Symmetry-driven phonon confinement in 2D halide perovskites

TL;DR

This study reveals how symmetry influences phonon confinement in 2D halide perovskites, providing insights into vibrational modes, and introduces Raman spectroscopy as a non-destructive method for nanoplatelet thickness measurement.

Contribution

It demonstrates the symmetry-driven behavior of phonons in 2D halide perovskites and establishes Raman fingerprints as a novel, non-invasive thickness metrology tool.

Findings

B1g modes intensify with phonon confinement

A1g modes are sensitive to surface disorder and size effects

Raman intensity ratio correlates with nanoplatelet thickness

Abstract

Quantum confinement not only reshapes electronic states but also reorganizes the vibrational landscape of low-dimensional semiconductors. In halide perovskites, however, the role of confinement in governing symmetry effects on vibrational modes has remained unresolved. Here we synthesize 2D CsPbBr3 nanoplatelets with atomically defined thicknesses for 2-5 monolayers (MLs) and perform exciton absorption and emission analysis, crystalline phase determination, and phonon analysis. The lowest dimensional structure (2 MLs) reveal a co-existence of cubic and orthorhombic structure, energetically converging to orthorhombic for 3 MLs and beyond. Through polarization-resolved Raman spectroscopy and first-principles theory for 2-5 MLs, a striking symmetry contrast is found: B1g modes intensify and evolve in line with the phonon-confinement model, while Ag modes deviate, reflecting their distinct spatial localization. First principles calculations show that B1g vibrational modes largely reside in the xy-plane, Pb-Br-Pb units connect octahedra along the xy-direction with increased lattice dynamics as inner layers accumulate, whereas A1g vibrations couple to out-of-plane distortions and remain susceptible to surface disorder and finite-size effects. This symmetry-driven dichotomy provides a general framework for understanding phonon localization in layered halide perovskites. Beyond mechanism, we establish Raman fingerprints, particularly the A1g/B1g intensity ratio in cross-polarized geometry, as a calibrated, non-destructive metrology for 2D nanoplatelet thickness through 2-5 MLs. These results bridge electronic and phonon confinement and highlight symmetry engineering as a route to understand and control phonons, excitons, and their interactions in low-dimensional optoelectronic materials.