Characterization of core-shell type nanoparticles in 3 dimensions (3D) by transmission electron microscopy (TEM) can be very challenging. Especially when both heavy and light elements co-exist within the same nanostructure, artefacts in the 3D reconstruction are often present. A representative example would be a particle comprising an anisotropic metallic (Au) nanoparticle coated with a (mesoporous) silica shell. To obtain a reliable 3D characterization of such an object, we collected high angle annular dark field scanning TEM (HAADF-STEM) and annular dark field tilt series (ADF-STEM) for tomography (Figure 1A and 1B respectively). Although the series acquired by ADF-STEM shows both the Au and the SiO₂, artefacts are clearly present in the 3D reconstruction (Figure 2). Since the observed artefacts may cause loss of information or may lead to misinterpretation, it is extremely challenging to obtain reliable 3D results for core-shell hybrid materials using conventional electron tomography. When selecting an optimal value for the collection angle, a compromise is needed between optimal contrast, produced by the atomic number of coexisting elements, and the minimization of diffraction contrast.  

We here overcome this limitation by exploiting the flexibility of modern TEM instruments that enable one to collect multiple (HA)ADF-STEM series simultaneously, by using different (HA)ADF detectors at the same time. This multi-mode approach is very dose-efficient, as one is able to collect 2 images while keeping the necessary electron dose the same. Tilt series were simultaneously acquired using an ADF detector with inner and outer collection angles of 35 and 125 mrad and a HAADF-STEM detector using inner and outer collection angles of 150 and 220 mrad, respectively. To remove the artefacts that appear in the ADF-STEM tilt series, we removed the complete Au nanoparticle from the ADF-STEM projection images. Next, a technique known as inpainting was applied[1]. This approach replaces the absent information by a continuation of the texture of the surrounding area. This procedure was performed for each projection image of the tilt series separately (Figure 3A).  The processed tilt series was then used as an input for 3D reconstruction using the SIRT algorithm implemented in the ASTRA toolbox[2]. Finally, the 3D HAADF-STEM and ADF-STEM reconstructions are combined into one single visualization using the AMIRA software as illustrated in Figure 3B [3]. In this manner, we were able to reliably characterize the structure of mesoporous SiO₂ Au nanoparticles. It must be noted that the methodology we propose here is generally applicable to a broad range of core shell hybrid nanostructures.

References

[1] G. Wang, D. Garcia, Y. Liu, R. de Jeu, A. J. Dolman, Environ. Modell. Softw. 2012 , 30 , 139.

[2] W. Van Aarle, W. J. Palenstijn, J. De Beenhouwer, T. Altantzis, S. Bals, K. J. Batenburg, J. Sijbers, Ultramicroscopy 2015 , 157 , 35

[3] D. Stalling, M. Westerhoff, H.-C. Hege, in The Visualization Handbook, (Eds: C. D. Hansen and C. R. Johnson), Academic Press, Elsevier, 2004 , pp. 749–767.

Figures:

Figure 1: TEM image of a Au@SiO2 particle. A) The image was acquired using HAADF-STEM. The core can be clearly seen but the intensity from the shell is too low. B) The image was acquired using ADF STEM, showing that the intensity from the SiO2 shell becomes prominent.

Figure 2: Slices through the silica-coated Au nanoparticle viewed in XZ (A) and XY (B) orientations. Dark and streaking artifacts can be observed in the vicinity of the Au nanorod and are highlighted with white arrows.

Figure 3: A) Slice through the mesoporous silica shell, viewed in XZ orientation. After optimization of reconstruction, dark and streaking artifacts (see Figure 2 A, B) were eliminated. B) Volume rendering of the combined HAADF-STEM and ADF-STEM reconstructions viewed along XZ orientation.

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EMC Abstracts - https://emc-proceedings.com/abstract/multi-adf-detector-tomography-for-3d-characterization-of-heterostructures/