Interplay of Phonon Directionality and Emission Polarization in Two-Dimensional Layered Metal Halide Perovskites

Abstract

ConspectusLayered metal halide perovskites represent a natural quantum well system for charge carriers that provides rich physics, and the organic encapsulation of the inorganic metal halide layers not only increases their stability in devices but also provides an immense freedom to design their functionality. Intriguingly, these organic moieties strongly impact the optical, electrical, and mechanical properties, not only through their dielectric, elastic, and chemical properties but also because of induced mechanical distortions in the inorganic lattice. This tunability makes two-dimensional layered perovskites (2DLPs) highly attractive as light emitters. Common consensus is that exciton-phonon coupling plays an important role in radiative recombination. For bulk and some two-dimensional (2D) materials, the band edge emission broadening can be described by the classic models for polar inorganic semiconductors, while for the temperature dependence of the self-trapped exciton emission, an analysis developed for color centers has been successfully applied. For many 2DLPs these approaches do not work because of the complexity of their vibrational spectra. However, their emission is still strongly determined by phonons, and therefore, an adequate understanding of the electron-phonon coupling needs to be developed.With polarized and angle-resolved Raman spectroscopy studies on single 2DLP flakes based on different ammonium molecules as organic cations, in 2020 we revealed very rich phonon spectra in the low-frequency regime. Although the phonon bands at low frequency can generally be attributed to the vibrations of the inorganic lattice, we found very different responses by only changing the type of organic cations. In addition, the intensity of the different phonon modes depended strongly on the angle of the linearly polarized excitation beam with respect to the in-plane axes of the octahedron lattice. In 2022, we mapped this angular dependence of the phonon modes, which allowed identification of the directionality of the different lattice vibrations. By correlating the phonon spectra with the temperature-dependent emission for a set of 2DLPs that featured very different self-trapped exciton (STE) emission, we demonstrated that the exciton relaxation cannot be related to coupling with a single (longitudinal-optical) phonon band and that several phonon bands should be involved in the emission process. To gain insights into the exciton-phonon coupling effects on the band edge emission, we performed both angle-resolved polarized emission and Raman spectroscopy on single 2D lead iodide perovskite microcrystals. These experiments revealed the impact of the organic cations on the linear polarization of the emission and corroborated that multiple phonon bands should be involved in the radiative recombination process. Analysis of the temperature-dependent line width broadening of the band edge emission showed that for many systems, the behavior cannot be described by assuming the involvement of only one phonon mode in the electron-phonon coupling process. Our studies revealed a wealth of highly directional low-frequency phonons in 2DLPs from which several bands are involved in the emission process, which leads to diverse optical and vibrational properties depending on the type of organic cation in the material.

Figure 1

Two-dimensional metal halide perovskites: architecture and typical emission and vibration spectra. (A) Schematic illustration of the organic–inorganic layered structure that results in a periodic lattice of quantum wells in the vertical direction. (B, C) Emission and Raman spectra recorded at cryogenic temperatures from single exfoliated (UDA)₂PbBr₄ flakes. Adapted from ref (2). CC BY-NC 4.0.

Figure 2

Emission tunability of 2DLP ensembles by stress-induced alignment of the microcrystals. (A, B) Change of the photoluminescence (PL) spectrum under compression of the ensemble by applying pressures in the MPa range. (C) Calculated transmittance of the microcrystal ensemble with and without compression. (D) Simulated absorption along the crystallographic directions, where ϵ_aa, ϵ_bb, and ϵ_cc are the diagonal elements of the dielectric tensor, and a, b, and c are the crystallographic axes of the structure with the octahedron lattice in the ab plane. Reproduced with permission from ref (53). Copyright 2018 Wiley-VCH. (E) Illustration of the stretchable PDMS film loaded with 2DLP microcrystals and emission spectra for various cycles under 5% and 70% stretch. Reproduced from ref (57). CC BY-NC 3.0.

Figure 3

Exciton and phonon bands of 2D lead bromide perovskites with different organic phases. (A, B) 2D perovskite powders with organic cations that have different amine bonding groups under (top) daylight and (bottom) UV light, showing different colors (A) and emission spectra (B). Reproduced from ref (43). CC BY 4.0. (C) Emission and absorption spectra recorded from powders of 2D perovskites with different primary ammonium cations. Reproduced from ref (58). CC BY-NC 3.0. (D) Raman spectra of 2D perovskites with different primary ammonium cations recorded at cryogenic temperatures, where phonon bands attributed to similar vibration modes are highlighted by the shaded background. Reproduced from ref (1). Copyright 2020 American Chemical Society.

Figure 4

Exfoliation of single 2D perovskite flakes and the directional dependence of the optical and vibrational spectra. (A) Optical and (B) atomic force microscopy images of exfoliated flakes, where the height profiles along the green, pink, and blue dashed lines reveal thicknesses of 20, 14, and 10 nm, respectively. (C) PL spectra recorded from powders and exfoliated crystals. Reproduced from ref (58). CC BY-NC 3.0. (D, E) Raman spectra recorded from a single flake with horizontal (D) or diagonal (E) orientation with respect to the detection optics. The black, blue, and red colors refer to unpolarized, polarized, and depolarized configurations. The blue dots indicate the measurement spot. Reproduced from ref (1). Copyright 2020 American Chemical Society.

Figure 5

Angle-resolved polarized Raman spectroscopy on single flakes. (A, B) Illustration of the relative orientation of the perovskite octahedron lattice with respect to the polarizers in the excitation/detection paths (V, H) and of the rotation of the linear polarization with respect to the sample. Schemes are superimposed on an optical microscopy image of a single microcrystal. (C, D) Color map of angle-resolved Raman spectra recorded in polarized (VV) and depolarized (HV) configurations. Reproduced from ref (2). CC BY-NC 4.0. (E) Relation of the Raman tensor to the angular mode intensities, resulting in isotropic, bipolar, or quadrupolar modes. Reproduced with permission from ref (3). Copyright 2022 Wiley-VCH.

Figure 6

Assignment of the directional behavior of the phonon modes detected by Raman spectroscopy from single flakes. (A–C) Raman spectra recorded from lead bromide 2D perovskites at cryogenic temperature. The directional properties color-coded in (A) and (B) and illustrated in (C) were extracted from angular intensity mapping in polarized and depolarized configurations. Reproduced from ref (2). CC BY-NC 4.0. (D) Angle-resolved Raman data from 2D double perovskites and classification of the modes with respect to the symmetries of the Raman tensor. Reproduced with permission from ref (3). Copyright 2022 Wiley-VCH.

Figure 7

Correlation of PL and Raman spectra recorded from the same lead iodide microcrystal. Insets show optical microscopy images of the investigated flakes, and the yellow dots indicate the measurement spot. (A, C) Angle-resolved PL spectra from single exfoliated (BA)₂PbI₄ and (UDA)₂PbI₄ microcrystals. Spectra at the angles indicated by the green (blue) dashed lines are shown in the top (bottom) panels. (B, D) Angle dependence of the phonon bands measured by resonant (B) and nonresonant (D) Raman spectroscopy. The bottom (top) panels show spectra recorded at the angles indicated by the red (white) dashed lines. Adapted from ref (4).

Figure 8

Temperature dependence of the band edge emission. (A) (a) PL recorded from 2D lead bromide microcrystal powders with BA, UDA, and N-MDDA as organic cations at cryogenic temperatures with laser excitation at 375 nm. (b–d) Fitting of the Gaussian line width broadening of the peak indicated by the blue arrow with eq 3. Reproduced from ref (35). CC BY-NC 3.0. (B, C) PL spectra recorded from single lead iodide microcrystals shown on a semilogarithmic scale. Adapted from ref (4).