Materials preparation

To deposit the perovskite MQWs films for optical characterization, the precursors of C₁₀H₇CH₂NH₃I (NMAI), CH₅IN₂ (FAI) and PbI₂ with a molar ratio of 2:1:2 were dissolved in DMF (40 wt.%). The obtained solution was stirred at 60 °C for 2 h in a nitrogen-filled glovebox. Then the films were prepared by spin coating the solution onto quartz substrates at 2,000 r.p.m., followed by annealing on a hot plate at 100 °C for 10 min. The 2D (NMA)₂PbI₄ and 3D FAPbI₃ films were prepared with the same methods described above by adding stoichiometric precursors in DMF (40 wt% for 2D, 20 wt% for 3D). All optical measurements were conducted in an optical cryostat under nitrogen-filled or vacuum.

The precursor NMAI was synthesized by adding 4.34 g hydroiodic acid (45 wt% in water) to a stirring solution of 1-naphthalenemethylamine (12.72 mmol) in tetrahydrofuran (THF, 50 ml) at 0 °C for 2 h. The solution was then evaporated at 50 °C to obtain the NMAI precipitate, which was washed three times with THF:CH₂Cl₂ (3:1) mixture and then dried under vacuum. The other precursors were bought from Sigma without any further purification.