Interlayer Interaction Research Articles

Three-dimensional ultrafast charge-density-wave dynamics in CuTe

Charge density waves (CDWs) involved with electronic and phononic subsystems simultaneously are a common quantum state in solid-state physics, especially in low-dimensional materials. However, CDW phase dynamics in various dimensions are yet to be studied, and their phase transition mechanism is currently moot. Here we show that using the distinct temperature evolution of orientation-dependent ultrafast electron and phonon dynamics, different dimensional CDW phases are verified in CuTe. When the temperature decreases, the shrinking of c-axis length accompanied with the appearance of interchain and interlayer interactions causes the quantum fluctuations (QF) of the CDW phase until 220 K. At T < 220 K, the CDWs on the different ab-planes are finally locked with each other in anti-phase to form a CDW phase along the c-axis. This study shows the dimension evolution of CDW phases in one CDW system and their stabilized mechanisms in different temperature regimes.

Variable Mechanical Properties in Layered Copper Bromide Hybrid Solids Based on Configuration of Organic Cations.

Organic-inorganic organohalide hybrid solids have garnered significant attention due to their unique structural diversity, desirable electronic structures, and excellent optoelectronic properties. Here, we investigate the mechanical properties of a homologous series of 2D copper bromide hybrids (A2CuBr4, ACuBr4) through nanoindentation to analyze the contribution of organic interlayer interactions to bulk mechanical response. We identify a direct correlation between the identity of the organic spacer and the bulk mechanical response, where stronger bonding interactions in the organic interlayer resulted in increased hardness and elasticity. Additionally, we uncover a unique conformational dependence within the series of arylammonium spacers which was found to periodically modulate pi-pi interactions between neighboring molecules, leading to an alternating even-odd bulk material response.

Unified understanding to the rich electronic-structure evolutions of two-dimensional black phosphorus under pressure

The electronic-structure evolutions of few-layer black phosphorus (BP) under pressure shows a wealth of phenomena, such as the nonmonotonic change of direct gap at the Γ point, the layer-number dependence, and the distinct responses to normal and hydrostatic pressures. A full and unified understanding to these rich phenomena remains lacking. Here, we provide a unified understanding from the competition between quasibonding (QB) interactions and chemical bonding interactions. The former decreases while the latter increases the band gap under pressure and the origin can be correlated to different combinations of inter- and intralayer antibonding or bonding interactions at the band edges. More interestingly, the interlayer QB interactions are a coexistence of two categories of interactions, namely, the coexistence of interactions between bands of the same occupancy (occupied-occupied and empty-empty interactions) and of different occupancies (occupied-empty interaction); and, the overall effect is a four-level interaction, which explains the anomalous interlayer-antibonding feature of the conduction band edge of bilayer BP. Our current study lays the foundation for the electronic-structure tuning of two-dimensional (2D) BP, and, our analysis method for multi-energy-level interactions can be applied to other 2D semiconductor homo- and heterostructures that have occupied-empty interlayer interactions. Published by the American Physical Society 2024

2D/0D multiscale interfacial engineering of carbon fiber composites for simultaneous achieving high mechanical and electromagnetic interference shielding properties

AbstractIn the face of escalating electromagnetic pollution, it is urgently needed to expedite the development of innovative carbon fiber reinforced polymer (CFRP) composites that not only demonstrate superior mechanical properties as well as durability but also boast exceptional electromagnetic interference shielding (EMI) performance. However, given the absence of robust interface interactions and the lack of an outstanding nanostructured conductive network, there remains a need to further elevate the overall performance of CFRP composites. Herein, a highly continuous conductive network by integrating 2D MXene and 0D Ag nanoparticles with strong interlayer interactions was constructed through point‐to‐plane conductive modes, facilitated by polydopamine (PDA). Thanks to the excellent electromagnetic wave loss and the unique multi‐dimensional structure arising from the optimizing interfacial engineering, the EMI shielding effectiveness of the modified CF‐PDA‐M‐Ag/EP composite achieved a remarkable 45.9 dB, obtaining a 99.5% enhancement compared with the neat CF/EP. The synergistic effect resulting from the multi‐dimensional structure and strong interaction can improve the interfacial shear strength (IFSS), interlaminar shear strength (ILSS) and flexure strength of the composite up to 81.9, 85.8 and 864.6 MPa, respectively, far superior to the unmodified composite materials. This research provides a dependable and convenient approach for exploiting high‐performance CFRP composites with intriguing electromagnetic interference (EMI) shielding properties.Highlights An optimizing multifunctional 2D/0D interface for carbon fiber was designed. A highly conductive network via the point‐to‐plane conductive mode. The EMI shielding effectiveness of the composite reached up to 45.9 dB.

Anomalous layer-dependent photoluminescence spectra of supertwisted spiral WS2.

Twisted stacking of two-dimensional materials with broken inversion symmetry, such as spiral MoTe2 nanopyramids and supertwisted spiral WS2, emerge extremely strong second- and third-harmonic generation. Unlike well-studied nonlinear optical effects in these newly synthesized layered materials, photoluminescence (PL) spectra and exciton information involving their optoelectronic applications remain unknown. Here, we report layer- and power-dependent PL spectra of the supertwisted spiral WS2. The anomalous layer-dependent PL evolutions that PL intensity almost linearly increases with the rise of layer thickness have been determined. Furthermore, from the power-dependent spectra, we find the power exponents of the supertwisted spiral WS2 are smaller than 1, while those of the conventional multilayer WS2 are bigger than 1. These two abnormal phenomena indicate the enlarged interlayer spacing and the decoupling interlayer interaction in the supertwisted spiral WS2. These observations provide insight into PL features in the supertwisted spiral materials and may pave the way for further optoelectronic devices based on the twisted stacking materials.

Electronic structures and quantum capacitance of twisted mixed-dimensional van der Waals heterostructures of graphene/C2H based on tight-binding model

Constructing twisted mixed dimensional graphene-based van der Waals heterostructure (vdWH) is an effective strategy to manipulate the electronic structures and improve the quantum capacitance (C q) of graphene. In this work, mixed dimensional vdWH of graphene/C2H has been proposed owing to similar Dirac semimetal character of one-dimensional C2H with that of graphene. Meanwhile, the influence of twisting angle (θ) and interlayer interaction strength on the electronic structures and the C q of the MD vdWH are systemically explored based on tight binding model. With the fitted hopping integral parameters, it is found that the linear dispersion of the graphene is basically preserved but the bandwidth is decreased with modulating twisting angle and interlayer interaction, and the C q of mixed dimensional vdWH is improved 5–19 times compared with graphene at zero bias. Moreover, the compressed strain could enhance the C q of mixed dimensional vdWH to 74.57 μF cm−2 at zero bias and broaden the low working voltage window of mixed-dimensional vdWH with considerable C q. Our results provide suitable tight-binding model parameters and theoretical guidance for exploring the twisted MD vdWH of graphene/C2H and offer an effective strategy to modulate the electronic structures and the C q of graphene through constructing the MD vdWH.

Strain-induced modification in thermal properties of monolayer 1T-ZrS2 and ZrS2/ZrSe2 heterojunction.

This paper systematically analyzes the phonon dispersion curves of single-layer ZrS2, ZrSe2, and ZrS2/ZrSe2 heterostructures under different strains. The phonon spectra and thermal parameters of the three structures were obtained based on the density functional perturbation theory method. The upper limits of strain that different monolayers and heterojunctions can withstand were studied. The monolayers ZrSe2 and ZrS2 can withstand up to 8% biaxial tensile strain, and the ZrS2/ZrSe2 heterojunction can withstand up to 6% biaxial tensile strain. In addition, the van der Waals force of the heterojunction may cause phonon tearing in the vertical direction. The application of biaxial tensile strain can adjust the thermal properties of the system to a large extent, which is similar to the strain effect in the pristine case. When the temperature rises, the entropy enthalpy of the three structures also gradually increases, the free energy gradually decreases, and the heat capacity of the system gradually increases until it tends to be stable. Taking single-layer ZrS2 as an example, we analyzed the change curve of thermal properties of single-layer ZrS2 under tensile strain. The results show that with the gradual increase of strain, the crystal's entropy, enthalpy, and free energy change differently. In addition, the heat capacity increases slowly under high temperatures. When all systems reach the limit strain, the phonon spectrum appears to have an imaginary frequency, and the thermal properties decrease significantly. This paper uses the first-principle calculation method based on density functional theory, and the PBE exchange-correlation function based on generalized gradient approximation (GGA) is selected for a specific calculation. The density functional perturbation theory (DFPT) calculates the full kinetic matrix. Because the lattice constants of ZrS2 and ZrSe2 are similar and have similar periodicity, the corresponding unit cells are used for structural optimization and property calculation. The Brillouin zone is integrated using the K points generated by the Monkhorst-pack method. For single-layers ZrS2 and ZrSe2, 8 × 8 × 1K-point grid is selected, and for ZrS2/ZrSe2 heterojunction, 8 × 8 × 2K-point grid is selected. A vacuum layer of 30Å was added in the vertical direction to avoid interlayer interaction. The non-conservative pseudopotential method is used to optimize the structure, and the optimization convergence is set as follows: the cutoff energy is set to 700eV, the convergence threshold of the maximum force between atoms is 0.01eV/Å, the convergence threshold of the maximum energy change is set to 1 × 10-9eV, and the convergence threshold of the maximum displacement is 0.001Å.

Understanding layered compounds under high pressure

This Tutorial focuses on the physics of layered compounds under high pressure. We have chosen h-BN and III–VI layered materials as representative materials. h-BN layers are strictly two-dimensional. Layers in III–VI compounds are more complex, and subtle details in their structural behavior play an important role in the evolution of high pressure properties. They are also interesting because they contain a different number of layers in their primitive unit cell and/or have a different ionic character. We begin describing the structural evolution. We discuss the experimental challenges encountered as well as the main findings related to intra- and interlayer compressibility, polytype influence, and geometrical modifications induced by pressure inside the layers. We then describe lattice vibrations. The origin of the modes is reviewed, paying attention to the relationships between atom motions in different layers. We discuss the convenience of redefining the Grüneisen parameter and describe the behavior of rigid layer modes, soft modes, and Davidov pairs. The last section is devoted to the electronic properties. We show that the changes observed when passing from a single layer to a three-dimensional BN are qualitatively similar to those induced by high pressure. The pressure behavior of electronic transitions in III–VI layered compounds is very rich, revealing the subtle balance between intra- and inter-layer interactions. Finally, we take advantage of high pressure studies to explain the formation of the Mexican hat type of valence band at ambient conditions in single layers of InSe and GaSe, but not in three-dimensional compounds.

Crystallization of molecular layers produced under confinement onto a surface

It is well known that molecules confined very close to a surface arrange into molecular layers. Because solid-liquid interfaces are ubiquitous in the chemical, biological and physical sciences, it is crucial to develop methods to easily access molecular layers and exploit their distinct properties by producing molecular layered crystals. Here we report a method based on crystallization in ultra-thin puddles enabled by gas blowing, which allows to produce molecular layered crystals with thickness down to the monolayer onto a surface, making them directly accessible for characterization and further processing. By selecting four molecules with different types of polymorphs, we observed exclusive crystallization of polymorphs with Van der Waals interlayer interactions, which have not been observed with traditional confinement methods. In conclusion, the gas blowing approach unveils the opportunity to perform materials chemistry under confinement onto a surface, enabling the formation of distinct crystals with selected polymorphism.

3D Graphene Straintronics for Broadband Terahertz Modulation

AbstractThe increasing utilization of terahertz (THz) bandwidth in both industrial and private sectors highlights the significance of efficient terahertz shielding and absorption devices. These devices play a crucial role in safeguarding electronic components from disruptive effects and rendering objects less detectable by radar systems. However, the limited availability of materials and devices hinders progress in this field. In this study, a strain engineering route is presented for the active control of terahertz shielding and absorption properties in 3D graphene through the application of mechanical strain. A straintronic modulator based on 3D graphene is demonstrated, capable of modulating absorption and reflection of THz radiation in real‐time over a wide range of 0.1–3 THz. The modulator can be tuned to exhibit either shielding capability with a specific shielding effectiveness of 0.3 × 105 dB cm2 g−1 or stealth characteristics with an average reflection loss of 25 dB and 99.4% absorption. These findings open new avenues for leveraging 2D materials in their 3D porous form, where strain‐induced changes in interlayer interactions enable control over the properties of these materials. This discovery unveils vast unexplored physical phenomena with immense potential for advanced THz imaging, radar, and electromagnetic applications.

MXene‐Based Semiconductor Materials for Various Applications in Photocatalysis Field

The escalating issues of energy crises and environmental pollution have drawn increasing attention for photocatalysis as a green and sustainable solution. In this context, the emergence of MXene materials has injected new vitality into the field of photocatalysis. These materials are garnering significant interest due to their rich surface functional groups, interlayer interactions, and tunable interlayer spacing. Hence, this review provides an overview of the role of MXene materials in photocatalytic technology, using various theoretical simulations analysis methods on these MXene materials and the various applications involved in photoreduction of CO2 to the valuable products, photocatalytic H2 evolution from water splitting, photodegradation of organic contaminants, and so forth. Furthermore, the current challenges and potential directions are discussed for perfecting the photocatalysis application of MXene materials. It is sincerely hoped that this review will stimulate more research attention to meet the needs of practical applications in the future.

Two-Dimensional Spin-Crossover Molecular Solid Solutions with Tunable Transition Temperatures across 90 K.

Spin-crossover (SCO) materials exhibit remarkable potential as bistable switches in molecular devices. However, the spin transition temperatures (Tc) of known compounds are unable to cover the entire ambient temperature spectrum, largely limiting their practical utility. This study reports an exemplary two-dimensional SCO solid solution system, [FeIII(H0.5LCl)2-2x(H0.5LF)2x]·H2O (H0.5LX = 5-X-2-hydroxybenzylidene-hydrazinecarbothioamide, X = F or Cl, x = 0 to 1), in which the adjacent layers are adhered via hydrogen bonding. Notably, the Tc of this system can be fine-tuned across 90 K (227-316 K) in a linear manner by modulating the fraction x of the LF ligand. Elevating x results in strengthened hydrogen bonding between adjacent layers, which leads to enhanced intermolecular interactions between adjacent SCO molecules. Single-crystal diffraction analysis and periodic density functional theory calculations revealed that such a special kind of alteration in interlayer interactions strengthens the FeIIIN2O2S2 ligand field and corresponding SCO energy barrier, consequently resulting in increased Tc. This work provides a new pathway for tuning the Tc of SCO materials through delicate manipulation of molecular interactions, which could expand the application of bistable molecular solids to a much wider temperature regime.

Spinodal decomposition and domain coarsening in a multilayer Cahn-Hilliard model for lithium intercalation in graphite.

During the intercalation of lithium in layered host materials such as graphite, lithium atoms can move within the plane between two neighboring graphene sheets, but cannot cross the sheets. Repulsive interactions between atoms in different layers lead to the existence of ordered phases called "stages," with stage n consisting of one filled layer out of n, the others being empty. Such systems can be conveniently described by a multilayer Cahn-Hilliard model, which can be seen as a mean-field approximation of a lattice-gas model with intra- and interlayer interactions between lithium atoms. In this paper, the dynamics of stage formation after a rapid quench to lower temperature is analyzed, both by a linear stability analysis and by numerical simulation of the full equations. In particular, the competition between stages 2 and 3 is studied in detail. The linear stability analysis predicts that stage 2 always grows the fastest, even in the composition range where stage 3 is the stable equilibrium state. This is borne out by the numerical simulations, which show that stage 3 emerges only during the nonlinear coarsening of stage 2. Some consequences of this finding for the charge-discharge dynamics of electrodes in batteries are briefly discussed.

Crucial role of interfacial interaction in 2D polar SiGe/GeC heterostructures

The planar charge transfer is a distinctive characteristic of the two-dimensional (2D) polar materials. When such 2D polar materials are involved in vertical heterostructures (VHs), in addition to the van der Waals (vdW) interlayer interaction, the interfacial interaction triggered by the in-plane charge transfer will play a crucial role. To deeply understand such mechanism, we conducted a comprehensive theoretical study focusing on the structural stability and electronic properties of 2D polar VHs built by commensurate SiGe/GeC bilayers with four species ordering patterns (classified as a C-group with patterns I and II and a Ge-group with patterns III and IV, respectively). It was found that the commensurate SiGe/GeC VHs are mainly stabilized by interfacial interactions (including the electrostatic interlayer bonding, the vdW force, as well as the sp 2/sp 3 orbital hybridization), with the Ge-group being the most energetically favorable than the C-group. A net charge redistribution occurs between adjacent layers, which is significant (∼0.23–0.25 e cell−1) in patterns II and IV, but slightly small (∼0.05–0.09 e cell−1) in patterns I and III, respectively, forming spontaneous p–n heterojunctions. Such interlayer charge transfer could also lead to a polarization in the interfacial region, with the electron depletion (accumulation) close to the GeC layer and the electron accumulation (depletion) close to the SiGe layer in the C-group (the Ge-group). This type of interface dipoles could induce a built-in electric field and help to promote photogenerated electrons (holes) migration. Furthermore, a semi-metal nature with a tiny direct band gap at the SiGe layer and a semiconducting nature at the GeC layer indicate that the commensurate SiG/GeC VHs possess a type-I band alignment of heterojunction and have a wide spectrum of light absorption capabilities, indicating its promising applications for enhancing light-matter interaction and interfacial engineering.

Chiral Magnetic Phases in Moire Bilayers of Magnetic Dipoles

AbstractIn magnetic insulators, the sense of rotation of the magnetization is associated with novel phases of matter and exotic transport phenomena. Aimed to find new sources of chiral magnetism rooted in intrinsic fields and geometry, twisted square bilayers of magnetic dipoles with easy plane anisotropy are studied. For no twist, each lattice settles in the zig‐zag magnetic state and orders antiferromagnetically to the other layer. The moire patterns that result from the mutual rotation of the two square lattices influence such zig‐zag order, giving rise to several phases that depict non‐collinear magnetic textures with chiral motifs that break both time and inversion symmetry. For certain moire angles, helical and toroidal magnetic orders arise. Changing the vertical distance between layers can further manipulate these novel phases. It is shown that the dipolar interlayer interaction induces an emergent twist‐dependent chiral magnetic field orthogonal to the direction of the zig‐zag chains, which is responsible for the internal torques conjugated to the toroidal orders.

First-principles study of the effects of doping B, N, and O on the photoelectric properties of Cr adsorbed GaS.

To lessen the impact of the dangerous metal Cr, this paper applies the first principles to investigate the adsorption behavior and photoelectric properties of GaS on Cr. The effects of doped GaS on Cr adsorption behavior are investigated with four GaS systems, which are pure, boron (B)-doped, nitrogen (N)-doped, and oxygen (O)-doped, in order to maximize the characteristics of GaS for use in novel sectors, to obtain understanding of the impact of doping on the electronic structure and optical properties of GaS adsorption of Cr, as well as to promote the development of the material. Four GaS adsorbed Cr systems, pure, B-doped, N-doped, and O-doped, are optimized, and the optimized results show that the stable adsorption position of Cr on both pure and doped GaS is the top position of Ga atoms, whereas doped elements B, N, and O can promote the adsorption of Cr on GaS, and the order of the strength of this promotion is B > N > O. In this paper, molecular simulation calculations and analyses using the CASTEP module in the software Materials Studio are performed to simulate the structure optimization of GaS-adsorbed Cr materials doped with B, N, and O atoms by using the generalized gradient approximation (GGA) plane-wave pseudopotential approach [1] and the Perdew-Burke-Ernzerhof (PBE) generalized function [2]. From the convergence test, it is reasonable to set the K-point network to 4 × 4 × 1 and the truncation energy to 500eV [3]. In this paper, a 3 × 3 × 1 supercell structure with 18 S atoms and 18Ga atoms is selected. The convergence value of the iterative accuracy is 1.0e - 5eV/atom, and all the atomic forces are less than 0.02eV/Å. A vacuum layer of 16Å is also set in the C direction to avoid interlayer interactions of GaS. First, we optimize the geometry of the model and then analyze the nature of the adsorption energy and electronic structure corresponding to the model.

Enhancing Hydrogen Evolution Reaction Activity of Palladium Catalyst by Immobilization on MXene Nanosheets.

Efficient catalysts with minimal content of catalytically active noble metals are essential for the transition to the clean hydrogen economy. Catalyst supports that can immobilize and stabilize catalytic nanoparticles and facilitate the supply of electrons and reactants to the catalysts are needed. Being hydrophilic and more conductive compared with carbons, MXenes have shown promise as catalyst supports. However, the controlled assembly of their 2D sheets creates a challenge. This study established a lattice engineering approach to regulate the assembly of exfoliated Ti3C2Tx MXene nanosheets with guest cations of various sizes. The enlargement of guest cations led to a decreased interlayer interaction of MXene lamellae and increased surface accessibility, allowing intercalation of Pd nanoparticles. Stabilization of Pd nanoparticles between interlayer-expanded MXene nanosheets improved their electrocatalytic activity. The Pd-immobilized K+-intercalated MXene nanosheets (PdKMX) demonstrated exceptional electrocatalytic performance for the hydrogen evolution reaction with the lowest overpotential of 72 mV (@10 mA cm-2) and the highest turnover frequency of 1.122 s-1 (@ an overpotential of 100 mV), which were superior to those of the state-of-the-art Pd nanoparticle-based electrocatalysts. Weakening of the interlayer interaction during self-assembly with K+ ions led to fewer layers in lamellae and expansion of the MXene in the c direction during Pd anchoring, providing numerous surface-active sites and promoting mass transport. In situ spectroscopic analysis suggests that the effective interfacial electron injection from the Pd nanoparticles strongly immobilized on interlayer-expanded PdKMX may be responsible for the improved electrocatalytic performance.

Probing Polymorphic Stacking Domains in Mechanically Exfoliated Two-Dimensional Nanosheets Using Atomic Force Microscopy and Ultralow-Frequency Raman Spectroscopy.

As one of the key features of two-dimensional (2D) layered materials, stacking order has been found to play an important role in modulating the interlayer interactions of 2D materials, potentially affecting their electronic and other properties as a consequence. In this work, ultralow-frequency (ULF) Raman spectroscopy, electrostatic force microscopy (EFM), and high-resolution atomic force microscopy (HR-AFM) were used to systematically study the effect of stacking order on the interlayer interactions as well as electrostatic screening of few-layer polymorphic molybdenum disulfide (MoS2) and molybdenum diselenide (MoSe2) nanosheets. The stacking order difference was first confirmed by measuring the ULF Raman spectrum of the nanosheets with polymorphic stacking domains. The atomic lattice arrangement revealed using HR-AFM also clearly showed a stacking order difference. In addition, EFM phase imaging clearly presented the distribution of the stacking domains in the mechanically exfoliated nanosheets, which could have arisen from electrostatic screening. The results indicate that EFM in combination with ULF Raman spectroscopy could be a simple, fast, and high-resolution method for probing the distribution of polymorphic stacking domains in 2D transition metal dichalcogenide materials. Our work might be promising for correlating the interlayer interactions of TMDC nanosheets with stacking order, a topic of great interest with regard to modulating their optoelectronic properties.

First principle study of the effect of doping on the optoelectronic properties of Cr-adsorbed MoS2.

This study explores, for the first time, using first principles, the impact of substitutional doping with boron (B), carbon (C), and nitrogen (N) on the adsorption of chromium (Cr) on monolayer MoS2. The effects of doping on the Cr adsorption behavior of MoS2 were investigated using four MoS2 systems, namely, pure, boron (B)-doped, carbon (C)-doped, and nitrogen (N)-doped, in order to gain an in-depth understanding of the mechanism of the effects of doping on the electronic structure and optical properties of Cr adsorbed by MoS2, to optimize the properties of MoS2, to explore new areas of application, and to promote the development of materials science. Four MoS2 adsorption systems of Cr adsorption on pure, B-doped, C-doped, and N-doped MoS2 were optimized, and the optimized results showed that the stable adsorption location of Cr on both pure and doped MoS2 was the hollow location at the top of the folded hexagon. The findings reveal that pure MoS2 has an adsorption effect on Cr, and doped elements B, C, and N can promote the adsorption of Cr on MoS2, and the strong and weak order of this promotion is B > C > N. In this paper, we use the CASTEP module in the simulation software Materials Studio to perform simulation calculations and analyses to optimize the simulation of Cr adsorption by MoS2 doped with B, C, and N atoms using the generalized gradient approximation (GGA) plane-wave pseudo-potential method (Perdew et al. Phys Rev Lett 77(18):3865-3968, 1996), as well as Perdew-Burke-Ernzerhof (PBE) generalized functionals (Segall et al. J Phys: Condens Matter 14(11):2717-2744, 2022). The convergence test reveals that it is more reasonable to set the K-point network to 3 × 3 × 1 and the truncation energy to 400 eV. In this paper, a 3 × 3 × 1 supercell structure with 18 S atoms and 9 Mo atoms is selected. The convergence value of the iteration accuracy is 1.0e - 5eV/atom, and all the atomic forces are less than 0.02eV/Å. Additionally, to prevent MoS2 interlayer interaction, a vacuum layer with a thickness of 18 Å is set in the C direction. The geometrical optimization of the model is performed first, and then the corresponding adsorption energies of the model and the nature of the electronic structure are analyzed.

Sliding-mediated ferroelectric phase transition in CuInP2S6 under pressure

Interlayer stacking order has recently emerged as a unique degree of freedom to control crystal symmetry and physical properties in two-dimensional van der Waals (vdW) materials and heterostructures. By tuning the layer stacking pattern, symmetry-breaking and electric polarization can be created in otherwise non-polar crystals, whose polarization reversal depends on the interlayer sliding motion. Herein, we demonstrate that in a vdW layered ferroelectric, its existing polarization is closely coupled to the interlayer sliding driven by hydrostatic pressure. Through combined structural, electrical, vibrational characterizations, and theoretical calculations, we clearly map out the structural evolution of CuInP2S6 under pressure. A tendency toward a high polarization state is observed in the low-pressure region, followed by an interlayer-sliding-mediated phase transition from a monoclinic to a trigonal phase. Along the transformation pathway, the displacive-instable Cu ion serves as a pivot point that regulates the interlayer interaction in response to external pressure. The rich phase diagram of CuInP2S6, which is enabled by stacking orders, sheds light on the physics of vdW ferroelectricity and opens an alternative route to tailoring long-range order in vdW layered crystals.