Type of presentation: Oral

MS-1-O-2269 HR-STEM investigations of metallic nanoparticles grown with superfluidal He-droplets

Knez D.¹, Volk A.², Thaler P.², Fisslthaler E.¹, Grogger W.¹, Ernst W. E.², Hofer F.¹

¹Institute for Electron Microscopy and Nanoanalysis, Graz University of Technology, Austria, ²Institute of Experimental Physics, Graz University of Technology, Austria

daniel.knez@felmi-zfe.at

Metallic nanoparticles have attracted more and more interest in recent years as they exhibit completely new physical and chemical properties compared to bulk materials. Over the years numerous synthesis methods, mostly based on wet chemical processes, pyrolysis or evaporation have been developed. In contrast, the nanoparticles used for our investigations were synthesized by using superfluid helium nanodroplets (composed of 10³ to 10⁶ helium atoms) at around 0.4 K under ultra-high vacuum (UHV) conditions.¹ This approach provides exceptional advantages over conventional methods like sequential addition of a wide range of materials.
Thus, nanoparticles can be synthesized with any composition and different structures, with extremely high purity, which cannot be achieved by other known methods. Figure 1 shows a schematic of the synthesis facility.² Knowledge of the morphology, dimension and composition of the produced particles are not only essential for understanding their physical and chemical properties, but also for optimizing the synthesis parameters. By using a probe corrected, monochromated FEI Titan³ 60-300 equipped with a Super-X detector (EDX) and a Gatan Quantum energy filter we performed analytical high resolution STEM (HR-STEM) in order to characterize metal nanoparticles with respect to their morphology and chemistry. The HR-STEM investigation of Ag nanoparticles on 3 nm carbon (prepared by the He-droplet method) reveals very small Ag clusters (3-6 nm in size) exhibiting a decahedral structure.

Furthermore, bimetallic clusters with a gold-silver core-shell structure were synthesized. STEM images (a and b in Fig. 3) of a AuAg particle reveal that it grew from single spherical particles inside the He droplet. Elemental analysis of the nanoparticles by EELS and EDX clearly showed that Ag and Au, which were added to the droplet sequentially, are not alloyed. The elemental distribution of this particle is shown in images f and g of Fig. 3.
Finally, the nano-optical properties of these metallic clusters will be studied via low-loss EELS measurements in the plasmon regime, which depend on their size, structure and morphology.^{3, 4} In order to quantify the influence of the underlying substrate, we will also compare conventional carbon films with mechanical exfoliated monolayer substrates (graphene and hexagonal boron nitride).

References:

1. P. Thaler et al., J. Chem. Phys. 140, 44326 (2014).

2. A. Volk et al., J. Chem. Phys. 138, 214312 (2013).

3. F.-P. Schmidt et al., Nano Lett. 12, 5780 (2012).

4. B. Schaffer et al., Micron 40, 269 (2009)

Our research is supported by the European Union within the 7^th Framework Programme (FP7/2007-2013) under Grant Agreement no. 312483 (ESTEEM2) as well as by the Austrian Research Promotion Agency (FFG).

Fig. 1: The helium droplets (blue) are produced in the source (1) by evaporation. After passing the skimmer (2) they collide with atoms or molecules (red) evaporated by the thermal evaporator (3). The particles congregate in the center of the droplet and finally land on the target (4) (e.g. a TEM-grid)	Fig. 2: HR-STEM images (a: HAADF, b: BF) of a silver nanoparticle on a 3 nm amorphous carbon film with decahedral morphology
Fig. 3: a: HAADF image of the AuAg core-shell particle; b: STEM BF image showing that the particle grew from single spherical particles; c-d: EDX elemental maps for Au (c) and Ag (d); e: EELS Ag map (calculated via MLLS fitting); f-g: RGB maps illustrating the elemental core-shell distribution with data from (c) and (e) in (f) and from (c) and (d) in (g)