(Invited) Atomic Imaging and Device Characterization of Molecularly Thin 2D Hybrid Perovskites
Authors: Leng Kai 
Published: February 2024

2D Organic-Inorganic Hybrid perovskites (OIHPs) possess alternating layers of organic molecules and inorganic quantum wells. By tuning the interaction between organic and inorganic layers, very interesting properties can be introduced. The successful isolation of hybrid perovskite monolayers with clean and flat surfaces are particularly suitable for atomic structure characterization and optoelectronic device fabrication. In this talk, I will present recent results on the excitonic properties and atomic structure of molecularly thin OIHPs1,2. We found that a reversible shift in excitonic energies can be induced upon laser irradiation in large-sized 2D hybrid perovskite monolayers which is attributed to the reversible structural reorientation of the surface BA cations in the easily deformable lattice3. The photodetection performance of monolayer, bilayer and thicker OIHPs were compared. The internal quantum efficiency was measured to be 34% for a monolayer (BA)2(MA)3Pb4I13 and 19% for the bulk crystal. In addition, 2D heterostructures of molecularly thin OIHPs/graphene were constructed which exhibited a lower barrier than gold for carrier injection, enabling applications in field effect transistors (FETs)4. Electron tunneling occurs across the interface of organic molecular layers on 2D perovskite and graphene, while photoinduced charge transfer occurs at femtosecond timescale (~50 fs).
Resolving the atomic structure of 2D hybrid perovskite is a challenging task because it is easily damaged by electron beam. Here we use scanning tunneling microscopy (STM) & Qplus AFM to directly visualize surface octahedral tilt in freshly exfoliated 2D Ruddlesden-Popper perovskites (RPPs)5. 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 promotes Rashba spin splitting for n > 1. Our studies demonstrate a protocol to investigate structure-property correlations in low dimensional 2D OIHPs, which will pave the way to better material design and device performance.
References

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Acknowledgement
K.L. would like to acknowledge the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. PolyU15305221 for GRF project funded in 2021/22 Exercise and Project No. PolyU25305222 for ECS project funded in 2022/23 Exercise).