Advanced Electron Microscopy of Clusters Grown Inside Superfluid Helium Droplets: Advances, Challenges, and Perspectives

Superfluid helium nanodroplets act as ultracold, nanoscale “cryostats” that enable the synthesis of exceptionally pure metallic, bimetallic, and hybrid nanoparticles with precise control over size and architecture. This review highlights how advanced electron microscopy—particularly aberration‐corrected scanning transmission electron microscopy combined with spectroscopy—reveals the structure, composition, and three‐dimensional morphology of these particles at near‐atomic resolution. In situ heating and cooling experiments uncover unique thermodynamic behaviors, such as nanowire breakup, alloying, and structural inversion, while studies on beam‐induced effects expose atomic displacements and radiolysis‐driven chemistry. The soft‐landing deposition of the helium droplet method preserves metastable configurations and facilitates the creation of nanoparticle architectures that are unattainable by conventional routes. Looking ahead, emerging low‐dose imaging techniques, phase‐sensitive methods, and machine learning‐driven analyses promise to further expand our ability to design and study functional nanomaterials for use in catalysis, plasmonics, and quantum technologies.