Experimental and numerical study of submonolayer sputter deposition of polystyrene fragments on silver for the storing matter technique.

In static secondary ion mass spectrometry (SIMS), quantification and high ionization probabilities are difficult to obtain. The Storing Matter technique has been developed to circumvent these issues and has already been applied to deposit inorganic and organic samples. For organic samples, the effect of fragmentation during sputter deposition and changing coverage on time-of-flight (TOF)-SIMS mass spectra has not been investigated. In this work, polystyrene (PS) was sputter deposited on silver using an argon ion beam in order to investigate these parameters and to get a better control of the whole process. For this purpose, we introduce a multitechnique characterization approach for the submonolayer deposition of PS. Experimental methods (TOF-SIMS, X-ray photoelectron spectroscopy (XPS)) were used in combination with simulations (density functional theory (DFT) calculations) in order to obtain information about the molecular and structural changes and the interactions of organic matter with the metal surface. Alterations of the PS surface and PS sputter deposit as a function of surface coverage and Ar(+) ion fluence are addressed. A major finding is that this approach can be used to identify surface reactions between different fragments on the collector surface. Indeed, in the dynamic regime, the ratio of large to small fragments is increasing although the fragmentation during the sputter deposition should lead to increasingly smaller fragments. Hence, for Storing Matter, the coverage on the collector must be kept low in order to minimize the reactions between fragments and to preserve the information on the original sample.

Reactive force field potential for carbon deposition on silicon surfaces.

In this paper a new interatomic potential based on the Kieffer force field and designed to perform molecular dynamics (MD) simulations of carbon deposition on silicon surfaces is implemented. This potential is a third-order reactive force field that includes a dynamic charge transfer and allows for the formation and breaking of bonds. The parameters for Si-C and C-C interactions are optimized using a genetic algorithm. The quality of the potential is tested on its ability to model silicon carbide and diamond physical properties as well as the formation energies of point defects. Furthermore, MD simulations of carbon deposition on reconstructed (100) silicon surfaces are carried out and compared to similar simulations using a Tersoff-like bond order potential. Simulations with both potentials produce similar results showing the ability to extend the use of the Kieffer potential to deposition studies. The investigation reveals the presence of a channelling effect when depositing the carbon at 45° incidence angle. This effect is due to channels running in directions symmetrically equivalent to the (110) direction. The channelling is observed to a lesser extent for carbon atoms with 30° and 60° incidence angles relative to the surface normal. On a pristine silicon surface, sticking coefficients were found to vary between 100 and 73%, depending on deposition conditions.