Valence EELS study of the composition of a liquid phase in a Janus Sn-Ge nanoparticle over a temperature range of 250–750 °C

Mapping the composition of liquid alloy nanoparticles in TEM at relatively low electron doses is essential for emerging nanotechnologies. In this work, we used volume and surface plasmon energies to determine the composition across different regions of a single Sn-Ge nanoparticle over a temperature range of 250-750 °C. A 53 nm Janus nanoparticle, composed of liquid Sn and solid Ge sides, was heated in a TEM, inducing the gradual dissolution of Ge into liquid Sn. Low-loss electron energy loss spectral images were acquired at 50 °C intervals, and plasmon energies were accurately measured using model-based fitting. We demonstrated that the free-electron gas Drude model, combined with Zen's law of alloy volume-concentration relation, enables the reliable determination of the composition of liquid Sn-Ge alloy from both surface and volume plasmon energy shifts. The determined compositions of the liquid alloy were consistent with EDX measurements and the liquidus line of the phase diagram. A homogeneous distribution of chemical elements in the liquid Sn-Ge alloy was revealed. At the same time, the composition on the Ge side of the nanoparticle was inhomogeneous, indicating the formation of a thin liquid shell over the solid Ge core. As a result, Ge in the Sn-Ge Janus nanoparticle exhibited highly tunable surface plasmon resonance, with its energy varying between 10.75 and 9.25 eV over a temperature range of 250-750 °C.