Film-substrate Interaction Research Articles

0‐3 magnetic nanocomposites via EPD: Current status for power component fabrication and future directions

AbstractInductors and transformers (here referred to as power components) for modern AC/DC switching power supplies require magnetic materials that have high power density and efficiency at high frequencies, with high magnetic saturation, low coercivity, and multi‐micrometer thicknesses to increase magnetic energy storage and power handling. Rather than using a single‐phase magnetic material in a polymer‐based composite, a composite formed from two magnetic phases (such as a 0‐3 nanocomposite) can simultaneously achieve all of the listed requirements and benefit from contributions by both the zero‐ and three‐dimensional phases to the magnetic properties. The fabrication of 0‐3 magnetic nanocomposites for power component applications requires a method to deposit magnetic nanoparticles into thick, physically stable yet porous films, and a subsequent method for infiltrating the magnetic nanoparticle film with another magnetic material. Here, the deposition of magnetic nanoparticles into micron‐thick films using electrophoretic deposition (EPD) is discussed. This is described along with a new method, to improve upon traditional EPD methods by increasing film–substrate interactions with chelating agents, therefore increasing film stability. Next, the use of electro‐infiltration for fully incorporating a secondary magnetic material within the nanoparticle film is presented, showing the cumulative fabrication process with the addition of a multilayered nanocomposite fabrication technique for increasing overall nanocomposite thickness. The subsequent cross‐sectional and magnetic characterization of the fabricated 0‐3 nanocomposites is also shown. Finally, future directions for 0‐3 magnetic nanocomposites are offered, with emphasis on potential materials synthesis techniques and on translating knowledge beyond power component applications.

Intrinsic flexoelectricity of van der Waals epitaxial thin films

The direct measurement of flexoelectric coefficients in epitaxial thin films is an unresolved problem, due to the clamping effect of substrates which induces a net strain (and hence parasitic piezoelectricity) in addition to strain gradients and flexoelectricity. Herein, we propose and demonstrate the use of van der Waals epitaxy as a successful strategy for measuring the intrinsic (clamping-free = flexoelectric coefficients of epitaxial thin films. We have made, measured, and compared ${\mathrm{BaTiO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$ thin film capacitor heterostructures grown both by conventional oxide-on-oxide epitaxy and by van der Waals oxide-on-mica epitaxy, and found that, whereas the former is dominated by parasitic piezoelectricity, the response of the latter is truly flexoelectric. The results are backed by theoretical calculations of the film-substrate mechanical interaction, as well as by direct measurements that confirm the strain-free state of the films. van der Waals epitaxy thus emerges as powerful new tool in the study of flexoelectricity and, in particular, they finally allow exploring flexoelectric phenomena at the nanoscale (where strain gradients are highest) with direct experimental knowledge of the actual flexoelectric coefficients of thin films.

Predicted Impacts of Catalyst Layer Ionomer Microstructure and Distribution on PEMFC Performance

Electric vehicles powered by proton exchange membrane fuel cells (PEMFCs) are a promising means to reduce environmentally harmful emissions. PEMFCs offer low weight, high range (miles traveled before refueling), and quick refueling times; however, persistent technical challenges limit commercialization of fuel cell electric vehicles (FCEVs). In addition to system-level challenges such as an immature refueling infrastructure and H2 production, it remains challenging to manufacture low-cost PEMFCs that meet durability and performance requirements. Addressing the performance issues of low-cost PEMFCs has proven difficult due to a gap in understanding physiochemical resistances within the heterogeneous catalyst layer (CL).Electrochemical reactions occurring in the CL—and therefore PEMFC performance—depend in complex ways upon the CL microstructure. Reactants and products are transported to and from Pt catalysts through nano-thin films of Nafion ionomer, which coat carbon-dispersed Pt catalysts. Through the use of neutron reflectometry (NR) [1,2] and conductivity measurements [3], previous studies show that the structure and material transport properties of Nafion are linked and depend on multiple factors including film thickness, substrate interactions, temperature, and relative humidity. Previously [4], we developed pseudo-2D Newman-type computational models that incorporate novel structure-property relationships derived from these studies. The results concluded that increasing the ionomer film thickness surrounding Pt-covered carbon particles in the CL may lead to a higher PEMFC performance due to an increased conductivity caused by increased ionomer volume fraction and increased Nafion water absorption.In this presentation, we extend our previous model to further advance understanding of how ionomer structure and distribution influence PEMFC performance. The current modeling efforts include two-phase water transport to understand the role of flooding in performance limits under low-Pt loading. Additionally, structure-property relationships from our previous work are updated to include more relevant structures taken from NR experiments performed with Nafion on technologically relevant substrates (Pt and carbon). Presented results will demonstrate the predicted impacts of non-uniform ionomer distribution within the cathode CL, including novel design concepts such as ionomer-rich regions to enhance ion transport for increased catalyst utilization further from the membrane. These predicted impacts provide insight for future PEMFC design modifications to accelerate FCEV commercialization.[1] J. A. Dura, V. S. Murthi, M. Hartman, S. K. Satija, and C. F. Majkrzak, “Multilamellar Interface Structures in Nafion,” Macromolecules, vol. 42, no. 13, pp. 4769–4774, 2009.[2] S. C. DeCaluwe, A. M. Baker, P. Bhargava, J. E. Fischer, and J. A. Dura, “Structure-property relationships at Nafion thin-film interfaces: Thickness effects on hydration and anisotropic ion transport,” Nano Energy, vol. 46, pp. 91–100, 2018.[3] D. K. Paul, R. McCreery, and K. Karan, “Proton Transport Property in Supported Nafion Nanothin Films by Electrochemical Impedance Spectroscopy,” Journal of The Electrochemical Society, vol. 161, no. 14, 2014.[4] C. R. Randall and S. C. DeCaluwe, “Physically Based Modeling of PEMFC Cathode Catalyst Layers: Effective Microstructure and Ionomer Structure-Property Relationship Impacts,” Journal of Electrochemical Energy Conversion and Storage, vol. 17, no. 4, 2020.

Graphene-substrate decoupling by S segregation. A LEEM/LEED study

The growth of graphene on Ni(110) is studied with low energy electron microscopy, low energy electron diffraction (LEED) and associated methods at several sulfur coverages obtained by surface segregation and compared with growth on the clean surface. On the clean surface graphene grows nearly epitaxial with small azimuthal alignment fluctuations. The segregated S reduces the interaction between graphene and Ni, which results in the formation of several azimuthal orientations. The reduced film-substrate interaction is evident in the details of the LEED pattern, in the work function change and the energy dependence of the specular reflected intensity. This study clarifies the differences between earlier studies of graphene growth on Ni(110) and suggests using impurity surface segregation as means for decoupling of graphene from the substrate.

Chelating Agent Functionalized Substrates for the Formation of Thick Films via Electrophoretic Deposition.

Incorporating nanoparticles into devices for a wide range of applications often requires the formation of thick films, which is particularly necessary for improving magnetic power storage, microwave properties, and sensor performance. One approach to assembling nanoparticles into films is the use of electrophoretic deposition (EPD). This work seeks to develop methods to increase film thickness and stability in EPD by increasing film-substrate interactions via functionalizing conductive substrates with various chelating agents. Here, we deposited iron oxide nanoparticles onto conductive substrates functionalized with three chelating agents with different functional moieties and differing chelating strengths. We show that increasing chelating strength can increase film-substrate interactions, resulting in thicker films when compared to traditional EPD. Results will also be presented on how the chelating strength relates to film formation as a function of deposition conditions. Yield for EPD is influenced by deposition conditions including applied electric field, particle concentration, and deposition time. This work shows that the functionalization of substrates with chelating agents that coordinate strongly with nanoparticles (phosphonic acid and dopamine) overcome parameters that traditionally hinder the deposition of thicker and more stable films, such as applied electric field and high particle concentration. We show that functionalizing substrates with chelating agents is a promising method to fabricate thick, stable films of nanoparticles deposited via EPD over a larger processing space by increasing film-substrate interactions.

Early-stage growth observations of orientation-controlled vacuum-deposited naphthyl end-capped oligothiophenes

We report on the real-time structure formation and growth of two thiophene-based organic semiconductors, $5,{5}^{\ensuremath{'}}$-bis(naphth-2-yl)-$2,{2}^{\ensuremath{'}}$-bi- and $5,{5}^{\ensuremath{'}\ensuremath{'}}$-bis(naphth-2-yl)-$2,{2}^{\ensuremath{'}}$:${5}^{\ensuremath{'}},{2}^{\ensuremath{'}\ensuremath{'}}$-terthiophene (NaT2 and NaT3), studied in situ during vacuum deposition by grazing-incidence x-ray diffraction and supported by atomic force microscopy and photoabsorption spectroscopy measurements on corresponding ex situ samples. On device-relevant silicon dioxide substrates, for both molecules the growth is observed to transition from two-dimensional (2D) layer-by-layer growth to three-dimensional (3D) growth after the formation of a few-molecule-thick wetting layer. The crystal structure of the NaT2 film is considerably more ordered than the NaT3 counterpart, and there is a significant collective change in the unit cell during the initial stage of growth, indicating strain relief from substrate induced strain as the growth transitions from two to three dimensions. In addition, the orientation of the film molecules are controlled by employing substrates of horizontally and vertically oriented few-layer molybdenum disulfide. Both molecules form needle-like crystals on horizontally oriented ${\mathrm{MoS}}_{2}$ layers, while the NaT3 molecules form tall, isolated islands on vertically oriented ${\mathrm{MoS}}_{2}$ layers. The molecules are standing on silicon dioxide and on vertically oriented ${\mathrm{MoS}}_{2}$, but lying flat on horizontally oriented ${\mathrm{MoS}}_{2}$. These results demonstrate the importance of film-substrate interactions on the thin-film growth and microstructure formation in naphthyl-terminated oligothiophenes.

(Invited) Developing the Electrodeposition of High Energy Density Anodes for 3D Architectures for Rechargeable Batteries

As Li-ion batteries continue to improve in all metrics, new markets are now opening to portable energy. However, the fundamental challenges are still the same: how do we improve how much energy a particular material can store, how quickly it can charge and discharge safely, while enabling effective communication between all components of a particular cell? It is not sufficient to simply discover and improve new materials, we have to also consider the development of efficient, manufacturable architectures that enable the best performance for a particular material.We are working to incorporate high surface area structures of novel anode materials into a 3D interdigitated solid-state battery architecture. The significant advantage to this architecture is that the diffusion length for Li+between the cathode and anode will be dramatically reduced, which should lead to much faster charging rates. The structure and composition of the anode is critical to the energy density, power density, cycle life, and safety of the overall battery. I will discuss electrodeposited M-Sb (where M = Ni, Cu, Zn) thin films and high surface area structures that have been investigated as alloy anodes for Li-ion batteries to elucidate the effects of both the film composition and substrate interactions on cycling stability and lifetime. The results discussed will demonstrate that important composition and substrate factors for anode cycling performance are being developed that are likely applicable to other systems. A key theme in this work is how to develop materials architectures that not only have attractive properties for the chosen application (energy storage), but that are also ideally suited to elucidating and understanding degradation mechanisms for high energy density materials. I will also discuss my opinions on the need for integrating a big-picture view of manufacturability for applied materials research, as well as the challenges and opportunities in moving research and development from an academic lab into the marketplace.

The role of the substrate on the mechanical and thermal stability of Pd thin films during hydrogen (de)sorption

In this work, we studied the mechanical and thermal stability of ~100 nm Pd thin films magnetron sputter deposited on a bare oxidized Si(100) wafer, a sputtered Titanium (Ti) intermediate layer, and a spin-coated Polyimide (PI) intermediate layer. The dependence of the film stability on the film morphology and the film-substrate interaction was investigated. It was shown that a columnar morphology with elongated voids at part of the grain boundaries is resistant to embrittlement induced by the hydride formation (α↔β phase transitions). For compact film morphology, depending on the rigidity of the intermediate layer and the adherence to the substrate, complete transformation (Pd-PI-SiO2/Si) or partly suppression (Pd-Ti-SiO2/Si) of the α to β-phase was observed. In the case of Pd without intermediate layer (Pd-SiO2/Si), buckling delamination occurred. The damage and deformation mechanisms could be understood by the analysis of the stresses and dislocation (defects) behavior near grain boundaries and the film-substrate interface. From diffraction line-broadening combined with microscopy analysis, we showed that in Pd thin films, stresses relax at critical stress values via different relaxation pathways depending on film-microstructure and film-substrate interaction. On the basis of the in-situ hydriding experiments, it was concluded that a Pd film on a flexible PI intermediate layer exhibits free-standing film-like behavior besides being strongly clamped on a stiff SiO2/Si substrate.

Surface‐enhanced resonance Raman scattering in partially oxidized thin copper film

AbstractBroad and low‐intensity Raman peaks are usually expected from nanocrystalline thin semiconductor films. The inherently weak Raman scattering phenomenon can be further deteriorated by unwanted background signals preventing the successful Raman analysis of an analyte. In this study, the resonant and surface‐enhanced Raman scattering techniques were combined to detect CuO and Cu2O phases in the partially oxidized nanocrystalline copper film that were otherwise undetectable. Heat treatment resulted in increased oxidation and phase transition from multiphase to single CuO phase that was in situ observed by temperature‐dependent Raman measurements. Detailed understanding of the film properties and substrate interaction was made by using several characterization techniques.

The Coalescence Behavior of Two-Dimensional Materials Revealed by Multiscale In Situ Imaging during Chemical Vapor Deposition Growth.

Wafer-scale monocrystalline two-dimensional (2D) materials can theoretically be grown by seamless coalescence of individual domains into a large single crystal. Here we present a concise study of the coalescence behavior of crystalline 2D films using a combination of complementary in situ methods. Direct observation of overlayer growth from the atomic to the millimeter scale and under model- and industrially relevant growth conditions reveals the influence of the film-substrate interaction on the crystallinity of the 2D film. In the case of weakly interacting substrates, the coalescence behavior is dictated by the inherent growth kinetics of the 2D film. It is shown that the merging of coaligned domains leads to a distinct modification of the growth dynamics through the formation of fast-growing high-energy edges. The latter can be traced down to a reduced kink-creation energy at the interface between well-aligned domains. In the case of strongly interacting substrates, the lattice mismatch between film and substrate induces a pronounced moiré corrugation that determines the growth and coalescence behavior. It furthermore imposes additional criteria for seamless coalescence and determines the structure of grain boundaries. The experimental findings, obtained here for the case of graphene, are confirmed by theory-based growth simulations and can be generalized to other 2D materials that show 3- or 6-fold symmetry. Based on the gained understanding of the relation between film-substrate interaction, shape evolution, and coalescence behavior, conditions for seamless coalescence and, thus, for the optimization of large-scale production of monocrystalline 2D materials are established.

Assessing the film-substrate interaction in germania films on reconstructed Au(111)

Purely amorphous germania bilayer films are grown on a reconstructed Au(111) surface. The presence of the film affects the native configuration of the Au soliton walls, as observed with scanning tunneling microscopy. They partly avoid the film islands, and partly penetrate under film patches. This behavior indicates a weaker film-substrate interaction than the one reported for other oxide films on reconstructed Au(111). Moreover, this new system highlights the impact of the metal support on the structure of ultrathin films of germania: With decreasing film-substrate interaction the amorphous phase is promoted. Density functional theory calculations confirm and rationalize the experimental observations. This work provides a useful generalization of the relationship between film structure and adhesion energy.

Carrier lifetime enhancement in halide perovskite via remote epitaxy

Crystallographic dislocation has been well-known to be one of the major causes responsible for the unfavorable carrier dynamics in conventional semiconductor devices. Halide perovskite has exhibited promising applications in optoelectronic devices. However, how dislocation impacts its carrier dynamics in the ‘defects-tolerant’ halide perovskite is largely unknown. Here, via a remote epitaxy approach using polar substrates coated with graphene, we synthesize epitaxial halide perovskite with controlled dislocation density. First-principle calculations and molecular-dynamics simulations reveal weak film-substrate interaction and low density dislocation mechanism in remote epitaxy, respectively. High-resolution transmission electron microscopy, high-resolution atomic force microscopy and Cs-corrected scanning transmission electron microscopy unveil the lattice/atomic and dislocation structure of the remote epitaxial film. The controlling of dislocation density enables the unveiling of the dislocation-carrier dynamic relation in halide perovskite. The study provides an avenue to develop free-standing halide perovskite film with low dislocation density and improved carried dynamics.

Interlayer Difference of Bilayer-Stacked MoS2 Structure: Probing by Photoluminescence and Raman Spectroscopy.

This work reports the interlayer difference of exciton and phonon performance between the top and bottom layer of a bilayer-stacked two-dimensional materials structure (BSS). Through photoluminescence (PL) and Raman spectroscopy, we find that, compared to that of the bottom layer, the top layer of BSS demonstrates PL redshift, Raman mode redshift, and lower PL intensity. Spatial inhomogeneity of PL and Raman are also observed in the BSS. Based on theoretical analysis, these exotic effects can be attributed to substrate-coupling-induced strain and doping. Our findings provide pertinent insight into film–substrate interaction, and are of great significance to researches on bilayer-stacked structures including twisted bilayer structure, Van der Waals hetero- and homo-structure.

Direct Comparison of PdAu Alloy Thin Films and Nanoparticles upon Hydrogen Exposure.

Nanostructured metal hydrides are able to efficiently detect hydrogen in optical sensors. In the literature, two nanostructured systems based on metal hydrides have been proposed for this purpose each with its own detection principle: continuous sub-100 nm thin films read out via optical reflectance/transmittance changes and nanoparticle arrays for which the detection relies on localized surface plasmon resonance. Despite their apparent similarities, their optical and structural response to hydrogen has never been directly compared. In response, for the case of Pd1–yAuy (y = 0.15–0.30) alloys, we directly compare these two systems and establish that they are distinctively different. We show that the dissimilar optical response is not caused by the different optical readout principles but results from a fundamentally different structural response to hydrogen due to the different nanostructurings. The measurements empirically suggest that these differences cannot be fully accounted by surface effects but that the nature of the film–substrate interaction plays an important role and affects both the hydrogen solubility and the metal-to-metal hydride transition. In a broader perspective, our results establish that the specifics of nanoconfinement dictate the structural properties of metal hydrides, which in turn control the properties of nanostructured devices including the sensing characteristics of optical hydrogen sensors and hydride-based active plasmonic systems.

Transport properties of a few nanometer-thick TiSe2 films grown by molecular-beam epitaxy

Layered materials are known to exhibit a variety of charge-density wave (CDW) phases due to their quasi-two dimensional nature. Of particular interest is the CDW phase in a prototypical layered transition-metal dichalcogenide (TMDC) TiSe2, where the CDW is known to form with commensurate 2a × 2a × 2c structural distortion at T = 200 K (where a and c are the lattice parameters). Recent experimental studies have revealed intriguing aspects of this material as represented by the emergence of superconductivity upon electron doping and possible existence of the excitonic insulator phase, making TiSe2 attractive as a model material for investigation of collective phenomena in TMDC. However, the evolution of the CDW phase at nanometer-scale thickness, at least below 10 monolayers (6 nm), has not been well investigated yet, in particular from transport viewpoints, presumably due to difficulty in fabrication of such ultrathin samples by conventional approaches. Here, we report the transport properties of a few nanometer-thick highly crystalline TiSe2 epitaxial thin films grown on insulating Al2O3 substrates by molecular-beam epitaxy, demonstrating robust CDW transitions down to 5 monolayers (3 nm). We also clarify an interesting aspect of van der Waals epitaxy, a “self-rotational” growth without strain, which should be realized only in a system having a weak substrate-film interaction.

Growth of <i>a</i>-axis Textured Pulsed Laser Deposited VO<SUB>2</SUB> Nanostructures on Glass Substrate

Vanadium dioxide thin film nanostructures were synthesized by pulsed laser deposition on soda lime glass at a substrate temperature of 600°C and an oxygen ambient pressure of 15 mTorr. The effect of cooling pressure on the crystalline orientation of VO2 nanostructures was investigated. As the cooling oxygen pressure is increased, the VO2 nanostructures exhibit sharp a-axis diffraction peaks, showing the growth of (1 0 0) oriented VO2 on glass, which is characteristic of the VO2 monoclinic phase and implies that pure highly a-axis textured VO2 was formed. We found that the growth mechanism and substrate–film interaction play important roles in the development of these well-textured films via the formation of an interlayer of SiO2.

Ionomer Thin Films for PEM Fuel Cells

Ion-conductive polymers (ionomers) such as perfluorosulfonic-acid (PFSA) have been widely adopted in polymer electrolyte fuel cells (PEFCs) as a polymer-electrolyte membrane (PEM) due to their ability to conduct ions in hydrated conditions without losing stability. The role of ionomers in PEFCs, however, is not limited to PEM as it also extends to the porous catalyst layers (CL) where ionomers exist as nanometer-thick electrolyte thin films binding the catalytic agglomerates and facilitating proton transport. Moreover, the ionomer’s properties differ from the bulk (PEM) when it is confined to these nanometer thicknesses where its behavior is influenced by its interfaces with the air and the catalytic sites, such as carbon/platinum. In particular, this phenomenon of confinement onto Pt sites could have important implications for the mass-transport limitations observed in fuel-cell catalyst layers, which have been related to the transport resistances at the ionomer thin-film. Thus, despite an ever-growing number of studies on ionomer membranes for PEM, for desired next-generation PEFCs, it is important to characterize catalyst ionomers and elucidate factors controlling their transport properties. In particular, if is of great interest to develop model systems for thin films that can mimic catalyst ionomer behavior and establish their structure/functionality to optimize ionomer interfaces and improve CL performance. This tutorial talk will present an overview of PFSA ionomer thin films and their current state of understanding with a focus on recent developments in advanced diagnostics and the information they can provide to establish their structure/function relationship. In addition, applicability of the various diagnostics techniques in the thin-film regime (5 to 500 nm) will be discussed, including the use of state-of-the-art techniques, such as Grazing-Incidence X-ray scattering (GIXS). First, thin films of dispersion-cast ionomers on model substrates will be explained in relation to catalyst ionomers. Swelling and water uptake behavior as well as various transport properties of these thin films will be examined. Then, it will be discussed how an ionomer’s phase-separated nanostructure and properties are correlated and impacted by film thickness (i.e., confinement effect) and substrate interactions, especially below 50 nm. Lastly, it will be explored how EW and side-chain chemistry influence the nature of ionomer-substrate interactions and thus the structure and properties of PFSA thin films. The collected data set will be analyzed to illustrate the bulk-to-film transition of ionomers of various equivalent weights used in fuel-cells. The results will be elucidated to highlight the key aspects of ionomer thin films, provide insight into optimization of their structure and functionalities for fuel-cell catalysts, and discuss implications for PEFC performance.

Effect of tacticity on the structure and glass transition temperature of polystyrene adsorbed onto solid surfaces.

We have carried out atomistic (all-atom) molecular dynamics simulations to investigate the effect of tacticity on the structure and glass transition temperature (Tg) of polystyrene (PS) thin films adsorbed on two distinct types of solid substrates. The systems consist of thin films made of atactic, isotactic, and syndiotactic PS chains supported by graphite or hydroxylated α-quartz substrates, which are known to be atomically flat but chemically and structurally different. We have observed a marked dependence of the film structure on substrate type as well as on tacticity. For instance, rings' orientation near substrate surfaces depends on substrate type for atactic PS and isotactic PS films, while no such dependence is observed for syndiotactic PS films whose interfacial structure seems to result from their propensity to adopt the trans conformation rather than their specific interaction with the substrates. Moreover, our results indicate that glass transition temperatures of substrate supported polystyrene films are higher compared to those of the corresponding free-standing films. More specifically, PS films on graphite exhibit larger Tg values than those on α-quartz, and we have noticed that syndiotactic PS has the largest Tg irrespective of the substrate type. Furthermore, the local Tg in the region of the film in contact with the substrates shows a strong tacticity and substrate dependence, whereas no dependencies were found for the local Tg in the middle of the film. Substrate-film interaction energy and chains' dynamics near substrate-film interfaces were subsequently investigated in order to substantiate the obtained Tgs, and it was found that films with higher Tgs are strongly adsorbed on the substrates and/or exhibit smaller interfacial chains' dynamics essentially due to steric hindrance.

Phonons and electron-phonon anomalies in ultra-thin Pb films on Si(111) and Ge(111)

The surface phonon dispersion curves of ultrathin (3–6 ML) Pb layers grown on Si(111) and Ge(111) substrates, measured with inelastic He atom scattering, reveal shallow Kohn anomalies in the surface acoustic branches. Ab-initio calculations based on density functional perturbation theory for the 3–6 monolayers of Pb on a rigid substrate, besides reproducing well the observed dispersion curves, allow for the assignment of the anomalies to well defined flat segments of the Fermi contours. Unlike Pb films grown on Cu(111), where both the optical surface and the interface phonon branches above the bulk frequency maximum are observed, for Si(111) and Ge (111) only the surface-localized branch is observed, indicating the comparatively weaker film-substrate interaction. Consistently with the observation of superconductivity in ultrathin Pb films on Si(111) down to one monolayer (wetting layer), the mass-enhancement factor derived from the specular scattering intensity dependence on the film thickness is found for both substrates in the range of unity, however with slightly larger values for the Ge(111) substrate.

Moisture effect on interfacial integrity of epoxy-bonded system: a hierarchical approach

The epoxy-bonded system has been widely used in various applications across different scale lengths. Prior investigations have indicated that the moisture-affected interfacial debonding is the major failure mode of such a system, but the fundamental mechanism remains unknown, such as the basis for the invasion of water molecules in the cross-linked epoxy and the epoxy-bonded interface. This prevents us from predicting the long-term performance of the epoxy-related applications under the effect of the moisture. Here, we use full atomistic models to investigate the response of the epoxy-bonded system towards the adhesion test, and provide a detailed analysis of the interfacial integrity under the moisture effect and the associated debonding mechanism. Molecular dynamics simulations show that water molecules affect the hierarchical structure of the epoxy-bonded system at the nanoscale by disrupting the film-substrate interaction and the molecular interaction within the epoxy, which leads to the detachment of the epoxy thin film, and the final interfacial debonding. The simulation results show good agreement with the experimental results of the epoxy-bonded system. Through identifying the relationship between the epoxy structure and the debonding mechanism at multiple scales, it is shown that the hierarchical structure of the epoxy-bonded system is crucial for the interfacial integrity. In particular, the available space of the epoxy-bonded system, which consists of various sizes ranging from the atomistic scale to the macroscale and is close to the interface facilitates the moisture accumulation, leading to a distinct interfacial debonding when compared to the dry scenario.