Colloidal Synthesis and Optical Properties of Perovskite-Inspired Cesium Zirconium Halide Nanocrystals

Optoelectronic
devices based on lead halide perovskites are processed in facile ways,
yet are remarkably efficient. There are extensive research efforts
investigating lead-free perovskite and perovskite-related compounds,
yet there are challenges to synthesize these materials in forms that
can be directly integrated into thin film devices rather than as bulk
powders. Here, we report on the colloidal synthesis and characterization
of lead-free, antifluorite Cs2ZrX6 (X = Cl,
Br) nanocrystals that are readily processed into thin films. We use
transmission electron microscopy and powder X-ray diffraction measurements
to determine their size and structural properties, and solid-state
nuclear magnetic resonance measurements reveal the presence of oleate
ligand, together with a disordered distribution of Cs surface sites.
Density functional theory calculations reveal the band structure and
fundamental band gaps of 5.06 and 3.91 eV for Cs2ZrCl6 and Cs2ZrBr6, respectively, consistent
with experimental values. Finally, we demonstrate that the Cs2ZrCl6 and Cs2ZrBr6 nanocrystal
thin films exhibit tunable, broad white photoluminescence with quantum
yields of 45% for the latter, with respective peaks in the blue and
green spectral regions and mixed systems exhibiting properties between
them. Our work represents a critical step toward the application of
lead-free Cs2ZrX6 nanocrystal thin films into
next-generation light-emitting applications.