Metal Bilayers Research Articles

3D‐Printed In Situ Growth of Bilayer MOF Hydrogels for Accelerated Osteochondral Defect Repair

AbstractRepairing osteochondral (OC) defect presents a significant challenge due to the intricate structural requirements and the unpredictable differentiation pathways of bone marrow mesenchymal stem cells (BMSCs). To address this challenge, a novel biomimetic OC hydrogel scaffold is developed that features a structure of soft and hard components. This scaffold incorporates bilayer metal–organic frameworks (MOFs), specifically ZIF‐67 in the upper layer and ZIF‐8 in the lower layer, achieved through an in situ printing process. This configuration enables the spatial and temporal modulation of BMSC differentiation by controlling the release of Co2⁺ and Zn2⁺. The results demonstrate that the bilayer MOF hydrogels significantly outperform hydrogels that either lack MOFs or contain a single type of MOF in enhancing repair outcomes in rabbit models of knee OC defects. The improved regenerative efficacy is attributed to the distinct chondrogenic and osteogenic differentiation cues provided by the bilayer MOFs, effectively guiding BMSCs toward enhanced tissue regeneration. This customizable biomimetic OC hydrogel scaffold not only opens new avenues for innovative therapeutic strategies but also holds great promise for widespread clinical applications.

Thickness dependence on dynamical spin injection driven by thermal effects in CoFeB/Pt bilayer

In ferromagnetic metal (FM)/non-magnetic metal (NM) bilayer structures, dynamical spin injection is primarily attributed to spin pumping at the interface. However, thermal effects, such as the spin (dependent) Seebeck effect (S(d)SE) caused by FMR heating effect, are also expected to contribute, particularly farther from the interface within the FM layer. In this study, the detailed mechanism of dynamical spin injection in CoFeB/Pt bilayer films has been investigated. We demonstrate the proper evaluation of the CoFeB thickness dependence of inverse spin Hall voltage by subtracting the signal from single CoFeB system. The dynamical spin injection and FMR heating effect were observed to significantly depend on thickness. Additionally, we separated the contributions of SSE and SdSE by focusing on their diffusion properties and found that SSE is larger than SdSE in the CoFeB/Pt bilayer film.

Nanosized-laser-induced sub-20 nm homogenous alloy nanoparticles

Alloy nanoparticles (NPs) have great potential in nanosized 3D-printing, surface coating, plasmonic enhancement, information coding, and so forth. However, chemical-pollution-free and homogeneous sub-20 nm NPs maintain still a challenge in preparation. Here we present a smart nanosecond laser scan strategy of alloy-NPs preparation on a bilayer metal film by using a nanosized focused beam, successfully realizing controllable fabrication of the sub-20 nm homogeneous alloy NPs without pollution. As a demonstration, various sub-20 nm AgCu NPs with different volume ratios have been prepared, all NPs show narrow size distribution and uniform interparticle spacing. This simple and cost-effective method is stable and adaptable for other alloy-NPs such as AuAg NPs. In addition, such alloy NPs exhibit two-peak plasma resonance feature and information coding capacity. We believe that homogenous alloy sub-20 nm NPs will provide new application opportunities in many fields.

Enhanced Thermoelectric Properties of Ti2CO2-Bismuthene-Ti2CO2: Optimized Power Factor and Reduced Thermal Conductivity.

In this study, bismuthene was intercalated between bilayer Ti2CTx to induce significant modifications in its electronic and phonon structures, thereby enhancing its thermoelectric properties. First-principles calculations reveal that the insertion of bismuthene transforms the Ti2CO2 system from a semiconductor into a metal and optimizes the thermoelectric properties of bilayer Ti2CO2 by enhancing its power factor and reducing its lattice thermal conductivity. Under the first-principles calculation parameters used in this study, the ZT of the Ti2CO2 system increased from 0.12 to 0.55. Conversely, for metallic bilayer MXenes, the introduction of bismuthene led to a substantial decrease in ZT (from 0.53 to 0.11 in the Ti2C system and from 0.07 to 0.05 in the Ti2CCl2 system). This study investigates the physical mechanisms underlying the enhancement of thermoelectric properties from both electronic and phononic perspectives and provides theoretical insights into the development and application of MXene-based thermoelectric materials.

Large-area and few-layered 1T′-MoTe2 thin films grown by cold-wall chemical vapor deposition

This study employs cold-wall chemical vapor deposition to achieve the growth of MoTe2 thin films on 4-inch sapphire substrates. A two-step growth process is utilized, incorporating MoO3 and Te powder sources under low-pressure conditions to synthesize MoTe2. The resultant MoTe2 thin films exhibit a dominant 1T′ phase, as evidenced by a prominent Raman peak at 161 cm−1. This preferential 1T′ phase formation is attributed to controlled manipulation of the second-step growth temperature, essentially the reaction stage between Te vapor and the pre-deposited MoO x layer. Under these optimized growth conditions, the thickness of the continuous 1T′-MoTe2 films can be precisely tailored within the range of 3.5–5.7 nm (equivalent to 5–8 layers), as determined by atomic force microscopy depth profiling. Hall-effect measurements unveil a typical hole concentration and mobility of 0.2 cm2 Vs−1 and 7.9 × 1021 cm−3, respectively, for the synthesized few-layered 1T′-MoTe2 films. Furthermore, Ti/Al bilayer metal contacts deposited on the few-layered 1T′-MoTe2 films exhibit low specific contact resistances of approximately 1.0 × 10−4 Ω cm2 estimated by the transfer length model. This finding suggests a viable approach for achieving low ohmic contact resistance using the 1T′-MoTe2 intermediate layer between metallic electrodes and two-dimensional semiconductors.

Thermal and Coherent Spin Pumping by Noncollinear Antiferromagnets.

Antiferromagnets attract much interest because of their potential for spintronic applications and open fundamental physics questions, but especially noncollinear antiferromagnets remain relatively unexplored. Here, we formulate the thermal and coherent pumping of spins in noncollinear antiferromagnets|normal metal bilayers. We find that the spin current polarization is a vector with components along both the Néel vector and net magnetic moment. The spin mixing conductance for the coherent spin pumping is a tensor with elements depending on the degree of noncollinearity and interface spin configuration. We explain the controversial sign problem of the antiferromagnetic spin Seebeck effect by interface effects and suggest that interface engineering may enhance the spin pumping efficiency.

Breaking the intrinsic activity barriers of bilayer metal oxides for catalytic CO2 reduction

The photocatalytic CO2 reduction reaction is severely limited by sluggish charge kinetics. To address this issue, a strategy utilizing non-metal-doped layered double hydroxide (LDH) has been developed to control the electronic structure of spindle-shaped nanoflowers, resulting in efficient photocatalytic CO2 reduction. The results demonstrate that the designed catalyst yields 263.16 μmol g−1 h−1 for the photoreduction of CO2 to CO. Furthermore, the in situ Fourier transform infrared spectrum (FT-IR) analysis demonstrate that the specific S-ligand (S-bridge) facilitates CO2 activation, ensuring the continuous production of *COOH. The hydrothermal-assisted ionic liquid method proposed in this study offers guidance for modifying catalysts.

Damage-free dry transfer method using stress engineering for high-performance flexible two- and three-dimensional electronics.

Advanced transfer printing technologies have enabled the fabrication of high-performance flexible and stretchable devices, revolutionizing many research fields including soft electronics, optoelectronics, bioelectronics and energy devices. Despite previous innovations, challenges remain, such as safety concerns due to toxic chemicals, the expensive equipment, film damage during the transfer process and difficulty in high-temperature processing. Thus a new transfer printing process is needed for the commercialization of high-performance soft electronic devices. Here we propose a damage-free dry transfer printing strategy based on stress control of the deposited thin films. First, stress-controlled metal bilayer films are deposited using direct current magnetron sputtering. Subsequently, mechanical bending is applied to facilitate the release of the metal bilayer by increasing the overall stress. Experimental and simulation studies elucidate the stress evolution mechanisms during the processes. By using this method, we successfully transfer metal thin films and high-temperature-treated oxide thin films onto flexible or stretchable substrates, enabling the fabrication of two-dimensional flexible electronic devices and three-dimensional multifunctional devices.

Widening the Gamut of Structural Colors of Gold via Insulator–Metal Bilayer Coatings

Tuning the color of Au has been a longstanding problem in the luxury industry. Conventional approaches, involving Au alloying, compromise purity and demand distinct alloy compositions for each hue. This study demonstrates a lithography‐free method for generating structural colors on a gold surface by adjusting the thickness of titanium dioxide, a high‐index dielectric. While color tuneability is limited if TiO2 is coated directly on the Au surface, a range of vivid colors can be generated if a 50−100 nm thick AuAl2 underlayer is used. AuAl2, an accepted alloy for purple gold, broadens the color gamut, providing a protective coating without diminishing gold purity. The reflectance dip of the bilayer structure exhibits a significant red shift with increasing thickness of the TiO2 layer, allowing diverse colors by TiO2 insulator tuning. Simulation studies corroborate experimental results, affirming that coating a TiO2 layer on the AuAl2 underlayer yields a wide range of colors. This method, based on thin‐film interference, shows promise for widespread use, offering a broad spectrum of structural colors in an industry striving for diverse Au color representation.

Giant Spin-Orbit Torque in Antiferromagnetic-Coupled Pt/[Co/Gd]N Multilayers with Suppressed Spin Dephasing and Robust Thermal Stability.

Manipulating magnetization via power-efficient spin-orbit torque (SOT) has garnered significant attention in the field of spin-based memory and logic devices. However, the damping-like SOT efficiency (ξDL) in heavy metal (HM)/ferromagnetic metal (FM) bilayers is relatively small due to the strong spin dephasing accompanied by additional spin polarization decay. Furthermore, the perpendicular magnetic anisotropy (PMA) originating from the HM/FM interface is constrained by the thickness of FM, which is unfavorable for thermal stability in practical applications. Consequently, it is valuable to develop systems that not only exhibit large ξDL but also balance thermal stability. In this work, we designed antiferromagnetic-coupled [Co/Gd]N multilayers, where staggered Co and Gd magnetic moments effectively suppress the spin dephasing and additional spin polarization decay. The ordered Co-Gd arrangements along the out-of-plane direction provide bulk PMA, endowing Pt/[Co/Gd]N high thermal stability. The SOT of Pt/[Co/Gd]N was systematically studied with N, demonstrating a significantly large ξDL of up to 0.66. The ξDL of Pt/[Co/Gd]N is greater than those of Pt/Co and Pt/ferrimagnetic alloys. This significant enhancement relies on the effective suppression of spin dephasing in [Co/Gd]N. Our work highlights that the antiferromagnetic-coupled [Co/Gd]N multilayer is a promising candidate for low-consumption and high-density spintronic devices.

Robust chiral spin transport in the antiferromagnetic iron oxide/heavy metal bilayers

We have observed robust chiral spin torques and non-reciprocal charge transport behaviors in the α-Fe2O3/Pt bilayers through a combination of magnetic field and current-dependent second longitudinal harmonic resistance measurements. The interfacial Dzyaloshinskii–Moriya interaction induced magnetic chirality has been predicted to account for the sign reversal characteristic of the second longitudinal harmonic resistance with increasing the current amplitude. A physical model that considers the chirality dependence of both the asymmetric scattering and the giant Rashba spin–orbit coupling has been set up to uncover the microscopic interactions between charge, spin, and magnetic chirality. Our comprehensive approach leverages the semiclassical Boltzmann theory to validate the consistency between this model and our experimental findings. Through our investigation, we have established the pivotal role of interfacial magnetic chirality in determining both charge and spin transport behaviors within antiferromagnetic insulator/heavy metal bilayer systems. Our work not only enhances the comprehension of spin–orbit torques and non-reciprocal charge transport but also contributes to the broader understanding of these phenomena. The outcomes of this study have broader implications for the advancement of spintronics and related fields.

Electron–phonon coupling and non-equilibrium thermal conduction in ultra-fast heating systems

A discrete unified gas kinetic scheme is developed to solve the Boltzmann transport equation (BTE) with coupled electron and phonon thermal transport, whose time step is not limited by the relaxation time. The key of the present scheme is that the electron/phonon advection, scattering and electron–phonon interactions are coupled together within one time step by solving the BTE again at the cell interface. Numerical results show that the present scheme can correctly predict the electron–phonon coupling effects, and is in agreement with typical two-temperature model and experimental results in existing literatures and our performed time-domain thermoreflectance technique. It can also capture the ballistic effects when the characteristic length is comparable to or smaller than the mean free path where the two-temperature model fails. In addition, two anomalous heat conduction phenomena are predicted by the present scheme, namely, heat flows from phonon to electron in transient thermal grating geometry and re-rise reflected signal appears in Au/Pt bilayer metal structure. The present work can provide theoretical guidance for the study of electron–phonon coupling and offer a useful tool for multi-scale thermal management.

Field-Free Spin-Orbit Torque Magnetization Switching in a Perpendicularly Magnetized Semiconductor (Ga,Mn)As Single Layer.

Current-induced spin-orbit torque (SOT) in a perpendicularly magnetized single layer has a strong potential to switch the magnetization using an extremely low current density, which is generally 2-3 orders of magnitude smaller than that required for conventional metal bilayer systems. However, an in-plane external magnetic field has to be applied to break the symmetry and achieve deterministic switching. To further enhance the high-density integration and accelerate the practical application of highly efficient SOT magnetic random-access memory (SOT-MRAM) devices, field-free SOT magnetization switching in a ferromagnetic single layer is strongly needed. In a spin-orbit ferromagnet (a ferromagnet with strong spin-orbit interaction) with crystal inversion asymmetry and a multi-domain structure, the internal Dzyaloshinskii-Moriya effective fields are considered to induce field-free switching. Here, combined with strong spin-orbit coupling and a tilted anisotropy axis induced by a nonuniform Mn distribution and a possible magnetocrystalline anisotropy resulting from a slight substrate tilting, we successfully achieve magnetization switching in a spin-orbit ferromagnet (Ga,Mn)As single layer by utilizing SOT without applying any external magnetic field. Our findings help to deeply elucidate the SOT switching mechanism and can advance the development of a highly efficient MRAM with better scalability.

The phases formed in Sn/Co thin bilayer upon heating

The structure and phases formed in Sn/Co thin films are interesting both from the solid-state chemistry point of view and due to applications of such a metallic bilayer. The phases forming in thin films Sn/Co obtained by thermal vacuum evaporation on two different substrates SiO2 and MgO(100) at different annealing temperatures have been studied. Annealing above 110°С results in intermetallics formation in the films. The hcp-cobalt is grown in the films on SiO2 substrate, and the fcc-Co is observed on MgO(100) substrate. It is found that the stable α-Co3Sn2 intermetallic is formed at higher annealing temperature in film on MgO(100) substrate. We show that transformations related to mass transfer in the Sn/Co bilayers were up to 500°С and were finished upon reaching the thermodynamically equilibrium phase composition at this temperature.

Dry synthesis of bi-layer nanoporous metal films as plasmonic metamaterial

Abstract Nanoporous metals are a class of nanostructured materials finding extensive applications in multiple fields thanks to their unique properties attributed to their high surface area and interconnected nanoscale ligaments. They can be prepared following different strategies, but the deposition of an arbitrary pure porous metal is still challenging. Recently, a dry synthesis of nanoporous films based on the plasma treatment of metal thin layers deposited by physical vapour deposition has been demonstrated, as a general route to form pure nanoporous films from a large set of metals. An interesting aspect related to this approach is the possibility to apply the same methodology to deposit the porous films as a multilayer. In this way, it is possible to explore the properties of different porous metals in close contact. As demonstrated in this paper, interesting plasmonic properties emerge in a nanoporous Au–Ag bi-layer. The versatility of the method coupled with the possibility to include many different metals, provides an opportunity to tailor their optical resonances and to exploit the chemical and mechanical properties of components, which is of great interest to applications ranging from sensing, to photochemistry and photocatalysis.

The relationship between deformation mechanisms and mechanical properties in nanocrystalline Cu/Ag-bilayer alloy

The nanocrystalline metallic bilayer structure can effectively enhance the strength of Cu alloy while maintaining excellent electrical conductivity. Based on the microstructure evolution of Cu/Ag alloy observed by transmission electron microscope (TEM), molecular dynamics (MD) simulations of Cu/Ag alloy with nanocrystalline metallic bilayer structure were established. The mechanical properties and deformation mechanisms of nanocrystalline Cu/Ag-bilayer alloys were investigated at different temperatures. A mixed Hall-Petch (H-P) relationship was observed between the ultimate stress and the increased layer thickness. In this paper, the deformation mechanism changes from dislocation accumulation at the Cu-Ag interface to dislocation through the Cu-Ag interface. In addition, the data analysis results of elastic modulus showed an approximately linear decrease trend with increasing temperature, and the deformation mechanism shifted from dislocation motion to a combination of dislocation motion and grain boundaries (GBs) diffusion. These findings provide guidance for the design of high magnetic field facility (HMFF) water-cooled magnets and very large-scale integration (VLSI) lead frames.

Interface Effect on the Out‐of‐Plane Spin‐Orbit Torque in the Ferromagnetic CoPt Single Layers

AbstractField‐free switching of a perpendicularly magnetized heavy metal/ferromagnetic metal bilayer or single alloy layer through spin‐orbit torque (SOT) provides a potential way for next‐generation spintronic devices with fast speed and high efficiency. Here, a sizable symmetry dependence of field‐free magnetization switching via an out‐of‐plane torque in CoxPt100−x single layers is reported. It is found that the sign of in‐plane SOT in CoxPt100−x when x ≤ 25 is positive. However, it changes to negative when x > 25, while the out‐of‐plane SOT changes its sign when x > 30. The polarized neutron reflectometry measurement further suggests an interface layer with rich Pt content near the substrate, which could have a strong interface effect on the out‐of‐plane SOT. The sign of the out‐of‐plane SOT, and then the polarity of the field‐free magnetization switching, is determined by the competition between the out‐of‐plane torques arising from the bulk and interface parts in the CoxPt100−x single layers. It is expected that such interface effect of the out‐of‐plane torque is desirable to the applications in spin logic devices using symmetry dependence of field‐free magnetization switching in the ferromagnetic CoPt single layers.

Temperature Evolution of Magnon Propagation Length in Tm3Fe5O12 Thin Films: Roles of Magnetic Anisotropy and Gilbert Damping.

The magnon propagation length, ⟨ξ⟩, of a ferro-/ferrimagnet (FM) is one of the key factors that controls the generation and propagation of thermally driven magnonic spin current in FM/heavy metal (HM) bilayer based spincaloritronic devices. For the development of a complete physical picture of thermally driven magnon transport in FM/HM bilayers over a wide temperature range, it is of utmost importance to understand the respective roles of temperature-dependent Gilbert damping (α) and effective magnetic anisotropy (Keff) in controlling the temperature evolution of ⟨ξ⟩. Here, we report a comprehensive investigation of the temperature-dependent longitudinal spin Seebeck effect (LSSE), radio frequency transverse susceptibility, and broad-band ferromagnetic resonance measurements on Tm3Fe5O12 (TmIG)/Pt bilayers grown on different substrates. We observe a significant drop in the LSSE voltage below 200 K independent of TmIG film thickness and substrate choice. This is attributed to the noticeable increases in effective magnetic anisotropy field, HKeff (∝Keff) and α that occur within the same temperature range. From the TmIG thickness dependence of the LSSE voltage, we determined the temperature dependence of ⟨ξ⟩ and highlighted its correlation with the temperature-dependent HKeff and α in TmIG/Pt bilayers, which will be beneficial for the development of rare-earth iron garnet based efficient spincaloritronic nanodevices.

High-responsivity photodetectors made of indium selenide with visible to near-infrared photodetection

This study demonstrates indium selenide (InSe) photodetectors (PDs) with a broadband photodetection range from the visible to the near-infrared (NIR) region (400–1000 nm). The InSe flakes exfoliated from bulk material are transferred to a sapphire substrate, followed by the deposition of Ni/Au bi-layer metal contact to fabricate the metal-semiconductor–metal (M-S-M) PDs. In addition to an absorption edge at around 1000 nm, a strong absorption edge at around 516 nm is observed from the typical spectral response taken from the fabricated InSe M-S-M PDs. Under a bias voltage of -1 V, the typical responsivity at 400 nm is as high as 1130 A W-1, corresponding to a photoconductive gain of approximately 3500. Thus, InSe has potential application in broadband photodetection from the NIR to the visible region.

Polymer-Metal Bilayer with Alkoxy Groups for Antibacterial Improvement.

Many bio-applicable materials, medical devices, and prosthetics combine both polymer and metal components to benefit from their complementary properties. This goal is normally achieved by their mechanical bonding or casting only. Here, we report an alternative easy method for the chemical grafting of a polymer on the surfaces of a metal or metal alloys using alkoxy amine salt as a coupling agent. The surface morphology of the created composites was studied by various microscopy methods, and their surface area and porosity were determined by adsorption/desorption nitrogen isotherms. The surface chemical composition was also examined by various spectroscopy techniques and electrokinetic analysis. The distribution of elements on the surface was determined, and the successful bonding of the metal/alloys on one side with the polymer on the other by alkoxy amine was confirmed. The composites show significantly increased hydrophilicity, reliable chemical stability of the bonding, even interaction with solvent for thirty cycles, and up to 95% less bacterial adhesion for the modified samples in comparison with pristine samples, i.e., characteristics that are promising for their application in the biomedical field, such as for implants, prosthetics, etc. All this uses universal, two-step procedures with minimal use of energy and the possibility of production on a mass scale.