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

1. 二维晶体我们已经证明了“单层”二维晶体（C_6H_<13>NH_3）_2PblI_4中的最低激子是接近理想的二维万尼尔激子。所谓的介电约束效应有效地增强了最低激子的大结合能。此外，我们还研究了各种23维晶体中的激子，并验证了空间和介电约束对激子中电子和空穴之间的库仑相互作用的影响。此外，通过群论分析和第一性原理能带计算，我们已经成功地理解了这些晶体中几乎全部的电子结构。基于这张图，我们建议存在三个激子（一个允许为E⊥，一个允许为E⊥，一个禁止（三重态）），并且三重态激子影响双激子的松弛过程。一维晶体我们测量了“共角”一维晶体[CH_3 CS(=NH_2)NH_2]_3 PbI_5的光谱，发现了具有很强一维特征的最低激子。基于群论分析，我们对光谱中的所有结构（激子）及其偏振依赖性进行了归因。该晶体中的激子为自困激子，并表现出较大的斯托克斯位移。激子的交换能为70 meV，表明系统具有较强的一维性。此外，能带计算表明，“共享类”（角共享、线共享或面共享）和晶格变形强烈影响碘化铅基一维晶体的带隙能量

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