Fig. 2

Possible scenarios for nucleation with mesoporous TiO₂ substrates. a GIWAXS pattern of BA₂MA₃Pb₄I₁₃ thin film formed on mesoporous TiO₂ (mp-TiO₂) substrates indicates a strong vertical orientation. The continuous ring at 1.8 Å is diffraction signal from mp-TiO₂ substrate. b–d Illustration of possible crystallization processes from three different nucleation sites: liquid–air interface b, within bulk liquid c, and substrate–liquid interface d. The gray circle stacks represent mp-TiO₂ substrate, the brown species represent BA₂MA₃Pb₄I₁₃ and the brown arrows represent the crystallization direction. With the mp-TiO₂ substrate, only the nucleation and growth from the liquid–air interface scenario is consistent with formation of a vertically oriented BA₂MA₃Pb₄I₁₃ thin film