Nanoscale Phase-Separated Structure in Core-Shell Nanoparticles of SiO-SiGeO Glass Revealed by Electron Microscopy.

SiO-based optical fibers are indispensable components of modern information communication technologies. It has recently become increasingly important to establish a technique for visualizing the nanoscale phase-separated structure inside SiO-GeO glass nanoparticles during the manufacturing of SiO-GeO fibers. This is because the rapidly increasing price of Ge has made it necessary to improve the Ge yield by clarifying the detailed mechanism of Ge diffusion into SiO. However, direct observation of the internal nanostructure of glass particles has been extremely difficult, mainly due to electrostatic charging and the damage induced by electron and X-ray irradiation. In the present study, we used state-of-the-art scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and energy dispersive X-ray spectroscopy (EDX) to examine cross-sectional samples of SiO-GeO particles embedded in an epoxy resin, which were fabricated using a broad Ar ion beam and a focused Ga ion beam. These advanced techniques enabled us to observe the internal phase-separated structure of the nanoparticles. We have for the first time clearly determined the SiO-SiGeO core-shell structure of such particles, the element distribution, the degree of crystallinity, and the quantitative chemical composition of microscopic regions, and we discuss the formation mechanism for the observed structure. The proposed imaging protocol is highly promising for studying the internal structure of various core-shell nanoparticles, which affects their catalytic, optical, and electronic properties.