Directional Anisotropy of the Vibrational Modes in 2D-Layered Perovskites

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 to 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 us 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 exciton-phonon interaction, and for optical switching of the optoelectronic properties of such 2D layered materials.

Keywords: 2D layered perovskite; Ruddlesden−Popper structure; exfoliation; low temperature; octahedral plane; polarized Raman spectroscopy; vibrational phonon modes.

Figure 1

Structure of the organic and inorganic layers. (a, c) Unit cell of the lattice of the (PEA)₂PbBr₄ (a) and (BA)₂PbBr₄ (c) layered perovskites generated from the structure parameters obtained by XRD, as in ref (13). (b, d) Top view of the octahedral layer where the anchoring of the organic moiety is depicted. For the (PEA)₂PbBr₄ system the phenethyl ring is oriented in diagonal direction such that Pb–Br−π stacking occurs. The arrows in (b) indicate the x,y directions of the pseudocubic octahedral lattice. For simplicity, the octahedra are sketched without distortions.

Figure 2

Raman spectra of exfoliated (PEA)₂PbBr₄ and (BA)₂PbBr₄ flakes. (a) Scheme of the 2D layered perovskite structure with PEA molecules as spacer and n = 1. (b) Optical microscope image of exfoliated flakes with different thickness, where the different colors result from optical interferences. Scale bar is 5 μm. (c, d) Raman spectra of individual (PEA)₂PbBr₄ (c) and (BA)₂PbBr₄ (d) flakes recorded at different temperatures in the range from 4 to 300 K under laser excitation at 633 nm. The temperature is given for each spectrum, and the vibrational bands M1–M5 are indicated by the transparent colored rectangles.

Figure 3

Ultralow-frequency Raman spectra of individual (PEA)₂PbBr₄ and (BA)₂PbBr₄ perovskite flakes recorded at T = 4 K. The individual flakes are oriented with one main axis of the pseudocubic octahedral lattice parallel to the linear polarization of the excitation laser. Spectra are offset vertically for clarity. Peak positions are given in Table 1, and Figure S7 and S8 show Raman spectra recorded for an extended frequency range. The excitation wavelength was 633 nm.

Figure 4

Polarization and orientation dependence of the vibrational bands. Polarized Raman spectra recorded from (PEA)₂PbBr₄ (a, b) and (BA)₂PbBr₄ (c, d) flakes at T = 4 K under laser excitation at 633 nm. Unpolarized spectra are plotted in black, polarized spectra with polarizer and analyzer in parallel direction in blue, and depolarized spectra with polarizer oriented perpendicular to the analyzer in red color. The insets show optical microscope images of the investigated flakes and their orientation, and the blue circles there indicate the positions chosen for collecting the spectra. The analyzer was kept in vertical direction with respect to these images.