All-inorganic two-dimensional (2D) halide perovskite nanoplatelets (PNPls) have recently garnered considerable interest in materials science. These materials exhibit outstanding optoelectronic properties, making them highly promising for applications in LEDs and photodetectors. The appeal of PNPls lies in their exceptional electronic and optical properties, which can be finely tuned across a wide range due to their unique structural characteristics. Initially, tuning the properties of PNPls focused on altering the elemental composition of the perovskite structure, specifically the halide component and/or the primary cation, shifting from organic to inorganic or between various organic molecules. In recent years, extensive research has indicated that another strategy for tuning PNPls' properties is based on the number of monolayers (MLs). A monolayer corresponds to a single microscopic layer of inorganic metal-halide octahedrons surrounded by large organic cations or ligands. While significant research has explored the optical properties of these systems, the role of phonons (lattice vibrations) in different dimensionalities remains underexplored. Additionally, a key scientific goal is a fundamental understanding of the collective carrier-phonon coupling in excited states, the thermalization process, initial charge separation, and final transport, including the mobility of electrons and holes and their relationship to charge carrier-lattice interactions. Since these phenomena are dimensionality- and phase-dependent, where the coupling between the excited state and phonons changes significantly with both dimensionality and phase, it is crucial to investigate the vibrational properties across different system dimensionalities and crystallographic phases to better understand the role of phonons in these materials.

In this work, our main goal is to employ Raman, photoluminescence, UV-Vis spectroscopy for the realization of, firstly, a model for the vibrational properties of 2-6 MLs NPls and larger nanocrystals of oleic acid- and oleylamine-capped CsPbBr₃. 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 2D 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-related 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. Lastly, we investigated the photoluminescence enhancement with different nanoplatelets thicknesses. Photoluminescence measurements revealed that, by changing the number of monolayers from 2-6, there is an enhancement of the photoluminescent intensity exponentially. This is attributed to the increase in the photoactive sites, where the number of the bright excitons increase with increasing the monolayers of the nanoplatelets, considering maintaining all the synthetic parameters the same, such as concentration.

These results provide an essential initial overview of the crucial vibrational and optical properties of the system, paving the way for a better understanding of carrier-phonon coupling, among other phenomena, 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.