Negative Pressure Engineering with Large Cage Cations in 2D Halide Perovskites Causes Lattice Softening
Negative Pressure Engineering with Large Cage Cations in 2D Halide Perovskites Causes Lattice Softening
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DOI：
10.1021/jacs.0c03860
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发表时间：
2020-07-01
影响因子：
15
通讯作者：
Kanatzidis, Mercouri G.
中科院分区：
文献类型：
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作者：
Li, Xiaotong;Fu, Yongping;Kanatzidis, Mercouri G.
有机-无机杂化卤化物钙钛矿是一种具有可定制光学和电学性质的有前途的半导体材料。选择A位阳离子来支撑三维(3D)钙钛矿结构AMX(3)(其中M是金属，X是卤化物)受到几何Goldschmidt容差因子的限制。然而，这种几何约束可以在二维(2D)钙钛矿中放松，这为我们提供了一个机会来了解不同的A位阳离子如何调制结构性质，从而调节光电性质。在这里，我们报道了单晶(BA)(2)(A)Pb2I7的合成和结构，其中BA=丁基铵，A=甲铵(MA)、甲酰胺(FA)、二甲基铵(DMA)或胍(GA)，其中A位的一系列阳离子的大小不同。单晶X射线衍射表明，MA、FA和GA结构在同一CMCM空间群中结晶，而DMA结构在CCMB空间群中结晶。我们观察到，随着A位阳离子的增大，Pb-I键不断拉长，使钙钛矿笼的体积扩大，相当于对钙钛矿结构施加了“负压”。光学研究和密度泛函理论计算表明，PbI键长的延长减少了PbPb2+和Ip-轨道的重叠，增加了光学带隙，而PbI-Pb3+倾斜角起次要作用。拉曼光谱显示晶格随A位阳离子尺寸的增大而软化。随着A离子的增大，这些结构变化导致光致发光强度和寿命显著降低，这与更明显的非辐射衰变相一致。瞬时吸收显微镜结果表明，光致发光的下降可能源于较高浓度的陷阱或声子辅助的非辐射复合。结果表明，扩大二维钙钛矿的Goldschmidt容差因子的范围是可以实现的，这使得进一步调整二维钙钛矿的结构-性质关系成为可能。
Organic-inorganic hybrid halide perovskites are promising semiconductors with tailorable optical and electronic properties. The choice of A-site cation to support a three-dimensional (3D) perovskite structure AMX(3) (where M is a metal and X is a halide) is limited by the geometric Goldschmidt tolerance factor. However, this geometric constraint can be relaxed in two-dimensional (2D) perovskites, providing us an opportunity to understand how various A-site cations modulate the structural properties and thereby the optoelectronic properties. Here, we report the synthesis and structures of single-crystal (BA)(2)(A)Pb2I7 where BA = butylammonium and A = methylammonium (MA), formamidinium (FA), dimethylammonium (DMA), or guanidinium (GA), with a series of A-site cations varying in size. Single-crystal X-ray diffraction reveals that the MA, FA, and GA structures crystallize in the same Cmcm space group, while the DMA imposes the Ccmb space group. We observe that as the A-site cation becomes larger, the Pb-I bond continuously elongates, expanding the volume of the perovskite cage, equivalent to exerting "negative pressure" on the perovskite structures. Optical studies and DFT calculations show that the Pb-I bond length elongation reduces the overlap of the Pb s- and I p-orbitals and increases the optical bandgap, while Pb-I-Pb tilting angles play a secondary role. Raman spectra show lattice softening with increasing size of the A-site cation. These structural changes with enlarged A cations result in significant decreases in photoluminescence intensity and lifetime, consistent with a more pronounced nonradiative decay. Transient absorption microscopy results suggest that the PL drop may derive from a higher concentration of traps or phonon-assisted nonradiative recombination. The results highlight that extending the range of Goldschmidt tolerance factors for 2D perovskites is achievable, enabling further tuning of the structure-property relationships in 2D perovskites.