Weak Interlayer Interaction Research Articles

Facilitating Charge Transfer via Ti-Knot Pathway in Electrochromic Three-Dimensional Metalated Covalent Organic Frameworks.

Due to the ordered one-dimensional channel as well as accessible redox sites, two-dimensional covalent organic frameworks (2D COFs) have garnered extensive attention in the field of electrochromism. However, organic 2D frameworks impose limitations on charge transfer and the weak interlayer interactions in 2D COFs, adversely affecting the stability during switching processes. Herein, we introduced Ti knots to construct three-dimensional metalated covalent organic frameworks (3D MCOFs), denoted as Ti-DHTA-Py. The Ti knots not only serve as templates for organizing organic units into unique 3D topological structures in a controlled manner but also establish charge transfer pathways conducive to electron delocalization and transmission within the framework. As a result, the 3D Ti-DHTA-Py MCOFs electrode exhibited a reduced band gap and remarkable electrochromic (EC) performances: electrochemical cyclic stability of 93.6% retention after 500 cycles, switching times (2.5 s/0.5 s), and a high coloration efficiency (423 cm2 C-1). This research underscores the potential of 3D MCOFs as promising candidates for advancing EC technologies, surmounting the limitations associated with traditional 2D COFs.

Crystal structures and electronic and magnetic properties of Janus bilayer Cl[formula omitted]Cr[formula omitted]I[formula omitted]

Two-dimensional (2D) magnetic material CrI3 has aroused extensive attention, because it could provide a new platform for investigating the relations between crystal structures and electronic and magnetic properties. Here, we study crystal structures and electronic and magnetic properties of three configurations (Cl-Cl I-I and Cl-I) of Janus bilayer Cl3Cr2I3 with two stacking orders (AB and AA1/3) by using the first principles approach incorporating the spin–orbit coupling (SOC) effect and the dipole correction. It is found that the spin polarization and the SOC effect can expand the lattice constant and the interlayer distance (ID) of the three configurations. Especially, the ID of the I-I configuration is 1 Å larger than that of the Cl-Cl configuration. The total energy calculation results show that the atomic configuration, the SOC effect and the stacking order play an important role in determining the magnetic ground states of the Janus layers. Our results indicate the atomic configurations of the Janus bilayers are not conducive to the increase of critical temperature. Interestingly, the Cl-I configuration has a vertical dipole moment, and its AFM state has large spin splitting. It is revealed that the non-periodic structure, the symmetry breaking of the average potential and the weak interlayer interaction lead to the vertical dipole moment and the abnormal AFM state that is not the most stable state.

Contributions of Edge and Internal Atoms to the Friction of Two-Dimensional Heterojunctions.

The weak interlayer interaction and strong intralayer bonding in Van der Waals (vdW) layered materials give rise to ultralow friction at incommensurate contact interfaces, a phenomenon known as structural superlubricity. This phenomenon is complicated by the interplay between atomic degrees of freedom, twist angle, and normal force. In this Letter, we exploit naturally occurring cracks in vdW crystals and microfabrication techniques to qualitatively separate the contributions of edge, complete moiré tile, and incomplete moiré rim regions. It is observed that friction from the incomplete moiré rim region, scaling linearly with the rim area, plays a dominant role in determining the twist angle dependence of friction. Interestingly, despite lower friction contributions from complete moirés, friction from a complete moiré tile is independent of moiré size; consequently, friction from the moiré tile scales linearly with the total number of moirés. By integrating the contributions from edge, moiré tile, and incomplete moiré rim, a friction law for vdW heterojunctions is proposed. Finally, friction changes from positively to negatively correlating with normal force, contingent on the suppression of moiré ridge and the dissipation from edge atoms.

Out-of-Plane Electron Transport in Graphite Intercalation Compounds with Applications in Battery Anodes

Graphite intercalation compounds (GICs) are commonly used as anode materials for alkali metal-ion batteries, such as Li-ion and K-ion batteries. Although typically prepared as a low-stage intercalation compound (with a high concentration of the intercalating metal), over several electrochemical cycles, the distribution of intercalate within the graphite sample is expected to be non-uniform with domains of high-stage regions where the intercalant fraction is low [1]. The mechanism of interlayer electron transport in GICs strongly depends on the intercalation staging and temperature, and is expected to vary over the lifetime of a battery.High-stage GICs and highly oriented pyrolytic graphite (HOPG), which is the upper limit of infinite-stage GIC, have an out-of-plane electrical conductivity that exponentially increases with temperature, whereas low-stage GICs have a conductivity that decreases as a function of temperature [2, 3, 4]. Although out-of-plane electron transport in low-stage GICs can be described by a metallic conduction model, the mechanism of out-of-plane electron transport in HOPG and high-stage GICs is a hitherto unresolved problem [5]. Out-of-plane electrical conductivity for HOPG and high-stage GICs increases exponentially with temperature, which cannot be described by metallic conduction, nor by conventional impurity or defect hopping mechanisms [6].We propose a new mechanism of electron transport that takes place in HOPG and high-stage GICs at temperatures relevant to battery operation. Electrons are transferred between graphite and intercalant layers through tunneling that is enhanced by large amplitude out-of-plane phonon modes in graphite layers. Due to weak interlayer interactions in adjacent graphite layers, GICs can accommodate large amplitude phonon modes in the out-of-plane direction that locally enhance the interlayer electron tunneling probability. The tunneling enhancement is temperature dependent, and exponentially depends on the population of out-of-plane phonon modes excited at a given temperature. The expression for the conductivity 𝜎 for the phonon amplitude-driven tunneling mechanism as a function of temperature T is shown in Eq. 1; the exponential dependence of conductivity with temperature qualitatively agrees with experimental observations in HOPG and high-stage GICs. On a quantitative level, we also observe from Fig.1 that the theory agrees with experimental values for out-of-plane conductivity in HOPG.The phonon amplitude-driven tunneling mechanism of electron transport is relevant in describing the kinetic over-potential in batteries that employ layered materials as anodes in ambient temperature operating conditions, and optimally sizing the anode accounting for both electron transport and ion diffusion rates.

Weak interlayer interaction and enhanced electron mobility in multilayer WS2 achieved through a layer-by-layer wet transfer approach

Weak interlayer interaction and enhanced electron mobility in multilayer WS2 achieved through a layer-by-layer wet transfer approach

Enhanced interlayer interaction in sulfonated CONs membrane by amino-rich CONs enabling ultrafast proton transport

Covalent organic framework nanosheets (CONs) with porous crystalline features and ultrathin thickness are ideal candidates as membrane building blocks to form well-defined transfer nanochannels. The formidable challenge behind self-supporting CONs membrane lies in weak non-covalent interlayer interactions and thus loose stacking, insufficient strength and structure stabilities. Herein, we propose the fabrication of interlayer force-strengthened freestanding CONs membrane through the electrostatic attraction bridge effect of positively-charged amino-rich CONs (CON–NH2) to negatively-charged sulfonated CONs (CON–SO3H). Ultrathin and large lateral sized CON–SO3H and CON–NH2 are synthesized, followed by restacking to prepare freestanding CONs membrane with CON–SO3H as the membrane bulk. Benefiting from effective interlayer interconnection due to strong electrostatic interaction, the obtained CON–SO3H/CON–NH2 membrane displays features of ultrahigh integrity, dense stacking, eminent water/acid/base/organic solvents stabilities and mechanical strength (109 MPa). The shortened –SO3H distance contributes to construct site-continuous transfer pathways, and the deprotonated –SO3H and protonated –NH2 form acid–base pairs to decrease interfacial resistance, which impart membrane superior proton conductivity of 486 mS cm−1 (80 °C, 100% RH). This interlayer force enhancement strategy offers a promising perspective on achieving densely-stacked CONs membrane with ultrahigh mechanical property and conduction performance for fuel cell application.

Tunable magnetism and band structure in kagome materials RETi3Bi4 family with weak interlayer interactions

Tunable magnetism and band structure in kagome materials RETi3Bi4 family with weak interlayer interactions

Weak interlayer interactions and nearly temperature independent electrical transport in p-type 1T′-MoTe2/Sb2Te3 superlattice-like films

Thermoelectric superlattices (SLs) are one of the important focuses in the thermoelectric field due to their ordered stacking structure and the potential to obtain extremely high thermoelectric performance. To understand the critical effects of interlayer interactions, this work fabricates p-type (1T′-MoTe2)x(Sb2Te3)y SLs and investigates their electrical properties as functions of SL period and temperature. The results discovered that the weak interlayer interactions could be driven by both the small work function difference and temperature. This brings forth hole donation and energy filtering and leads to increased hole density and the carrier effective mass of (1T′-MoTe2)x(Sb2Te3)y SLs. Strikingly, the SL structure is beneficial for achieving temperature independent electrical properties, owing to the temperature driven interlayer interaction. Several SLs acquire the largest average power factor among all SLs, which is about 1.10 mWm−1K−2 within 300∼473 K, favorable for thermoelectric applications near room temperature. This work points out the beneficial effect of the SL structure on obtaining high thermoelectric properties in a wide temperature range.

Thermally Activated Photoluminescence Induced by Tunable Interlayer Interactions in Naturally Occurring van der Waals Superlattice SnS/TiS2.

van der Waals (vdW) superlattices, comprising different 2D materials aligned alternately by weak interlayer interactions, offer versatile structures for the fabrication of novel semiconductor devices. Despite their potential, the precise control of optoelectronic properties with interlayer interactions remains challenging. Here, we investigate the discrepancies between the SnS/TiS2 superlattice (SnTiS3) and its subsystems by comprehensive characterization and DFT calculations. The disappearance of certain Raman modes suggests that the interactions alter the SnS subsystem structure. Specifically, such structural changes transform the band structure from indirect to direct band gap, causing a strong PL emission (∼2.18 eV) in SnTiS3. In addition, the modulation of the optoelectronic properties ultimately leads to the unique phenomenon of thermally activated photoluminescence. This phenomenon is attributed to the inhibition of charge transfer induced by tunable intralayer strains. Our findings extend the understanding of the mechanism of interlayer interactions in van der Waals superlattices and provide insights into the design of high-temperature optoelectronic devices.

Leaf-inspired hybrid membrane assembled by covalent organic framework and cellulose nanocrystals for efficient desalination

Covalent organic framework (COF) membranes hold great promise in molecular separations. However, COF membranes fabricated from COF nanosheets only often suffer from boundary defects with undesirable separation performance due to the small lateral size and weak interlayer interaction of COF nanosheets. Inspired by the 2D lamellar structure of leaf where 1D veins distributed in the leaflets render them integrity and robustness, vein-mimic 1D sulfonated cellulose nanocrystals (S–CNC) were assembled with the leaflet-mimic COF nanosheets to fabricate S–CNC–COF hybrid membranes. The hydrogen bonding between crystalline COF nanosheets and hydrophilic S–CNC could effectively eliminate boundary defects. More importantly, S–CNC with high rigidity could ensure the oriented assembly of COF nanosheets during the membrane formation process. With ordered structure, high hydrophilicity, and reduced mass transport resistance, the optimal S–CNC–COF membrane exhibits an excellent water flux of 314.93 kg m−2 h−1 and rejection of NaCl (99.99%). This leaf-inspired strategy sets up a platform for designing advanced hybrid COF membranes for molecular separations.

Electric-field-controlled Schottky barriers in BSe/M2CF2 (M = Ta, W) van der Waals heterostructures: A computational study

Constructing two-dimensional (2D) van der Waals (vdW) heterostructures is regarded as an effective strategy to explore novel properties and enhance the performance of 2D materials. Herein, taking full advantage of boron selenide (BSe) and MXenes, 2D BSe/MXene vdW heterostructures are designed and their electronic and interfacial properties are investigated via first-principles calculations, by using BSe/Ta2CF2 and BSe/W2CF2 as models. The band structures of the BSe and M2CF2 (M = Ta, W) layers in the BSe/M2CF2 heterostructures are preserved quite well due to the weak vdW interlayer interaction. The Fermi level shifts towards the conduction band minimum of the BSe layer, leading to an n-type Schottky contact. Interestingly, the interface charge redistribution in BSe/W2CF2 is more pronounced compared to BSe/Ta2CF2, which can be attributed to the disparity in electronegativity between Ta and W. In particular, the Schottky barrier heights of both heterostructures vary linearly with the vertical electric field within a certain range. This allows for the tuning of the Schottky contact from n-type to p-type. However, the variation rate in BSe/W2CF2 is comparatively lower than that in BSe/Ta2CF2. Moreover, when the positive electric field exceeds 1.0 V/Å, a transition from the Schottky contact to the Ohmic contact in BSe/W2CF2 occurs, whereas the contact remains the Schottky contact in BSe/W2CF2. This work offers valuable insights into developing electronic nano-devices with tunable Schottky barriers using vdW heterostructures.

Improving the electrical performances of InSe transistors by interface engineering

InSe has emerged as a promising candidate for next-generation electronics due to its predicted ultrahigh electrical performance. However, the efficacy of the InSe transistor in meeting application requirements is hindered due to its sensitivity to interfaces. In this study, we have achieved notable enhancement in the electrical performance of InSe transistors through interface engineering. We engineered an InSe/h-BN heterostructure, effectively suppressing dielectric layer-induced scattering. Additionally, we successfully established excellent metal–semiconductor contacts using graphene ribbons as a buffer layer. Through a methodical approach to interface engineering, our graphene/InSe/h-BN transistor demonstrates impressive on-state current, field-effect mobility, and on/off ratio at room temperature, reaching values as high as 1.1 mA/μm, 904 cm2⋅V−1⋅s−1, and >106, respectively. Theoretical computations corroborate that the graphene/InSe heterostructure shows significant interlayer charge transfer and weak interlayer interaction, contributing to the enhanced performance of InSe transistors. This research offers a comprehensive strategy to elevate the electrical performance of InSe transistors, paving the way for their utilization in future electronic applications.

Laser manufactured-liquid metal nanodroplets intercalated Mxene as oil-based lubricant additives for reducing friction and wear

Two-dimensional (2D) material MXene is a research hotspot in lubricating materials because of its unique layered structure, which provides weak interlayer interaction and easy shear ability. Herein, the liquid metal nanodroplets intercalated MXene were successfully prepared via pulsed laser treatment and self-assembly method. First, zero-dimensional (0D) nano gallium-based liquid metal (GLM) was synthesized by pulsed laser irradiation of bulk Ga80In20 in acetone. Then, the GLM nanodroplets were loaded onto 2D MXene nanosheets via the effect of electrostatic adsorption to prepare MXene@GLM composite material. The as-obtained GLM not only widens the interlayer distance between MXene nanosheets, making it more susceptible to interlayer shear, but also enhances its lipophilicity. The friction test results showed that the MXene@GLM has the best lubrication performance with 1.0 wt% additive. The coefficient of friction (COF) of base oil PAO-6 can decrease from 0.79 to 0.097, the wear volume is reduced by 90.3 %, and the maximum load sustained reaches 950 N. The good tribological properties are mainly owing to the synergistic lubrication of GLM and MXene, which can form a continuous and firm tribofilm on the friction contact surface and avoid the direct contact of the friction pair.

Charge density fluctuation determined the interlayer friction in bilayer MN4 (M = Be, Mg, and Pt).

MN4 (M = Be, Mg, and Pt) represents a new class of van der Waals materials. These materials are characterized by exceptional electrical and thermal conductivities, remarkable intralayer mechanical strength, and weak interlayer interactions, making them prone to shearing and slipping. Therefore, MN4 has significant potential applications as a solid lubricant. However, until now, there have been only limited comprehensive theoretical investigations focusing on the frictional properties of MN4 systems. Here, the frictional performances of MN4 are systematically analyzed by applying first-principles high-throughput calculations. The results reveal that interlayer friction of MN4 decreases from MgN4 to BeN4 and then to PtN4. The friction is directly determined by charge density variations during the sliding processes. The periodic formation and breaking of quasi-σ bonds in bilayer MgN4 leads to substantial variations in charge density and large interlayer friction. In contrast, the weak charge density alternations in PtN4 lead to rather low frictions in PtN4. Moreover, surface functionalization effectively diminishes friction within bilayer MgN4, but amplifies interlayer friction within bilayer PtN4, and under surface functionalization interlayer friction can be efficiently modulated by out-of-plane polarizations. Interestingly, HBr-MgN4 exhibits two orders of magnitude lower COF compared to intrinsic bilayer MgN4, leading to a phenomenon resembling superlubricity. These results significantly contribute to our understanding of the friction properties, offering valuable guidance for the practical implementation of MN4 in solid lubricants.

Structural superlubricity at the interface of penta-BN2.

Two-dimensional (2D) materials have been widely used as lubricants due to their weak interlayer interaction and low shear resistance for interlayer sliding. Composed entirely of five-membered rings, penta-BN2 monolayer has excellent thermal and mechanical stability, higher hardness and a negative Poisson's ratio. In this work, we investigate the frictional properties at both the commensurate and incommensurate contacting interfaces of penta-BN2 by adopting the molecular dynamics (MD) simulation method. Our calculations demonstrate robust superlubricity at the incommensurate contacting interface of penta-BN2. The ultra-low friction is explained by the potential energy surface (PES) fluctuations, interlayer binding energy and out-of-plane motion amplitude of the sliding layer. In addition, our calculations show that the anisotropy of friction at the commensurate contacting interface is more obvious compared with that at the incommensurate contacting interface. Finally, the influences of the size of the Moiré pattern, normal force, temperature and sliding velocity on the friction are examined. Our results show that 2D penta-BN2 is a promising solid lubricant, enriching the family of 2D lubrication materials.

Rhombohedrally stacked layered transition metal dichalcogenides and their electrocatalytic applications.

Layered transition metal dichalcogenides (TMDCs) are extensively investigated as catalyst materials for a wide range of electrochemical applications due to their high surface area and versatile electronic and chemical properties. Bulk TMDCs are van der Waals solids that possess strong in-plane bonding and weak inter-layer interactions. In the few-layer 2D TMDCs, several polymorphic structures have been reported as each individual layer can either retain octahedral or trigonal prismatic coordination. Among them, 1T (tetragonal), 2H (hexagonal) and 3R (rhombohedral) phases are very common. These polymorphs can display discrepancies in their catalytic activity as their electronic structure diverges due to different d orbital filling states. The broken inversion symmetry and large exposed edge sites of some of the 3R-phase TMDCS such as MoS2, NbS2 and TaS2 appear to be advantageous for electrocatalytic water reduction and battery applications. We describe recent studies in phase engineering of 2D TMDCs, particularly aiming at the 3R polytype and their electrocatalytic properties. Redox ability primarily depends on a distinct polymorphic phase in which TMDCs are isolated, and hence, with rich polymorphic structures being reported, numerous new catalytic applications can be realized. Phase conversion from 2H to 3R phase in some TMDCs enhances structural integrity and establishes robustness under harsh chemical conditions.

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.

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.

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.

Interfacing differently oriented biaxial van der Waals crystals for negative refraction

Abstract Negative refraction has a wide range of applications in diverse fields such as imaging, sensing, and waveguides and typically entails the fabrication of intricate metamaterials endowed with hyperbolic features. In contrast to artificially engineered hyperbolic materials, natural van der Waals (vdW) materials are more accessible owing to their inherent strong in-plane covalent bonding and weak interlayer interactions. However, most vdW materials manifest uniaxial crystal properties, which restrict their behavior solely to out-of-plane hyperbolicity. This characteristic poses a considerable challenge to their seamless integration via planar fabrication techniques, unless a suitable pattern is employed. Recent advances have identified natural biaxial α-MoO3 as a promising vdW material capable of exhibiting in-plane hyperbolicity. In this study, we performed numerical simulations demonstrating that negative refraction could be achieved by interfacing differently oriented α-MoO3 slabs coated with tunable graphene on a gold substrate. Our comprehensive analysis yielded three notable outcomes: negative refraction, simultaneous positive and negative refractions, and diffractionless propagation. These outcomes could be operated in a broad range of frequencies and achieved at all angles to offer a superior platform for the flexible manipulation of mid-infrared polaritons. Our findings provide valuable insights into the potential application of other two-dimensional vdW materials for advances in nanoscale super-resolution imaging, molecular sensing, and on-chip photonic integrated circuits.