| AVS 66th International Symposium & Exhibition | |
| Chemical Analysis and Imaging Interfaces Focus Topic | Thursday Sessions |
| Session CA-ThP |
| Session: | Chemical Analysis and Imaging at Interfaces Poster Session |
| Presenter: | Stanislav Verkhoturov, Texas A&M University |
| Authors: | S.V. Verkhoturov, Texas A&M University D.S. Verkhoturov, Texas A&M University E.A. Schweikert, Texas A&M University |
| Correspondent: | Click to Email |
SIMS is a method of choice for elemental and molecular surface thin layer analysis. We present here significant enhancements in SIMS capability with regards to a) detection sensitivity, and b) applicability to liquid samples.
Detection of atto to zeptomole amounts of analyte is demonstrated with deposits of fractional monolayers of organics on double layer graphene. Three innovations make ultrasensitive analysis possible. The graphene support minimizes interference between analyte and substrate signals. Secondly, the analysis is run in a unique experimental setup. The sample on graphene is bombarded with 50 keV C60+ in transmission mode, i.e. the ejecta are mostly in the forward direction where they are collected in a time-of-flight mass spectrometer, ToF MS. Thirdly, the bombardment is reduced to a series of single C60 impacts, each coupled with ToF measurement. This event-by-event bombardment-detection mode allows the selection of specific impacts on analyte at the exclusion of signals from supporting materials. In this approach, unusually high ionization of analyte molecules (e.g. ~10%) was observed. A distinct ejection-ionization mechanism operates here. The molecules “trampoline” off the graphene following a C60 impact. The mass spectra contain abundant peaks of molecular ions. The proposed mechanism of ionization involves tunneling of electrons from the vibrationally exited area around the hole to the molecules-. Another proposed mechanism is a direct proton transfer exchange.
Liquid samples are inherently incompatible with SIMS, which operates under vacuum. We demonstrate the feasibility of storing liquids in carbon nanotube, CNT, sponges. We found that within a practical experimental time of 30 min, liquids with vapor pressure of < 1 torr can be analyzed. The CNT sponge (97% porosity) was made from multiwall carbon nanotubes (CNT cross-section ~6 layers). Hydrophilic and hydrophobic sponges were examined. For the hydrophobic case, we examined the lubricant fluids (vapor pressure ~100 millitorr). The mass spectra of pure fluids were used as a blank for comparison with mass spectra of wear tracks on metals. For the hydrophilic case, pure glycerol and the glycerol-water mixture have been examined. Glycerol evaporates slowly from the sponge. The method allows to investigate the organic molecules dissolved in glycerol and glycerol water mixture. Thus, the event-by-event technique applied on CNT sponges is a promising method for analysis of liquid and soft materials by Cluster SIMS.
Work supported by NSF grant CHE-130832