Two-dimensional Van Der Waals Research Articles

Anisotropic 2D van der Waals Magnets Hosting 1D Spin Chains.

The exploration of one-dimensional (1D) magnetism, frequently portrayed as spin chains, constitutes an actively pursued research field that illuminates fundamental principles in many-body problems and applications in magnonics and spintronics. The inherent reduction in dimensionality often leads to robust spin fluctuations, impacting magnetic ordering and resulting in novel magnetic phenomena. Here, we explore structural, magnetic, and optical properties of highly anisotropic two-dimensional (2D) van der Waals antiferromagnets that uniquely host spin chains. First-principles calculations reveal that the weakest interaction is interchain, essentially leading to 1D magnetic behavior in each layer. With the additional degree of freedom arising from its anisotropic structure, we engineer the structure by alloying, varying the 1D spin chain length using electron beam irradiation, or twisting for localized patterning, and calculate spin textures, predicting robust stability of the antiferromagnetic ordering. Comparing with other spin chain magnets, we anticipate these materials to bring fresh perspectives on harvesting low-dimensional magnetism. This article is protected by copyright. All rights reserved.

Giant magneto-optical effect in van der Waals room temperature ferromagnet Fe3GaTe2

Abstract The discovery of ferromagnetic two-dimensional (2D) van der Waals (vdWs) materials provides an opportunity to explore intriguing physics and develop innovative spin electronic devices. However, the main challenge for practical applications of vdWs ferromagnetic crystals lies in the weak intrinsic ferromagnetism and small perpendicular magnetic anisotropy (PMA) above room temperature. Here, we report the intrinsic vdWs ferromagnetic crystal Fe3GaTe2, synthesized by the self-flux method, exhibiting a Curie temperature (TC) of 370 K, a high saturation magnetization of 33.47 emu/g, and a large PMA energy density of approximately 4.17 × 105 J/m3. Furthermore, for the first time, the magneto-optical effect has been systematically investigated in Fe3GaTe2. The doubly degenerate E2g(Γ) mode reverses the helicity of incident photons, indicating the existence of pseudoangular momentum (PAM) and chirality. Meanwhile, the non-degenerate non-chiral A1g(Γ) phonon exhibits a significant magneto-Raman effect under an external out-of-plane magnetic field. These results lay the groundwork for studying phonon chirality and magneto-optical phenomena in 2D magnetic materials, providing the feasibility for further fundamental research and application in spintronic devices.

A synapse with low power consumption based on MoTe2/SnS2heterostructure.

The use of two-dimensional materials and van der Waals heterostructures holds great potential for improving the performance of memristors Here, we present SnS2/MoTe2heterostructure synaptic transistors. Benefiting from the ultra-low dark current of the heterojunction, the power consumption of the synapse is only 19 pJ per switching under 0.1 V bias, comparable to that of biological synapses. The synaptic device based on the SnS2/MoTe2demonstrates various synaptic functionalities, including short-term plasticity, long-term plasticity, and paired-pulse facilitation (PPF). In particular, the synaptic weight of the excitatory postsynaptic current (EPSC) can reach 109.8%. In addition, the controllability of the long-term potentiation (LTP) and long-term depression (LTD) are discussed. The dynamic range (Gmax/Gmin) and the symmetricity values of the synaptic devices are approximately 16.22 and 6.37, and the non-linearity is 1.79. Our study provides the possibility for the application of 2D material synaptic devices in the field of low-power information storage.
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Growth of Quasi-Two-Dimensional CrTe Nanoflakes and CrTe/Transition Metal Dichalcogenide Heterostructures

Two-dimensional (2D) van der Waals layered materials have been explored in depth. They can be vertically stacked into a 2D heterostructure and represent a fundamental way to explore new physical properties and fabricate high-performance nanodevices. However, the controllable and scaled growth of non-layered quasi-2D materials and their heterostructures is still a great challenge. Here, we report a selective two-step growth method for high-quality single crystalline CrTe/WSe2 and CrTe/MoS2 heterostructures by adopting a universal CVD strategy with the assistance of molten salt and mass control. Quasi-2D metallic CrTe was grown on pre-deposited 2D transition metal dichalcogenides (TMDC) under relatively low temperatures. A 2D CrTe/TMDC heterostructure was established to explore the interface’s structure using scanning transmission electron microscopy (STEM), and also demonstrate ferromagnetism in a metal–semiconductor CrTe/TMDC heterostructure.

Intrinsic valley-polarized quantum anomalous Hall effect in a two-dimensional germanene/MnI2 van der Waals heterostructure

Abstract Valley-polarized quantum anomalous Hall effect (VQAHE), combined nontrivial band topology with valleytronics, is of importance for both fundamental sciences and emerging applications. However, the experimental realization of this property is challengeable. Here, by using first-principles calculations and modal analysis, we predict a mechanism of producing VQAHE in two-dimensional ferromagnetic van der Waals germanene/MnI2 heterostructure. This heterostructure exhibits both valley anomalous Hall effect and VQAHE due to the joint effects of magnetic exchange effect and spin-orbital coupling with the aid of anomalous Hall conductance and chiral edge state. Moreover interestingly, through the electrical modulation of ferroelectric polarization state in In2Se3, the germanene/MnI2/In2Se3 heterostructure can undergo reversible switching from a semiconductor to a metallic behavior. This work offers a guiding advancement for searching for VQAHE in ferromagnetic van der Waals heterostructures and exploiting energy-efficient devices based on the VQAHE.

Double Superconducting Dome of Quasi Two-Dimensional TaS2 in Non-Centrosymmetric van der Waals Heterostructure.

Two-dimensional van der Waals heterostructures (2D-vdWHs) based on transition metal dichalcogenides (TMDs) provide unparalleled control over electronic properties. However, the interlayer coupling is challenged by the interfacial misalignment and defects, which hinders a comprehensive understanding of the intertwined electronic orders, especially superconductivity and charge density wave (CDW). Here, by using pressure to regulate the interlayer coupling of non-centrosymmetric 6R-TaS2 vdWHs, we observe an unprecedented phase diagram in TMDs. This phase diagram encompasses successive suppression of the original CDW states from alternating H-layer and T-layer configurations, the emergence and disappearance of a new CDW-like state, and a double superconducting dome induced by different interlayer coupling effects. These results not only illuminate the crucial role of interlayer coupling in shaping the complex phase diagram of TMD systems but also pave a new avenue for the creation of a novel family of bulk heterostructures with customized 2D properties.

Magnetic domain structures in ultrathin Bi2Te3/CrTe2 heterostructures

Abstract Chromium tellurium compounds are important two-dimensional van der Waals ferromagnetic materials with high Curie temperature and chemical stability in air, which is promising for applications in spintronic devices. Here, high-quality spin-orbital-torque (SOT) device, Bi2Te3/CrTe2 heterostructure was epitaxially grown on Al2O3 (0001) substrates. Anomalous Hall measurements indicate the existence of strong ferromagnetism in this device with the CrTe2 thickness down to 10 nm. In order to investigate its micromagnetic structure, cryogenic magnetic force microscope (MFM) was utilized to measure the magnetic domain evolutions at various temperatures and magnetic fields. The virgin domain state of the device shows a worm-like magnetic domain structure with the size around 0.6 ~ 0.8 µm. Larger irregular-shape magnetic domains (＞1µm) can be induced and pinned, after the field is increased to coercive field and ramped back to low fields. The temperature-dependent MFM signals exhibit a nice mean-field-like ferromagnetic transition with Curie temperature around 201.5 K, indicating a robust ferromagnetic ordering. Such device can be potentially implemented in future magnetic memory technology.

Electrical engineering of topological magnetism in two-dimensional heterobilayers

The emergence of topological magnetism in two-dimensional (2D) van der Waals (vdW) magnetic materials and their heterostructures is an essential ingredient for next-generation information technology devices. Here, we demonstrate the all-electric switching of the topological nature of individual magnetic objects emerging in 2D vdW heterobilayers. We show from the first principles that an external electric field modifies the vdW gap between CrTe2 and (Rh, Ti)Te2 layers and alters the underlying magnetic interactions. This enables switching between ferromagnetic skyrmions and meron pairs in the CrTe2/RhTe2 heterobilayer while it enhances the stability of frustrated antiferromagnetic merons in the CrTe2/TiTe2 heterobilayer. We envision that the electrical engineering of distinct topological magnetic solitons in a single device could pave the way for novel energy-efficient mechanisms to store and transmit information with applications in spintronics.

Nonvolatile ferroelectric resistive switching in α-In2Se3(2H) ferroelectric semiconductor junctions

Emerging two-dimensional van der Waals (vdW) based ferroelectric semiconductor junctions (FSJs) are promising for low-power and high-density applications. In this work, we report the vdW α-In2Se3(2H) based FSJs of different metal-In2Se3 interfaces. All vertical devices emerge with the nonvolatile ferroelectric resistive switching behavior. The Cr/α-In2Se3(2H)/Au FSJs show a typical ferroelectric resistive switching behavior under ∼1.0 V applied voltage. The switching voltage of the Ag/α-In2Se3(2H)/Au FSJs could be further decreased from 1.0 V to 0.8 V because of Ag migrations and ferroelectric reversal. Further, by optimizing the metal-In2Se3 interfaces, the Ag/α-In2Se3(2H)/Pt FSJs exhibit the ultralow switching voltage (∼0.3 V), excellent endurance (>225 cycles) and retention (>1 × 104 s). The results suggest that vdW α-In2Se3(2H) ferroelectric has great potential for low-power memristive devices.

Graphene-integrated microring cavity for electronically controlled molecular fingerprinting

Microring cavities supporting whispering-gallery modes (WGMs) have an exceptionally high quality factor (Q) and a small mode volume, greatly improving the interaction between light and matter, which has attracted great attention in various microscale/nanoscale photonic devices and potential applications. Recently, two-dimensional van der Waals (vdW) materials such as graphene have emerged as a potential platform for next-generation biosensing by enabling the confinement of light fields at the nanoscale. Here, we propose what we believe to be a novel approach to achieve molecular fingerprint retrieval by integrating graphene into a microring cavity and conducting numerical simulations using the finite-difference time-domain (FDTD) method. The hybrid cavity exhibits high-quality WGMs with a high Q factor of up to 800. Moreover, the resonant wavelength can be electronically controlled through modulation of graphene’s Fermi level, enabling coverage of the entire free spectral range at infrared frequencies. By depositing a thin layer of biomolecular material (e.g., CBP) onto the surface of our hybrid cavity, we are able to accurately read out the absorption spectrum at multiple spectral points, thereby achieving broadband fingerprint retrieval for the targeted biomolecule. Our results pave the way for highly sensitive, chip-integrated, miniaturized, and electrically modulated infrared spectroscopy biosensing.

Computational Study on Interlocked-Ferroelectricity-Contributed High-Performance Memristors Based on Two-Dimensional van der Waals Ferroelectric Semiconductors.

In order to realize the prevailing artificial intelligence technology, memristor-implemented in-memory or neuromorphic computing is highly expected to break the bottleneck of von Neumann computers. Although high-performance memristors have been vigorously developed in labs or in industry, systematic computational investigations on memristors are seldom. Hence, it is urgent to provide theoretical or computational support for the exploration of memristor operating mechanisms or the screening of memristor materials. Here, a computational method based on the main input parameters learned from the first-principles calculations was developed to measure resistance switching of two-terminal memristors with sandwiched metal/ferroelectric semiconductor/metal architectures, which strikingly agrees with the experimental measurements. Based on our developed method, the diverse multiterminal memristors were designed to fully exploit the application of interlocked ferroelectricity of a ferroelectric semiconductor and realize their heterosynaptic plasticity, and their heterosynaptic behaviors can still be well described. Our developed method can provide a paradigm for the emulation of ferroelectric memristors and inspire subsequent computational exploration. Furthermore, our study also supplies a device optimization strategy based on the interlocked ferroelectricity and easy processing of two-dimensional van der Waals ferroelectric semiconductors, and our proposed heterosynaptic memristors still await further experimental exploration.

Moiré band renormalization due to lattice mismatch in bilayer graphene

We investigated the band renormalization caused by the compressive-strain-induced lattice mismatch in parallel AA stacked bilayer graphene using two complementary methods: the tight-binding approach and the low-energy continuum theory. While a large mismatch does not alter the low-energy bands, a small one reduces the bandwidth of the low-energy bands along with a decrease in the Fermi velocity. In the tiny-mismatch regime, the low-energy continuum theory reveals that the long-period moiré pattern extensively renormalizes the low-energy bands, resulting in a significant reduction of bandwidth. Meanwhile, the Fermi velocity exhibits an oscillatory behavior and approaches zero at specific mismatches. However, the resulting low-energy bands are not perfectly isolated flat, as seen in twisted bilayer graphene at magic angles. These findings provide a deeper understanding of moiré physics and offer valuable guidance for related experimental studies in creating moiré superlattices using two-dimensional van der Waals heterostructures.

First-Principles Simulation and Materials Screening for Spin-Orbit Torque in 2D van der Waals Heterostructures.

Recent advancements in spin-orbit torque (SOT) technology in two-dimensional van der Waals (2D vdW) materials have not only pushed spintronic devices to their atomic limits but have also unveiled unconventional torques and novel spin-switching mechanisms. The vast diversity of SOT observed in numerous 2D vdW materials necessitates a screening strategy to identify optimal materials for torque device performance. However, such a strategy has yet to be established. To address this critical issue, a combination of density functional theory and non-equilibrium Green's function is employed to calculate the SOT in various 2D vdW bilayer heterostructures. This leads to the discovery of three high SOT systems: WTe2/CrSe2, MoTe2/VS2, and NbSe2/CrSe2. Furthermore, a figure of merit that allows for rapid and efficient estimation of SOT is proposed, enabling high-throughput screening of optimal materials and devices for SOT applications in the future.

Dual-band high quality-factor perfect absorber based on uniaxial/biaxial anisotropic van der Waals polar crystals

Phonon polaritons (PhPs) in two-dimensional van der Waals crystals, such as hexagonal boron nitride (hBN) and α-phase molybdenum trioxide (α-MoO3), exhibit remarkable spatial confinement and extremely low optical losses, offering possibilities for the development of novel micro and nanophotonic devices at mid-infrared wavelengths. Here, we demonstrate the potential of the proposed Tamm structure to achieve dual-band high quality-factor perfect absorption within the mid-infrared range. This structure utilizes upper and lower spacer layers to isolate the anisotropic materials hBN and α-MoO3 from one-dimensional photonic crystals, where the Tamm phonon polaritons (TPhPs) are respectively excited at the interfaces, facilitating strong coupling with incident light and leading to significant dual-band perfect absorption phenomena. The results demonstrate that due to polariton resonances, the absorption peaks exhibit extremely narrow absorption bandwidths, with the maximum quality-factor reaching up to 297. In addition, tunable resonant wavelengths and polarization-dependent absorption properties are also demonstrated in the proposed Tamm structure. This study provides an alternative and straightforward approach for constructing dynamic and multifunctional optical devices such as optical modulators, tunable multi-band filters and biosensors.

Superstructure of Fe5-xGeTe2 Determined by Te K-Edge Extended X-ray Absorption Fine Structure and Te Kα X-ray Fluorescence Holography.

The local structure of the two-dimensional van der Waals material, Fe5-xGeTe2, which exhibits unique structural/magnetic phase transitions, was investigated by Te K-edge extended X-ray absorption fine structure (EXAFS) and Te Kα X-ray fluorescence holography (XFH) over a wide temperature range. The formation of a trimer of Te atoms at low temperatures has been fully explored using these methods. An increase in the Te-Fe distance at approximately 150 K was suggested by EXAFS and presumably indicates the formation of a Te trimer. Moreover, XFH displayed clear atomic images of Te atoms. Additionally, the distance between the Te atoms shortened, as confirmed from the atomic images reconstructed from XFH, indicating the formation of a trimer of Te atoms, i.e., a charge-ordered superstructure. Furthermore, Te Kα XFH provided unambiguous atomic images of Fe atoms occupying the Fe1 site; the images were not clearly observed in the Ge Kα XFH that was previously reported because of the low occupancy of Fe and Ge atoms. In this study, EXAFS and XFH clearly showed the local structure around the Te atom; in particular, the formation of Te trimers caused by charge-ordered phase transitions was clearly confirmed. The charge-ordered phase transition is fully discussed based on the structural variation at low temperatures, as established from EXAFS and XFH.

Two-dimensional transition metal dichalcogenides van der Waals heterojunctions with broken-gap for tunnel field-effect transistors applications

The scaling down of transistors is nearing the limits dictated by Moore's Law, primarily due to the quantum tunneling effect. This challenge has spurred considerable interest in exploring novel materials characterized by broken-gap band alignment. Among these materials, two-dimensional (2D) materials-based van der Waals heterojunctions (vdWHs) have emerged as promising candidates, owing to their nanoscale dimensions, which render them suitable for transistor construction. Type-III vdWHs, in particular, have been investigated for their ability to facilitate rapid quantum Band-To-Band Tunneling (BTBT), thereby enabling low-power charge transport. In our study, we conducted a detailed analysis of four vdWHs comprised of HfSe2 and WXY (where X and Y represent Se or Te). Our findings indicate that 2D WSeTe (or WTe2)/HfSe2 vdWHs exhibit a broken-gap band alignment (type-Ⅲ) under the influence of an external electric field, thus showcasing potential for integration into future tunnel field-effect transistors (TFETs). Additionally, our investigation revealed that the band gap of two other vdWHs, WTeSe (or WSe2)/HfSe2, varies linearly with the external electric field, fluctuating between 0.4 eV and 1.0 eV. This characteristic makes them well-suited for application as infrared detectors. By unveiling the distinct properties of these vdWH configurations, our research not only anticipates the emergence of a notable type-III band alignment vdWH but also underscores the potential of 2D vdWHs in optical sensor applications.

Controllable memory window in two-dimensional hybrid van der Waals heterostructured devices

Van der Waals (vdW) heterostructures based on inorganic layered materials have been demonstrated as potential candidates for a variety of electronic applications due to their flexibility in energy band engineering. However, the presence of unstable charge-trapping states in atomically thin two-dimensional (2D) materials may limit the performance of devices. Here, we aim to conduct a systematic investigation on hybrid heterostructured memory devices that consist of 2D layered organic and inorganic materials. The objective is to explore the potential of these devices in offering efficient charge-trapping states. Molybdenum disulfide (MoS2) is employed as a channel, while N, N′-Dimethyl-3,4,9,10-perylenedicarboximide (Me-PTCDI) serves as the charge-trapping layer to store charges from MoS2. The hysteresis window of these heterostructured devices can be effectively modified within a range of 13–70 V by manipulating both the thickness of the organic layer and the gate voltages. The largest hysteresis window is found in a combination of a few-layer Me-PTCDI (12.6 nm) and MoS2 (6 nm), showing a high on/off current ratio (>104) and a long retention time (104 s). Furthermore, the endurance test, which lasts for over 1000 cycles, demonstrates an exceptional level of stability and reliability. In addition, multilevel memory effects can be observed when gate pulses with different widths and amplitudes are applied. These 2D hybrid heterostructured devices have the capability to broaden the scope of material systems and present substantial potential for functional neuromorphic applications.

Giant and controllable nonlinear magneto-optical effects in two-dimensional magnets

The interplay of polarization and magnetism in materials with light can create rich nonlinear magneto-optical (NLMO) effects, and the recent discovery of two-dimensional (2D) van der Waals magnets provides remarkable control over NLMO effects due to their superb tunability. Here, based on first-principles calculations, we reported giant NLMO effects in CrI3-based 2D magnets, including a dramatic change of second-harmonics generation (SHG) polarization direction (90°) and intensity (on/off switch) under magnetization reversal and a 100% SHG circular dichroism effect. We further revealed that these effects could not only be used to design ultra-thin multifunctional optical devices but also to detect subtle magnetic orderings. Remarkably, we analytically derived conditions to achieve giant NLMO effects and proposed general strategies to realize them in 2D magnets. Our work not only uncovers a series of intriguing NLMO phenomena but also paves the way for both fundamental research and device applications of ultra-thin NLMO materials.

First-principles study of perpendicular magnetic anisotropy of CrI3/In2Se3 Van der Waals heterostructure

Two-dimensional multiferroic van der Waals (vdW) heterostructures are promising for nanoscale and multifunctional spintronic devices. Controllable perpendicular magnetic anisotropy in vdW heterostructure often plays a central role in stabilizing ferromagnetic order and lowering energy consumption. Here, we carry out first-principles calculations for CrI3/In2Se3 vdW heterostructure and investigate how its magnetic anisotropy is interrelated to the electronic structure. We show that the semiconducting nature of CrI3 is preserved but band gap decreases in monolayer-CrI3/In2Se3 and bilayer-CrI3/In2Se3 vdW heterostructures. The magnetoelectric effect of CrI3/In2Se3 vdW heterostructure enhances the magnetic moment of CrI3, but decreases the perpendicular magnetic anisotropy of CrI3. I-p orbital coupling matrix element is noteworthy in regard to the modulation of magnetic anisotropy. The MAE contribution of Cr atom slightly enhances but is still negligible by the reason of countervailing contribution of d-orbital coupling matrix elements. By reversing the ferroelectric polarization of In2Se3, the easy magnetization axis of monolayer CrI3 can be switched. These results will help to understand the magnetic anisotropy of multiferroic vdW heterostructure and may enable the development of nonvolatile memory devices.

Electrochemical Generation of Te Vacancy Pairs in PtTe for Efficient Hydrogen Evolution.

Two-dimensional (2D) van der Waals materials are increasingly seen as potential catalysts due to their unique structures and unmatched properties. However, achieving precise synthesis of these remarkable materials and regulating their atomic and electronic structures at the most fundamental level to enhance their catalytic performance remain a significant challenge. In this study, we synthesized single-crystal bulk PtTe crystals via chemical vapor transport and subsequently produced atomically thin, large PtTe nanosheets (NSs) through electrochemical cathode intercalation. These NSs are characterized by a significant presence of Te vacancy pairs, leading to undercoordinated Pt atoms on their basal planes. Experimental and theoretical studies together reveal that Te vacancy pairs effectively optimize and enhance the electronic properties (such as charge distribution, density of states near the Fermi level, and d-band center) of the resultant undercoordinated Pt atoms. This optimization results in a significantly higher percentage of dangling O-H water, a decreased energy barrier for water dissociation, and an increased binding affinity of these Pt atoms to active hydrogen intermediates. Consequently, PtTe NSs featuring exposed and undercoordinated Pt atoms demonstrate outstanding electrocatalytic activity in hydrogen evolution reactions, significantly surpassing the performance of standard commercial Pt/C catalysts.