SIMS2015 Session SM1-FrM: SIMS Analysis of Materials

Although fossils traditionally are assumed to be completely mineralized imprints of ancient organisms, recent studies have shown that remains of the biomolecular contents of the fossilized animal can be retained. Pigment molecules are of particular interest because their presence in different parts of the body (skin, eye, feather, etc) can be expected to provide important clues about the behavior and habits of the living animal as well as evolutionary relationships. TOF-SIMS is well suited for this type of analysis owing to the capability to provide detailed chemical information about small sample structures and the possibility to correlate the molecular signals with structural features in the sample, with minimal modification or destruction. We have previously used TOF-SIMS to identify eumelanin, which is an abundant polymer pigment in all vertebrates, in the eye of a fossil fish (54 million years)[1] and in the skin of three distantly related marine reptiles (55-190 million years)[2]. Here, we present results for the presence of eumelanin in fossil feathers of a small dinosaur, Anchiornis, from the Jurassic period (161 million years). Exceptional spectral agreement was observed between specific structures in the fossil feather and synthetic and natural (modern) eumelanin standards. SEM analysis revealed that the eumelanin-containing fossil structures closely resemble structures in modern feathers that correspond to elongated (0.5-2 µm in size) pigment particles, so called melanosomes, embedded in a keratin fibrous structure. The chemical identification of eumelanin and the close correlation to these structural features provide compelling evidence that the microbodies observed in the fossil are, in fact, preserved melanosomes from the the Anchiornis feather. This observation confirms previous studies where similar particles in Anchiornis feathers were interpreted as melanosomes [3]. However, these interpretations were based on structural information alone (SEM data), which is problematic because of the similar size and shape of microorganisms that makes it difficult to distinguish between melanosomes and microorganisms in fossil samples without chemical information [4].

[1] J. Lindgren, et al. Nat. Commun. 3, 824 (2012)

[2] J. Lindgren, et al. Nature 506, 484–488 (2014)

[3] Q. Li, et al. Science 327, 1369–1372 (2010)

[4] A.E. Moyer, et al. Sci. Rep. 4, 4233 (2014)

The analysis of multiple sulfur isotopes (³²S^-, ³³S^- & ³⁴S^-) has led to several important discoveries regarding the evolution of the earth’s atmosphere. Most studies utilize bulk analysis by SF₆ for 3- and 4-isotope sulfur analysis. The recent advancement in the analysis of multiple sulfur isotopes using the Cameca 1280/1270 large radius secondary ion mass spectrometer has allowed the sampling of zones that are too small for separation using bulk analysis, with precision approaching that of conventional techniques. While ³⁴S^- and ³²S^- sulfur are routinely measured using the smaller radius Cameca f series mass spectrometers, a 3 isotope sulfur analytical technique has yet to be developed. Increasing the mass resolving power (MRP) or using extreme energy filtering (EEF) can minimize interferences when measuring sulfur isotopes. An increased MRP is accomplished by narrowing the exit slits to allow only a narrow mass range through to the detector, while EEF involves applying a voltage offset between the sample and the secondary column (usually a few hundred volts).

Previous studies have compared the MRP technique relative to EEF for measuring ³⁴S/³²S isotope ratios using a small radius SIMS. They concluded that the EEF technique is more robust, because results were less affected by instrument drift and differences between sample mounts. In the current study, we compare three techniques that utilize MRP and EEF for measuring 3-isotope sulfur in pyrite using a Cameca 7f SIMS.

The first technique utilized a MRP ~4000 and a sample voltage offset of 0. These parameters produced spot-to-spot reproducibility for ³⁴S/³²S and ³³S/³²S of 0.5‰ and 0.9‰, respectively. For the second technique, a 300v offset was applied for all sulfur isotopes and a low mass resolution was used (MRP = 347). Spot-to-spot reproducibility for ³⁴S/³²S and ³³S/³²S was 0.5‰ & 1.3‰, respectively. The final technique also utilized a MRP of 347 and a 300v offset for ³²S^-. However, the voltage offset for the less abundant isotopes (³³S^- and ³⁴S^-) was changed to 200v. These parameters increased the precision for both³⁴S/³²S and ³³S/³²S, producing spot-to-spot reproducibility of 0.2‰ and 0.3‰, respectively.

Basalt is a common rock throughout the solar system and the primary rock type on Mars, and it is a likely rock type that subsurface life might exploit. Consequently, it is a suitable material for the study of the methods required to detect and analyse organic material in rock. Telluric basalts represent an analogue for extra-terrestrial rocks where the incumbent organic material could be analysed for molecular biosignatures. The biogenicity of these remains a subject of debate, since active biological activity or molecular biosignatures from past activity have yet to be directly associated with micro-tubule features. Therefore it is worth researching using surface analysis techniques for molecular biomarkers in rock samples where at least the possibility of geological biomarkers exists, and in addition may provide further evidence for the origin of these structures.

To this end, this study investigates organic material s in submarine basalt . The basalt core sample from the approximately 120 million years old Ontong Java Plateau (OJP) was selected for analysis as it possess the distinctive tubule alteration textures, which are proposed as being the product of biological activity. This study focuses on tube-like features on the basalt with the use of Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS) and Pyrolysis gas chromatography mass spectrometry (Py-GC-MS) to characterize organic molecules.

The data obtained in this study demonstrates organic molecules were heterogeneously distributed and characterise the organic materials in rock samples. This indicates that a positive correlation exists between the presence of the tube alteration textures in the basalt and the organic compounds detected. From these results we propose that ToF-SIMS is effective at detecting organic materials in geological samples, and ToF-SIMS combined with XPS may prove a useful technique in the study of extra-terrestrial organic material, and hence extra-terrestrial life. In addition, we recommend applying principal component analysis (PCA) method for processing of massive ToF-SIMS data, especially imaging data, to reduce data-processing time and to suggest meaningful fragments.

Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) was used in the static mode to probe the surface of a 2 at.% Ga-Pu alloy metal sample. The relative intensities of positive and negative SIMS fragments (Pu⁺, PuO⁺, PuO^-, PuO₂^-, etc.) were recorded from the surface in a fully oxidized state (Pu 4⁺) and from the surface in a reduced state (Pu 3⁺). Argon ion bombardment was used to remove the oxide layer to obtain spectra from a nearly clean plutonium metal surface, which revealed a series of PuH_x^- peaks from mass 240 to 243 amu. The relation of these peaks to possible hydroxide inclusions is discussed. The data also provide information regarding the state of the gallium at the surface of the plutonium. Results are compared to data recorded from similar surfaces by X-ray photoelectron spectroscopy and Auger electron spectroscopy. To our knowledge, this is the first reported systematic analysis of the surface of a monolithic plutonium metal sample with ToF-SIMS.

The paper presents experimental data on 'storing matter' technique applied in SIMS. Tested are several types of collectors including functionalized substrates, applied to store ion sputtered material for SIMS analysis. 'Storing matter' procedure allows to separate two processes: ion sputtering and ionisation [1]. Sputtered material is collected at the substrate surface so as to cover it with submonolayer deposit. Thereafter this material is analyzed by SIMS, and it can be assumed that the stored atoms do not interact with each other during the ionization process. In this way we reduce the so-called matrix effects.

We present recently developed procedure which is carried out within one analytical chamber of quadrupole type SIMS [2]. The process of sputtering of the sample is carried out using a 5 keV, 1μA, Ar⁺ ion beam, rastered over 1.6mm x 1.6mm area. The sputtered material is deposited through 1mm diaphragm at a 25mm diameter substrate rotating at 1rev./hour. For tests we use also stationary substrates as collectors.

After deposition of the sputtered material, the collector is placed in the position for the SIMS analysis. However, for analysis we use reduced current ion beam: 5keV, 0.1μA, Ar⁺. For stationary collectors we raster the beam over 2mm x 2mm area and for rotating collectors we use 2mm linear scan.

We show results of several of collectors made of titanium, copper and nickel. As functionalized collectors we tested substrates coated with gold and partly covered by CsCl salt. Also TiO₂, SnO₂, MoO₃, WO₃ and indium tin oxide have been tested. Oxygen flooding technique is applied both during sputter deposition process and during the analysis of the stored material.

The results show that functionalized substrates used as collectors enhance the adhesion of sputtered material. Moreover, ionization of the stored material is improved compared to the pure metal collectors. However, the use of the multi-elemental substrates limits range of possible elements for the analysis, since the mass spectrum of the substrate is far richer than the mass spectrum of pure elemental substrates.

Acknowledgements:

Authors thank The National Centre for Research and Development, Poland, for project no PBS1/A9/9/2012 founded in years 2012-2015 and Ministry of Science and Higher Education, Poland, for project no DI2013 013943 founded in years 2014-2018.

References:

[1] T. Wirtz, H-N. Migeon; Applied Surface Science, vol. 255.4, pp. 1498-1500, 2008.

[2] P. Konarski, M. Miśnik, A. Zawada, and H.H. Brongersma; Surface and Interface Analysis, vol. 46, no. S1, pp. 360–363, 2014.

Photovoltaic device based on chalcopyrite Cu(In,Ga)Se₂ (CIGS) absorber layer is one of the most promising thin film solar cell due to its low cost, high efficiency, variety of growth methods available, and compatibility with flexible substrates enabling roll-to-roll manufacturing. Recently, CIGS solar cells show excellent light-to-power conversion efficiencies exceeding 20%, which is superior to the efficiencies achieved using multi-crystalline silicon solar cells.^1,2 CIGS absorber materials for high efficiency solar cells are critically influenced by the deviation of their stoichiometry. The incorporation and diffusion of alkali metals, especially sodium and potassium, have been widely shown to be beneficial for CIGS absorber layers, serving to improve p-type conductivity and concomitant cell efficiencies.³

In this study, CIGS thin films were prepared on bilayer molybdenum back contacts deposited on soda-lime glass substrate via 3-stage evaporation. We compared layer structures, interfaces, compositions, and impurities of CIGS thin films looking for differences that might explain the efficiency difference. CIGS solar cell structures with different cell efficiencies were examined by SEM and TEM cross-section images. Surface analyses via AES, magnetic SIMS, and TOF-SIMS were used to obtain the elemental distribution of the CIGS thin film solar cells and compare their depth profiles. Atom probe tomography (APT), a sub-nanometer resolution characterization technique, was used to obtain three-dimensional elemental mapping and compositional distribution at the grain boundaries of CIGS layer. To characterize impurities in a CIGS layer, the distribution of trace elements was also obtained according to depth by magnetic SIMS, TOF-SIMS, and APT.

[1] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (version 41). Prog. Photovolt. 21, 1 (2013).

[2] P. Jackson, D. Hariskos, R. Wuerz, W. Wischmann, M. Powalla, Phys. Status Solidi RRL 8, 219 (2014).

[3] A. Chirila, P. Reinhard, F. Pianezzi, P. bloesch, A.R. Uhl, C. Fella, L. Kranz, D. Keller, C. Gretener, H. Hagendorfer, D. Jaeger, R. Erni, S. Nishiwaki, S. Buecheler, A.N. Tiwari, Nat. Mater. 12, 1107 (2013).

For lithium-ion batteries to widely power plug-in hybrid electric and all-electric vehicles (PHEVs and EVs), they must meet a range of stringent criteria: energy densities high enough to allow for more than 100 miles of travel autonomy at low costs (in terms of $/Wh or $/kg), as well as moderate, but consistent, power densities throughout the entire state-of-charge (SOC) range. Other requirements include good safety, a 10 year calendar-life and a cycle-life of up to a few thousand charge and discharge cycles.^1-3 However, battery life requirements remain a serious challenge for this class of material when repeatedly cycled or held at high voltages for extended periods. At high operating cell potentials (> 4.0 V vs. Li⁺/Li), the electrolyte and its constituents are exposed to strongly reducing and oxidizing conditions on the negative electrode (anode) and positive electrode (cathode) surfaces, respectively.

In cells containing Li_1.05(Ni_1/3Co_1/3Mn_1/3)_0.95O₂ based positive and graphite-based negative electrodes, a significant portion of the cell performance degradation can be attributed to the negative electrode. Dynamic SIMS was performed on fresh, formed, and aged graphite electrodes to provide a comparison of Mn, Co, and Ni contaminants on the surface and in the bulk of the electrode. The SIMS results show significant accumulation of transition-metal elements at the negative electrode-electrolyte interface, which is a possible cause for cell capacity fade and impedance rise.⁴

One method to mitigate degradation and improve cell longevity is to minimize the dissolution of transition metal elements from the positive electrode by application of alumina coatings via atomic layer deposition (ALD). SIMS results, obtained on high-capacity Li-ion cells containing Li_1.2Ni_0.15Mn_0.55Co_0.1O₂-based positive and graphite based negative electrodes, show that the alumina coating inhibits, but does not prevent, transition metal dissolution. In addition, significant accumulation of Al on the graphite negative electrode indicates chemical instability of the ALD coatings during extended cycling. Even with transition metal dissolution, electrochemical cycling reveals that capacity retention is better, and impedance rise is smaller for cells containing ALD-coated electrodes.⁵

¹ M.M. Thackeray, C. Wolverton, E.D. Isaacs, Energy Eviron. Sci. 5 (2012) 7854.

² J.-M. Tarascon, Phil.Trans. R. Soc. A 368 (2010) 3227.

³ B. Kang, G. Ceder, Nature 458 (2009) 190.

⁴ D.P. Abraham, T. Spila, M.M. Furczon, E. Sammann, Electrochem. Solid State Let. 11 (2008) A226.

⁵ M. Bettge, Y. Li, B. Sankaran, N. Dietz Rago, T. Spila, R.T. Haasch, I. Petrov, D.P. Abraham, J. Power Sources 223 (2013) 346.

Time of flight secondary ion mass spectrometry (ToF-SIMS) is an appropriate analytical technique for the investigation of thin film Li ion battery components, especially when the depth and spatial distribution of lithium inside a battery component or even the whole thin film cell are of interest. Analytical techniques using electron or x-ray beams for elemental detection are not able to access lithium in a sufficient way due to its low atomic number of Z=3.

The low detection limits of ToF-SIMS and its eminent spatial resolution make this technique a suitable candidate to investigate the interface between different functional battery layers or to visualize the lithium distribution within e.g. the active cathode layer. Therefore, a layered structure of a thin film all solid state battery system with a typical thickness of a few micrometers is an ideal model system for a detailed ToF-SIMS study. While operating in the dual beam mode, the ToF-SIMS is able to investigate all electrochemical active layers of the battery in one depth profile.

The starting point of this study is the investigation of thin films of the commonly used cathode material LiCoO₂. The thin film cathodes are prepared by a radio frequency sputter deposition process. The first aspect is the investigation of the deposition process by ToF-SIMS and by other analytical methods like e.g. scanning electron microscopy or x-ray diffraction. Using these techniques, the impact of different deposition parameters like e.g. the deposition temperature or the effect of an additional interlayer as diffusion barrier and adhesion layer is investigated. This knowledge is applied to enable the reproducible production of samples.

The key aspect of this study is monitoring the lithiation and delithiation process of the LiCoO₂ cathode material using post mortem ToF-SIMS analysis. Therefore, different thin film batteries with LiCoO₂ cathodes in combination with liquid electrolytes and also solid state electrolytes are cycled to different states of charge. Afterwards elemental distributions (especially Li) within the cathode are measured by ToF-SIMS and the different states of charge (SOC) are compared in a semi quantitative way. It is discussed, how the deintercalation of lithium during charging affects the matrix environment and hence the characteristics of the depth profiles. In order to get also a quantitative insight into the Li distribution within the thin films, 2 MeV p nuclear reaction analysis and glow discharge optical emission spectroscopy are used as quantitative comparison methods.

This contribution deals with a ToF-SIMS study of dye-sensitized nanostructured titanium dioxide (TiO₂) and tin dioxide (SnO₂) electrodes, designed for photoinduced water splitting cells. Untreated substrates consist in a several micrometers thick layer of nanoporous TiO₂ or SnO₂ deposited on FTO slides. The functional electrodes were prepared according to a stepwise methodology, which includes a preliminary surface priming¹ aimed to create a stable platform for the subsequent anchoring of a phosphonate-derivatized polypirydinic ruthenium dye. The dye-sensitized nanostructured electrodes were further functionalized with a molecular water oxidation catalyst (WOC). Using Time-of-Flight SIMS depth profiling we were able to characterise the in-depth chemical composition of the nanostructured electrodes after each preparation step, and we gained useful information for the assessment of the experimental protocol adopted for the electrodes preparation. ToF-SIMS data allowed us for the optimisation of the functionalisation strategy, in order to achieve a uniform sensitization along the whole nanostructured oxide layer.

1. Spampinato, V.; Tuccitto, N.; Quici, S.; Calabrese, V.; Marletta, G.; Torrisi, A.; Licciardello, A., Functionalization of Oxide Surfaces by Terpyridine Phosphonate Ligands: Surface Reactions and Anchoring Geometry. Langmuir 2010, 26 (11), 8400-8406.