Secondary Neutral Mass Spectrometry Research Articles

In Situ Helium Isotope Microimaging of Meteorites.

Isotope imaging is commonly used to investigate the localization of trace elements and their isotopes. In situ noble gas analysis of meteorites revealed the distribution of primordial noble gases that were trapped in the building blocks of asteroids and planets during the early stage of the solar system evolution. Solar wind noble gases are among the primordial gases present in meteorites and were trapped through exposure to solar wind. Micrometer-resolution in situ noble gas analysis has not been achieved due to the lack of sensitivity and spatial resolution. The microscale imaging technique is crucial for identifying the carrier phase of the solar wind noble gases. We have developed 4He isotope imaging utilizing secondary neutral mass spectrometry with strong field postionization. This technique achieved a lateral resolution of 2 μm and a 4He detection limit of 2 × 1017 cm-3. This development allows for the study of a solar wind gas-rich meteorite, Northwest Africa 801 carbonaceous chondrite, with micrometer resolution. The solar wind 4He carriers are fine-grained particles and are sparsely scattered in the matrix region.

Alkali-metal-based molecular-ion secondary ion mass spectrometry for precise quantitative analysis of low-dimensional materials without standards

Advancement in the understanding of fundamental aspects of Secondary Ion Mass Spectrometry (SIMS) has made this technique to be extremely powerful for quantitative materials analysis. The secondary ion emission process is a complex phenomenon involving various mechanisms to understand the mechanism. Among the existing mechanisms, the “electron-tunneling model” is the most accepted concept in the understanding of ionization probability for positive and negative ions. This model essentially concerns the survival probability of an ion emitted from the surface. As the intensity of secondary ions of a certain element depends strongly on the efficiency of ionization of a sputtered atom or molecule, the “local surface chemistry” plays a significant role in the secondary ion emission. This is purported as the “Matrix Effect” in SIMS, making the technique extremely challenging for quantification of materials in spite of the fact that it has the highest detection sensitivity (<ppb) and exceptional depth-resolution (<1[Formula: see text]nm). Therefore, it is required to correct the “matrix effect” for quantitative analysis. If the probing element (M) in its close proximity sees an alkali element (such as Li, Rb, K, Na and Cs, referred to as “A”) on the sample surface, the sputtered neutral atom (M0) of the probing element can attach an alkali-ion forming a quasi-molecular (MA)[Formula: see text] ion. Such a phenomenon can also occur if an alkali ion beam is chosen as the primary beam for sputtering. The MA[Formula: see text] molecular ion formation is directly related to the atomic polarizability of the element M. Since the sputtering of neutral atom (M0) is unconnected with the formation of MA[Formula: see text] ion, the “matrix effect” decreases dramatically in the analysis using MA[Formula: see text] ions. The phenomenon is very similar to the mechanism of “secondary neutral mass spectrometry” (SNMS). Although these (MA)[Formula: see text] molecular ions are potentially applicable in the direct quantification of materials without the need for “calibration standards”, it generally has a weak intensity. In such a case, the (MA)[Formula: see text] molecular ions are preferred in the analysis because of their higher detection sensitivity (by a few orders of magnitude in some cases), compared to that of (MA)[Formula: see text] molecular ions. Monitoring of the molecular ions is routinely preferred in conventional SIMS experiments to discriminate the sputtered ion species which are affected by mass interference or have poor dynamic ranges. For example, while making the SIMS analysis of GaAs, carbon as an impurity element is detected by monitoring (AsC)[Formula: see text] molecular ions instead of C− ions, because the C− ions have high background arising from the carbon-containing residual-gas species in the analysis chamber. Caesium is decidedly chosen for MCs[Formula: see text] or MCs[Formula: see text] molecular ions in SIMS because of the strongest reactivity and electropositive nature of caesium. This paper deals with a comprehensive discussion on the ion-emission phenomena in sputtering, the “matrix effect” and its compensation, and the potential application of the “MCs[Formula: see text]-SIMS” ([Formula: see text]) in the effective quantitative chemical analysis of materials. A special prominence has been given to the quantification of low-dimensional materials, superlattices and quantum-confined structures using this state-of-the-art “MCs[Formula: see text]-SIMS” approach in all complexities.

Ar cluster bombardment of phenylalanine deposited on graphene-Effect of kinetic energy and projectile size.

This study investigates the effects of kinetic energy and projectile size on particle emission from a phenylalanine overlayer adsorbed on a graphene bilayer during argon projectile bombardment. Projectiles of various sizes (1-1000 atoms) and kinetic energies (0.5-40keV) are used to study their impact on the ejection efficiency, mass distribution, and kinetic and angular spectra of the emitted substances. Particle ejection mechanisms are discussed in scenarios with and without sample perforation. Additionally, this study investigated the potential use of graphene in secondary ion mass spectrometry or neutral secondary mass spectrometry analyses.

Effect of the kinetic energy on particle ejection process from carbon nanotubes bombarded by kilo-electron-volt C60

Molecular dynamics computer simulations are employed to investigate processes leading to particle ejection from single-wall carbon nanotubes bombarded by keV C60 projectiles. The effect of the primary kinetic energy, the incidence angle, and the nanotube diameter on the ejection process is studied. Armchair nanotubes with diameters of 3.26, 5.4, and 8.2 nm are tested. C60 projectiles bombard these targets with kinetic energy between 3 and 50 keV and the angle of incidence ranging between 0° and 75°. The particle ejection yield is a result of the interplay between the amount of kinetic energy available for breaking interatomic bonds, the size of the bombarded area, and the size and form of projectiles hitting this area. Much of the initial kinetic energy is dissipated in the nanotubes as waves, especially for low-energy impacts. Computer simulations are used to find the optimal conditions leading to the gentle ejection of unfragmented organic molecules adsorbed on nanotube substrates. This knowledge may be helpful in the potential application of nanotube substrates in secondary ion mass spectrometry or secondary neutral mass spectrometry.

Atomic transport in amorphous Mo-Cu and Ta-Cu immiscible systems

Metastable binary systems with positive enthalpy of mixing have practically no diffusional data in literature. Experimental data on Cu diffusion in amorphous immiscible systems such as Mo-Cu and Ta-Cu are presented in this study. These studies were conducted in sputtered Ni/Mo1−xCux/Ni and Ni/Ta1−xCux/Ni tri-layers, where the nanocrystalline Ni layers acted as effective sinks for Cu atoms located in the amorphous layers. The secondary neutral mass spectrometry (SNMS) technique allowed us to observe the out-diffusion of Cu into the Ni sink layer at temperatures between 100 °C and 250 °C. Additionally, high-resolution transmission electron microscopy (HR-TEM) and energy dispersive spectroscopy (EDX) was used to characterize Mo-Cu and Ta-Cu layers. As a result of a coupled diffusion equation being solved for both amorphous (source) and polycrystalline (sink) layers, we were able to construct best-fit concentration profiles and calculate Cu diffusion parameters for both layers. We assumed bulk diffusion in amorphous films, while transport along moving grain boundaries was assumed in polycrystalline Ni layers. With this model, we were able to replicate the observed profile features at the amorphous/polycrystalline interface. Based on these evaluations, the pre-exponential factors and activation energies are 5 × 10−14 m2/s, 55.3 ± 12 kJ/mol in Mo-Cu and 1.2 × 10−12 m2/s, 76 ± 19 kJ/mol in Ta-Cu systems.

Multi-element isotopic analysis of hot particles from Chornobyl

Microscopic fuel fragments, so-called “hot particles”, were released during the 1986 accident at the Chornobyl nuclear powerplant and continue to contaminate the exclusion zone in northern Ukraine. Isotopic analysis can provide vital information about sample origin, history and contamination of the environment, though it has been underutilized due to the destructive nature of most mass spectrometric techniques, and inability to remove isobaric interference. Recent developments have diversified the range of elements that can be investigated through resonance ionization mass spectrometry (RIMS), notably in the fission products. The purpose of this study is to demonstrate the application of multi-element analysis on hot particles as relates to their burnup, particle formation in the accident, and weathering. The particles were analysed with two RIMS instruments: resonant-laser secondary neutral mass spectrometry (rL-SNMS) at the Institute for Radiation Protection and Radioecology (IRS) in Hannover, Germany, and laser ionization of neutrals (LION) at Lawrence Livermore National Laboratory (LLNL) in Livermore, USA. Comparable results across instruments show a range of burnup dependent isotope ratios for U and Pu and Cs, characteristic of RBMK-type reactors. Results for Rb, Ba and Sr show the influence of the environment, retention of Cs in the particles and time passed since fuel discharge.

Corrosion of Passive Aluminum Anodes in a Chloroaluminate Deep Eutectic Solvent for Secondary Batteries: The Bad, the Good, and the Ugly.

The passivity of aluminum is detrimental to its performance as an anode in batteries. Soaking of native oxide-covered aluminum in a chloroaluminate deep eutectic solvent gradually activates the electrode surface, which is reflected in a continuously decreasing open circuit potential. The underlying processes were studied by analyzing the 3 to 7 nm thick layer of native oxide after increasing periods of soaking with secondary neutral mass spectrometry, X-ray photoelectron spectroscopy, and energy-dispersive spectroscopy in a transmission electron microscope. They consistently show permeation of electrolyte species into the layer associated with gradual swelling. After extended periods of soaking at open circuit potentials, local deposits of a range of foreign metals have been found in scanning electron microscopy images of the electrode surface. The pitting corrosion is caused by trace metal ion impurities present in the electrolyte and results in highly nonuniform current density distribution during discharge/charge cycling of battery cells as shown by local deposits of aluminum. The processes during soaking at open circuit potentials have been monitored by electrochemical impedance spectroscopy and could be analyzed by fitting an equivalent circuit model for pitting corrosion.

Production and characterization of standard particles for rL-SNMS

In this work, uranium-and plutonium-baring particles were produced by fast iron co-precipitation for the purpose of creating homogeneous multi-element standards. A set of single isolated particles showing no inhomogeneities in the element distribution were selected. These particles were used to determine the maximal achievable suppression ratios for uranium in Resonant Laser Secondary Neutral Mass Spectrometry (rL-SNMS) measurements of plutonium. It was shown for the first time directly that suppression-ratios in the order of three magnitudes are achievable with a resonant two-step excitation scheme for non-destructive measurements.

Grain boundary diffusion of Si in polycrystalline copper film

Grain boundary (GB) diffusion of Si in polycrystalline Cu film was studied in the temperature range of 403–453 K, in the C-type kinetic regime. The amorphous Si layer (80 nm) and polycrystalline Cu layer (40 nm) were successively deposited by magnetron sputtering at room temperature onto a Si (111) wafer. Appearance of Si atoms on the copper surface due to GB diffusion through the copper layer was detected by low energy ion scattering spectroscopy with high sensitivity. The depth distribution of Si in Cu grain boundaries was revealed by secondary neutral mass spectrometry. Surface morphology of Cu films was investigated by scanning tunneling microscopy. Identification of Si chemical bonds on the surface layer was made by X-ray photoelectron spectroscopy. GB diffusion coefficients were estimated by the relation used for calculation of diffusant distribution from a constant source in assumption that the diffusion path equals to the film thickness at the moment of appearing Si atoms on the Cu surface. At 453 K we estimated the surface segregation factor and detected formation of Cu–O–Si atomic bonds on the Cu film surface.

A Study on Decisive Early Stages in White Etching Crack Formation Induced by Lubrication

The reliability of rolling bearings is affected by white etching crack (WEC) or white structure flaking (WSF) failures, causing tremendous commercial burdens for bearing manufacturers and operators. The research for the underlying failure mechanism has attracted interest from a large scientific community over decades. Despite the significant amount of efforts, a root cause of white etching cracking is still missing. Amongst other factors, lubricant chemistry is considered to be essential in WEC formation. The authors aim to elucidate this key parameter by provoking white etching crack formation on a FE8 bearing test rig using a well-described set of chemicals in high- and low-reference lubricants. Scanning electron microscopy and energy dispersive X-ray analysis prove the presence of a patchy tribofilm on the surface of bearing washers, leading most likely to a higher frictional torque at the early stages of operation when the low reference oil is used. Secondary neutral mass spectrometry (SNMS) shows a hydrogen containing tribofilm in the shallow subsurface of about 30 nm depth, suggesting that hydrogen proliferating into bearing material may subsequently facilitate crack propagation via dislocation pileups, leading to premature bearing failure.

Actinide imaging in environmental hot particles from Chernobyl by rapid spatially resolved resonant laser secondary neutral mass spectrometry

Radioactive particles, so-called hot particles, are of great importance in the risk assessment of environmental pollution and human health hazards. Since most of these particles are only a couple of micrometers in size and thus have correspondingly small atomic inventory, information on origin and composition is difficult to obtain. Many analytical methods do not give insight into isotopic details or require the complete use up of the particle. Resonant laser secondary neutral mass spectrometry (rl-SNMS) has been proven to be a well-suited method for quasi-non-destructive analysis of trace elements. The technique provides high suppression of isobaric interferences and high spatial resolution without necessitating extensive chemical preparation. To measure multiple elements, it is often necessary to switch between laser excitation schemes, requiring elaborate adaptation of the laser setup. Here we present a novel rL-SNMS system capable of multi-element measurements in short succession without any manual adjustment to the laser system. For demonstration, ten micrometer sized hot particles from the Chernobyl Exclusion Zone are analyzed and the obtained isotopic ratios of uranium, plutonium and americium are discussed.

Imaging of I, Re and Tc plant uptake on the single-cell scale using SIMS and rL-SNMS

In radioecological studies, there is a significant need for understanding the plant uptake of radionuclides on a cellular level. The present work applies mass spectrometry to image the radionuclide distribution within the cellular structures of plants at varying concentrations. In a first step, plants of Daucus carota and Pisum sativum labelled with iodine and rhenium were examined, at concentrations in the range of 10 mM. Cross sections of several plant parts were imaged by secondary ion mass spectrometry (SIMS) after cryogenation in order to preserve cell structure. In a second step, the distribution of 99Tc in the two plant species was determined. For radiological reasons, a concentration three orders of magnitude lower was used, rendering measurements with SIMS impossible. Therefore, resonant laser secondary neutral mass spectrometry (rL-SNMS) was used for the first time to image 99Tc with suppression of molecular isobaric interferences. The measurement of only about 1010 atoms of 99Tc atoms is demonstrated and the distribution of 99Tc within a single epidermal cell is imaged.

Development, characterization, and first application of a resonant laser secondary neutral mass spectrometry setup for the research of plutonium in the context of long-term nuclear waste storage

Plutonium is a major contributor to the radiotoxicity in a long-term nuclear waste repository; therefore, many studies have focused on interactions of plutonium with the technical, geotechnical, and geological barriers of a possible nuclear waste storage site. In order to gain new insights into the sorption on surfaces and diffusion of actinides through these complex heterogeneous materials, a highly sensitive method with spatial resolution is required. Resonant laser secondary neutral mass spectrometry (Laser-SNMS) uses the spatial resolution available in time-of-flight secondary ion mass spectrometry (TOF-SIMS) in combination with the high selectivity, sensitivity, and low background noise of resonance ionization mass spectrometry (RIMS) and is, therefore, a promising method for the study and analysis of the geochemical behavior of plutonium in long-term nuclear waste storage. The authors present an approach with a combined setup consisting of a commercial TOF-SIMS instrument and a Ti:sapphire (Ti:Sa) laser system, as well as its optimization, characterization, and improvements compared to the original proof of concept by Erdmann et al. (2009). As a first application, the spatial distributions of plutonium and other elements on the surface of a pyrite particle and a cement thin section were measured by Laser-SNMS and TOF-SIMS, respectively. These results exemplify the potential of these techniques for the surface analysis of heterogeneous materials in the context of nuclear safety research.

Single and Multilayer Atomic Layer Deposition on and into Mesoporous Silicon Layers

Surface functionalization and pore filling of porous layers are essential key aspects for successful application of complex porous matrices. Atomic layer deposition (ALD) in deep macroporous trenches with high aspect ratio using different coating materials was investigated for a wide field of applications, such as energy storage, catalysis, and membrane technology1-3. Experimental data for ALD into mesopores, especially in the low mesoporous range (up to 20 nm) are rare4, 5. ALD on and into mesoporous silicon layers has different functions: 1st Surface passivation (sealing) of the underlying mesoporous structures, such as electrical barrier, diffusion barrier or corrosion protection. In this case complete pore filling is necessary to avoid transport processes through the passivated mesoporous interface. 2nd Pore size reduction (narrowing), i.e. adjustment of pore size for mesoporous membranes or single pores, e.g. for universally applicable, label-free, and nanopore-based sensor systems for the detection of small molecules in the nanofiltration range. Additionally, pore narrowing can be used for fine tuning of porous silicon-based sensors or optical filter properties5. 3rd Surface functionalization, e.g. in biomimetic or bioinspired systems or in fuel cells as catalytic layer.This contribution gives an extensive investigation of single and multi-layer depositions on and into mesoporous silicon layers and analyzes the material distribution and penetration profile of the corresponding atomic compositions.Mesoporous silicon layers were fabricated using highly doped (0.01-0.02 Ωcm) p-type substrates. Electrochemical etching (anodization) was performed in a mixture of 50 m% HF and absolute ethanol in volume ratio 1:1, to generate a 5 µm thick mesoporous layer. A current density of 70 mA/cm2 was used. 48 % porosity was measured by gravimetric technique. Nitrogen adsorption-desorption isotherms were measured in a Belsorp-mini from Rubotherm/Bel GmbH at 77 K using native porous silicon layers. Pore radius (rp) and pore radius distribution were evaluated using Barrett-Joyner-Halenda (BJH) theory. An average pore size radius of 4.68 nm, and pore radius range up to 12 nm were measured. However, surface oxidation will change the layer morphology, especially the pore size, porosity, and specific surface area.The layer deposition process was carried out with different ALD single layers, such as thermal Al2O3, plasma TiO2, thermal HfO2, thermal ZnO, and layer combinations as multilayer ALD, such as plasma Al2O3/TiO2 in an Oxford FlexAl reactor. Typical ALD cycle consists of two half-cycles, sequential precursor and co-reactant doses, which are separated by purge and pump steps, leading to self-limited layer growth6. Co-reactants and oxidizers are typically oxygen sources (H2O or oxygen plasma). When using an oxygen-containing medium, vacuum oxidation occurs, which leads to partial oxidation of the huge inner surface of the mesoporous layer5.Characterization of ultrathin layer deposited in complex 3D mesoporous structures is challenging and demands special analysis methods. In our experiments the penetration depth and depth distributions of atomic compositions for different ALD single and multilayers were measured using secondary neutral mass spectrometry (SNMS). Fig. 1 shows one example of the material distribution for Al2O3 atomic compositions on silicon substrate (Fig. 1a) and on and into mesoporous silicon substrate (Fig. 1b) with the same nominal Al2O3 layer thickness of 96.89 nm, which was measured on the silicon substrate with a Woollan M-2000FTM ellipsometer. The influence of both half-cycles of the ALD process must be considered at the evaluation, both the deposition effect on and into mesoporous silicon layers, and the transformation of the inner pore surface by oxidation. In the mesoporous layer the Al atomic composition profile follows the initial Al atomic profile up to 110 nm and subsequently shows a decreasing tendency up to 225 nm. The O atomic composition shows a decreasing tendency from 125 nm to 225 nm, and a saturation at 3.5 atomic percent. The existence of O in the deeper (>250 nm) mesoporous layers (Fig. 1b) indicates the partial oxidation of the underlying mesoporous layer occurring mainly during the H2O dose half-cycle process sequence. Penetration depths and profiles will be presented for the investigated single ALD layer. Further experimental results show that for multilayer ALD, e.g. with Al2O3/TiO2 layer combination, the first ALD-layer of the multilayer and its specific properties are dominating in the pore filling of the mesoporous layer.

Demonstration of single-phase wurtzite BAlN with over 20% boron content by metalorganic chemical vapor deposition

Wurtzite BAlN alloys are emerging ultrawide bandgap III-nitride semiconductors promising for optical and electronic devices. Yet the boron compositions of the grown alloys have been limited. In this Letter, we report on the demonstration of a thick single-phase wurtzite BAlN film with a boron composition over 20%. The growth was conducted at 1010 °C and 150 Torr with continuous flows of group-III precursors and ammonia with a growth rate of 2.2 μm/h by metalorganic chemical vapor deposition. The boron composition was studied by x-ray diffraction (XRD), secondary neutral mass spectrometry (SNMS), and Rutherford backscattering spectrometry (RBS). The XRD 2θ scan exhibited the clear wurtzite BAlN peak 1.82° larger than the AlN peak, indicating the boron composition of 30.9% based on the lattice constants of wurtzite AlN and BN. The SNMS and RBS experiments, independent of strain and defects, revealed that the boron content was 22%. The microstructures of the wurtzite BAlN film were further studied by transmission electron microscopy, showing an initial 5 nm thick layer free of crystal twinning followed by widespread crystal twinning with lattice rotations of 60° clockwise and anti-clockwise. The optical transmission experiment manifested that the bandgap of the smaller-lattice BAlN film was 5.1 eV, smaller than that of larger-lattice AlN. This trend was the opposite of the conventional InGaAlN but consistent with theoretical predictions. This study would greatly facilitate the research of material, physics, and devices incorporating the wurtzite BAlN alloys.

Two-color symmetry breaking in laser-based secondary neutral mass spectrometry.

An alternative method of analysis for laser-based secondary neutral mass spectrometry is introduced. By mixing a fundamental 1240 nm wavelength of the near-infrared laser field with an intensity of 1 × 1015 W/cm2 with its second harmonic generation wavelength of 620 nm, control is attained over the ion yield in secondary neutral mass spectrometry for a native silver surface and a surface covered with silver nanoparticles. The ion yields of selected species in the mass spectra, such as a C2 H4 O2 surfactant fragment ion and the Ag ion, are enhanced or suppressed by a factor of 8 or 23, respectively, in comparison with single color post-ionization. In addition, a significant suppression of ion fragmentation is demonstrated for both the silver sample and silver nanoparticles. Periodic variation of the ion yield with a modulation depth of 4% is observed depending upon the relative phase between the fundamental and second harmonic generated fields. These variations are explained in terms of tunneling ionization with symmetry-broken fields. An additional degree of freedom in a form of additional second color is introduced in laser-based secondary neutral mass spectrometry. Ion yield control is extended in a form of symmetry-broken fields.

Structural phase transformations in annealed Pt/Mn/Fe trilayers

Thermally-activated phase transitions in Pt/Mn/Fe thin films were investigated by a combination of x-ray diffraction, transmission electron microscopy, secondary neutral mass spectrometry depth profiling, atomic force microscopy, and magnetic properties measurements. Post-annealing was carried out in vacuum to different temperatures up to 620 °C. Initially, at temperatures between 280 °C–450 °C first L10-MnPt is formed at the Mn/Pt interface followed by the most likely formation of metastable bcc Fe3Pt, which gets transformed by further annealing to fcc Fe3Pt and eventually to chemically ordered L12-Fe3Pt. The final product after annealing at 620 °C consists of two interesting phases, which are relevant for spintronic applications, antiferromagnetic L10-MnPt with addition of Fe and ferromagnetic L12-Fe3Pt, consistent with the initial element composition.

Thermally-induced phase transitions in Pt/Tb/Fe trilayers

Structural phase formation induced by thermally-activated diffusion processes and intermixing of layered structures is a promising reaction pathway for the synthesis of unexpected (metastable) thin film materials. Furthermore, the sequence of structural phase transitions during annealing can substantially differ from those predicted by corresponding bulk phase diagrams due to the underlying grain boundary diffusion mechanisms as well as surface and interface effects. In this study, phase transitions in Pt/Tb/Fe trilayers occurring during post-annealing up to 620°C were investigated by various techniques including x-ray diffraction, transmission electron microscopy, secondary neutral mass spectrometry, Auger electron spectroscopy depth profiling, and magnetic properties measurements. At an annealing temperature of 215°C pronounced Pt and Tb diffusion intermixing is observed leading to the formation of the binary Pt2Tb phase. Further annealing up to 280 °C is accompanied by the appearance of the chemically disordered A1-FePt phase and significant Tb segregation to the outer surface forming an oxide layer. The chemically ordered L10-FePt phase starts to form in parallel to the disappearance of the Pt2Tb phase at 450°C. The final phase product at 620°C is characterized by the coexistence of two remaining phases, L10-FePt and TbO2. Structural phase transitions have a significant impact on the films magnetic properties – heat treatment leads to an enhancement of the coercive field. This fact is associated with the formation of the hard magnetic L10-FePt phase, which might be accompanied by grain isolation induced by partially Tb/Tb-O grain boundary filling.

Investigation of Ytterbium Incorporation in Lithium Niobate for Active Waveguide Devices

In this work, we report on an investigation of the ytterbium diffusion characteristics in lithium niobate. Ytterbium-doped substrates were prepared by in-diffusion of thin metallic layers coated onto x- and z-cut congruent substrates at different temperatures. The ytterbium profiles were investigated in detail by means of secondary neutral mass spectroscopy, optical microscopy, and optical spectroscopy. Diffusion from an infinite source was used to determine the solubility limit of ytterbium in lithium niobate as a function of temperature. The derived diffusion parameters are of importance for the development of active waveguide devices in ytterbium-doped lithium niobate.

Investigation of the ASPN process of low alloy steel by using Ni or Cr coated active screens

The mechanism of active screen plasma nitriding (ASPN) and the influence of the active screen's coating material on the surface properties of the nitrided layer was investigated on low alloyed steel samples, using chromium or nickel-coated active screens. ASPN experiments were performed at 510 °C, for 5 h in a 75% N2 + 25% H2 gas mixture using tempered 42CrMo4 type low alloy steel. The base material of the screens was unalloyed steel, but the screens were electroplated with chromium or nickel. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD), atomic force microscopy (AFM), and secondary neutral mass spectrometry (SNMS) were used for the characterization of surface properties of the samples and the screen. SNMS results show that the coating material of the active screen (chromium or nickel) was deposited on the surface of the samples in the form of compounds, with layer thicknesses between 160 and 200 nm (chromium) and 290–420 nm (nickel). Chemical analysis by X-ray photoelectron spectroscopy (XPS), performed at the interface of the steel sample and the deposited compounds reveals that iron-nitride was only formed in the case of samples that were treated with a Cr-coated active screen. Here the decomposition and oxidation of the unstable chromium-nitride provided the nitrogen atoms and the chemical drive for the formation of iron-nitride. In the case of the Ni-coated active screen, only molecular nitrogen was observed in the treated steel samples. The results prove that – contrary to widespread models on ASPN nitriding mechanisms – physically adsorbed nitrogen plays little role in the nitriding process of iron.