SIMS Experiments Research Articles

Titanium pre-sputtering for an enhanced secondary ion mass spectrometry analysis of atmospheric gas elements

Gettering properties of titanium used as a pre-sputtering target provide better detection limits of hydrogen and oxygen is SIMS experiments.

Data acquisition and control system for an evolving nuclear microprobe

The application possibilities of the nuclear microprobe are constantly evolving which places new requirements on the data acquisition and microbeam control systems. MeV SIMS experiments for example, require the ability to pulse a continuous beam and steer it in the X-Y plane. For channeling RBS and STIM experiments, the sample needs to be rotated over two axes. Once it is positioned in the channeling direction, patterned irradiation may be needed. Additionally, with the evolution of digital electronics, it is now possible and sometimes preferred to perform signal processing in the digital domain as oppose to analogue signal chains in NIM modules.In this work we present the most recent upgrades to the data acquisition and control system developed at Rudjer Boskovic Institute. A data acquisition/control system based on a Xilinx Virtex 6 FPGA was developed which can evolve with the microprobe and be reconfigured for various applications. The real time reprogrammable nature of the FPGA coupled with a modular design approach, allow for the ADCs, processing algorithms and communication protocols to be interchanged and upgraded while keeping a constant user interface through the SPECTOR software package.

C-O Bond Dissociation and Induced Chemical Ionization Using High Energy (CO2)n+ Gas Cluster Ion Beam.

A gas cluster ion beam (GCIB) source, consisting of CO2 clusters and operating with kinetic energies of up to 60keV, has been developed for the high resolution and high sensitivity imaging of intact biomolecules. The CO2 molecule is an excellent molecule to employ in a GCIB source due to its relative stability and improved focusing capabilities, especially when compared to the conventionally employed Ar cluster source. Here we report on experiments aimed to examine the behavior of CO2 clusters as they impact a surface under a variety of conditions. Clusters of (CO2)n+ (n= 2000~10,000) with varying sizes and kinetic energies were employed to interrogate both an organic and inorganic surface. The results show that C-O bond dissociation did not occur when the energy per molecule is less than 5eV/n, but that oxygen adducts were seen in increasing intensity as the energy is above 5eV/n, particularly, drastic enhancement up to 100 times of oxygen adducts was observed on Au surface. For Irganox 1010, an organic surface, oxygen containing adducts were observed with moderate signal enhancement. Molecular dynamics computer simulations were employed to test the hypothesis that the C-O bond is broken at high values of eV/n. These calculations show that C-O bond dissociation occurs at eV/n values less than the C-O bond energy (8.3eV) by interaction with surface topological features. In general, the experiments suggest that the projectiles containing oxygen can enhance the ionization efficiency of surface molecules via chemically induced processes, and that CO2 can be an effective cluster ion source for SIMS experiments. Graphical Abstract.

On-Tissue Chemical Derivatization of Catecholamines Using 4-( N-Methyl)pyridinium Boronic Acid for ToF-SIMS and LDI-ToF Mass Spectrometry Imaging.

The analysis of small polar compounds with ToF-SIMS and MALDI-ToF-MS have been generally hindered by low detection sensitivity, poor ionization efficiency, ion suppression, analyte in-source fragmentation, and background spectral interferences from either a MALDI matrix and/or endogenous tissue components. Chemical derivatization has been a well-established strategy for improved mass spectrometric detection of many small molecular weight endogenous compounds in tissues. Here, we present a devised strategy to selectively derivatize and sensitively detect catecholamines with both secondary ion ejection and laser desorption ionization strategies, which are used in many imaging mass spectrometry (IMS) experiments. Chemical derivatization of catecholamines was performed by a reaction with a synthesized permanent pyridinium-cation-containing boronic acid molecule, 4-( N-methyl)pyridinium boronic acid, through boronate ester formation (boronic acid-diol reaction). The derivatization facilitates their sensitive detection with ToF-SIMS and LDI-ToF mass spectrometric techniques. 4-( N-Methyl)pyridinium boronic acid worked as a reactive matrix for catecholamines with LDI and improved the sensitivity of detection for both SIMS and LDI, while the isotopic abundances of the boron atom reflect a unique isotopic pattern for derivatized catecholamines in MS analysis. Finally, the devised strategy was applied, as a proof of concept, for on-tissue chemical derivatization and GCIB-ToF-SIMS (down to 3 μm per pixel spatial resolution) and LDI-ToF mass spectrometry imaging of dopamine, epinephrine, and norepinephrine in porcine adrenal gland tissue sections. MS/MS using collision-induced dissociation (CID)-ToF-ToF-SIMS was subsequently employed on the same tissue sections after SIMS and LDI mass spectrometry imaging experiments, which provided tandem MS information for the validation of the derivatized catecholamines in situ. This methodology can be a powerful approach for the selective and sensitive ionization/detection and spatial localization of diol-containing molecules such as aminols, vic-diols, saccharides, and glycans along with catecholamines in tissue sections with both SIMS and LDI/MALDI-MS techniques.

Effects of plasma immersion ion implantation on fatigue properties of titanium alloy surfaces

This paper reports on nanocharacterization of commercial Ti-6Al-4V and sintered Ti-Si-B alloys after surface treatment by high temperature plasma immersion ion implantation (HTPIII) to investigate fatigue in addition to hardness and wear properties. The results show that HTPIII is an effective method for improvement of the mechanical and tribological properties of the conventional Ti-6Al-4V alloy, however higher temperatures are necessary to produce the same gain on sintered Ti-Si-B alloys. For the Ti-6Al-4V sample after HTPIII the SIMS experiments identify a nitrogen-rich layer with a thickness of about 1 ?m. The untreated samples show a barely visible nitrogen signal below the surface oxide, indicating a large nitrogen uptake and retention during the HTPIII process. Both HTPIII treated alloys presented higher hardness and wear resistance, but – surprisingly - lower fatigue resistance.

Hydrogen diffusion process in the oxides formed on zirconium alloys during corrosion in pressurized water reactor conditions

During the corrosion in primary water of zirconium alloys, a fraction of hydrogen from the water diffuses through the oxide into the alloy. To better understand the hydriding process, isotopic tracers and SIMS experiments were performed on oxides grown on Zircaloy-4 alloy under simulated primary water conditions. Results showed the oxide film in pre-transition is divided into two sub-layers, an external one, highly permeable to hydrogen and an inner protective one. Thermal Desorption Spectroscopy analyses revealed two interaction sites of hydrogen, located in each oxide sublayer.Apparent diffusion coefficients of hydrogen in both oxide sublayers were estimated with different methods.

CO2 Cluster Ion Beam, an Alternative Projectile for Secondary Ion Mass Spectrometry.

The emergence of argon-based gas cluster ion beams for SIMS experiments opens new possibilities for molecular depth profiling and 3D chemical imaging. These beams generally leave less surface chemical damage and yield mass spectra with reduced fragmentation compared with smaller cluster projectiles. For nanoscale bioimaging applications, however, limited sensitivity due to low ionization probability and technical challenges of beam focusing remain problematic. The use of gas cluster ion beams based upon systems other than argon offer an opportunity to resolve these difficulties. Here we report on the prospects of employing CO2 as a simple alternative to argon. Ionization efficiency, chemical damage, sputter rate, and beam focus are investigated on model compounds using a series of CO2 and Ar cluster projectiles (cluster size 1000-5000) with the same mass. The results show that the two projectiles are very similar in each of these aspects. Computer simulations comparing the impact of Ar2000 and (CO2)2000 on an organic target also confirm that the CO2 molecules in the cluster projectile remain intact, acting as a single particle of m/z 44. The imaging resolution employing CO2 cluster projectiles is improved by more than a factor of two. The advantage of CO2 versus Ar is also related to the increased stability which, in addition, facilitates the operation of the gas cluster ion beams (GCIB) system at lower backing pressure. Graphical Abstract ᅟ.

Protocol of single cells preparation for time of flight secondary ion mass spectrometry

There are several techniques like time of flight secondary ion mass spectrometry (ToF SIMS) that require a special protocol for preparation of biological samples, in particular, those containing single cells due to high vacuum conditions that must be kept during the experiment. Frequently, preparation methodology involves liquid nitrogen freezing what is not always convenient. In our studies, we propose and validate a protocol for preparation of single cells. It consists of four steps: (i) paraformaldehyde fixation, (ii) salt removal, (iii) dehydrating, and (iv) sample drying under ambient conditions. The protocol was applied to samples with single melanoma cells i.e. WM115 and WM266-4 characterized by similar morphology. The surface and internal structures of cells were monitored using atomic force, scanning electron and fluorescent microscopes, used to follow any potential protocol-induced alterations. To validate the proposed methodology for sample preparation, ToF SIMS experiments were carried out using C60+ cluster ion beam. The applied principal component analysis (PCA) revealed that chemical changes on cell surface of melanoma cells were large enough to differentiate between primary and secondary tumor sites.Subject category: Mass spectrometry.

SIMS—A precursor and partner to contemporary mass spectrometry

Significant events driving the development of SIMS over the last 50 years are reviewed. The discussion includes recollections of dynamic and static SIMS from the 1970s, of the emergence of TOF–SIMS during the 1980s and of the incorporation of cluster ion bombardment during most recent times. Advances in theoretical understanding of the sputtering phenomenon and of the ionization process that accompanied these advances are also included. Many early discoveries were focused upon the stimulated desorption of organic and bioorganic molecules, first via static SIMS and next via fast atom bombardment, that were important precursor experiments to modern day mass spectrometry. Today, submicron molecule-specific imaging and molecular depth profiling represent unique aspects of SIMS experiments. Developments that led to the optimization of these modalities are also emphasized in the review. In general, the characteristics of SIMS that make it a contemporary partner to modern day mass spectrometry are highlighted.

Diffusion of cesium in silicon during SIMS experiments investigated by numerical simulations

In the present paper, we focus on the diffusion behaviour of cesium atoms implanted into silicon. This aspect has not been investigated directly up to now but is of utmost importance to understand the ionisation mechanisms in more detail. Density functional theory calculations combined with ab initio molecular dynamics simulations have been used to study the different aspects of cesium diffusion to the sample surface as a function of cesium concentration and sample temperature. In general, the solubility of cesium atoms in a silicon matrix is small, and the diffusion of the cesium atoms towards the surface is almost immediate, depending on how and where the cesium atom is incorporated in the silicon lattice. Depending on the defects introduced into the Si substrate, the diffusion could be largely reduced or cannot be observed during the few picosecond time range that can be explored by ab initio molecular dynamics simulations. Copyright © 2014 John Wiley & Sons, Ltd.

SIMS measurement of nano-sized Au agglomerates in Si

The conditions required for SIMS measurement, namely the detection depth of the SIMS signal and an appropriate measurement cycle, to obtain a reliable shape, size and atom number of nano-sized agglomerates in Si have been investigated. Au agglomerates in Si generated during annealing at 900 °C for 360 h are measured by an appropriate SIMS method and it is found that the Au agglomerate has a spherical shape with a concentration of 5.4 × 1021 Au atoms/cm3. The SIMS results agree well with the calculation of spherical agglomerates. The size of agglomerates larger than 20 nm and the Au atom number in agglomerates larger than several nm can be measured in a usual SIMS experiment.

Examination of Two P-Type 4H SiC Samples Having Similar Resistivity but Very Different Radiation Damage and Annealing Characteristics

In the course of studying by low temperature photoluminescence spectroscopy a wide range of electron-irradiated samples of p(Al)-type epitaxial layers of 4H SiC, from a variety of different sources of supply, the results were found to fit into two very different categories. The origin of these differences has been explored using a wide range of experimental techniques and found to result from the degree of compensation of the aluminium by nitrogen in the layers. Nitrogen concentrations deduced by SIMS experiments on these materials were found to be unreliable. The two different categories of material, called V and AB here, showed marked differences in their subsequent annealing behaviour and the implications of this distinction are discussed.

Focused ion beam implantation of Ga in InP studied by SIMS and dynamic computer simulations

AbstractFocused ion beam implantation of 30 keV Ga+ ions in single‐crystalline InP was investigated by SIMS, using Cs+ primary ions for sputtering. Nine different implantation fluences ranging from 1 × 1013 to 1 × 1017 Ga+ ions/cm2 were used, with implanted areas of 50 × 50 µm2. The Ga concentration distributions of these implants were determined by SIMS depth profiling. Such 30 keV Ga implantations were also simulated by a dynamic Monte Carlo simulation code that takes into account the gradual change of the near‐surface composition due to the Ga incorporation. In both approaches, an essentially linear increase of the Ga content with fluence is found up to ∼1 × 1016 cm−2; for higher fluences, the Ga peak concentration approaches a saturation level which is reached at about (0.5–1) × 1017 cm−2 and amounts to ∼10%. The measured and simulated concentrations of the Ga distributions are in good agreement. The most probable range of the Ga+ ions in InP obtained from the SIMS experiments is ∼23 nm and matches closely with the value from the simulations. Copyright © 2010 John Wiley & Sons, Ltd.

ToF‐SIMS depth profiling of trehalose: the effect of analysis beam dose on the quality of depth profiles

AbstractIn static SIMS experiments, an analysis dose of 1012 ions/cm2 typically produces optimum results. However, the same dose used in dual beam depth profiling can significantly degrade the signal. This is because during each analysis cycle a high‐energy beam is rastered across the same x‐y location on the sample. If a sufficient amount of sample is not removed during each sputter cycle, the subsequent analysis cycle will sample a volume degraded by the previous analysis cycles. The dimensionless parameter, R′, is used to relate the amount of damage accumulated in the sample to the amount of analysis beam dose used relative to the etching beam. Depth profiles from trehalose films spin‐cast onto silicon wafers acquired using Bi1+ and Bi3+ analysis beams were compared. As R′ increased, the depth profile and the depth resolution (interface width) both degraded. At R′ values below 0.04 for both Bi1+ and Bi3+, the shape of the profile as well as the depth resolution (9 nm) indicated that dual beam analysis can be superior to C60 single beam depth profiling. Copyright © 2010 John Wiley & Sons, Ltd.

Time of flight mass spectrometry imaging of samples fractured in situ with a spring-loaded trap system.

An in situ freeze fracture device featuring a spring-loaded trap system has been designed and characterized for time of flight secondary ion mass spectrometry (TOF SIMS) analysis of single cells. The device employs the sandwich assembly, which is typically used in freeze fracture TOF SIMS experiments to prepare frozen, hydrated cells for high-resolution SIMS imaging. The addition of the spring-loaded trap system to the sandwich assembly offers two advances to this sample preparation method. First, mechanizing the fracture by adding a spring standardizes each fracture by removing the need to manually remove the top of the sandwich assembly with a cryogenically cooled knife. A second advance is brought about because the top of the sandwich is not discarded after the sandwich assembly has been fractured. This results in two imaging surfaces effectively doubling the sample size and providing the unique ability to image both sections of a cell bifurcated by the fracture. Here, we report TOF SIMS analysis of freeze fractured rat pheochromocytoma (PC12) cells using a Bi cluster ion source. This work exhibits the ability to obtain single cell chemical images with subcellular lateral resolution from cells preserved in an ice matrix. In addition to preserving the cells, the signal from lipid fragment ions rarely identified in single cells are better observed in the freeze-fractured samples for these experiments. Furthermore, using the accepted argument that K(+) signal indicates a cell that has been fractured though the cytoplasm, we have also identified different fracture planes of cells over the surface. Coupling a mechanized freeze fracture device to high-resolution cluster SIMS imaging will provide the sensitivity and resolution as well as the number of trials required to carry out biologically relevant SIMS experiments.

Molecular approaches to p- and n-nanoscale doping of Ge 1−ySn y semiconductors: Structural, electrical and transport properties

We report the development of practical doping protocols via designer molecular sources to create n- and p-type doped Ge 1− y Sn y layers grown directly upon Si(1 0 0). These materials will have applications in the fabrication of advanced PIN devices that are intended to extend the infrared optical response beyond that of Ge by utilizing the Sn composition as an additional design parameter. Highly controlled and efficient n-doping of single-layer structures is achieved using custom built P(GeH 3) 3 and As(GeH 3) 3, precursors containing preformed Ge–As and Ge–P near-tetrahedral bonding arrangements compatible with the structure of the host Ge–Sn lattice. Facile substitution and complete activation of the P and As atoms at levels ∼10 17–10 19 cm −3 is obtained via in situ depositions at low temperatures (350 °C). Acceptor doping is readily achieved using conventional diborane yielding carrier concentrations between 10 17–10 19 cm −3 under similar growth conditions. Full activation of the as-grown dopant concentrations is demonstrated by combined SIMS and Hall experiments, and corroborated using a contactless spectroscopic ellipsometry approach. RTA processing of the samples leads to a significant increase in carrier mobility comparable to that of bulk Ge containing similar doping levels. The alloy scattering contribution appears to be negligible for electron carrier concentrations beyond 10 19 cm −3 in n-type samples and hole concentrations beyond 10 18 cm −3 in p-type samples. A comparative study using the classical lower-order hydrides PH 3 and AsH 3 produced n-doped films with carrier densities (up to 9 × 10 19 cm −3) similar to those afforded by P(GeH 3) 3 and As(GeH 3) 3. However, early results indicate that the simpler PH 3 and AsH 3 sources yield materials with inferior morphology and microstructure. Calculations of surface energetics using bond enthalpies suggest that the latter massive compounds bind to the surface via strong Ge–Ge bonds and likely act as “retardants” that moderate surface diffusion of the reactions species, thereby promoting layer-by-layer growth leading to thick, atomically smooth films, particularly in the case of P(GeH 3) 3. Furthermore, the intact incorporation of the Ge 3P and Ge 3As molecular cores results in highly uniform compositional, strain and doping profiles at the atomic level.

Towards quantitative depth profiling with high spatial and high depth resolution

Nowadays, fundamental SIMS physics limits the quantitative compositional analysis of new semiconductor structures with high depth resolution and high spatial resolution. Therefore, a new concept (Zero-energy SIMS) is introduced which is based on electron beam inducted etching (EBIE) of the substrate surface and subsequent post-ionization of the volatile species by short high-power laser pulses. Silicon electron beam induced etching with SF 6 as precursor species with an etching yield of 0.003 Si atoms/electron is demonstrated. Zero-energy SIMS experiments in a magnetic sector instrument with SF 6 in positive mode show both electron-induced processes (ESD of fluorine) and collision-induced removal of Si atoms. These sputter events are caused by the impact of accelerated SF 6 fragment anions which are formed by an electron-attachment process in the gas phase. In negative mode, a large contribution of cluster ions is observed due to the high flux of incoming polyatomic cations, formed by electron-impact ionization of SF 6 molecules in the gas phase. The kinetic energy distributions of these particles are in agreement with the recombination model for cluster emission.

Which is more important in bioimaging SIMS experiments—The sample preparation or the nature of the projectile?

Sample preparation is central to acquiring meaningful molecule-specific images with SIMS, especially when submicron lateral resolution is involved. The issue is to maintain the distribution of target molecules while attempting to introduce biological cells or tissue into the high vacuum environment of the mass spectrometer. Here we compare freeze-drying, freeze-etching, freeze-fracture and trehalose vitrification as possible strategies for these experiments. The results show that the prospects for successful imaging experiments are greatly improved with all of these methods when using cluster ion bombardment, particularly C 60 + ions, not only due to increased sensitivity of this projectiles, but also since it removes contamination overlayers without insult to the underlying chemistry. The emergence of 3-dimensional imaging capabilities also suggests that sample preparation should not perturb the 3-dimensional morphology of the cell, a situation not generally possible during freeze-drying. Hence, sample preparation and projectile type are strongly coupled parameters for bioimaging with mass spectrometry.

Superlubricity mechanism of diamond-like carbon with glycerol. Coupling of experimental and simulation studies

We report a unique tribological system that produces superlubricity under boundary lubrication conditions with extremely little wear. This system is a thin coating of hydrogen-free amorphous Diamond-Like-Carbon (denoted as ta-C) at 353 K in a ta-C/ta-C friction pair lubricated with pure glycerol. To understand the mechanism of friction vanishing we performed ToF-SIMS experiments using deuterated glycerol and 13C glycerol. This was complemented by first-principles-based computer simulations using the ReaxFF reactive force field to create an atomistic model of ta-C. These simulations show that DLC with the experimental density of 3.24 g/cc leads to an atomistic structure consisting of a 3D percolating network of tetrahedral (sp3) carbons accounting for 71.5% of the total, in excellent agreement with the 70% deduced from our Auger spectroscopy and XANES experiments. The simulations show that the remaining carbons (with sp2 and sp1 character) attach in short chains of length 1 to 7. In sliding simulations including glycerol molecules, the surface atoms react readily to form a very smooth carbon surface containing OH-terminated groups. This agrees with our SIMS experiments. The simulations find that the OH atoms are mostly bound to surface sp1 atoms leading to very flexible elastic response to sliding. Both simulations and experiments suggest that the origin of the superlubricity arises from the formation of this OH-terminated surface.

ToF-SIMS characterization of uranium hydride

Time-of-flight secondary-ion mass spectrometry (ToF-SIMS) has been employed for the first time to investigate uranium hydride. The deuterated form of uranium hydride (UD3) was formed on a polycrystalline uranium sample by exposure to high-purity D2 gas at room temperature. The characteristic positive and negative secondary-ion fragments observed from uranium hydride are reported and assigned. Our investigations show that negative-ion fragments provide the most unambiguous method for identification of uranium hydride, through the detection of and fragments. In addition, density functional theory (DFT) calculations are presented which confirm the identity and stability of, and also assign structural geometries to, the ion fragments observed within the SIMS experiment. The DFT calculations show that ions adopt a linear arrangement, whereas positive tri- and tetrahydride ions () adopt structures based on square planar geometries, although negative tri- and tetrahydride ions () adopt structures based on tetrahedral geometries.