Nanoscale Phase-Separated Structure in Core–Shell\nNanoparticles of SiO₂–Si_1–<i>x</i>Ge_<i>x</i>O₂ Glass Revealed\nby Electron Microscopy

Abstract

SiO₂-based optical fibers are indispensable components\nof modern information communication technologies. It has recently\nbecome increasingly important to establish a technique for visualizing\nthe nanoscale phase-separated structure inside SiO₂–GeO₂ glass nanoparticles during the manufacturing of SiO₂–GeO₂ fibers. This is because the rapidly increasing\nprice of Ge has made it necessary to improve the Ge yield by clarifying\nthe detailed mechanism of Ge diffusion into SiO₂. However,\ndirect observation of the internal nanostructure of glass particles\nhas been extremely difficult, mainly due to electrostatic charging\nand the damage induced by electron and X-ray irradiation. In the present\nstudy, we used state-of-the-art scanning electron microscopy (SEM),\nscanning transmission electron microscopy (STEM), and energy dispersive\nX-ray spectroscopy (EDX) to examine cross-sectional samples of SiO₂–GeO₂ particles embedded in an epoxy resin,\nwhich were fabricated using a broad Ar ion beam and a focused Ga ion\nbeam. These advanced techniques enabled us to observe the internal\nphase-separated structure of the nanoparticles. We have for the first\ntime clearly determined the SiO₂–Si_1–<i>x</i>Ge_<i>x</i>O₂ core–shell\nstructure of such particles, the element distribution, the degree\nof crystallinity, and the quantitative chemical composition of microscopic\nregions, and we discuss the formation mechanism for the observed structure.\nThe proposed imaging protocol is highly promising for studying the\ninternal structure of various core–shell nanoparticles, which\naffects their catalytic, optical, and electronic properties.