Secondary Ion Yield Enhancement in organic samples due to Au/Pt nanoparticle condensation and their substrate effects

Upon atomic ion bombardment, metal nanoparticles at the surface of organic materials (polymers, additives, biomolecules) induce a strong increase of the analytically significant sample ion yields in secondary ion mass spectrometry. In order to unravel the physics behind the Metal Assisted SIMS (MetA SIMS) effect, this study focuses, first, on the comparison between thin layers of polystyrene (PS) on gold-covered substrates and thick layers of PS covered with different thicknesses of Au. The evolutions of the characteristic fragment ion yields are almost identical up to 8 nm equivalent Au thickness but the yields of Au-cationized oligomers are consistently one order of magnitude larger when gold is used as substrate. This difference might be due to the limited diffusion of PS on the Au condensates. Second, the effect of Au and Pt on the ion yield enhancement has been studied by casting thin layers of PS on Au and Pt-covered Si substrates. The Au and Pt-induced fragment ion yields evolve similarly with the metal thickness up to 6 nm. The yields of metal cationized oligomers, however, are 7-8 times lower in the case of Pt compared to that of Au. Tentative interpretation of this effect involves the nature of the Me-PS bond and the abundance of the respective species.