We investigate the formation, diffusion, and motion of Au-Ge nanodroplets on atomically flat Ge(111) terraces under a thermal gradient. The observations are carried out in real time, by low-energy electron microscopy. Au deposition leads to nucleation of mobile nanodroplets that exhibit biased diffusion toward the hot side of the substrate. Quantitative analysis of the droplet trajectories reveals that the diffusion coefficient scales with the droplet's radius as $D_{\mathrm{drop}}\propto R^{-2}$, consistent with a rate-limiting mechanism dominated by atom attachment/detachment at the droplet-substrate interface, as a possible interpretation. The droplet velocity shows no statistically significant dependence on droplet radius within the investigated range. In contrast, the velocity increases with increasing temperature. The effective activation energy for the droplet motion is 0.5 $\pm 0.3\mathrm{eV}$. These results provide insights into the atomic-scale mechanisms governing nanodroplet dynamics on semiconductor surfaces.

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• Diffusion
• Island
• Surface diffusion
• Thermophoresis
• Alloys
• Liquid metals
• Nanoparticles
• Electron microscopy
• Physical deposition