Scanning tunneling microscopy observation of graphite surfaces irradiated with size-selected Ar cluster ion beams

A cluster ion irradiation system with cluster size selection has been developed to study the effects of the cluster size in surface processing using cluster ions. A permanent magnet with a magnetic field of 1.2T was installed for size separation of large cluster ions. Traces formed on a graphite surface by the impact with Ar cluster ions under an acceleration energy of 30 keV were investigated by, scanning tunneling microscopy. The nature of the traces is strongly affected by the number of constituent atoms of the irradiating cluster ion. When the cluster ion is composed of 100-3000 atoms, crater like traces are observed on the irradiated surfaces. In contrast, such traces are not observed at all with the irradiation of the cluster ions composed of over 5000 atoms. This behavior is discussed on the basis of the kinetic energy per constituent atom of the cluster ion.