Site-specific and advanced preparation of ultra-thin TEM samples of nanostructured materials

Abstract text (incl. figure legends and references)

Advanced microanalytical investigation techniques allow to characterize the structure as well as the chemical composition of materials down to atomic scale. Especially, nanostructures with their confined length scales require a well-defined and extremely precise sample preparation to elucidate their unique characteristics [1]. Newest dual focused ion beam scanning electron microscopy (FIB-SEM) enables such indispensable site-specific sample preparation with high accuracy and precision on the nm-scale. However, beam sensitive materials and nonconductive substrates increase the challenges of an accurate preparation.

In this work, we illustrate the possibilities and challenges of targeted preparation of nanostructures using the latest FIB-SEM techniques. A selection of investigated nanostructures is presented in Fig. 1. Each unique nanostructure is analyzed precisely in SEM (first column) to identify the desired TEM lamella orientation with respect to the morphology and arrangement of the nanostructure as well as its orientation towards the substrate.

The first sample system are gold bipyramids (Fig. 1a) which exhibit a five-fold-twinned crystal structure with interesting optical properties [2,3,4]. The maximum extent of about ≈65nm limits significantly the position of the lamella and a high accuracy during the preparation is needed. The desired cross section must be perfectly in the center to study the present structure on the atomic scale. The final TEM studies reveal the fivefold twinned atomic structure. The lamella delimits a thin region around the center of the bipyramid, with their surfaces parallel to the central base. In this way, the diameter at the central base can be accurately determined, which shows a rounded geometry compared to the optimal structure.

Fig. 1b shows Au_xNi_1-x nanoparticles on a α-Al₂O₃ substrate. In this system, the interface between the ceramic substrate and the metal is of interest due to great scientific and technological importance, with applications in electro-ceramic devices, structural composites and catalysis [5]. Therefore, the nanoparticle orientation with respect to that of the underlying substrate is identified by electron backscatter diffraction (EBSD) measurements. Afterwards, the cross section is prepared in such a way that the center of the particle is located exactly in the center of the lamella (Fig. 1b). Besides the extreme precision, the non-conductive substrate is very challenging. Additionally, advanced STEM methods are needed to image the interface between high and low atomic number materials [5]. For this purpose, lamellae with a thickness of less than 40nm and a very good surface quality are required.

Besides one-dimensional structures, also single nanowires (NW) and the corresponding networks are analyzed. The electrical properties of such networks or NW in contact can be strongly improved by heat treatment to weld the corresponding contact points [6]. The presented example (Fig. 1c) shows two identified and select Ag NW after heat treatment. To investigate the welding zone between both wires, the corresponding cross section has to be precisely prepared. The achieved lamella reveals that the intersecting NW unite with each other and creating a constant internal crystalline microstructure.

[1] 10.1017/S1431927613013780

[2] 10.1021/ac900276n

[3] 10.1021/jacs.6b12143

[4] U. Frank et al. ACS Nano, under review

[5] 10.1038/s41598-018-20377-2

[6] 10.1002/aelm.202100787