The properties of nanocrystals are highly dependent on their morphology,composition and structure.To obtain full control over their properties,the behavior of nanocrystals under external stimuli,such as heat treatment,needs to be understood.Herein,to in situ observe their microstructure and morphology changes,Fe3O4-Ag heterodimers were selected as a model system.Their structural changes after heat treatment were investigated by in situ transmission electron microscopy.A combination of real-time imaging with elemental analysis enabled observation of the transformation of Fe3O4-Ag heterodimers having a loose interface configuration to those with a Janus structure at the atomic scale after heating from room temperature to 600 ℃.After incubation at 600 ℃ for 32 min,two kinds of Janus structures could be seen,including a clear linear interface in the Fe3O4-Ag heterodimers and a semi-crescent-shaped interface between the Ag and Fe3O4 nanoparticles (NPs).These dynamic observations provide unique insights into NP growth mechanisms,which are essential for understanding and controlling the structure and morphology of nanoparticles.