Interlayer Interaction Research Articles

Highly Efficient Spin-Orbit Torque Switching in Bi2Se3/Fe3GeTe2 van der Waals Heterostructures.

Topological insulators (TIs) have shown promise as a spin-generating layer to switch the magnetization state of ferromagnets via spin-orbit torque (SOT) due to charge-to-spin conversion efficiency of the TI surface states that arises from spin-momentum locking. However, when TIs are interfaced with conventional bulk ferromagnetic metals, the combination of charge transfer and hybridization can potentially destroy the spin texture and hamper the possibility of accessing the TI surface states. Here, we fabricate an all van der Waals (vdW) heterostructure consisting of molecular beam epitaxy grown bulk-insulating Bi2Se3 and exfoliated 2D metallic ferromagnet Fe3GeTe2 (FGT) with perpendicular anisotropy. By detecting the magnetization state of the FGT via anomalous Hall effect and magneto-optical Kerr effect measurements, we determine the critical switching current density for magnetization switching to be Jc ≈ 1.2 × 106 A/cm2, the lowest reported for the switching of a perpendicular anisotropy ferromagnet using Bi2Se3. From second harmonic Hall measurements, we further determine the SOT efficiency (ξDL) to be in the range of 1.8 ± 0.3 and 1.4 ± 0.08 between 5 and 150 K, comparable to the highest values reported for Bi2Se3. Our density functional theory calculations find that the weak interlayer interactions at the Bi2Se3/FGT interface lead to a weakened dipole at the interface and suppress the proximity induced magnetic moment on Bi2Se3. This enables direct access to the TI surface states contributed by the first quintuple layer, where the spins are singly degenerate with significant net in-plane spin polarization. Our results highlight the clear advantage of all-vdW heterostructures with weak interlayer interactions that can enhance SOT efficiency and minimize critical current density, an important step toward realizing next generation low-power nonvolatile memory and spintronic devices.

Computational Screening of 2D All‐Inorganic Lead‐Free Halide Perovskites A3B2X9 for Photovoltaic and Photocatalytic Applications

AbstractExploration of low‐dimensional Pb‐free halide perovskites with high stability and outstanding properties is still a pursuing target for developing integrated optoelectronic devices. Herein, comprehensive computational screening of a new class of two‐dimensional (2D) all‐inorganic Pb‐free A3B2X9 perovskites is performed based on the first‐principles calculations. The results indicate that the structural and electronic properties of 2D A3B2X9 structures strongly depend on the B‐X bonding interactions, which makes that their thermodynamic stability follows the trend of A3Bi2X9≈A3Sb2X9>A3In2X9>A3Ga2X9 and their interlayer interactions show a reversal trend. Owing to the lack of lone‐pair electron effect, A3In2X9 indicate direct bandgap characteristics and present the relatively smaller bandgaps and higher electron mobilities than A3Sb2X9 and A3Bi2X9. Benefit from optimal bandgaps (0.8–2.1 eV) and large absorption coefficients (104–105 cm−1) in the visible region, A3B2I9 (B = In, Sb, Bi) exhibit high power conversion efficiency up to 18.2%. Moreover, A3B2I9 (B = Sb, Bi) is verified as efficient photocatalysts for overall water splitting. The theoretical solar‐to‐hydrogen efficiency of Rb3Bi2I9 and Cs3Bi2I9 are >16%. This work suggests huge potential of 2D A3B2X9 perovskites for photovoltaic and photocatalytic applications.

Thickness-dependent variations of atomic vibration, band-edge excitonic emission, and valleytronic response in layered Mo0.55W0.45S2 ternary compounds

Two-dimensional (2D) van der Waals transition metal dichalcogenides (TMDs) are highly attractive due to their novel phenomena and potential device applications, but most of the studies are focused on binary TMDs except a few reports on ternary TMDs. Here, we report thickness-dependent mono-to-multi-layer transition behaviors of mechanically-exfoliated ternary TMD Mo0.55W0.45S2. Dependences of Raman modes and their peak intervals on thickness (d) are divided into three regions (1.5 ≤ d ≤ 9.2, 9.2 < d ≤ 34, and > 34 nm), mostly resulting from the increase of van der Waals force and long-range Coulomb force in interlayer interactions at larger d. Photoluminescence (PL) peak and its intensity/full width at half maximum show similar three-region behaviors, which are associated with direct-to-indirect-bandgap transition as d increases. In particular, critical changes in the Raman scattering and PL occur at d = 9.2 nm (8–13 layers), meaning a 2D-to-3D transition. The effect of the valley polarization is observed up to d = 8.1 nm (7–11 layers), resulting from random mixing-induced suppression of the inversion symmetry in the multilayer. These results suggest that the critical layer number for the 2D-to-3D dimensional crossover in ternary Mo0.55W0.45S2 is considerably larger than in binary TMDs, thereby exhibiting the direct-bandgap nature in an extended range of layer number, more useful for their optoelectronic device applications.

Photocatalytic water splitting properties of GeC/InS van der Waals heterostructure: first-principles calculations

Based on first-principles, we conducted an in-depth study of the GeC/InS van der Waals heterostructure formed by GeC and InS and discussed its structure, electronic properties and optical properties. First, we observe that this heterostructure has negative binding energy, indicating that the interlayer interactions are mainly affected by van der Waals forces. Through band structure and density of state analysis, we confirmed its type-II band alignment characteristics, which means that photogenerated carriers have the ability to automatically separate in space. Moreover, the average charge density difference and Bader charge analysis show that there is a built-in electric field in the heterostructure, and further proves that GeC/InS forms a Z-scheme charge transfer mechanism. Interestingly, the band edge position spans the water redox potential and can fully induce the redox reaction of water splitting, indicating that it is a potential photocatalyst. The high light absorption coefficient shown in the absorption spectrum also further confirms its excellent photocatalytic activity. The most striking thing is that the solar hydrogen production efficiency of GeC/InS heterostructure is as high as 44.39%. Our research demonstrates the theoretical basis for GeC/InS heterostructure as a photocatalyst.

An ab initio study of vertical heterostructures formed by CdO and SnC monolayers

Assembling two dimensional (2D) materials in vertical heterostructures is one of the main techniques for tuning electronic and optical properties. In most cases, known as van der Waals heterostructures (vdWHs), the interlayer distances are larger than typical covalent bond lengths resulting in weak interlayer interactions. It has been shown that reducing the distance between the layers can significantly alter the properties of separated layers, which is not so noticeable in vdWHs and thus creates a new platform for controlling the physical properties of 2D materials. Motivated by enhanced properties of 2D vertical heterostructures, employing ab-initio calculations based on density functional theory we examined CdO/SnC systems in four different configurations. Our results reveal that in spite of thermodynamic and mechanical stabilities of all considered structures, according to the calculated phonon frequencies, only the structure formed by placing the Sn atom on the O atom and the C atom on the Cd atom is dynamically stable at zero temperature. This structure has an interlayer distance of 2.52 Å which is smaller than the interlayer distance in typical vdWHs. We investigated the electronic and optical properties of this dynamically stable structure utilizing GW approximation and solving Bethe–Salpeter equation. Unlike the monolayer CdO which possesses a single optical absorption peak close to the red light energy, the considered CdO/SnC structure has an optical band gap of 1.14 eV, and it can absorb 13% of incident light in the blue light region.

Investigation of effects of interlayer interaction and biaxial strain on the phonon dispersion and dielectric response of hexagonal boron arsenide

In this study, the effects of interlayer interaction and biaxial strain on the electronic structure, phonon dispersion and optical properties of monolayer and bilayer BAs are studied, using first-principles calculations within the framework of density functional theory. The interlayer coupling in bilayer BAs causes the splitting of out-of-plane acoustic (ZA) and optical (ZO) mode. For both structures, positive phonon modes across the Brillouin zone have been observed under biaxial tensile strain from 0 to 8%, which indicate their dynamical stability under tensile strain. Also, the phonon band gap between longitudinal acoustic (LA) and longitudinal optical (LO)/transverse optical (TO) modes for monolayer and bilayer BAs decreases under tensile strain. An appreciable degree of optical anisotropy is noticeable in the materials for parallel and perpendicular polarizations, accompanied by significant absorption in the ultraviolet and visible regions. The absorption edge of bilayer BAs is at a lower energy with respect to the monolayer BAs. The results demonstrate that the phonon dispersion and optoelectronic properties of BAs sheet could as well be tuned with both interlayer interaction and biaxial strain that are promising for optoelectronic and thermoelectric applications.

Optical properties and Landau quantisations in twisted bilayer graphene

ABSTRACT The optical properties and Landau quantisations in twisted bilayer graphene are investigated using a generalised tight-binding model that takes into account all interlayer and intralayer atomic interactions in the Moire superlattice. The system is a zero-gap semiconductor with double-degenerate Dirac-cone structures, and saddle-point energy dispersions appear at low energies for small twisting angles. The magnetic quantisation phenomena in this system are rich and unique, with Landau-level subgroups specified owing to specific Moire zone folding through modulation of the stacking angles. The Landau-level spectrum shows hybridised characteristics associated with those in monolayer, as well as AA and AB stackings. The detailed analysis explores the complex relations among the different sublattices on the same and different graphene layers.

Electronic Properties and Interlayer Interactions in Antimony Oxide Homo‐ and Heterobilayers

Antimony shows promise as a 2D mono‐elemental crystal, referred to as antimonene. When exposed to ambient conditions, antimonene layers react with oxygen, forming new crystal structures, leading to significant changes in electronic properties. These changes are influenced by the degree of oxidation. Utilizing density‐functional theory calculations, stable configurations of bilayer antimony oxide and their corresponding electronic properties are studied. Additionally, different stacking arrangements and their effects on the physical properties of the materials are investigated. Furthermore, the analysis encompasses strain‐free heterobilayers containing both pristine and oxidized antimonene layers, aiming to understand the interplay between these materials and their collective impact on the bilayer properties. In the results, insight is provided into how the properties of antimony‐based bilayer structures can be modified by adjusting stoichiometry and stacking configurations.

Reduction of Interlayer Interaction in Multilayer Stacking Graphene with Carbon Nanotube Insertion: Insights from Experiment and Simulation

Reduction of Interlayer Interaction in Multilayer Stacking Graphene with Carbon Nanotube Insertion: Insights from Experiment and Simulation

Topology and spectral interconnectivities of higher-order multilayer networks

Multilayer networks have permeated all areas of science as an abstraction for interdependent heterogeneous complex systems. However, describing such systems through a purely graph-theoretic formalism presupposes that the interactions that define the underlying infrastructures are only pairwise-based, a strong assumption likely leading to oversimplification. Most interdependent systems intrinsically involve higher-order intra- and inter-layer interactions. For instance, ecological systems involve interactions among groups within and in-between species, collaborations and citations link teams of coauthors to articles and vice versa, and interactions might exist among groups of friends from different social networks. Although higher-order interactions have been studied for monolayer systems through the language of simplicial complexes and hypergraphs, a systematic formalism incorporating them into the realm of multilayer systems is still lacking. Here, we introduce the concept of crossimplicial multicomplexes as a general formalism for modeling interdependent systems involving higher-order intra- and inter-layer connections. Subsequently, we introduce cross-homology and its spectral counterpart, the cross-Laplacian operators, to establish a rigorous mathematical framework for quantifying global and local intra- and inter-layer topological structures in such systems. Using synthetic and empirical datasets, we show that the spectra of the cross-Laplacians of a multilayer network detect different types of clusters in one layer that are controlled by hubs in another layer. We call such hubs spectral cross-hubs and define spectral persistence as a way to rank them, according to their emergence along the spectra. Our framework is broad and can especially be used to study structural and functional connectomes combining connectivities of different types and orders.

Theoretical insight on the effect of middle layer specifications on electronic properties of SnS2/MX2/SnS2 Trilayer heterostructure (M = Mo, w; X = S, Se, Te)

Recently, the van der Waals (vdW) heterostructure design has gained paramount importance for realizing artificial two-dimensional (2D) material systems with tunable electronic/opto-electronic properties for different technological applications. Driven by this paradigm, in this work, a detailed theoretical analysis has been performed on the structural and electronic properties of different tri-layer van der Waals (vdW) heterostructure of SnS2/MX2/SnS2 (M = Mo, W; X = S, Se, Te) via density functional theory(DFT) based Ab-initio calculation. Moreover, for individual vdW tri-layers, two characteristics of interlayer stacking configurations corresponding to natural stacking configurations of tri-layer SnS2 and MX2 have been considered. The structural properties are evaluated from the interlayer distance, bond angle, bond length, electrostatic difference potential, electron difference density, and Mulliken charge distributions. The electronic properties of different vdW tri-layers are assessed from the interlayer charge transfer, energy band structures, and the density of states profiles, which are quantified from the energy bandgap and effective masses, which are finally compared and benchmarked with homogenous tri-layer SnS2. It has been observed that the chalcogen specification of the middle layer predominantly influences the interlayer interaction and thereby, structural/electronic properties of vdW tri-layers. The vdW tri-layers demonstrate metallic, semi-metallic, and semiconducting energy band structures (bandgap ranging from 0.138 to 0.729 eV, electron effective mass ranging from 0.286/0.286 m0 to 1.299/0.342 m0, hole effective mass ranging from 0.216/0.216 m0 to 2.825/2.927 m0) based on the middle layer MX2 specifications and stacking orientations.

Condensation of preformed charge density waves in kagome metals

Charge density wave (CDW) is a spontaneous spatial modulation of charges in solids whose general microscopic descriptions are yet to be completed. Kagome metals of AV3Sb5 (A = K, Rb, Cs) provide a chance to realize CDW intertwined with dimensional effects as well as their special lattice. Here, based on a state-of-the-art molecular dynamics simulation, we propose that their phase transition to CDW is a condensation process of incoherently preformed charge orders. Owing to unavoidable degeneracy in stacking charge orders, phases of preformed orders on each layer are shown to fluctuate between a limited number of states with quite slower frequencies than typical phonon vibrations until reaching their freezing temperature. As the size of interfacial alkali atom increases, the fluctuations are shown to counterbalance the condensation of orderings, resulting in a maximized transition temperature for RbV3Sb5. Our results resolve controversial observations on their CDWs, highlighting a crucial role of their interlayer interactions.

In situ ligand synthesis affording a new Co(ii) MOF for photocatalytic application

Abstract In situ disulfide bond formation affords a new flexible dicarboxylic acid ligand of 3,3′-dithiobisbenzoic acid (3,3′-H2DTBA), which assembled with Co(ii) ions under hydrothermal conditions, generating a novel metal–organic framework (MOF), namely, [Co(3,3′-DTBA)2(H2O)2] n (1). This compound is assembled from layered structures, featuring weak interlayer interactions that can reduce pore blockage through relative sliding. The results of photocatalytic experiment indicate that MOF 1 is a good semiconducting material and it can function as photocatalyst for degrading methylene blue under the ultraviolet light irradiation. Cyclic experiments demonstrated its good stability and long-lasting photocatalytic performance, indicating its potential application value.

Anisotropic Interlayer Force Field for Group-VI Transition Metal Dichalcogenides.

An anisotropic interlayer force field that describes the interlayer interactions in homogeneous and heterogeneous interfaces of group-VI transition metal dichalcogenides (MX2, where M = Mo, W, and X = S, Se) is presented. The force field is benchmarked against density functional theory calculations for bilayer systems within the Heyd-Scuseria-Ernzerhof hybrid density functional approximation, augmented by a nonlocal many-body dispersion treatment of long-range correlation. The parametrization yields good agreement with the reference calculations of binding energy curves and sliding potential energy surfaces. It is found to be transferable to transition metal dichalcogenide (TMD) junctions outside of the training set that contain the same atom types. Calculated bulk moduli agree with most previous dispersion-corrected density functional theory predictions, which underestimate the available experimental values. Calculated phonon spectra of the various junctions under consideration demonstrate the importance of appropriately treating the anisotropic nature of the layered interfaces. Considering our previous parametrization for MoS2, the anisotropic interlayer potential enables accurate and efficient large-scale simulations of the dynamical, tribological, and thermal transport properties of a large set of homogeneous and heterogeneous TMD interfaces.

Alumina intercalation to enhance the performance of BAW magnetic sensors: Micromagnetic and finite element analysis

The bulk acoustic wave (BAW) magnetic sensor, which is based on a thin-film bulk acoustic wave resonator and magnetoelectric coupling, has the technological advantage of passive wireless magnetic detection and can be applied to medical diagnostics and resource exploration to achieve ultra-weak magnetic field detection. However, the enhancement of BAW magnetic sensors' performance is severely limited by the localized magnetization phenomenon and the significant eddy current loss suffered by single-layer magnetic films. In this paper, the effect and internal physical mechanism of inserting different alumina intercalation layers into a magnetic film on the performance of the magnetic film and the BAW sensor were investigated, and the optimal design of the BAW magnetic sensor was achieved. First, single-layer FeGaB films and composite FeGaB/(Al2O3 5 nm/FeGaB)n films (n = 1, 2, 4, 6, and 10) were fabricated to characterize the effect of intercalation on the static and dynamic magnetic properties of the films. When four layers of 5 nm alumina were inserted, the magnetic film was found to have the finest soft magnetic properties. Further micromagnetic simulation and microstructural characterization demonstrated that the alumina intercalation enhances the magnetic film's microstructure, resulting in a complex interlayer interaction. Second, the BAW magnetic sensor's finite element simulation model was developed using COMSOL Multiphysics. The relaxation response mechanism of the magnetic film to an external alternating magnetic field was investigated using micromagnetism, and the energy transfer and coupling between the magnetic film and the piezoelectric layer were further analyzed. The effect of the alumina intercalation on the magnetic sensor's performance, including sensitivity and linearity, was quantified. Eventually, the operating frequency of the BAW magnetic sensor was matched to 2.5 GHz by refining the design conditions of inserting four layers of 5 nm alumina into FeGaB thin films. Within the magnetic field testing range of 50000–60000 A/m, simulation results indicate that the sensitivity can reach 0.0028 Vm/A, and the linearity of the output voltage is better than 3.85%.

Analyzing the bending deformation of van der Waals-layered materials by a semi-discrete layer model

Van der Waals (vdW)-layered materials, such as graphite, exhibit unique mechanical properties owing to their structural and mechanical anisotropies. This study reports the development of a mechanical model that reproduces the characteristics of the nonlinear and reversible bending deformation of vdW-layered materials, while taking into account the microscopic mechanism of the discrete interlayer slips. The vdW-layered material was modeled as a stack of interacting discrete deformable layers (semi-discrete layer model), and the interlayer interaction was modeled using a cohesive zone model that reproduced the localized interlayer slip. Using the finite-element method, out-of-plane bending deformation analyses were performed on the cantilevers of the highly oriented pyrolytic graphite (HOPG) and MoTe2, and the validity of the model was verified by comparing it with the experimental results. The model accurately reproduced the loading and unloading behaviors in the experiments for the submicron HOPG cantilevers or the large nonlinear and reversible deformation with a hysteresis loop. Furthermore, the model reproduced well the characteristics of the bending experiments for the micro-MoTe2 cantilevers, or the intermittent decrease in stiffness during the loading process and deformation restoration during the unloading process. These results demonstrated that the designed semi-discrete layer model can be universally applied to reproduce the bending deformation characteristics of a variety of vdW-layered materials and can be employed to effectively elucidate the underlying deformation mechanisms.

Emerging Characteristics and Properties of Moiré Materials.

In recent years, scientists have conducted extensive research on Moiré materials and have discovered some compelling properties. The Moiré superlattice allows superconductivity through flat-band and strong correlation effects. The presence of flat bands causes the Moiré material to exhibit topological properties as well. Modulating electronic interactions with magnetic fields in Moiré materials enables the fractional quantum Hall effect. In addition, Moiré materials have ferromagnetic and antiferromagnetic properties. By tuning the interlayer coupling and spin interactions of the Moiré superlattice, different magnetic properties can be achieved. Finally, this review also discusses the applications of Moiré materials in the fields of photocurrent, superconductivity, and thermoelectricity. Overall, Moiré superlattices provide a new dimension in the development of two-dimensional materials.

Converting the Bulk Transition Metal Dichalcogenides Crystal into Stacked Monolayers via Ethylenediamine Intercalation.

We report the synthesis of ethylenediamine-intercalated NbSe2 and Li-ethylenediamine-intercalated MoSe2 single crystals with increased interlayer distances and their electronic structures measured by means of angle-resolved photoemission spectroscopy (ARPES). X-ray diffraction patterns and transmission electron microscopy images confirm the successful intercalation and an increase in the interlayer distance. ARPES measurement reveals that intercalated NbSe2 shows an electronic structure almost identical to that of monolayer NbSe2. Intercalated MoSe2 also returns the characteristic feature of the monolayer electronic structure, a direct band gap, which generates sizable photoluminescence even in the bulk form. Our results demonstrate that the properties and phenomena of the monolayer transition metal dichalcogenides can be achieved with large-scale bulk samples by blocking the interlayer interaction through intercalation.

Leveraging Interlayer Interaction in M-N-C Catalysts for Enhanced Activity in Oxygen Reduction Reactions.

Atomically dispersed metal-nitrogen-carbon (M-N-C) materials are deemed promising catalysts for the oxygen reduction reaction (ORR) in fuel cells. Yet the multilayer nature of M-N-C has been largely neglected in computational analysis. To bridge the gap, we conducted a first-principles investigation using bilayer M-N-C models (TMNx/G-TMNy/G, TM = Mn, Fe, Co, Ni, Cu, G = graphene, x, y = 3 or 4), where the TMs on the top serves as the active center. While in-plane TMN4 at the bottom has a minimal impact on the ORR, out-of-plane TMN3 substantially influences the adsorption free energy of OH through a strong interlayer bonding interaction. By leveraging interlayer interactions, we appreciably lowered the overpotential of selected TMN4 (TM = Co, Ni, Cu) and achieved a minimum of 0.40 V on CoN4/G-CuN3/G. Constant potential calculations revealed weak dependence of OH binding energy on external voltage and obtained results comparable to constant charge calculation. This study provided new physical insight into modulating naturally occurring multilayer M-N-C catalysts.

Defects induced the bilayer graphene-copper hybrid and its effect on mechanical properties of graphene reinforced copper matrix composites

Graphene/Cu composites have shown great potential in the fields of mechanical engineering, electronic devices and so on. In this study, the impacts of vacancies on the interlayer interaction of bilayer graphene (BLG) and the resultant influence of graphene interlayer bonding on the interfacial bonding, tensile strength, and slipping mechanism of the graphene/Cu composites are comprehensively investigated based on first-principles calculations. It was revealed that clustering of cross-layer vacancies (CV) can initiate the formation of distinct hybrid types (sp2 hybrid, sp3 hybrid) of interlayer bonds, resulting in enhanced interlayer bonding of BLG. Unlike the van der Waals interaction between graphene and Cu, vacancies cause the Cu atoms on the surfaces to be embedded in the BLG, forming the graphene-copper hybrid that can improve the combination of Cu and graphene. The notable hybridization between the 2p orbitals of the unsaturated C atoms neighboring the vacancies and the 4s, 3d orbitals of the Cu atoms on the surfaces drives this effect. BLG with interlayer bonds can substantially improve the interfacial bonding and mechanical properties of the composites. This research provides a novel outlook for the interface engineering research and innovative development of graphene/Cu composites by regulating the interlayer bonding of grapheme.