Confined acoustic phonons in CsPbI3 nanocrystals explored by resonant Raman scattering on excitons

TL;DR

This study investigates how confined acoustic and optical phonons interact with excitons in CsPbI3 nanocrystals, revealing size-dependent phonon energies and nanocrystal shape influences through resonant Raman scattering and theoretical modeling.

Contribution

It provides a combined experimental and theoretical analysis of exciton-phonon interactions in CsPbI3 nanocrystals, including phonon spectra modeling for various shapes and sizes.

Findings

Optical phonon energies are size-independent.

Confined acoustic phonon energies increase as nanocrystals become smaller.

Nanocrystals are predominantly spherical or spheroidal in shape.

Abstract

Optical properties of the lead halide perovskites nanocrystals are controlled by confined excitons and rich spectrum of confined acoustic and optical phonons. We study experimentally and theoretically the exciton-phonon interaction in CsPbI3 perovskite nanocrystals embedded in a glass matrix. Energies of phonon modes allowed by selection rules are detected by resonant Raman scattering for nanocrystals with sizes of 4-13 nm, covering exciton energies of 1.72-2.25 eV. While optical phonon energies remain size-independent, the energies of confined acoustic phonons increase in smaller nanocrystals. Acoustic phonons are modeled within the continuum approximation using elastic constants computed by density functional theory. The model provides the energy spectra of confined phonons for nanocrystals of various shapes (cube, sphere, spheroid), crystal symmetries (orthorhombic and tetragonal), and sizes. Exciton confinement restricts efficient coupling to only few phonon modes observable in Raman spectra. By comparing experimental data with model predictions, we conclude that the nanocrystals in our samples predominantly have spherical or spheroidal shapes.