Thin Bilayer Research Articles

Physical properties of aluminum doped Zinc Oxide thin films and single wall carbon nanotubes bilayers with potential application in photovoltaics devices

A study of transparent thin films of Zinc Oxide doped with Aluminum (AZO) processed by sputtering with potentials application as substrate for single wall carbon nanotubes processing by spray-coating is discussed in this work. AZO Thin films have been deposited by radiofrequency magnetron sputtering technique at room temperature in high vacuum ambient and using an Argon atmosphere. The AZO thin films and the processed bilayers of Zinc Oxide: Aluminum/Single wall carbon nanotubes were characterized by scanning electron microscopy, Raman spectroscopy, x-ray diffraction, ultraviolet-visible spectrophotometry, stylus profilometry and four-terminal method for the electrical resistivity measurement. The feasibility of the Zinc Oxide doped with Aluminum thin films for its use as single wall Carbon Nanotubes substrate processed by a low-cost technique, with potential application in the configuration of photovoltaics devices is presented, analyzed and discussed as function of its electrical, morphological and optical properties through Haacke figure of merit quantification. A two-order reduction in resistivity values, average acceptable transmittance values (∼80%), and crystallinity expected for polycrystalline films were obtained for AZO/Single wall carbon nanotubes bilayers; although the homogeneity of the single wall carbon nanotubes distribution on AZO thin films processed by RF-Sputtering should be improved, the possible application as transparent electrode in a usual configuration of solar cells of AZO/Single wall carbon nanotubes can be performed.

Coupling of Ferromagnetic and Antiferromagnetic Spin Dynamics in Mn_{2}Au/NiFe Thin Film Bilayers.

We investigate magnetization dynamics of Mn_{2}Au/Py (Ni_{80}Fe_{20}) thin film bilayers using broadband ferromagnetic resonance (FMR) and Brillouin light scattering spectroscopy. Our bilayers exhibit two resonant modes with zero-field frequencies up to almost 40GHz, far above the single-layer Py FMR. Our model calculations attribute these modes to the coupling of the Py FMR and the two antiferromagnetic resonance (AFMR) modes of Mn_{2}Au. The coupling strength is in the order of 1.6T nm at room temperature for nm-thick Py. Our model reveals the dependence of the hybrid modes on the AFMR frequencies and interfacial coupling as well as the evanescent character of the spin waves that extend across the Mn_{2}Au/Py interface.

Investigation of the high-temperature oxidation behavior of Fe-14Cr-9Mn-2.5Ni austenitic stainless steel in N2-21 vol%O2 environment

The high-temperature oxidation behavior of Fe-14Cr-9Mn-2.5Ni austenitic stainless steel in N2-21 vol％O2 environment at 900–1100 °C has been investigated. As the oxidation temperature rises, the scale expands from a thin bilayer to a thicker multilayer composed of an outer Fe-rich layer and an internal oxidation zone. The inner oxide consists of island Cr-rich spinel precipitates surrounded by remaining Ni-rich metallic. At 1000 °C, we observed an embedded Cr-rich dense layer that can slow down the oxidation rate. At a higher temperature (1100 °C), the outer oxide scale (Fe, Mn)2O3/(Fe, Mn)3O4/(Fe, Mn)O forms, and the oxidation rate significantly increases.

A Quartz Crystal Microbalance (QCM) Study on the Formation of Aqueous κ-Carrageenan-Chitosan Composite Bilayers with NaCl and Graphene Oxide

Biomaterials and thin biofilms play a fundamental role in the medical, food and pharmaceutical industries. The Quartz crystal microbalance (QCM) measurement technique is one of the attractive techniques which have been used for monitoring the thin-film formation process. In the current work described here, κ-carrageenan and chitosan solutions were used for the layer-by-layer (LBL) deposition of polyelectrolyte multilayers (PEMs) on the gold surface of a 5 MHz AT-cut (thickness-shear mode) quartz crystal in a flow module. A sensitive QCM system was designed to detect 0.1 Hz differences in the resonance frequency (Δf) and 10−7 changes in the energy dissipation (ΔD) values, which are the measures of the deposited mass and the film rigidity, respectively. Negatively charged carrageenan and positively charged chitosan biomolecules in the solutions were used to build up sequential and very thin bilayers on the modified gold surface. The effects of NaCl and graphene oxide (GO) addition on the mass of the deposited films were also investigated. Intermolecular interactions between the biopolymer chains, Na+ cations and GO sheets were explained by interactions between molecules, such as electrostatic forces and hydrogen bondings.

Stability Criteria for Self-Propagating Reaction Waves in Co/Al Multilayers.

The propagation of self-sustained formation reactions in sputter-deposited Co/Al multilayers is known to exhibit a design-dependent instability. Multilayers having thin bilayers (<55 nm period) exhibit stable propagating waves, whereas those with a larger period react unstably. The specific two-dimensional (2D) instability observed involves the transverse propagation of a band in front of a stalled front commonly referred to as a "spin band." Previous finite-element studies have shown that these instabilities are thermodynamically driven by the forward conduction of heat away from the flame front. However, the magnitude of that loss is inherently tied to the bilayer design in traditional bimetallic multilayers, which couples any proposed stability criteria to a varying critical diffusion distance. This work utilizes a recently developed class of materials known as "inert-mediated reactive multilayers" to decouple the thermodynamic and kinetic contributions to propagating wave stability by reducing the stored chemical energy density in normally stable bilayer designs. By depositing an inert product phase (B2-CoAl) within the mid-plane of Co and Al reactant layers, spin instabilities arise as a function of both diluted volume and critical diffusion distance. From there, a stability criterion is determined for Co/Al multilayers based on enthalpy loss from the reaction zone, and its physical significance is explored.

Surface and interface properties of TiO2/CuO thin film bilayers deposited by rf reactive magnetron sputtering

Thin film bilayers of TiO2/CuO with intentionally varied thickness of the top layer of TiO2(25–150 nm) were grown by means of sequential rf magnetron reactive sputtering from Cu and Ti metallic targets in a flow rate-controlled Ar + O2 gas mixture. XRD diffraction in GID configuration, SEM top and cross-sectional images, EDS combined with SEM, XRR profiles, UV–VIS-NIR spectrophotometry and synchrotron soft radiation XAS in TEY mode were used to characterize crystal structure, morphology, the mode of growth, chemical composition, density and roughness as well as the optical and electronic properties of the synthesized thin film bilayers. CuO bottom layer turned out to crystallize in a monoclinic structure while TiO2 top layer remained amorphous. Film roughness of CuO decreased from about 12 nm with the increasing thickness of TiO2 thin film to about 1–2 nm. X-ray absorption spectroscopy demonstrated a presence of Cu+ and Cu2+ in TiO2 top layer of TiO2/CuO heterostructure. The photoelectrochemical measurements revealed that thin film bilayers of TiO2/CuO behaved as photocathodes in the process of water splitting. The photocurrent density was found to be an increasing function of the top TiO2 layer thickness.

Temperature-Dependent Re-alignment of the Short Multifunctional Peptide BP100 in Membranes Revealed by Solid-State NMR Spectroscopy and Molecular Dynamics Simulations.

BP100 is a cationic undecamer peptide with antimicrobial and cell-penetrating activities. The orientation of this amphiphilic α-helix in lipid bilayers was examined under numerous conditions using solid-state 19 F, 15 N and 2 H NMR. At high temperatures in saturated phosphatidylcholine lipids, BP100 lies flat on the membrane surface, as expected. Upon lowering the temperature towards the lipid phase transition, the helix is found to flip into an upright transmembrane orientation. In thin bilayers, this inserted state was stable at low peptide concentration, but thicker membranes required higher peptide concentrations. In the presence of lysolipids, the inserted state prevailed even at high temperature. Molecular dynamics simulations suggest that BP100 monomer insertion can be stabilized by snorkeling lysine side chains. These results demonstrate that even a very short helix like BP100 can span (and thereby penetrate through) a cellular membrane under suitable conditions.

Instability and rupture of surfactant-laden bilayer thin liquid films.

The stability of surfactant-laden bilayer thin films, where the top layer is subject to van der Waals driven breakup, is of particular relevance to applications where one thin liquid layer is spread on another, such as film-forming firefighting foams and multilayer coatings. Although there has been much prior modeling work on the stability of thin liquid bilayers, additional physical effects and assumptions were incorporated in those studies, making it difficult to isolate the influence of surfactant on the rupture of the top layer. The present work addresses this issue through application of the lubrication approximation to derive a coupled system of nonlinear evolution equations describing the perturbations to the liquid-liquid and liquid-air interfaces and the surfactant interfacial concentrations. The surfactant is assumed to be insoluble and can be present at each interface. Linear stability analysis suggests, and nonlinear simulations confirm, that by using surfactant that adsorbs to both interfaces, the rupture time can be increased by an order of magnitude relative to the surfactant-free case. However, we find it crucial to have the right amount of surfactant to generate strongly stabilizing Marangoni stresses without reducing the interfacial tension too much. Nonlinear simulations and linear stability analysis provide insight into the mechanisms of the delayed rupture and show how the direction and strength of the Marangoni stresses strongly depend on the viscosity ratio of the layers. These results can help guide the choice and design of surfactants to achieve more effective firefighting foams and more stable liquid coatings.

Towards lithium-free solid-state batteries with nanoscale Ag/Cu sputtered bilayer electrodes.

Enhancing the reversible Li growth efficiency in "Li-free" solid-state batteries is key for the deployment of this technology. Here, we demonstrate a nanoscale material design path that enables the reversible cycling of a lithium-free solid-state battery, using Li7La3Zr2O12 (LLZO) electrolyte. By means of nanometric Ag-Cu bilayers, directly sputtered onto the LLZO, we can effectively control Li deposition. The robust thin film bilayer, which is compatible with LLZO, enables stable cycling, accommodating the volume changes without the need for extra external pressure.

Improvement in soft magnetic properties of thin bilayer ribbons using magnetoelastic effect

We have prepared Fe-Ni-system bilayer ribbons with different magnetostriction (compositions) and investigated the improvement of soft magnetic properties using the magnetoelastic effect. A toroidal core with D = 10 mm was made from the Fe6Ni94/Fe56Ni44 bilayer ribbon, and the B-H loop of the core was measured. The shape of the hysteresis loop dramatically changed depending on the inner layer (inner magnetic phase). This result indicates that the direction of the anisotropy induced by bending stress was changed depending on the inner layer. The slope of the B-H loop and coercivity reduced when the Fe56Ni44 layer was on the inner side. From the experimental results, we found that domain rotation was dominant for the magnetization process. Consequently, the increase in the coercivity over frequency could be suppressed by controlling the magnetization process. From these results, we found that a thin bilayer ribbon with positive and negative magnetostriction constant is an attractive material for reducing iron losses under high frequency.

3D Asymmetric Bilayer Garnet-Hybridized High-Energy-Density Lithium–Sulfur Batteries

Lithium garnet Li7La3Zr2O12 (LLZO), with high ionic conductivity and chemical stability against a Li metal anode, is considered one of the most promising solid electrolytes for lithium-sulfur batteries. However, an infinite charge time resulting in low capacity has been observed in Li-S cells using Ta-doped LLZO (Ta-LLZO) as a solid electrolyte. It was observed that this cell failure is correlated with lanthanum segregation to the surface of Ta-LLZO that reacts with a sulfur cathode. We demonstrated this correlation by using lanthanum excess and lanthanum deficient Ta-LLZO as the solid electrolyte in Li-S cells. To resolve this challenge, we physically separated the sulfur cathode and LLZO using a poly(ethylene oxide) (PEO)-based buffer interlayer. With a thin bilayer of LLZO and the stabilized sulfur cathode/LLZO interface, the hybridized Li-S batteries achieved a high initial discharge capacity of 1307 mA h/g corresponding to an energy density of 639 W h/L and 134 W h/kg under a high current density of 0.2 mA/cm2 at room temperature without any indication of a polysulfide shuttle. By simply reducing the LLZO dense layer thickness to 10 μm as we have demonstrated before, a significantly higher energy density of 1308 W h/L and 257 W h/kg is achievable. X-ray diffraction and X-ray photoelectron spectroscopy indicate that the PEO-based interlayer, which physically separates the sulfur cathode and LLZO, is both chemically and electrochemically stable with LLZO. In addition, the PEO-based interlayer can adapt to the stress/strain associated with sulfur volume expansion during lithiation.

Discovery of Type II Interlayer Trions.

In terms of interlayer trions, electronic excitations in van der Waals heterostructures (vdWHs) can be classified into Type I (i.e., two identical charges in the same layer) and Type II (i.e., two identical charges in the different layers). Type I interlayer trions are investigated theoretically and experimentally. By contrast, Type II interlayer trions remain elusive in vdWHs, due to inadequate free charges, unsuitable band alignment, reduced Coulomb interactions, poor interface quality, etc. Here, the first observation of Type II interlayer trions is reported by exploring band alignments and choosing an atomically thin organic-inorganic system-monolayer WSe2 /bilayer pentacene heterostructure (1L+2L HS). Both positive and negative Type II interlayer trions are electrically tuned and observed via PL spectroscopy. In particular, Type II interlayer trions exhibit in-plane anisotropic emission, possibly caused by their unique spatial structure and anisotropic charge interactions, which is highly correlated with the transition dipole moment of pentacene. The results pave the way to develop excitonic devices and all-optical circuits using atomically thin organic-inorganic bilayers.

Spin pumping at interfaces with ferro- and paramagnetic Fe60Al40 films acting as spin source and spin sink

We present a study of spin pumping efficiency and determine the spin mixing conductance and spin diffusion length in thin bilayer films based on 3d transition metal alloy Fe60Al40. Due to its magnetostructural phase transition, Fe60Al40 can be utilized as a ferromagnetic (FM) or paramagnetic (PM) material at the same temperature depending on its structural order; thus a thin Fe60Al40 film can act as a spin source or a spin sink when interfaced with a paramagnet or a ferromagnet, respectively. Ferromagnetic resonance measurements were performed in a frequency range of 5–35 GHz on bilayer films composed of FM–Fe60Al40/Pd and PM–Fe60Al40/Ni80Fe20 (permalloy). The increase in damping with the thickness of the paramagnetic layer was interpreted as a result of spin pumping into the paramagnet. We determine the spin mixing conductance gPd↑↓=(3.8±0.5)×1018m−2 at the FM–Fe60Al40/Pd interface and the spin diffusion length λPd=9.1±2.0nm in Pd. For the PM–Fe60Al40/permalloy interface, we find a spin mixing conductance gFeAl↑↓=(2.1±0.2)×1018m−2 and a spin diffusion length λFeAl=11.9±0.2nm for PM–Fe60Al40. The demonstrated bi-functionality of the Fe60Al40 alloy in spin pumping structures may be promising for spintronic applications.

Cumulative polarization in conductive interfacial ferroelectrics.

Ferroelectricity in atomically thin bilayer structures has been recently predicted1 and measured2-4 in two-dimensional materials with hexagonal non-centrosymmetric unit-cells. The crystal symmetry translates lateral shifts between parallel two-dimensional layers to sign changes in their out-of-plane electric polarization, a mechanism termed 'slide-tronics'4. These observations have been restricted to switching between only two polarization states under low charge carrier densities5-12, limiting the practical application of the revealed phenomena13. To overcome these issues, one should explore the nature of polarization in multi-layered van der Waals stacks, how it is governed by intra- and interlayer charge redistribution and to what extent it survives the addition of mobile charge carriers14. To explore these questions, we conduct surface potential measurements of parallel WSe2 and MoS2 multi-layers with aligned and anti-aligned configurations of the polar interfaces. We find evenly spaced, nearly decoupled potential steps, indicating highly confined interfacial electric fields that provide a means to design multi-state 'ladder-ferroelectrics'. Furthermore, we find that the internal polarization remains notable on electrostatic doping of mobile charge carrier densities as high as 1013 cm-2, with substantial in-plane conductivity. Using density functional theory calculations, we trace the extra charge redistribution in real and momentum spaces and identify an eventual doping-induced depolarization mechanism.

Possible Role of Bent Structure of Methylated Lithocholic Acid on Artificial and Plasma Membranes.

Bile acids form micelles that are essential for the absorption of dietary lipids. However, excessive bile acid micelles can disrupt the plasma membrane by removing phospholipids, resulting in cell death. We hypothesized that the bent geometrical structure of the steroid scaffold of bile acids decreases the lipid order (similar to unsaturated phospholipids with cis double bonds), disrupting the plasma membrane. Here, lithocholic acid (LCA), a bile acid, was methylated to prevent micellization. Methylated lithocholic acid (Me-LCA) was mixed with a thin phase-separated lipid bilayer comprising 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and cholesterol (Chol). Me-LCA was not localized in the DPPC-rich rigid phase but localized in the DOPC-rich fluid phase, and excess Me-LCA did not affect the phase separation. Me-LCA is distributed in the plasma and organelle membranes. However, Me-LCA with bent structure did not affect the membrane properties, membrane fluidity, and hydrophobicity of liposomes composed of DOPC, DPPC, and Chol and also did not affect the proliferation of cells.

Microfluidic construction of cytoskeleton-like hydrogel matrix for stabilizing artificial cells

Giant unilamellar liposomes have been largely used as artificial cell models to imitate the structures of biological cells. However, liposomes only consist of a thin bilayer of phospholipids with 2–5 nm in thickness, and thus are fragile, easy to burst, and not ready to endure environmental changes. Here we present a microfluidic strategy of embedding micro-hydrogel networks inside liposomes to improve their stability. We use droplet microfluidics to prepare liposomes encapsulated with hydrogel precursors which are subsequently polymerized into a gel phase. The hydrogel matrices are able to enhance the resistance of liposomes to osmotic shocks and can be functionalized by immobilizing nanoparticles. Finally, we demonstrate the internal gel networks support enzymatic reactions. Overall, our microfluidic approach to the construction of cytoskeleton-like hydrogel matrix for stabilizing liposome-based artificial cells provides new opportunities in micro-compartmentalization in synthetic cells, and may find applications in various fields ranging from bio-microreactors to drug delivery.

3D Asymmetric Bilayer Garnet Hybridized High-Energy-Density Lithium-Sulfur Batteries

Li7La3Zr2O12 (LLZO) has high ionic conductivity and stability against a Li metal anode making it a promising solid electrolyte for Lithium-Sulfur batteries. However, typically reported discharge capacity does not achieve theoretical. Therefore, we addressed this by forming a stable sulfur cathode/LLZO interface through a poly(ethylene oxide) (PEO)-based interlayer, with a small amount of catholyte used to maintain physical contact and ionic conduction between sulfur cathode and the PEO-based interlayer. With a thin bilayer LLZO and the stabilized sulfur cathode/LLZO interface, the hybridized Li-S batteries achieved an initial discharge capacity of 1307 mAh/g at a current density of 0.2 mA/cm2 (0.03C) at room temperature (22°C) without any indication of a polysulfide shuttle. With specific optimization, an energy density of 1308 Wh/L and 485 Wh/kg is achievable.The mechanisms regarding the cell stabilization are presented: 1). X-ray Diffraction and X-ray Photoelectron Spectroscopy indicate that the PEO-based interlayer which physically separates the sulfur cathode and LLZO, is both chemically and electrochemically stable with LLZO. 2). Due the physical separation between liquid electrolyte and LLZO through the PEO-based film, the proton/Li+ exchange with the moisture in liquid electrolytes was avoided. 3). PEO-based interlayer can adapt to the stress/strain associated with sulfur volume expansion during lithiation to protect LLZO from failure.

(Digital Presentation) High Energy Density Stable Lithium-Sulfur Batteries Enabled By 3D Bilayer Garnet Electrolytes

The cubic-garnet (Li7La3Zr2O12, LLZO) Lithium-Sulfur battery (GLSB) shows great promise in the pursuit of achieving high energy densities. The sulfur used in the cathodes are abundant and inexpensive and possess high specific capacity. As well, LLZO displays excellent chemical stability with Li metal. By using unique porous/dense/porous LLZO “trilayer” and dense/porous LLZO “bilayer” architectures developed by our group, an exceptionally high areal current density of 10 mA/cm2 in Li-Li symmetric cells without applied pressure was achieved. However, instability in the sulfur cathode/LLZO interface can cause cell performance issues. Therefore, it is critical to resolve the sulfur cathode/LLZO interfacial challenge to achieve stable cycling.Here, we created an innovative gel polymer (GPE) buffer layer to stabilize the sulfur cathode/LLZO interface. With a thin bilayer LLZO architecture as a solid electrolyte, stable cycling was achieved with a high initial discharge capacity of 1542 mAh/g corresponding to an energy density of 223 Wh/kg and 769 Wh/L under a discharge current density of 0.87 mA/cm2 without applied pressure. Moreover, the addition of the GPE interlayer also allowed the GLSB cells to maintain an average discharge capacity of 1218 mAh/g over 265 cycles with 80% capacity retention at discharge current density of 1.74 mA/cm2 under a sulfur loading of 5.2 mg/cm2 at 22 (Figure 1). Achieving such stability is a major step in the development of commercial garnet lithium sulfur batteries. Figure 1

A Bilayer SnO2/MoS2-Coated Evanescent Wave Fiber Optic Sensor for Acetone Detection-An Experimental Study.

The need for sensors that measure the acetone content of exhaled breath for diabetes severity has recently increased. Clinical researchers have reported less than 0.8 ppm acetone concentration in the exhaled breath of an average individual, while that for a diabetic patient is higher than 1.8 ppm. This work reports the development of two sets of evanescent wave-based fiber optic sensor coated with SnO2 thin film and bilayer of SnO2/MoS2 to detect different acetone concentrations (0–250 ppm). In each set, we have studied the effect of clad thickness (chemical etch time 5min, 10 min, 15 min, 25 min, 40 min, and complete clad removal) to optimize the clad thickness for a better response. In Set 1, SnO2 thin film was used as the sensing layer, while in Set 2 a bilayer of SnO2 thin film/ MoS2 was used. Enhanced sensor response of ~23.5% is observed in the Set 2 probe with a response and recovery time of ~14 s/~17 s. A SnO2/MoS2-coated sensor prototype is developed using LEDs of different wavelength and intensity detector; its potential to detect different concentrations of acetone is tested. X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), Ultraviolet (UV) Spectroscopy, and Ellipsometry were used to study the structural, morphological and optical properties of the sensing layers. The present study indicates that the SnO2/MoS2-coated sensor has the potential to create a handheld sensor system for monitoring diabetes.

Residual stresses in adhesively bonded wood determined by a bilayer flexion reporter system

The application of adhesives in modern timber engineering often introduces moisture into the wood, leading to permanent residual stresses after hardening. This paper proposes a novel approach to assess these residual stresses by using wooden bilayers as a reporter system. For thin bilayers, moisture-induced stresses lead to pronounced visible flexion that can be used to identify the stress-driving parameters of the adhesive’s gelation process. These parameters depend solely on the wood/adhesive combination and are inversely determined by fitting a finite element method model on the experimentally obtained flexion state. In a subsequent step, the determined parameters are used to calculate the residual stresses in the adhesive bondline of cross-laminated timber plates, emphasizing this approach’s scale independence and general applicability to larger scale structures. All combinations of European beech and Norway spruce with the adhesives Melamine–Urea–Formaldehyde (MUF), Phenol–Resorcinol–Formaldehyde (PRF), and Polyurethane (PUR) were investigated.