Seed-induced penetration: a new tool for the synthesis of core-shell nanoparticles using superfluid helium droplets

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Helium nanodroplets are very small drops of liquid helium, with diameters at 1/1000 ~ 1/100,000 of a human hair. They are extremely cold, with a temperature as low as 0.38 K. When molecules and atoms encounter helium droplets, they will be captured, and in most cases, migrate to the interior of helium droplets. Helium droplets are superfluid, and the bonding between helium atoms and the dopants are often very weak. Therefore, when more than one atom or molecule are picked up, molecular clusters will be formed. When more dopants are added to helium droplets molecule by molecule, or atom by atom, and different materials are added in sequence, core-shell nanoparticles (the nano-onions ), will be formed. Core-shell nanoparticles can have exotic properties depending on the size and compositions. The unique properties of helium droplets make it ideal for fabrication of nanoparticles because they allow almost any combination materials to be grown.However, not everything goes to the interior of helium droplets. For high-spin metal atoms, e.g., Na and K, they will reside on the surface of helium nanodroplets and will hinder the growth of sizeable nanoparticles using helium droplets. A seeding technique will be therefore introduced in this research to overcome this major difficulty for the formation of nanoparticles with high-spin metals. A number of core-shell and core-multiple shell nanoparticles will be synthesized which have potential for a number of applicatoins in material science, biomedical science, imaging and strorage devices.