Low energy impact of size selected FeCo nanoparticles with a W(1 1 0) surface

Deposited mass-filtered FeCo nanoparticles were investigated in situ by means of scanning tunneling microscopy (STM). The particles are generated in the gas-phase by a continuously working, UHV compatible Arc Cluster Ion Source (ACIS) and subsequently mass-, i.e. size-selected by an electrostatic quadrupole. After size-selection these particles were deposited on a clean W(1 1 0)-crystal surface. The size of the deposited FeCo particles is tuned between 5 and 15 nm. Because of the preparation, deposition and characterization is carried out under UHV conditions neither oxidation nor contamination takes place. The kinetic energy of the nanoparticles can be varied by applying an external field at the substrate because they are charged thus allowing an acceleration or deceleration. We focus on the landing behavior of these nanoparticles and their possible structural change after the collision with the target surface. It is shown that for different deposition energies slightly above and below the one for softlanding condition a significant change of the crystallographic structure occurs. We found an unexpected thermal induced phase transition of the nanoparticles after the impact on the surface at kinetic energies being ≈0.2eV/atom.