MOLECULAR DYNAMICS SIMULATIONS OF ATOMIC AND CLUSTER BOMBARDED SURFACES

Open Access
- Author:
- Smiley Jr., Edward J
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 17, 2006
- Committee Members:
- Nicholas Winograd, Committee Chair/Co-Chair
Barbara Jane Garrison, Committee Chair/Co-Chair
Peter C Jurs, Committee Member
Vincent Henry Crespi, Committee Member - Keywords:
- molecular dynamics
secondary ion mass spectrometry
cluster
C60 - Abstract:
- The mechanism of enhanced desorption initiated by 15 keV C60 cluster ion bombardment of a Ag single crystal surface is examined using molecular dynamics computer simulations. The size of the model microcrystallite of 165,000 atoms and the sophistication of the interaction potential function yields data which are directly comparable with experiment. The C60 model is chosen since this source is now being used in secondary ion mass spectrometry experiments (SIMS) in many laboratories. The results show that a crater is formed on the Ag surface that is about 10 nm in diameter. The yield of Ag atoms is about 16 times larger than for corresponding atomic bombardment with 15 keV Ga atoms, and the yield of Ag3 is enhanced by a factor of 35. The essential mechanistic reasons for these differences is that the C60 kinetic energy is deposited closer to the surface, with the deeply penetrating energy propagation occurring via a non-destructive pressure wave. The calculations have been designed to provide fundamental insight into the experimental techniques of SIMS. The calculated kinetic energy distributions of Ag monomers and Ag2 dimers compare favorably with experimental results. The approach is extended to include the study of molecular solids, a situation of growing importance to the SIMS community. Emission of benzene molecules by 5 keV cluster bombardment of a range of carbon projectiles from C6H6 to C180 is studied by a coarse-grained (CG) molecular dynamics (MD) technique. This approach permits calculations that are not feasible using more complicated potential energy functions, particularly as the interesting physics associated with the ion impact event approaches the mesoscale. These calculations show that the highest ejection yields are associated with clusters that deposit their incident energy 15 to 20 Å below the surface. The maximum yield for the projectiles is produced by the C20 and C60 projectiles. The results from the MD simulations are also compared favorably to an analytical model based on fluid dynamics to describe the energy deposition. The analytical model is then utilized to extend the range of the calculations to larger incident energies. The issue of the relative amount of chemical fragmentation and intact molecular desorption is also examined for the benzene crystal. These results show that damage accumulation at high incident fluence should not be problematic and that it should be possible to perform molecular depth profiling via SIMS experiments. In general, the approach presented here illustrates the power of combining a simplified MD method with analytical strategies for describing a length scale that is difficult to achieve with traditional MD calculations.