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Wafer curvature measurements reported in literature for polycrystalline (often textured) and epitaxial fcc metal thin films on hard substrates show a characteristic “signature” in the stress-temperature evolution for either type of films. While epitaxial films reveal characteristic elastic – ideal plastic deformation with no dislocation storage and highly repeatable cycles, polycrystalline films show considerable hardening upon cooling in addition to the relaxation by diffusional creep at elevated temperatures. In the present study, we study the deformation characteristics of an electron beam deposited epitaxial nanotwinned Ag on Si (111) substrate. The twin spacing λ of the nanotwinned Ag is controlled by suitable heat treatment and the “signature” thermomechanical deformation curves by wafer curvature measurements are recorded for twin spacings varying from 20 nm to 1 μm. Further, deformation is compared to other small scale deformation studies on fcc metals such as epitaxial bicrystal films, bicrystal micropillars containing a coherent twin boundary and nanotwinned micropillars.

Preprint version of the open access article Merola, C., Cheng, H., Dworschak, D., Ku, C., Chiang, C., Renner, F. U., & Valtiner, M. (2019). Nanometer Resolved Real Time Visualization of Acidification and Material Breakdown in Confinement. Advanced Materials Interfaces, 6(10), 1802069. https://doi.org/10.1002/admi.201802069, Localized surface reactions in confinement are inherently difficult to visualize in real-time. Herein multiple-beam-interferometry (MBI) is extended as a real-time monitoring tool for corrosion of nanometer confined bulk metallic surfaces. The capabilities of MBI are demonstrated, and the initial crevice corrosion mechanism on confined nickel and a Ni75Cr16Fe9 model material is compared. The initiation of crevice corrosion is visualized in real time during linear sweep polarization in a 1 × 10−3 m NaCl solution. Pre- and post-experiment analysis is performed to complementarily characterize the degraded area with atomic force microscopy (AFM), optical microscopy, nano-Laue diffraction (nano-LD), scanning electron microscopy (SEM)/electron backscatter diffraction (EBSD), and X-ray photoelectron spectroscopy (XPS). Overall, Ni75Cr16Fe9 displays a better corrosion resistance; however, MBI imaging reveals 200 nm deep severe localized corrosion of the alloy in the crevice opening. Chromium rich passive films formed on the alloy contribute to accelerated corrosion of the confined alloy by a strongly acidifying dissolution of the passive film in the crevice opening. Nano-LD further reveals preferential crystallographic defect and corrosion migration planes during corrosion. MBI provides nanometer accurate characterization of topologies and degradation in confined spaces. The technique enables the understanding of the initial crevice corrosion mechanism and testing modeling approaches and machine-learning algorithms.

Multiple beam interferometry (MBI) evolved as a powerful tool for the simultaneous evaluation of thin film thicknesses and refractive indices in Surface Forces Apparatus (SFA) measurements. However, analysis has relied on simplifications for providing fast or simplified analysis of recorded interference spectra. Here, we describe the implementation of new optics and a generalized fitting approach to 4 × 4 transfer matrix method simulations for the SFA. Layers are described by dispersive complex refractive indices, thicknesses, and Euler angles that can be fitted, providing modeling for birefringent or colored layers. Normalization of data by incident light intensities is essential for the implementation of a fitting approach. Therefore, a modular optical system is described that can be retrofit to any existing SFA setup. Real-time normalization of spectra by white light is realized, alignment procedures are considerably simplified, and direct switching between transmission and reflection modes is possible. A numerical approach is introduced for constructing transfer matrices for birefringent materials. Full fitting of data to the simulation is implemented for arbitrary multilayered stacks used in SFA. This enables self-consistent fitting of mirror thicknesses, birefringence, and relative rotation of anisotropic layers (e.g., mica), evaluation of reflection and transmission mode spectra, and simultaneous fitting of thicknesses and refractive indices of media confined between two surfaces. In addition, a fast full spectral fitting method is implemented for providing a possible real-time analysis with up to 30 fps. We measure and analyze refractive indices of confined cyclohexane, the thickness of lipid bilayers, the thickness of metal layers, the relative rotation of birefringent materials, contact widths, as well as simultaneous fitting of both reflection and transmission mode spectra of typical interferometers. Our analyses suggest a number of best practices for conducting SFA and open MBI in an SFA for increasingly complex systems, including metamaterials, multilayered anisotropic layers, and chiral layers. © 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)

The interfacial decomposition products forming the so-called solid-electrolyte interphase (SEI) significantly determine the destiny of a Li-ion battery. Ultimate knowledge of its detailed behavior and better control are required for higher rates, longer life-time, and increased safety. Employing an electrochemical surface force apparatus, it is possible to control the growth and to investigate the mechanical properties of an SEI in a lithium-ion battery environment. This new approach is here introduced on a gold model system and reveals a compressible film at all stages of SEI growth. The demonstrated methodology provides a unique tool for analyzing electrochemical battery interfaces, in particular in view of alternative electrolyte formulations and artificial interfaces. B.M. acknowledges travel support by InnoEnergy Ph.D. School. The authors thank Carsten Dosche for additional XPS support. Furthermore, the funding of the Escalab250 Xi XPS spectrometer at Carl von Ossietzky University of Oldenburg by the Deutsche Forschungsgemeinschaft (INST 184/106-1 FUGG and INST 184/144-1 FUGG) is acknowledged. B.M. and H.W.C. performed the SFA experiments. C.M. and Q.H. helped with the experiments and produced thin films. B.M., Q.H., and M.O. provided XPS measurements. M.V. and F.U.R. conceived and designed the experiments. M.S. designed and carried out theoretical kinetic analysis. All authors analyzed data, discussed the results, co-wrote the paper, and commented on the manuscript.

Reactivity in confinement is central to a wide range of applications and systems, yet it is notoriously difficult to probe reactions in confined spaces in real time. Using a modified electrochemical surface forces apparatus (EC-SFA) on confined metallic surfaces, we observe in situ nano- to microscale dissolution and pit formation (qualitatively similar to previous observation on nonmetallic surfaces, e.g., silica) in well-defined geometries in environments relevant to corrosion processes. We follow “crevice corrosion” processes in real time in different pH-neutral NaCl solutions and applied surface potentials of nickel (vs. Ag|AgCl electrode in solution) for the mica–nickel confined interface of total area ∼0.03 mm2. The initial corrosion proceeds as self-catalyzed pitting, visualized by the sudden appearance of circular pits with uniform diameters of 6–7 μm and depth ∼2–3 nm. At concentrations above 10 mM NaCl, pitting is initiated at the outer rim of the confined zone, while below 10 mM NaCl, pitting is initiated inside the confined zone. We compare statistical analysis of growth kinetics and shape evolution of individual nanoscale deep pits with estimates from macroscopic experiments to study initial pit growth and propagation. Our data and experimental techniques reveal a mechanism that suggests initial corrosion results in formation of an aggressive interfacial electrolyte that rapidly accelerates pitting, similar to crack initiation and propagation within the confined area. These results support a general mechanism for nanoscale material degradation and dissolution (e.g., crevice corrosion) of polycrystalline nonnoble metals, alloys, and inorganic materials within confined interfaces.

Tuning chemical structure and molecular layering of ionic liquids (IL) at solid interfaces offers leverage to tailor performance of ILs in applications such as super-capacitors, catalysis or lubrication. Recent experimental interpretations suggest that ILs containing cations with long hydrophobic tails form well-ordered bilayers at interfaces. Here we demonstrate that interfacial bilayer formation is not an intrinsic quality of hydrophobic ILs. In contrast, bilayer formation is triggered by boundary conditions including confinement, surface charging and humidity present in the IL. Therefore, we performed force versus distance profiles using atomic force microscopy and the surface forces apparatus. Our results support models of disperse low-density bilayer formation in confined situations, at high surface charging and/or in the presence of water. Conversely, interfacial structuring of long-chain ILs in dry environments and at low surface charging is disordered and dominated by bulk structuring. Our results demonstrate that boundary conditions such as charging, confinement and doping by impurities have decisive influence on structure formation of ILs at interfaces. As such, these results have important implications for understanding the behavior of solid/IL interfaces as they significantly extend previous interpretations.

Tuning chemical structure and molecular layering of ionic liquids (IL) at solid interfaces offers leverage to design performance of ILs in emerging technologies including super-capacitors, heterogeneous catalysis or lubrication. Recent experimental interpretations suggest that ILs containing cations with long hydrophobic tails form well-ordered bilayers at interfaces. Here we demonstrate that interfacial bilayer formation is insignificant and not an intrinsic quality of hydrophobic ILs. In contrast, bilayer formation is triggered by boundary conditions including confinement and humidity present in the IL. Therefore, we performed force versus distance profiles using atomic force microscopy and the surface forces apparatus. Our results support models of disperse low-density bilayer formation in confined situations, and at high surface charging in the presence of water. Conversely, interfacial structuring of long-chain ILs in dry environments and at low surface charging is disordered and bulk structuring also dominates interface structures in confinement. Our results demonstrate that boundary conditions such as charging, confinement and the presence of impurities drive structure formation of ILs at interfaces, while at low surface charge solvophobic interactions have a much weaker influence on interface structuring than generally believed. These results have important implications for understanding solid/IL interfaces and clarify previous interpretations.

The unsatisfactory behaviour of the solid electrolyte interphase (SEI) on silicon-based anodes still prevents the commercial introduction of silicon in lithium-ion batteries – a milestone which would be a huge leap forward towards a sustainable energy use.[1] Our group has recently developed an electrolyte mixture improving the cycling performance of a silicon-based lithium-ion battery. For understanding the stabilizing effect of this novel electrolyte on the SEI, advanced characterization techniques had to be employed, such as a modified Surface Forces Apparatus (SFA) and synchrotron-based Hard X-ray Photoelectron Spectroscopy (HAXPES). So far, the SFA was used to research fundamental biologic and solution based interfacial processes.[2] Here we present a detailed investigation of the SEI on a silicon anode by designing a battery SFA cell for in-situ electrochemical measurements. This enabled us to monitor the SEI thickness during the different stages of the SEI formation on both silicon and graphite surfaces. Additionally, we report the mechanical properties of the SEI for different electrolytes which can also be addressed by the SFA setup. HAXPES data collected on a modified SEI on silicon based anodes reveal a complex nanostructure and provide an additional insight into the stabilization mechanism of the electrolyte mixture. Coin-cell testing confirm the stabilizing effect of the novel electrolyte mixture on the battery performance. The electrochemical battery SFA will be an important tool for evaluating novel electrolyte compositions and their effect on the mechanical properties and growth mechanism of the interfacial SEI. The introduction of this in-situ battery test cell for SFA opens the way for both fundamental interfacial insights as well as applied material studies in battery research. REFERENCES 1. E.C. Evarts, To the limits of lithium, Nature, 2015, 526, 593-595. 2. J. Israelachvili et al., Recent advances in the surface forces apparatus (SFA) technique, Rep. Prog. Phys., 2010, 73, 1-16. Figure 1

Corrosion of nickel and its alloys in confined areas remains a serious concern for the safety of structural materials. We uniquely combined white light interferometry and microscopy with high spatial and time resolution for a real-time study of the corrosion mechanism of confined nickel surfaces in alkali metal chloride (e.g., NaCl) electrolytes (c = 1 – 100 mM) at pH = 7. A mica surface is used as crevice former in the electrochemical surface forces apparatus to establish 600-1000 µm2 confined dry areas on extended nickel surfaces. NaCl is the wetting medium of the confined zones and the electrolyte layer spreads out along all the surface. The nickel surfaces form interfacial oxides films at the beginning when no electrochemical potential is applied. Subsequent application of an electrochemical potential ramp from -400 mV to +800 mV initiates severe corrosion of the formed crevices. Surprisingly, the corrosion inside the crevices proceeds as preferential and self-catalyzed “pitting” corrosion (inhomogeneous localized dissolution with the formation of pits) within in the confined zone. Moreover, different behavior of the Nickel corrosion is observed depending on the concentration: inside the confined zone at low concentration (10mM). Our data reveal a surprising and detailed view into the initial corrosion mechanism of nickel surfaces in confined zones, and may contribute to a deeper understanding and, ultimately, prevention of localized corrosion of other metallic surfaces. A double-layer theory coupled with diffusion-limited effects provides a plausible explanation for the observed phenomena.

Crevice corrosion (CC) of engineering metals remains a serious concern for structural materials. Yet combining both, a real-time and high-resolution in-situ visualization of corrosion within confined geometries remains very challenging. In this contribution, we will discuss how multiple-beam interferometry (MBI) and simultaneous microscopy can be utilized to directly visualize corrosion processes in real-time and with Ångstrom resolution within well-defined confinement geometries. Here we will directly compare results of accelerated corrosion tests of confined Aluminum and Nickel surfaces in a modified surface forces apparatus1. With MBI, we can detect and track active sites of aluminum corrosion in NaCl solution within confined geometries in real time. We find that CC of aluminum randomly initiates in the confined crevice mouth, where the distance between apposing surfaces is between 20-300 nm. We can directly track oxide dissolution/formation, and corrosion-rates as well as their retardation due to sodium vanadate inhibitors present in solution. Formation of a compacted oxide layer effectively inhibits CC in 5 mM NaCl solutions with 2.5 mM of added NaVO3, while inhibition rapidly breaks down at chloride concentrations above 50 mM. Breakdown of the inhibition-layers is initiated by rapid dissolution of the protective oxide within the confined zone. Interestingly, Nickel CC shows distinctly different and unexpected corrosion mechanism depending on the NaCl concentration. We find that low Chloride concentration lead to “preferential pitting corrosion” inside of confined zones, while high Chloride concentrations lead to “crevice corrosion in the crevice mouth”. We will discuss these surprising results and we will show real time high-resolution videos of CC in confined geometries that detail this behavior. Literature 1. Shrestha, B. R.; Hu, Q.; Baimpos, T.; Kristiansen, K.; Israelachvili, J. N.; Valtiner, M., Real-Time Monitoring of Aluminum Crevice Corrosion and Its Inhibition by Vanadates with Multiple Beam Interferometry in a Surface Forces Apparatus. Journal of the Electrochemical Society 162 2015, C327-C332