Síntese e propriedades ópticas de cristais CsPb₂Br₅

Abstract

CsPb2Br5 is an inorganic halide with chemical formula AB2X5, conmonly known as "2D perovskite". In recent years, low-dimensional perovskite-related structures gained attention in the scientific community due to its interesting photoluminescense properties, low cost and facilitated synthesis. CsPb2Br5 stands out as a promising candidate for light-emitting devices and lasing applications. This crystal is a semiconductor that exhibits an indirect bandgap around 3,09 eV. The structure is composed by [PbBr5]− ions layers intercalated by Cs+, in a tetragonal lattice that belongs in the I4/mcm (D16 4h space group. This work consists in the synthesis of CsPb2Br5 crystals and investigation of their optical properties. The crystals were synthetized by aqueous evaporation at constant temperature (ETC) and anti-solvent vapor-assisted crystallization (AVC). Crystal growth by ETC at room temperature turned out to be the most favorable method to obtain purest CsP2Br5 crystals, with plain white-colored platelets. The other crystals presented clusters of layers in their structures, with yellow or orange tones. The chemical compositions were confirmed semi- quantitatively by scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) in the Cs:Pb:Br proportion of 1:2:5. The Raman vibrational modes positions of CsPb2Br5 are in accordance with the theoric calculated values by DFT. We observed the A1g, B1g and B2g modes at the ab crystallographic plane, and in the ac or bc plane two Eg modes were identified for the first time in accordance with the theorical values. In one of the samples grown at 40 °C we detected Raman modes from the CsPbBr3 perovskite, suggesting that this crystal formed together with CsP2Br5 during the synthesis. The angular dependence of the Raman modes intensities was investigated with polarized Raman in a AVC crystal. Due to the clusters of layers in the structure a superposition of the polarizations took place and the expected behavior of the intensities wasn’t obtained, but an approximated one. All crystals exhibited photoluminescent peaks with values from 2,34 to 2,38 eV. This range corresponds to the visible region which is not in accordance with CsPb2Br5 theorical bandgap at the UV region. We propose that the origin of the emissions are due to defects on the crystals or CsPbBr3 on the structure, depending on the temperature growth.