Computer Simulation of keV Cluster Interaction with Molecular Solid
Supervisor: Roger Webb
Research Group: Ion Beam Centre
Molecular Dynamics (MD) is an effective computer program that is able to simulate the atomic interactions that occur during this sputtering process. It makes use of Newton’s laws of motion to govern inter-atomic interactions, and hence is able to provide invaluable insight into the mechanisms involved during these cluster-surface interactions. However, MD simulations require ~20 days to provide just 5ps (1ps = 10-12 s) worth of information, giving rise to a real-time to simulation time ratio of ~1017.
In our extremely fast paced world today, the MD simulation program is a massively time constraining one. Hence, there lies a demand for SIMS experimentalists for a tool by their side that can be used in a purely predictive manner with slight compromise on precision. This is where our Energy Deposition approach comes in. The concept here is that of an attempt to replicate outcomes of an impact-crater by means of an explosion-crater to match the outcome in terms of SIMS relevant information such as surface topography, sputter yield, fragmentation patterns, etc... The method of energy deposition involves using a Gaussian Energy Distribution for the kinetic energy and using a Random Isotropic Function for distributing the velocities of the atoms.
SIMS (Secondary Ion Mass Spectrometry) is a well-known surface analysis technique. It uses a primary beam of energetic single ions to bombard material surfaces and cause ejection or sputtering of species. This sputtered species is then analysed using a Mass Spectrometer to generate a mass spectrum which can be analysed further to determine the surface composition of the target material. A relatively modern approach is to use a beam of cluster ions instead of single ions due to its clear benefits in terms providing a higher sputter yield and a higher proportion of high-mass fragments, which is essential for good surface characterisation.
The images on the right show a 2 nm slice comparison of the impact and the explosion craters, concerning a 10keV C60 impact on a Benzene (C6H6) target. There is reasonable match in surface topography as well as the atom tracks shown in red. This result was arrived at after numerous testing of the sensitivity of the Energy Deposition to the final outcome. Further work requires demonstrating its applicability towards varying the impact energy of the projectile, the impact angle of the projectile as well as varying target materials.
Jaydeep D Mody