Nano-Domain Analysis Via Massive Cluster Secondary Ion Mass Spectrometry in the Event-by-Event Mode
Secondary ion mass spectrometry (SIMS) is a surface analysis technique which characterizes species sputtered by an energetic particle beam. Bombardment with cluster projectiles offers the following notable advantages over bombardment with atomic ions or small clusters: enhanced emission of molecular ions, low damage cross-section, and reduced molecular fragmentation. Additionally, in the case of Au4004 and C60 impacts, desorption originates from nanometric volumes. These features make clusters useful probes to obtain molecular information from both nano-objects and nano-domains. The "event-by-event bombardment/detection mode" probes nano-objects one-at-a-time, while collecting and storing the corresponding secondary ion (SI) information. Presented here are the first experiments where free-standing nano-objects were bombarded with keV projectiles of atomic to nanoparticle size. The objects are aluminum nano-whiskers, 2 nm in diameter and ~250 nm in length. Au4004 has a diameter of ~2 nm, comparable to the nominal diameter of the nanowhiskers. There are notable differences in the SI response from sample volumes too small for full projectile energy deposition. The whisker spectra are dominated by small clusters?the most abundant species being AlO- and AlO2-. Bulk samples have larger yields for AlO2- than for AlO-, while this trend is reversed in whisker samples. Bulk samples give similar abundances of large SI clusters, while whisker samples give an order of magnitude lower yield of these SIs. Effective yields were calculated in order to determine quantitative differences between the nano-objects and bulk samples. The characterization of individual nano-objects from a mixture is demonstrated with negatively charged polymer spheres that are attracted to and retained by the nano-whiskers. The spheres are monodisperse polystyrene nanoparticles (30nm diameter). Our results show that the event-by-event mode can provide information on the nature, size, relative location, and abundance of nano-objects in the field of view. This study presents the first evidence of quantitative molecular information originating from nano-object mixtures. Biologically relevant systems (solid-supported lipid bilayers) were also characterized using Au5 , Au4004 and C60 . Organization-dependent SI emission was observed for phosphocholine bilayers. Lipid domain formation was also investigated in bilayers formed from cholesterol and a mixed lipid system. Trends in the correlation coefficient suggest that cholesterol segregates from the surrounding lipid environment during raft formation.