Shao Yan, Gao Wei, Yan Hejin, Li Runlai, Abdelwahab Ibrahim, Chi Xiao, Rogée Lukas, Zhuang Lyuchao, Fu Wei, Lau Shu Ping, Yu Siu Fung, Cai Yongqing, Loh Kian Ping, Leng Kai
Department of Chemistry, National University of Singapore, Singapore, Singapore.
Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China.
Nat Commun. 2022 Jan 10;13(1):138. doi: 10.1038/s41467-021-27747-x.
Molecularly soft organic-inorganic hybrid perovskites are susceptible to dynamic instabilities of the lattice called octahedral tilt, which directly impacts their carrier transport and exciton-phonon coupling. Although the structural phase transitions associated with octahedral tilt has been extensively studied in 3D hybrid halide perovskites, its impact in hybrid 2D perovskites is not well understood. Here, we used scanning tunneling microscopy (STM) to directly visualize surface octahedral tilt in freshly exfoliated 2D Ruddlesden-Popper perovskites (RPPs) across the homologous series, whereby the steric hindrance imposed by long organic cations is unlocked by exfoliation. The experimentally determined octahedral tilts from n = 1 to n = 4 RPPs from STM images are found to agree very well with out-of-plane surface octahedral tilts predicted by density functional theory calculations. The surface-enhanced octahedral tilt is correlated to excitonic redshift observed in photoluminescence (PL), and it enhances inversion asymmetry normal to the direction of quantum well and promotes Rashba spin splitting for n > 1.
分子软有机-无机杂化钙钛矿易受晶格动态不稳定性的影响，这种不稳定性称为八面体倾斜，它直接影响其载流子传输和激子-声子耦合。尽管与八面体倾斜相关的结构相变在三维杂化卤化物钙钛矿中已得到广泛研究，但其在二维杂化钙钛矿中的影响尚不清楚。在这里，我们使用扫描隧道显微镜（STM）直接观察了新鲜剥离的二维Ruddlesden-Popper钙钛矿（RPPs）同系物表面的八面体倾斜，通过剥离消除了长有机阳离子带来的空间位阻。从STM图像中确定的n = 1到n = 4的RPPs的八面体倾斜实验值与密度泛函理论计算预测的面外表面八面体倾斜非常吻合。表面增强的八面体倾斜与光致发光（PL）中观察到的激子红移相关，并且对于n > 1，它增强了垂直于量子阱方向的反演不对称性并促进了Rashba自旋分裂。
