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Advances in Transmission Electron Microscopy for 2D and 3D Investigations of Complex Nanomaterials

Host: Petra de Jongh 

Abstract: 
It is well known that the properties of nanomaterials are intrinsically connected to their size, shape, composition, and crystal structure. 
Their characterization at the nanoscale is therefore essential to enable the optimization of a controlled synthesis, as well as to tune the structure-property connection, leading to materials with specific, predefined properties. (Scanning) transmission electron microscopy ((S)TEM) has proven to be an indispensable tool for the characterization of nanomaterials. Over the last decades, the field has significantly evolved, with most developments focusing on hardware optimisation. These efforts have e.g. enabled drastically improved spatial resolution, even down to the atomic scale and more sensitive detectors, enabling the investigation of materials that would otherwise suffer from damage causing by the electron beam.
An important evolution in the field has been to push the characterization by TEM from 2D to 3D. Electron tomography (ET) enables one to determine the 3D structures of nanomaterials from 2D images. These 2D projection images are acquired over a large tilt range and combined in a 3D reconstruction of the structure of interest through a mathematical algorithm. During past decades, ET in high-angle annular dark-field STEM (HAADF-STEM) mode has become a popular technique to investigate the overall morphology of nanomaterials, to determine the nature of surface facets, and even to characterize the atomic structure in 3D.
Although these experiments are already at the state-of-the-art, several open questions remain. These questions are often related to the fact that 3D characterization by TEM is typically performed using the conventional conditions of a TEM: ultrahigh vacuum and room temperature. Since it is known that the morphology and consequently, the activity of nanomaterials will transform at higher temperatures or pressures, this poses 
a fundamental limitation. It is therefore not surprising that much effort has been devoted to monitoring nanoparticle transformations upon application 
of external stimuli by TEM.
Another challenge is related to the fact that the acquisition of a conventional tilt series for ET is a time-consuming process that requires at least 
1 h for a standard experiment. In addition, after the acquisition, a postprocess reconstruction step is required to evaluate the final 3D shape of the nanomaterial. Consequently, one can typically analyze approximately 10 NPs in a time frame of 1 day. This restriction further limits a thorough understanding of the structure–property relations, especially because the properties of nanomaterials are mostly measured by ensemble techniques.
A final challenge concerns the beam sensitivity of specific nanostructured materials. For example, metal halide nanocrystals of MOFs are extremely sensitive to the electron beam. In order to distinguish degradation effects caused by the electron beam from those related with external triggers, new low-dose electron microscopy techniques need to be applied. One such technique exploits the use of four-dimensional scanning transmission electron microscopy, a technique during which for every scan position a diffraction pattern is collected.
In this presentation, I will discuss the progress in overcoming these different challenges and will give examples on how modern TEM can contribute to a better understanding in the field of catalysis, optoelectronics and nanoscience in general.