Bilayer System Research Articles

Theoretical studies of sliding ferroelectricity, magnetoelectric couplings, and piezo-multiferroicity in two-dimensional magnetic materials

It has been predicted that for a series of two-dimensional (2D) materials, their van der Waals bilayers may possess sliding ferroelectricity with vertical polarizations switchable via interlayer sliding. Such intriguing ferroelectricity has been recently experimentally confirmed in a series of nonmagnetic bilayer systems like BN, MoS2, etc., while magnetic bilayers remain unexplored in those reports. In this paper we show first-principles evidence that three most prevalent 2D ferromagnetic materials, CrI3, Cr2Ge2Te6 and Fe3GeTe2, can all be entailed with sliding ferroelectricity (FE) with vertical polarizations over 0.1 pC/m in their bilayers and are thus multiferroic. In particular, Fe3GeTe2 bilayer may exhibit magnetoelectric coupling effect with a net magnetization moment of 0.01 μB per unit cell switchable upon ferroelectric switching. Moreover, it is shown to be both piezoelectric and piezomagnetic as both its vertical electric polarization and the net magnetic moment can be approximately doubled when the interlayer distance is compressed by 10%, giving rise to unprecedented “piezo-multiferroic” effect where the magnetoelectric coupling effect is intensified with reduced interlayer distance.

Investigating the Effect of Medium Chain Triglycerides on the Elasticity of Pulmonary Surfactant

In recent years, vaping has increased in both popularity and ease of access. This has led to an outbreak of a relatively new condition known as e-cigarette/vaping-associated lung injury (EVALI). This injury can be caused by physical interactions between the pulmonary surfactant (PS) in the lungs and toxins typically found in vaping solutions, such as medium chain triglycerides (MCT). MCT has been largely used as a carrier agent within many cannabis products commercially available on the market. Pulmonary surfactant ensures proper respiration by maintaining low surface tensions and interface stability throughout each respiratory cycle. Therefore, any impediments to this system that negatively affect the efficacy of this function will have a strong hindrance on the individual's quality of life. Herein, neutron spin echo (NSE) and Langmuir trough rheology were used to probe the effects of MCT on the mechanical properties of pulmonary surfactant. Alongside a porcine surfactant extract, two lipid-only mimics of progressing complexity were used to study MCT effects in a range of systems that are representative of endogenous surfactant. MCT was shown to have a greater biophysical effect on bilayer systems compared to monolayers, which may align with biological data to propose a mechanism of surfactant inhibition by MCT oil.

Raman spectroscopic studies on the evolution of interlayer coupling and stacking order in twisted bilayers and polytypes of WSe2

Twisted 2D bilayers of van der Waals materials, a new class of quantum materials, offer pioneering advances in the field of nanoelectronics and photonics. As these layered materials can have various preferential stacking configurations with varying electronic behavior, it is important to have a characterization technique that can unambiguously probe the stacking order and interlayer interactions in 2D materials and twisted 2D homobilayers. In this work, we show that by using Raman spectroscopy, we can probe variations in the interlayer coupling of bilayer WSe2 stacked at different twist angles. The interlayer interactions are weakest at a twist angle of 30°, and the twisted bilayer system is almost equivalent to two decoupled monolayers of WSe2. Also we demonstrate Raman mapping as a quick imaging tool with capabilities of clear distinction between 2H and 3R polytypes of bilayer WSe2 and can be used to study various kirigami structures and bilayer nucleation centers commonly observed during chemical vapor deposition-based growth of WSe2. This work proves to be beneficial in the characterization of twisted bilayers of 2D materials and offer key insights into the optoelectronic properties of 2D materials and heterostructures.

Lipid Bicelles in the Study of Biomembrane Characteristics.

Simulations of lipid membranes typically make use of periodic boundary conditions to mimic macroscopically sized membranes and allow for comparison to experiments performed e.g. on planar lipid membranes or on unilamellar lipid vesicles. However, the lateral periodicity partly suppresses membrane fluctuations or membrane remodeling, processes that are of particular importance in the study of asymmetric membranes-i.e. membranes with integral or associated proteins and/or asymmetric lipid compositions. Here, we devised a simple albeit powerful lipid bicelle model system that (i) displays similar structural, dynamical, and mechanical properties compared to infinite periodic lipid membrane systems and allows (ii) for the study of asymmetric lipid bilayer systems and (iii) the unperturbed formation of local spontaneous curvature induced by lipids or proteins in molecular dynamics simulations. In addition, the system is characterized by largely unbiased thermal fluctuations as opposed to standard bilayer systems. Application of the bicelle system for an asymmetric lipid composition resembling the plasma membrane reveals that the cholesterol density for a tension-free plasma membrane with a vanishing spontaneous curvature is larger by 28% within the extracellular leaflet compared to the cytosolic leaflet.

Self-Wrinkling in Polyacrylamide Hydrogel Bilayers.

Swelling of a gel film attached to a soft substrate can induce surface instability, which results in the formation of highly ordered patterns such as wrinkles and folds. This phenomenon has been exploited to fabricate functional devices and rationalize morphogenesis. However, obtaining centimeter-scale patterns without immersing the film in a solvent remains challenging. Here, we show that wrinkles with wavelengths of up to a few centimeters can be spontaneously created during the open-air fabrication of film-substrate bilayers of polyacrylamide (PAAm) hydrogels. When the film of an aqueous pregel solution of acrylamide prepared on the PAAm hydrogel substrate undergoes open-air gelation, hexagonally packed dimples initially emerge on the surface, which later evolve into randomly oriented wrinkles. The formation of such self-organized patterns can be attributed to the surface instability resulting from autonomous water transport in the bilayer system during open-air fabrication. The temporal evolution of the patterns can be ascribed to an increase in overstress in the hydrogel film due to continued water uptake. The wrinkle wavelength can be controlled in the centimeter-scale range by adjusting the film thickness of the aqueous pregel solution. Our self-wrinkling method provides a simple mechanism for the generation of swelling-induced centimeter-scale wrinkles without requiring an external solvent, which is unachievable with conventional approaches.

Universal Quantum Computation Based on Nanoscale Skyrmion Helicity Qubits in Frustrated Magnets.

We propose a skyrmion-based universal quantum computer. Skyrmions have the helicity degree of freedom in frustrated magnets, where twofold degenerated Bloch-type skyrmions are energetically favored by the magnetic dipole-dipole interaction. We construct a qubit based on them. A skyrmion must become a quantum-mechanical object when its size is of the order of nanometers. It is shown that the universal quantum computation is possible based on nanoscale skyrmions in a magnetic bilayer system. The one-qubit quantum gates are materialized by controlling the electric field and the spin current. The two-qubit gate is materialized with the use of the Ising-type exchange coupling. The merit of the present mechanism is that external magnetic field is not necessary. Our results may open a possible way toward universal quantum computation based on nanoscale topological spin textures.

Linear response based theories for Dzyaloshinskii-Moriya interactions

We investigate abilities of various linear response based techniques for extracting parameters of antisymmetric Dzyaloshinskii-Moriya (DM) interactions from the first-principles electronic structure calculations. For these purposes, we further elaborate the idea of Sandratskii [Phys. Rev. B 96, 024450 (2017)], which states that $z$ component of the DM vector can be computed by retaining only spin-diagonal part of the spin-orbit (SO) coupling. We start our analysis with the magnetic force theorem (MFT), which relies on additional approximations resulting in the linear dependence of the exchange interactions on the response tensor, and compare it with the exact approach formulated in terms of the inverse response. We propose the downfolding procedure transferring the effect of these ligand spins into parameters of effective interactions between the localized spins. These techniques are applied for the series of CrCl$_3$ and CrI$_3$ based materials, including bulk, monolayer, bilayer, and three-layer systems. Particularly, we discuss how the DM interactions are induced by the inversion symmetry breaking at the surface or by the electric field. As long as the SO interaction of the heavy ligand atoms is taken into account in the calculations of the response tensor between the Cr $3d$ states, the MFT appears to be a good approximation for the DM interactions, being in contrast with the isotropic exchange, for which MFT and the exact method provide quite a different description. Finally, we discuss the relevance of our approach to other techniques ever proposed for calculations of the DM interactions. Particularly, we argue that the spin-current model for the DM interactions can be derived from the MFT based expression and is the relativistic counterpart of the double exchange, occurring in metallic systems in the limit of infinite exchange splitting.

Unidirectional Spin Hall Magnetoresistance in Antiferromagnetic Heterostructures.

Unidirectional spin Hall magnetoresistance (USMR) has been widely reported in the heavy metal/ferromagnet bilayer systems. We observe the USMR in Pt/α-Fe_{2}O_{3} bilayers where the α-Fe_{2}O_{3} is an antiferromagnetic (AFM) insulator. Systematic field and temperature dependent measurements confirm the magnonic origin of the USMR. The appearance of AFM-USMR is driven by the imbalance of creation and annihilation of AFM magnons by spin orbit torque due to the thermal random field. However, unlike its ferromagnetic counterpart, theoretical modeling reveals that the USMR in Pt/α-Fe_{2}O_{3} is determined by the antiferromagtic magnon number with a non-monotonic field dependence. Our findings extend the generality of the USMR which pave the ways for the highly sensitive detection of AFM spin state.

Spin Hall magnetoresistance in quasi-two-dimensional antiferromagnetic-insulator/metal bilayer systems

We study the temperature dependence of spin Hall magnetoresistance (SMR) in antiferromagnetic insulator (AFI)/metal bilayer systems. We calculate the amplitude of the SMR signal by using a quantum Monte Carlo simulation and examine how the SMR depends on the amplitude of the spin, thickness of the AFI layer, and randomness of the exchange interactions. Our results for simple quantum spin models provide a useful starting point for understanding SMR measurements on atomic layers of magnetic compounds.

In Situ Study of the Interface-Mediated Solid-State Reactions during Growth and Postgrowth Annealing of Pd/a-Ge Bilayers.

Ohmic or Schottky contacts in micro- and nanoelectronic devices are formed by metal-semiconductor bilayer systems, based on elemental metals or thermally more stable metallic compounds (germanides, silicides). The control of their electronic properties remains challenging as their structure formation is not yet fully understood. We have studied the phase and microstructure evolution during sputter deposition and postgrowth annealing of Pd/a-Ge bilayer systems with different Pd/Ge ratios (Pd:Ge, 2Pd:Ge, and 4Pd:Ge). The room-temperature deposition of up to 30 nm Pd was monitored by simultaneous, in situ synchrotron X-ray diffraction, X-ray reflectivity, and optical stress measurements. With this portfolio of complementary real-time methods, we could identify the microstructural origins of the resistivity evolution during contact formation: Real-time X-ray diffraction measurements indicate a coherent, epitaxial growth of Pd(111) on the individual crystallites of the initially forming, polycrystalline Pd2Ge[111] layer. The crystallization of the Pd2Ge interfacial layer causes a characteristic change in the real-time wafer curvature (tensile peak), and a significant drop of the resistivity after 1.5 nm Pd deposition. In addition, we could confirm the isostructural interface formation of Pd/a-Ge and Pd/a-Si. Subtle differences between both interfaces originate from the lattice mismatch at the interface between compound and metal. The solid-state reaction during subsequent annealing was studied by real-time X-ray diffraction and complementary UHV surface analysis. We could establish the link between phase and microstructure formation during deposition and annealing-induced solid-state reaction: The thermally induced reaction between Pd and a-Ge proceeds via diffusion-controlled growth of the Pd2Ge seed crystallites. The second-phase (PdGe) formation is nucleation-controlled and takes place only when a sufficient Ge reservoir exists. The real-time access to structure and electronic properties on the nanoscale opens new paths for the knowledge-based formation of ultrathin metal/semiconductor contacts.

Large Tunable Perpendicular Magnetic Anisotropy in Ultrathin Co -Based Ferromagnetic Films Induced by Antiferromagnetic δ−Mn

We report on the observation of large and tunable perpendicular magnetic anisotropy (PMA) in $\mathrm{Co}$-based ferromagnetic (FM) films induced by an antiferromagnetic (AFM) $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Mn}$ layer. The perpendicular anisotropic energy of the bilayers can be manipulated by varying the thickness of the $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Mn}$ layer and the maximum reaches 2.41 \ifmmode\times\else\texttimes\fi{} ${10}^{7}$ erg/cm${}^{3}$. The coercivity of bilayers is also effectively regulated over a broad range from 0.02 to 7.09 T. We demonstrate that this large PMA originates from strong interfacial exchange coupling and the AFM anisotropic energy. In addition, the antisymmetric longitudinal magnetoresistance occurs in the bilayers in the absence of asymmetric geometry or magnetic field. These results enrich our understanding of AFM-induced large PMA in an AFM/FM bilayer system and provide a promising bilayer structure for exploring high-density magnetic memory devices.

Evaluation of stress and elastic energy relief efficiency in a hard coating with a metal interlayer—Using TiN/Ti as a model system

This study measured the stress relief extent of a hard coating by a metal interlayer in a bilayer system. An energy-balance model was used to evaluate the energy relief efficiency (ξtot) by the interlayer. The objective of this study was to understand the relationship between plastic deformation and the energy relief efficiency of the metal interlayer in a bilayer thin film system. A TiN/Ti bilayer thin film was chosen as the model system. TiN/Ti samples were prepared with different interlayer thicknesses and under different stress levels in TiN coating using unbalanced magnetron sputtering. The overall stress of the bilayer samples was determined by the laser curvature method, and the stress in the individual layer was measured by average x-ray strain combined with nanoindentation method. For the TiN/Ti sample with Ti interlayer thickness >78 nm, a maximum ξtot was reached at an interlayer thickness about 110 nm; further increasing the interlayer thickness may decrease ξtot. This was mainly due to plastic deformation of the Ti interlayer being localized near the TiN/Ti interface. The results also showed that ξtot increased with increasing stress in the TiN coating. The model analyses revealed that the energy relief was mostly contributed from the TiN coating, while less than 30% was from curvature relaxation of the Si substrate. For the sample with insufficient thickness (52 nm) of an Ti interlayer, the stress of the TiN coating could not be effectively relieved and the interlayer was subjected to compressive stress. In this case, the energy-balance model was not valid, while our previous elastic model could be used to account for the stress state transition. The residual stress state of the Ti interlayer can serve as an index to assess the effectiveness in relieving film stress by the interlayer. The interlayer is functioning by sustaining tensile stress, whereas it is ineffective if the interlayer is subjected to compressive stress.

Understanding the spatial and functional link between the IL-2R and TCR at the immunological synapse.

The immunological synapse is the molecular platform for signal integration that controls T cell activation in response to antigen presentation. Interleukin 2 (IL-2) is an essential signal for effective T cell activation and proliferation after T cell receptor (TCR) engagement. However, the molecular mechanisms that link IL-2 signaling with TCR activation are not fully understood. Here, we investigate the spatial and temporal relationship between the IL-2 receptor (IL-2R) and the TCR function by visualizing their localization and activity during the formation of a synaptic contact. We use a supported lipid bilayer (SLB) system to mimic the surface of a virus-infected cell and we visualize antigen-specific CD8+ T cells forming a contact with it. The SLB system allows us to manipulate the two-dimensional accessibility of IL-2R and TCR ligands at the contact. For this purpose, we use the novel biologic therapeutics Immuno-STATs™. The full Immuno-STATs™ framework consists of a Fc-formatted peptide-HLA complex and a modified IL-2 with reduced affinity. Using structural variants of these compounds, we can manipulate the spatial link between IL-2 and TCR signaling pathways and evaluate its relevance for an efficient synapse formation. Our data sheds light into the mechanisms of early IL-2 action and IL-2R assembly during the immunological synapse.

Phosphatidylinositol-4-phosphate domain properties in asymmetric giant unilammelar vesicles.

Phosphatidylinositol-4-phosphate (PI(4)P) is a precursor for higher phosphorylated phosphoinositide species and mediates directly many cellular processes at the plasma membrane and in the Golgi. Cellular membranes are vertically asymmetric and, with respect to lipid distribution, laterally heterogenous. It has been shown that many physiological functions affected by PI(4)P are associated with clustering of the lipid. Studies using symmetric lipid vesicle model systems have shown that PI(4)P forms domains in the presence of physiological levels of Ca2+ and Mg2+. However, at present it is unclear to what extent PI(4)P domain behavior translates from symmetric to asymmetric lipid bilayer systems. Asymmetric giant unilamellar vesicles (aGUVs) are model systems that allow for the visualization of lipid interactions within a phospholipid bilayer using confocal fluorescence microscopy. Novel techniques have only recently enabled the fabrication of aGUVs. We are using the hemifusion method to obtain aGUVs with PI(4)P containing lipid mixtures in one leaflet and varying lipid mixtures in the opposing leaflet. Our experiments are designed to determine the conditions under which PI(4)P domain formation is favored and we are investigating the registration of PIP domains with raft domains in the opposite leaflet.

2-d liquid-liquid phase separation of GM130 Golgin protein bound to supported bilayer.

Golgins are long (50-400 nm) rod-like membrane proteins, enveloping the Golgi apparatus (GA). They initially capture cognate cargo vesicle, to facilitate Snare-mediated membrane fusion. Most of them can phase separate and this may result to internal organization of the GA. Golgi matrix protein 130 (GM130), is the most abundant Golgin located at the cis Golgi. In our study of recombinant GM130 on supported bilayer system, we observed that GM130 self-assembles and phase separates into two distinct phases: A Mesh phase and a Condensate phase. Here we will describe the precise nature of these phases and their hypothetical physiological roles in the GA.

Analysis of direct transmembrane permeability of mono-amino acids by the planar membrane system.

Amino acids are delivered to living cells by direct permeation, endocytosis, or assisted diffusion. Direct permeation does not require carriers or energy to deliver amino acids into the cytoplasm. Revealing how mono-amino acids permeate through the membrane will help to understand the permeation mechanism of cell-penetrating peptides and elucidate the way protocells take up amino acids. The relationship between the direct permeability of amino acids and their physicochemical properties is poorly understood, and their interactions with membranes remain controversial. In this study, we used the electrophysiological method, a label-free permeation measurement that avoids the influence of label molecules on the permeation, to analyze the transmembrane permeation through the planar lipid bilayer system. Considering the relationship between the shape of the current signal and the permeation models in the membrane, we attempt to estimate the membrane direct permeability of all twenty L-amino acids. As the result, the current signals were detected for amino acids at 20 mM. To confirm the transmembrane permeation of amino acids through the membrane, UV spectroscopy and chromogenic reaction were also measured. Three kinds of signals corresponding to the direct permeation (spike-like current) and the formation of membrane defects (erratic and multiple currents) were observed and assigned to each model. We analyzed the frequency and ratio of signal occurrence the spike-like signals and assessed the direct permeation of amino acids. The results show that hydrophobic amino acids exhibited higher current frequency and shorter duration, suggesting higher permeation. Through the relationship of current value and duration, we found differences in pore-forming ability and membrane permeation rate between amino acids. We will discuss factors that contribute to differences in membrane permeation.

Study of cell-cell adhesion formation and dynamics in cancer cells using a hybrid cell-lipid bilayer system.

The formation and regulation of connections between cells play a crucial role in maintaining tissue homeostasis and tumour repression. In many tissues, particular importance comes to adherens junctions that are formed by the binding of E-cadherin proteins at the surface of adjacent cells, followed by their clustering and recruitment of catenins and other adaptor proteins on the intracellular site linking the protein complex to the actomyosin cytoskeleton. This process depends on multiple parameters such as the mobility, density, type of cadherins, and the mechanical forces acting on the cadherin-catenin complexes and can change dramatically during developmental stages or in diseases such as cancer. To study the initial steps of adherens junction formation in a controlled environment in cancer cells, we have established a reconstituted system of cells adhering to supported lipid bilayers decorated with extracellular domains of E-cadherin proteins. This reconstituted system allowed us to capture the initiation and dynamics of cadherin interactions, the real-time localisation and clustering of adaptor proteins and the maturation of adherens junction in cancer cells over time using live cell imaging with confocal and total internal reflection microscopy. To understand the importance of the different parameters and to mimic the cell/tissue abnormalities occurring in cancer scenarios, we then perturb the controlled system by varying cadherin density, membrane mobility and inducing mechanical stress by confining the cell system over time. We find, for example, that reduced E-cadherin mobility leads to the formation of stable regions showing actin enrichment and that confinement increases the steady state adhesion zone. This approach provides new insights about cell surface dynamics during cell-cell adhesion and can improve our understanding of the role of cell mechanics and E-cadherin organisation patterns in cancer progression.

Protective Characteristics of TiO2 Sol-Gel Layer Deposited on Zn-Ni or Zn-Co Substrates

This study aimed to present the differences in the corrosion properties and protective ability of two bi-layer systems obtained on low-carbon steel in a model corrosive medium of 5% NaCl solution. These newly developed systems consist of Zn-Co (3 wt.%) or Zn-Ni (10 wt.%) alloy coatings as under-layers and a very thin TiO2 sol-gel film as a top-layer. Scanning electron microscopy (SEM) is used for characterization of the surface morphology of the samples indicating that some quantitative differences appear as a result of the different composition of both zinc alloys. Surface topography is investigated by means of atomic force microscopy (AFM), and the hydrophobic properties are studied by contact angle (CA) measurements. These investigations demonstrate that both sample types possess grain nanometric surface morphology and that the contact angle decreases very slightly. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are used for characterization of the chemical composition and electronic structure of the samples. The roughness Rq of the Zn-Ni/TiO2 is 49.5 nm, while for Zn-Co/TiO2, the Rq value is 53.4 nm. The water contact angels are 93.2 and 95.5 for the Zn-Ni/TiO2 and Zn-Co/TiO2 systems, respectively. These investigations also show that the co-deposition of Zn and Ni forms a coating consisting entirely of Ni2Zn11, while the other alloy contains Zn, Co and the intermetallic compound CoZn13. The corrosion resistance and protective ability are estimated by potentiodynamic polarization (PDP) curves, as well as polarization resistance (Rp) measurements for a prolonged test period (35 days). The results obtained are compared with the corrosion characteristics of ordinary zinc coating with an equal thickness. The experimental data presents the positive influence of the newly developed systems on the enhanced protective properties of low-carbon steel in a test environment causing a localized corrosion—lower corrosion current density of about one magnitude of order (~10−6 A.cm−2 for both systems and ~10−5 A.cm−2 for Zn) and an enhanced protective ability after 35 days (~10,000–17,000 ohms for the systems and ~900 ohms for Zn).

Mechanism of paramagnetic spin Seebeck effect

We have theoretically investigated the spin Seebeck effect (SSE) in a normal metal (NM)/paramagnetic insulator (PI) bilayer system. Through a linear response approach, we calculated the thermal spin pumping from PI to NM and backflow spin current from NM to PI, where the spin-flip scattering via the interfacial exchange coupling between conduction-electron spin in NM and localized spin in PI is taken into account. We found a finite spin current appears at the interface under the difference in the effective temperatures between spins in NM and PI, and its intensity increases by increasing the density of the localized spin $S$. Our model well reproduces the magnetic-field-induced reduction of the paramagnetic SSE in Pt/${\mathrm{Gd}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$ experimentally observed when the Zeeman energy is comparable to the thermal energy, which can be interpreted as the suppression of the interfacial spin-flip scattering. The present finding provides an insight into the mechanism of paramagnetic SSEs and the thermally induced spin-current generation in magnetic materials.

Barrier behaviour of partially N-acetylated chitosan layers in aqueous media

Chitosan coatings of 353 ± 12 nm thickness were prepared on glass and zinc substrates by dip-coating method to study their barrier-behaviour. The coatings were chemically modified to increase their degree of acetylation (DA) from ca. 44 % up to ca. 98 % resulting a quasi-chitin coating. The effect of the acetylation reaction was studied by infrared spectroscopy, and the structural changes of the native and acetylated coatings were investigated by UV–Vis spectrophotometry and X-ray diffraction. The surface properties of the coated samples were characterized by wettability measurements – advancing water contact angle decreased from ca. 80° (native) to ca. 43° (fully acetylated) – and microscopic (SEM, AFM) studies. The barrier behaviour of the chitosan layer depending on the DA was evaluated by electrochemical impedance spectroscopy studies and with a special mesoporous silica – chitosan bilayer system by measuring the amount of dye (Rhodamine 6G) accumulated in the silica through the chitosan coating during an impregnation step. These methods showed significant decrease in the barrier-effect of the coatings with increasing DA (accumulation of approximately six times more dye and a reduction of charge transfer resistance by an order of magnitude), due to the structural and ionization changes in the coatings.