Synthesis, Structural, Linear, and Nonlinear Optical Studies of Inorganic–Organic Hybrid Semiconductors (R–C6H4CHCH3NH3)2PbI4, (R = CH3, Cl)

Synthesis, crystal structure, and optical properties of two-dimensional (2D) layered structurally slightly different inorganic–organic (IO) hybrid semiconductors (R–C6H4C2H4NH3)2PbI4 (R = CH3, Cl) are presented. They are naturally self-assembled systems where two (RNH3)+ moieties are sandwiched between two infinitely extended 2D layers of the [PbI6]4– octahedral network and treated as natural IO multiple quantum wells. While the former compound crystallizes into an orthorhombic system in the Cmc21 space group, the latter crystallizes into a monoclinic system in the space group P21/c. As a thin film, they are well-oriented along the (l00) direction. Both single crystals and thin films show strong room-temperature Mott type exciton features that are highly sensitive to the self-assembly and crystal packing. Linear (one-photon) and nonlinear (two-photon) optical probing of single crystals for exciton photoluminescence imaging and spectral spatial mapping provide deep insight into the layered re-arrangement and structural crumpling due to organic conformation. The strongly confined excitons, within the lowest band gap of inorganic, show distinctly different one- and two-photon excited photoluminescence peaks: free excitons from perfectly aligned 2D self-assembly and energy down-shifted excitons originated from the locally crumpled layered arrangement. Their structural aspects are successfully presented with proper correlation that emphasize various differences in physical and optical properties associated between these novel IO hybrids.本研究报道了两种结构略有差异的二维（2D）层状无机-有机（inorganic–organic，简称IO）杂化半导体(R–C6H4C2H4NH3)2PbI4（R=CH3、Cl）的合成、晶体结构及光学性质。它们属于天然自组装体系，两个(RNH3)+基团被夹在两个无限延伸的[PbI6]4–八面体网络二维层之间，可被视为天然IO多量子阱（multiple quantum wells）。其中前者晶体属于正交晶系（orthorhombic system），空间群（space group）为Cmc21；后者晶体属于单斜晶系（monoclinic system），空间群为P21/c。以薄膜形式存在时，它们沿(l00)方向高度取向。无论是单晶还是薄膜，均表现出显著的室温莫特型（Mott type）激子特征，且该特征对自组装过程与晶体堆积方式极为敏感。对单晶开展线性（one-photon，单光子）与非线性（two-photon，双光子）光学探测，用于激子光致发光（photoluminescence）成像与光谱空间测绘，可深入揭示由有机构象变化引发的层状重排与结构褶皱现象。在无机相最低带隙（band gap）范围内的强受限激子，表现出显著不同的单光子与双光子激发光致发光峰：完美对齐的二维自组装结构产生自由激子（free excitons），而局部褶皱的层状排列则引发能量下移的激子。本研究通过合理关联两种杂化半导体的结构特征，成功阐明了这类新型IO杂化材料在物理与光学性质上的诸多差异。