Study on Excitons and biexcitons in Lead-Iodide-Based Perovskite-Type Low-Dimensional Crystals

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1. Two-three-dimensional crystalsWe have shown that lowest excitons in a "monolayer" two-dimensional crystal (C_6H_<13>NH_3)_2PblI_4 are nearly ideal two-dimensional Wannier excitons. The large binding energy of the lowest exciton is enhanced effectively by the so-called dielectric confinement effect. In addition, we have investigated excitons in a variety of 23-dimensional crystals, and have verified that the spatial and the dielectric confinements affects the Coulomb interlaction between electons and holes in the excitons. Moreover, we have successfully understood almost whole electronic structures in these crystals by group-theory analysis and first-principle band calculations. Based on this picture, we suggest three excitons (one allowed for E ⊥L c, one aloowed for E ‖ c, one forbiden (triplet)) exist and the triplet excitons affect,the relaxation process of the biexcitons.2. One-dimensional crystalsWe have measured optical spectra of the "corner-sharing" one-dimensional crystal [CH_3 CS(=NH_2)NH_2]_3 PbI_5, and have found lowest excitons with very strong one-dimensional characters. Based on the group-theory analysis, we have attributed all the structures (excitons) in the spectra including their polarization dependences. Excitons in this crystal are self-trapped ones and exhibit large Stokes shift. Exchange energy of the excitons have proved to be 70 meV which indicates the strong one-dimensionality of the system. Moreover, band calculations have shown that "sharing class" (corner-, line-, or face-sharing) and the lattice deformation strongly affect the bandgap energy of the lead-iodide-based one-dimensional crystals