SIMS2015 Session NN1+FN2-ThA: Nanomaterials and Nanotechnology (2:00-3:40 pm) + Fundamentals (4:00-5:40 pm)

Although nanomaterials have found their way into daily products, knowledge about the risks involved for human health and environment is far from complete. Therefore, studies with controlled exposition of nanoparticles to model systems are conducted. In particular, we focus on the distribution of nanomaterials via the respiratory system and its influences on the surrounding lung tissue material. Especially low dose expositions with realistic concentrations of nanoparticles are a matter of interest. Therefore, for SIMS analyses high secondary ion yields are mandatory. A standard way to improve the limits of detection for small metal materials is the use of surface oxidizing methods. However, in order to maintain the chemical information while performing depth profiling, the use of primary cluster ions is required. Ultimately, the use of oxygen cluster ions as primary ions for depth profiling is intended. This presentation discusses the preparatory work using different sample treatment methods including atomic oxygen and UV-ozone as well as the combination of Ar_x cluster ion sputtering and O₂⁺ sputtering. Secondary ion yields and erosion rates for the detection of metal nanoparticles in organic and biological matrices are investigated. Optimized analysis conditions are identified and applied to the analysis of particles in model systems, for example, gelatine and macrophages. Subsequently, the analysis of lung tissues samples for toxicological studies as the ultimate objective is intended.

It has been shown that secondary ion emission from ultra-thin foils is notably enhanced in the forward direction (J. Phys. Chem.C 2012, 116, 8138). This feature should be of interest for examining nano-objects. A pre-requisite is to deposit them on as thin a support as possible. For this study we chose graphene. We present here data from bombarding free-standing graphene alone and with deposition of dispersed nanoparticles in a setup enabling sample bombardment at 0° and secondary ion (SI) detection in transmission in-line with the incident projectiles. For primary ions we used C₆₀^1,2+ and Au₄₀₀⁴⁺ at impact energies of ~0.4, 0.8 and 1.2 keV/atom respectively. The experiments were run as a sequence of single projectile impacts with each time separate recording of the SIs identified via ToF-MS. The methodology has been described in Int. J. Mass Spectrom. 2013, 334, 43. All experiments were run in the negative ion mode. We observed significantly enhanced SI yields, attributed to a varying ionization probability affected by the characteristics of the projectile, the graphene and the nanoparticle.

The bombardment of graphene alone provided some surprising data. C₆₀^1,2+ on graphene represents the unique case of a 2D projectile on a 2D target. The C_n^- yields are above 10% for n ≤ 4. Similarly, the bombardment of graphene with Au₄₀₀⁴⁺ produced C_n^- yields in excess of 10% for n ≤ 4. The experimental observations indicate at least 4 different emission processes in this projectile-target combination. (J. Chem. Phys. 2015, 142, 044308) With both C₆₀^1,2+ and Au₄₀₀⁴⁺, the ion yields show only a small difference between 1 and 4 layer graphene. We infer that the abundant ionization is a surface phenomenon.

Gold nanoparticles (AuNPs) on graphene in a sub-monolayer of 5 nm dodecanethiol-coated AuNPs were examined with C₆₀²⁺. The Au^- peak from NPs was measured with an effective yield of 1.2%. S^-, SH^- and C₁₂H₂₅SO₃^- from coated dodecanethiol were also observed. The same sample was also run under Au₄₀₀⁴⁺ bombardment. Except for a prominent Au^- peak (effective yield of 35%, in part attributed to the projectile), we observed Au₂^-, Au₃^-, their adducts, and S^-, SH^- and C₁₂H₂₅SO₃^- from dodecanethiol. Experiments with different size nanoparticles suspended on graphene indicate that the dependence of the ionization probability on the projectile-target characteristics will be a key issue for accurate characterization. The enhanced detection sensitivity afforded by transmissions SIMS applies for NPs on graphene and of sizes that are destroyed with one impact. Work supported by NSF grant CHE-1308312.

The prospects for SIMS to detect nano-size inhomogeneities is examined here with a study of gold aggregates containing 55, 147, or 225 atoms encapsulated in poly-amido amine dendrimers. We show that the secondary ion yield is dependent on the number of Au atoms, that the number of gold inclusions can be determined, and that the dendrimer branch closest to the Au inclusion can be identified.

The experiments were run on dendrimers samples with their Au agglomerates deposited on silicon wafers. Bombardment was with 520 keV Au₄₀₀⁴⁺ in the event-by-event bombardment-detection mode, one projectile at a time, for a total of ~3 million impacts. The ejecta from each impact were recorded separately. This methodology allows to identify different types of impacts (e.g., on Au aggregates, dendrimers, or interfacial regions). The respective analyte-characteristic secondary ions were summed for statistics. The fundamental methodology has been described in Science 1990, 248, 988.

The dependence of the secondary ion emission on the number of Au atoms was tested with the co-emission of Au₂^- and Au₂CN^-. They result from a specific projectile-Au inclusion impact parameter. From the co-emission data we obtained the effective yields for Au₂CN^- on Au₅₅, Au₁₄₇, and Au₂₂₅ which show a linear increase with the number of Au atoms (Figure 1 supplemental). It is important to note that for the sizes of Au inclusions examined here the ionization probability is constant, contrary to a trend occurring on larger size nanoparticles (≥5 nm) where differences in impact parameters can result in varying ionization probabilities (Int J Mass Spectrom. 2013, 334, 43).

The number of Au aggregates in the dendrimer samples was determined using a surface coverage procedure described earlier(Int J Mass Spectrom. 2011, 303, 97). The number of Au₅₅, Au₁₄₇, and Au₂₂₅ aggregates over an area of ~500 µm in diameter were 8.7x10¹⁰, 2.2x10¹⁰, and 2.1x10¹⁰, respectively. We estimate that the three dendrimers tested contain ~15% Au₅₅, ~4% Au₁₄₇, or ~4% Au₂₂₅, respectively.

The ability to detect co-emitted secondary ions was also applied to gain insight into the preferred location of the Au inclusions in the dendrimer structure. The co-emission of Au₂^- is correlated with m/z 84 and 99 and anti-correlated with m/z 109 (Figure 2 supplemental), suggesting that the inclusions locate preferentially between two branches of the dendrimer structure, rather than along a single branch. The approach demonstrated on Au inclusions in dendrimers is applicable to other materials containing nano-sized inclusions. The scope and limitations of our methodology will be reviewed. Work supported by NSF grant CHE-1308312.

Most industrial laboratories study a wide variety of material systems such as polymers or inorganic thin films and many of these material systems require analysis not just of the as received surface, but also of and through the depth of a thin film. A successful sputter depth profile precisely identifies the composition and layer thickness(es) of materials as a function of depth. Historically, in the case of inorganic thin films, monoatomic argon ion beam depth profiling is the preferred primary ion beam even though there are issues with preferential sputtering, material migration, and chemical reduction during sputtering which can alter the apparent profile of the analyzed material.

In recent years other ion sources have become routinely available such as C₆₀⁺ and most recently, gas cluster ion sources (Ar⁺_x). From a practical standpoint, i t is important to understand the sputter induced chemistry that may be created by these sources and the trade-offs for applying these different primary ion sources for routine surface chemical analyses. We shall compare and contrast the effects of preferential sputtering and chemical changes or reactions, and sputter rates of metal oxides. The different types of samples to be discussed will include niobium oxides, titanium nitride and multilayer thin films.

It has been shown that combining Cs bombardment with positive secondary molecular ion detection (MCs⁺) can extend the analysis capability of SIMS from the dilute limit (<1%) to matrix elements[1]. The MCs⁺ technique has had great success in quantifying the sample composition of III-V semiconductors as well as dopants and or impurities[1,2]. However, the MCs⁺ has been less effective at reducing the matrix effect for column IV compounds, particularly Si containing compounds[3], due to high Cs surface concentrations overloading the sample surface and impacting ion yields. The Cs overload issue is attributed to the mobility and relocation of the implanted Cs to the surface during the analysis. This effect happens almost instantaneously and once the surface is overloaded with Cs, the excess Cs begins to limit the Cs ionization[4]. Recent studies have shown a temperature dependent component of the Cs mobility[5]. Understanding the material dependent success of the MCs⁺ technique and elucidating the Cs mobility depends on answering two questions: 1) how is Cs incorporated and distributed into the sample; and 2) how the Cs surface concentration affects the ionization processes. With the addition of a custom-built variable temperature stage on our Cameca 7f-GEO SIMS and other surface and sub-surface characterization techniques we will answer those two questions.

This study provides new insight for improving the MCs⁺ technique by investigating the Cs retention, up-take, and distribution differences between group III-V and IV materials and provides a greater understanding of the temperature dependent relocation of Cs. The Cs retention and distribution differences are determined by measuring the Cs concentration using heat treatment-XPS and heat treatment-MEIS. Cs SIMS build-up curves are acquired to assess the Cs up-take differences. By utilizing the newly developed heating stage on our SIMS, we use Cs build up curves acquired at elevated temperatures to show the temperature dependent relocation of Cs and the effect it has on the ionization processes. Additionally, the Cs ionization and neutralization as a function of Cs fluence and temperature were measured. Our results allow for a more thorough understanding of the material dependence on the Cs⁺-sample interaction and the temperature component of the Cs mobility.

[1] Y. Gao, J. Appl. Phys. 64 (1988) 3760-3762.

[2] C. Magee, W. Harrington, E. Botnick, Int. J. Mass Spectrom. Ion Prosses. (1990) 45-56.

[3] K. Wittmaack, Nucl. Instruments Methods Phys. Res. B. 85 (1994) 374–378.

[4] K. Wittmaack, Surf. Sci. 606 (2012) L18–L21.

[5] A. Giordani, J. Tuggle, C. Winkler, J. Hunter, Surf. Interface Anal. (2014) 31–34.

We report on the angular distribution of large organic species sputtered with argon gas cluster ion beams (GCIB) in the range of E/n=1-5. The transport of neutrals emitted from the sample surface after primary ion impact is of particular importance for laser-based secondary neutral mass spectrometry (SNMS) techniques, which critically depend upon the effective overlap of the expanding plume with the laser irradiated volume. Some theoretical work on angular distributions of ejecta has been published on the use of GCIB with organic layers,[1] but there is a lack of experimental data for large argon clusters.

An ION-TOF TOF-SIMS IV and a Kratos AXIS Supra, both equipped with argon GCIB, were used without modification. Organic layers of Irganox 1010 on silicon were prepared by physical vapor deposition. The thickness of the deposited layers was measured by ellipsometry. Craters of few hundred square micrometers were sputtered with argon GCIB in DC mode at various projectile kinetic energies of 5-20 keV, incidence angles from 25-62° and cluster sizes in the range of n=2000-5000. Clean silicon wafers were mounted perpendicularly to the surface of the organic layer and used for capturing the ejected material. Ellipsometry was used to determine the silicon oxide layer thickness on the capturing wafers before the experiment. Maps of the thickness of the captured organic layer were recorded using ellipsometry, and the data corrected for the variation in capturing area with distance and impact angle from the sputtering site. An in-house holder was used to enable the precise and reproducible positioning of target and capturing silicon wafers.

We present data illustrating a pronounced anisotropy with a strong forward direction for the plume of ejected material. Our data reveal that a significant fraction of the sputtered organic material is ejected within an azimuthal solid angle of ±45°. Typically half of the sputtered material is captured on the wafers, presumably because a fraction of material does not stick to the capture wafer. This may be due to a significant fraction of ejecta being low molecular mass fragments or impinging at angles and velocities that do not permit sticking. The data also suggest that ejecta distributions have minimal dependence on the incidence angle of the primary ion beam. SIMS depth profiles on the captured organic layer demonstrate that a significant proportion of molecules survive the ejection and capture process intact and enable an estimate of the degree of fragmentation during the sputtering event to be made.

[1] B. Czerwinski, L. Rzeznik, R. Paruch, B.J. Garrison, Z. Postawa. Nucl. Instrum. Meth. Phys. Res. B 269 (2011) 1578.

A study of cluster emission from the ternary alloy Au₇Cu₅Al₄ was conducted. This study was focused on the effect of surface modification via ion irradiation, subjecting the sample to different modes of ion bombardment: (1) 10 keV obliquely incident (60˚) Ar⁺ and (2) normally incident Ar⁺ with variable energy, from 5 keV to below 100 eV. These experiments utilized high resolution dual-beam sputter depth profiling technique with single-photon ionization by a 7.9 eV F₂ laser.

The yield of Al-containing clusters is larger after applying a dose of high energy (10 keV) Ar⁺, while the yield of AuCu clusters is larger after applying a dose of a low energy (≤ 200 eV) Ar⁺. The effect is general for all clusters detected, up to Au₄Al₂.The yield of Al-containing clusters decreases as the dosing ion energy decreases, while the opposite is seen for Au_n and Au_nCu_m clusters. The effect is strongest at the lowest energies, i. e. below about 300 eV.

These observations are out of modern cluster formation models and require further theoretical and experimental studies. Possible ways and models will be discussed.

This work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division

SIMS depth profiling of polymers is nowadays achievable thanks to the introduction of cluster beams, that allow the in-depth detection of the typical fragments of the material under investigation well beyond the static limit. Such capability depends on several physical and chemical processes following the cluster impact. The capability to perform molecular depth profiling can be roughly connected with the high sputtering yield and the very low penetration depth of the polyatomic ions that prevent damage accumulation. However, several experiments show that feasibility of molecular depth profiling is dependent on the ion-beam-induced chemistry of the sample. Paradigmatic, in this respect, is the opposite behavior of PS and PMMA under SF₅⁺ or C₆₀⁺ and, by contrast, the “profilability” of both polymers by GCIB’s. There is nowadays a general consensus that ion-beam induced chemistry cannot be neglected for a full interpretation of the behavior of polymer and organic systems under cluster ion beam irradiation. Parallel to the development of studies on the fundamentals of cluster sputtering of organics, there is clearly the need of tools for the practical interpretation of the experimental profiles, possibly in a quantitative or semi-quantitative way. In the more traditional field of inorganic depth profiling several models have been developed, such as the mixing-roughness-information depth (MRI) model of Hofmann et al. In the case of organics, Krantzman and Wucher developed the statistical sputtering model (SSM) for connecting information from the molecular dynamic simulations to depth profiles and Paruch et al. computed a molecular depth profile for cluster beam bombardment of a molecular solid.

In this contribution, we propose a new approach that is able to incorporate the beam induced reactivity, so leading to a reasonable simulation of depth profiles of polymers and organic solids. This could provide a tool for assessing the role of different hypothesized processes occurring in the target. Basically, the approach is similar to those that are successfully applied for the modelling of the advection, diffusion and reaction phenomena in fluid media. The continuum equation is used for modelling erosion rate, ion beam induced mixing and reactions during the sputtering process. The model we are going to outline allows to include the influence of temperature as well as the effects of the reactive gas dosing on sputtering yield and damage accumulation during profiling. Comparison with experimental data confirms the goodness of the model and strengthens the proposed approach.