Utilizing gradient samples to investigate solid state dewetting of bimetallic alloys

Abstract text (incl. figure legends and references)

Solid-state-Dewetting (SSD) is a typical degradation process of thin metallic films deposited on a substrate when annealed below the melting point. The process is driven by the minimization of total energy through surface- and interfacial energy reduction. While it used to be an undesirable effect, nowadays it is utilized as a powerful tool to fabricate bimetallic alloyed nanostructures for e.g. catalysis [1]. Here, switching from a single component film to binary systems makes the whole process more complex. A detailed understanding of this process is, however, unavoidable in order to fabricate nanostructures with targeted composition, shape and size.

In the present study, we present an elegant route for the fabrication of single-gradient (Fig.1 A) and triple gradient (Fig.1 B) samples via e-beam evaporation of Au and Ni on (0001) oriented α-Al2O3. Subsequently, rapid thermal annealing is utilized at 890°C for 120s in reducing atmosphere (Ar/5% H2). The temperature is chosen in order to anneal the samples above the miscibility gap of the Au-Ni system for the production of AuxNi1-x alloys. This type of sample preparation allows i) the preparation of AuNi alloys with fine concentration differences to study the effect of alloying on SSD and ii) to study alloying effects and film thickness effects using a single sample.

Ex-situ scanning electron microscopy (SEM) studies of the thermal annealed single-gradient sample are displayed in Fig. 2A. The homogeneous back-scatter contrast in the particles confirm a successful formation of an AuNi solid solution for all compositions ranging from 1 at% Au up to 99 at% Au. Via segmentation of the SEM images, the ratio between the exposed and unexposed are extracted to visualize the Au concentration dependent dewetting kinetics (Fig. 2B). An increased dewetting kinetic can be found in the range of 50 at% to 70 at% which is in good agreement with our recent studies [2]. Using EBSD measurement the texture of the formed particles is investigated (Fig. 2 C). The pole figures show a strong 111 out-of-plane texturing for a wide concentration range. Furthermore, the microstructure is fibre-like with tendencies towards epitaxial orientation relationships of AuNi (111)[1-10] || α-Al2O3 (0001)[10-10]. Interestingly this texture is substanial lost for the sample with 60at% Au leading to a texture free microstructure of the particles.

With the presented approach, the influence of a multitude of different factors like atomic size mismatch during alloying onto the SSD of bimetallic alloys can be investigated in just one single sample. Furthermore, we demonstrate, that the concentration can be tuned continuously over a concentration range from 1 at% to 99 at% Au, which allows a systematic investigation of the influence of the misfit between metal particles and ceramic substrate, which is currently part of further investigations.

[1] Shahvaranfard et al., CS Appl. Mater. Interfaces 2020, 12, 34, 38211–38221

[2] Dierner et al., unpublished work