Fig. 3. Optimal 2D intermediate.

Scanning Electron Microscopy (SEM) images showing the microstructure of 2D-intermediate α-FAPbI₃ layers when a series of 2D intermediate processed from THF with varying R_BA-MA+ are converted sequentially. R_BA-MA+ = (a) 3.00 (inset: photograph showing macroscopic cracking of 3D perovskite layers converted from R_BA-MA+ = 3.00 intermediates due to extreme volume contraction), (b) 2.50, (c) 2.00, (d) 1.75, (e) 1.50, (f) 1.25, (g) 1.00, (h) 0.75. Scale bars: 5 μm. Schematic diagrams depicting the conversion of (i) BA₂PbI₄ (n = 1) (j) BA₂MAPb₂I₇ (n = 2), and (k) BA₂MA₃Pb₄I₁₃ (n = 4) into 3D α-FAPbI₃. FA⁺ highlighted in green has replaced BA⁺ in the organic channels of the 2D intermediate, while FA⁺ highlighted in yellow has replaced MA⁺ in the inorganic layers. Double-headed arrows display layer thickness. l Plot showing lead iodide (PbI₂) content as a phase fraction in comparison to 2D-intermediate α-FAPbI₃ extracted from XRD diffraction patterns (Supplementary Fig. 15) via peak integration of 6R-PbI₂ (001) and α-FAPbI₃ (100) scattering peaks. Inset shows an expanded view for low-PbI₂ region. m ¹²⁷I nuclear quadrupole resonance (NQR) spectra of 2D-intermediate FAPbI₃ and FAPbI₃ (processed from DMF:DMSO) thin films, which have been mechanically exfoliated and powdered for measurement.