Stress effects on vibrational spectra of cubic hybrid perovskite: A
probe of local strain
- Author(s): Talit, Kuntal
- Strubbe, David A
- et al.
Strain plays an important role in semiconductor performance and stability. Hybrid organic metal-halide perovskites are made in thin films and inhomogeneous residual strain may develop in the material due to thermal expansion mismatch with the substrate, polycrystallinity in the thin films or even by light soaking. These strains can affect carrier mobility, non-radiative recombination, degradation, and other optoelectronic properties. Measuring spatially varying strains is difficult but of prime importance for understanding these effects. Vibrational frequencies shift due to strain in a material, and we want to enable use of this phenomenon to map local strain within a perovskite material via Raman spectroscopy, as is done in crystalline silicon. In this work, we have used density functional theory and density functional perturbation theory to investigate the effect of applied strain on the vibrations of pseudo-cubic methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$) via Raman spectroscopy. Small uniaxial strains are applied along the three cubic crystallographic directions ,  and  to determine the frequency changes. Frequency vs strain graphs can be used as a calibration curve to probe local strain in Raman microscopy. We identify the modes most favorable for this application. We analyze the different behaviors vs. strain observed and the relation to mode characters, and structural changes under strain. We also calculate the mode Gr\"uneisen parameters in each direction, giving information about anharmonicity within the crystal. The negative Gr\"uneisen parameter in the c-direction implies an anisotropic negative thermal expansion in that crystal axis, as recently reported for other crystal phases.
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