Large lattice distortions and size-dependent bandgap modulation in epitaxial halide perovskite nanowires

Eitan Oksenberg,Amnon Rothman,Ifat Kaplan-Ashiri,Ivan G Scheblykin,Aboma Merdasa,Eva L Unger,Lothar Houben,Ernesto Joselevich

doi:10.1038/s41467-020-14365-2

Abstract

Metal-halide perovskites have been shown to be remarkable and promising optoelectronic materials. However, despite ongoing research from multiple perspectives, some fundamental questions regarding their optoelectronic properties remain controversial. One reason is the high-variance of data collected from, often unstable, polycrystalline thin films. Here we use ordered arrays of stable, single-crystal cesium lead bromide (CsPbBr3) nanowires grown by surface-guided chemical vapor deposition to study fundamental properties of these semiconductors in a one-dimensional model system. Specifically, we uncover the origin of an unusually large size-dependent luminescence emission spectral blue-shift. Using multiple spatially resolved spectroscopy techniques, we establish that bandgap modulation causes the emission shift, and by correlation with state-of-the-art electron microscopy methods, we reveal its origin in substantial and uniform lattice rotations due to heteroepitaxial strain and lattice relaxation. Understanding strain and its effect on the optoelectronic properties of these dynamic materials, from the atomic scale up, is essential to evaluate their performance limits and fundamentals of charge carrier dynamics.

Highlights

Metal-halide perovskites have been shown to be remarkable and promising optoelectronic materials
Employing nanobeam scanning electron diffraction (SED) and geometric phase analysis (GPA), we find a large and uniform lattice rotation, which relates to octahedral tilting
We discuss the origin of the lattice distortions and conclude that they stem from heteroepitaxial mismatch, accentuated by a significant difference between the thermal expansion coefficients of the sapphire substrate and the CsPbBr3 surface-guided nanowires

Summary

Introduction

Metal-halide perovskites have been shown to be remarkable and promising optoelectronic materials. We found that as the height of the nanowire decreases from ~1.5 μm to ~45 nm, the photoluminescence (PL) emission peak blue-shifts from ~530 nm to ~510 nm, respectively Such a size-dependent emission shift has been reported only a few times in the literature, but was observed for both methylammonium lead iodide (MAPbI3) and CsPbBr3 and for different material structures, including platelets, nanowires, and thin films[8,35,36,37]. Wang et al reported an emission blue-shift from ~530 to ~516 nm with decreasing thickness of CsPbBr3 platelets and suggested that this stems from strain that accumulates due to nontrivial strength of the van der Waals epitaxial interaction between the platelets and the mica substrate[37] The recurrence of this phenomenon in different MHPs and different structures implies that this is a general feature that probably exists in many other MHPs systems. In accordance with recent reports that highlight the impact of strain on the optoelectronic properties and stability of polycrystalline MHPs thin films[39,40,41,42,43,44,45,46], our observations of large lattice distortions at the atomic scale, could greatly contribute to better understanding MHPs and pushing them towards their performance limits

Methods

Results

Conclusion