SIMS2015 Session DO1-ThA: Depth Profiling/Organics

Over the last decade the beneficial properties of hydrogels as artificial cell culture supports have been extensively investigated¹. Certain synthetic hydrogels have been proposed to be similar in composition and structure to the native extracellular matrix of the stem cell niche, their in vivo cell habitat, which is a powerful component in controlling stem cell fate². The stem cell differentiation pathway taken is influenced by a number of factors. When culturing cells within or upon hydrogels this choice can be strongly dependent on the underlying 3D hydrogel chemistry which strongly influences hydrogel-cell interactions³. The interrelationship between hydrogel chemistry and that of biomolecules in controlling cellular response ideally requires analysis methods to characterise the chemistry without labels and often in 3D.

Time-of-flight secondary ion mass spectrometry (ToF SIMS) has the potential to be utilised for through thickness characterisation of hydrogels. The frozen-hydrated sample format is well suited to minimise changes associated with dehydration or ‘fixation’, a challenging aspect in vacuum analysis conditions⁴. Frost formation can occur in the ambient atmosphere preventing ready depth profiling of the frozen hydrogels⁵. We develop a simple method to remove this frost by blowing with gas prior to entry into the instrument which is shown to produce remarkably good profiles on a poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogel film where a model protein, lysozyme, is incorporated to demonstrated how biomolecule distribution within hydrogels can be determined. A comparison of lysozyme incorporation is made between the situation where the protein is present in the polymer dip coating solution and lysozyme is a component of the incubation medium. It is shown that protonated water clusters H(H₂O)_n⁺ where n=5-11 that are indicative of ice are detected through the entire thickness of the pHEMA and the lysozyme distribution through the pHEMA hydrogel films can be determined using the intensity of characteristic fragment secondary ions.

(1) Lutolf, Biomaterials: Spotlight on Hydrogels. Nat. Mater. 2009 8 451.

(2) Kobel & Lutolf Biomaterials Meet Microfluidics: Building the next Generation of Artificial Niches. Curr. Opin. Biotechnol. 2011 22 690.

(3) Yang et al. Mechanical Memory and Dosing Influence Stem Cell Fate. Nat. Mater. 2014 13 645.

(4) Robinson & Castner Characterization of Sample Preparation Methods of NIH/3T3 Fibroblasts for ToF-SIMS Analysis. Biointerphases 2013 8 15.

(5) Piwowar et al. Investigating the Effect of Temperature on Depth Profiles of Biological Material Using ToF-SIMS. Surf. Interface Anal. 2011 43 207

Up until now, the crucial issue of organic depth-profiling has driven the evolution of ion sputter sources. One major issue is the degradation of organic materials under ion irradiation such as cross-linking or depolymerization of polymers which have been illustrated among others on polystyrene (PS) and poly(methyl methacrylate) (PMMA) [1]. The use of ultra-low energy cesium ion sputtering has demonstrated an interest for organic depth profiling, as described by Houssiau and Mine [2]. Also new ion sources such as fullerene (C₆₀^q+) or Ar gas clusters (Ar_n⁺) have been developed for the analysis of organic films. Although C₆₀^q+ sputtering can cause cross-linking, it has been shown that the use of nitric oxide (NO) gas dosing can prevent this effect, making successful depth profiling of polystyrene films possible [3].

In this work, we compare these 3 different sputter guns for depth profiling of PS-b-PMMA block-copolymers (BCPs) which self-assemble into nano-domains [4]. These samples were previously characterized using ultra-low energy cesium sputtering and other techniques [5] and are an interesting model system to compare the 3 sputter guns. The results show that all three sputter guns, in optimized conditions, enable successful depth profiles of the in-depth morphology of PS-b-PMMA BCPs.

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In the case of argon clusters, the variation of energy and cluster size at constant energy per atom leads to unexpected intensity variation at the interface between the substrate and the organic film, consistent with the occurrence of degradation of the block-copolymer film when using larger argon cluster ions although with an equivalent energy per atom. This is confirmed by analyzing the ToF-SIMS crater using XPS and AFM.

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In conclusion, all three projectiles can provide good quality depth profiles of the investigated block copolymer, but care should be taken in choosing the appropriate sputtering conditions to minimize damage, even in the case of Ar gas clusters.

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Acknowledgements: This work was supported by the French "Recherches Technologiques de Base" Program and was performed on the Nano Characterization Platform (PFNC) of the CEA Grenoble.

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References:

1. V. Zaporojtchenko, et al. , Nucl.Instrum. Meth. B, 208, 155, (2003).

2. L. Houssiau, N. Mine,Surf. Interface Anal., 42, 1402, (2010).

3. R. Havelund, et al. , Anal. Chem, 85, 5064, (2013).

4. Y.-C. Tseng, S. B. Darling,Polymers, 2, 470, (2010).

5.T. Terlier, et al. , Surf. Interface Anal., 46, 83, (2014).

Trehalose thin films were employed as a model organic system to investigate and compare the sputtering characteristics of two cluster projectiles, argon gas cluster ion beams (Ar-GCIBs) and C₆₀. It has been reported that Ar-GCIBs produce less chemical damage and better depth resolution than C₆₀. However, in a recent study, distorted depth profiles of trehalose are observed using Ar-GCIBs with E/n ≤ 5 eV/atom. In this study, 20 kV Ar₄₀₀₀⁺ and 40 kV C₆₀⁺, were partially overlapped and were used to alternately depth profile though the sample.

The results clearly show that the two projectile ions have different sputtering properties, such as erosion rate and ion beam induced chemical damage. In addition, it shows that the two ion beams are complementary: C₆₀ is able to mitigate distorted depth profile seen with pure argon gas clusters bombarding, while Ar-GCIBs are able to help recover the damage caused by C₆₀.

Argon cluster ion sputtering has become the preferred sputter ion source for 3D SIMS imaging of organics. The importance of this approach is clear and several studies have been conducted to characterise and model the sputtering process, including studies of the effect of the beam energy and cluster size [1] and studies of the sputtering yield material dependence [2]. Here, we extend on these studies and characterise the effect of A) the angle of incidence, B) the sample temperature, and C) the sample composition.

The ion beam angle of incidence is an important parameter for argon cluster sputter depth profiling. Most commercial instruments have the argon cluster ion source at 45° to the sample surface but other configurations may be advantageous. We have measured the sputtering yields of Irganox 1010 using argon gas cluster ion beams of 5 and 10 keV energy, E, with cluster sizes, n, from 1000 to 5000. The data shows that the angular dependence depends on E/n with an enhancement at 45° that changes between 2 and 10 times that at 0°, the strongest enhancement being for low E/n [3]. The effect of the angle of incidence on the depth resolution will also be discussed.

The effect of the sample temperature on sputter depth profiling has been measured for two organic materials (NPB and Irganox 1010) using a 5 keV Ar₂₀₀₀⁺ cluster ion beam. It is shown that the sputtering yields are constant with depth and increase at a rate <0.05 nm³/K with temperature up to a transition temperature which for NPB and Irganox 1010 are 15 ºc and 0 ºc, respectively. Above this temperature, the rate of increase of the yield rises by an order of magnitude. The secondary ion spectra also change with temperature with the intensities of the molecular entities increasing least. It is recommended that for consistent results, measurements are always made at temperatures significantly below the transition temperature.

For interpretation of the Z dimension in 3D SIMS images it is necessary to know the sputtering yield, as well as the variation of sputtering yield within the sample. Presently, sputtering yields are only known for a limited set of materials. Here, the sputtering yields of mixed systems have been measured for binary mixtures of Irganox 1010 with either Irganox 1098 or Fmoc-pentafluoro-L-phenylalanine. The sputtering yields are shown to be lower than those deduced from a linear interpolation from the pure materials. We describe this using a simple model.

[1] MP Seah, J. Phys. Chem. C, 2013, 117, 12622–12632

[2] V Cristaudo et al, Surf. Interface Anal. 2014, 46, 79–82

[3] MP Seah, AG Shard, SJ Spencer, J. Phys. Chem. B. 2015, 119, 3297-303

Thanks to the development of Ar cluster ion beam sputtering, nano level depth profiles by time-of-flight secondary ion mass spectrometry (ToF-SIMS) have become feasible. However, the depth profiles could be influenced by the conditions of samples and measurements, which are often obscure. The objective of this study is to examine the degree of matrix effects depending on the combination of polymers in a sample for depth profiles.

Two model polymer samples composed of polymer layers, one contains Irganox1010 and Irganox1098 and the other contains Irganox1010 and Fmoc-pentafluoro-L-phenylalanine, were measured with 30 Kv Bi₃⁺⁺ while it was sputtered using 5 or 10 Kv Ar₁₀₀₀⁺ cluster beams for depth profiling. And then, for finding positive secondary ions specific to each polymer, multivariate analysis which was principal component analysis (PCA) and multivariate curve resolution (MCR) was employed. Finally matrix effects on specific secondary ions suggested by multivariate analysis were examined.

As a result, PCA and MCR indicate components corresponding to each polymer for each sample. It was not easy to find out specific secondary ions to each polymer in positive ToF-SIMS spectra by simple comparison of the spectra though some were found in negative spectra. By using PCA and MCR, positive secondary ions specific to each polymer could be found. Three layers containing 0%, 50% and 100% of Irganox1098, respectively, were investigated in terms of the relationship between each peak intensity and the concentration ratio of the layers. For instance, the peak m/z 98 specific to Irganox1098 shows stronger intensity than expected from concentration ratio, while another Irganox1098 specific peak m/z 377 shows weaker intensity than expected from concentration ratio.

In conclusion, it is suggested that multivariate analysis is useful to investigate depth profiles of mixed samples when specific peaks to each material in the samples are not known.

Matrix effects in SIMS are known to vary the ionization probability of materials which are sputtered from a sample surface. These effects can be challenging when using SIMS since the ion signal in the spectra do not accurately represent the abundance of the different species in the sample. Such issues can lead to misinterpretation of spectral data and for this reason ionization methodologies which are free from matrix effects are sought. One such method to overcome these issues is to decouple the ionization process from the emission process by post-ionization of sputtered neutrals. For this purpose, intense ultrafast laser pulses in the near IR region with powers up to several 10¹⁵ W/cm² and a wavelength between 1200-2000nm are used for post-ionization of sputtered materials. It has been previously shown that ultrashort pulses in this wavelength region can effectively ionize a wide variety of molecules with minimized fragmentation [1]. In this study we report the latest results from our experimentation on large organic molecules such as Irganox 1010 and the polymer system poly-lactic acid. Spectral data and depth profiling analyses are provided which compare the SIMS and post-ionization signal. Successful ionization of the Irganox 1010 molecular ion (1178 m/z) with laser post-ionization is reported for the first time.

[1] Andrew Kucher, Andreas Wucher and Nicholas Winograd, J. Phys. Chem. C, 2014, 118 (44), pp 25534–25544

The lithographic performance of photoresists is a function of the distribution of formulation components, such as photoacid generator (PAG) molecules, in photoresist thin films and how these components undergo chemical modification and migrate within the film during the lithography processing steps. This presentation will discuss how GCIB-SIMS depth profiles were used to monitor the PAG and quencher base distributions before and after exposure and post-exposure bake processing steps for different PAG/photoresist formulations.

Organic bulk heterojunction (BHJ) photovoltaics have received substantial interest as promising sources of renewable energy. To better understand their performance, it is important to have an accurate picture of the material distribution through the depth of the photoactive layer. Recent advances in argon cluster ion beams as sputter sources in conjunction with ToF-SIMS analysis have enabled gentle depth profiling of organic layers.

The photoactive layer of a BHJ photovoltaic cell is typically a blend of a donor and acceptor material. The most widely used acceptor material in such devices is PCBM. Commonly employed donors are P3HT, MDMO PPV, PCDTBT and DPP5T.

In this study, we extend systematically previous investigations on the thiophene-containing donor materials to one without (MDMO PPV) thiophene in order to study the role of this unit on observed ionization effects This allows us to propose general guidelines regarding the optimisation of critical parameters, such as the choice of analysis beam, dose ratio between the analysis and the sputter beam, cluster size and energy of the sputter beam that provide plausible depth profiles. For instance, in each of these systems, it has been observed that sputter beams exceeding the value of 10 eV/atom cause irreversible damage [1] in these organic layers.

Quantification of the depth profiles is posed with several challenges. Firstly, there is a strong matrix effect on the steady state intensity of the fullerene ion, which peaks at different sample composition for each donor:acceptor system. This matrix effect has a complex relation with the concentration of strongly electronegative elements in the donor molecule. Secondly, we observe a large intensity enhancement of the fullerene ion at the interface between the organic layer and the substrate. We elucidate the origin of this enhancement by comparing Ar_n⁺ depth profiles obtained by ToF-SIMS with those obtained from XPS. Taking these effects into consideration, we are able to fully quantify all donor:acceptor systems from this study and to confirm segregation effects at the surface and at the interface.

There is no doubt that the bulk heterojunction (BHJ) is the most optimal structure providing high performance organic thin-film solar cells.[1] Casting a solution of donor and acceptor molecules is the most common method used in fabrication of the active layer of polymer based solar cells. Self-assembly of the phase domains into BHJ takes place usually during solvent evaporation. Understanding of thermodynamic phenomena leading to the phase separation is crucial for the optimization of the morphology of active layers within polymer solar cells.

Secondary Ion Mass Spectrometry (SIMS) provides information on the distribution of components within thin films and thus it is particularly useful in such studies. [2] The complexity of sputtering of multicomponent systems imposes the need to pay special attention to the selection of experimental parameters in case of depth profiling of active layers of polymer solar cell.

Recently, it was shown that phenyl-C61-butyric acid methyl ester (PCBM), fullerene derivatives acting as acceptor in most of organic solar cells, can diffuse into layer of conjugated polymer (donor) upon thermal annealing.[3] In addition to diffusion, crystallization of the polymer often occurs at elevated temperatures. The competition between these processes leads to a reduction in the mobility of PCBM molecules in polymer film . Here we show, for the first time, that exposition to a solvent vapor, known as solvent vapor annealing[4], can also trigger diffusion in PCBM/conjugated polymer systems. SIMS depth profiles have shown that the diffusion of PCBM molecules, polymer molecules or both is forced by appropriate selection of the solvent. In order to reveal the real distribution of interdiffused components, which differ strongly in sputtering rate, sputtering with low energy Cs beam, C60 beam and co-sputtering with C60 and Cs beams [5] were carried.

This work was partially supported by the Polish National Science Centre project no. 2013/11/B/ST5/04473.

[1] J. Peet, A. Heeger, and G. Bazan Accounts Of Chemical Research 2009;42(11):1700-1708

[2] C.M. Björström-Svanström, J. Rysz, A. Bernasik, A. Budkowski, F. Zhang, O. Inganäs, M.R. Andersson, K.O. Magnusson, J.J. Benson-Smith, J. Nelson, E. Moons, Advanced Materials 2009, 21:4398–4403

[3] N.D. Treat, M.A. Brady, G. Smith, M.F. Toney, E.J. Kramer, C.J. Hawker, M.L. Chabinyc Advanced Energy Materials 2011, 1:82-89

[4] Y. Zhao, Z. Y. Xie, Y. Qu, Y. H. Geng, L. X. Wang, L. X. Appl. Phys. Lett. 2007, 90: 043504

A hair cuticle, which consists of flat overlapping scales that surround the hair fiber, protects inner tissues against external stimuli. The outermost surface of the cuticle is covered with the epicuticle that is a thin membrane contains lipids and proteins. Beneath the epicuticle, three layers, which are referred to as the A-layer, the exocuticle and the endcuticle, form a multilayer structure. In our previous study, depth profile analysis of the amino acid composition was conducted in order to reveal the multilayer structure of the hair cuticle tissue using TOF-SIMS with a bismuth primary ion source combined with the C₆₀ sputtering technique [1]. It was confirmed that the lipids and the cysteine rich layer exists on the outermost surface of the cuticles which is considered to be the epicuticle, though the detailed structure of the epicuticle has not been clarified. In this study, depth profiling analysis of the cuticle surface was carried out using the argon gas cluster beam (GCIB) sputtering technique in order to characterize the structure of the epicuticle. The shallow depth profiling of the cuticle surface up to the A-layer was managed to be obtained by optimizing acceleration voltage and current density of GCIB. The profile of lipids differs from that of proteins. It was suggested that the lipid layer exists above the protein layer.

In conclusion, it was suggested that the outermost surface of cuticles has a multilayer containing lipid layers, which is consistent with the previously proposed structure [2]. And, it was indicated that molecular level characterization of the layer structure is possible by using the GCIB depth profile analysis with the low sputtering rate.

[1] M. Okamoto, K. Ishikawa, N. Tanji, S. Aoyagi, I. Kita and C. T. Migita, e-J. of Surf. Sci. Nanotechnology, 10, 2012, 234.

[2] A. P. Negri, H. J. Cornell, and D. E. Rivett, Textile Res. J. 63, 1993, 109.