All-inorganic halide perovskite nanoplatelets (PNPls) have recently gained significant attention in the field of materials science. These materials exhibit remarkable optoelectronic properties, making them promising candidates for applications in solar cells, LEDs, and photodetectors. The interest in PNPls stems from their exceptional electronic and optical properties, which can be tuned over a wide range that are deeply intertwined with their unique structural characteristics. Tuning the properties of PNPls has relied at first on changing of the elemental compositional of the perovskite structure, whether the halide part and/or the 1^st cation from organic to inorganic, or between different organic molecules. Over the last few years, studies have shown that optical tuning of PNPls based on the number of monolayers (MLs) is a viable strategy, where the monolayer corresponds to a single microscopic layer of inorganic metal-halide octahedrons surrounded by large organic cations or ligands. While extensive research has explored the optical properties of these systems, the role of phonons (lattice vibrations) in different dimensionalities remains underexplored. This is important for a fundamental understanding of the collective carrier-phonon coupling in the excited states, thermalization process, initial charge separation and final transport that includes the mobility of electrons and holes and their interconnection to the charge carrier-lattice interactions. Since all the above-mentioned phenomena are dimensionality- and phase-related, where the coupling between the excited state and phonons changes dramatically with both the dimensionality and phase, the vibrational properties over different system dimensionalities and/or crystallographic phases should be revealed for a better understanding of the role of phonon in the materials.

In this work, we employed optical and non-resonant Raman spectroscopy for the realization of, firstly, a model for the vibrational properties of 2-6 MLs of CsPbBr₃ NPls in comparison with larger nanocrystals. Our Raman measurements showed that, by systematically varying the number of monolayers of the nanoplatelets, there is distinct changes in the relative intensities of the vibrational modes that are sensitive to the number of monolayers. These observations can be attributed to the quantum confinement effect, which becomes more pronounced as the thickness of the nanoplatelets decreases. In addition, there is strain generated in the materials upon the formation of lower thickness NPls. This can be identified through the shift of Pb-Br vibrational mode. Secondly, we established a model of the vibrational properties over a wide temperature range to identify changes in phase, strain, and anharmonicity for different system dimensionalities. Raman measurements revealed the tetragonal phase formation at room temperature for all prepared nanocrystal systems is dominant with the orthorhombic and cubic phases at low and high temperatures, respectively. This in addition to the phase related and/or the anharmonicity of the shift of phonon modes with temperature. Furthermore, there were changes in the peak intensities’ ratios, which provide valuable insights into the structural variations induced by the number of monolayers.

Strain, phase transitions, anharmonicity, and their dependence on dimensionality are important for electrical and thermal conductivity. Moreover, it helps to clarify the mechanism and pathways of electronic energy into heat in 2D lead halides. Therefore, these results provide an essential initial overview of the crucial vibrational properties of the system, paving the way for a better understanding of carrier-phonon coupling in these materials. The ability to determine these structural parameters via Raman spectroscopy establishes it as an indispensable characterization technique for rapid and accurate analysis of thickness and confinement regimes in perovskite nanoplatelets and suggests its potential for dimensionality analysis in other nanocrystal families.