Thermal stability and optical properties of Si-Ge nanoparticles.

Silicon-germanium is an important alloy mainly used in thermoelectricity and electronics. However, its thermal and optical properties still need further investigation at the nanoscale. That is why in this study, the size and shape effect on the silicon-germanium phase diagram is investigated through the nano-thermodynamics methodology. As expected, the phase diagram undergoes a shift down in temperature when the size decreases. However, it is demonstrated and explained why the size effect on the solidus-liquidus curves is much stronger than the one on the miscibility gap. Moreover, the shape effect is investigated for various faceted polyhedral nanoparticles as well as for the sphere. Phase maps are then provided as a function of the number of facets, at 4 and 10 nm, in order to determine the structure of the alloy. Furthermore, the size and shape effects on the energy bandgap are also studied. The energy bandgap increases when the size is reduced. The cube and tetrahedral shapes exhibit the largest size effect on the thermal and optical properties of the silicon-germanium alloy. Finally, this paper provides a useful roadmap for experimentalists willing to tune the properties of this alloy.