Nano assemblies are two- or three-dimensional (3D) collections of nanoparticles. The properties of the assemblies are determined by the number of particles, their position, shape and chemical nature as well as the bonding between them. If we are able to determine these parameters in 3D, we will be able to provide the necessary input for predicting the properties and we can guide the synthesis and development of new assemblies or superstructures. A detailed structural characterization is therefore of utmost importance.

Electron tomography is a versatile and powerful tool that has been increasingly used in the field of materials science. Also for the investigation of nanoassemblies, electron tomography has been of high value as illustrated in Figure 1 [1-3]. However, to extract quantitative information, optimization of the electron tomography experiment is required, especially for large assemblies or assemblies consisting of more than one type of nanoparticle. Here, we will show how advanced electron microscopy,  in combination with novel reconstruction algorithms, enables us to determine the number of particles, their stacking, interparticle distances and outer morphology. This will be illustrated for very large assemblies consisting of spherical nanoparticles [4], assemblies that contain anisotropic particles (Figure 2) and binary assemblies (Figure 3) consisting of particles with different sizes or different compositions [5,6].

Going a step further is the investigation of self assembly and oriented attachment at the atomic scale. Using annular bright field scanning transmission electron microscopy, the possible effect of polarity can be investigated, whereas high angle annular dark field scanning transmission electron microscopy in combination with advanced computational techniques enables the investigation of the interfacial planes in two-dimensional superlattices from nanocrystals [7]. Finally, we will discuss the use of exit wave reconstructions to investigate the effect of surface ligands on self assembly.

[1] S. Bals, B. Goris, L.M. Liz-Marzán, G. Van Tendeloo, Angewandte Chemie 53 (2014) 10600

[2] T. Altantzis, B. Goris, A. Sánchez-Iglesias, M. Grzelczak, L.M. Liz-Marzán, S. Bals, Particle and Particle Systems Characterization 30 (2013) 84

[3] M.P. Boneschanscher, W. Evers, J.J.Geuchies, T. Altantzis, B. Goris, F.T. Rabouw, S.A.P. van Rossum, H.S.J. van der Zant, L.D.A. Siebbeles, G. Van Tendeloo,I. Swart, J. Hilhorst, A. Pethukov, S. Bals, D. Vanmaekelbergh, Science 344 (2014) 1377

[4] D. Zanaga, F. Bleichrodt, T. Altantzis, N. Winckelmans, W.J. Palenstijn, J. Sijbers, B. de Nijs, M.A. van Huis, A. Sánchez-Iglesias, L.M. Liz-Marzán, A. van Blaaderen, K.J. Batenburg, S. Bals, G. Van Tendeloo, Nanoscale 8 (2016) 292

[5] M. Grzelczak, A. Sanchez-Iglesias, L. M. Liz-Marzán, Soft Matter 9 (2013) 9094-9098

[6] T. Altantzis, Z. Yang, S. Bals, G. Van Tendeloo, M.-P. Pileni, Chemistry of Materials 28 (2016) 716

[7] E. Javon, M. Gaceur, W. Dachraoui, O. Margeat, J. Ackermann, M. Ilenia Saba, P. Delugas, A. Mattoni, S.Bals, G. Van Tendeloo, ACS Nano 9 (2015) 3685

Acknowledgements

S.B., D.Z. and N.C. acknowledge financial support from European Research Council (ERC Starting Grant # 335078-COLOURATOMS). The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreements 312483 (ESTEEM2). 

Figures:

3D reconstruction of a) a 3D assembly of Au nanoparticles [2] b) a 2D assembly of CdSe nanoparticles [3].

a) Tomographic reconstruction of an assembly of Au nanorods. b) Quantitative analysis of the orientation of the nanorods within the assembly.

3D reconstruction of a) a binary assembly of Au nanoparticles with different sizes b) an assembly in which a Au nanostar (yellow) is surrounded by Fe3O4 nanoparticles (blue).

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