Formation process of alloy nanoparticles synthesized by sputtering: noncontact monitoring using resonant ultrasound spectroscopy

PdAu alloy nanoparticles show unique properties, and several studies have been performed on PdAu alloy nanoparticles. For example, it is reported that the cross-coupling reaction using PdAu alloy nanoparticles is more efficient than that using Pd nanoparticles, and PdAu nanoparticles are used as a catalyst to reduce sulfite in water. It is expected that PdAu nanoparticles enhance the surface-enhanced Raman scattering effect. Application of PdAu nanoparticles to hydrogen sensors is also studied. Alloy nanoparticles can be synthesized by several methods. Sputtering of two metals simultaneously on a substrate is one of the methods. Using the sputtering, the composition can be changed by changing target metals, and the composition ratio can be changed by controlling the sputtering rates. These features are suitable for fabricating alloy nanoparticles of desired composition. However, formation process of alloy nanoparticles by sputtering has not been understood completely. When the metals are sputtered on a substrate, atoms diffuse on the substrate and alloy nanoparticles are formed through nucleation and nucleus growth. Internal structure of alloy nanoparticles then becomes disordered structure (Fig. 1 (a)), core-shell structure (Fig. 1 (b)), and so on, depending on the composition and sputtering conditions. Because the properties of alloy nanoparticles change depending on the internal structure, it is important to understand the formation process of alloy nanoparticles to obtain desired internal structures and properties. However, it is not straightforward to observe the formation process of alloy nanoparticles during sputtering. For this reason, the formation process has not been clarified. Under the circumstances, in this study, we observed the formation process of PdAu alloy nanoparticles in real time during sputtering using the resistive spectroscopy method, and internal structure of PdAu alloy nanoparticles is investigated.