Structural Features and Optical Properties of All-Inorganic Zero-Dimensional Halides Cs₄PbBr_6–xI_x Obtained by Mechanochemistry

Abstract

Despite the great success of hybrid CH 3 NH 3 PbI 3 perovskite in photovoltaics, ascribed to its excellent optical absorption properties, its instability toward moisture is still an insurmountable drawback. All-inorganic perovskites are much less sensitive to humidity and have potential interest for solar cell applications. Alternative strategies have been developed to design novel materials with appealing properties, which include different topologies for the octahedral arrangements from three-dimensional (3D, e.g., CsPbBr 3 perovskite) or two-dimensional (2D, e.g., CsPb 2 Br 5 ) to zero-dimensional (0D, i.e., without connection between octahedra), as the case of Cs 4 PbX 6 (X = Br, I) halides. The crystal structure of these materials is complex, and their thermal evolution is unexplored. In this work, we describe the synthesis of Cs 4 PbBr 6−x I x (x = 0, 2, 4, 6) halides by mechanochemical procedures with green credentials; these specimens display excellent crystallinity enabling a detailed structural investigation from synchrotron X-ray powder diffraction (SXRD) data, essential to revisit some features in the temperature range of 90−298 K. In all this regime, the structure is defined in the trigonal R3̅ c space group (#167). The presence of Cs and X vacancies suggests some ionic mobility into the crystal structure of these 0D halides. Bond valence maps (BVMs) are useful in determining isovalent surfaces for both Cs 4 PbBr 6 and Cs 4 PbI 6 phases, unveiling the likely ionic pathways for cesium and bromide ions and showing a full 3D connection in the bromide phase, in contrast to the iodide one. On the other hand, the evolution of the anisotropic displacement parameters is useful to evaluate the Debye temperatures, confirming that Cs atoms have more freedom to move, while Pb is more confined at its site, likely due to a higher covalency degree in Pb−X bonds than that in Cs−X bonds. Diffuse reflectance ultraviolet−visible (UV−vis) spectroscopy shows that the optical band gap can be tuned depending on iodine content (x) in the range of 3.6−3.06 eV. From density functional theory (DFT) simulations, the general trend of reducing the band gap when Br is replaced by I is well reproduced.

Dimension and Thickness Control Synthesis of Strongly Confined Cesium Lead Iodide Perovskites with Excellent Two-Photon Absorption Properties

“…Using EDX data as reported in Figure F and ICP-MS data reported in Table S1 in the Supporting Information, we have determined the ratio of Cs, Pb, and I for 0D-NCs, which is 4.1:1:6.2, indicating the structure as Cs 4 PbI 6 . Figure G shows the XRD analysis of 0D Cs 4 PbI 6 , showing the presence of (012), (113), (131), (214), (223), (314), and (224) indices for the hexagonal lattice and very similar to Cs 4 PbBr 6 lattice (98–002–5124), as reported in the literature. − Figure J shows the absorption spectra of 0D Cs 4 PbI 6 , which shows the presence of a strong sharp band at 365 nm, matching very well with the reported absorption data for 0D Cs 4 PbI 6 . − …”

“…We have determined the photoluminescence quantum yield (PLQY) for Cs 4 PbI 6 NCs using eq . − PLQY 0 normalD = N emit N absorb For this purpose, the number of emitted ( N emit ) photons for 0D Cs 4 PbI 6 NCs and absorbed ( N absorb ) photons for 0D Cs 4 PbI 6 NCs were determined using an absolute PLQY spectrometer (Hamamatsu). − Table indicates that the PLQY value for 0D Cs 4 PbI 6 NCs is 49%.…”

“…Figure L shows the time-resolved photoluminescence decay plots for 0D Cs 4 PbI 6 NCs. Using eq 3, we have calculated the average lifetime <τ>. − false⟨ τ false⟩ 0 Ds = A 1 τ 1 2 + A 2 τ 2 2 + A 3 τ 3 2 A 1 τ 1 + A 2 τ 2 + A 3 τ 3 Here, A 1 is the amplitude component for lifetime τ 1 for 0D Cs 4 PbI 6 NCs. A 2 is the amplitude component for lifetime τ 2 for 0D Cs 4 PbI 6 NCs, and A 3 is the amplitude component for lifetime τ 3 for 0D Cs 4 PbI 6 NCs.…”

“…Cesium halide perovskites with the structure of CsPbX 3 (X = Cl, Br, or I) have been demonstrated to be promising materials for next-generation optoelectronic and photovoltaic devices. − We can classify CsPbX 3 (X = Cl, Br, or I) perovskite systems as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D), depending on the connectivity of octahedral [BX 6 ] −4 in the building block. − As we and others have discussed before, for the 3D perovskite system, octahedral [BX 6 ] −4 are arranged in a 3D corner-sharing network. In the case of 3D CsPbI 3 , each cavity is occupied by one Cs-site cation. − …”

“…In the case of 2D, 1D, and 0D perovskite materials, one can tune their photoluminescence (PL) and optoelectronic properties by reducing their diameter or thickness to below the excitonic Bohr radius and enhancing the quantum confinement effect. − Recent studies indicate that the quantum confinement effects and the optical properties of cesium halide perovskite materials can be tuned by controlling their dimensions, and they exhibit narrow emission peaks regardless of their lateral dimensions. − As a result, the design of 0D, 1D, and 2D nanostructures of CsPbX 3 (X = Cl, Br, or I) has stimulated huge interest via the hot injection and ligand-assisted reprecipitation method. − But until now, controlled synthesis has been challenging for low-dimensional CsPbI 3 as compared to CsPbBr 3 perovskites. − Driven by the need, in this manuscript, we report a systematic investigation of the synthesis of 0D Cs 4 PbI 6 perovskite nanocrystals (NCs), 1D CsPbI 3 nanowires (NWs), and 2D CsPbI 3 nanoplatelets (NPLs), which exhibit excellent two-photon absorption (TPA) properties. As reported in Figure , we show that temperature is the key influencing factor to develop highly confined 2D CsPbI 3 NPL, 1D CsPbI 3 NWs, and 0D Cs 4 PbI 6 NCs.…”