DOI: https://doi.org/10.29363/nanoge.emlem.2024.007
Publication date: 13th July 2024
Quantum confinement is one of the effects dictating semiconductors’ electronic and optical properties. This effect, regulated by the spatial extension of the semiconductor, has the potential to significantly modulate optical and electronic properties of interest. For instance, quantum confinement effects imposed by the small thickness of 2D nanoplatelets (NPLs) result in enhanced oscillator strength, reduced dielectric screening, and increased exciton binding energy. The relative effects are material-dependent and are modulated by the number of electrons and their orbital occupation in the specific material. Here, we investigate the effects of thickness on the vibrational and electronic structure properties of 2D halide perovskite (Csn+1PbnBr3n+1) nanoplatelets (NPLs) with n = 3, 4, and 5. We report the change in electronic structure as a function of platelet thickness as well as changes in vibrations. Our hypothesis is that the Raman intensities ratio between two-dimensionally dependent modes would vary with the layer thickness of the NPLs due to the different vibrational-induced polarizability over n = 3, 4, and 5. To quantify and understand such an effect, Phonon dispersion, electronic structure and Raman intensities of the vibrational modes at the point are computed for each thickness case using density functional perturbation theory (DFPT) and density functional theory (DFT). For this task, we have built slab models from the tetragonal CsPbBr3 phase with a vacuum in the z direction to avoid (as much as possible) the interaction between periodic images. The changes in relative intensities between the modes are in agreement with our experimental data from Raman spectroscopy, revealing the same trend in intensity change between the Raman active modes upon confinement. The effect is attributed to the reported change in electronic structure and subsequent polarizability change upon quantum confinement of the material into fewer layers.
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