Phase segregation in NiAu-nanoalloys induced by swift electrons

Session

PST.6 - In-situ and in-operando microscopy

Authors

Daniel Knez (1, 3), Martin Schnedlitz (2), Maximilian Lasserus (2), Andreas W. Hauser (2), Wolfgang E. Ernst (2), Ferdinand Hofer (1, 3), Gerald Kothleitner (1, 3)

Affiliations

1. Institute for Electron Microscopy and Nanoanalysis, Graz University of Technology
2. Institute for Experimental Physics, Graz University of Technology
3. Graz Center for Electron Microscopy

Keywords

in situ STEM, Molecular Dynamics, NiAu cluster, phase segregation,Thermodynamics of nanoalloys

Abstract text

With recent advances in aberration corrected electron optics, scanning transmission electron microscopy (STEM) has proven its excellent capability of characterising nanomaterials. In principle, with STEM chemical information can be extracted at the level of individual atoms. Due to the high current densities occurring in a highly focused electron beam, however, interpretation of STEM data is often impeded by continuous beam induced sample changes. This is especially true for systems with a high percentage of weakly bound surface or interface atoms, which is characteristic for nanoscaled materials [1]. 

In previous work we studied beam induced mass loss in metallic clusters, surface diffusion of adatoms on crystalline surfaces and hollowing of Ni clusters, for instance [2-4]. Furthermore, we developed a computational scheme for the simulation of such phenomena based on a combination of molecular dynamics and Monte Carlo techniques [2].

Here, we will report on the electron beam induced segregation of alloyed Ni-Au clusters into a Ni and a Au rich phase at temperatures above the miscibility gap of the system. The bimetallic nanoparticles with diameters of less than 10 nm are grown fully inert in superfluid helium droplets with a Ni-Au core-shell morphology [5]. Upon heating above 400 °C, the clusters are alloyed (first image in Figure 1a). Under subsequent irradiation with 300 keV electrons, however, they transform to a Janus-type morphology, as shown in Figure 1a. This process is reversible and the cluster transform back to the initial, alloyed state after electron irradiation is stopped.

We study these processes via transient in situ STEM and find that the segregation kinetics is highly temperature-dependent following an exponential relationship, which strongly indicates the occurrence of diffusive relaxation processes. 

The observed phenomenon is further elucidated by using our computational framework (see Figure 1b) [2, 6]. Thereby, we found that Au atoms exhibit an approximately ten times higher displacement cross section compared to Ni and identified weakly bound atoms inside the cluster (marked by a green arrow in Figure 1b). We suggest that electron beam induced displacements of such Au atoms are followed by diffusive relaxation processes of adjacent Ni neighbours, resulting in phase segregation at elevated temperatures [7].

References

[1] R.F. Egerton, Microsc. Res. Tech. 75, (2012), p. 1550.

[2] D. Knez et al., Ultramicroscopy 182 (2018), p. 69.

[3] T. Furnival et al., Appl. Phys. Lett. 113, (2018) p. 183104.

[4] D. Knez et al., Ultramicroscopy 176, (2017), p. 105.

[5] M. Schnedlitz et al., Chem. Mater. 30, (2018), p. 1113.

[6] D. Knez et al., Appl. Phys. Lett. 115, 12 (2019), p. 123103.

[7] The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 823717-ESTEEM3.