טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentCohen Yossi
SubjectReactive Collisions of C60- Ions at KeV Energies with
Transition Metal Surfaces
DepartmentDepartment of Chemistry
Supervisor Professor Eliezer Kolodney


Abstract

Reactive ion-surface scattering involving cluster ions in the keV kinetic energy regime is a rarely explored area. In the following we describe the reactive collisions of C60- ions with transition metals by analyzing both the emitted clusters in the gas phase and the thin films formed in the solid phase under these collisions. In order to address this subject we have focused on several different metals which span a broad spectrum of reactivity towards carbon. Ranging from gold, which does not form stable carbide, to tantalum which does form highly stable carbide films.

The main achievements in this work are:

-          A novel approach for the formation of unique carbon based materials was found. High kinetic energy collisions of fullerene ions with metal surfaces impulsively created numerous metal-carbon compounds. Positively and negatively charged gold, silver, copper and tantalum carbide cluster ions, were observed following the impact of C60- ions on clean metal surfaces at kinetic energies of 0.8-14 keV. Using surface analysis measurements we have found that during fullerene bombardment on copper and tantalum surfaces, ultra-thin (nanometers) of carbidic polycrystallinic films were grown.

-          Time dependence of signal intensity for scattered ions was measured. A few orders of magnitude immediate carbide signal jump instantaneous with the C60- ion beam opening was observed for all targets. It was concluded that the process of impulsive carbides formation can be described as an Eley-Rideal - like reactive scattering event in which several surface atoms are being simultaneously collected by fullerene's fragment within sub-picosecond to one picosecond time scale.

-          Using density functional theory we have calculated the structure of the experimentally found carbidic species. M3C2 ion which is the most abundant carbide in the mass spectra during keV C60- bombardment of all coinage metals targets was found to possess a unique structure of acetylenic M2C2 structure with a π-coordinated third metal atom. The calculations show meaningful contributions of direct metal-metal bonding to the overall stability of the M3C2 complex.

-          Kinetic energy distribution measurements of emitted atoms and clusters have shown two main formation mechanisms for the carbidic species. The small carbidic species (AuC, AuC2 and AgC) formation is via a recombination between independently emitted, moving gold/silver and carbon species, within the surface vicinity. The larger carbidic species (AuCm, AgCm, m>9) formation is via a thermic mechanism in which the colliding C60- creates a super-hot volume from which atoms and clusters are evaporated.