Symmetric Films Research Articles

Curvature Potential Unveiled Topological Defect Attractors

We consider the theoretical and positional assembling of topological defects (TDs) in effectively two-dimensional nematic liquid crystal films. We use a phenomenological Helfrich–Landau–de Gennes-type mesoscopic model in which geometric shapes and nematic orientational order are expressed in terms of a curvature tensor field and a nematic tensor order parameter field. Extrinsic, intrinsic, and total curvature potentials are introduced using the parallel transport concept. These potentials reveal curvature seeded TD attractors. To test ground configurations, we used axially symmetric nematic films exhibiting spherical topology.

Optical and surface energy probe of Hamaker constant in copper oxide thin films for NEMS and MEMS stiction control applications

Copper oxide films hold substantial promise as anti-stiction coatings in micro-electromechanical (MEMS) devices and with shrinking dimensions on the nanometre scale on nano electromechanical (NEMS) devices. The Hamaker constant will play a very significant role in understanding stiction and tribology in these devices. We used an approximate but sufficiently accurate form of the Lifshitz theory using the multiple oscillator model to calculate the Hamakers constant of symmetric copper oxide thin films based on experimentally obtained dielectric data in the wavelength range 190-850 nm using spectroscopic ellipsometry. We also used the Tabor–Winterton approximation (TWA) and Surface energy measurements to determine the Hamaker constant. There was better agreement in the Hamaker constant values obtained by the limited Lifshitz theory and TWA approach than with the Surface energy approach. The difference is explained through the influence of surface roughness on the surface energy using extensions of the stochastic KPZ growth model and the Family-Vicsek scaling relation and rigorous treatment of the Cassie-Baxter and Wenzel models as optimisations of a surface free energy functional linking roughness and surface tension. The dominance of the Cu2O phase in the films and of the London dispersion force on the surface of the films was previously confirmed by FTIR Cu(I)–O vibrational mode observation and XPS Cu 2p3/2 binding energy peak and its fitted satellites. The use of the limited Lifshitz theory and ellipsometry data would seem to provide a suitable best first approximation for determining the Hamaker constant of predominantly dispersive anti-stiction coatings in technologically important MEMS/NEMS devices.

Design of a Hyperbolic Metamaterial as a Waveguide for Low-Loss Propagation of Plasmonic Wave

A stratiform hyperbolic metamaterial comprises multiple units of symmetrical metal-dielectric film, stacked to have a precisely equivalent refractive index, admittance, and iso-frequency curve. A metamaterial that is composed of stacks of symmetrical films as a waveguide to couple a diffracted wave into a horizontally propagating plasmonic wave is designed herein. By tuning the parameters of the constituent thin films within a hyperbolic metamaterial, both the loss of the plasmonic wave and admittance matching are minimized and optimized, respectively.

Conformation and dynamics of ring polymers under symmetric thin film confinement.

Understanding the structure and dynamics of polymers under confinement has been of widespread interest, and one class of polymers that have received comparatively little attention under confinement is that of ring polymers. The properties of non-concatenated ring polymers can also be important in biological fields because ring polymers have been proven to be a good model to study DNA organization in the cell nucleus. From our previous study, linear polymers in a cylindrically confined polymer melt were found to segregate from each other as a result of the strong correlation hole effect that is enhanced by the confining surfaces. By comparison, our subsequent study of linear polymers in confined thin films at similar levels of confinements found only the onset of segregation. In this study, we use molecular dynamics simulation to investigate the chain conformations and dynamics of ring polymers under planar (1D) confinement as a function of film thickness. Our results show that conformations of ring polymers are similar to the linear polymers under planar confinement, except that ring polymers are less compressed in the direction normal to the walls. While we find that the correlation hole effect is enhanced under confinement, it is not as pronounced as the linear polymers under 2D confinement. Finally, we show that chain dynamics far above Tg are primarily affected by the friction from walls based on the monomeric friction coefficient we get from the Rouse mode analysis.

A numerical investigation of convective condensation in micro-fin tubes of different geometries

A numerical investigation was conducted to determine the effects of fin shape on heat transfer coefficients during in-tube condensation of R410A. Four micro-fin tubes of different geometries were selected for this study, including a straight-finned one and three helical tubes, and two different helical angles of 18° and 31° were included to explore the influence of helical angles. The numerical results were verified by previous experimental results. It was found that there was a positive relationship between heat transfer coefficients and mass velocities and vapor quality, while the helical angle seemed to improve the heat transfer coefficients especially for medium vapor quality. A nearly symmetrical liquid film distribution was found along the circumference for all three micro-fin tubes due to the centrifugal force, while the liquid tends to accumulate at the bottom of the straight-finned tube.

On the Impact of the Degree of Fluorination on the ORR Limiting Processes within Iron Based Catalysts: A Model Study on Symmetrical Films of Barium Ferrate.

In this study, symmetrical films of BaFeO2.67, BaFeO2.33F0.33 and BaFeO2F were synthesized and the oxygen uptake and conduction was investigated by high temperature impedance spectroscopy under an oxygen atmosphere. The data were analyzed on the basis of an impedance model designed for highly porous mixed ionic electronic conducting (MIEC) electrodes. Variable temperature X-ray diffraction experiments were utilized to estimate the stability window of the oxyfluoride compounds, which yielded a degradation temperature for BaFeO2.33F0.33 of 590 °C and a decomposition temperature for BaFeO2F of 710 °C. The impedance study revealed a significant change of the catalytic behavior in dependency of the fluorine content. BaFeO2.67 revealed a bulk-diffusion limited process, while BaFeO2.33F0.33 appeared to exhibit a fast bulk diffusion and a utilization region δ larger than the electrode thickness L (8 μm). In contrast, BaFeO2F showed very area specific resistances due to the lack of oxygen vacancies. The activation energy for the uptake and conduction process of oxygen was found to be 0.07/0.29 eV (temperature range-dependent), 0.33 eV and 0.67 eV for BaFeO2.67, BaFeO2.33F0.33 and BaFeO2F, respectively.

Self‐assembled nanostructures of diblock copolymer films under homopolymer topcoats

AbstractTo mitigate the interface energies of block copolymers with high interaction parameters, a topcoat strategy has been experimentally proposed to produce perpendicular oriented domains on a length scale of sub‐10 nm. However, the origin of perpendicular oriented domains and the effect of topcoats on the self‐assembled nanostructures remain to be uncovered. Herein, we use the dynamic self‐consistent field theory to explore the self‐assembly behaviors of symmetric block copolymer films under homopolymer topcoats. It is clearly demonstrated that the introduction of homopolymer topcoats enables a wide formation range of perpendicular oriented lamellae, originating from the fluctuating diblock copolymer/homopolymer interfaces in the process of in‐plane microphase separation of block copolymer films. Our simulation results also demonstrate that the formation range of perpendicular oriented lamellae can be tuned by changing the wetting properties of homopolymer topcoats and the thickness of block copolymer films, but is weakly dependent upon the chain length of homopolymers. Our theoretical findings have wide implications for understanding the formation of perpendicular oriented domains of block copolymer films, which are important for the rational design of self‐assembled nanostructures with new horizons for block copolymer lithography. © 2020 Society of Chemical Industry

Effect of initial adsorption coverage and dynamic adsorption layer formation at bubble surface in stability of single foam films

The paper presents the experimental studies on influence of degree of initial adsorption coverage and structure of the dynamic adsorption layer over a rising bubble on stability of single foam films formed at surfaces of solutions of surface-active-substances namely n-octanol, n-hexadecyltrimethylammonium bromide (CTAB) and polyoxyethylenesorbitan monooleate (Tween80). Stability of single foam films formed by the colliding bubble under different experimental conditions were determined on the basis of systematic measurements of a single bubble lifetime in self-elaborated set-up. The experiments were supplemented by theoretical calculations of values of rupture thicknesses of the foam films, determined according to the Radove-Dimitrov-Ivanov model of drainage, which allows to consider formation of symmetrical and unsymmetrical foam films, depending on estimated structure of the dynamic adsorption layer. It was found that film lifetime depends strongly on the structure (stage of development) of motion-induced dynamic adsorption layer over the rising bubble surface. Moreover, it was shown that the bubble with completely different degree of initial adsorption coverage can form foam film of similar stability. This effect was attributed to the fact that eventual degree of the bubble adsorption coverage can reach comparable (and in some cases even equilibrium) value before film formation with identical structure of the DAL and degree of liquid/gas interface immobilization.

Design a Stratiform Metamaterial with Precise Optical Property

In this work, a stratiform metamaterial is arranged as multiple periods of metal-dielectric symmetrical film stack to provide precise equivalent refractive index and admittance. There are multiple solutions of equivalent refractive index retrieved from the characteristic matrix of the film stack. The correct refractive index is derived by connecting different branches of solution at different ranges of wavelength or thickness of the dielectric layer. The refractive index of an Ag-TiO2 five-layered symmetrical film stack shown in previous work is demonstrated to be positive real instead of negative real. The associated type I iso-frequency curve supports negative refraction. In order to extend the operating wavelength of type I metamaterial, the number of the metal-dielectric symmetrical film stack is increased to reduce the thickness of the dielectric film to approach subwavelength requirement.

Effect of Substrate Surface Roughening on the Capacitance and Cycling Stability of Ni(OH)2 Nanoarray Films

The effect of substrate surface roughening on the capacitance of Ni(OH)2/NiOOH nanowall array samples produced via chemical bath deposition for 2, 4, 6, 24 and 48 h on an as-received stainless steel substrate and the same substrate after sandblasting has been investigated. Symmetric cells were subjected to 120,000 charge-discharge cycles to access changes in their capacitance. Specific capacitances were derived from cyclic voltammetry and charge-discharge cycling under a three electrode setup. Substrate roughening significantly increases the capacitance of symmetric cells and film stability since film exfoliation does not occur to the same degree as on the as-received substrate. Interestingly, films deposited on a roughened substrate for 6, 24 and 48 h also exhibit self-recovery of capacitance, which could be related to an electrodissolution-electrodeposition effect. With the use of a roughened substrate, the thinnest film gives the highest specific capacitance, 1456 F g−1, whilst the thickest one shows the highest areal capacitance, 235 mF cm−2, after 20,000 cycles. These results reveal the promise of surface roughening toward increasing the capacitance and stability of Ni(OH)2/NiOOH films.

Regions of existence of surface spin-waves in surface Brillouin zones of hexagonal simple ferromagnetic thin films

Abstract A quantum statistic Green's-function method is used to study the surface in-plane propagating spin-waves (k// ≠ 0) in a symmetric magnetic film consisting of twelve atomic layers with damping. The effects of temperature, external magnetic field, surface exchange coupling, surface anisotropy and thickness on the regions of existence of low- and high-frequency surface spin-waves in the surface two-dimensional Brillouin zone have been investigated. The regions of existence of surface spin-waves increase first and then decrease with increasing temperature or external magnetic field. There is a special temperature or external magnetic field, at which the regions of existence of surface spin-waves exhibit a maximum. Additionally, the spatial distribution of spin-waves in different spin-wave frequency range is also studied. For a film with low-frequency surface spin-waves, there is a special spin-wave frequency range, in which no spin-waves propagate in surfaces, the spin-waves propagate only in internal layers of the film. Namely, in this frequency range, the film is a spin-waves surface insulator . For a film with high-frequency surface spin-waves, there is also a special spin-wave frequency range, in which the spin-waves propagate only in surfaces. Namely, in this frequency range, the film is a spin-waves internal insulator . In the present work, a quantum statistic approach is developed to study the surface spin-wave of magnetic film. The results show the method to adjust the regions of existence of surface spin-waves in the surface two-dimensional Brillouin zone and are beneficial for building future microwave device based upon magnetic film.

Dynamical Behavior Analysis of Rubbing Rotor System under Asymmetric Oil Film Force

Rubbing is one of the most common and significant faults that exist in the rotor system. It exhibits extremely complicated dynamic behavior. Existing dynamic models are primarily based on the symmetrical rotor structure, in which the oil film forces at both ends of support are considered identical. However, in practice, the oil film force may be differently affected by multiple factors (e.g., viscosity of lubricants, the thickness of oil film, and clearance of journal bearing). In this study, a novel dynamic model of the rubbing rotor system under asymmetric oil film force was developed. Furthermore, based on the nonlinear rotor dynamic theory, its dynamic behavior with different parameters was analyzed, and the corresponding chaotic features were extracted. The results indicated that the evolution law of chaotic motion was more complicated, and the chaotic region of system response was obviously wider under the asymmetric oil film force than the symmetrical oil film force.

Elastic property characterization of soft substrate-supported thin films using multiscale digital image correlation

We present an original experimental approach to characterizing the Young’s modulus of nanoscale thin films on a soft substrate. A microtester is developed to permit pre-stretching of the polymer substrate and symmetric film deposition on both sides of its half surface. During compression and tensile tests related to the as-deposited residual stress state, the full-field macrostrains of the film-substrate composite and the uncoated substrate can be measured simultaneously with the film microstrains using multiscale digital image correlation. The macrostrain difference allows extraction of the elastic properties of thin films. Combined with microstrains, a perfect strain transfer at the film-substrate interface, minus buckling of the composite, is evidenced. No film delamination or cracking phenomena are observed, and the measured Young’s modulus agrees well with the literature. Our approach could potentially be used to explore the mechanical properties and behaviors of emerging functional thin films, both crystalline and amorphous.

Single-Layer Graphene-Based Transparent and Flexible Multifunctional Electronics for Self-Charging Power and Touch-Sensing Systems.

Applications in the field of portable and wearable electronics are becoming multifunctional, and the achievement of transparent electronics extensively expands the applications into devices such as wearable flexible displays or skin-attachable mobile computers. Moreover, the self-charging power system (SCPS) is the core technique for realizing portable and wearable electronics. Here, we propose a transparent and flexible multifunctional electronic system in which both an all-in-one SCPS and a touch sensor are combined. A single-layer graphene (SLG) film was adapted as an electrode for the supercapacitor, touch sensor, and a triboelectric nanogenerator (TENG), thus making an electronic system that is ultrathin, lightweight, transparent, and flexible. Capacitive-type transparent and flexible electronic devices can be simultaneously used as an electrochemical double-layer capacitance-based supercapacitor and as a sensitive, fast-responding touch sensor in a single-device architecture by inserting a separator of polyvinyl alcohol-lithium chloride-soaked polyacrylonitrile electrospun mat on polyethylene naphthalate between two symmetric SLG film electrodes. Furthermore, a transparent all-in-one SCPS was fabricated by laminating a TENG device with a supercapacitor, and high-performance electric power generation/storage capability is demonstrated.

Switching dynamics enhancement in P(VDF-TrFE) copolymer ultrathin films with symmetric organic film electrodes

Abstract The polarization switching behaviors of ultrathin films of ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) capacitors with different thicknesses of top PEDOT-PSSH layer have been studied. The polarization switching results are related to an effective internal electric field, and a large, permanent, internal electric field can be ensured by introducing a symmetric PEDOT-PSSH layer into the capacitors. The switching speed is potentially influenced by the PEDOT-PSSH layer and the asymmetric PEDOT-PSSH layer leads to a different density of surface charge, resulting in a build-in field. As a result, these enhanced switching dynamics in spin-coating 25 nm P(VDF-TrFE) copolymer films are achieved by introduced a symmetric electroactive layer to optimize morphology and the internal electric field. At high temperature (60 °C), we also show that the enhanced switching speed is two times as fast as what is measured at room temperature (25 °C) because the temperature depended active field is related to the switching kinetic energy. Switching time of P(VDF-TrFE) films with an electroactive layer as a function of the number of switching cycles show the excellent switching characteristics of ferroelectric capacitors.

Special wave solutions in the theory of waves of a mixed spectrum in a planar gradient bianisotropic nanocrystalline structure

The propagation of surface and pseudosurface waves of a mixed spectrum in a symmetric nanocrystalline film structure with continuously-inhomogeneous anisotropic material characteristics was considered. The substrate and coating of the compositional structure had the same scalar dielectric and magnetic permeabilities. The artificial medium of the waveguide layer was made from a bianisotropic material with an anisotropy of the magneto-dielectric bond, characterized by four tensors of the dielectric, magnetic, and magneto-dielectric permeabilities with nonzero diagonal elements. They depended on spatial coordinates with a biquadratic law. A system of coupled quasi-differential wave equations was obtained toward to the transverse components of the electric and magnetic fields from Maxwell’s equations with taking into account specific material relationships. It had linearly independent solutions, which were represented in the form of generalized series. Recurrence relations were determined for the determination of their terms. Taking into account the continuity of the tangential components of the electric and magnetic fields, a dispersion equation for calculating the transverse wave numbers was found. We obtained the existence of four proper surface and pseudosurface waves with different polarizations, which possess a whole series of fundamentally new characteristics, from the structure of the system of wave equations and their solutions.

Symmetric Cladding Thin Film Waveguides: From Lossy Media to Disordered Nanostructures

Intrinsic absorption in a waveguiding material is often considered an undesirable effect that severely limits wave propagation. However, with appropriate design of the cladding, waveguides can be made from highly absorbing materials, to the degree that stronger absorption actually increases the propagation length. Here, we discuss the permittivity landscape showing accessible optical properties of lossy materials together with their fitness for symmetric cladding thin film waveguides. The concept of Zenneck wave-like propagation in absorbing materials is extended to thin films of disordered nanocomposites. In an experimental demonstration, we show that despite intrinsic absorption and disorder, a metal–polymer composite itself can support long-range propagation of light. We have used a nanoparticle filled poly(N-isopropylacrylamide) (PNIPAM) polymer brush that can serve as a functionalized surface. The material supports bounded optical modes with superior propagation length, exceeding its bulk absorption ...

Research on tunable distributed SPR sensor based on bimetal film.

In order to overcome the limitations in range of traditional prism structure surface plasmon resonance (SPR) single-point sensor measurement, a symmetric bimetallic film SPR multi-sensor structure is proposed. Based on this, the dual-channel sensing attenuation mechanism of SPR in gold and silver composite film and the improvement of sensing characteristics were studied. By optimizing the characteristics such as material and thickness, a wider range of dual-channel distributed sensing is realized. Using a He-Ne laser (632.8nm) as the reference light source, prism-excited symmetric SPR sensing was studied theoretically for a symmetrical metal-clad dielectric waveguide using thin-film optics theory. The influence of the angle of incidence of the light source and the thickness of the dielectric layer on the performance of SPR dual formant sensing is explained. The finite-difference time-domain method was used for the simulation calculation for various thicknesses and compositions of the symmetric combined layer, resulting in the choice of silver (30nm) and gold (10nm). When the incident angle was 78deg, the quality factor reached 5960, showing an excellent resonance sensing effect. The sensitivity reached a maximum of 5.25×10-5 RIU when testing the water content of an aqueous solution of honey, which proves the feasibility and practicality of the structure design. The structure improves the theoretical basis for designing an SPR multi-channel distributed sensing system, which can greatly reduce the cost of biochemical detection and significantly increase the detection efficiency.

Interlayer coupling in symmetric and asymmetric CoFeB based trilayer films with different domain structures: Role of spacer layer and temperature

We report the study of interlayer coupling between Co20Fe60B20 (CoFeB262) layers and the resulting magnetic properties of trilayer [CoFeB262 (y nm)/Cr (x nm)/CoFeB262 (20 nm)] films with symmetric (y = 20) and asymmetric (y = 100) structures. All as-deposited CoFeB262 films exhibit amorphous structure. Surface topography studies display clear and uniform surfaces with very fine and sparsely dispersed nanosized grains. The average surface roughness of the CoFeB262 films increases marginally as y is increased from 20 to 100 nm. The shape of magnetic hysteresis (M-H) loop of single-layer CoFeB262 (y nm) films transforms from rectangular shape for CoFeB262 (20 nm) film to transcritical one for CoFeB262 (100 nm) film. This is mainly due to change in magnetic domain structure from in-plane magnetization to dense stripe domain with increase in y. On the other hand, the shape of the M-H loops in trilayer films strongly depends on x, y and temperature. All symmetric trilayer films show single magnetization reversal process at room temperature with a kink in the first quadrant for films with x < 1. Coercivity (HC), saturation field (Hsat) and remanence ratio (MR/MS) show oscillatory behaviors as a function of x. For asymmetric films, the transcritical loop changes into rectangular one after the introduction of Cr interlayer. This also causes a large reduction in Hsat and enhances MR/MS sharply. Temperature dependent M-H loops at low temperatures reveal the disappearance of the kink in symmetric trilayer films and the formation of additional steps in asymmetric trilayer films with x > 0.5. The relative increase in HC with decreasing temperature is found to be larger for symmetric films as compared to asymmetric ones. The observed results are explained on the basis of change in the interlayer coupling between CoFeB262 layers with change in x, y and temperature dependent interfacial strain, which modifies the nature of interlayer coupling. These results also demonstrate that dense stripe domain of thicker CoFeB262 layers can be easily converted into simple in-plane magnetization using trilayer thin films.

Quantum confinement and heavy surface states of Dirac fermions in bismuth (111) films: An analytical approach

Recent high-resolution angle-resolved photoemission spectroscopy experiments have given a reason to believe that pure bismuth is a topologically nontrivial semimetal. We derive an analytic theory of surface and size-quantized states of Dirac fermions in Bi(111) films taking into account the new data. The theory relies on a new phenomenological momentum-dependent boundary condition for the effective Dirac equation. The boundary condition is described by two real parameters that are expressed by a linear combination of the Dresselhaus and Rashba interface spin-orbit interaction parameters. In semi-infinite Bi(111), near the $\overline{\mathrm{M}}$ point the surface states possess anisotropical parabolic dispersion with very heavy effective mass in the $\overline{\mathrm{\ensuremath{\Gamma}}}\text{\ensuremath{-}}\overline{\mathrm{M}}$ direction order of ten free electron masses and light effective mass in the $\overline{\mathrm{M}}\text{\ensuremath{-}}\overline{\mathrm{K}}$ direction order of one hundredth of free electron mass. In Bi(111) films with equivalent surfaces, the surface states from top and bottom surfaces are not split. In such a symmetric film with arbitrary thickness, the bottom of the lowest quantum confinement subband in the conduction band coincides with the bottom of the bulk conduction band in the $\overline{\mathrm{M}}$ point.