Revealing the Origin of Luminescence Center in 0D Cs₄PbBr₆ Perovskite

Abstract

Zero dimensional perovskite Cs4PbBr6 has attracted considerable attention recently not only because of its highly efficient green photoluminescence (PL), but also its two highly debated opposing mechanisms of the luminescence: embedded CsPbBr3 nanocrystals versus intrinsic Br vacancy states. After a brief discussion on the root cause of the controversy, we provide sensitive but noninvasive methods that can not only directly correlate luminescence with the underlying structure, but also distinguish point defects from embedded nanostructures. We first synthesized both emissive and non-emissive Cs4PbBr6 crystals, obtained the complete Raman spectrum of Cs4PbBr6 and assigned all Raman bands based on density functional theory simulations. We then used correlated Raman-PL as a passive structure-property method to identify the difference between emissive and non-emissive Cs4PbBr6 crystals and revealed the existence of CsPbBr3 nanocrystals in emissive Cs4PbBr6. We finally employed a diamond anvil cell to probe the response of luminescence centers to hydrostatic pressure. The observations of fast red-shifting, diminishing and eventual disappearance of both green emission and Raman below Cs4PbBr6 phase transition pressure of ~3 GPa is compatible with CsPbBr3 nanocrystal inclusions as green PL emitters and cannot be explained by Br vacancies. The resolution of this long-lasting controversy paves the way for further device applications of low dimensional perovskites, and our comprehensive optical technique integrating structure-property with dynamic pressure response is generic and can be applied to other emerging optical materials to understand the nature of their luminescent centers.

Elucidating the Weakly Reversible Cs-Pb-Br Perovskite Nanocrystal Reaction Network with High-Throughput Maps and Transformations

“…We synthesized cesium lead bromide and lead bromide species using a modified version of the method reported by Imran et al 40 By varying reagent concentrations, we observed formation of nine species that could be differentiated based on their UV-Vis absorption spectra, and made reference samples to characterize and confirm their identity by UV-Vis absorption, Xray diffraction and transmission electron microscopy (TEM) ( Figure 1). CsPbBr 3 nanocubes, 22 nanoplatelets between 1 and 4 lead bromide layers 23,46 and Cs4PbBr6 17 have been thoroughly described elsewhere and our results match literature properties. We will describe the properties of CsPb 2Br5, PbBr2 nanocrystals and molecular lead halide species here in more detail, as we have found ambiguities in the literature regarding their characteristics.…”

Elucidating the Weakly Reversible Cs-Pb-Br Perovskite Nanocrystal Reaction Network with High-Throughput Maps and Transformations

“…We synthesized cesium lead bromide and lead bromide species using a modified version of the method reported by Imran et al 40 By varying reagent concentrations, we observed formation of nine species that could be differentiated based on their UV-Vis absorption spectra, and made reference samples to characterize and confirm their identity by UV-Vis absorption, Xray diffraction and transmission electron microscopy (TEM) ( Figure 1). CsPbBr 3 nanocubes, 22 nanoplatelets between 1 and 4 lead bromide layers 23,46 and Cs4PbBr6 17 have been thoroughly described elsewhere and our results match literature properties. We will describe the properties of CsPb 2Br5, PbBr2 nanocrystals and molecular lead halide species here in more detail, as we have found ambiguities in the literature regarding their characteristics.…”

Elucidating the Weakly Reversible Cs–Pb–Br Perovskite Nanocrystal Reaction Network with High-Throughput Maps and Transformations

“…45 While we observe photoluminescence from these species ( Figure S 2.1 B), it is similar to that of CsPbBr 3 species and could easily arise due to small impurities of perovskite cubes or plates in the sample, as has been indicated for the Cs 4PbBr6 phase. 46 PbBr 2 nanocrystals, as determined by XRD (Figure 1 C2), have a variable absorption onset, occurring between 345 and 370 nm with peaks or shoulders between 330 and 350 nm (Figure 1 B2). This is close to the reflectance spectrum observed for PbBr 2 in bulk, 47 and would suggest that nanocrystals observed (Figure 1 A2) do not display strong quantum or dielectric confinement, unlike smaller structures others have prepared.…”