Thermal Phase Transitions in Superlattice Assemblies of Cuboidal CH3NH3PbI3 Nanocrystals Followed by Grazing Incidence X-ray Scattering

Colloidal nanocrystals of CH3NH3PbI3 (MAPI) with the tetragonal crystal structure and cuboidal shape terminated by {110} and {002} facets were assembled into superlattices with cubatic structure and heated under nitrogen while collecting in situ grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS). The nanocrystals have completed a tetragonal-to-cubic phase transition by 60 degrees C similar to bulk films. GISAXS shows that the superlattice remains stable until 90 degrees C. At this temperature, GIWAXS reveals the evolution of PbI2. In situ photoluminescence shows that PbI2 begins to form at 75 degrees C, which is about 10-25 degrees C lower than in bulk MAPI films. Another difference compared to bulk MAPI is that both hexagonal and rhombohedral phases of PbI2 are observed as thermal degradation products, when the nanocrystals are heated up to 150 degrees C.