Subnanometric Depth Resolution Research Articles

Bulk-independent surface oxide composition controls the electrochemical performance of high-entropy alloys.

Multi-element alloys and high-entropy alloys show promising electrocatalytic behavior for water splitting and other catalytic reactions, due to their highly tunable composition. While preparation and synthesis of these materials are thoroughly investigated, the true reactive surface composition is still not well understood, as it may significantly differ from the bulk composition. Precise knowledge and understanding of resulting surface composition is crucial for effective control of the electrocatalytic performance. In this work, low energy ion scattering spectroscopy was applied to determine the surface oxide composition of a series of Ni-based multi-metallic alloys with Mn, Fe, Co, and Cr under alkaline, neutral and acidic conditions. The composition of the surface oxide was investigated with sub-nanometer depth resolution. In electrochemical tests, good catalytic activity was found for the oxygen evolution reaction, although a strong dependence on the selected reaction conditions was observed. The surface composition under OER conditions deviates significantly from the bulk composition. No significant benefit of high entropy alloying compared with binary or ternary alloys concerning catalytic OER performance was found.

Absolute local conformation of poly(methyl methacrylate) chains adsorbed on a quartz surface.

Polymer chains at a buried interface with an inorganic solid play a critical role in the performance of polymer nanocomposites and adhesives. Sum frequency generation (SFG) vibrational spectroscopy with a sub-nanometer depth resolution provides valuable information regarding the orientation angle of functional groups at interfaces. However, in the case of conventional SFG, since the signal intensity is proportional to the square of the second-order nonlinear optical susceptibility and thereby loses phase information, it cannot be unambiguously determined whether the functional groups face upward or downward. This problem can be solved by phase-sensitive SFG (ps-SFG). We here applied ps-SFG to poly(methyl methacrylate) (PMMA) chains in direct contact with a quartz surface, shedding light on the local conformation of chains adsorbed onto the solid surface. The measurements made it possible to determine the absolute orientation of the ester methyl groups of PMMA, which were oriented toward the quartz interface. Combining ps-SFG with all-atomistic molecular dynamics simulation, the distribution of the local conformation and the driving force are also discussed.

High-Sensitivity Low-Energy Ion Spectroscopy with Sub-Nanometer Depth Resolution Reveals Oxidation Resistance of MoS2 Increases with Film Density and Shear-Induced Nanostructural Modifications of the Surface.

For decades, density has been attributed as a critical aspect of the structure of sputter-deposited nanocrystalline molybdenum disulfide (MoS2) coatings impacting oxidation resistance and wear resistance. Despite its importance, there are few examples in the literature that explicitly investigate the relationship between the density and oxidation behaviors of MoS2 coatings. Aging and oxidation are primary considerations for the use of MoS2 coatings in aerospace applications as they inevitably experience prolonged storage in water and oxygen-rich environments prior to use. Oxidation that is either limited to the first few nanometers or through the bulk of the coating can result in seizure due to high initial coefficients of friction or component failure from excessive wear. High-sensitivity low-energy ion spectroscopy (HS-LEIS) and Rutherford backscattering spectrometry (RBS) are both used to understand the extent of oxidation throughout the first ∼10 nanometers of the surface of pure sputtered nanocrystalline MoS2 coatings after high-temperature aging and how it is impacted by the density of coatings as measured by RBS. Results show that low-density coatings (ρ = 3.55 g/cm3) exhibit a more columnar microstructure and voiding, which act as pathways for oxidative species to penetrate and interact with edge sites, causing severe surface and subsurface oxidation. Furthermore, HS-LEIS of surfaces sheared prior to oxidation reveals that the oxidation resistance of low-density MoS2 coatings can be significantly improved by shear-induced reorientation of the surface microstructure to a basal orientation and elimination of pathways for oxygen into the bulk through compaction of surface and subsurface voids.

Time- and Depth-Resolved Chemical State Analysis of the Surface-to-Subsurface Oxidation of Cu by X-ray Absorption Spectroscopy at Near Ambient Pressure.

Metal surface oxidation is a well-known phenomenon, but the oxidation states of metal surfaces have not been observed in situ and in real time because most techniques obtaining surface chemical states require high-vacuum conditions. Here, we achieved the real-time, in situ observation of the initial stages of surface Cu oxidation using fluorescence-yield wavelength-dispersive X-ray absorption spectroscopy (XAS) in the soft X-ray region at sub-nanometer depth resolution. Further, the XAS data suggest a unique oxidation mechanism: CuO forms on the top surface, and subsequently, Cu2O forms in the subsurface layers (>1 nm from the surface), accompanied by the interdiffusion of Cu from the inner layer and that of Cu2O to the inner layer. The reported technique has applications for the analysis of surface phenomena at ambient pressure, especially oxidation processes, whose understanding is crucial in many fields, from chemistry to structural engineering.

Precise localization of contaminants in graphene with secondary ion mass spectrometry

Integration of graphene with the existing fabrication lines may be possible only if the growth process fulfills the high purity requirements of the front-end-of-line integration approaches. While many analytical techniques may provide information about the concentration of contaminants, their dimensional sensitivity is usually poor and thus it is not possible to assess which contaminants are located above/below graphene and which are incorporated into its structure. In this work, an optimized secondary ion mass spectrometry measurement procedure that combines excellent detection limits (0.8–2.9 ppm) and subnanometer depth resolution is presented. It confirms the presence of sodium and sulfur at the surface of the graphene layer, the formation of graphene–copper composite, and allows the detection of molybdenum and tungsten at the interface between graphene and the germanium substrate. Such precision may prove to be valuable for further optimization of the growth process of 2D materials.

Real-Time Observation of Surface Chemical Reactions Proceeding in the Depth Direction by Wavelength-Dispersive Soft X-ray Absorption Spectroscopy.

It has long been a challenging task to observe surface chemical reactions proceeding in the depth direction without stopping the reaction (i.e., real-time observation), with subnanometer depth resolution. X-ray absorption spectroscopy (XAS) in the soft X-ray region is one of the most crucial methods to analyze the chemical states of light elements; however, simultaneous time- and depth-resolved XAS measurements during the reaction have not been realized before. Herein, we show the real-time observation of the time-evolving oxidation process of a Co thin film with a fluorescence-yield wavelength-dispersive XAS method, which also has subnanometer depth resolution. The series of Co L-edge XAS spectra show that the oxide component increases from the surface to deeper regions as the reaction proceeds. The progress of oxidation in the depth direction was interpreted by the reaction with O2 gas at the surface and the interlayer reaction by oxygen migration.

X-ray Standing Waves and Molecular Dynamics Studies of Ion Surface Interactions in Water at a Charged Silica Interface

The adsorption of monovalent ions in millimolar concentrations at the silica solution interface has been investigated using X-ray standing waves, a method which combines element sensitivity and subnanometer depth resolution. Results were analyzed using a model free analysis and a Gouy–Chapman–Stern model giving access to interfacial concentration profiles. We find the concentration of potassium cations in the Stern layer to be slightly higher than that of cesium cations. They also come closer to the interface, in agreement with our molecular dynamics simulations which suggests that our experimental results can be explained by the higher ability of smaller cations to share their hydration shell with the surface. Extension of the method to different systems can provide the necessary experimental basis to benchmark ion surface potentials, a necessary step to predict interfacial behavior of ions which is of high relevance to several areas of science and technology.

A Comparison of Electrode Surface Films Formed with Different Oxide Cathodes for Lithium-Ion Batteries

High-nickel, Li-rich, and high-voltage spinel electrodes are lithium-ion battery cathodes commonly considered as candidates for near-term commercialization. Each of these cathode materials can offer higher energy densities than conventional lithium cobalt oxide cathode materials, at least during initial cycles. High-Ni cathodes of the form LiNiaMnbCo1-a-bO2 have the same layered structure as current commercial cathodes with alternating transition metal and lithium layers between oxide slabs. With increasing Ni content, High-Ni cathode materials generally can provide higher discharge capacities, reaching upwards of 240 mAh g-1. Li-rich electrode materials (Li1+xMO) have a similar layered structure as High-Ni materials but include excess Li+ in the transition metal layer that can be accessed for much higher discharge capacities, even exceeding 300 mAh g-1. High-voltage spinel electrodes of the form LiMnaNi2-aO4 operate at high voltages providing high energy densities, though at much lower discharge capacities, ca. 140 mAh g-1. While all three of these electrode classes offer exciting possibilities for future lithium-ion battery development, they all suffer from the inherent instability of their surface reactivities. High-Ni cathodes suffer from high surface reactivity and a slow growth of the cubic rocksalt phase throughout cycling that impedes lithium intercalation. The activation of Li-rich electrodes involves a complex surface passivation involving the evolution of molecular oxygen and the formation of a spinel-like phase. High-voltage spinel electrodes operate outside the electrolyte stability region, so also suffer from rapid oxidative breakdown of electrolyte components and impedance growth. Aside from the cathode interphase, the solid electrolyte interphase formed at the anode can be affected by the surface chemistry of these various cathodes; for instance, transition-metal ions may dissolve from the cathode, migrate to the anode side where they are reduced, and act as a catalyst for further electrolyte reduction. This presentation describes our recent characterization of cathode and anode surface electrolyte interphases from cells with High-Ni, high-voltage spinel, and Li-rich electrodes. Half cells vs. lithium anodes, as well as full cells with up to 1,500 cycles are presented. For these studies, we employ time of flight – secondary ion mass spectrometry (TOF-SIMS), a powerful characterization technique pioneered by our group for characterization of battery materials. TOF-SIMS provides sub-nanometer depth resolution of specific chemical fragments over micron-sized image areas that can also be used in a depth profiling method, through layer-by-layer ablation of surface species to form a 3D image (Figure 1). TOF-SIMS is used in conjunction with standard characterization techniques, such as X-ray photoelectron spectroscopy (XPS), galvanostatic cycling data, and electrochemical impedance spectroscopy. Figure 1: A 3D TOF-SIMS image of cycled high-Ni cathode particles with transition metals indicated in green, as well as decomposed organics (C-, CH-, CH2 - ) indicated in red. Figure 1

Oxygen out-diffusion and compositional changes in zinc oxide during ytterbium ions bombardment

Oxygen release and out-diffusion in zinc oxide crystals during heavy ions bombardment has been suggested by many experimental techniques. In this work we have employed secondary ion mass spectrometry to study ZnO implanted with ytterbium ions. Our measurements confirm formation of an oxygen-depleted layer and oxygen out-diffusion and agglomeration at the surface. Moreover, an average compositional change in a heavily damaged near-surface region can also be monitored. This reproducible measurement procedure with subnanometer depth resolution allows to localize precisely these altered layers at the depth of 14–28 nm (oxygen-depleted layer) and 9 nm (maximum of the amorphized region). Such precise measurements may prove to be valuable for further characterization of ion beam induced defects in wide bandgap compound semiconductors and optimization of optoelectronic devices based on these materials.

3D Molecular ToF-SIMS Imaging of Artificial Lipid Membranes Using a Discriminant Analysis-Based Algorithm.

Artificial lipid membranes play a growing role in technical applications such as biosensors in pharmacological research and as model systems in the investigation of biological lipid films. In the standard procedure for displaying the distribution of membrane components, fluorescence microscopy, the fluorophores used can influence the distribution of the components and usually not all substances can be displayed at the same time. The discriminant analysis-based algorithm used in combination with scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) enables marker-free, quantitative, simultaneous recording of all membrane components. These data are used for reconstruction of distribution patterns. In the model system used for this survey, a tear fluid lipid layer, the distribution patterns of all lipids correlate well in calculated ToF-SIMS images and epi-fluorescence microscopic images. All epi-fluorescence microscopically viewable structures are visible when using both positive and negative secondary ions and can be reproduced with high lateral resolution in the submicrometer range despite the very low signal intensity and a very low signal-to-noise ratio. In addition, three-dimensional images can be obtained with a subnanometer depth resolution. Furthermore, structures and the distribution of substances that cannot be made visible by epi-fluorescence microscopy can be displayed. This enables new insights that cannot be gained by epi-fluorescence microscopy alone.

Interface between Al2O3 and 4H-SiC investigated by time-of-flight medium energy ion scattering

The formation of interfacial oxides during heat treatment of dielectric films on 4H-SiC has been studied. The 4H-SiC surface has been carefully prepared to create a clean and abrupt interface to Al2O3. An amorphous, 3 nm thick, Al2O3 film has been prepared on 4H-SiC by atomic layer deposition and rapid thermal annealing was then performed in N2O ambient at 700 °C and 1100 °C during 1 min. The samples were studied by time-of-flight medium energy ion scattering (ToF-MEIS), with sub-nanometer depth resolution and it is seen that, at both annealing temperatures, a thin SiOx (1 ⩽ x ⩽ 2) is formed at the interface. Our results further indicate that carbon remains in the silicon oxide in samples annealed at 700 °C. Additional electrical capacitance voltage measurements indicate that a large concentration of interface traps is formed at this temperature. After 1100 °C annealing, both MEIS and XRD measurements show that these features disappear, in accordance with electrical data.

Towards Quantification of Contaminants in Electrodeposited Cu Films

A Cu electroplating process is typically used in semiconductor industry for the on-chip fabrication of interconnects on non-planar (patterned) wafer surfaces with feature dimensions ranging from the sub-10nm level (advanced Damascene applications) up to the micrometer level (Through Silicon Via technology). The presence of particular additives in the plating bath during the electroplating is a crucial prerequisite for the successful fill of vias and trenches with Cu. The accumulation of contaminants in the resulting Cu film is known to significantly decelerate the post-deposition recrystallization that is mandatory to improve the conductivity of these interconnects. It is therefore vital to develop new analytical techniques allowing a precise (true) quantification of the contamination level in the Cu deposit. Classical Secondary Ion Mass Spectrometry (SIMS) depth profiling approaches only provide semi-quantitative information on the contamination level with a poor depth resolution. In this contribution we report on a recently developed quantitative depth profiling technique with sub-nanometer depth resolution based on laser ablation ionization mass spectrometry (LIMS). The high vertical depth resolution, a high dynamic range >108 coupled with a high detection sensitivity of ~ 10 ppb allows the identification/quantification of trace amounts of contaminants localized at grain boundaries of the Cu deposit. Results of the LIMS techniques are compared with complementary 3D atom probe tomography experiments. To demonstrate the capabilities of these analytical techniques we use copper films electrodeposited in the presence of SPS (bis-(sodium-sulfopropyl)-disulfide) and Imep (polymerizates of imidazole and epichlorohydrin) additives as model system. This approach allows the targeted insertion of grain boundaries into the Cu film perpendicular to the growth (depth profiling) direction.

Aqueous solution/metal interfaces investigated in operando by photoelectron spectroscopy

We describe a new in operando approach for the investigation of heterogeneous processes at solid/liquid interfaces with elemental and chemical specificity which combines the preparation of thin liquid films using the meniscus method with standing wave ambient pressure X-ray photoelectron spectroscopy [Nemšák et al., Nat. Commun., 5, 5441 (2014)]. This technique provides information about the chemical composition across liquid/solid interfaces with sub-nanometer depth resolution and under realistic conditions of solution composition and concentration, pH, as well as electrical bias. In this article, we discuss the basics of the technique and present the first results of measurements on KOH/Ni interfaces.

Quantitative confocal phase imaging by synthetic optical holography.

We demonstrate quantitative phase mapping in confocal optical microscopy by applying synthetic optical holography (SOH), a recently introduced method for technically simple and fast phase imaging in scanning optical microscopy. SOH is implemented in a confocal microscope by simply adding a linearly moving reference mirror to the microscope setup, which generates a synthetic reference wave analogous to the plane reference wave of wide-field off-axis holography. We demonstrate that SOH confocal microscopy allows for non-contact surface profiling with sub-nanometer depth resolution. As an application for biological imaging, we apply SOH confocal microscopy to map the surface profile of an onion cell, revealing nanoscale-height features on the cell surface.

Future directions in standing-wave photoemission

Over the past decade, standing-wave photoemission (SW-XPS) has evolved into a powerful and versatile non-destructive technique for probing element-specific electronic, magnetic, and structural properties of buried layers and interfaces with sub-nanometer depth resolution. In this article, I will discuss several promising future directions in this emergent field stemming from experimental and theoretical studies wherein SW-XPS is combined with other X-ray techniques, such as magnetic circular dichroism (MCD), hard X-ray photoemission spectroscopy (HAXPES), angle-resolved photoemission (ARPES), and photoemission microscopy (PEEM), adding extra dimensions to the measurement and thus widening the scope of scientific and technological questions accessible via the use of standing waves. I will further discuss examples of recently developed methods for X-ray standing-wave data analysis, which yield layer-resolved matrix-element-weighted densities of states at interfaces as well as Ångstrom-level changes in periodicity of synthetic superlattices. Finally, I will explore the possibility of localizing the standing waves near the surface and within a buried layer by the use of aperiodic superlattices, total reflection, and X-ray waveguide effects.

Interference microscopy of subnanometer depth resolution: Experimental study

Subnanometer-range roughness parameters were measured using an automated interference Linnik microscope with a white-light source. The sensitivity of the developed microscope in depth was shown to be 0.25 nm.

Interference microscopy of subnanometer depth resolution: Numerical simulation

We present results of numerical simulation performed with the aim to study the possibility of measuring subnanometer-range roughness parameters using an automated interference Linnik microscope with a white-light source. It is shown that reconstruction noises of phase images can be reduced to 0.1 nm when using averaging of interferograms and phase images.

Fe diffusion in amorphous Si studied using x-ray standing wave technique

Diffusion of Fe impurity in amorphous Si at the intermediate concentration range of 25 at.% Fe has been studied. A combination of x-ray standing wave technique and secondary ion mass spectrometry provides unambiguous determination of the concentration profiles of the constituent species with sub-nanometer depth resolution. X-ray standing waves are generated using total external reflection from an underlying W layer. It is found that up to 573 K, Fe diffusivity is less than 10−23m2/s. This is in stark contrast to isolated Fe impurity diffusion in Si or to the interdiffusion at Fe/Si interface, which are orders of magnitude higher. An interesting phenomenon is observed, when a Pt buffer layer is used instead of W for generating standing waves: With thermal annealing, as the Pt atoms move into Si layer and cross the marker layer containing Fe atoms, Fe atoms also move along. This results in an upwards shift of the concentration profile of Fe.

Determination of layer-resolved composition, magnetization, and electronic structure of an Fe/MgO tunnel junction by standing-wave core and valence photoemission

Spin-dependent tunneling across a highly textured MgO insulating barrier has received much attention due to its potential applications in various spintronic devices. However, the interfacial magnetic and electronic structure of a prototypical realization of this in Fe/MgO/Fe and the effective band gap of the MgO layer are still under debate. In order to resolve these issues, we have employed standing-wave excited core and valence photoemission, as well as core-level magnetic circular dichroism (MCD) in photoemission, to study the Fe/MgO interface with subnanometer depth resolution. For our synthetic procedure, we show that the Fe/MgO interface is linearly intermixed in composition over a length of \ensuremath{\sim}8 \AA{} (\ensuremath{\sim}4 monolayers) and that there is a magnetic dead layer \ensuremath{\sim}2--3 \AA{} thick. The unambiguous extraction of depth-resolved density of states (DOS) reveals that the interfacial layer composition is mostly metallic and nonmagnetic FeO${}_{\mathrm{x}}$, with x \ensuremath{\cong} 1, which accounts for a smaller magnetoresistance compared to theoretical predictions. The formation of the magnetic dead layer (FeO) at the interface should also reduce the tunneling spin polarization. The analysis of our data also shows a clear valence band edge of ultrathin MgO layer at \ensuremath{\sim}3.5 eV below the Fermi level (${E}_{\mathrm{F}}$) that is very close to that of single crystal bulk MgO. An analysis that does not consider the interdiffused region separately exhibits the valence band edge for MgO layer \ensuremath{\sim}1.3 eV below ${E}_{\mathrm{F}}$, which is significantly closer to the MgO barrier height estimated from magnetotransport measurements and further suggests that the Fe/MgO interdiffusion effectively reduces the MgO band gap.

Asymmetric diffusion at the interfaces in Fe/Si multilayers

Nanoscale diffusion at the interfaces in multilayers plays a vital role in controlling their physical properties for a variety of applications. In the present work depth-dependent interdiffusion in a Si/Fe/Si trilayer has been studied with subnanometer depth resolution using x-ray standing waves. High depth selectivity of the present technique allows one to measure diffusion at the two interfaces of Fe, namely, Fe on Si and Si on Fe independently, yielding an intriguing result that Fe diffusivity at the two interfaces is not symmetric. While the values of activation energy at the two interfaces are comparable, the main difference is found in the pre-exponent factor suggesting different mechanisms of diffusion at the two interfaces. This apparently counterintuitive result has been understood in terms of an asymmetric structure of the interfaces as revealed by depth selective conversion electron M\"ossbauer spectroscopy. This asymmetry is in turn explained by peculiarities during Si/Fe/Si layer formation by sputter deposition, in particular due to the difference between the surface free energies of Fe and Si.