Directional Anisotropy of the Vibrational Modes in 2D Layered Perovskites

The vibrational modes in organic/inorganic layered perovskites are of fundamental importance for their optoelectronic properties. The hierarchical architecture of the Ruddlesden-Popper phase of these materials allows for distinct directionality of the vibrational modes withrespect to the main axes of the pseudocubic lattice in the octahedral plane. Here, we study the directionality of the fundamental phonon modes in single exfoliated Ruddlesden-Popper perovskite flakes with polarized Raman spectroscopy at ultralow-frequencies. A wealth of Raman bands is distinguished in the range from 15-150 cm-1 (2-15 meV), whose features depend on the organic cation species, on temperature, and on the direction of the linear polarization of the incident light. By controlling the angle of the linear polarization of the excitation laser with respect to the in-plane axes of the octahedral layer, we gain detailed information on the symmetry of the vibrational modes. The choice of two different organic moieties, phenethylammonium (PEA) and butylammonium (BA) allows to discern the influence of the linker molecules, evidencing strong anisotropy of the vibrations for the (PEA)2PbBr4 samples. Temperature dependent Raman measurements reveal that the broad phonon bands observed at room temperature consist of a series of sharp modes, and that such mode splitting strongly differs for the different organic moieties and vibrational bands.

Abstract. The vibrational modes in organic/inorganic layered perovskites are of fundamental importance for their optoelectronic properties. The hierarchical architecture of the Ruddlesden-Popper phase of these materials allows for distinct directionality of the vibrational modes with respect to the main axes of the pseudocubic lattice in the octahedral plane. Here, we study the directionality of the fundamental phonon modes in single exfoliated Ruddlesden-Popper perovskite flakes with polarized Raman spectroscopy at ultralow-frequencies. A wealth of Raman bands is distinguished in the range from 15-150 cm -1 (2-15 meV), whose features depend on the organic cation species, on temperature, and on the direction of the linear polarization of the incident light. By controlling the angle of the linear polarization of the excitation laser with respect to the in-plane axes of the octahedral layer, we gain detailed information on the symmetry of the vibrational modes. The choice of two different organic moieties, phenethylammonium (PEA) and butylammonium (BA) allows to discern the influence of the linker molecules, evidencing strong anisotropy of the vibrations for the (PEA)2PbBr4 samples. Temperature dependent Raman measurements reveal that the broad phonon bands observed at room temperature consist of a series of sharp modes, and that such mode splitting strongly differs for the different organic moieties and vibrational bands. Softer molecules such as BA result in lower vibrational frequencies and splitting into fewer modes, while more rigid molecules such as PEA lead to higher frequency oscillations and larger number of Raman peaks at low temperature. Interestingly, in distinct bands the number of peaks in the Raman bands is doubled for the rigid PEA compared to the soft BA linkers. Our work shows that the coupling to specific vibrational modes can be controlled by the incident light polarization and choice of the organic moiety, which could be exploited for tailoring excitonphonon interaction, and for optical switching of the optoelectronic properties of such 2D layered materials.
Two-dimensional (2D) Ruddlesden-Popper lead halide perovskites have emerged as a highly versatile material for optoelectronic applications 1, 2 due to their bright emission, 3,4 strong excitonic and dielectric confinement, [5][6][7] and photostability in solar energy harvesting under ambient conditions. [8][9][10][11] This class of 2D layered perovskites can be fabricated using simple and low-cost strategies in the form of atomically thin layers, 12 single crystals, 13 and ensembles. 14,15 They consist of layers of corner-sharing [ % ] '( octahedra that are separated by organic ammonium-based cations (e.g. phenethylammonium (PEA) and butylammonium (BA)), which are too large to fit into a three-dimensional (3D) octahedral structure, 5,16,17 and form a natural superlattice of octahedral planes linked by interdigitated bilayers of the organic moieties. This hierarchical architecture has a variety of highly appealing properties that are different from their 3D framework: the inorganic octahedral layers form a quantum well potential for the electronic carriers, in which the confinement can be tuned by the number n of adjacent octahedral planes, 15,16,18,19 an aspect that is also extremely interesting for fundamental studies; 13, 20, 21 the electron-phonon coupling (and distance) between the inorganic layers can be modified by the choice of the organic moiety; 14,[22][23][24] and the electronic level structure as well as the band gap can be tailored by choice of the halide anion and via lattice deformations. 5,[25][26][27] Furthermore, the presence of long hydrophobic organic moieties intercalated between the octahedral layers protects the layered perovskites from moisture permeation, conferring them structural and functional stability. 6,28,29 The intrinsic structural orientation of the 2D Ruddlesden-Popper perovskites results in distinct optoelectronic and thermal properties due to the extremely different environments that the electrical, optical, and vibrational excitations encounter in-and out of plane of the layers. 20,30 For example, the [ % ] '( layers behave as semiconductors, while the organic spacer is insulating. 31 Such structural anisotropy has also mechanical advantages: the photoluminescence of macroscopic stacks of 2D layered perovskites can be tuned by moderate external pressures due to the related anisotropy of the transition dipole moments, 32 and the layered structure allows for mechanical exfoliation of single flakes with a defined number of octahedral layers, and their transfer on suitable substrates. 14 The organic/inorganic bilayer structure behaves mechanically as an organic composite material reinforced by the octahedral layers, with out-of-plane weak bonds (van-der-Waals) and in-plane strong bonds (covalent or ionic), generating rigid cages when increasing the number of inorganic layers. 1,[33][34][35] Concerning the exciton binding energy, this hybrid structure has the peculiar consequence that the dielectric environment within the octahedral planes is similar to that of an inorganic crystal, leading to strongly bound 2D Wannier-Mott excitons, 36,37 while the exciton confinement out-of-plane is defined by the organic moieties. 6 Such entangled exciton dynamics critically depend on coupling to phonons and distortions of the relatively soft lattice. 4,38 Moreover, the exciton-phonon coupling strongly affects the scattering and decay channels of the excitons, and thus determines the absorbance and emission properties of the material. 13,36,39 Together with the thermal relaxation, these are essential for improving their current performance The layered structure of the perovskite flakes with one octahedral layer (n=1) separated by the organic cation molecules is schematically shown in Figures 1 and 2a, and depicts the anchoring and coupling of the organic molecules. The conformation of the unit cells was plotted using the structure parameters that we obtained from our XRD analysis, which agree with those in ref. 13 In the top view in Figure 1b we oriented the bent PEA molecule in diagonal direction such that Pb -Br -stacking is formed, which is motivated by our polarized Raman spectroscopy results, as will be discussed in detail later. While the PEA molecules contain phenethyl complexes that are expected to be relatively rigid, BA spacer molecules consist of a linear alkyl chain that is soft and more flexible. Therefore, torsional and bending oscillations of the molecules, 41 can be seen from the optical microscopy images in Figure S2. The X-ray diffraction patterns and optical characterization of the flakes are given in Figure S1 of the Supporting Information (SI), and show that the (001) planes are parallel to the substrate, with a spacing of 1.4 nm and 1.7 nm of the octahedral layers for (BA)2PbBr4 and (PEA)2PbBr4, respectively. These values correspond to n = 1, as we reported in our previous work. 14 Individual flakes manifest a single emission peak around 410 nm due to quantum confinement, 43 and evaluation of the height of the flake by atomic force microscopy allows to assign the number of organic/inorganic bilayers in the stack (see Figure   S3). 14 Photoluminescence (PL) spectra of single (PEA)2PbBr4 flakes with different thicknesses are reported in Figure S4, and a slight redshift of the emission peak with increasing thickness can be observed, due most likely to self-absorption.   44,45 it is interesting to investigate the influence of the organic moieties. Figure 2d depicts a similar temperature series of Raman spectra recorded from a single 2D layered perovskite flake with BA as the organic linker molecule. At room temperature, the Raman bands of (BA)2PbBr4 are less resolved with respect to the PEA system. Also, in this case, the Raman peaks become more defined with decreasing temperature, and at T=4K a series of sharp peaks can be resolved. Figure   3 shows the Raman spectra of (PEA)2PbBr4 and (BA)2PbBr4 flakes at low temperature (T=4K).
The M1 band consists of a single peak for both BA and PEA systems, which is in line with its assignment to a twisting/rocking motion of the octahedra that should not depend significantly on the organic moiety. 39 Interestingly, the number of peaks for (BA)2PbBr4 is much smaller than for   The orientation dependence is also strongly present in the Raman spectra recorded at room temperature, as shown in Figure S9, Furthermore, the multilayer structures manifest a larger number of modes with respect to the single layers, which is more prominent for the (PEA)2PbBr4 system. The latter is in good agreement with our experimental Raman results and supports that mode splitting due to interlayer coupling occurs.
Based on the temperature, polarization and orientation dependent Raman spectra and our modeling results, we make a tentative assignment of the observed Raman peaks to the possible vibrational modes of the 2D layered perovskite flakes. The lowest frequency band, M1, can be associated to a rocking/twisting vibration of the heaviest subsystem, represented by the The frequencies of the different peaks that were identified in the low-temperature Raman spectra, the symmetry of the modes from group theory analysis (performed on a single octahedron), and the assignment of the vibrational motion (from the discussion above) is summarized in Table 1.
We note that even within this simple representation, we are able to assign an irreducible representation of the D2h symmetry group to the main vibrational motions. This assignment also agrees with those from ref. 44,45 . However, the polarization dependent intensities that can be derived from the Ag, B1g, B2g and B3g symmetry of the modes (see group theory discussion in the SI) in some parts does not agree with our experimental findings, which can be rationalized by the additional symmetry-breaking that is induced by the organic molecules. Measurements down to 5 cm -1 for each excitation wavelength were achieved by three BragGrate notch filters from OptiGrate Corp with optical density of 3~4 with full width of half maximum of 5-10 cm -1 . Two vertical polarizers were used for the polarized and depolarized measurements, and a half wave plate was inserted to rotate the laser polarisation to be parallel (polarized) or perpendicular (depolarized) to the analyzer. The laser power was kept below 500 µW to avoid laser induced damage. In the temperature dependent measurements, the sample was first cooled to 4 K and then the temperature was raised stepwise to the indicated values.
The photoluminescence spectra were recorded with the Raman spectroscopy setup, using a grating with 600 lines/mm and excitation with a laser at 325 nm wavelength.
Modeling. DFT simulations were carried out using the CP2K software. 46 The experimental structures from ref. 13 were allowed a tight relaxation (10 -6 Bohr/Hartree threshold for forces, and 10 -10 Hartree threshold for energy) to the ground state geometry. Goedecker-Teter-Hutter pseudopotentials and the MOLOPT basis set were employed within the Gaussian and plane waves (GPW) formalism. Group theory based on DFT calculations on a single Cs4PbBr6 octahedron in vacuum after geometry optimization using the ADF software were performed to analyze the symmetry of the Raman active modes. Cs was used as A-cation to keep the charge balance neutral and to reduce the computational costs.
Supporting Information. The following files are available free of charge.
The Supporting Information (PDF) contains: Figure S1. X-ray diffraction spectra of the 2D layered perovskite materials.