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 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.有机/无机层状钙钛矿中的振动模式对其光电子学性质具有至关重要的基础性意义。这类材料的Ruddlesden–Popper相层级结构，使得振动模式在八面体平面内相对于伪立方晶格主轴呈现出明确的方向性。本研究采用极低频偏振拉曼光谱，对单片剥离的Ruddlesden–Popper钙钛矿薄片中的本征声子模式方向性展开探究。在15至150 cm⁻¹（2至15 meV）的波段范围内可观测到丰富的拉曼峰，其特征取决于有机阳离子种类、测试温度以及入射光的线偏振方向。通过调控激发激光的线偏振角相对于八面体层内轴的夹角，我们可获取振动模式对称性的详细信息。本研究选用两种不同的有机基团：苯乙胺（phenethylammonium, PEA）与丁胺（butylammonium, BA），以此区分连接分子的影响，结果证实(PEA)₂PbBr₄样品的振动具有显著的各向异性。变温拉曼测量结果表明，室温下观测到的宽频声子带由一系列尖锐的振动模式组成，且不同有机基团与振动带对应的模式劈裂差异显著。相较于柔性更强的丁胺（BA）分子，其振动频率更低且劈裂出的模式更少；而刚性更强的苯乙胺（PEA）分子则在低温下呈现更高频率的振动，且拉曼峰数量更多。值得注意的是，在特定波段中，刚性PEA连接分子对应的拉曼峰数量是柔性BA连接分子的两倍。本研究证实，可通过入射光偏振与有机基团的选择来调控与特定振动模式的耦合，这一发现可用于调控激子-声子相互作用，以及实现此类二维层状材料光电子性质的光学开关。