Extended Data Fig. 2: Configuration and FTIR studies.

a, b, Built slab containing two inorganic layers (a) and a vacuum space of 17 Å. For the ab-initio molecular dynamics simulations, this model was expanded to a 4×3×1 supercell (b). Color code: Sn in grey, I in purple, S in yellow, C in black, H in pink, O in red, N in blue. c, AIMD snapshoot of a stable dimer at a perovskite grain boundary. The distances are in Å. Color code: Sn in grey, I in purple, S in yellow, C in black, H in pink, O in red, N in blue. d, Valeric acid dimer on top of a pristine TEA₂SnI₄ surface. The protons are shared between two oxygens with distances of 1.0 Å and 1.6 Å. Valeric acid only contains one functional group - thus if the carboxylic acid group is used to form dimers, the molecule can no longer bind to the perovskite surface, as observed with ab initio MD simulations. e, Dimer formation energies (blue) and adsorption energies (red) for CA and valeric acid (VA). f, Vibrational power spectra of the CA keto and enol monomers and dimers in gas phase. The mixed enol-keto form can also explain the observed CO stretch in FTIR. This figure shows the vibrational power spectra of the gas phase molecules of monomers and dimers of the pure keto and pure enol CA form. In the monomeric form, the CN and CO modes overlap at 1587 cm⁻¹ and 1775 cm⁻¹ for the enol and keto forms, respectively. Upon dimerization, these peaks split into three components with the CO contribution blue shifted compared to the CN one. From these results we can conclude that the experimental spectra contain contributions from mixed keto/enol dimers. Furthermore, Fig. 1 shows the computed vibrational power spectra of the most stable trimer configuration in vacuum and adsorbed on the perovskite surface. When the trimer is adsorbed on the surface, a blue shift of 25 cm⁻¹ and 15 cm⁻¹ is reported for CN and CO modes, respectively, in agreement with the experimental data. Another clear signature that CA adsorbs in the form of trimers and/or dimers is the peak around 3000~3300 cm⁻¹ due to OH and NH groups involved in H-bond interactions (CO-HNC); indeed, this kind of peaks are not present for monomers.