Surface sputtering with nanoclusters: the relevant parameters

Large Molecular Cluster Formation from Liquid Materials and Its Application to ToF-SIMS

Since molecular cluster ion beams are expected to have various chemical effects, they are promising candidates for improving the secondary ion yield of Tof-SIMS. However, in order to clarify the effect and its mechanism, it is necessary to generate molecular cluster ion beams with various chemical properties and systematically examine it. In this study, we have established a method to stably form various molecular cluster ion beams from relatively small amounts of liquid materials for a long time by the bubbling method. Furthermore, we applied the cluster ion beams of water, methanol, methane, and benzene to the primary beam of SIMS and compared the molecular ion yields of aspartic acid. The effect of enhancing the yields of [M+H]+ ion of aspartic acid was found to be the largest for the water cluster and small for the methane and benzene clusters. These results indicate that the chemical effect contributes to the desorption/ionization process of organic molecules by the molecular cluster ion beam.

Improvement of the gas cluster ion beam-(GCIB)-based molecular secondary ion mass spectroscopy (SIMS) depth profile with O2(+) cosputtering

Over the last decade, cluster ion beams have displayed their capability to analyze organic materials and biological specimens. Compared with atomic ion beams, cluster ion beams non-linearly enhance the sputter yield, suppress damage accumulation and generate high mass fragments during sputtering. These properties allow successful Secondary Ion Mass Spectroscopy (SIMS) analysis of soft materials beyond the static limit. Because the intensity of high mass molecular ions is intrinsically low, enhancing the intensity of these secondary ions while preserving the sample in its original state is the key to highly sensitive molecular depth profiles. In this work, bulk poly(ethylene terephthalate) (PET) was used as a model material and analyzed using Time-of-Flight SIMS (ToF-SIMS) with a pulsed Bi3(2+) primary ion. The optimized hardware of a 10 kV Ar2500(+) Gas Cluster Ion Beam (GCIB) with a low kinetic energy (200-500 V) oxygen ion (O2(+)) as a cosputter beam was employed for generating depth profiles and for examining the effect of beam parameters. The results were then quantitatively analyzed using an established erosion model. It was found that the ion intensity of the PET monomer ([M + H](+)) and its large molecular fragment ([M - C2H4O + H](+)) steadily declined during single GCIB sputtering, with distortion of the distribution information. However, under an optimized GCIB-O2(+) cosputter, the secondary ion intensity quickly reached a steady state and retained >95% intensity with respect to the pristine surface, although the damage cross-section was larger than that of single GCIB sputtering. This improvement was due to the oxidation of molecules and the formation of -OH groups that serve as proton donors to particles emitted from the surface. As a result, the ionization yield was enhanced and damage to the chemical structure was masked. Although O2(+) is known to alter the chemical structure and cause damage accumulation, the concurrently used GCIB could sufficiently remove the surface layer and allow the damage to be masked by the enhanced ionization yield when the ion-solid interaction volume was kept shallow with a low O2(+) energy. This low O2(+) energy (200 V) cosputtering also produced a smoother surface than a single GCIB. Because the oxidized species were produced by O2(+) and removed by GCIB simultaneously, a sufficiently high O2(+) current density was required to produce adequate enhancements. Therefore, it was found that 10 kV with 2 × 10(-6) A per cm(2) Ar2500(+) and 200 V with 3.2 × 10(-4) A per cm(2) O2(+) produced the best profile.

Mass spectrometric imaging of brain tissue by time-of-flight secondary ion mass spectrometry--How do polyatomic primary beams C₆₀⁺, Ar₂₀₀₀⁺, water-doped Ar₂₀₀₀⁺ and (H₂O)₆₀₀₀⁺ compare?

RATIONALE

To discover the degree to which water-containing cluster beams increase secondary ion yield and reduce the matrix effect in time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging of biological tissue.

METHODS

The positive SIMS ion yields from model compounds, mouse brain lipid extract and mouse brain tissue together with mouse brain images were compared using 20 keV C60(+), Ar2000(+), water-doped Ar2000(+) and pure (H2O)6000(+) primary beams.

RESULTS

Water-containing cluster beams where the beam energy per nucleon (E/nucleon) ≈ 0.2 eV are optimum for enhancing ion yields dependent on protonation. Ion yield enhancements over those observed using Ar2000(+) lie in the range 10 to >100 using the (H2 O)6000 (+) beam, while with water-doped (H2O)Ar2000(+) they lie in the 4 to 10 range. The two water-containing beams appear to be optimum for tissue imaging and show strong evidence of increasing yields from molecules that experience matrix suppression under other primary beams.

CONCLUSIONS

The application of water-containing primary beams is suggested for biological SIMS imaging applications, particularly if the beam energy can be raised to 40 keV or higher to further increase ion yield and enhance spatial resolution to ≤1 µm.

Computer simulations of cluster impacts: effects of the atomic masses of the projectile and target

Cluster secondary ion mass spectrometry is now widely used for the characterization of nanostructures. In order to gain a better understanding of the physics of keV cluster bombardment of surfaces and nanoparticles (NPs), the effects of the atomic masses of the projectile and of the target on the energy deposition and induced sputtering have been studied by means of molecular dynamics simulations. 10 keV C60 was used as a model projectile and impacts on both a flat polymer surface and a metal NP were analyzed. In the first case, the mass of the impinging carbon atoms was artificially varied and, in the second case, the mass of the NP atoms was varied. The results can be rationalized on the basis of the different atomic mass ratios of the projectile and target. In general, the emission is at its maximum, when the projectile and target have the same atomic masses. In the case of the supported NP, the emission of the underlying organic material increases as the atomic mass of the NP decreases. However, it is always less than that calculated for the bare organic surface, irrespective of the mass ratio. The results obtained with C60 impacts on the flat polymer are also compared to simulations of C60 and monoatomic Ga impacts on the NP.