[Paper Review] Halide perovskites under polarized light: Vibrational symmetry analysis using polarized Raman

Rituraj Sharma, Matan Menahem+10 more|arXiv (Cornell University)|Dec 1, 2019

Perovskite Materials and Applications54 references30 citations

TL;DR

This study combines polarized Raman spectroscopy with density functional perturbation theory (DFPT) to unambiguously assign vibrational mode symmetries in low-temperature orthorhombic methylammonium lead iodide (MAPI). By using linear and circularly polarized light excitation and iterative analysis of polarization-dependent intensity, the authors determine the crystal orientation and assign all Raman-active modes at 10 K, resolving long-standing challenges in mode symmetry assignment due to birefringence and complex lattice dynamics.

ABSTRACT

In the last decade, hybrid organic-inorganic halide perovskites have emerged as a new type of semiconductor for photovoltaics and other optoelectronic applications. Unlike standard, tetrahedrally bonded semiconductors (e.g. Si and GaAs), the ionic thermal fluctuations in the halide perovskites (i.e. structural dynamics) are strongly coupled to the electronic dynamics. Therefore, it is crucial to obtain accurate and detailed knowledge about the nature of atomic motions within the crystal. This has proved to be challenging due to low thermal stability and the complex, temperature dependent structural phase sequence of the halide perovskites. Here, these challenges are overcome and a detailed analysis of the mode symmetries is provided in the low-temperature orthorhombic phase of methylammonium-lead iodide. Raman measurements using linearly- and circularly- polarized light at 1.16 eV excitation are combined with density functional perturbation theory (DFPT). By performing an iterative analysis of Raman polarization-orientation dependence and DFPT mode analysis, the crystal orientation is determined. Subsequently, accounting for birefringence effects detected using circularly polarized light excitation, the symmetries of all the observed Raman-active modes at 10 K are assigned.

Motivation & Objective

• To overcome the challenges in assigning vibrational mode symmetries in halide perovskites due to low thermal stability and complex phase transitions.
• To determine the crystallographic orientation of MAPI single crystals at 10 K, where XRD at room temperature is insufficient due to symmetry breaking.
• To resolve the effects of birefringence on Raman signals using circularly polarized light excitation.
• To unambiguously assign the symmetries of all Raman-active modes in the orthorhombic phase of MAPI through combined experimental and theoretical analysis.
• To establish a robust framework for vibrational mode assignment in complex perovskite systems using polarization-dependent Raman and first-principles calculations.

Proposed method

• Conducting polarization-orientation (PO) Raman scattering measurements using linearly polarized 1.16 eV laser excitation at 10 K on MAPI single crystals.
• Measuring angular dependence of Raman intensity in parallel and cross-polarization configurations to extract Raman tensor components via I(θ) ∝ |es(θ)RT RRei(θ)|².
• Performing density functional perturbation theory (DFPT) calculations on a √2×2×√2 supercell with LDA and Grimme-d2 corrections to compute harmonic Raman tensors.
• Using iterative comparison between experimental PO data and theoretical Raman tensor predictions to determine crystal orientation and mode symmetries.
• Applying circularly polarized light excitation to detect and account for birefringence effects that distort polarization-dependent signals.
• Fitting experimental angular intensity data to a model based on Raman tensor symmetry and rotation matrices, with normalization to the largest tensor component.

Experimental results

Research questions

• RQ1 How can the crystallographic orientation of low-temperature MAPI be unambiguously determined when conventional XRD fails due to phase transition-induced symmetry breaking?
• RQ2 What is the role of birefringence in distorting polarization-dependent Raman intensity measurements, and how can it be experimentally corrected?
• RQ3 Which irreducible representations correspond to the observed Raman-active modes in the orthorhombic phase of MAPI at 10 K?
• RQ4 Can the Raman tensor components for each mode be quantitatively assigned using a combined experimental-theoretical approach?
• RQ5 Why does the mode at 98.7 cm⁻¹ show a discrepancy between experimental data and theoretical modeling despite correct symmetry assignment?

Key findings

• The crystal orientation of MAPI at 10 K was successfully determined as (010) and (110) through iterative comparison of PO Raman data with DFPT predictions.
• All 18 Raman-active modes in the orthorhombic phase (Γ = 5Ag + 4B1g + 5B2g + 4B3g) were unambiguously assigned to specific symmetries using polarization-dependent intensity analysis.
• The mode at 98.7 cm⁻¹, assigned to B3g symmetry, showed a 90° periodicity in intensity and exhibited a mismatch with the theoretical model, likely due to peak overlap with background.
• Birefringence effects were experimentally detected and corrected using circularly polarized light, which revealed deviations in polarization response not accounted for by standard models.
• The Raman tensor components were normalized and their relative ratios were determined, with the sign of components deduced by combining experimental data with theoretical DFPT tensors.
• The agreement between experimental and theoretical angular dependence for most modes (e.g., 26.9 cm⁻¹, 33.1 cm⁻¹, 42.6 cm⁻¹) confirmed the validity of the combined approach for mode assignment in complex perovskites.