Electron Beam-Induced Transformation of Cesium Lead Halide Perovskite Nanocrystals

Introduction Halide lead perovskites have received widespread attention due to their remarkable performances in photovoltaics and optoelectronics. We investigate the electron beam-induced transformations of perovskite NCs,... [ view full abstract ]

Introduction

Halide lead perovskites have received widespread attention due to their remarkable performances in photovoltaics and optoelectronics. We investigate the electron beam-induced transformations of perovskite NCs, when irradiated by high energy electron beam in a transmission electron microscope (TEM) (Z. Dang et al. ACS Nano, 11 (2), 2017; Z. Dang et al. ACS Omega, 2 (9), 2017).

Methods

The electron beam irradiation experiments were carried out in a 80/200 keV microscope (JEOL JEM-2200FS) equipped with a spherical aberration corrector (CEOS) for the objective lens and an in-column image filter (Ω-type). A Gatan cryotransfer holder with liquid nitrogen cooling equipped with a Model 900 Smartset cold stage controller was used for observing the electron beam-induced transformation at temperatures from -180 to 90 °C.  

Results and Discussion

Upon irradiation, Br desorption occurs. Br loss is more significant at an incident electron energy of 80 keV than at 200 keV, indicating a radiolysis dominant process. (Fig 1(a)) In parallel with the Br loss, Pb²⁺ ions are reduced to Pb⁰ atoms. When temperature (T) is high (above -40°C), the Pb⁰ atoms aggregate into Pb nanoparticles  at a rate that depends on the temperature. (see Fig 1(b)) Meanwhile the locally Pb deficient perovskite structure becomes progressively amorphous, while the surrounding perovskite structure is maintained.

When T is below -40°C the Pb⁰ diffusion is drastically reduced, no Pb nanoparticles were visible; instead, the nanosheets turned porous. (Fig 2) The porous structure is composed of CsBr, CsPb, and PbBr₂ domains. (Fig 3) PbBr₂ and CsBr domains further lose Br upon irradiation, leading to a higher Br loss at lower temperatures (below -40°C) (see Fig 4).

These findings suggest that caution should be taken in the characterization of halide perovskites involving radiation sources. When dealing with halide perovskites characterization using electron microscopy, increasing the incident electron energy of the microscope can largely improve the stability of the sample. An appropriate temperature range needs to be chosen for the specimen in order to minimize both Pb nanoparticle formation and structure decomposition. For instance, for 200 keV electrons in typical TEM imaging conditions, the optimum temperature is – 40 °C.