Proceedings of nanoGe Spring Meeting 2022 (NSM22)
DOI: https://doi.org/10.29363/nanoge.nsm.2022.307
Publication date: 7th February 2022
Metal halide double perovskites, especially the Cs2AgBiBr6, have attracted the attention of researchers in the search for less toxic candidates as active materials in optoelectronic devices.[1,2] From this 3D structure, low-dimensional double perovskites are fabricated by introducing large organic cations, resulting in organic/inorganic architectures with one or more inorganic-octahedra layers separated by organic cations.[3] Here, we synthesize a series of layered double perovskites from Cs2AgBiBr6 that consist of double (2L) or single (1L) inorganic octahedra layers, using ammonium cations of different size and chemical structure. By carrying out temperature-dependent Raman spectroscopy measurements, we highlight phase transition signatures in both inorganic lattice and organic moieties by detecting changes in the position and linewidth of the vibrational modes. Variations in the conformational arrangement of the organic cations from an ordered to disordered state match with a phase transition in the 1L systems with propyl- and butyl-ammonium moieties. Density functional theory calculations of the band structure reveal that the bandgap is direct only for the 1L crystal structures, and it becomes indirect in 2L systems. The direct bandgap character in 1L compounds stems from an extremely flat lowest conduction band, which enables small octahedral distortions to produce significant changes in the band structure across the bandgap. This translates to the optical properties, where we observe significant changes of photoluminescence intensity around the transition temperature as result of octahedral tilting rearrangement emerging at the phase transition.[4] Our work provides novel insights into the structure-optical properties relationship in layered double perovskites, and demonstrates how the proper selection of the organic cations in the layered perovskites is relevant in terms of thermal stability or the use of the phase transition for active switching in future applications.
This work is supported by funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 785219 (GrapheneCore2), the German Research Foundation (DFG) within the project ESSENCE, the Spanish MINECO under the María de Maeztu Units of Excellence Programme (MDM-2016-0618) and the Spanish MICINN under project PID2019-108153GA-I00. B.M-G. thanks also to Gipuzkoa Council (Spain) in the frame of Gipuzkoa Fellows Program.
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