Bilayer Formation Research Articles

Direct observation of van der Waals stacking-dependent interlayer magnetism.

Controlling the crystal structure is a powerful approach for manipulating the fundamental properties of solids. In van der Waals materials, this control can be achieved by modifying the stacking order through rotation and translation between the layers. Here, we observed stacking-dependent interlayer magnetism in the two-dimensional (2D) magnetic semiconductor chromium tribromide (CrBr3), which was enabled by the successful growth of its monolayer and bilayer through molecular beam epitaxy. Using in situ spin-polarized scanning tunneling microscopy and spectroscopy, we directly correlate the atomic lattice structure with the observed magnetic order. Although the individual monolayer CrBr3 is ferromagnetic, the interlayer coupling in bilayer depends on the stacking order and can be either ferromagnetic or antiferromagnetic. Our observations pave the way for manipulating 2D magnetism with layer twist angle control.

Preparation of an integrated porous substrate of 11-mercaptoundecanoic acid and chitosan on gold for electrochemical study of pores and pore forming interactions in lipid bilayers

In this work formation of a planar phospholipid bilayer of 1,2-dioleoyl-sn-glycero-3-phospho-L-serine on an integrated porous cushion and its interaction with the ionic liquid 1-ethyl-3-methylimidazolium tosylate was studied. The integrated porous cushion was prepared by grafting chitosan covalently on porous 11-mercaptoundecanoic acid self-assembled monolayer on gold. Pores on the self-assembled monolayer were created by partial electro-oxidative desorption of the thiol molecules from the gold surface. Chitosan was grafted on the porous self-assembled monolayer by EDC/NSH activation procedure. Lipid bilayer was formed through solvent exchange method and evidenced with QCM-D, fluorescence microscopy, and electrochemical impedance spectroscopy. A lowering of QCM frequency of 26.4±1.5 Hz was observed during the bilayer formation. Impedance measurement showed that the lipid bilayer deposited integrated membrane can attain a high electrical resistance of 27 MΩcm2. The said ionic liquid created pores in the bilayer through interaction that was evidenced by significant decrease in charge transfer resistance in the impedance spectra and corroborated with fluorescence imaging. The work has indicated that the ionic liquid 1-ethyl-3-methylimidazolium tosylate can be cytotoxic. The new strategy for the formation of polymer cushion on a porous tether has shown new possibility of studying membrane attack interaction and ion diffusion in lipid bilayers through simple impedance analysis technique.

Dynamics of Defects in van der Waals Epitaxy of Bismuth Telluride Topological Insulators

Potential applications in spintronics and quantum information processing have motivated much recent research in epitaxial films of bismuth telluride. This system is also an example of van der Waals (vdW) epitaxy, where the interface coherence between the film and substrate is based on vdW bonds instead of strong ionic or covalent bonds. Because of the weakness of the vdW bonds, the overall quality of the epitaxial films is difficult to control and structural defects are easily introduced with a significant impact on the electronic phase diagram of the epitaxial films. To elucidate the evolution of defects as a function of the growth parameters, we combine nondestructive methods for electrical and structural analysis, as well as to establish intercorrelations between structural features and density of free charge carriers. It clearly shows that point defects and twinned domains favor p-type of charge carriers. Passivation of points defects by formation of metallic bismuth bilayers (BLs) drastically changes...

Electronic structure and magnetic properties of few-layer Cr2Ge2Te6: the key role of nonlocal electron–electron interaction effects

Atomically-thin magnetic crystals have been recently isolated experimentally, greatly expanding the family of two-dimensional materials. In this Article we present an extensive comparative analysis of the electronic and magnetic properties of , based on density functional theory (DFT). We first show that the often-used approaches fail in predicting the ground-state properties of this material in both its monolayer and bilayer forms, and even more spectacularly in its bulk form. In the latter case, the fundamental gap decreases by increasing the Hubbard-U parameter, eventually leading to a metallic ground state for physically relevant values of U, in stark contrast with experimental data. On the contrary, the use of hybrid functionals, which naturally take into account nonlocal exchange interactions between all orbitals, yields good account of the electronic gap as measured by ARPES. We then calculate all the relevant exchange couplings (and the magneto-crystalline anisotropy energy) for monolayer, bilayer, and bulk with a hybrid functional, with super-cells containing up to 270 atoms, commenting on existing calculations with much smaller super-cell sizes. In the case of bilayer , we show that two distinct intra-layer second-neighbor exchange couplings emerge, a result which, to the best of our knowledge, has not been noticed in the literature.

Influence of brain gangliosides on the formation and properties of supported lipid bilayers

Gangliosides are glycolipids that are enriched on the outer surface of cell membranes. Gangliosides are receptors for a number of signaling molecules and toxins, and therefore are often incorporated into biosensors. Many of these biosensors incorporate gangliosides into supported lipid bilayers which are formed by the spontaneous rupture of unilamellar vesicles on glass or SiO2 substrates. In this work, we used quartz crystal microbalance with dissipation monitoring (QCM-D) to investigate how the presence of the four major brain gangliosides (GM1, GD1a, GD1b, and GT1b) influences the process of supported lipid bilayer formation on SiO2 surfaces. We show that the rate of supported bilayer formation is dependent on both the charge and position of sialic acid moieties on ganglioside molecules. Additionally, Ca2+ can accelerate ganglioside-rich supported bilayer formation, but the degree of acceleration differs for vesicles containing different gangliosides. Fluorescence recovery after photobleaching measurements show that the presence of all gangliosides reduces lipid diffusion coefficients in a concentration-dependent manner, and that Ca2+ slows lipid diffusion in membranes with and without gangliosides. Finally, we use ganglioside-rich supported bilayers to measure binding constants for a GD1a-binding antibody that has similar properties to antibodies present in a variant of Guillain-Barré syndrome.

Excitation of Fluorescent Lipid Probes Accelerates Supported Lipid Bilayer Formation via Photosensitized Lipid Oxidation.

Fluorescent lipid probes are commonly used to label membranes of cells and model membranes like giant vesicles, liposomes, and supported lipid bilayers (SLB). Here, we show that excitation of fluorescent lipid probes with BODIPY-like conjugates results in a significant acceleration of the rupture and SLB formation process for unsaturated phospholipid vesicles on SiO2 surfaces. The resulting SLBs also have smaller measured masses, which is indicative of a reduction in membrane thickness and/or membrane density. The excitation of fluorescent probes with NBD and Texas Red conjugates does not accelerate the SLB formation process. In the absence of fluorescent probes or light, the inclusion of oxidized phospholipids also accelerates SLB formation. The excitation-induced acceleration caused by BODIPY-like probes is eliminated when the probes are present with saturated phospholipids not susceptible to oxidation, and it is attenuated when a lipophilic antioxidant (α-tocopherol) is present. These results suggest that BODIPY-phospholipid conjugates are photosensitizers, and their excitation causes oxidation of lipid membranes, which significantly alters membrane properties.

Structural transitions and bilayer formation of CTAB aggregates

Controlled CTAB self-assembly is an essential prerequisite for the formation of gold nanorods with tailored shape and monodispersity. In this paper, we exploit the use of salt concentration and co-surfactant decanol for the preparation of CTAB aggregates with different morphologies. To this end we use a model of CTAB recently developed by ourselves, and perform electronic structure calculations in order to improve current point charge parametrization of decanol. Using molecular dynamics simulations with the new models, we find a sequence of structural transitions of CTAB aggregates induced by salt concentration and added cosurfactant. In pure solutions, CTAB aggregates form spherical micelles with a compact ionic shell and a diffuse double layer that can be qualitatively described with Poisson–Boltzmann theory. Addition of decanol as a cosurfactant induces a sequence of dramatic structural transitions at low surfactant concentration and allows the stabilization of compact ordered bilayers in a well defined range of intermediate decanol/CTAB ratios. At low and high decanol/CTAB ratios spherical micelles are transformed into wormlike cylindrical micelles. At intermediate decanol/CTAB ratios, fully formed bilayers are observed, with surfactants exhibiting a compact structure with strong positional and orientational order. We discuss how the controlled self-assembly of compact CTAB bilayers at low global CTAB concentration can pave the way for the selective passivation of gold facets and the controlled formation of monodisperse gold nanorods.

Surface Interactions between Boundary Layers of Poly(ethylene oxide)-Liposome Complexes: Lubrication, Bridging, and Selective Ligation †.

Poly(ethylene oxide), PEO, is widely exploited in biomedical applications, while phosphatidylcholine (PC) lipids (in the form of bilayers or liposomes) have been identified as very efficient boundary lubricants in aqueous media. Here we examine, using a surface force balance (SFB), the interactions between surface-adsorbed layers of PEO complexed with small unilamellar vesicles (SUVs, i.e. liposomes) or with bilayers of PC lipids, both well below and a little above their main gel-to-liquid phase-transition temperatures TM. The morphology of PEO layers (adsorbed onto mica), to which liposomes were added, was examined using atomic force microscopy (AFM) and cryo-scanning electron microscopy (cryo-SEM). Our results reveal that the PC lipids could attach to the PEO either as vesicles or as bilayers, depending on whether they were above or below TM. Under water (no added salt), excellent lubrication, with friction coefficients down to 10-3-10-4, up to contact stresses of 6.5 MPa (comparable to those in the major joints) was observed between two surfaces bearing such PEO-PC complexes. At 0.1 M KNO3 salt concentration (comparable to physiological salt levels), the friction between such surfaces was considerably higher, attributed to bridging by the polymer chains. Remarkably, such bridging could be suppressed and the friction could be restored to its previous low value if the KNO3 was replaced with NaNO3, as a result of the different PEO-mica ligation properties of Na+ compared to those of K+. Our results provide insight into the properties of PEO-PC complexes in potential applications, and large interfacial effects that can result from the seemingly innocuous replacement of K+ by Na+ ions.

Fabrication of Nanosized Ag3PO4 and Self-Assembly on Cotton Fabrics to Enhance Visible Light Photocatalytic Activities

A facile route has been developed for the preparation of stable and recyclable Ag3PO4 nanoparticles to upgrade the photocatalytic capacity and practicality. Ag3PO4 nanoparticles are first in situ deposited on a negatively charged potassium alginate/CNT colloidal solution, and then Ag3PO4–CNT and TiO2 assemble alternately on cotton fabric with the attraction of positive and negative charges. Taking advantage of the excellent absorptive capacity and recyclable nature of cotton fabric, the Ag3PO4–CNT/TiO2 photocatalytic coating presents great photocatalytic performance and recyclability for both liquid rhodamine B and gas toluene decomposition. The 7 bilayer (BL) form is the best for RhB degradation with a minimum residual of 3.94% within 5 min under visible light, and the 5BL form presents the highest toluene degradation rate (95.96%) within 60 min under simulated sunlight. Furthermore, the stability of Ag3PO4 has been greatly improved. The photocatalytic mechanism of this coating system is analyzed by ESR measurement. This work provides a nice strategy to realize the practical application of nanoscale powder photocatalysts in both liquid and gas pollutant removal.

Two-dimensional ordering in 1-propanol-graphite-oxide intercalates: isotopic effects

ABSTRACTIntercalation of fully deuterated 1-propyl alcohol into graphite oxide is studied by means of the unique capabilities offered by the NIMROD neutron diffractometer. We develop methods to estimate the amount of intercalant present within the neutron irradiated volume using for the purpose of the set of constraints implicit in the outputs of the Gudrun diffraction data treatment software. The results are compared to those from our previous study on of fully hydrogenated 1-propanol intercalation [C. Cabrillo, F. Barroso-Bujans, R. Fernandez-Perea, F. Fernandez-Alonso, D. Bowron and F.J. Bermejo, Carbon 100, 546 (2016)] where a two-dimensional ordering transition under slow heating from 100 K was discovered. The rearrangement of the 1-propanol molecular pillars did allow for the formation of bilayer intercalation. In contrast, sample deuteration leads to remarkable differences in both the intercalation kinetics and intercalate thermal response, avoiding the two-dimensional transition which would lead to bilayer formation. Finally, neutron diffraction results have enabled us to disentangle the highly anomalous behaviour exhibited by hitherto unpublished broad band dielectric spectroscopy studies which are now understood on the basis of the two-dimensional transition just referred to.

BaAs3: a narrow gap 2D semiconductor with vacancy-induced semiconductor–metal transition from first principles

Searching for novel two-dimensional (2D) materials is highly desired in the field of nanoelectronics. We here propose a new 2D crystal barium tri-arsenide ($\mathrm{BaAs_3}$) with a series of encouraging functionalities. Being kinetically and thermally stable, the monolayer and bilayer forms of $\mathrm{BaAs_3}$ possess narrow indirect band gaps of 0.87 eV and 0.40 eV, respectively, with high hole mobilities on the order of ~$\mathrm{10^3\ cm^{2}\,V^{-1}\,s^{-1}}$. The electronic properties of 2D $\mathrm{BaAs_3}$ can be manipulated by controlling the layer thickness. The favorable cleavage energy reveals that layered $\mathrm{BaAs_3}$ can be produced as a freestanding 2D material. Furthermore, by introducing vacancy defects monolayer $\mathrm{BaAs_3}$ can be transformed from a semiconductor to a metal. 2D $\mathrm{BaAs_3}$ may find promising applications in nanoelectronic devices.

Spontaneous intercalation of Ga and In bilayers during plasma-assisted molecular beam epitaxy growth of GaN on graphene on SiC

The formation of a self-limited metallic bilayer is reported during the growth of GaN by plasma-assisted molecular beam epitaxy on graphene on (0001) SiC. Depending on growth conditions, this layer may consist of either Ga or In, which gets intercalated between graphene and the SiC surface. Diffusion of metal atoms is eased by steps at SiC surface and N plasma induced defects in the graphene layer. Energetically favorable wetting of the (0001) SiC surface by Ga or In is tentatively assigned to the breaking of covalent bonds between (0001) SiC surface and carbon buffer layer. As a consequence, graphene doping and local strain/doping fluctuations decrease. Furthermore, the presence of a metallic layer below GaN opens the way to the development of devices with a spontaneously formed metallic electrode on their back side.

Lipid bilayer formation on an ion image sensor and measurement of time response of potential dependency on ion concentration

We aimed to establish an ion channel measurement system using a charge transfer type ion image sensor on which an artificial lipid bilayer membrane was formed. A supported lipid bilayer (SLB) was formed on the K+-sensitive membrane composed of polyvinyl chloride on the ion image sensor. Fluid SLB fully covered the sensor surface and consequently reduced the output potential of the ion sensor, keeping the Nernst response of the ion sensor. We measured the time course after changing K+ concentrations before and after the formation of a SLB. The temporal response of the output potential was decelerated after the SLB formation, because ion diffusion to the sensor surface was shielded by the SLB. We demonstrated the ion channel reconstruction and inhibitor addition using gramicidin and Ca2+.

Highly Reproducible Physiological Asymmetric Membrane with Freely Diffusing Embedded Proteins in a 3D-Printed Microfluidic Setup.

Experimental setups to produce and to monitor model membranes have been successfully used for decades and brought invaluable insights into many areas of biology. However, they all have limitations that prevent the full in vitro mimicking and monitoring of most biological processes. Here, a suspended physiological bilayer-forming chip is designed from 3D-printing techniques. This chip can be simultaneously integrated to a confocal microscope and a path-clamp amplifier. It is composed of poly(dimethylsiloxane) and consists of a ≈100 µm hole, where the horizontal planar bilayer is formed, connecting two open crossed-channels, which allows for altering of each lipid monolayer separately. The bilayer, formed by the zipping of two lipid leaflets, is free-standing, horizontal, stable, fluid, solvent-free, and flat with the 14 types of physiologically relevant lipids, and the bilayer formation process is highly reproducible. Because of the two channels, asymmetric bilayers can be formed by making the two lipid leaflets of different composition. Furthermore, proteins, such as transmembrane, peripheral, and pore-forming proteins, can be added to the bilayer in controlled orientation and keep their native mobility and activity. These features allow in vitro recapitulation of membrane process close to physiological conditions.

Study on the Adsorption Behavior between an Imidazolium Ionic Liquid and Na-Montmorillonite.

Interactions between 1-butyl-3-methylimidazolium tetrafluoroborate (IL), an ionic liquid, and Na-montmorillonite (Na-MMT) were studied under different kinetic conditions to investigate the adsorption behavior of IL by Na-MMT. The adsorption of IL by Na-MMT was rapid, with a fast rate, reaching a capacity of 0.43 mmol/g, lower than Na-MMT’s cation exchange capacity (CEC) of 0.90 mmol/g. Meanwhile, the highest adsorption rate occurred at the IL concentration of 1000 mg/L. The exchangeable cation of Na-MMT could not be completely substituted by the cation group of IL regardless of the IL concentration. Stoichiometric desorption experiments confirmed that the cation exchange was the dominating adsorption mechanism for the IL adsorption by Na-MMT. The pH value of the solution between 2 and 11 had a negligible effect on the adsorption amount of IL by Na-MMT. The cation group of IL interacted into the interlayer of Na-MMT successfully, resulting in the change in the wettability of Na-MMT. A bilayer formation of the cationic group should occur in the interlayer of the modified Na-MMT and the configuration of IL was dependent on the adsorption amount of IL. Furthermore, the thermal stability of the modified Na-MMT was also dependent on the adsorption amount of IL.

Microfluidic liquid cell chamber for scanning probe microscopy measurement application.

In this paper, we present a universal microfluidic liquid chamber device platform for atomic force microscopy (AFM), which enables to fabricate the uniform lipid bilayer on the hydrophilic surface using the solvent-assisted lipid bilayer formation method. Using this device enables us to acquire the various properties of delicate soft matter, including morphological data, and mechanical property measurements, using high-resolution AFM systems. The proposed technology is expected to provide an understanding of complicated biological materials.

Alternative stacking sequences in hexagonal boron nitride

The relative orientation of successive sheets, i.e. the stacking sequence, in layered 2D materials is central to the electronic, thermal, and mechanical properties of the material. Often different stacking sequences have comparable cohesive energy, leading to alternative stable crystal structures. Here we theoretically and experimentally explore different stacking sequences in the van der Waals bonded material hexagonal boron nitride (h-BN). We examine the total energy, electronic bandgap, and dielectric response tensor for five distinct high symmetry stacking sequences for both bulk and bilayer forms of h-BN. Two sequences, the generally assumed AA′ sequence and the relatively unknown (for h-BN) AB (Bernal) sequence, are predicted to have comparably low energy. We present a scalable modified chemical vapor deposition method that produces large flakes of virtually pure AB stacked h-BN; this new material complements the generally available AA′ stacked h-BN.

Microfabricated silicon chip as lipid membrane sample holder for serial protein crystallography

Abstract Rapid progress in protein crystallography techniques at X-ray free electron lasers (X-FELs) requires the development of new methods for sample delivery. Recording dynamic changes in protein structures is one of the aims. To assess protein dynamics, sample holders for crystallography need to become active devices, providing possibilities to induce in situ changes of the protein conformation during the experiments. We propose an integrated device for crystallographic studies on 2D crystals of membrane proteins. The conceptual device can support physiological conditions and provide a platform for electrophysiological measurements, as well as for electrical stimulation during the diffraction experiments at X-FELs. This allows for triggering conformational changes in membrane proteins, ultimately permitting to take series of crystallographic snapshots of the dynamic behavior of proteins. The device integrates a microfabricated silicon chip between two microfluidic transport layers. We demonstrate the fabrication of the device, describe its key components and show the method of sample loading and lipid bilayer formation with the use of the microfluidic delivery system. Electrochemical impedance spectroscopy (EIS) has been used to characterize the properties of lipid bilayers formed in the microfabricated silicon nitride support. Minimizing the sample consumption, mimicking physiological conditions during crystallographic data collection and an interface for electrochemical characterization and addressing protein 2D crystals are the key properties of the proposed device.

Mechanism of fabrication and nano-mechanical properties of α-lactalbumin/chitosan and BSA/κ-carrageenan nanotubes through layer-by-layer assembly for curcumin encapsulation and determination of in vitro cytotoxicity

An exciting technology which still has many unknowns is the fabrication of hollow cylindrical polyelectrolyte complexes that can function at different pHs. With two-open ends and hollow interior, bionanotubes can find application in nanodevices, pharmaceutics, and biology. This study focused on fabricating nanotubes using chitosan (CHI) with α-lactalbumin (LAC), and bovine serum albumine (BSA) with κ-carrageenan (CAR). The ζ-potential charge difference was largest and therefore optimum at pH 7.0 for CHI/LAC and pH 4.0 for BSA/CAR. Strong electrostatic interactions enable the formation of bilayers from oppositely charged polyelectrolytes. ITC was able to show differences in the strength and spontaneity of electrostatic bonding interactions between both pairs of polyelectrolytes at these pHs. Nanotubes with 400, 600 and 800 nm diameter were achieved with a total of 5 bilayers for the 600 and 800 nm diameter nanotubes and 4 bilayers with the 400 nm diameter nanotubes using polycarbonate membranes. SEM images showed the formation of well-defined nanotubular structures that were affected by the type of polyelectrolytes used. The mechanical strength of the walls of the nanotubes was dependent on the polyelectrolytes used and the diameter of each nanotube, as shown by atomic force microscopy (AFM). Young's moduli, in the range of 25–55 MPa were obtained for LAC/CHI, and were significantly different from BSA/CAR nanotubes with values around 30–80 MPa. Both nanotubes systems proved to be potential candidates for the encapsulation and delivery of curcumin. These nanotubes achieved entrapment efficiencies around 40–45% with subsequent releasing in physiological conditions up to 300 μg/ml, with no significant cytotoxicity.

Role of low energy transition metal ions in interface formation in ZnO thin films and their effect on magnetic properties for spintronic applications

In this study, X-ZnO/ZnO/Si(100) (X = Co, Ni and Cu) bilayer structure is fabricated using low energy ion implantation technique. Five different fluences 1 × 1015, 5 × 1015, 1 × 1016, 2.5 × 1016 and 5 × 1016 ions/cm2 with 100 keV ion-beam energy were selected in order to implant the ions up to the depth of ≈44 nm as calculated through Stopping Range of Ion in Matter and Transport Range of Ions in Matter software. Structural modification was investigated by high resolution X-ray diffraction measurements in ZnO bilayer system. An observed systematic 2θ shift in (002) peak with increasing fluence implies increased density of implanted metal ions in ZnO matrix revealing the substitution of implanted ion at Zn-site. The mechanism of bilayer formation by ion-beam implantation technique has been discussed for metal-ions by investigating their interface properties. Atomic force microscopy reveals the morphological modification after ion implantation. Near-edge X-ray absorption fine-structure (NEXAFS) measurements at metal K- and L3,2-edges have been used to investigate the nature of implanted ions in terms of their valance state and local electronic environment. Further, O K-edge NEXAFS measurement for Ni-ZnO/ZnO/Si bilayer is highly sensitive to incident beam angles whereas no spectral change is seen for Zn L-edge measurements. The magnetic measurements were performed via vibrating sample magnetometer that showed the films are ferromagnetic at room temperature. The origin of ferromagnetism has been understood through defect mediated bound magnetic polaron model. Further, perpendicular magnetic anisotropy is also observed for Ni-ZnO/ZnO/Si bilayer structure at room temperature, which is correlated with the angle dependent O K-edge NEXAFS measurements. Fabrication of ZnO bilayer via ion implantation and investigation of above properties may prove useful in spin related and optoelectronic applications.