Double perovskites are considered for future photovoltaic and electro-optic applications as a toxic-free alternative to lead halide perovskites. Alas, due to the lower efficiency of lead-free devices, material properties need to improve to compete. In this work, the self-healing and annealing of crystal voids is reported. Experiments are conducted on nanocrystals and in situ a transmission electron microscopy (TEM) microscope. The setup enables creation of crystal voids and to monitor their dynamics in real time. Void trajectories and velocities are calculated for TEM videos. An inaccessible, protected volume for migration near the nanocrystal outer surface is discovered, confining the migration of voids to inner crystal parts. Once surface passivation in the form of organic ligands is removed, void dynamics changes, to enable annealing of the voids and self-healing of the crystal. It is determined that surface ligand protection against void migration is extending several atomic layers below the crystal surface. Modeling based on these results predict equilibrium positions for the voids, which are discovered in the data. The study suggests that tuning of organic ligand density influences structural stability and crystal defect tolerance in double perovskites. Engineering surfaces with inherent self-healing properties may increase efficiencies in future devices based on these materials.
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.
Perovskite nanocrystal superlattices (NC SLs), made from millions of ordered crystals, support collective optoelectronic phenomena. Coupled NC emitters are highly sensitive to the structural and spectral inhomogeneities of the NC ensemble. Free electrons in scanning electron microscopy (SEM) are used to probe the cathodoluminescence (CL) properties of CsPbBr3 SLs with a ∼20 nm spatial resolution. Correlated CL-SEM measurements allow for simultaneous characterization of structural and spectral heterogeneities of the SLs. Hyperspectral CL mapping shows multipole emissive domains within a single SL. Consistently, the edges of the SLs are blue-shifted relative to the central domain by up to 65 meV. We discover a relation between NC building block colloidal softness and the extent of the CL shift. Residual uniaxial compressive strains accompanying SL formation are contributors to these emission shifts. Therefore, precise control over the colloidal softness of the NC building blocks is critical for SL engineering.
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