Inverse size-dependent Stokes shift in strongly quantum confined CsPbBr 3 perovskite nanoplates

Colloidal semiconductor nanocrystals (NCs) are used as bright chromatic fluorophores for energy-efficient displays. We focus here on the size-dependent Stokes shift for CsPbBr3 nanocrystals. The Stokes shift, i.e., the difference between the wavelengths of absorption and emission maxima, is crucial for display application, as it controls the degree to which light is reabsorbed by the emitting material reducing the energetic efficiency. One major impediment to the industrial adoption of NCs is that slight deviations in manufacturing conditions may result in a wide dispersion of the product's properties. A data-driven analysis of over 2000 reactions comparing two data sets, one produced via standard colloidal synthesis and the other via high-throughput automated synthesis is discussed. We show that differences in the reaction conditions of colloidal CsPbBr3 nanocrystals yield nanocrystals with opposite Stokes shift size-dependent trends. These match the morphologies of two-dimensional nanoplatelets (NPLs) and nanocrystal cubes. The Stokes shift size dependence trend of NPLs and nanocubes is non-monotonic indicating different physics is at play for the two nanocrystal morphologies. For nanocrystals with cubic shape, with the increase of edge length, there is a significant decrease in Stokes shift values. However, for NPLs with the increase of thickness (1-4 ML), Stokes shift values will increase. The study emphasizes the transition from a spectroscopic point of view and relates the two Stokes shift trends to 2D and 0D exciton dimensionalities for the two morphologies. Our findings highlight the importance of CsPbBr3 nanocrystal morphology for Stokes shift prediction

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

How Does Local Strain Affect Stokes Shifts in Halide Double Perovskite Nanocrystals?

Lead-free perovskite nanocrystals are of interest due to their nontoxicity and potential application in the display industry. However, engineering their optical properties is nontrivial and demands an understanding of emission from both self-trapped and free excitons. Here, we focus on tuning silver-based double perovskite nanocrystals' optical properties via two iso-valent dopants, Bi and Sb. The photoluminescence quantum yield of the intrinsic Cs2Ag1-yNayInCl6 perovskite increased dramatically upon doping. However, the two dopants affect the optical properties very differently. We hypothesize that the differences arise from their differences in electronic level contributions and ionic sizes. This hypothesis is validated through absorption and temperature dependence photoluminescence measurements, namely, by employing the Huang-Rhys factor, which indicates the coupling of the exciton to the lattice environment. The larger ionic size of Bi also plays a role in inducing significant microstraining verified via synchrotron measurements. These differences make Bi more sensitive to doping concentration over antimony which displays brighter emission (QY ∼40%). Such understanding is important for engineering optical properties in double perovskites, especially in light of recent achievements in boosting the photoluminescence quantum yield