Argon Gas Cluster Ion Beam Research Articles

Enhancing Ion Signals and Improving Matrix Selection in Time-of-Flight Secondary Ion Mass Spectrometry with Microvolume Expansion Using Large Argon Clusters.

The molecular environment has an important impact on the ionization mechanism in time-of-flight secondary ion mass spectrometry (ToF-SIMS). In complex samples, desorption/ionization, and thus the detection of a molecular signal, can be hampered by molecular entanglement, ionization-suppressive neighbors, or even an unfavorable sample substrate. Here, a method called microvolume expansion is developed to overcome these negative effects. Large argon clusters are able to transfer biomolecules from a target to a collector in vacuum. In this study, argon gas cluster ion beams (Arn+-GCIB with n centered around 3000 or 5000) are used to expand a microvolume from the sample to a collector, which is a material ideally enhancing the ionization yield. The collector is then analyzed using a liquid metal ion gun. The signal amplification factor corresponding to the expansion of phosphatidylcholine (PC) lipid on collectors partially covered with acidic matrices was evaluated as an initial proof of concept. In one experiment, the PC expansion on a pattern of four drop-casted matrix-assisted laser desorption/ionization matrices led to the selection of α-cyano-4-hydroxycinnamic (CHCA) as the optimal candidate for cationic PC detection. The ion signal is increased by at least three orders of magnitude when PC was expanded using 10 keV Ar3000+ and Ar5000+ on a sublimated layer of CHCA. Finally, the expansion of the gray matter of a mouse on different materials (Si, Au-coated Si, CHCA, and polyethylene) was achieved with varying degrees of success, demonstrating the potential of the method to further analyze complex and fragile biological assemblies.

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.

XPS investigation of monoatomic and cluster argon sputtering of zirconium dioxide

The surfaces of zirconium dioxide and yttria-stabilized zirconia (YSZ) have been analyzed using x-ray photoelectron spectroscopy after ion sputtering with monoatomic Ar+ or an argon gas cluster ion beam (GCIB). The O/Z ratio and new components in the Zr 3d lines show reduction to lower oxidation states when sputtered with monoatomic Ar+, but significantly less damage is observed when GCIB sputtering is used. The damaged surface layer caused by Ar+ sputtering can be removed by subsequent GCIB sputtering. However, the depth resolution observed in depth profiles of thin YSZ films was significantly better when Ar+ sputtering is used. Differences in the Sn content in the oxidized Zr-4 specimen were also observed when comparing Ar+ and GCIB sputtering, suggesting preferential sputtering.

Gas Cluster Ion Beams as a Versatile Soft-Landing Tool for the Controlled Construction of Thin (Bio)Films

Surface biofunctionalization with proteins is the key to many biomedical applications. In this study, a solvent-free method for the controlled construction of protein thin films is reported. Using large argon gas cluster ion beams, proteins are sputtered from a target (a pool of pure proteins), and collected on a chosen substrate, being nearly any solid material. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) revealed the presence of intact protein molecules on the collectors. Furthermore, lowering the energy per atom in the cluster projectiles down to 1 eV/atom allowed more than 60% of bradykinin molecules to be transferred intact. This protein deposition method offers a precise control of the film thickness as the transferred protein quantity is proportional to the argon clusters ion dose reached for the transfer. This major feature enables building protein films from (sub)mono- to multilayers, without upper limitation of the thickness. A procedure was developed to measure the film thickness in situ the ToF-SIMS instrument. The versatility and potential of this soft-landing alternative for further applications is demonstrated on the one hand by building a protein thin film at the surface of paper, a substrate hardly compatible with solution-based adsorption methods. On the other hand, the possibility to achieve alternated multilayer buildup is demonstrated with the construction of a bilayer composed of bradykinin and Irganox, with the two layers well separated. These results lay the first stone toward original and complex multilayers that could previously not be considered with solution-based adsorption methods, and this regardless of the substrate nature.

In-depth analysis of iodine in artificial biofilm model layers by variable excitation energy XPS and argon gas cluster ion sputtering XPS.

Here, we present a study on agarose thin-film samples that represent a model system for the exopolysaccharide matrix of biofilms. Povidone-iodide (PVP-I) was selected as an antibacterial agent to evaluate our x-ray photoelectron spectroscopy (XPS)-based methodology to trace specific marker elements, here iodine, commonly found in organic matrices of antibiotics. The in-depth distribution of iodine was determined by XPS analyses with variable excitation energies and in combination with argon gas cluster ion beam sputter cycles. On mixed agarose/PVP-I nanometer-thin films, both methods were found to solve the analytical task and deliver independently comparable results. In the mixed agarose/PVP-I thin film, we found the outermost surface layer depleted in iodine, whereas the iodine is homogeneously distributed in the depth region between this outermost surface layer and the interface between the thin film and the substrate. Depletion of iodine from the uppermost surface in the thin-film samples is assumed to be caused by ultrahigh vacuum exposure resulting in a loss of molecular iodine (I2) as reported earlier for other iodine-doped polymers.

Microscopic Structure and Binding Mechanism of the Corrosion-Protective Film of Oleylpropanediamine on Copper in Hot Water

The structure of the film formed by oleylpropanediamine (OLDA) on the copper (Cu) metal surface in water at 150 °C was investigated by combining quantitative NMR and surface characterization methods. We succeeded in quantifying the amount of film formation by precisely determining all mass balances in the systems examined. 2D IR microscopic mapping showed that the film thickness is uneven in the horizontal direction with a length scale of ∼100 μm and hundreds of OLDA layers. This film thickness was also confirmed by AFM. The analysis of the C–H stretching vibrational frequency disclosed that the alkyl chains are highly ordered in the layers close to the Cu surface and are conformationally disordered in the layers distant from the Cu surface in the thicker portion of the film. Combining XPS measurements using argon gas cluster ion beam etching with the ICP-AES analysis, we revealed that the key to multiple layering is the formation of a coordination complex of the unprotonated amino groups of OLDA with Cu that presumably results in polymer chain-like network structures. Contact angle measurements at different OLDA concentrations and treatment times showed that the water repellency of the film originated from the thick layering of OLDA molecules with disordered hydrophobic chains.

Ion distribution in iron oxide, zinc and manganese ferrite nanoparticles studied by XPS combined with argon gas cluster ion beam sputtering

In this work, the influence of X-ray irradiation and two types of surface sputtering methods is thoroughly studied. The first one, monoatomic sputtering, resulted in the reduction of iron oxide to the metallic iron in a short time of sputtering. The second studied technique is argon cluster sputtering, which uses lower energies per atom, hence the modification of the structure of the samples is greatly reduced. Based on the XPS characterization of Fe3O4 single crystal the fitting procedure for the Fe 2p spectra for nanoparticles is established. The preliminary results showed that X-rays combined with cluster sputtering resulted in alteration of the morphology and solidification of the surface of the samples. In the response to the morphology modification, the Spot-by-Spot Cluster Procedure is proposed. It allows to thoroughly characterize the layer-by-layer structure of the nanoparticles, without changing it. The Procedure was used to determine the structure of zinc, manganese and zinc-manganese ferrite nanoparticles. Finally, the results show, that depending on the used dopant, its incorporation and the state of iron are changed.

Effects of Cs+ and Arn+ ion bombardment on the damage of graphite crystals

Intercalation mechanisms and diffusion or segregation phenomena in graphitic materials play a crucial role in different applied science fields. The investigation of such phenomena is usually accomplished through depth profiling experiments. Ar-GCIBs (Argon- Gas Cluster Ion Beams) are commonly adopted for in-depth concentration profiling of organic or soft materials; on the other hand, cesium ions are in general more suitable for the sputtering of inorganics. During such experiments, the beam-target interaction could alter chemistry and structure of the material. In this work, we define the optimal conditions in terms of both sputtering ion source and energy to preserve the crystal features. HOPG was used as a model system to compare morphological, physical, and chemical effects induced by different Arn+ clusters, and ultra-low energy Cs+ beam during ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometry) depth profiling experiments. We demonstrated, through in-situ AFM (Atomic Force Microscopy) analysis, that the monoatomic Cs+ beam alters to a lower extent the HOPG structure. On the contrary, Ar-GCIBs strongly modify the graphite surface basal plane and underlying layers. However, HOPG crystals treated with the cesium monoatomic source undergo a chemistry modification leading to the formation of graphite oxide (GOx) together with the presence of hydrogen, and cesium adducts.

Neutral gas effect on the surface potential and charge compensation of an insulating sample

Surface charging of an insulating sample has been a significant issue in surface analysis. In time-of-flight secondary ion mass spectrometry, the charge also accumulates on the surface of an insulating sample by the primary ion beam, such as an argon gas cluster ion beam. Such charging can lead to not only degraded mass resolution but also low image resolution, and thus it is necessary to compensate for surface charging of insulating sample. Therefore, several efficient methods have been proposed for charge compensation, such as using a low-energy electron flood gun, applying a bias voltage to the sample stage, and using a gas flooding system. Here, we measured the surface potential of a polymer film to investigate the gas effect on the charge compensation using a Kelvin probe. During the measurements, the chamber pressure was gradually increased by injecting gases, such as He, Ne, Ar, N2, and O2. We found that the injection gas may reduce the surface charging of the sample, depending on the type of gas.

Internal Diesel Injector Deposit Chemical Speciation and Quantification Using 3D OrbiSIMS and XPS Depth Profiling

<div class="section abstract"><div class="htmlview paragraph">The impact of internal diesel injector deposits (IDIDs) on engine performance, efficiency and emissions remains a major concern in the automotive industry. This has been compounded in recent years by fuel injection equipment developments and changes to diesel fuel towards ultra-low sulfur diesel (ULSD) and biodiesel as well as the introduction of new fuels such as hydrotreated vegetable oil (HVO). Prevention and mitigation of such deposit formation requires an understanding of the formation process, which demands a chemical explanation. The chemistry of these deposits therefore remains a key research interest to the industry using the latest analytical methodologies to inform and build further on previous investigations. In this work, 3D OrbiSIMS analysis using a time of flight-secondary ion mass spectrometer (ToF-SIMS) with hybrid Orbitrap<sup>TM</sup> functionality has been employed for chemical speciation and depth profiling of IDIDs in-situ on two injector needle samples from field failures in China and Eastern USA. The instrument’s soft gas cluster ion beam (GCIB) and high mass resolution enables unequivocal identification of chemistries from several classes of compounds. Here, chemistries identified include alkylbenzene sulfonates, zinc oxides, sodium and calcium salts, carboxylic acids, carbonaceous/polyaromatic hydrocarbons and metal substrate related material. With depth profiling, the distributions of these materials are traced throughout the deposit thickness. Given the semi-quantitative nature of SIMS data, X-ray photoelectron spectroscopy (XPS) depth profiling using an argon GCIB has also been employed to provide parallel data of atomic concentration through the deposit thickness. This data is complementary to SIMS as it places the chemical information in a quantitative context, most notably showing that despite the extreme intensities of salt material in the SIMS data, carbon is the main element throughout both IDIDs. Together the techniques show in general the more functionalized chemistries are at the deposit surface, with underlying salt layers and innermost layers of more amorphous organic material. These techniques represent a new method of comprehensive IDID characterization that affords diagnostic chemical information alongside elemental quantification, thus complementing previous studies.</div></div>

Ion yield enhancement at the organic/inorganic interface in SIMS analysis using Ar-GCIB

Argon gas cluster ion beams (Ar-GCIBs) provide new opportunities for molecular depth profiling and imaging of organic materials and biological samples, thanks to recent technological developments that have led to the construction of new SIMS spectrometers where the traditional issues related to their low lateral resolution along with poor mass resolution and mass accuracy are overcome. The present fundamental contribution on SIMS molecular depth profiling investigates the variations of the secondary ion signals observed at the organic–inorganic hybrid interface when using Ar-GCIB as the analytical beam. With this in mind, depth profiling experiments were performed with a ToF-SIMS spectrometer using different analysis beams: 30-keV Bi5+ versus 10-keV Arn+ with a cluster size (n) of 800, 1500, 3000 and 5000 atoms, respectively. A 10-keV Ar3000+ beam was used for sputtering in all the experiments. Irganox 1010 and model polymers such as polystyrene (PS) and poly (methyl methacrylate) (PMMA) oligomers were chosen. Silicon wafer and a polymer-based substrates were employed to test materials with different stiffness, which is directly related to their Young’s moduli, an important parameter in this study. Ar-GCIB depth profiles systematically show ion signal enhancement of the characteristic fragments of PS and Irganox 1010 when approaching the interface with the silicon substrate, that can reach up to 60% for [M1010-H]− in Irganox films deposited onto silicon wafers. This enhancement increases with increasing n in both ion polarities. These results point out some ionization effects on the observed signal enhancement at the interface. The experimental observations will be explained on the basis of the physics of the impact of large argon clusters on different target materials and the energy confinement of the ion projectile in the organic overlayers. Finally, the thickness of organic films on rigid substrates in the nanoscale appears to be a crucial parameter to dramatically improve the sensitivity to molecular fragments when using large Ar cluster ions as analysis beams.

Surface topography and composition of NiPd alloys under oblique and normal gas cluster ion beam irradiation

Polycrystalline NiPd and Ni5Pd alloys were irradiated with argon gas cluster ion beam and atomic ion beam. In situ XPS measurements showed surface enrichment with Ni. For cluster ions, the degree of the enrichment was significantly higher, and the ion current density influenced on it. Under oblique cluster ion beam, ripples developed on the surface, and the elements of the alloy redistributed along the surface according to the ripple pattern. For normal cluster ion beam direction, sputtering rate was determined by a crystalline orientation, which limited the smoothing effect.

Argon gas cluster ion beam used to identify intact proteins through enzymatic digestion

Using the beam with time-of-flight secondary ion mass spectrometry, researchers improved the signal-to-noise ratio and showed on-surface detection was identical to in-solution detection.

Direct characterization of vertical molecular distributions of organic bulk heterojunction structure by photoemission spectroscopy combined with argon gas cluster ion beam sputtering

The characterization of the vertical molecular distribution of organic bulk heterojunction (BHJ) structures is crucial to the development of high-performance organic photovoltaic (OPV) devices. Herein, we report a novel and direct method for the characterization of the vertical composition gradient of a BHJ structure. Ar gas cluster ion beam (GCIB) sputtering provided a uniform sputtering yield that preserved the chemical structure of the organic semiconducting materials. The combination of X-ray photoelectron spectroscopy (XPS) and Ar GCIB sputtering facilitated the accurate analysis of the vertical molecular distribution of a regioregular poly(3-​hexylthiophene) (P3HT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) BHJ structure. The J-V characteristics and depth profiles of the as-deposited, ethanol-annealed, and chloroform-annealed devices were examined to demonstrate the usefulness of Ar GCIB sputtering in optimizing the BHJ morphology. Chloroform-annealed device exhibited both the best performance and the most homogeneous morphology among the three prepared samples, and the thickness of the P3HT-rich region was substantially reduced from 20 to 10 nm. Consequently, these results provide important information about the correlation between the vertical molecular distribution of a BHJ blend structure and power conversion efficiency of an OPV device.

Time-of-flight secondary ion mass spectrometry analysis of hair samples using unsupervised artificial neural network.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is extensively employed for the structural analysis of the outermost surfaces of organic materials, including biological materials, because it provides detailed compositional information and enables high-spatial-resolution chemical mapping. In this study, a combination of TOF-SIMS and data analysis was employed to evaluate biological materials composed of numerous proteins, including unknown ones. To interpret complicated TOF-SIMS data of human hair, an autoencoder, a dimensionality reduction method based on artificial neural networks, was applied. Autoencoders can be used to perform nonlinear analysis; therefore, they are more suitable than principal component analysis (PCA) for analyzing TOF-SIMS data, which are influenced by the matrix effect. As a model sample data, the TOF-SIMS depth profile of human hair, acquired via argon gas cluster ion beam sputtering and Bi3 2+ primary ion beam, was employed. Useful information, including the characteristic distributions of amino acids and permeated surfactants on the outermost surface of the hair, was extracted from the results obtained from the autoencoder. Furthermore, the autoencoder extracted more detailed features than did PCA. Therefore, autoencoders can become a powerful tool for TOF-SIMS data analysis.

ToF‐SIMS depth profiling of oral drug delivery films for 3D visualization of active pharmaceutical particles

The size, shape, and spatial distribution of active pharmaceutical ingredient (API) are important physical characteristics of drug delivery systems that can affect the performance, stability, appearance, and even bulk properties of the end product. This study explores the feasibility of using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) for the 3D characterization of API particles in two commercially available oral dissolvable drug delivery films. It was found that ToF‐SIMS imaging with argon gas cluster ion beam (GCIB) sputtering allowed production of 3D chemical maps that could be utilized to obtain size distributions of buprenorphine particles whose effective diameters ranged from approximately 6 μm to 41 μm, with shapes that were generally spherical with a few nonspherical structures. The particles were heterogeneously distributed both laterally and as a function of depth in the film. In addition, ToF‐SIMS was able to differentiate between different oral drug delivery films based on differences in the spatial distribution of buprenorphine; in one case, the particles were distributed throughout the depth of the film, whereas the particles in the other case were localized close to the surface. Preliminary studies suggest that ToF‐SIMS with argon GCIB sputtering may also allow us to provide a very rough estimate of the concentration of the APIs (factors of 2 to 4), namely buprenorphine and naloxone, at pharmacologically relevant concentrations inside organic drug delivery systems with a thickness of hundreds of micrometers.

In-Depth Spectroscopy and New Heights for Organic Solar Cells

In this issue of Joule, Lami et al. describe a method that enables UV photoemission spectroscopy (UPS) in the transverse dimension of polymeric semiconductor layers with nanometer-scale resolution. The approach is based on the use of Argon gas cluster ion beam (GCIB) etching instead of monoatomic ion bean bombardment. The use of GCIB reduces surface damage, enabling in depth UPS. The method is applied to the study of critical electronic levels and photovoltage in organic solar cells.

Development of an Argon Gas Cluster Ion Beam for ToF‐SIMS Analysis

Argon gas cluster ion beams (Ar‐GCIBs) are widely used as a sputter and primary beam for analyzing soft materials such as organic and biological samples in secondary ion mass spectrometry. We designed a novel ionization source using Simion simulation to improve the beam generation efficiency, and compared with the experimental results. Simulation results of different operation voltages, which are acceleration and extraction from the ionization source were in good agreement with the experimental results. At Ar pressure of 25 bar, total ion beam currents from the ionization source reached 80 μA. The highest current of cluster in the ion beam was found at cluster sizes of 1400. Simple linear time‐of‐flight mass spectrometer was constructed and then a secondary ion mass spectrum signal of Rhodamine 6G was obtained by using a Ar1400+ (@10 keV) cluster ion beam.

Cluster ion beam bombardment and Q-ToF-SIMS analysis of large biomolecules

Cluster ion beam effect on large biomolecules is interesting due to differences in ion beam characters from normal ion beams and applications in analysis of the molecules as the cluster ion beam should be more effective in fragmenting large and complicated biomolecules to make study of the complex molecular structure possible. Argon gas cluster ion beam (Ar-GCIB) at 10 keV bombarded samples of DSPC (1,2-distearoyl-sn-glycero-3-phosphocholine, C44H88NO8P, monoisotopic molecular weight 789.6 Da), angiotensin II (C50H71N13O12, monoisotopic molecular weight 1045.5 Da) and leucine enkephalin (C28H37N5O7, monoisotopic molecular weight 555.6 Da). The bombardment-generated fragments were in-situ analyzed and measured using the tandem quadrupole – time-of-flight secondary ion mass spectrometry (Q-ToF-SIMS) technique. The experiment was operated within an integrated system of Ar-GCIB and Q-ToF-SIMS. Two modes, i.e. MS (quadruple off) and MS/MS (quadrupole on), were applied in the ToF-SIMS measurement. Particularly, the secondary ion fragmentation with the collision energy dependence was focused in the study.

Sputter Depth‐Profile Study of Accelerated Interface Mixing by Thermal Annealing in Solution‐Processed Organic Light‐Emitting Diodes

AbstractThe effect of thermal annealing on interfacial mixing of solution‐processed organic light‐emitting diodes (OLEDs) using direct sputter‐depth profiling techniques is investigated. X‐ray and ultraviolet photoelectron spectroscopy and argon gas cluster ion beam sputtering are used to investigate the distribution of chemical species near the interface. Extensive interfacial mixing is found in solution‐processed OLEDs after the thermal annealing at temperatures below the glass transition temperature of the organic material, while such mixing is not evident in vapor‐deposited devices. It is found that there is a partially mixed interface in the solution‐processed devices prior to the annealing, which seems to promote effective interdiffusion during the annealing. Surprisingly, the extensive interfacial mixing after the thermal annealing appears to significantly enhance the efficiency of OLEDs.