Symmetry-Driven Phonon Confinement in 2D Halide Perovskites

Quantum confinement in low-dimensional semiconductors modifies not only electronic states but also the underlying lattice dynamics. In halide perovskite nanoplatelets, however, how confinement influences vibrational behavior alongside structural and optical properties remains an open question. Here, we synthesize CsPbBr₃ nanoplatelets with atomically defined thicknesses from 2 to 5 monolayers and examine their structural, optical, and vibrational responses. Structural characterization combined with photoluminescence spectroscopy and first-principles calculations confirms reliable thickness control and structural stability across the series. Using polarization-resolved Raman spectroscopy, we observe gradual and systematic changes in the intensities of low-frequency lattice vibrational modes as the nanoplatelet thickness increases. First-principles calculations reveal that these trends arise from thickness-dependent changes in the spatial distribution of atomic displacements, with in-plane Pb-Br-Pb bending modes evolving more strongly than modes involving out-of-plane distortions. Similar behavior is found in iodine-based perovskite nanoplatelets, suggesting that the observed trends reflect a general feature of confined halide perovskites. Together, these results show that Raman intensity ratios, particularly in cross-polarized geometries, offer a physically grounded and nondestructive way to track thickness-dependent lattice dynamics in the 2-5 monolayer regime. More broadly, dimensional control provides a pathway to modulate vibrational dynamics and related energy relaxation processes, with implications for 2D materials at interfaces and in miniaturized devices.