Due to their layered structure, two-dimensional Ruddlesden–Popper perovskites (RPPs), composed of multiple organic/inorganic quantum wells, can in principle be exfoliated down to few and single layers. These molecularly thin layers are expected to present unique properties with respect to the bulk counterpart, due to increased lattice deformations caused by interface strain. Here, we have synthesized centimetre-sized, pure-phase single-crystal RPP perovskites (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 (n = 1–4) from which single quantum well layers have been exfoliated. We observed a reversible shift in excitonic energies induced by laser annealing on exfoliated layers encapsulated by hexagonal boron nitride. Moreover, a highly efficient photodetector was fabricated using a molecularly thin n = 4 RPP crystal, showing a photogain of 105 and an internal quantum efficiency of ~34%. Our results suggest that, thanks to their dynamic structure, atomically thin perovskites enable an additional degree of control for the bandgap engineering of these materials
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K.L. thanks the Solar Energy Research Institute of Singapore (SERIS) for scholarship support. I.A. acknowledges the NUS–Imperial Joint PhD programme. K.P.L. acknowledges A* star DST funding under the project ‘Flexible and High Performance Based Perovskite Solar Cells on Graphene’ (no. R-143000-598-305). K.L. thanks I.-H. Park for help with solving single-crystal data, L. Wang for discussions of this work and H. Zhu for help with photoluminescence lifetime tests. G.E. acknowledges the Singapore Ministry of Education Tier 2 grant (MOE2015-T2-2-123).
Supplementary Text, Supplementary Figures 1–13, Supplementary Tables 1–8, Supplementary Reference 1
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