknesses ranging from 2 to 5 monolayers, alongside bulk-like nanocrystals as references, and meticulously characterized their structural and optical properties. X-ray diffraction and electron microscopy confirmed the formation of orthorhombic frameworks with discrete layer counts, while optical absorption and photoluminescence revealed the expected blue shift and linewidth changes associated with quantum confinement. Building on this foundation, the team employed polarization-resolved Raman spectroscopy, coupled with first-principles calculations, to investigate how vibrational symmetry evolves with decreasing thickness. Experiments revealed a striking thickness-dependent enhancement of B1g Raman modes relative to Ag modes, a trend not predicted by simple confinement models.

B1g mode behavior aligned with phonon-confinement expectations, exhibiting red shifts and asymmetric low-frequency tails as confinement increased, while Ag mode trends deviated from this pattern. Detailed eigenvector analyses clarified this dichotomy, demonstrating that B1g vibrations largely reside within the xy-plane, with Pb, Br, Pb units connecting octahedra and increasing lattice dynamics with accumulating inner layers. Conversely, Ag vibrations couple to out-of-plane distortions and remain susceptible to surface disorder and finite-size effects. This breakthrough delivers a calibrated, non-destructive method for determining nanoplatelet thickness using Raman spectroscopy.

Polarization-resolved measurements disentangle Ag and B1g channels, quantify interlayer coupling and surface localization, and yield robust observables, notably the B1g/Ag intensity ratio and a lineshape asymmetry parameter. These symmetry-resolved metrics establish Raman spectroscopy as a precise tool for tracking thickness-induced confinement in halide perovskite nanoplatelets, linking structure to optical response and providing a new pathway for controlling phonons, excitons, and their interactions in low-dimensional optoelectronic materials. Measurements confirm that the B1g/Ag intensity ratio provides a reliable thickness readout across the 2 to 5 monolayer range.

Confinement Dictates Perovskite Nanoplatelet Vibrations

This research establishes a detailed understanding of how structural confinement impacts vibrational modes in two-dimensional halide perovskite nanoplatelets. Scientists synthesized atomically thin crystals of CsPbBr3, ranging from two to five monolayers, and meticulously analyzed their structural and vibrational properties using a combination of spectroscopic techniques and theoretical calculations. The team discovered a striking symmetry-driven dichotomy in phonon behavior, where certain vibrational modes intensify with confinement as predicted by existing models, while others deviate due to their unique spatial localization. These findings reveal that the vibrational landscape of these materials is intricately linked to their crystal structure and the degree of confinement, providing a framework for understanding phonon localization in layered perovskites. Importantly, the researchers established a non-destructive method for determining nanoplatelet thickness based on Raman spectroscopy, utilizing the intensity ratio of specific vibrational modes. Extending this work to iodine-based perovskites confirmed the generality of these trends and validated the model’s applicability to a broader range of layered materials.