Microscopic study of (assemblies of) nano-objects

2D and 3D imaging of supra-particles

Introduction

Nanoparticles of various shapes can self-assemble into new structuring material, offering better controlled optical (e.g. polarization of light emission) and electronic (e.g. conductivity) properties in finely tuned 3D structures at multiple scales.

In order to functionalise such structuring matter, it is essential to control the length scales of the self-assemblies and the inner crystallographic orientation.

In this case, self-assemblies of non-spherical particles with size and shape uniformity are introduced since they can form various hierarchical structures with controlled orientations that matter to the overall material properties. [1,2].

Example 1

Here is an example of Fe3O4 nanocubes with an average core side length of 22.7 nm (24.1 nm total side length due to ligands).

By altering the synthetic conditions, the cubes could become round-edged, ending up with a different 3D structure when self-assembled.

This SEM image is of the self-assembled superstructure of nanocubes in drying emulsion droplets. It can be seen that the nanocubes on the exterior appear aligned through the spherical confinement.

Further TEM and simulation studies have revealed that the inner core exhibits a simple-cubic structure. [2]

Example 2

This is an example of assemblies of silica-coated gold nanorods. The rods have an average length of 119 nm and a cross-section diameter of 16 nm.

3D imaging of such nanorod assemblies can be achieved using FIB-SEM tomography at high resolution.

Furthermore, the position and orientation of the individual nanorods in assemblies could be obtained, which is essential when studying the relationship between the assembly structure and its physical property enhancement compared to single rods. [3]

Challenges of tomographic acquisition

Compared to conventional electron tomography in TEM, FIB-SEM serial sectioning tomography allows for much larger assembly supra-particles to be imaged, e.g. 500–600 nm in diameter with hundreds of nanoparticles assembled.

No special preparation of the sample beforehand is needed, such as applying a dye which is common for TEM imaging.

However, due to their potential delicate structure, extra precautions are needed to preserve the sample’s initial status during the long acquisition process.

If possible, imaging in cryogenic conditions is advantageous to maintain the structure and protect the sample under the attack of either or both beams.

References

[1] Bart de Nijs, Simone Dussi, Frank Smallenburg, Johannes D. Meeldijk, Dirk J. Groenendijk, Laura Filion, Arnout Imhof, Alfons van Blaaderen and Marjolein Dijkstra (2014) 'Entropy-driven formation of large icosahedral colloidal clusters by spherical confinement'. Nature Materials 14, 56–60, DOI

[2] Wang D, Hermes M, Kotni R, Wu Y, Tasios N, Liu Y, de Nijs B, van der Wee EB, Murray CB (2018) 'Interplay between spherical confinement and particle shape on the self-assembly of rounded cubes'. Nature Communications 9, (1), DOI

[3] van der Hoeven JES, van der Wee EB, de Winter DAM, Hermes M, Liu Y, Fokkema J, Bransen M, van Huis MA, Gerritsen HC (2019) 'Bridging the gap: 3D real-space characterization of colloidal assemblies via FIB-SEM tomography'. Nanoscale 11, (12) 5304-5316, DOI.