Thin Interfacial Region Research Articles

The dielectric function profile across the water interface through surface-specific vibrational spectroscopy and simulations

The dielectric properties of interfacial water on subnanometer length scales govern chemical reactions, carrier transfer, and ion transport at interfaces. Yet, the nature of the interfacial dielectric function has remained under debate as it is challenging to access the interfacial dielectric with subnanometer resolution. Here we use the vibrational response of interfacial water molecules probed using surface-specific sum-frequency generation (SFG) spectra to obtain exquisite depth resolution. Different responses originate from water molecules at different depths and report back on the local interfacial dielectric environment via their spectral amplitudes. From experimental and simulated SFG spectra at the air/water interface, we find that the interfacial dielectric constant changes drastically across an ∼1 Å thin interfacial water region. The strong gradient of the interfacial dielectric constant leads, at charged planar interfaces, to the formation of an electric triple layer that goes beyond the standard double-layer model.

Understanding Degradation Mechanism between Garnet Solid Electrolyte Li7La3Zr2O12 and LiNi0.6Mn0.2Co0.2O2 Cathode

All-solid-state Li-ion batteries (ASSLBs) using oxide based solid electrolytes have been considered as a promising candidates for the next generation batteries due to their excellent thermal and electrochemical stability. Especially, garnet-type Li7La3Zr2O12 (LLZO) has gained much attention due to its good conductivity and large electrochemical window. Since the electrochemical window of LLZO allows usage of high voltage cathodes such as NMC (LiNixMnyCo1-x-yO2) and Li anode, it is possible to design high voltage, high capacity and safe energy storage medium using the eletrolyte.However, current attempts on exploiting LLZO as a solid electrolyte have been hindered by high resistance at the interface, especially between cathode and the solid electrolyte. Even though the importance of the issue is well known, there has not been a comprehensive understanding yet on the topic. The lack of understanding comes from the difficulty of characterizing buried interface. It is difficult to study thin interfacial region between cathode and solid electrolyte with conventional techniques since the signal from the interface is often overwhelmed by the bulk, and destructive characterization techniques ruin the signal itself.We developed model system by sputtering thin-film cathode on top of pre-prepared solid state electrolyte pellets to overcome this problem. Samples were annealed afterwards to simulate sintering process. Since the interfacial region is near surface, we could use XANES and EXAFS which are surface-sensitive and non-destructive. Those techniques gave us valuable information regarding oxidation state and local chemical environment of each element, which were used to evaluate migration tendency. Moreover, those results can be used to precisely characterize secondary phases by complementing complex XRD results. Results obtained from those characterization techniques were correlated with electrochemical analysis techniques such as EIS and chronovoltammetry to evaluate the effect of the interfacial degradation on cell performance.In this work, we aimed to obtain fundamental understanding on the instability of cathode|electrolyte interface during two different stages: manufacturing stage and operation stage. We systematically varied variables related to manufacture (Sintering temperature, sintering time, gas environment during sintering), and operation (Electrochemical potential, current density). We chose NMC622 (LiNi0.6Mn0.2Co0.2O2) as a cathode, and Al-doped cubic LLZO (Li7La3Zr2O12) as a solid electrolyte.To study degradation of NMC622|LLZO during manufacturing stage, we varied temperature condition (300°C, 500°C, 700°C for 4h respectively) and gas environment (air, O2, humidified O2, N2, CO2) during annealing. Samples annealed in air showed high interfacial resistance and poor performance, which are signs for interfacial degradation. At 700°C, formation of detrimental phase such as La2Zr2O7 and La(Ni,Co)O3 led to complete blockage of Li ions. XANES and EXAFS showed severe change of oxidation state and chemical environment of Ni and Co, which indicated that those species escaped out of the cathode and participate in the secondary phase formation. In contrast, NMC622|LLZO interface remained chemically stable up to 700°C with no formation of detrimental phase when the sample was annealed in O2. On the other hand, when the sample was annealed in humidified O2 at 700°C, detrimental phase(La2Zr2O7) could be found. In addition, we could characterize substantial amount of Li2CO3 from samples annealed in humidified O2, which could have formed by reaction between LiOH and CO2 during sample characterization done in air. Annealing in CO2 condition led to the crystallization of detrimental phases such as Li2CO3, La2O2CO3, NiCO3 at 500°C. In addition, we could see formation of La2Zr2O7 and La2(Ni,Co)O4 from the sample annealed at 700°C. In both air and CO2 environment, we could find La-(Ni,Co)-O secondary phase from annealed samples, which indicates interdiffusion at the interface. However, extent of the degradation was much higher in CO2 environment. This could be seen by the intensity of XRD peaks corresponding to secondary phases, which overwhelmed the ones corresponding to bulk LLZO. Co L-edge XANES and XRD data for samples annealed at 700°C in different gas conditions could be seen in the attached figure.These findings suggest that interfacial degradation is strongly dependent on the sintering temperature and gas environment. Sintering temperature should be kept as low as possible to avoid formation of detrimental phase. In terms of gas environment, O2 environment is the most promising since the sample remained stable up to 700°C. On the other hand, we found that CO2 and H2O(vapor) are detrimental to the sample. This could be because that they could both form phases which could extract Li out of the system, which led to formation of delithiated secondary phases such as La2Zr2O7. Findings from the work can be used to design optimal conditions to manufacture and operate all solid Li-batteries using LLZO with minimal interfacial degradation. Figure 1

Quadrupolar mechanism for vibrational sum frequency generation at air/liquid interfaces: Theory and experiment

Vibrational sum frequency generation (VSFG) spectroscopy has been widely utilized to investigate various interfaces through molecular vibration. VSFG is usually attributed to the breakdown of the inversion symmetry at the interface within the electric dipole approximation. Although the electric dipole approximation is a very good approximation in the isotropic bulk, its validity is questionable at the interface because a large electric field gradient exists in the thin interface region. Thus, the quadrupole contribution may become significant at the interface. Here, we discuss the quadrupole contribution in VSFG from theoretical and experimental viewpoints. We describe a theory as to how the quadrupole contribution appears in experimental VSFG spectra by deriving the vibrational selection rule and discussing the interface selectivity. With this theoretical framework, we examine the mechanism of VSFG at the air/benzene and air/decane interfaces. The accurate determination of the vibrational frequencies realized by heterodyne-detected VSFG spectroscopy reveals that VSFG at the air/benzene interface arises from the quadrupolar mechanism. This means that the observation of VSFG does not imply a molecular alignment so that interfacial benzene molecules may be randomly oriented. Meanwhile, at the air/decane interface, it is concluded that the VSFG signal arises from the ordinary dipolar mechanism. This implies that decane molecules are aligned in a preferential direction at the interface despite their low polarity. This study demonstrates the importance of examining the mechanism of VSFG before discussing the interfacial structure based on VSFG spectra. A strategy to distinguish different mechanisms is also proposed. The present study further shows that the quadrupolar mechanism, if properly taken into consideration, enables us to obtain information about interfacial molecules beyond the restriction of the inversion symmetry breaking required by the dipolar mechanism, through the large electric field gradient localized at the interface.

Structural, chemical, and magnetic properties of cobalt intercalated graphene on silicon carbide

We report on a study of the Co intercalation process underneath the R30° reconstructed 6H-SiC(0001) surface for Co film-thicknesses in a range of 0.4–12 nm using a combination of surface sensitive imaging, diffractive, and spectroscopic methods. In situ photoemission electron microscopy reveals a dependence of the intercalation temperature on the Co film-thickness. Using low energy electron diffraction and photoemission spectroscopy (XPS), we find that the SiC surface reconstruction is partially lifted and transformed. We show that the R30° reconstruction does not prevent silicide formation for Co film-thicknesses ≥0.4 nm according to XPS and x-ray absorption spectra. Our results indicate that the silicide formation is self-limited to a thin interface region and is followed by Co intercalation between graphene and silicide. Furthermore, we analyze the magnetic properties using x-ray magnetic circular dichroism at the Co L-edge. In-plane magnetization is observed for all analyzed film-thicknesses. For ultra-thin Co films, self-assembled magnetic wires with a width of the order of 100 nm form at the step-edges.

Turbulence aspects of mass transfer in the thin interfacial region of the concentration boundary layer in gas–liquid systems

Turbulence aspects of mass transfer in the thin interfacial region of the concentration boundary layer in gas–liquid systems

Atomically modified thin interface in metal-dielectric hetero-integrated systems: control of electronic properties

We have performed first-principles studies of the electronic properties of Cu-diamond hetero-integrated systems, particularly placing emphasis on elucidating the effects of surface modification of diamond with H or O. It is found that the electronic properties crucially depend on the chemical compositions of the modified atomically thin interface region. The local density of states (LDOS) of the H-terminated diamond moiety near the Cu surface exhibits a clearly different distribution from that near the vacuum region, whereas the LDOS of the O-terminated diamond is almost independent of the Cu deposition. In other words, the effects of the electronic interactions between Cu and diamond on the electronic properties in the interface region are readily controlled by surface modification with only one atomic (i.e. H or O) layer. Electric field (EF) effects on the Cu-diamond systems also strongly depend on the electronic details, i.e. atomistic modification in the interface regions. In particular, at the interface between the H-terminated diamond moiety and the vacuum region, its conduction band energy is strongly affected by an applied EF much more than the valence band energy; that is, the band gap can be varied with an applied EF. The band gap variation is found to be attributed to an atomistic level difference in the spatial extension of the valence and conduction bands and thus is not explained with a macroscopic band diagram model. It has been demonstrated that the electronic properties of hetero-integrated systems are described and controlled well by carefully designing atomically thin interface regions.

Studies of the Electrode/Electrolyte Interface in Magnesium Batteries Using Ambient Pressure X-Ray Photoelectron Spectroscopy

Introduction With the high volumetric energy density, high negative reduction potential, and relatively good safety features, magnesium metal has been considered as an attractive battery anode.1 As the reversible charge and discharge is one of the most important electrochemical properties of a battery, understanding the processes of plating and stripping of the metal across the electrode/electrolyte interface is crucial in the advancement of battery performance.2 However, the current knowledge of the chemical and electrochemical interactions/reactions at the interface is limited because of the paucity of suitable in situ and operando techniques to probe these issues. In this talk, we present how we combine an ambient pressure X-ray photoelectron spectroscopy (AP-XPS) technique, a “dip & pull” approach3, with the “tender” X-ray synchrotron source to access the solid/liquid interface and to obtain in situ chemical specific information in Mg batteries. The ability to access the solid/liquid interface stems from that the photoelectron probing depth reaches 10 nm with a ‘tender’ X-ray photon source. The sensitivity to the thin interface region is also maintained in this photon energy range.3 Results In this study, magnesium(II) bis(trifluoromethane sulfonyl) imide [Mg(TFSI)2] dissolved in diglyme solvent is used as the electrolyte solution. The operating pressure stabilizes at 1-2 Torr with diglyme vapor when the electrolyte solution is introduced into the AP-XPS analysis chamber at beamline 9.3.1 at the Advanced Light Source, Lawrence Berkeley National Laboratory. A three-electrode electrochemistry sample holder is used to facilitate electrochemistry measurements and to create a stable nanometers-thick liquid electrolyte film on the Mg working electrode (WE) surface for investigation. Electrochemical measurements are conducted by using a potentiostat with potentials referenced to the Mg reference electrode. Figure 1 shows an actual photograph of the three Mg electrodes immersed in the electrolyte while obtaining XPS spectra. A representative Mg 1s spectra at the electrode/electrolyte interface in presence of 0.8 M Mg(TFSI)2 electrolyte solution is shown in Figure 2. Initially, a thin oxide/hydroxide layer with small presence of carbonate covers the Mg metal. After a thin film of electrolyte is created and stabilized on the electrode by the “dip & pull” method, distinct photoemission signal from the Mg(TFSI)2 electrolyte solution is observed at the binding energy of 1303.5 eV. In this talk, we will provide details about how this spectroscopy technique is used in probing the solid/liquid interface and provides in situ chemical information. We will also address the behavior of Mg(TFSI)2 electrolyte at this interface and its response to different electrochemical conditions.

Controlled Topological Transitions in Thin-Film Phase Separation

In this paper the evolution of a binary mixture in a thin-film geometry with a wall at the top and bottom is considered. By bringing the mixture into its miscibility gap so that no spinodal decomposition occurs in the bulk, a slight energetic bias of the walls toward each one of the constituents ensures the nucleation of thin boundary layers that grow until the constituents have moved into one of the two layers. These layers are separated by an interfacial region where the composition changes rapidly. Conditions that ensure the separation into two layers with a thin interfacial region are investigated based on a phase-field model. Using matched asymptotic expansions a corresponding sharp-interface problem for the location of the interface is established. It is then argued that this newly created two-layer system is not at its energetic minimum but destabilizes into a controlled self-replicating pattern of trapezoidal vertical stripes by minimizing the interfacial energy between the phases while conserving their area. A quantitative analysis of this mechanism is carried out via a thin-film model for the free interfaces, which is derived asymptotically from the sharp-interface model.

The Effects of Unresolved Scalar Fluctuations During Homogeneous Nucleation

The effects of the subgrid-scale (SGS) scalar interactions on nanoparticle nucleation are investigated via a priori analysis of direct numerical simulation data. The formation of dibutyl-phthalate (DBP) particles via homogeneous nucleation is simulated in a planar wake. Classical nucleation theory is used to model particle nucleation and the Navier-Stokes equations are coupled with the scalar transport equations to provide the fluid, thermal, and chemical fields. The data shows that particle nucleation is initially confined to the thin interfacial region or shear layers, where molecular diffusion is dominant. As the flow becomes turbulent nucleation increases significantly and the rate of particle formation increases by several orders of magnitude. To assess the effect of SGS scalar interactions on DBP particle nucleation, the temperature and mass-fractions are filtered and the resulting quantities are used to compute the nucleating particle field. Two filter widths are used to obtain varying levels of SG...

Reorientation transition of the magnetic proximity polarization in Fe/(Ga,Mn)As bilayers

Recently, it has been observed that thin ferromagnetic Fe films deposited on top of (Ga,Mn)As layers induce a significant proximity polarization in the (Ga,Mn)As film even at room temperature. Furthermore, it was found that a thin interfacial region of the (Ga,Mn)As film is coupled antiferromagnetically to the Fe layer. Here we report a series of combined x-ray magnetic dichroism and superconducting quantum interference device magnetometer measurements for Fe/(Ga,Mn)As bilayers where the (Ga,Mn)As layer thickness is varied between 5 and 50 nm. We find a reorientation transition of the magnetic proximity polarization as a function of the (Ga,Mn)As thickness. The data are compared to results obtained performing ab initio calculations. A varying concentration of Mn interstitials as a function of (Ga,Mn)As layer thickness is responsible for this reorientation. Furthermore, exchange bias is studied in the fully epitaxial bilayer system. We find a rather strong ferromagnetic exchange bias. The strength of the exchange bias can be estimated by using a simple partial domain wall model.

Interplay between magnetism and chemical structure at spinel-spinel interfaces

By utilizing a graded wedge sample geometry in combination with surface sensitive soft x-ray techniques, we explain the enhanced magnetic properties observed at the interface between two dissimilar magnetic spinel oxide thin films in terms of the chemical and magnetic character of the constituent cations. Through x-ray absorption spectroscopy and magnetic circular dichroism studies, we find that the interfacial cations exhibit chemical valences and site-occupancies which differ remarkably from the bulk of either film. This results in enhanced magnetic properties localized to the interface region. While this phenomena likely arises from cation interdiffusion of 1–2 nm near the interface due to the open spinel crystal structure, this dramatic change in the magnetic properties localized to a thin interface region may provide a route to obtaining isolated interfacial properties in other spinel-structured heterostructures.

Liquid separation by a graphene membrane

The behavior of liquids separated by a single graphene membrane has been studied with extensive molecular dynamics (MD) simulations at ambient conditions. With the help of appropriate empirical potentials, we have exploited two liquid phases forming distinct systems; say XGY, where G stands for graphene and X (Y) represents water (W), benzene (B), or acetonitrile (A). Our MD simulations revealed important changes in the wettability patterns of these liquids near the graphene surface. For instance, WGW exhibits strong density oscillations in a thin interfacial region with thickness of ∼2.4 nm. In the case of BGB and AGA the oscillating-density interfacial region extends beyond ∼3 nm and ∼5 nm, respectively, under ambient conditions. More interestingly, our findings indicate that a liquid at one side of the graphene sheet can affect the degree of wetting on the other side, by means of dispersion interactions through the graphene membrane. These systems can offer a useful framework to understand the structural as well as thermodynamic properties of interfaces by considering a real two-dimensional substrate.

Effects of deposition temperature and elemental layer thickness on the properties of Fe/Co multilayers grown by molecular beam epitaxy

Fe/Co multilayers were grown at different temperatures up to 200 °C onto MgO-(1 0 0) monocrystalline substrates, varying both the Co and Fe elemental layer thicknesses. All samples show a multilayered structure where the elemental layers are separated by a thin interfacial region which, for high-temperature growths, is constituted by an equiatomic FeCo compound. The multilayers grown at room temperature show an in-plane uniaxial magnetic anisotropy. The increase in the growing temperature determines an improvement of the sample smoothness and the appearance of two preferred in-plane orientations of the easy magnetization axis having different strengths. By increasing the Co layer thickness, the in-plane magnetic behaviour becomes isotropic, while by increasing the Fe layer thickness, an out-of-plane contribution to the magnetization vector is established, which is responsible for the appearance of a well-defined magnetic morphology constituted by stripe-like domains.

Effect of structural modifications on the switching voltage of a holographic polymer-dispersed liquid crystal lattice

The effect of temperature-induced variations of the nematic director field on the switching voltage of a 2D square holographic polymer-dispersed liquid crystal (HPDLC) lattice was investigated for a mixture prepared from UV-curable commercially available constituents. At temperatures far below the nematic–isotropic phase transition, the switching voltage is governed by the domain structure of the director field, while at temperatures close to the phase transition it is governed by elastic and dielectric properties of the liquid crystal (LC) material. The crossover occurs at the temperature at which a thin interface region between the LC domains and the polymer pilasters is melted into the isotropic phase. We also performed a comparative study of the square lattices recorded with the use of four and three coherent beams and resolved numerous differences in their structural and switching properties that arise from different spatial profiles of the associated interference fields.

Monte-Carlo modeling of dense polymer melts near nanoparticles

Using Monte-Carlo techniques, we study polymer chain conformations near nanoparticles for dense melts of high molecular weight. Our results indicate the presence of a thin interfacial region (1–2nm in thickness) within which polymer segments orient tangentially to the particle surface causing a stretching and widening of the chain ellipsoid. That region is also characterized by an accumulation of chain ends and a decrease in polymer density. Nanoparticles also affect polymer properties far away into the bulk. Thus, at small particle radius, we observe an overall swelling of the polymer chains when the distance between the centers of mass of nearest-neighbor particles becomes smaller than the radius of gyration of the chains.

Connection of Fields across the Interface in the Fluid Particle Dynamics Method for Colloidal Dispersions

Unlike in conventional hydrodynamics, the viscosity is assumed to smoothly change across a thin interfacial region between two fluid-phases in the fluid particle dynamics (FPD) method, which was proposed to facilitate the numerical study of colloidal dynamics. We study how the pressure and velocity fields are connected in the limit of the thin interfacial region by use of the steady Stokes approximation. We obtain connection formulas to calculate the fields in and around a droplet, far from which the uniform flow or the linear shear flow is imposed. We also show, around a spherical rigid particle in a steady state where the particle center moves uniformly, that the FPD method yields the flow consistent with that of conventional hydrodynamics.

Thin Germanium-Carbon Layers on Silicon for MOS Applications

We report the growth and materials characterization of thin (<35 nm) germanium-carbon alloy (GeC) layers grown directly on Si by ultra-high-vacuum chemical vapor deposition. We show that the presence of C atoms is limited to a thin interfacial region at the Si substrate, and interpret this result as a mechanism for strain relief. We also show that the GeC films exhibit remarkably low threading dislocation densities despite significant relaxation.

Steady Stokes Flow in and around a Droplet Calculated Using Viscosity Smoothened across Interface

Velocity continuity and force balance are usually required at the interface of two fluid phases in conventional hydrodynamics, where the viscosity is assumed to be constant in a single fluid phase. These boundary conditions connect the pressure and velocity fields across the interface. An alternative way to achieve this connection, where the viscosity is assumed to smoothly change across a thin interfacial region, was proposed to facilitate the numerical study of colloidal dynamics. We study the steady Stokes flow in and around a single droplet by use of the smoothened viscosity, imposing a purely extensional flow far from the droplet. In the limit of the thin interfacial region, we analytically obtain a set of connection formulas, which yields the fields that are different from those obtained in conventional hydrodynamics unless the droplet is a rigid body.

Electrocatalytic Properties of Thin Mesoporous Platinum Films Synthesized Utilizing Potential‐Controlled Surfactant Assembly

A new electrochemical strategy has been developed for the production of mesoporous Pt films (see Figure). The method utilizes only a thin interfacial region of surfactant–inorganic assemblies formed on the working electrode to produce nanostructured films. The resulting films possess high surface areas that lead to enhanced electrocatalytic properties toward methanol oxidation compared to those of non‐mesoporous Pt films.

Preparation of bipolar membranes (I)

AbstractThe bipolar membranes were prepared by charged material of polysulfone as a base. The bipolar membranes were composed of a solvent‐resistant anion exchange layer with a crosslinking matrix prepared by the reaction of chloromethylated polysulfone in DMF with diamine, an interfacial layer made from chloromethylated polysulfone solution in DMF containing cation exchange resin and amine, and a cation exchange layer made from sulfonated polysulfone dispersing cation resin powder. The prepared bipolar membrane can exhibit lower voltage drop over 100 mA/cm2. The critical requirement for producing bipolar membranes of low potential drop is the creation of a thin interfacial region with a low electrical resistance and a suitable chemical structure, which act to catalyze water splitting. © 2001 John Wiley &amp; Sons, Inc. J Appl Polym Sci 80: 1658–1663, 2001