Characterization of Nano-Environments by Hypervelocity Projectile Secondary Ion Mass Spectrometry
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The purpose of this study was to explore the performance of a secondary ion mass spectrometry (SIMS) technique for probing nanovolumes. The variant of SIMS used involves bombardment with individual massive projectiles (Au400 and C60) at ∼ 1 keV/atom energy coupled with separate recording of the ionized ejecta from each projectile impact. Under these conditions of event-by-event bombardment/detection, each projectile acts as a nanoprobe and secondary ion emission is from an area of 10-15 nm in diameter and a depth of up to 10 nm. The data from ∼ 1x106 impacts can be searched for a specific ion or ions and their coemitted species, revealing the molecular environment around a selected moiety. The methodology was applied to study the coverage of surfactant coatings on gold nanorods. The presented study adds a new instrumental technique for the determination of nanoparticle coverage to this discussion. While nanorod surface density is ∼ 50%, the analysis shows that solvent washing of the nanorods does not result in the removal of the surfactant coating where coverage remains at ∼ 90%. SIMS in the event-by-event bombardment/detection mode displays its promise as an analytical technique due to ease of sample preparation and drastically reduced sample requirements. The ability to probe chemical homogeneity at the nano level allows for the characterization of the chemical environment around nanoparticles. Ultra-small gold nanoparticles, with only 55 to 225 atoms, were encapsulated in a dendrimer structure and analyzed. The comparison of mass spectra of these samples shows that the secondary ion yield of Au moieties vary linearly with the number of Au atoms. Preferential colocation of the nanoparticles and undamaged dendrimer structure was observed, while reductively damaged dendrimer branches are shown be segregated from the nanoparticles. The preference of colocation opens new possibilities for the directed growth of gold nanoparticles within a support structure. The interaction of carbon delivered by hypervelocity projectiles and impacted surfaces was also studied as an analogue to micrometeorite impacts. First, the difference between crystalline and condensed film samples were investigated, determining that secondary ion emission from pressed powders is enhanced over samples produced through vapor deposition. The bombardment with isotopically labeled 13^C60 on inorganic powders enabled the study of recombination products. Here, recombination CN and CNO ions are produced but only CN shows increasing production with higher impact velocities. The study of the interaction of hypervelocity nanoparticles with a 2D material and ultra-thin targets (single layer graphene, multi-layer graphene, and amorphous carbon foils) has been performed using Au4+ 400. The ejected area is much larger (∼60 nm2 ) than that predicted by molecular dynamic simulations and a large ionization rate (∼1%) is observed. The interaction proceeds in an entirely different manner for the process in 3D materials. The experimental observations indicate at least four different emission processes for the observed secondary ions: direct interaction with the projectile which produces high kinetic energy secondary ions in the transmission direction; emission of high velocity secondary ions from the rim of the rupture; emission of H+ n and C+ due to a high charge around the rupture; and emission of low velocity carbon clusters due to a propagation of tears and defects in the graphene foil.
Clubb, Aaron Bryant (2017). Characterization of Nano-Environments by Hypervelocity Projectile Secondary Ion Mass Spectrometry. Doctoral dissertation, Texas A & M University. Available electronically from