Advanced Electron Tomography for Colloidal Self-Assemblies

The controlled organization of nanoparticles (NPs) into assemblies has gained increasing scientific interest. By modifying e.g. the size and shape of the individual NPs, the inter-particle distances or the chemical composition of the building blocks, assemblies with various shapes and functionalities have been obtained. Since the properties of these nanostructures are inseparably connected to their three-dimensional (3D) structure and shape, a thorough structural and morphological characterization is of great importance.<br/><br/>Transmission Electron Microscopy (TEM) is an ideal technique to investigate materials at the nanometer level or even at the atomic scale. The technique has therefore been widely used in the study of nano-assemblies. However, TEM images only correspond to a two-dimensional (2D) projection of a 3D object. Therefore, electron tomography, a technique that derives a 3D reconstruction from a tilt series of 2D projection images, has become a standard technique to characterize nano-assemblies in 3D. When assemblies of metallic nanoparticles are investigated, acquisition of electron tomography series is usually performed in High Angle Annular Dark-Field Scanning TEM (HAADF-STEM) mode. In this manner, diffraction contrast present in Bright-Field TEM (BF-TEM) images is minimized, and the projection requirement for tomography is fulfilled. For relatively small (consisting of 100 particles or less) or loose assemblies, in which the distance between the composing building blocks is rather large, conventional electron tomography techniques enable one to determine the morphology and the inner structure of the assemblies. Moreover, a manual segmentation allows one to determine, e.g. the 3D stacking of the nanoparticles in a quantitative manner. On the other hand, when the size and density (atomic number Z) of the investigated assembly increases, several challenges appear that hamper a reliable interpretation of the structure.<br/><br/>In this contribution, we will discuss how acquisition, reconstruction and segmentation during an electron tomography experiment can be optimised to enable a quantitative analysis of the structure under investigation [1-4]. Another challenge is related to the investigation of assemblies of nanoparticles under realistic conditions. We will therefore discuss how to combine a 3D characterization of nanoparticle assemblies with in situ and cryo TEM studies [5,6].<br/><br/>[1] Altantzis T, Wang D, Kadu A, van Blaaderen A, Bals S, Journal of Physical Chemistry C 125, 26240 (2021).<br/>[2] Zanaga D, Bleichrodt F, Altantzis T, Winckelmans N, Palenstijn WJ, Sijbers J, de Nijs B, van Huis MA, Sanchez-Iglesias A, Liz-Marzán LM, van Blaaderen A, Joost Batenburg K, Bals S, Van Tendeloo G, Nanoscale 8, 292 (2016).<br/>[3] Wang D, van der Wee EB, Zanaga D, Altantzis T, Wu Y, Dasgupta T, Dijkstra M, Murray CB, Bals S, van Blaaderen A, Nature Communications 12, 3980 (2021).<br/>[4] Wang D, Dasgupta T, van der Wee EB, Zanaga D, Altantzis T, Wu Y, Coli GM, Murray CB, Bals S, Dijkstra M, van Blaaderen A, Nature Physics, 17, 128 (2021).<br/>[5] Kumar J, Eraña H, López-Martínez E, Claes N, Martín VF, Solís DM, Bals S, Cortajarena AL, Castilla J, Liz-Marzán LM, Proceedings of The National Academy of Sciences of The United States Of America 115, 3225 (2018).<br/>[6] Baginski M, Pedrazo-Tardajos A, Altantzis T, Tupikowska M, Vetter A, Tomczyk E, Suryadharma RNS, Pawlak M, Andruszkiewicz A, Górecka E, Pociecha D, Rockstuhl C, Bals S, Lewandowski W, ACS Nano 15, 4916 (2021).