Piezoelectric Resonance Method for In Situ Monitoring of Formation of Pd-Based Bimetallic Nanoparticles Synthesized by Sputtering

When metallic material is sputtered on a solid surface, isolated nanoparticles are formed by nucleation, and a continuous film is formed after the nanoparticles grow and contact with each other. Just before the continuous film is formed, gaps of the order of a few nanometers appear between nanoparticles. This state is called nanogap nanoparticles. The nanogap nanoparticles show electrical properties different from those of isolated nanoparticles and continuous films, and it has been applied to hydrogen gas sensors.<br/>The nanogap nanoparticles can be synthesized by interrupting the sputtering just before the nanoparticles contact. However, the nanoparticles are formed when the height of the nanoparticles becomes around a few nanometers, and it is difficult to identify the moment to interrupt the sputtering. To solve this problem, we developed the piezoelectric resonance method. This method identifies the formation of nanogap nanoparticles by utilizing the resonant vibration of a piezoelectric material without contacting the substrate or the nanoparticles. Using this method, the gap size can be controlled, and it has been demonstrated to improve the performance of hydrogen gas sensors.<br/>After the above studies, we found that the piezoelectric resonance method is also applicable for in situ monitoring of the formation processes of bimetallic nanoparticles synthesized by sputtering. Bimetallic nanoparticles have attracted attention due to their properties, which are different from those observed in bulk materials or nanoparticles composed of a single metallic element. Since the properties change depending on the internal structure (core-shell structures, mixed structures, and intermetallic alloy structures), it is important to understand the internal structure and the associated formation process. However, observing the formation process during the synthesis is difficult, and it has not been clarified completely. In this presentation, we show that the formation process of core-shell nanoparticles can be monitored using the piezoelectric resonance method. Pd-based bimetallic core-shell nanoparticles are synthesized by sequential sputtering of two metals, and their growth is monitored. In the experimental results, sputtering of metal A followed by metal B tended to form B-shell/A-core nanoparticles. However, in the Pd-Au system, restructuring occurs during the synthesis, and core and shell turnover occurs. To validate the experimental results, we performed molecular dynamics simulations, and the availability of the developed method is demonstrated.