TOF-SIMS Analysis Using C 60 . Effect of Impact Energy on Yield and Damage

Langmuir-Blodgett multilayers of alternating barium arachidate and barium dimyristoyl phosphatidate are characterized by secondary ion mass spectrometry employing a 40 keV buckminsterfullerene (C 60 ) ion source. These films exhibit well-defined structures with minimal chemical mixing between layers, making them an intriguing platform to study fundamental issues associated with molecular depth profiling. The experiments were performed using three different substrates of 306 nm, 177 nm, and 90 nm in thickness, each containing six subunits with alternating chemistry. The molecular subunits are successfully resolved for the 306 nm and 177 nm films by cluster ion depth profiling at cryogenic temperatures. In the depth profile, very little degradation was found for the molecular ion signal of the underneath layers compared with that of the top layer, indicating that the formation of chemical damage is removed as rapidly as it is formed. The resolving power decreases as the thickness of the alternating subunits decrease, allowing a depth resolution of 20 to 25 nm to be achieved. The results show the potential of LB films as an experimental model system for studying fundamental features of molecular depth profiling. (J Am Soc Mass Spectrom 2008, 19, 96 -102)

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confidence: 84%

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confidence: 99%

Chemically alternating langmuir-blodgett thin films as a model for molecular depth profiling by mass spectrometry

Zheng

Wucher

Winograd

2008

“…In a second instance, a thin film of neat dipalmitoyl-phosphatidylcholine (DPPC) has been examined in a similar fashion. This system is of interest since the film is less-well characterized but represents an important class of biomolecules of interest in 3D imaging experiments [20,23]. The results show that in all cases the C 60 ϩ projectile yields larger molecular ion signals than Au 3 ϩ during depth profiling.…”

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confidence: 99%

“…The gas ion source requires apertures to define the beam size, sacrificing beam current. Recently, a C 60 source has been introduced [20] that achieves a submicron probe size with enough current to acquire images in a reasonable amount of time. In general, however, higher lateral resolution is achieved with the LMIG design if there is enough sensitivity in the mass spectrum associated with the smallest resolvable pixel.…”

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confidence: 99%

Direct comparison of Au₃⁺ and C₆₀⁺ cluster projectiles in SIMS molecular depth profiling

Cheng

Kozole

Hengstebeck

et al. 2007

The sputtering properties of two representative cluster ion beams in secondary ion mass spectrometry (SIMS), C 60 ϩ and Au 3 ϩ , have been directly compared. Organic thin films consisting of trehalose and dipalmitoylphosphatidylcholine (DPPC) are employed as prototypical targets. The strategy is to make direct comparison of the response of a molecular solid to each type of the bombarding cluster by overlapping the two ion beams onto the same area of the sample surface. The ion beams alternately erode the sample while keeping the same projectile for spectral acquisition. The results from these experiments are important to further optimize the use of cluster projectiles for SIMS molecular depth profiling experiments. For example, Au 3 ϩ bombardment is found to induce more chemical damage as well as Au implantation when compared with C 60 ϩ . Moreover, C 60 ϩ is found to be able to remove the damage and the implanted Au effectively. Discussions are also presented on strategies of enhancing sensitivity for imaging applications with cluster SIMS. (J Am Soc Mass Spectrom 2007, 18, 406 -412) © 2007 American Society for Mass Spectrometry E nergetic cluster ion bombardment and secondary ion mass spectrometry (SIMS) experiments have opened new applications in molecular depth profiling and 3D chemical imaging [1][2][3]. With this approach, it has been reported for many systems, that cluster projectiles remove large amounts of material from molecular solids without the damage accumulation associated with atomic projectiles [4 -12]. Moreover, erosion of the material occurs without significant interlayer mixing and/or the formation of topographical features, allowing depth resolution of 10 to 30 nm to be achieved [8 -10]. These attributes have compelled a majority of SIMS workers to quickly adopt this new technology.Although many different cluster projectiles have been examined, practical considerations have led to wide adaptation of liquid metal ion sources consisting of Au or Bi projectiles (Au 3 ϩ and Bi 3 ϩ , respectively) [13][14][15][16], and gas ion sources consisting of SF 5 ϩ or C 60 ϩ projectiles [6,17,18]. The physics of the ion/solid interaction is likely to be quite different between the carbon and metal-type sources since at 20 keV incident energy, each carbon atom carries 333 eV of kinetic energy while each metal atom in a trimer carries 6667 eV of kinetic energy. Moreover, there is a large mass variation between the atoms comprising these cluster species.The imaging capability of the two types of sources is quite different. The liquid metal ion gun (LMIG) technology yields very bright ion beams with a probe size of less than 100 nm in diameter [19]. The gas ion source requires apertures to define the beam size, sacrificing beam current. Recently, a C 60 source has been introduced [20] that achieves a submicron probe size with enough current to acquire images in a reasonable amount of time. In general, however, higher lateral resolution is achieved with the LMIG design if there is enough sensitivity in the mass...

“…One projectile impact at a velocity of a few tens of km/s, or energies≥keV per projectile atom, can generate notable SI emission [18,19]. The critical criterion for projectile selection is the probability of emitting analyte-specific ions.…”

Section: One-on-one Bombardmentmentioning

confidence: 99%

Mass Spectrometry of Nanoparticles is Different

Liang

Eller

Verkhoturov

et al. 2015

Abstract. Secondary ion mass spectrometry, SIMS, is a method of choice for the characterization of nanoparticles, NPs. For NPs with large surface-to-volume ratios, heterogeneity is a concern. Assays should thus be on individual nano-objects rather than an ensemble of NPs; however, this may be difficult or impossible. This limitation can be side-stepped by probing a large number of dispersed NPs one-by-one and recording the emission from each NP separately. A large collection of NPs will likely contain subsets of like-NPs. The experimental approach is to disperse the NPs and hit an individual NP with a single massive cluster (e.g., C-60, Au-400). At impact energies of~1keV/atom, they generate notable secondary ion (SI) emission. Examination of small NPs (≤20nm in diameter) shows that the SI emission is sizedependent and impacts are not all equivalent. Accurate identification of the type of impact is key for qualitative assays of core or outer shell composition. For quantitative assays, the concept of effective impacts is introduced. Selection of co-emitted ejecta combined with rejection (anticoincidence) of substrate ions allows refining chemical information within the projectile interaction volume. Last, to maximize the SI signal, small NPs (≤5nm in diameter) can be examined in the transmission mode where the SI yields are enhanced~10-fold over those in the (conventional) reflection direction. Future endeavors should focus on schemes acquiring SIs, electrons, and photons concurrently.