Van Der Waals Gap Research Articles

Atomic‐Scale Order and Disorder Induced Diverse Topological Spin Textures in Self‐Intercalated Van der Waals Magnets Cr1+δTe2

AbstractThe intercalation of magnetic atoms into van der Waals (vdW) gaps offers a versatile approach for manipulating local magnetic ordering in vdW magnets. However, these intercalated magnetic atoms are often intricately positioned within the gaps, resulting in an ambiguous impact on local magnetic interactions and spin configurations. Herein, the atomic and magnetic structures of self‐intercalated prototype materials Cr1+δTe2 are comprehensively investigated using transmission electron microscopy, elucidating the correlation between the concentration and distribution of intercalated Cr atoms and the evolution of spin textures. The observed transitions from topological Néel‐type skyrmions to non‐topological type‐II bubbles, and ultimately to trivial ferromagnetic domains, underscore the complex nature of these magnetic spin textures. The complexity of the local structural arrangements is further revealed through integrating atomic‐resolution imaging, revealing the disorder‐order‐disorder transitions manifested by the distribution of intercalated atoms. The modulation of intercalated atomic structure shapes the observed spin textures by affecting the Dzyaloshinskii−Moriya interaction (DMI), inter/intra‐layer coupling, and magnetic anisotropy. These findings provide profound insights into the structure‐magnetism relationship, offering promising avenues for engineering the magnetic properties of vdW magnets at the atomic level.

Combining graph deep learning and London dispersion interatomic potentials: A case study on pnictogen chalcohalides.

Machine-learning interatomic potential models based on graph neural network architectures have the potential to make atomistic materials modeling widely accessible due to their computational efficiency, scalability, and broad applicability. The training datasets for many such models are derived from density-functional theory calculations, typically using a semilocal exchange-correlation functional. As a result, long-range interactions such as London dispersion are often missing in these models. We investigate whether this missing component can be addressed by combining a graph deep learning potential with semiempirical dispersion models. We assess this combination by deriving the equations of state for layered pnictogen chalcohalides BiTeBr and BiTeI and performing crystal structure optimizations for a broader set of V-VI-VII compounds with various stoichiometries, many of which possess van der Waals gaps. We characterize the optimized crystal structures by calculating their x-ray diffraction patterns and radial distribution function histograms, which are also used to compute Earth mover's distances to quantify the dissimilarity between the optimized and corresponding experimental structures. We find that dispersion-corrected graph deep learning potentials generally (though not universally) provide a more realistic description of these compounds due to the inclusion of van der Waals attractions. In particular, their use results in systematic improvements in predicting not only the van der Waals gap but also the layer thickness in layered V-VI-VII compounds. Our results demonstrate that the combined potentials studied here, derived from a straightforward approach that neither requires fine-tuning the training nor refitting the potential parameters, can significantly improve the description of layered polar crystals.

Layered hybrid superlattices as designable quantum solids.

Crystalline solids typically show robust long-range structural ordering, vital for their remarkable electronic properties and use in functional electronics, albeit with limited customization space. By contrast, synthetic molecular systems provide highly tunable structural topologies and versatile functionalities but are often too delicate for scalable electronic integration. Combining these two systems could harness the strengths of both, yet realizing this integration is challenging owing to distinct chemical bonding structures and processing conditions. Two-dimensional atomic crystals comprise crystalline atomic layers separated by non-bonding van der Waals gaps, allowing diverse atomic or molecular intercalants to be inserted without disrupting existing covalent bonds. This enables the creation of a diverse set of layered hybrid superlattices (LHSLs) composed of alternating crystalline atomic layers of variable electronic properties and self-assembled atomic or molecular interlayers featuring customizable chemical compositions and structural motifs. Here we outline strategies to prepare LHSLs and discuss emergent properties. With the versatile molecular design strategies and modular assembly processes, LHSLs offer vast flexibility for weaving distinct chemical constituents and quantum properties into monolithic artificial solids with a designable three-dimensional potential landscape. This opens unprecedented opportunities to tailor charge correlations, quantum properties and topological phases, thereby defining a rich material platform for advancing quantum information science.

Microenvironment Control over Electrocatalytic Activity of g-C3N4/2H-MoS2 Superlattice-like Heterostructures for Hydrogen Evolution.

Microenvironments in heterogeneous catalysis have been recognized as equally important as the types and amounts of active sites for regulating catalytic activity. Two-dimensional (2D) nanospaces between van der Waals (vdW) gaps of layered materials provide an ideal microenvironment to create novel functionalities. Here, we explore a facile method for fabricating g-C3N4/2H-MoS2 superlattice-like heterostructures based on thermochemical intercalation and polymerization reactions of formamide within enlarged vdW gaps of 2H-MoS2 nanosheets without any transfer process. DFT calculations demonstrate that the interlayer electron-electron correlations due to the intercalation effect of g-C3N4, rather than high-κ dielectric environments, lead to the improvement of intrinsic conductivity of 2H-MoS2 nanosheets. As the proof of concept in applications for the electrocatalysis field, the heterostructure for hydrogen electrochemical reaction (HER) exhibits high stability and catalytic activity in both acid and alkaline media, such as a quite low onset overpotential of 98 mV, a high exchange current density of 77.6 μA cm-2, and a small Tafel slope (52.9 mV dec-1) in an acid medium. The enhanced HER activity is attributed to the improved conductivity and nanoconfinement effect of 2D nanospaces that decrease the reaction activation energy and activate the inert basal planes.

Two-Dimensional ABS4 (A and B = Zr, Hf, and Ti) as Promising Anode for Li and Na-Ion Batteries.

Metal ion intercalation into van der Waals gaps of layered materials is vital for large-scale electrochemical energy storage. Transition-metal sulfides, ABS4 (where A and B represent Zr, Hf, and Ti as monolayers as anodes), are examined as lithium and sodium ion storage. Our study reveals that these monolayers offer exceptional performance for ion storage. The low diffusion barriers enable efficient lithium bonding and rapid separation while all ABS4 phases remain semiconducting before lithiation and transition to metallic states, ensuring excellent electrical conductivity. Notably, the monolayers demonstrate impressive ion capacities: 1639, 1202, and 1119 mAh/g for Li-ions, and 1093, 801, and 671 mAh/g for Na-ions in ZrTiS4, HfTiS4, and HfZrS4, respectively. Average voltages are 1.16 V, 0.9 V, and 0.94 V for Li-ions and 1.17 V, 1.02 V, and 0.94 V for Na-ions across these materials. Additionally, low migration energy barriers of 0.231 eV, 0.233 eV, and 0.238 eV for Li and 0.135 eV, 0.136 eV, and 0.147 eV for Na make ABS4 monolayers highly attractive for battery applications. These findings underscore the potential of monolayer ABS4 as a superior electrode material, combining high adsorption energy, low diffusion barriers, low voltage, high specific capacity, and outstanding electrical conductivity.

Facilitating Ion Storage and Transport Pathways by In Situ Constructing 1D Carbon Nanotube Electric Bridges between 2D MXene Interlayers.

Multiple van der Waals (vdW) gaps invoke abundant opportunities for contriving artificial architectures and tailoring desired properties via the intercalation route beyond the reach of conventional concepts. Intriguingly, the electrochemical intercalation strategy can precisely and reversibly tune the intercalation stage of charged functional species. This study presents a valid structural editing protocol facilitated by electrochemical intercalation to engineer MXene interlayers, ultimately incorporating in situ constructed carbon nanotube (CNT) electric bridges for enhanced ion storage and transport pathways. The method allows for the precise modulation of electrochemical forces to tailor materials for specific applications. Deep intercalation and in situ growth processes establish robust anchoring sites and connectivity hubs between MXenes and CNTs, ensuring structural homogeneity and stability in advanced electrode materials. The results demonstrate the effectiveness of electrochemistry-mediated interlayer nanoengineering in MXenes, offering a versatile approach to design vdW heterostructures with tailored functionalities for energy storage and conversion applications. This work highlights the potential of electrochemical modulation in advancing materials engineering strategies for next-generation energy storage technologies.

Gate-Controlled Potassium Intercalation and Superconductivity in Molybdenum Disulfide.

Intercalation of guest ions into a van der Waals (vdW) gap in layered materials is a powerful route to create novel material phases and functionalities. Ionic gating is a technique to control the motions and configuration of ions for both intercalation and surface electrostatic doping. The advance of ionic gating enables the in situ probe of dynamics of ion diffusion, carrier doping, and transport properties. Here we performed in situ resistivity and Raman experiments on the potassium ion (K+) intercalation of single-crystal MoS2 and constructed a temperature-carrier density phase diagram. The K+-intercalation induces a structural transition from the prismatically coordinated phase to the octahedrally coordinated phase, where anisotropic three-dimensional superconductivity and a possible charge density wave state were observed. The present ionic gating offers a comprehensive view of the intercalated phases and proves that the electrostatically induced superconductivity is distinct from that in the intercalated phase.

Modulation performance of terahertz waves by titanium disulfide nanosheets towards 6G

AbstractThe TiS2 compound has attracted considerable attention among researchers in the fields of energy storage and conversion, as well as semiconductor materials. This is due to its remarkable capability to incorporate various elements into its van der Waals gap, as well as its potential to adjust the carrier concentration within the material to optimize its electrically relevant properties. In our study, we employed a terahertz (THz) time‐domain spectroscopy system to measure the spectra of TiS2 within the THz frequency range. Through this analysis, we examined the optical parameters, such as transmittance and refractive index, of the TiS2 samples. Additionally, we conducted optical pumping correlation experiments using THz detection technology to investigate the optical pumping effects on TiS2. By analyzing the THz energy transmission process in different states, we were able to calculate and fit the carrier recombination time. The findings of our research hold significant implications for the advancement of THz devices operating in the 6G frequency band.

Two-band superconductivity in Pt-intercalated ZrTe2, a Dirac semimetal

This study presents compelling evidence of superconductivity with 3.5 K onset and zero-resistance state at 2.25 K in the Dirac semimetal compound ZrTe2, upon intercalation with platinum in the van der Waals gap. The temperature dependence of the upper critical field and the normalized superfluid density can be modeled effectively using a two-gap model, consistent with the possibility of topological superconductivity.

Observation of a V-Shape Superconductivity Evolution on Tungsten-Intercalated 2H-Type Niobium Diselenide.

van der Waals-layered niobium diselenide (NbSe2) intercalated by d-electron transition metals is an ideal test bed for the exploration of their diversiform evolution of ground states. These intercalations are mostly viewed as ordered structures aligned with periodicities of their host materials that enable control of the electronic phases via gradually changing of intercalation ratios. Here, we present the structure and superconductivity in tungsten (W)-intercalated 2H-NbSe2 crystals, which reveals an order to disorder distribution of W atoms with increasing confined intercalating amounts, leading to an approximate V-shape suppression of superconductivity. Aided by density functional theory calculations, we demonstrate that the local magnetic moment around W intercalants induced by the charge redistribution gives rise to the quick superconductivity suppression in 2H-NbSe2 below a certain dilute amount (W% = 0.06). Simultaneously, W intercalants also induce structural aberration due to aggregation effects and inhibit the generation of an ordered structure in 2H-NbSe2, resulting in a recovery of its superconductivity. The alteration of structure and electronic phases in 2H-NbSe2 via intercalation of nonmagnetic transition metals in the van der Waals gap enables the exploration of combined magnetic quantum criticality, superconductivity, and other related electronic correlations.

When van der Waals Met Kagome: A 2D Antimonide with a Vanadium-Kagome Network.

2D materials showcase unconventional properties emerging from quantum confinement effects. In this work, a "soft chemical" route allows for the deintercalation of K+ from the layered antimonide KV6Sb6, resulting in the discovery of a new metastable 2D-Kagome antimonide K0.1(1)V6Sb6 with a van der Waals gap of 3.2 Å. The structure of K0.1(1)V6Sb6 was determined via the synergistic techniques, including X-ray pair distribution function analysis, advanced transmission electron microscopy, and density functional theory calculations. The K0.1(1)V6Sb6 compound crystallizes in the monoclinic space group C2/m (a = 9.57(2) Å, b = 5.502(8) Å, c = 10.23(2) Å, β = 97.6(2)°, Z = 2). The [V6Sb6] layers in K0.1(1)V6Sb6 are retained upon deintercalation and closely resemble the layers in the parent compound, yet deintercalation results in a relative shift of the adjacent [V6Sb6] layers. The magnetic properties of the K0.1(1)V6Sb6 phase in the 2-300 K range are comparable to those of KV6Sb6 and another Kagome antimonide KV3Sb5, consistent with nearly temperature-independent paramagnetism. Electronic band structure calculation suggests a nontrivial band topology with flat bands and opening of band crossing afforded by deintercalation. Transport property measurements reveal a metallic nature for K0.1(1)V6Sb6 and a low thermal conductivity of 0.6 W K-1 m-1 at 300 K. Additionally, ion exchange in KV6Sb6 via a solvothermal route leads to a successful partial exchange of K+ with A+ (A = Na, Rb, and Cs). This study highlights the tunability of the layered structure of the KV6Sb6 compound, providing a rich playground for the realization of new 2D materials.

Enhancing perovskite solar cell efficiency to 28.17% by Integrating Dion-Jacobson 2D and 3D phase perovskite Absorbers

Efficiency of power conversion for both kinds of inorganic as well as organic perovskites-based cells has increased in the last few years. To overcome this stability-related issue of 3D perovskite cells,2D perovskite has become an alternative option.The structure and the device, both are destabilized by the weak interactions observed in between the layers of the layered perovskite since the Ruddlesden Popper (RP) phase of 2D perovskite has a weak Van der Waal gap present in between the monoammonium cation layer.The PCE of perovskite cells has risen to 25 %. Commercialization of perovskites on a large scale as solar is a challenging task because lead is toxic.Keeping the toxic property of lead in mind we have used FTO (window layer)/TiO2(ETL)/(PDA)(MA)n-1PbnI3n+1(absorber layer)/Csx(FA0.4MA0.6)1-xPbI2.8Br0.2(absorber layer)/ spiro-OMeTAD (HTL).It has been observed that the DJ and RP phases of 2D PSC are individually found much more stable than their 3D counterpart used for perovskite-based solar cells. DJ and RP are merged to become proof of the efficiency as well as environmental stability of 2D perovskite-based solar cells.Several material parameters that affect the performance result obtained from the device,like the electron transport layer(ETL), hole transport layer(HTL), the thickness of the absorbing layer,etc were considered,studied,and also have been optimized.

Emerging Two-Dimensional Materials for Proton-Based Energy Storage.

The rapid diffusion kinetics and smallest ion radius make protons the ideal cations toward the ultimate energy storage technology combining the ultrafast charging capabilities of supercapacitors and the high energy densities of batteries. Despite the concept existing for centuries, the lack of satisfactory electrode materials hinders its practical development. Recently, the rapid advancement of the emerging two-dimensional (2D) materials, characterized by their ultrathin morphology, interlayer van der Waals gaps, and distinctive electrochemical properties, injects promises into future proton-based energy storage systems. In this perspective, we comprehensively summarize the current advances in proton-based energy storage based on 2D materials. We begin by providing an overview of proton-based energy storage systems, including proton batteries, pseudocapacitors and electrical double layer capacitors. We then elucidate the fundamental knowledge about proton transport characteristics, including in electrolytes, at electrolyte/electrode interfaces, and within electrode materials, particularly in 2D material systems. We comprehensively summarize specific cases of 2D materials as proton electrodes, detailing their design concepts, proton transport mechanism and electrochemical performance. Finally, we provide insights into the prospects of proton-based energy storage systems, emphasizing the importance of rational design of 2D electrode materials and matching electrolyte systems.

Thermal-Driven Cobalt Intercalation Enhances Thermoelectric ZT of n-Type Bi2Te2.7Se0.3.

Layered materials have emerged as stars in the realm of nanomaterials, showcasing exceptional versatility in various fields. This investigation employed a thermally driven method to intercalate cobalt (Co) into the van der Waals gaps of (CuI)0.002Bi2Te2.7Se0.3 crystals and investigated the mechanism by which the intercalated Co enhances the thermoelectric performance of the material. Co intercalation decreases the carrier concentration, thereby improving the Seebeck coefficient and decreasing both the mobility and the electrical conductivity. These effects result in a significant enhancement of the power factor above 400 K. Theoretical electronic structure calculations provide insights into the role of Co in this material. Additionally, the presence of intercalated Co significantly enhances phonon scattering, thereby boosting the thermoelectric figure-of-merit, ZT to 1.33 at 350 K for 0.17% Co intercalation. These findings highlight the potential of Co incorporation for improving the thermoelectric energy efficiency of n-type Bi2Te2.7Se0.3, offering avenues for further optimization in thermoelectric applications.

Unveiling charge transport in monolayer and few-layer CoPS3/metal contact: Insight from C-AFM

Monolayer and few-layer CoPS3 have been successfully synthesized via an Au-assisted exfoliation technique, with their transport properties subsequently investigated by conductive atomic force microscopy. Rectification effects can be observed in CoPS3 nanosheets due to the Schottky junction formation at the Au/CoPS3 interface, especially in configurations comprising two or more layer flakes. The distinct transport properties between monolayer and few-layer samples demonstrate that the charge transport behavior in the vertical direction is associated with the van der Waals gap. The calculated electron affinity for CoPS3 is about 4.84 eV, as revealed by the thermionic emission model. The results obtained contribute to the knowledge base concerning the transport characteristics of ultrathin MPX3, facilitating further exploration of the transport behavior of these materials and their potential applications in novel electronic devices.

Phase transition arising from order-disorder motion in stable layered two-dimensional perovskite

In recent years, organic-inorganic hybrid materials are widely designed and synthesized as switching materials for temperature response. However, due to the change of molecular arrangement inside the crystal during solid-solid phase transition, the distortion of crystal lattice and the great change of lattice parameters are often caused, which result in a poor repeatability and short life. Thus, designing phase change materials with small lattice changes helps to improve product life. In this article, a novel organic-inorganic hybrid material 3HDMAPAPbBr4 (1, 3HDMAPA is 3-(hydroxydimethylammonio)propan-1-aminium) was successfully synthesized and characterized. For 1, organic cations filled in the van der Waals gap are connected by hydrogen bonds with halogens in the two-dimensional inorganic layer, forming a stable sandwich structure. During the solid-solid phase transition driven by temperature, the changes of inorganic skeleton are relatively small, and the disorder movement of organic cations does not affect the existence of hydrogen bonds, maintaining a relatively stable crystal structure. In addition, electrical property, optical property and crystal structures are analyzed and discussed in detail. We believe that our work will contribute to the development and application of phase change materials in response materials.

Identifying the Origin of Thermal Modulation of Exchange Bias in MnPS3/Fe3GeTe2 van der Waals Heterostructures.

The exchange bias phenomenon, inherent in exchange-coupled ferromagnetic and antiferromagnetic systems, has intrigued researchers for decades. Van der Waals materials, with their layered structures, offer an ideal platform for exploring exchange bias. However, effectively manipulating exchange bias in van der Waals heterostructures remains challenging. This study investigates the origin of exchange bias in MnPS3/Fe3GeTe2 van der Waals heterostructures, demonstrating a method to modulate nearly 1000% variation in magnitude through simple thermal cycling. Despite the compensated interfacial spin configuration of MnPS3, a substantial 170 mT exchange bias is observed at 5 K, one of the largest observed in van der Waals heterostructures. This significant exchange bias is linked to anomalous weak ferromagnetic ordering in MnPS3 below 40 K. The tunability of exchange bias during thermal cycling is attributed to the amorphization and changes in the van der Waals gap during field cooling. The findings highlight a robust and adjustable exchange bias in van der Waals heterostructures, presenting a straightforward method to enhance other interface-related spintronic phenomena for practical applications. Detailed interface analysis reveals atom migration between layers, forming amorphous regions on either side of the van der Waals gap, emphasizing the importance of precise interface characterization in these heterostructures.

High‐Stability van Der Waals Structures of GeTe/Sb2Te3 Superlattices for 100× Increased Durability Phase‐Change Memory (PCM) by Low‐Temperature Atomic Layer Deposition

AbstractThe superlattices phase‐change memory (SL‐PCM), based on GeTe/Sb2Te3 superlattices (SLs), garners considerable attention within the academic community owing to its exceptional electrical properties. While the exact mechanism remains a topic of ongoing debate, researchers widely acknowledge that the presence of van der Waals (vdW) gap structures within SLs plays a pivotal role. However, the formation of vertical nanoparticles (VNPs) from the random orientation of Sb2Te3 and the substantial intermixing between GeTe and Sb2Te3 due to high‐temperature fabrication processes leads to pronounced elemental intermixing during electrical operations, causing a loss of SLs properties. This investigation uses atomic layer deposition techniques, specifically Sb‐atomic seed layer and periodic quintuple layers stacking, to address issues from random orientation Sb2Te3 grains and anomalous VNPs. This approach yields Sb2Te3 crystalline films with a high‐stability vdW gap structures. By leveraging these films, SLs with a preferred crystallographic orientation at a notable low temperature of 90 °C are successfully fabricated. The optimized SLs exhibit remarkable structural stability and mitigate alloy phase emergence during electrical operations, resulting in a 100‐fold enhancement in device durability. This work advances reliable SL‐PCM understanding and implementation, propelling this memory technology into the next generation.

Synthesis, structural phase transition and weak itinerant magnetism in NixNbSe2

The article presents the synthesis, structural and magnetic properties of NixNbSe2 (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25). The intercalation limit of Ni in NbSe2 is found to be 25 mol%, above which NiSe appears as a binary impurity phase. Single crystal X-ray diffraction studies show that all the compounds crystallize in the hexagonal P63/mmc space group. However, there is a distinct phase transition beyond x = 0.20 with Ni intercalation in NixNbSe2. The NixNbSe2 compounds crystallized in the parent 2H phase with x value between 0 ≤ x ≤ 0.2. Subsequently, the phase transforms into the 2H(I) phase with an order superstructure of 2 ao × 2 ao × 1 co for 0.20 ≤ x ≤ 0.25 in NixNbSe2. The superstructure formation is attributed to the occupation of Ni at the alternate octahedral voids present in the van der Waals gap of the NbSe2. The XPS results and theoretical analysis clearly show that the intercalation of Ni results in a more metallic character in NixNbSe2. Magnetic measurements on NixNbSe2 (x = 0.05, 0.1, 0.2, 0.25) shows very small magnetic moment at low temperature for all the compounds. Theoretical calculation on Ni0.25NbSe2 shows that the Ni-3d electrons are delocalized and are very weakly spin-polarized, which may be the reason for the experimentally observed weak magnetism in these compounds.

Chemical Trends of the Bulk and Surface Termination-Dependent Electronic Structure of Metal-Intercalated Transition Metal Dichalcogenides.

The addition of metal intercalants into the van der Waals gaps of transition metal dichalcogenides has shown great promise as a method for controlling their functional properties. For example, chiral helimagnetic states, current-induced magnetization switching, and a giant valley-Zeeman effect have all been demonstrated, generating significant renewed interest in this materials family. Here, we present a combined photoemission and density-functional theory study of three such compounds: , , and , to investigate chemical trends of the intercalant species on their bulk and surface electronic structure. Our resonant photoemission measurements indicate increased hybridization with the itinerant NbS2-derived conduction states with increasing atomic number of the intercalant, leading to pronounced mixing of the nominally localized intercalant states at the Fermi level. Using spatially and angle-resolved photoemission spectroscopy, we show how this impacts surface-termination-dependent charge transfers and leads to the formation of new dispersive states of mixed intercalant-Nb character at the Fermi level for the intercalant-terminated surfaces. This provides an explanation for the origin of anomalous states previously reported in this family of compounds and paves the way for tuning the nature of the magnetic interactions in these systems via control of the hybridization of the magnetic ions with the itinerant states.