Secondary Yield Research Articles

Ultrahigh-resolution imaging of biogenic phosphorus and molybdenum in palaeoproterozoic gunflint microfossils

Phosphorus and molybdenum play important roles in the formation of microbial cell structures and specific enzymes crucial for metabolic processes. Nevertheless, questions remain about the preservation of these elements within ancient microfossils. Here, we present shape-accurate ion images capturing phosphorus and molybdenum on Palaeoproterozoic filamentous microfossils by pioneering a methodology using lateral high-resolution secondary ion mass spectrometry. Introducing electrically conductive glass for mounting isolated microfossils facilitated clearer observations with increased secondary ion yields. Phosphorus was detected along the contours of microfossils, providing direct evidence of phospholipid utilization in the cell membrane. Trace amounts of molybdenum were detected within microfossil bodies, suggesting potential remnants of molybdenum-bearing proteins, such as nitrogenase. These findings align with the hypothesized cyanobacterial origin of filamentous gunflint microfossils. Our methodology introduces a groundbreaking tool for obtaining crucial insights into the cellular evolution and metabolic pathways of microorganisms, allowing comparisons of their morphological characteristics.

Reactive Gas Cluster Ion Beams for Enhanced Drug Analysis by Secondary Ion Mass Spectrometry.

In this study, we investigate the formation and composition of Gas Cluster Ion Beams (GCIBs) and their application in Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis. We focus on altering the carrier gas composition, leading to the formation of (Ar/CO2)n or (H2O)n GCIBs. Our results demonstrate that the addition of a reactive species (CO2) to water GCIBs significantly enhances the secondary ion yield of small pharmaceutical compounds in the positive ion mode. In negative ion mode, the addition of CO2 resulted in either a positive enhancement or no effect, depending on the sample. However, an excess of CO2 in the carrier gas leads to the formation of carbon dioxide clusters, resulting in reduced yields compared to that of water cluster beams. Cluster size also plays a crucial role in overall yields. In a simple two-drug system, CO2-doped water clusters prove effective in mitigating matrix effects in positive ion mode compared to pure water cluster, while in negative ion mode, this effect is limited. These clusters are also applied to the analysis of drugs in a biological matrix, leading to more quantitative measurements as shown by a better fitting calibration curve. Overall, the doping of water clusters with small amounts of a reactive gas demonstrates promising benefits for higher sensitivity, higher resolution molecular analysis, and imaging using ToF-SIMS. The effectiveness of these reactive cluster beams varies depending on the experimental parameters and sample type.

Quantitative aspects of ToF-SIMS analysis of metals and alloys in a UHV, O2 and H2 atmosphere

Although secondary ion mass spectrometry (SIMS) is a versatile method used in the fields of surface analysis, depth profiling and elemental and molecular mapping, it also lacks quantification capabilities. The main reason for this is the matrix effect, which influences the ionization yield of secondary ions with respect to the substrate from which the analyzed compounds originate. There are several approaches to reduce the matrix effect, and gas flooding is one of the easiest methods to apply. In this work, we have investigated the possibilities of the ToF-SIMS method for the quantification of selected metals and alloys containing these metals in different ratios by reducing the matrix effect in the presence of different atmospheres. The measurements were performed in the ultra-high vacuum (UHV) environment, H2 and O2 atmospheres. H2 flooding shows the most significant improvements compared to the UHV analysis, while O2 is also promising but has some limitations. Improvements are most evident for the transition metals Ti, Cr, Fe, Co and Ni employed in our study, while the p-block elements such as Al and Si do not change so extensively. The deviations from the true atomic ratios of selected transition metals in different alloys reach a maximum of only 46 % when analyzed in the H2 atmosphere. In contrast, these values are 66 and 228 % for the O2 atmosphere and UHV environment, respectively. Our results suggest that gas adsorption and consequent formation of a new matrix on the surface, especially in the case of hydrogen, reduces the differences between the different chemical environments and electronic structures of the surface. In this way, the quantitative aspects of the SIMS method can be improved.

ToF-SIMS analysis of ultrathin films and their fragmentation patterns.

Organic thin films are of great interest due to their intriguing interfacial and functional properties, especially for device applications such as thin-film transistors and sensors. As their thickness approaches single nanometer thickness, characterization and interpretation of the extracted data become increasingly complex. In this study, plasma polymerization is used to construct ultrathin films that range in thickness from 1 to 20 nm, and time-of-flight secondary ion mass spectrometry coupled with principal component analysis is used to investigate the effects of film thickness on the resulting spectra. We demonstrate that for these cross-linked plasma polymers, at these thicknesses, the observed trends are different from those obtained from thicker films with lower degrees of cross-linking: contributions from ambient carbon contamination start to dominate the mass spectrum; cluster-induced nonlinear enhancement in secondary ion yield is no longer observed; extent of fragmentation is higher due to confinement of the primary ion energy; and the size of the primary ion source also affects fragmentation (e.g., Bi1 versus Bi5). These differences illustrate that care must be taken in choosing the correct primary ion source as well as in interpreting the data.

Gold-Assisted Molecular Imaging of Organic Tissue by MeV Secondary Ion Mass Spectrometry.

The quality of molecular imaging by means of MeV primary ion-induced secondary ion mass spectrometry by coating with gold was evaluated on different reference organic molecules and plant samples. The enhancement of the secondary ion yield was evident for the majority of the studied analytes, reaching the highest values at gold thicknesses between 0.5 and 2 nm, and increased the intensity up to 5-fold for reference samples and >2-fold for specific peaks within the plant sample. Improved propagation of the electric field due to the target potential on otherwise electrically insulating plant samples was also evident through improved image resolution and by reducing the background in mass spectra. However, detection of several molecules was significantly decreased at even at 1 nm thick gold layer. The results indicated that an optimized sequence of analysis is required to reliably interpret results.

Comparing sputter rates, depth resolution, and ion yields for different gas cluster ion beams (GCIB): A practical guide to choosing the best GCIB for every application

Molecular gas species for gas cluster ion beams (GCIBs), such as carbon dioxide and water, were examined with a range of beam energies and cluster sizes to compare with the “universal relation” of the sputter yield, Y, per cluster atom against incident beam energy, E, per cluster atom of Arn cluster beam using Irganox 1010. In this work, we compare Arn, (CO2)n, and (H2O)n gas clusters to the universal equations for Arn clusters. To discuss molecular gas species for GCIBs, energy per nucleon (E/N) needs to replace energy per atom. We monitored sputter rate, depth resolution, and secondary ion yield as a function of the beam parameters: gas species, beam energy, and cluster size. (H2O)n GCIB shows reduced sputter rates and improved depth resolution with high sensitivity compared to Arn and (CO2)n GCIBs. These initial results indicate the potential to achieve high-depth resolution with high sensitivity and suggest that (H2O)n cluster ion beam has the potential to play a significant role in surface analysis techniques with organic materials. Results also show that no single set of conditions will provide the “best gas cluster ion beam” for all applications. However, it is possible to choose a set of conditions that will be more or less optimal depending on the experimental goals, such as maximizing the sputter rate, depth resolution, and molecular ion yield. In this work, we recommend the following three guidelines for GCIB users to set their own conditions: (1) to maximize the sputter rate, select a smaller cluster (higher E/N), but be aware that this will increase fragmentation and reduce molecular ion yield; (2) to maximize the depth resolution, select a larger cluster (lower E/N), and use (H2O)n GCIB, if possible; and (3) to maximize the molecular ion signal, use the highest beam energy available, and select a cluster with 0.15–0.25 eV/nucleon for Ar and (CO2)n GCIBs or around 0.1 eV/nucleon if using (H2O)n GCIB. These results are valid for XPS, SIMS, and any technique that utilizes GCIBs.

Secondary ion mass spectrometry analysis of metal oxides using 70 keV argon, carbon dioxide, and water gas cluster ion beams

Manganese (II) oxide (MnO), manganese (IV) oxide (MnO2), cobalt (II,III) oxide (Co3O4), and nickel (II) oxide (NiO) were analyzed with time-of-flight secondary ion mass spectrometry using 70 keV gas cluster ion beams. The obtained mass spectra are influenced by projectile chemistry and to a lesser extent velocity. Gas cluster ion beams containing CO2 or H2O enhanced the relative yield of metal oxide and metal hydroxide secondary ions compared to beams containing only Ar. For all gas cluster ion beams tested, steady-state ion ratios [MxOy]+/[Mx]+ were reached. For manganese oxides, the [MnxOy]+/[Mnx]+ ratio reflected the metal oxidation state whereas the [MnxOyHz]+/[Mnx]+ ion ratios did not. This study demonstrates that secondary ion mass spectrometry using 70 keV gas cluster ion beams provides a novel approach to the quantitative analysis of the surface and subsurface regions of metal oxides related to energy-storage materials.

Detection limit for secondary ions of organic molecules under MeV ion bombardment

In the present work we have investigated the chemical sensitivity of a mass spectrometry imaging method MeV – SIMS. Primary ion beam within the MeV energy range domain was employed to bombard samples of two organic compounds; amino acid arginine, and peptide hormone angiotensin II (human), with average molecular weights of 174.2 u and 1046.2 u respectively. Secondary ion yield was measured as a function of number of molecules per area unit, and the detection limit was determined. For both molecular compounds and two different energies of primary 35Cl ion beam, the secondary ion yield exhibited a significant decrease below the area density of 1015 molecules/cm2, while the density of 1013 molecules/cm2 resulted in molecular peak / background ratio being lesser than 3, which is below the commonly used sensitivity threshold in other techniques. Other experiment related drawbacks to sensitivity were also discussed.

Matrix‐enhanced secondary ion mass spectrometry: Effects of aliphatic carboxylic acid matrices on the sensitivity enhancement of biological phospholipids

Matrix‐enhanced secondary ion mass spectrometry (ME‐SIMS) is an effective pre‐treatment method for the sensitivity enhancement of large molecules. Recently, matrix‐assisted laser desorption/ionization (MALDI) matrices, which consist of aromatic acids with benzene rings, have been developed using this technique. However, there are several differences in the desorption and ionization processes of SIMS and MALDI. In this study, the sensitivities of phospholipids mixed with aliphatic carboxylic acids were investigated using Bi‐cluster time‐of‐flight SIMS (TOF‐SIMS). Trans‐aconitic acid (tri‐carboxylic acid) and citric acid (hydroxycarboxylic acid) were used as the matrices. 2,5‐Dihydroxybenzoic acid (DHB), which is a typical aromatic MALDI matrix, was selected as the reference matrix. When trans‐aconitic acid and DHB were used as matrices, the secondary ion yields of [M + H]+ and [2 M + H]+ for phospholipids were approximately 10–150 times higher than that of the pristine lipid. For samples with citric acid, the yield enhancements of [M + H]+ and [2 M + H]+ were approximately 400–1000 times higher than those of pristine lipids. Furthermore, the protonated ionization of citric acid was mostly suppressed because of mixture with phospholipids. These results indicate that the matrix effect between phospholipids and the matrix contributes to the sensitivity enhancement of the target molecules. Hence, aliphatic carboxylic acid was equally or more effective than the MALDI matrix for the sensitivity enhancement of phospholipids in Bi‐cluster TOF‐SIMS.

Review of Recent Advances in Gas-Assisted Focused Ion Beam Time-of-Flight Secondary Ion Mass Spectrometry (FIB-TOF-SIMS).

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a powerful chemical characterization technique allowing for the distribution of all material components (including light and heavy elements and molecules) to be analyzed in 3D with nanoscale resolution. Furthermore, the sample's surface can be probed over a wide analytical area range (usually between 1 µm2 and 104 µm2) providing insights into local variations in sample composition, as well as giving a general overview of the sample's structure. Finally, as long as the sample's surface is flat and conductive, no additional sample preparation is needed prior to TOF-SIMS measurements. Despite many advantages, TOF-SIMS analysis can be challenging, especially in the case of weakly ionizing elements. Furthermore, mass interference, different component polarity of complex samples, and matrix effect are the main drawbacks of this technique. This implies a strong need for developing new methods, which could help improve TOF-SIMS signal quality and facilitate data interpretation. In this review, we primarily focus on gas-assisted TOF-SIMS, which has proven to have potential for overcoming most of the aforementioned difficulties. In particular, the recently proposed use of XeF2 during sample bombardment with a Ga+ primary ion beam exhibits outstanding properties, which can lead to significant positive secondary ion yield enhancement, separation of mass interference, and inversion of secondary ion charge polarity from negative to positive. The implementation of the presented experimental protocols can be easily achieved by upgrading commonly used focused ion beam/scanning electron microscopes (FIB/SEM) with a high vacuum (HV)-compatible TOF-SIMS detector and a commercial gas injection system (GIS), making it an attractive solution for both academic centers and the industrial sectors.

Cationization of organic molecules under keV and MeV primary ion bombardment

The present work reports on cationization characteristics of organic molecules in alkali metal environment. Samples of arginine and polyethylene glycol polymers with average masses of 600, 1000 and 1500 were mixed with sodium- and potassium trifluoroacetate of various concentrations, and analyzed through secondary ion mass spectrometry. Bombardment proceeded with ions within both keV and MeV energy domains. Added salts positively affected the secondary ion yield through attachment of Na or K to organic molecule. Ionization differences between swift (MeV) ions and cluster ions, commonly used for secondary ion mass spectrometry analysis, were studied for arginine and PEG 600. While for arginine, secondary ion yield enhancement was only by a factor of 2, when analysing with MeV ions, and no enhancement was observed with keV Bi3 clusters, all polyethylene glycol samples showed increase of the secondary ion yield by factors between 3 and 40, depending on the amount of potassium or sodium that was mixed into the matrix. Additionally, higher amounts of salts also resulted in decreased fragmentation probability for organic molecules, reducing the intensities of specific fragments by more than two orders of magnitude.

Mechanism and Equivalence of Single Event Effects Induced by 14 MeV Neutrons in High-Speed QDR SRAM

Single-bit upset (SBU) and multiple-cell upset (MCU) features of high-speed QDR-SRAM are revealed under the 14 MeV neutron irradiation. By comparing with the high-altitude real atmosphere test results directly, the equivalence of 14 MeV neutrons for atmospheric neutron-induced single event effect (SEE) evaluation is investigated. It is found that, compared with the 65 nm planar device, the SBU cross-section of 14 nm FinFET SRAM decreases to 1/58 and the proportion of MCU shows little difference, which results from the narrow channel between fin and substrate caused by shallow channel isolation in 14 nm FinFET process, and the charge sharing effect between fins is weakened. The SBU and MCU cross-sections under the 14 MeV neutron irradiation are underestimated by 22.8% and 85.7%, respectively. Besides, the probability and maximum size of MCU are both smaller than those in the real atmosphere. The MCU shape tends to be vertical, resulting from the smaller vertical spacing of sensitive volumes (about 100 nm). Further Monte-Carlo simulation shows that the total yield of secondary ions produced by atmospheric neutrons is higher than that produced by 14 MeV neutrons. Major Components of the “useful” products are p, Si, α, etc., which are the main cause of SBU events. Besides, compared with 14 MeV neutrons, atmospheric neutrons generate more kinds of secondary ions in the SV within the scope from p to W, and the diverse high-Z elements, such as W, Ta, Hf, etc., are the main cause of MCU events. Moreover, the maximum LET of secondary ions can reach 31.5 MeV·cm2/mg. The equivalence of using 14 MeV neutrons for atmospheric neutron-induced SEE evaluation is closely related to the critical charge of the device under test.

Sputtering of GaAs target under Bim+ cluster ions bombardment

The yield of secondary ions and neutral Ga atoms, converted into positive ions on the surface of an Ir emitter was measured during the bombardment of a GaAs target with cluster ions Bim+ (m = 1–5) in the energy range 1–10 keV. A non-additive increase in the yield of secondary ions was observed with an increase in the number of atoms in the bombarding ions. An estimated calculation of the ionization probability of gallium during sputtering has been carried out. A decrease in the ionization probability with an increase in the number of atoms in the bombarding cluster ions was found.

Depth profiling of Cr-ITO dual-layer sample with secondary ion mass spectrometry using MeV ions in the low energy region

This work explores the possibility of depth profiling of inorganic materials with Megaelectron Volt Secondary Ion Mass Spectrometry using low energy primary ions (LE MeV SIMS), specifically 555 keV Cu2+, while etching the surface with 1 keV Ar+ ions. This is demonstrated on a dual-layer sample consisting of 50 nm Cr layer deposited on 150 nm In2O5Sn (ITO) glass. These materials proved to have sufficient secondary ion yield in previous studies using copper ions with energies of several hundred keV. LE MeV SIMS and keV SIMS depth profiles of Cr-ITO dual-layer are compared and corroborated by atomic force microscopy (AFM) and time-of-flight elastic recoil detection analysis (TOF-ERDA). The results show the potential of LE MeV SIMS depth profiling of inorganic multilayer systems in accelerator facilities equipped with MeV SIMS setup and a fairly simple sputtering source.

NanoSIMS Analysis of Rare Earth Elements in Silicate Glass and Zircon: Implications for Partition Coefficients

We have developed a method to analyze all rare earth elements in silicate glasses and zircon minerals using a high lateral resolution secondary ion mass spectrometer (NanoSIMS). A 2nA O− primary beam was used to sputter a 7–8-μm diameter crater on the sample surface, and secondary positive ions were extracted for mass analysis using an accelerating voltage of 8 kV. A high mass resolving power of 9,400 at 10% peak height was attained to separate heavy REE from oxide of light REE. A multi-collector system combined with peak-jumping by magnetic field was adjusted to detect REEs and silicon-30 for calibration. Based on results of NIST SRM610 glass, sensitivities of REEs vary from 3 cps/ppm/nA of Lu to 13 cps/ppm/nA of Eu. Reproducibility of REE/Si ratios is better than 18% at 2σ. Secondary ion yields of REEs show positive relationships with their ionization potential of second valence. REEs of AS3, QGNG, and Torihama zircons were measured and calibrated against those of 91500 standard zircon. SIYs of REEs of zircon are identical to those of the glass standard. AS3 and QGNG data are generally consistent with those of previous work. Torihama REE data combined with the whole rock data provide partition coefficients of REEs between silicate melt and zircon. The relationship between these coefficients and ionic radius is explained by an elastic moduli model.

Sensitivity enhancement using chemically reactive gas cluster ion beams in secondary ion mass spectrometry (SIMS)

We report for the first time on significant molecular secondary ion yield increases by modifying the chemistry of a water cluster primary ion beam. This was demonstrated using 70‐keV ion beams of 0.15 eV/amu. For the neutral drug Bezafibrate, secondary ion yield enhancements ×5–10 were observed when replacing the Ar carrier gas in a water gas cluster ion beam (GCIB) source with a mixture containing 12% CO2 and 2% O2 in Ar. For the cationic drug Ranitidine, the ion yield enhancements using the CO2‐containing carrier gas were up to ×20–50 in positive mode and ×2–4 in negative mode. The extent of molecular fragmentation was very similar from both cluster beams. We conclude that additional chemically reactive species are present in the impact zone using the (H2O/CO2)n projectile, which promote the formation of secondary ions of both polarity through projectile impact‐induced chemical reactions. This methodology can be applied to further extend the capabilities of high‐resolution 3‐dimensional mass spectral imaging using reactive GCIB‐SIMS.

Sputtering, Cluster Primary Ions and Static SIMS

Sputtering using cluster primary ion beams is very important for the future development of static SIMS and the SIMS depth profiling of organic layers. However, different results from different laboratories may be confusing. Analytical models have an important function for enabling the prediction of behaviour for practical analysis. Sigmund's model for sputtering, often used in surface analysis, is helpful and accurate in the linear cascade regime. However, for cluster sputtering this is no longer the case and spike effects need evaluation. Evidence will be presented of the spike model validity for clusters of up to more than 10 atoms over 3 orders of magnitude in sputtering yield. Using data from one primary ion, extremely good descriptions of measurements reported with other primary ions can then be achieved. This theory is then used to evaluate the molecular ion yield behaviour of interest in the static SIMS of organics. This leads to universal dependencies for the de-protonated molecular ion yields, relating all primary ions, both single atom and cluster, which are illustrated by experimental data over 5 decades of emission intensity. This formulation permits the prediction of the (M-H)- secondary ion yield for different, or new, primary ion sources. It is shown how further gains are predicted. For analysing materials, raising the molecular secondary ion yield is extremely helpful but it is the ratio of this yield to the disappearance cross-section (the efficiency) that is critical. The relation of the damage and disappearance cross sections is formulated. Data are evaluated and a description is given to show how these cross sections are related and to provide a further universal relation for the efficiency/yield dependence of all cluster ions.

Probing Surface Information of Alloy by Time of Flight-Secondary Ion Mass Spectrometer

In recent years, time of flight-secondary ion mass spectrometer (ToF-SIMS) has been widely employed to acquire surface information of materials. Here, we investigated the alloy surface by combining the mass spectra and 2D mapping images of ToF-SIMS. We found by surprise that these two results seem to be inconsistent with each other. Therefore, other surface characteristic tools such as SEM-EDS were further used to provide additional supports. The results indicated that such differences may originate from the variance of secondary ion yields, which might be affected by crystal orientation.

Molecular speciation analysis of oxidized metal surfaces by TOF SIMS

Molecular speciation by Time of Flight Secondary Ion Mass Spectrometry (TOF SIMS) was applied to oxidized bismuth, lead, tin, and tellurium. In their pure elemental form, they are basic constituent elements of both topological insulators and topological crystalline insulators, which are very hot topics in the last decade.The range of specific ions masses used to evaluate the valence of investigated oxides has been significantly expanded towards high-mass clusters MexOy (2 ≤ x ≤ 5). The significant enhancement in the secondary high-mass ion yield by using the Bi cluster ion source reflects the valence of primary ion clusters used in molecular speciation of inorganic species. The results show that chemical composition of clusters differs in three 2, 3, and 4 oxidation states studied. This implies that each oxidation state possesses its own set of the high-mass clusters extracted from oxidized metal surfaces during measurement.Such an approach allows for the first time to perform a semi-quantitatively molecular speciation of oxides exploiting the empirical Plog's model. It describes the secondary ion yield as a function of the metal valency in the oxide. We found that the estimated lattice valence state G0 is close to the valence state of the metal in stable oxides.

Chemoheteroepitaxy of 3C‐SiC(111) on Si(111): Influence of Predeposited Ge on Structure and Composition

Secondary ion mass spectroscopy, Fourier transformed infrared spectroscopy, ellipsometry, reflection high energy diffraction and transmission electron microscopy are used to gain inside into the effect of Ge on the formation of ultrathin 3C‐SiC layers on Si(111) substrates. Accompanying the experimental investigations with simulations it is found that the ultrathin single crystalline 3C‐SiC layer is formed on top of a gradient Si1–x–yGexCy buffer layer due to a complex alloying and alloy decomposition processes promoted by carbon and germanium interdiffusion and SiC nucleation. This approach allows tuning residual stress at very early growth stages as well as the interface properties of the 3C‐SiC/Si heterostructure. Useful yields of secondary ions of Ge in Si matrix and Si dimer are estimated.