Transformations of 2D to 3D Double-Perovskite Nanoplates of Cs2AgBiBr6 Composition

Double-perovskite (elpasolite) structures with Cs2AgBiBr6 composition are suggested as emerging inorganic semiconductors for solar energy conversion. We show how colloidal synthesis provides a methodological basis for investigating single monolayer two-dimensional (2D) materials. We then use the monolayers as building blocks for a more stable bilayer structure (quasi 2D) and thicker nanoplates. Each derivative's structure, composition, and morphology are studied, and a growing mechanism for the three-dimensional (3D) nanoplates is hypothesized. High-resolution powder X-ray diffraction (HR-PXRD) synchrotron data reveal that the unit cell volume contracts by ∼2% when transitioning from a monolayer to a bilayer structure. The monolayer's and bilayer's thermal stability and thermal expansion coefficients are investigated using in situ temperature-dependent X-ray diffraction (XRD) measurements. Our colloidal approach to two-dimensional perovskites enables the use of high-resolution transmission electron microscopy (HRTEM) to detect structural defects. We found a typical structural defect in Cs2AgBiBr6 nanoplates with big lateral dimensions in the form of elongated voids. We hypothesize that these defects are reminiscent of an oriented attachment formation step accentuated in the final annealing step of the synthesis. The colloidal approach is essential for improving the properties of bismuth-based lead-free double perovskites, bringing them one step closer to real-life photovoltaic (PV) implementation.