AVS2010 Session AS-WeA: Surface Mass Spectrometry: SIMS and Beyond

When a pulsed Bi_n⁺ analysis beam and a direct current C₆₀ etching beam are used in ToF-SIMS dual-beam depth profiling, we observed that increasing the ion dose of the analysis beam decreased the molecular signal as a function of depth. This decay was seen to occur much earlier in the depth profile for less clustered ions, suggesting that damage is related to the implantation depth of the primary ion. Recent experimental and simulation results in this field indicate that implantation depth is inversely related to the clustering of the primary ion, and are observed to affect outcomes such as sputter yield and escape depth (sampling depth). There is a need to characterize these parameters on organic systems, since a better understanding of these parameters will help us choose experimental parameters for the optimal analysis of samples.

In this study, the above mentioned quantities for Bi_n^q+ (n = 1, 3, 5, q = 1, 2) and C₆₀⁺⁺ ions were characterized on molecular and polymeric organic films; a spin-cast trehalose and a plasma polymerized tetraglyme, both about 100 nm thick. The implantation depths of Bi ions were determined using C₆₀ single-beam depth profiling of the same area bombarded by bismuth. The depth scale was calibrated using AFM. The sputter yield was obtained by AFM imaging of the sputter crater. The molecular escape depth was determined by capping a protein-adsorbed mica with tetraglyme overlayers of varying thicknesses and monitoring the protein fragment intensities as a function of thickness. Results show that an ion’s penetration depth is directly proportional to the energy-per-constituent of the ion. For example, increasing the number of Bi atoms from 1 to 3 (Bi₁⁺ to Bi₃⁺) for 25 keV single charged ions decreased the average penetration depth from 18 to 10 nm. When the energy of Bi₃ was increased from 25 to 50 keV, the implantation depth increased from 10 to 14 nm. When implanting bismuth ions, we observed etching of the sample surface, with the crater depths proportional to the cluster size of the bismuth ion. These crater depths were also proportional to the escape depth of the protein fragments. Bi₁⁺ at 25 keV was found to be the most surface sensitive; its sampling depth was only 1.8 nm despite its implantation of 18 nm. The second most surface sensitive was C₆₀⁺⁺ at 20 keV with a molecular escape depth of 2.4 nm. It turned out that the single most important factor that determined the surface sensitivity of the ion source was its sputter depth; both Bi₁⁺ and C₆₀⁺⁺ eroded 0.3 and 1.0 nm, respectively, whereas Bi₅⁺⁺ with a sputter depth of 1.8 nm was the least surface sensitive with a molecular escape depth of 4.7 nm.

The introduction of C₆₀⁺ as a sputter beam for TOF-SIMS made it possible to acquire molecular depth profiles on a wide variety of polymers. Previous studies by many different groups have demonstrated that not all polymers can be successfully depth profiled and that certain classes of polymers undergo sputter-induced chemical reduction when bombarded by C₆₀⁺ ions. If the polymer sputter yield is not high enough, the subsurface sputter-induced damage will accumulate as a function of sputter ion dose and the secondary molecular ion signals will not be stable. A number of different analytical parameters have been previously explored in attempts to improve depth profiling of these difficult polymers including sample temperature, beam energy, and incidence angle. We reported last year that glancing C₆₀⁺ incidence angle (76° from surface normal) significantly improved the ability to depth profile polycarbonate and polystyrene, which were previously unsuccessful at a typical incidence angle of 48°. However, even under these optimized conditions, the depth profiles eventually fail after several hundred nanometers due to accumulated sputter damage to the polymer and a concurrent reduction of the secondary ion and sputter yields. In the present study, we report the efficacy of an argon gas cluster ion beam (GCIB) for steady-state molecular depth profiling the same polymer systems. Depth profiles using different GCIB experimental conditions will be reported and compared to C₆₀⁺ depth profiles acquired under optimized conditions.

Due to their extraordinary properties nanoparticles enter more and more fields of industrial production and daily life. They facilitate the protecting effect of sun creams, make car coatings and spectacle lenses scratch resistant and reliably protect shoes from wetness. Nanoparticles with antibacterial properties are used in clothing and refrigerators.

Nanoparticles’ properties are influenced by their high surface to volume ratio. In addition, specifically tailored nanoparticles are increasingly composed of core shell structures. In order to understand and further develop the properties of nanoparticles it is therefore important to develop tools to investigate the elemental and molecular composition of their surfaces and interfaces.

Two ion beam technologies show promising features in this respect: On the one hand Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is able to probe the outermost 3-5 atomic layers of a sample. It reveals the elemental as well as the molecular composition of the samples’ surface in a semi-quantitative manner. On the other hand, Low Energy Ion Scattering (LEIS) is a quantitative technique to probe the elemental composition of the outermost atomic layer. Combining these two complementary techniques allows a comprehensive overview on the sample composition.

In our study we will present combined ToF-SIMS and LEIS data on model-nanoparticle systems and will give examples for their prospects to enable nanoparticle development, product control and failure analysis. Amongst others, the layer growth of a shell of Al₂O₃ on LiMn₂O₄ nanoparticles deposited by an ALD process was investigated. Both techniques are able to determine the increased coverage of the core. While ToF-SIMS yields additional information about organic residues, LEIS is able to quantify the surface composition and assess the integrity of the layer.

Nanoparticles are front-runner nanotechnologies key to high innovation products, such as biodiagnostics, drug delivery, medical imaging (contrast agents), cosmetics, sunscreens and catalysts. The surface and bulk chemistries of nanoparticles are crucial for their unique properties. There is also increasing concern over the potential health, environmental and safety risks of nanomaterials. Robust and consistent methodologies for characterising nanoparticle surface and bulk chemistries are urgently needed to support standardisation, regulatory requirements and toxicology studies [1].

Secondary ion mass spectrometry (SIMS) has been applied in nanoparticle characterisation [2,3]. However, the fundamentals of the sputtering of nanoparticles are not yet well understood. The sputtering yield is expected to be significantly different for nanoparticles compared to bulk materials, due to the dimension of nanoparticles being similar to the size of the SIMS collision cascade, and the larger available surface area for secondary emission. In this study, SIMS depth profiles were obtained for model Au nanoparticles, with diameters ranging from 10 nm – 100 nm, dispersed and mounted onto a silanised silicon substrate. The sputtering yields and profile shapes are evaluated for both atomic and cluster primary ion beams (Ar⁺, Bi⁺, Bi₃⁺ and C₆₀ⁿ⁺), using a range of primary ion energies. The nanoparticles are characterised by SEM and AFM both before and after sputtering. There is a significant increase in the sputtering yield for nanoparticles compared with bulk materials, in agreement with predictions from a recent molecular dynamics study [4]. In addition, for the first time, we observed the sputtering of nanoparticles in real time using a focused ion beam system with simultaneous SEM imaging. This provides valuable information for interpreting the SIMS depth profiles of nanoparticles and an interesting phenomenon of nanoparticle coalescence was observed. These results contribute towards developing the essential underpinning metrology and providing a practical procedure to analysts for the chemical characterisation and compositional depth profiling of nanoparticles.

References

[1] D R Boverhof and R M David, Anal Bioanal Chem 2010, 396, 953.

[2] D R Baer, D J Gaspar, P Nachimuthu, S D Techane and D G Castner, Anal Bioanal Chem 2010, 396, 983.

[3] W Szymczak, N Menzel, W G Kreyling, K Wittmaack, Int. J. Mass Spectrom. 2006, 254, 70.

[4] S Zimmermann and H M Urbassek, Int. J. Mass Spectrom. 2008, 272, 91.

Over the last decade time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been increasingly used for examining (imaging) biological samples ranging from surface immobilized proteins to tissue sections to single cells. Due to the ultra high vacuum requirement of the ToF-SIMS technique careful and stringent sample preparation protocols are necessary to preserve the native state of these samples. There are a variety of preparation procedures that have been developed to address this need. However, there is still debate as to which procedure best preserves cell chemistry and morphology. The goal of this study is to further elucidate the differences in information that can be obtained when comparing cellular samples that have been plunge cooled and then freeze dried versus those that have been plunge cooled and analyzed in the frozen hydrated state.

Freeze drying biological samples after cryofixation is a much simpler method of sample preparation compared to frozen hydrated. We have shown that freeze drying cryofixed samples preserves the native morphology as well as the K⁺/Na⁺ gradient in single NIH 3T3 fibroblasts, as shown in Figures 1a and 1b. Given optimal preparation procedures, images of subcellular compartments can be realized, as seen in Figure 1c. A single cell is shown after C₆₀⁺⁺ sputtering (C₆₀⁺⁺ ion dose = 4.3x10¹⁴ ions/cm²). The nucleus can be seen in the circular area within the cell that lacks C₃H₈N⁺ signal. A nucleolus is observed as the bright spot within the nucleus. Also noticeable is the C₃H₈N⁺ signal is the Golgi apparatus in the perinuclear region.

The question that remains is what type of information can be obtained from samples that have been freeze dried? Some groups have reported that freeze drying will cause cellular damage [1], while others have reported detrimental effects from frozen hydrated analysis [2]. Freeze drying continues to be the most popular preparation method after cryofixation, most likely because it is less complex than frozen hydrated analysis, as well as the fact that few ToF-SIMS instruments are setup to easily perform this type of investigation. We are adapting a liquid nitrogen cooled sample stage to allow for the rapid entry of cellular samples at liquid nitrogen temperatures to avoid sample warming and ice crystal formation. Ice crystal formation must be avoided in frozen hydrated analysis of biological material, as ice crystal formation will rupture cell membranes causing a loss of morphological and chemical information.

The results from this study shows the differences in information obtained when analyzing similar cellular samples in the freeze dried state versus the frozen hydrated state.

Because of the complexity of the structure and chemical composition in living organisms, much progress has been made in realizing mass imaging for biological samples. Secondary ion mass spectrometry (SIMS) imaging technique has the significant advantage of high spatial resolution, free of matrix deposition and with good reproducibility. In conventional SIMS the primary probes are keV-energy ion beams, which deposit their energy to atomic nuclei. As a result, primary ions break molecular bonding, and molecular information is lost. On the other hand, MeV-energy ions enhance ionization of high mass molecules, because of their dense electronic excitation. Molecular images with lateral resolution of around 5 mm have been demonstrated for peptides or lipids using MeV-energy ion beams [1].

In addition, MeV-energy ions, which have a long projection range in matter, can be utilized for material analysis under low vacuum. The low-vacuum condition prevents evaporation of water from the biological sample and helps maintain the shape of the sample. A new instrument has been developed to realize SIMS measurements under low vacuum. Biological samples were analyzed with swift heavy ions, and secondary molecular ions were introduced into a quadrupole ion guide combined with a differential pumping system. An orthogonal time-of-flight (o-ToF) retaining high vacuum was employed to measure secondary ion mass spectra. This system allows the use of continuous beam, because of collisional cooling and orthogonal extraction. The mass resolution of this system is better than 6000, and is independent on the primary beam. Molecular imaging of cells and tissues was observed with a sample-scanning system. Secondary emitted biomolecular ions, such as lipids or cholesterols, were clearly observed with sufficiently high intensity. This new technique opens new possibilities for SIMS analysis, and we called it “wet SIMS”.

Recent progress in this novel technique will be presented and discussed in view of its possible application in the analysis of biological matter.

[1] Y. Nakata, et al. J. Mass Spectrom. (2009) 44, 128–136

Even though significant progress has been achieved in molecular characterization by secondary ion mass spectrometry (SIMS) of organic compounds, some challenges still exist. One important limitation concerns the sensitivity the technique, often insufficient for high resolution molecular imaging. Several approaches have been proposed to increase the useful molecular signal intensities. The first one involves specific sample preparation procedures, such as the use of special matrices or metal-assisted SIMS (MetA-SIMS), in which metal nanoparticles are condensed on the sample surface. The second type of improvement directly involves the choice of the projectile, i.e. atomic projectiles are replaced by clusters, such as Au_n⁺, Bi_n⁺, C₆₀⁺ and SF₅⁺.

In this contribution, we investigate a new protocol to improve the analytical performance in organic SIMS, using water vapor injection at relatively high pressure near the sample surface. A significant enhancement of the positive secondary ion intensities is observed for organic materials irradiated with 12 keV Ga⁺ ions in the presence of H₂O gas. In addition, our results show that the enhancement depends on the type of secondary ion and that the formation of protonated ions is specifically favored. For the low molecular weight additive Irgafos 168 (C₄₂H₆₃O₃P), the normalized [M+H]⁺ intensity increases by more than 50 times upon H₂O flooding. The enhancement factors are lower with high molecular weight polymers. Nevertheless, the intensity of the protonated monomer ions is also enhanced preferentially in comparison with those of the deprotonated and the bare monomer ions. In the next phase of this work, we consider the combination of H₂O vapor flooding with polyatomic projectile bombardment.