Application of coincidence ion mass spectrometry for chemical and structural analysis at the sub-micron scale

Abstract

Surfaces can be probed with a variant of secondary ion mass spectrometry (SIMS) where the bombardment is with a sequence of single keV projectiles, each resolved in time and space, coupled with the separate record of the secondary ions (SIs) ejected from each projectile impact. The goal of this study was to demonstrate an efficient mode of SIMS where one obtains valid analytical information with a minimum of projectiles and hence a minimum of sample consumption. An inspection of the ejected SIs from individual bombardment events will reveal ??super efficient?? collision cascades i.e., events, where two or more secondary ions were emitted simultaneously. It has been shown that these coincidental emissions can provide information about the chemical composition of nano-domains. Previous studies using coincidence counting mass spectrometry (CCMS) indicated an enhancement of identifying correlations between SIs which share a common origin. This variant of SIMS requires an individual projectile impact thus causing SI emission from a surface area of ~5 nm in radius. Thus, in an event where two or moreSIs are ejected from a single projectile impact, they must originate from atoms and molecules co-located within the same nano-domain. Au nanorods covered by a 16-mercaptohexadecanoic acid (MHDA) monolayer were analyzed using this methodology. A coincidence ion mass spectrum was obtained for the MHDA monolayer covered Au nanorods which yielded a peak for a Au adduct. Similar results were obtained for a sample with a MHDA monolayer on a Au coated Si wafer. A series of samples consisting of Cu aggregates and AuCu alloys were investigated by SIMS to demonstrate that this technique is appropriate for characterizing nanoparticles. The mass spectra of these samples indicated that Au200 4+ is an effective projectile to investigate the surface of the target because it was able to penetrate through the poly(vinylpyrrolidone) (PVP) stabilizer that coated the surface of these nanoparticles. Coincidence mass spectra of the Cu aggregates yielded molecules colocated within the same nano-domain. Finally, this methodology was used to investigate surface structural effects on the occurrence of ??super-efficient?? events. The results indicated that it is possible to distinguish between two phases of ??-ZrP compounds although the stoichiometry remains the same.