Unlocking surface octahedral tilt in two-dimensional Ruddlesden-Popper perovskites

Yan Shao,Hejin Yan,Runlai Li,Siu Fung Yu,Kian Ping Loh,Lukas Rogée,Wei Fu,Wei Gao,Yongqing Cai,Lyuchao Zhuang,Xiao Chi,Ibrahim Abdelwahab,Kai Leng,Shu Ping Lau

doi:10.1038/s41467-021-27747-x

Abstract

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.

Highlights

Soft organic-inorganic hybrid perovskites are susceptible to dynamic instabilities of the lattice called octahedral tilt, which directly impacts their carrier transport and excitonphonon coupling
This surface-enhanced octahedral tilt consists of a rigid rotation of the anion cage which can be divided into the out-of-plane tilt and in-plane tilt
The results collected above allow us to examine the microscopic mechanism of surface octahedral tilt and its dependence on the nth dimensionality of 2D hybrid perovskites

Summary

Introduction

Soft organic-inorganic hybrid perovskites are susceptible to dynamic instabilities of the lattice called octahedral tilt, which directly impacts their carrier transport and excitonphonon coupling. 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. XRD provides bulk averaged information with no surface sensitivity, it cannot be used to probe the presence of surface reconstruction that occurs on exfoliated RPPs. Here, we directly imaged octahedral tilts on the surface of 2D RPPs (BA)2(MA)n−1PbnI3n+1 (n = 1–4; BA: CH3(CH2)3NH3, n refers to the number of inorganic slabs per unit cell of the homologous series) using scanning tunneling microscopy. The out-of-plane octahedral tilt of the exfoliated surface is correlated to the redshifted emission peak alongside the primary exciton in the photoluminescence spectra, and contributes to Rashba spin splitting

Methods

Results

Conclusion