In-situ observation of structural and optical properties changes in Ag-Cu plasmonic nanoparticles by using specimen heating holder

Abstract text (incl. figure legends and references)

[Introduction] Plasmonic nanoparticles are widely studied and used in various fields because they show unique optical properties. The optical properties, due to the surface plasmon resonances around the nanoparticles, are known to be dependent on their sizes, shapes, and dielectric functions of materials. Previously we fabricated several kinds of nanoparticles with different elemental compositions using the dewetting method[1, 2], and studied their optical properties and nanostructures by Transmission Electron Microscope (TEM). However, there are still unclear parts in the formation process of the nanoparticles during the heating procedure. In order to control and optimize the optical properties properly, the knowledge of the fabrication process of the nanoparticles becomes to be essential and important information.

[Objectives] We aim to clarify the process of nanoparticle formations and their structural changes during the dewetting procedure using a TEM and a dedicated specimen heating holder.

[Materials and Methods] The initial state of the sample was prepared by co-evaporating pure Ag and pure Cu on a thin SiO2 film for TEM observation. In order to characterize the structural changes of nanoparticles, we performed in-situ TEM observation by elevating the temperature from room temperature (RT) to 400 °C by a specimen heating holder. Besides, we measured the bulk optical properties of the nanoparticles during the heating process, comparing to their structural changes.

[Result] Figure 1 shows the in-situ measurement results of the optical properties of Ag-Cu nanoparticles. The peaks of the extinction spectrum were shifted to the shorter wavelength during the heating process and the double peaks were observed at higher temperatures. Figure 2 shows the elemental distributions of Ag-Cu nanoparticles by EELS at RT and 400 °C. The core-shell type nanoparticles were formed at RT. After the heating process, we found the nanoparticles changed to the Janus-type phase-separated nanoparticles.

[Conclusion] We investigated the structural changes of Ag-Cu nanoparticles by in-situ TEM observation. From the in-situ TEM results, we found that the size of the nanoparticles increased notably and in addition the Ag and Cu phases are more clearly separated at 400 °C. The red-shift of extinction spectrum is thought to be caused by the changes in the sizes and shapes of nanoparticles. The double peaks of the extinction spectrum are concluded to be related to the phase separation of the nanoparticles.

Figure 1. Extinction spectrum of Ag-Cu binary nanoparticles during the heating process

Figure 2. Elemental maps of Ag-Cu binary metal nanoparticles at room temp (a) and 400 °C (b) measured by Electron Energy Loss Spectroscopy (EELS). Green and red colors in elemental maps indicate Cu and Ag elemental distribution, respectively.

[References]

[1] C. Wadell et al., J. Phys. Chem. C. 121 (48), 27029-27035, 2017,

[2] A. Yasuhara et al., J. Phys. Chem. C. 124 (28), 15481–15488, 2020