Directionality of Vibrations and Emission in Two-Dimensional Metal Halide Perovskites
Roman Krahne a, Mehrdad Faraji a, Alexander Schleusener a, Miao-Ling Lin b, Ping-Heng Tan b
a Istituto Italiano Di Tecnologia, Via Morego, 30, Genova, Italy
b Institute of Semiconductors, Chinese Academy of Sciences, China
Proceedings of International Conference on Emerging Light Emitting Materials (EMLEM23)
Peyia, Cyprus, 2023 November 13th - 15th
Organizers: Grigorios Itskos, Maksym Kovalenko and Maryna Bodnarchuk
Invited Speaker, Roman Krahne, presentation 021
Publication date: 18th August 2023

Metal-halide perovskite materials demonstrated extraordinary performance in solar cells and light emission in recent years, and their layered low-dimensional counterparts promise increased stability and even greater tunability due to the huge variety of molecules available for the organic phase. [1-3] Single or double octahedra-layer (1L and 2L) structures show strong confinement and large exciton binding energies, and the band gap and light emission depends sensitively on the lattice distortions induced by the organic cations.[4-7] In particular, the broadband emission from trapped excitons in combination with the band-edge emission can lead to white light emission from a single material with tunable color temperature. [8]

We investigate the emission from several Ruddlesden-Popper metal-halide perovskites with single octahedra layers (n=1) that differ by their organic cations and lattice distortions.[9, 10] The impact of phonons on the optical emission dynamics can be assessed by the temperature-dependence of the emission linewidth broadening.[11, 12] However, only in some cases this analysis allows to identify the contribution of specific phonon modes.[9, 10] We therefore combine angle-dependent polarized Raman and photoluminescence spectroscopy on single microcrystalline flakes that reveal a strong directionality of the signal along the major axes of the octahedra lattice, to get a deeper insight into the photophysics of these materials. With non-resonant Raman spectroscopy in the ultralow frequency range (from 10 to 200 cm-1), we observe a very large number of phonon bands with different vibrational symmetries, which makes it plausible that for certain materials not only one single dominant phonon mode impacts the emission process.

Interestingly, also the light emission of single two-dimensional metal halide perovskite flakes shows a strong angle-dependence in polarization. Here the band-edge emission along one major axis of the octahedra lattice is significantly stronger than along the perpendicular one, which indicates that the in-plane distortions of the lattice lead to emission polarization. Therefore, the directional behavior of the emission of single perovskites flakes evidences the impact of the lattice strain on the excitonic properties and opens new opportunities for the design of light-emitting devices, for example by tailoring optical cavities along these directions or optimizing the structures for efficient outcoupling of the light.

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