Nanocrystals are important for a wide range of applications because of their unique properties, which are strongly connected to their three-dimensional (3D) structure. Electron tomography has therefore been used in an increasing number of studies. Most of these investigations resulted in 3D reconstructions with a resolution at the nanometer scale, but also atomic resolution was achieved in 3D. However, the increasing complexity of nanomaterials has driven the development of even more advanced 3D characterization techniques, which will be discussed in this contribution.

For example, 3D characterization of structural defects in nanoparticles by transmission electron microscopy is far from straightforward since the presence of diffraction contrast in a tilt series of images violates the projection requirement for tomography. However, being able to visualize defects is of great importance to understand e.g. the initial growth of metallic nanoparticles or the effect of pulsed laser irradiation on the crystal structure. By simultaneous acquisition of tilt series using different annular detectors, we were able to visualize both the morphology and the defect structure of several types of nanostructures [1]. 

In order to preserve the carefully designed morphologies and functionalities, understanding the stability of nanomaterials during application is of equal importance. It is hereby important to note that most electron tomography investigations have been performed at the conventional conditions of an electron microscope. An emerging challenge is therefore to fully understand the connection between the 3D structure and properties under realistic conditions, including high temperatures as well as in the presence of liquids and gases. Therefore, innovative methodologies are required to track the fast 3D changes of nanomaterials that occur under such conditions.

Recently, we proposed an acquisition approach where a tilt series of projection images is acquired within a few minutes. By continuously tilting the holder and simultaneously acquiring projection images while focusing and tracking the particle, we were able to reduce the total acquisition time for a tilt series by a factor of ten. In this manner, we were able to study the 3D morphological evolution of anisotropic Au(Pd) nanocrystals as a function of both heating time and temperature [2,3]. Moreover, we measured the elemental diffusion dynamics of individual anisotropic Au-Ag nanoparticles in 3D [4]. We conclude that for a given composition, the shape of the nanoparticle does not influence the alloying process significantly and that other factors such as surface diffusion need to be taken into account.