Time-of-flight Secondary Ion Mass Spectrometry Depth Profiling Research Articles

New insight into degradation mechanisms of conductive and thermally resistant polyaniline films

Conductive polyaniline (PANI) coatings find application in various fields, such as electrostatic dissipation, anticorrosion coatings, actives delivery, batteries, and solar control. Improving the thermal and electrical stability of PANI coatings at high temperatures and challenging environments is of growing interest. In this study, a novel polymer blend formulation was developed by adding polyurethane (PU) and sulfonyldiphenol (SDP) to a dinonylnaphthalene sulfonic acid (DNNSA)-doped PANI matrix. Films of the PANI-PU/SDP formulation and unmodified PANI control films were prepared on ITO glass substrates using spin coating, followed by thermal treatment. Characterization of the films was performed using thermogravimetric analysis-mass spectrometry (TGA-MS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) with depth profiling. Results showed that unmodified PANI films aged at 150 °C exhibited rapid thermo-oxidative degradation, resulting in a significant loss in electrical conductivity after 24 h. The inclusion of PU and SDP remarkably improved the thermal stability of the films, maintaining desirable conductivity levels over a week. ToF-SIMS depth profiling helped identify potential degradation mechanisms and monitor changes in chemical composition throughout the film thickness. This study provides insights into the functionality, optimization, and deployment of these new film types.

AFM Study of Roughness Development during ToF-SIMS Depth Profiling of Multilayers with a Cs+ Ion Beam in a H2 Atmosphere.

The influence of H2 flooding on the development of surface roughness during time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling was studied to evaluate the different aspects of a H2 atmosphere in comparison to an ultrahigh vacuum (UHV) environment. Multilayer samples, consisting of different combinations of metal, metal oxide, and alloy layers of different elements, were bombarded with 1 and 2 keV Cs+ ion beams in UHV and a H2 atmosphere of 7 × 10–7 mbar. The surface roughness Sa was measured with atomic force microscopy (AFM) on the initial surface and in the craters formed while sputtering, either in the middle of the layers or at the interfaces. We found that the roughness after Cs+ sputtering depends on the chemical composition/structure of the individual layers, and it increases with the sputtering depth. However, the increase in the roughness was, in specific cases, approximately a few tens of percent lower when sputtering in the H2 atmosphere compared to the UHV. In the other cases, the average surface roughness was generally still lower when H2 flooding was applied, but the differences were statistically insignificant. Additionally, we observed that for the initially rough surfaces with an Sa of about 5 nm, sputtering with the 1 keV Cs+ beam might have a smoothing effect, thereby reducing the initial roughness. Our observations also indicate that Cs+ sputtering with ion energies of 1 and 2 keV has a similar effect on roughness development, except for the cases with initially very smooth samples. The results show the beneficial effect of H2 flooding on surface roughness development during the ToF-SIMS depth profiling in addition to a reduction of the matrix effect and an improved identification of thin layers.

Determination of Sr2+ mobility in viscous bovine bone marrow by cryo-time-of-flight secondary ion mass spectrometry.

In osteoporosis research, strontium ions (Sr2+ ) have emerged as promising therapeutic agent in modified bone cements for better fracture healing. Modeling of Sr2+ dispersion in bone could be used as a predictive tool for the evaluation of functionalized biomaterials in future. Therefore, determination of experimental parameters for Sr2+ transport in bone is essential. In this study, we focus on the determination of Sr2+ diffusion in viscous bovine bone marrow by time-of-flight secondary ion mass spectrometry (ToF-SIMS). For this comparatively fast diffusion (FD) experiment, a specific experimental protocol of ToF-SIMS depth profiling under cryogenic conditions was developed. The validity of our experimental approach is proven by a time-dependent experimental series. Furthermore, 2D and 3D mass spectrometric imaging analysis was used to study Sr2+ surface and bulk distribution within bovine bone marrow. Detailed 2D and 3D mass spectrometric imaging analysis revealed that Sr2+ diffusion is slower in bone marrow areas with high intensity of lipid and fatty acid signals than in areas with less lipid content. The Sr2+ transport within this passive model can be described by Fickian diffusion. Average diffusion coefficients of Sr2+ in bovine bone marrow were obtained from diffusion profiles in FD areas (Dbovine,FD = [2.09 ± 2.39]·10-9 cm2 s-1 ), slow diffusion areas (Dbovine,SD = [1.52 ± 1.80]·10-10 cm2 s-1 ), and total area diffusion (Dbovine,TA = [1.94 ± 2.40]·10-9 cm2 s-1 ). We were able to show that cryo-ToF-SIMS is a useful tool for the characterization of rapid diffusion in water-containing highly viscous media. To the best of our knowledge, this is the first reported experimental approach for the investigation of the distribution of low concentrated therapeutic agents in bone marrow. Overall, our results provide important insights about Sr2+ diffusion in bovine bone marrow.

Co-solvented solution filling and interfacial phenomena of sublimation transferred emitting layer for high-resolution OLED fabrication

We investigated co-solvented solution properties and relations in the sublimation transfer process and optimized intense pulse light (IPL) using time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis. The capillary force induced co-solvented solution into the microchannel, with the co-solvented solution properties influencing the thickness and morphology of the sublimation transferred patterns of the emitting layer. The sublimation transferred pattern was optimized by tuning the ratio of solvents that were mixtures of chlorobenzene and ortho-dichlorobenzene. In addition, an analysis of the interface between pre-deposited layers and the evaporated layer fabricated by variation of IPL energy was conducted by means of ToF-SIMS depth profiling. Host emissions or the occurrence of the intermixing of layers was detected due to insufficient or excessive IPL energy exposure. With the optimized IPL energy, the current efficiency and external quantum efficiency were maximized without a recombination zone shift or intermixing during the IPL evaporation process.

Surface confinement induced amorphization of polyethylene oxide in high-performance porous polyethylene films

We report a new nanoconfinement phenomenon whereby surface confinement leads to the amorphization of polyethylene oxide (PEO) in porous ultrahigh molecular weight polyethylene membrane (UHMWPE). The surface confinement was achieved through a monolayer attachment of PEO onto the UHMWPE nanofibril surfaces‒pore bounding edges‒in the membrane via amphiphilic interactions of polypropylene oxide (PPO) blocks in the triblock copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO). Full amorphization of PEO is corroborated through the lack of phase transitions under differential scanning calorimetry (DSC), and liquid-like vibrational absorption responses under Fourier transform infrared spectroscopy (FTIR). Monolayer confinement of PEO in the membrane is quantified using a combination of Brunauer-Emmett-Teller (BET), time-of-flight secondary ion mass spectrometry (ToF-SIMS) molecular depth profiling and scanning electron microscopy (SEM). The total amount of PEO confined in the porous membrane is 7.7 wt% agreeing with that expected from the total BET surface area of the UHMWPE membrane; ToF-SIMS depth profiling using argon cluster ion reveals the homogeneous PEO distribution along the membrane depth; and surface topology analysis by SEM shows no obvious topological changes after PEO confinement.

Time-of-flight secondary ion tandem mass spectrometry depth profiling of organic light-emitting diode devices for elucidating the degradation process.

Organic light-emitting diode (OLED) products based on display applications have become popular in the past 10 years, and new products are being commercialized with rapid frequency. Despite the many advantages of OLEDs, these devices still have a problem concerning lifetime. To gain an understanding of the degradation process, the authors have investigated the molecular information for deteriorated OLED devices using time-of-flight secondary ion mass spectrometry (TOF-SIMS). TOF-SIMS depth profiling is an indispensable method for evaluating OLED devices. However, the depth profiles of OLEDs are generally difficult due to the mass interference among organic compounds, including degradation products. In this study, the tandem mass spectrometry (MS/MS) depth profiling method was used to characterize OLED devices. After degradation, defects comprised of small hydrocarbons were observed. Within the defect area, the diffusion of all OLED compounds was also observed. It is supposed that the source of the small hydrocarbons derives from decomposition of the OLED compounds and/or contaminants at the ITO interface. The true compound distributions have been determined using MS/MS depth profiling methods. The results suggest that luminance decay is mainly due to the decomposition and diffusion of OLED compounds, and that OLED decomposition may be accelerated by adventitious hydrocarbons present at the ITO surface.

Metals and Environment: Chemical Outputs From the Interaction Between Gilded Copper-Based Objects and Burial Soil

Three-dimensional chemical mapping was adopted to investigate an ancient fire-gilded buckle found in Rome. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) were used to detect and locate degradation products aiming to identify the alteration processes. Inorganic and organic compounds present in the outermost part of such a class of cultural heritage objects can be considered the result of long-term interaction with the burial environment. ToF-SIMS depth profiling experiments can provide chemical information at the molecular level and high resolved spatial information (about 1 μm laterally, and 1 nm in depth). In this work, the attention was focused on the identification and localization of the ionic and molecular species involved in the degradation process. Results showed the presence of copper oxides, chlorides, and sulfides as common corrosion products but also the presence of species related to copper and bronze corrosion process such as atacamite and its polymorphs. 3D maps for all the relevant molecular species allowed to visualize at the same time the eruption of copper chlorides throughout the micro/nanochannels present on the gold surface, the recrystallization of compounds of minor elements from the substrate, a pathway followed by silico-aluminates from the surface towards the internal corrosion layers, but mostly the evidence of biological activity of Sulphur Reducing Bacteria (SRB) living in anaerobic conditions.

Mitigating Measurement Artifacts in TOF-SIMS Analysis of Perovskite Solar Cells.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is one of the few techniques that can specifically distinguish between organic cations such as methylammonium and formamidinium. Distinguishing between these two species can lead to specific insight into the origins and evolution of compositional inhomogeneity and chemical gradients in halide perovskite solar cells, which appears to be a key to advancing the technology. TOF-SIMS can obtain chemical information from hybrid organic-inorganic perovskite solar cells (PSCs) in up to three dimensions, while not simply splitting the organic components into their molecular constituents (C, H, and N for both methylammonium and formamidinium), unlike other characterization methods. Here, we report on the apparently ubiquitous A-site organic cation gradient measured when doing TOF-SIMS depth-profiling of PSC films. Using thermomechanical methods to cleave perovskite samples at the buried glass/transparent conducting oxide interface enables depth profiling in a reverse direction from normal depth profiling (backside depth profiling). When comparing the backside depth profiles to the traditional front side profiled devices, an identical slight gradient in the A-site organic cation signal is observed in each case. This indicates that the apparent A-site cation gradient is a measurement artifact due to beam damage from the primary ion beam causing a continually decreasing ion yield for secondary ions of methylammonium and formamidinium. This is due to subsurface implantation and bond breaking from the 30 keV bismuth primary ion beam impact when profiling with too high of a data density. Here, we show that the beam-generated artifact associated with this damage can mostly be mitigated by altering the measurement conditions. We also report on a new method of depth profiling applied to PSC films that enables enhanced sensitivity to halide ions in positive measurement polarity, which can eliminate the need for a second measurement in negative polarity in most cases.

Analysis of the Chemical and Tribological Properties of Phosphate Glass Layers Developing during Metalworking Processes on Manual Transmission Synchronizers*

Abstract Optimization of machining processes to improve the tribological properties of surface layers has become a subject of industrial development. The modifications caused by manufacturing commonly include changes of topography, hardness or residual stresses. The present study investigates modification of the tribological properties of gear synchronizers by the formation of chemical layers on the metal surface within the final grinding process. These layers similar to phosphate glass emerge from the metalworking fluid additive zinc dialkyldithiophosphate (ZnDTP), which is applied in the process in different concentrations. The layers generated at the surface were measured by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Tribological experiments were conducted on a special synchronizer test facility. ToF-SIMS depth profiling revealed that higher ZnDTP concentrations in the metalworking fluid led to thicker phosphate glass layers on the machined surface. The layer that was generated by the addition of 5 % ZnDTP to the metalworking fluid led to a more homogenous coefficient of friction and a lower amount of wear in the tribological experiments. However, the addition of further ZnDTP, up to a total amount of 10 %, led to higher fluctuations in the coefficient of friction and increasing wear, likely related to the occurrence of friction oscillations resulting from thicker phosphate glass layers. However, the results indicate a good potential for improving surface layer properties of metal workpieces by adjusting the chemical composition of the metalworking fluid applied in the final machining process.

Evaluation of sputtering induced surface roughness development of Ni/Cu multilayers thin films by Time-of-Flight Secondary Ion Mass Spectrometry depth profiling with different energies O2+ ion bombardment

As a follow up on a previous paper on the quantitative evaluation of sputtering induced surface roughness and its influence on Auger electron spectroscopy depth profiling of polycrystalline Ni/Cu multilayers thin films, the ion sputtering induced surface roughness of these multilayered Ni/Cu polycrystalline thin films under low energy (0.5, 1.0 and 2.0 keV) O2+ ion bombardment and 1.0 keV Cs+ ion sputtering were investigated with Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Atomic Force Microscopy (AFM). The lowest energy (0.5 keV) O2+ ion sputtering gave the best depth resolution for the different energy ion sputtering. Furthermore the 1.0 keV Cs+ ion sputtering led to a better resolution than the O2+ ion sputtering. The ToF-SIMS depth resolution was also better than the AES depth profiling with a 2.0 keV Ar+ ion sputtering. The Mixing-Roughness-Information depth (MRI) model used for fitting of the ToF-SIMS depth profiles accounts for the broadening upon experimental depth profiling owing to the effects of atomic mixing, surface roughness and information depth. The MRI fitted roughness parameters obtained from the ToF-SIMS depth profiling are in good agreement with the root-mean-square roughness obtained from the AFM topography scans of the crater bottom. Ion sputtering induced roughness and depth resolution during ToF-SIMS depth profiling of Ni/Cu multilayers were quantitatively evaluated according to the MRI fitting parameters.

Characterization of advanced ALD-based thin film barriers for organic electronics using ToF-SIMS analysis

Abstract To address all the concerns of organic electronics device development and especially ageing issues, analytical techniques must be able to provide extremely fine characterization of the composition and thickness of layers and interfaces, while retaining appropriate spatial resolution. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is able to provide a detailed characterization of the in-depth composition of hybrid organic/inorganic multilayer structures which is crucial for such investigations. The study of an aged AlQ3 film after an exposition at the ambient atmosphere permitted to identify the secondary ions that are potentially indicative of the hydrolysis reaction of AlQ3. The investigation of the organic/inorganic multilayer devices by ToF-SIMS depth profiling also confirmed a confinement of the moisture in the gas barrier layer(s). It was possible to determine the moisture ratio present in the different barrier architectures to compare the efficiency of the encapsulation. In particular, the double barrier layer has shown a lower and a more localized diffusion of the moisture in the barrier which was consistent with the results of the performance of the devices and with the data reported in the literature such as water vapor transmission rate ones.

Differences between Li, Na, and K migration in thin SiO2 films during ToF-SIMS O2+ depth profiling

Distribution of alkali metals within thin insulating films as recorded by time of flight secondary ion mass spectrometry (ToF-SIMS) dual beam depth profiling using O2+ as the sputter projectile is usually exhibiting artifacts. Positive charges appear on the sample surface upon impact of sputter ions. This leads to large electric fields within the insulator, causing alkali metal ions to migrate toward the interface with an adjacent conductive layer. Additionally, the sample temperature is a main contributor to migration processes. By varying the temperature during measurements, the mobility of alkali metal ions can be regulated. This report addresses the difference between Li, Na, and K migration during ToF-SIMS depth profiling. It is shown that there is no significant difference between Li and Na migration within a thermally grown SiO2 layer on Si. K migration, however, is reduced, leading to less distorted depth profiles. At elevated temperatures above T = 200 °C, there is the onset to a second artifact, resulting in surface segregation of K and superposing the migration artifact in ToF-SIMS depth profiles. It is shown that the use of low energy electron flooding has an important role in the occurrence of alkali metal ion surface segregation.

Formation of hydroxyapatite on titanium implants in vivo precedes bone-formation during healing

The bone material interface has been an area of intense study over many decades, where studies of the healing process ranging from simple mineral deposition in vitro to actual healing in vivo have given important clues to the importance of calcium minerals in the bone/implant interface. Here, the authors use a combination of in vitro cell culture methods and in vivo implantation to study how the role of the spontaneously formed hydroxyapatite layer on Ti-implants for the in vivo-healing into the bone tissue of rat tibia. Initial experiments were made in reduced systems by incubation of TiO2 in cell culture medium and analysis by time of flight secondary ion mass spectrometry (ToF-SIMS) and energy-dispersive x-ray spectroscopy followed by subsequent exposure of human embryological stem cells analyzed by von Kossa staining and environmental scanning electron microsopy. In vivo studies of the bone-material interface was analyzed by ToF-SIMS depth profiling using both C60+ ions as well as a gas cluster ion source beam, Ar1500+ as sputter source. The low ion yield of the Ar1500+ for inorganics allowed the inorganic/organic interface of the implant to be studied avoiding the erosion of the inorganic materials caused by the conventional C60+ beam.

Decomposition of ultrathin LiF cathode underlayer in organic-based devices evidenced by ToF-SIMS depth profiling

In this work we investigate the chemical composition of an archetypal thin-film organic device with the Ag/LiF cathode using the time-of-flight secondary ion mass spectrometry (ToF-SIMS) with depth profiling. The LiF cathode underlayer is partly decomposed because a significant amount of lithium is released into the bulk of the multilayer device. The released lithium diffuses all the way to the substrate, accumulating, as revealed by ToF-SIMS depth profiles, at the interfaces rather than uniformly doping the underlying layers. Particularly, the bottom anode becomes chemically modified.

Chemical imaging of molecular changes in a hydrated single cell by dynamic secondary ion mass spectrometry and super-resolution microscopy.

Chemical imaging of single cells at the molecular level is important in capturing biological dynamics. Single cell correlative imaging is realized between super-resolution microscopy, namely, structured illumination microscopy (SIM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) using a multimodal microreactor (i.e., System for Analysis at the Liquid Vacuum Interface, SALVI). SIM characterized cells and guided subsequent ToF-SIMS analysis. Lipid fragments were identified in the cell membrane via dynamic ToF-SIMS depth profiling. Positive SIMS spectra show intracellular potassium and sodium ion transport due to exposure to nanoparticles. Spectral principal component analysis elucidates differences in chemical composition among healthy alveolar epithelial mouse lung C10 cells, cells that uptake zinc oxide nanoparticles, and various wet and dry control samples. The observation of Zn(+) gives the first direct evidence of ZnO NP uptake and dissolution by the cell membrane. Our results provide submicron chemical mapping for investigating cell dynamics at the molecular level.

Combining plasma profiling TOFMS with TOF-SIMS depth profiling for microelectronic applications

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is routinely used for depth profiling of microelectronic materials but interpretation of results can sometimes be hampered by artifacts such as matrix effects. Plasma profiling time-of-flight mass spectrometry (PP-TOFMS) provides direct measurement of the elemental composition of materials as a function of depth, with nanometer resolution and higher sputter rates than for TOF-SIMS. Here, the two techniques are compared on a range of multilayers used for microelectronic applications. Although depth resolution and sensitivity are less in the PP-TOFMS profiles, both techniques were able to determine the composition, detect contamination, and characterize diffusion as a function of annealing. Indeed, the use of both techniques on the same sample is complementary: (1) the PP-TOFMS profiles can be in most cases easily semiquantified using the ion beam ratio, whereas standards are required for the SIMS analysis and (2) simpler PP-TOFMS mass spectra with low contributions from molecular adducts allow for spontaneous detection of unexpected contaminants.

Al2O3 Coatings Deposited By ALD on Al Alloys

The key question that was addressed in this work is the following: would a uniform surface layer of aluminium oxide on aluminium alloys be protective against corrosion despite the presence of intermetallic Al-Cu particles? To answer this question, well defined aluminium oxide coatings were deposited by atomic layer deposition (ALD) on the surface of a model binary aluminium-copper alloy (Al-Cu5%wt.) in which copper-rich particles were produced by ageing. The corrosion resistance of uncoated, mechanically polished aluminium alloy samples covered by a native oxide layer was compared with the one of aluminium alloy samples protected by a 20 nm thick Al2O3 layer deposited by Atomic Layer Deposition (ALD). Detailed surface analysis was performed by X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Scanning Electron Microscopy (SEM) on the samples before and after immersion in 0.01M NaCl + 0.3 % vol. H2O2 (pH=6.2). The polished Al-Cu alloy with a thin native oxide layer (around 5 nm, as measured by XPS), exhibits a copper enrichment at the oxide/Al alloy interface, clearly evidenced by ToF-SIMS depth profiling. The ToF-SIMS chemical images and EDX analysis confirmed the presence of copper-rich intermetallics (Al2Cu) in the interfacial oxide/substrate region. A short immersion (~7 min) of this alloy in a near neutral NaCl solution lead to severe surface modifications: formation of mixed aluminium-copper oxide layer rich in CuO and Cu(OH)2 (as shown by XPS), and formation of an inhomogeneous, thick aluminium oxide deposit (as shown by SEM and ToF-SIMS imaging). Localized corrosion in the vicinity of the intermetallic particles (well known as trenching around particles) is clearlyobserved by SEM and ToF-SIMS imaging. A significant improvement on corrosion resistance was observed after deposition of the 20 nm-thick Al2O3 ALD coating. Despite the presence of intermetallics at the oxide/substrate interface (observed in the SEM) no significant corrosion occurred in the chloride solution, the Al2Cu particles remained intact, and no trenching around them was observed. In conclusion, the formation of a good quality alumina layer on the Al-5%Cu alloy is beneficial for its corrosion resistance. The coating can be deposited by ALD without prior removal of the intermetallic particles.

Accurate and precise measurement of oxygen isotopic fractions and diffusion profiles by selective attenuation of secondary ions (SASI).

The accuracy and precision of isotopic analysis in Time-of-Flight secondary ion mass spectrometry (ToF-SIMS) relies on the appropriate reduction of the dead-time and detector saturation effects, especially when analyzing species with high ion yields or present in high concentrations. Conventional approaches to avoid these problems are based on Poisson dead-time correction and/or an overall decrease of the total secondary ion intensity by reducing the target current. This ultimately leads to poor detection limits for the minor isotopes and high uncertainties of the measured isotopic ratios. An alternative strategy consists of the attenuation of those specific secondary ions that saturate the detector, providing an effective extension of the linear dynamic range. In this work, the selective attenuation of secondary ion signals (SASI) approach is applied to the study of oxygen transport properties in electroceramic materials by isotopic labeling with stable (18)O tracer and ToF-SIMS depth profiling. The better analytical performance in terms of accuracy and precision allowed a more reliable determination of the oxygen surface exchange and diffusion coefficients while maintaining good mass resolution and limits of detection for other minor secondary ion species. This improvement is especially relevant to understand the ionic transport mechanisms and properties of solid materials, such as the parallel diffusion pathways (e.g., oxygen diffusion through bulk, grain boundary, or dislocations) in electroceramic materials with relevant applications in energy storage and conversion devices.

Role of surface oxides in the formation of solid-electrolyte interphases at silicon electrodes for lithium-ion batteries.

Nonaqueous solvents in modern battery technologies undergo electroreduction at negative electrodes, leading to the formation of a solid-electrolyte interphase (SEI). The mechanisms and reactions leading to a stable SEI on silicon electrodes in lithium-ion batteries are still poorly understood. This lack of understanding inhibits the rational design of electrolyte additives, active material coatings, and the prediction of Li-ion battery life in general. We prepared SEI with a common nonaqueous solvent (LiPF6 in PC and in EC/DEC 1:1 by wt %) on silicon oxide and etched silicon (001) surfaces in various states of lithiation to understand the role of surface chemistry on the SEI formation mechanism and SEI structure. Anhydrous and anoxic techniques were used to prevent air and moisture contamination of prepared SEI films, allowing for more accurate characterization of SEI chemical stratification and composition by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling. Additionally, multivariate statistical methods were used to better understand TOF-SIMS depth profiling studies. We conclude that the absence of native-oxide layer on silicon has a significant impact on the formation, composition, structure, and thickness of the SEI.

Interphase Chemistry and Li-Ion Transport in Negative Electrode Materials Studied by XPS and ToF-SIMS

Understanding the electrode processes occurring at the electrode/electrolyte interface and in the bulk electrode material is necessary for developing the electrochemical performances of lithium-ion (LIB), sodium-ion, sulfur or metal-air batteries. The main electrode processes are insertion/extraction reactions that induce changes in the host electrode materials; they are accompanied by decomposition of electrolyte that leads to formation of a solid electrolyte interphase (SEI) layer [1]. Apart from the electrode processes, the electrochemical performance of batteries can also be significantly dependent on ionic transport, thus a good understanding of diffusion processes is also necessary.Two principal surface-sensitive techniques are particularly suitable for analyzing surface reactions at electrodes: X–ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). As XPS provides information on chemical composition on the first few nanometers at the electrode surface, depth profiling analysis is also necessary for the characterization of thicker surface layers and bulk materials. Depth profiling can be either performed by XPS or ToF–SIMS using Ar+ or Cs+ ions, respectively, for sputtering. ToF-SIMS is a highly sensitive surface analytical technique where a pulsed primary ion beam (e.g. Bi+) is used to extract secondary ions that are analyzed by time–of–flight spectrometry. Interlaced with a sputtering ion beam (e.g. Cs+), elemental depth profiles with excellent depth resolution (monolayer) and high sensitivity (ppb) can be readily obtained. ToF-SIMS can also serve as a new and more direct method than the commonly used electrochemical methods (i.e. electrochemical impedance spectroscopy, cyclic voltammetry or galvanostatic methods) for precise measurement of ion diffusion in electrode materials.Our approach is to use model thin-film electrodes (i.e. from few tens to few hundreds of nm directly grown on a current collector) with a good surface finishing and having a roughness suitable for analysis by surface sensitive techniques. The application of model thin-film electrode having enlarged surface-to-volume ratio provides clearer and more comprehensible insight into electrode/electrolyte interface reactions without complications from current percolators or binding agents that are used in bulk composite electrode materials. This allows for studies of intrinsic electrochemical and interfacial processes occurring on the electrode surfaces.In the present work, XPS and ToF-SIMS have been used to investigate the chemical and volume modifications of thin-film negative-electrode materials. As an example, two types of high capacity negative-electrode materials will be presented:a conversion-type electrode consisting of iron oxide (with a theoretical capacity of 1007 mAh/g), andan alloying-type electrode consisting of silicon (with a theoretical capacity of 3579 mAh/g). Both negative-electrode materials are interesting and important candidates for application in LiBs due to their abundance and environmental friendliness.For the first time, ToF-SIMS depth profiling have been applied to measure an apparent diffusion coefficient of lithium ions (DLi+) into these electrodes. The DLi+was calculated from the finite integration of Fick’s second law for one-dimensional diffusion.Regardless of the type of electrode material, XPS analysis shows strong intensity attenuation and binding energy positive shift of the principal core level peak corresponding to the main electrode component (Si2p in the case of Si-electrode and Fe2p in the case of Fe2O3-one), confirming the formation of the SEI layers on the surfaces after cycling in PC-LiClO4. The chemical composition of the SEI formed on the two different electrodes is essentially identical, with a principal Li2CO3 constituent and a minor quantity of alkyl carbonates (ROCO2Li) as evidenced by the C1s, O1s and Li1s core level peaks [2,3]. The SEI layer chemistry principally depends on the electrolyte composition [2].Both, XPS and ToF-SIMS, evidence a dynamic increase/decrease of the SEI layer thickness upon lithiation/delithiation, superimposed to a continuous increase upon cycling. The data reveal the formation of duplex-like SEI layer structure, with the organic carbonate species in the outer region and inorganic salt decomposition products in the inner part of the SEI layer. The ToF-SIMS depth profiles also show irreversible Li trapping in the bulk electrodes and volume variations with the degree of lithiation and the number of cycles [2,4]. These modifications can be detrimental to electrode cycling and capacity retention.A low DLi+ in the order of 10-15 cm2/s calculated from ToF-SIMS depth profiles for these two types of electrodes, indicates that the rate of lithiation is slower than for intercalation-type materials and can have an important influence on their electrochemical performance.