Emile A. Schweikert oral presentation (SN3-Wed2-2-5)
SIMS on ultra-small single nanoparticles.
Texas A&M University, Department of Chemistry, TX 77843 College Station, United States
Physical characterization of NPs refers usually to individual entities. In contrast chemical assays are most often on bulk amounts of NPs. Yet there are unavoidable limitations in ensemble averaging. The present study focuses on SIMS of individual NPs in dimensions below 20 nm. It follows that such measurements are on dispersed entities. Given the size of the objects, the projectile-target interaction must be considered at the level of a single impact. The challenge is to maximize the information obtained under such conditions. A prerequisite is to apply a projectile generating a high ion multiplicity. The studies discussed here are based on bombardment with Au400+4 of 520 keV, generating SI yields 2 to 3 orders of magnitude above those of equal velocity atomic ions. We have shown earlier that impacts on nano-objects ranging from 5 to 20 nm are not all equivalent. Thus the data acquired from one projectile impact must not only be recorded separately but analyzed as a function of the impact parameter. The approach is to run a few million impacts, identifying each time the SIs via ToF-MS. The individual SI records can then be sorted by the specific ions emitted. It is possible to further sort ejecta by kinetic energies or the number of co-emitted electrons. For instance, some SIs carry higher kinetic energies than others, they might arise from a “bulls- eye” impact shattering the NP, in contrast to lower energy species likely originating from grazing impacts. The latter can also provide information about the chemical environment of isolated NPs. Finally, records from individual bombardment-detection events reveal the mass distribution of co-emitted SIs which can correlate with the shape of nano-objects.
For NP in sizes below 5 nm, a massive projectile impact causes complete destruction, all impacts are again equivalent. Here the number of atoms per NP scales with the SI yield for particles of the same composition but varying in size. Limitations due to the vanishingly small amount of sample and contribution from the substrate can be minimized by depositing the NPs on graphene. The 2D substrate enables to run experiments in transmission with collection of the SIs in the forward direction where emission is enhanced up to ten-fold in comparison to the conventional reflection emission. Examples will be shown demonstrating that SIMS with a nanoprojectile, like Au400+4 run in the event-by-event bombardment-detection mode, can detect moieties in the ag range with remarkably detailed information. Work supported by NSF Grant CHE-1308312.