# Stress effects on vibrational spectra of a cubic hybrid perovskite: A probe of local strain

## TL;DR

This study investigates how uniaxial strain affects vibrational spectra of methylammonium lead iodide perovskite, identifying key vibrational modes for local strain mapping to enhance understanding of strain effects on optoelectronic properties.

## Contribution

The paper provides a detailed DFT analysis of strain-induced vibrational frequency shifts and calibration curves for methylammonium lead iodide, aiding local strain mapping in hybrid perovskites.

## Key findings

- Identified vibrational modes suitable for strain mapping (86, 97, 1457, 1537 cm$^{-1}$).
- Provided calibration curves linking vibrational frequency shifts to strain.
- Analyzed structural changes and mode Gr"uneisen parameters related to strain effects.

## Abstract

Inhomogeneous strain may develop in hybrid organic metal-halide perovskite thin films due to thermal expansion mismatch with a fabrication substrate, polycrystallinity or even light soaking. Measuring these spatially varying strains is difficult but of prime importance for understanding the effects on carrier mobility, non-radiative recombination, degradation and other optoelectronic properties. Local strain can be mapped using the shifts in vibrational frequencies using Raman or infrared microscopy. We use density functional theory to investigate the effect of uniaxial strain on the vibrations of pseudo-cubic methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$), and identify the vibrational modes most favorable for local strain mapping (86 cm$^{-1}$, 97 cm$^{-1}$, 1457 cm$^{-1}$, and 1537 cm$^{-1}$) and provide calibration curves. We explain the origin of the frequency changes with strain using dynamical matrix and mode eigenvector analysis and study strain-induced structural changes. We also calculate mode Gr\"uneisen parameters, giving information about anharmonicity and anisotropic negative thermal expansion as recently reported for other phases. Our results provide a basis for strain mapping in hybrid perovskites to further the understanding and control of strain, and improve stability and photovoltaic performance.