Evolution in morphology and particle size of germanium nanoparticles with increasing temperature: transmission electron microscopy (TEM) images and corresponding particle size distributions for samples, sintering at (a) 25, (b) 475, (c) 625 and (d) 775 °C. The scale bar is 100 nm. Typical HRTEM images: insert (b) Ge crystals in a GeOx amorphous matrix, sintered at 25–475 °C; insert (d): pure Ge crystal nanoparticles, obtained at high temperature synthesis (above 625 °C); inserts (a), (c): a typical SAED pattern of prepared germanium nanoparticles with temperature sintering from 25 to 775 °C; (f): the dependence of the melting temperature on the diameter of the germanium nanoparticles; (g): typical normalized EDX spectra of nanoparticles sintered at low (25–475 °C) and high (above 625 °C) temperatures and corresponding XRD spectra obtained from a Ge NP on cellulose filter (insert).