Controlled Assembly and Anomalous Thermal Expansion of Ultrathin Cesium Lead Bromide Nanoplatelets

Quantum confined lead halide perovskite nano platelets are anisotropic materials displaying strongly bound excitons with spectrally pure photoluminescence. We report the controlled assembly of CsPbBr3 nanoplatelets through varying the evaporation rate of the dispersion solvent. We confirm the assembly of superlattices in the face-down and edge-up configurations by electron microscopy, as well as X-ray scattering and diffraction. Polarization-resolved spectroscopy shows that superlattices in the edge-up configuration display significantly polarized emission compared to face-down counterparts. Variable temperature X-ray diffraction of both face-down and edge-up superlattices uncovers a uniaxial negative thermal expansion in ultrathin nanoplatelets, which reconciles the anomalous temperature dependence of the emission energy. Additional structural aspects are investigated by multilayer diffraction fitting, revealing a significant decrease in superlattice order with decreasing temperature, with a concomitant expansion of the organic sublattice and increase of lead halide octahedral tilt.

Automated summary

Using different evaporation rates of the dispersion solvent, controlled assembly of CsPbBr3 nanoplatelets was achieved. Electron microscopy, X-ray scattering, and diffraction confirmed the superlattice structures in both the face-down and edge-up configurations. Polarization-resolved spectroscopy revealed significantly polarized emission in the edge-up superlattices compared to the face-down counterparts. Variable temperature X-ray diffraction revealed a uniaxial negative thermal expansion in ultrathin nanoplatelets, explaining the anomalous temperature dependence of the emission energy. Additional structural aspects were investigated by multilayer diffraction fitting, showing a decrease in superlattice order and an expansion of the organic sublattice and lead halide octahedral tilt with decreasing temperature.