TEMcompression of nano‐particles in environmental mode and with atomic resolution observations

Abstract

Characterization of nanomaterials or materials at the nanoscale has drastically increased during the last decades. This increase can be explained by (i) the necessity to obtain materials with nanometer‐size grains, for instance nanocomposites, and by (ii) the use of nanoparticles in different fields, for instance lubrication applications. A challenge lies in thein situmicrostructural characterization of such materials as it can give access to valuable pieces of information regarding the microstructural changes induced by their use.The availability of dedicated TEM (Transmission Electron Microscopy) holders equipped with force and displacement sensors is of a very high interest to test,in situ,the size‐dependent mechanical properties of nanometer‐sized objects [1,2]. On crystalline nano‐objects, Molecular Dynamics simulations have shown that dislocations nucleate at the surface [3,4]. Therefore, the surface state is of utmost importance in determining the nucleation stresses and types of dislocations. For materials which undergo surface reconstruction or changes in the surface chemistry under vacuum, it is necessary to perform experiments in a controlled environment (i.e. under gas pressure) which reproduces the real one.Recently a Hysitron PI 95 Picoindenter has been installed on a Cs‐corrected FEI TITAN ETEM (Environmental TEM) microscope. It opens the possibility of performingin situcompression under gas pressure, with high resolution imaging. We will presentin situtests of cubic CeO2, a multifunctional oxide widely used in catalysis. Nanocubes are compressed along either under vacuum or under air pressure. Introducing oxygen inside the chamber limits or avoids the reduction of CeO2nanocubes induced more or less rapidly by the electron beam. A comparison of slopes of load‐displacement curves obtained under vacuum at different electron doses and under air pressure (see Figure 1) strongly suggests that ceria reduced as Ce2O3under the effect of an intense electron flux has a smaller Young modulus than unreduced or 'oxidized' ceria. Atomic resolution observations performed during the compression tests reveal the formation of dislocations and stacking faults (see Figure 2). Simulations are planned to further understand the deformation mechanisms as a function of the oxidation state (native, unreduced or oxidized states), as well as their reversibility [5].