vdW Materials Research Articles

Exploring Bonding Configurations in MnBi2Te4-Type Materials.

We perform a systematic investigation of several crystal structures, based on monolayer MnBi2Te4, of the form MnBiBiiXi2Xii2 using first-principles calculations. Our analysis shows that the most energetically favorable bonding configuration of the constituent elements in monolayer MnBiBiiXi2Xii2 is determined by the bond length between the Mn atom and its nearest X-site atoms. Tuning the bonding configuration of the material alters the magnetic, electronic, and topological properties. We also calculate the magnetic exchange parameters and magnetic anisotropy energy of the predicted structures. The calculations show that the elements at the X sites mainly determine the magnetic properties. Finally, we propose a stable phase of monolayer MnBi2S2Te2 (i.e., γ-MnBi2S2Te2) that exhibits the quantum anomalous Hall effect (QAHE). This study demonstrates that the bonding configuration of MnBi2Te4-type materials provides avenues for tuning the magnetic, electronic, and topological properties of van der Waals (vdW) materials.

Enhancing X-ray generation from twisted multilayer van der Waals materials by shaping electron wavepackets

We study twisted bilayer van der Waals (vdW) materials as a platform to generate versatile bremsstrahlung X-rays, and show that the twist angle in bilayer vdW materials provides an unprecedented degree of controllability over various properties of bremsstrahlung radiation from these materials. Specifically, we combine the waveshaping of the free electron’s quantum wavepacket with the unique crystalline atomic positioning of twisted bilayers to realize shaped bremsstrahlung X-rays, which feature enhancements in directionality and intensity. In the process, we present a theoretical model for bremsstrahlung radiation that is applicable to twisted multilayer vdW materials in general. We also investigate the dependence of our X-ray emission mechanism on physical parameters, including the interlayer spacing and number of layers. Our findings pave the way for the use of twisted multilayer van der Waals materials in the generation of tailored X-ray spectra for applications like X-ray imaging, X-ray fluorescence, and X-ray treatment.

Designing a 2D van der waals oxide with lone-pair electrons as chemical scissor

Abstract 2D van der Waals (vdW) materials are known for their intriguing physical properties, but their rational design and synthesis remain a great challenge for chemists. In this work, we successfully synthesized a new non-centrosymmetric oxide, i.e. InSbMoO6, with Sb3+ lone-pair electrons serving as chemical scissor to generate its 2D vdW crystal structure. Monolayer and few-layer InSbMoO6 flakes are readily obtained via facile mechanical exfoliation. They exhibit strong second-harmonic generation (SHG) response with an effective second-order nonlinear optical susceptibility $\chi _{{\rm{eff}}}^{{\rm{(2)}}}\ $of 32.4 pm·V−1. Meanwhile, the SHG response is in-plane anisotropic and directly proportional to the layer thickness, independent of layer parity. In addition, the InSbMoO6 flakes exhibit excellent thermal and atmospheric stability, along with pronounced anisotropy in Raman spectroscopy. This work implies that using lone-pair electrons as chemical scissor is an effective strategy for designing and synthesizing new 2D vdW materials for integrated photonic applications.

Narrowband Electroluminescence from Color Centers in Hexagonal Boron Nitride.

Defects in wide bandgap materials have emerged as promising candidates for solid-state quantum optical technologies. Electrical excitation of single emitters may lead to scalable on-chip devices and therefore is highly sought after. However, most wide bandgap materials are not amenable to efficient doping, posing challenges for electrical excitation and on-chip integration. Here, we demonstrate narrowband electroluminescence from visible and near-infrared color centers in hexagonal boron nitride (hBN). We harness van der Waals tunnel junctions of graphene-hBN-graphene. Charge carriers are electrically injected into hBN, exciting localized defects that emit nonclassical light, as characterized by the second order correlation measurement. Remarkably, the devices operate at room temperature and produce robust, narrowband emission spanning from visible to the near-infrared. Our work marks an important milestone in vdW materials and their promising attributes for integrated quantum technologies and on-chip photonic circuits.

Large Magnetic Anisotropy in van der Waals Ferromagnet Fe3GaTe2 above Room Temperature.

Discoveries of above-room-temperature intrinsic ferromagnetism in two-dimensional (2D) van der Waals (vdW) materials offer a platform for studying fundamental 2D magnetism and spintronic devices, especially the recently discovered above-room-temperature 2D vdW Fe3GaTe2 (FGaT). However, the magnetic mechanism in FGaT remains elusive. Here, a detailed investigation using magnetic force microscopy on the thickness-dependent magnetic behavior of FGaT single crystals is reported. The Heisenberg exchange interaction constant (J) at room temperature is determined to be 1.32836 × 10-12 J/m. Our study combining angle-resolved photoemission spectroscopy and density functional theory suggests that the high Curie temperature in FGaT is attributed to the shift of the localized Fe d band toward the Fermi level as well as the enhanced magnetic exchange effect due to the strong itinerant ability of Fe. This work sheds light on the understanding of magnetism in FGaT and provides a promising platform to investigate the mechanisms of 2D magnetic materials.

Unidirectional ray polaritons in twisted asymmetric stacks.

The vast repository of van der Waals (vdW) materials supporting polaritons offers numerous possibilities to tailor electromagnetic waves at the nanoscale. The development of twistoptics-the modulation of the optical properties by twisting stacks of vdW materials-enables directional propagation of phonon polaritons (PhPs) along a single spatial direction, known as canalization. Here we demonstrate a complementary type of directional propagation of polaritons by reporting the visualization of unidirectional ray polaritons (URPs). They arise naturally in twisted hyperbolic stacks with very different thicknesses of their constituents, demonstrated for homostructures of -MoO3 and heterostructures of -MoO3 and -Ga2O3. Importantly, their ray-like propagation, characterized by large momenta and constant phase, is tunable by both the twist angle and the illumination frequency. Apart from their fundamental importance, our findings introduce twisted asymmetric stacks as efficient platforms for nanoscale directional polariton propagation, opening the door for applications in nanoimaging, (bio)-sensing, or polaritonic thermal management.

Strong Surface-Enhanced Coherent Phonon Generation in van der Waals Materials.

Terahertz (THz) coherent phonons have emerged as promising candidates for the next generation of high-speed, low-energy information carriers in atomically thin phononic or phonon-integrated on-chip devices. However, effectively manipulating THz coherent phonons remains a significant challenge. In this study, we investigated THz coherent phonon generation in exfoliated van der Waals (vdW) flakes of Fe3GeTe2, Fe5GeTe2, and FePS3. We successfully generated the THz A1g coherent phonon mode in these vdW flakes. An innovative approach involved partially exfoliating vdW flakes on a gold substrate and partially on a silicon (Si) substrate to compare the THz coherent phonon generation between both sides. Interestingly, we observed a significantly enhanced THz coherent phonon in the vdW/gold area compared with that in the vdW/Si area. Frequency-domain Raman mapping across the vdW flakes corroborated these findings. Numerical simulations further indicated a stronger enhanced surface field in vdW/gold structures than in vdW/Si structures. Consequently, we attribute the observed enhancement in THz coherent phonon generation to the increased surface field on the gold substrate. This enhancement was consistent across the three different vdW materials studied, suggesting the universality of this strategy. Our results hold promise for advancing the design of THz phononic and phonon-integrated devices.

Multistate Ferroelectric Diodes with High Electroresistance Based on van der Waals Heterostructures.

Some van der Waals (vdW) materials exhibit ferroelectricity, making them promising for novel nonvolatile memories (NVMs) such as ferroelectric diodes (FeDs). CuInP2S6 (CIPS) is a well-known vdW ferroelectric that has been integrated with graphene for memory devices. Here we demonstrate FeDs with self-rectifying, hysteretic current-voltage characteristics based on vertical heterostructures of 10 nm thick CIPS and graphene. By using vdW indium-cobalt top electrodes and graphene bottom electrodes, we achieve a high electroresistance (on- and off-state resistance ratios) of ∼106, an on-state rectification ratio of 2500 for read/write voltages of 2 V/0.5 V, and a maximum output current density of 100 A/cm2. These metrics compare favorably with state-of-the-art FeDs. Piezoresponse force microscopy measurements show that stabilization of intermediate net polarization states in CIPS leads to stable multibit data retention at room temperature. The combination of two-terminal design, multibit memory, and low-power operation in CIPS-based FeDs is potentially interesting for compute-in-memory and neuromorphic computing applications.

Atomic Spalling of a van der Waals Nanomembrane.

ConspectusThe vertical integration of van der Waals nanomembranes (vdW NMs), composed of two-dimensional (2D) layered materials and three-dimensional (3D) freestanding films with vdW surfaces, opens new avenues for exploring novel physical phenomena and offers a promising pathway for prototyping ultrathin, superior-performance electronic and optoelectronic applications with unique functionalities. Achieving the desired functionality through vdW integration necessitates the production of high-quality individual vdW NMs, which is a fundamental prerequisite. A profound understanding of the synthetic strategies for vdW NMs, along with their fundamental working principles, is crucial in guiding the experimental design toward 3D integrated heterostructures. The foremost synthetic challenges in fabricating high-quality vdW NMs are achieving exact control over thickness and ensuring surface planarity on the atomic scale. Despite the development of numerous chemical and mechanical approaches to tackle these issues, an all-encompassing solution has yet to be realized. To address these challenges, we have developed advanced spalling techniques, specifically known as atomic spalling or 2D material-based layer transfer, which emerge as a promising technology for achieving both atomically precise thickness-engineered and atomically smooth vdW NMs. These techniques involve engineering the interfacial fracture toughness and strain energy in the vdW system, allowing for precise control over the initiation and the propagation of cracks within the vdW material based on controlled spalling theory.In this Account, we summarize our recent advancements in the atomic precision spalling technique for the preparation of vdW NMs and their applications. We begin by introducing the fundamentals of advanced spalling techniques, which are based on spalling mode fracture in bilayer systems. Following this, we succinctly describe the preparation methods for source materials for vdW NMs, with a primary focus on chemical synthesis approaches. We then delve into the working principles underlying our recent contributions to advanced spalling techniques, providing insights into how this method attains unprecedented atomic-precision control compared to other fabrication methods with a particular emphasis on tuning the interface between the stressor and the vdW system. Subsequently, we highlight cutting-edge applications based on vdW heterostructures, which combine our spalled vdW NMs. Finally, we discuss the current challenges and future directions for advanced spalling techniques, underscoring their potential to be established as a robust methodology for the preparation of high-quality vdW NMs. Our advanced spalling strategy not only ensures the reliable production of vdW NMs with exceptional control over thickness and atomic-level flatness but also provides a robust theoretical framework essential for producing high-quality vdW NMs.

Twisted hyperbolic van der Waals crystals for chip-scale full Stokes mid-infrared polarization detection

Abstract Investigating the polarization properties of light in the mid-infrared (mid-IR) spectrum is crucial for molecular sensing, biomedical diagnostics, and IR imaging system technologies. Traditional methods, limited by bulky size and complicated fabrication process, utilize large rotating optics for full Stokes polarization detection, impeding miniaturization and accuracy. Naturally occurring hyperbolic van der Waals (vdW) material based devices can address these challenges due to their lithography-free fabrication, ease of integration with chip-scale platforms and room-temperature operation. This study designs a chip-integrated polarimeter by performing multi-objective optimization for efficient exploration of the design parameter space. The spatial division measurement scheme used incorporates six precisely designed linear and circular polarization filters, achieving high extinction ratios exceeding 30 dB and transmittance surpassing 50%, with fabrication tolerance of film thickness up to 100 nm. The proposed device represents a significant advancement in polarimetric detection, providing a compact, cost-effective solution and opens new avenues for on-chip mid-IR polarimetric detection in next-generation ultra-compact optical systems.

Magnetism Measurements of Two-Dimensional van der Waals Antiferromagnet CrPS4 Using Dynamic Cantilever Magnetometry

Abstract Recent experimental and theoretical work has focused on two-dimensional van der Waals (2D vdW) magnets due to their potential applications in sensing and spintronics devises. In measurements of these emerging materials, conventional magnetometry often encounters challenges in characterizing the magnetic properties of small-sized vdW materials, especially for antiferromagnets with nearly compensated magnetic moments. Here, we investigate the magnetism of 2D antiferromagnet CrPS4 with a thickness of 8 nm by using dynamic cantilever magnetometry (DCM). Through a combination of DCM experiment and the calculation based on a Stoner–Wohlfarth-type model, we unravel the magnetization states in 2D CrPS4 antiferromagnet. In the case of H ∥ c, a two-stage phase transition is observed. For H ⊥ c, a hump in the effective magnetic restoring force is noted, which implies the presence of spin reorientation as temperature increases. These results demonstrate the benefits of DCM for studying magnetism of 2D magnets.

Full‐vdW Heterosynaptic Memtransistor with the Ferroelectric Inserted Functional Layer and its Neuromorphic Applications

AbstractThe emergence of 2D van der Waals (vdW) materials, owing to highly tunable electrical conductivity, remarkably free stackability, and excellent compatibility for heterojunction integration, has provided abundant research diversity of artificial synapses. Here, an innovative 2D vdW heterosynaptic memtransistor (vdW‐HT) synapse is proposed with a ferroelectric CuInP2S6 inserted layer. Neuromorphic synaptic weight change of the vdW‐HT synapse in this work are modulated by the synergistic effects of interlayer coupling and ferroelectric polarization reversal. It is the first time to evaluate the required initial energy consumption of ferroelectric vdW‐HT synapses with matrixed biomimetic characteristics of “Learning–Forgetting–Re‐learning–Memorizing.” The required initial energy consumption is only ≈3.06 pJ, which provided supporting evidence to indicate the promising potential of vdW‐HT synapses in exploring neuromorphic applications. A vdW‐HT computing system with artificial recognition capability for intelligent automobiles is established and demonstrated outstanding recognition abilities for multiple targets in various environments. The highest recognition accuracy for pedestrians is 98%. In addition, the excellent recognition property is integrated with a robotic arm, successfully achieving high‐precision grasping behavior and designated position transmission for identified targets. These results provide promising strategies for the integrated development of emerging neuromorphic electronics and industrial applications.

Extended Kataura plot of single-walled carbon nanotubes and on-demand fabrication of mixed-dimensional heterostructures

We present a comprehensive optical spectroscopic study on air-suspended single-walled carbon nanotubes (SWNTs) with diameters ranging from 1.7 nm to 3.4 nm and at higher-order optical transitions, a previously inaccessible regime. We establish an atlas of resonant radial breathing modes (RBMs) for more than 100 new SWNTs in this unknown regime, thus significantly extending the RBM dataset in Kataura plot and enhancing our knowledge of SWNTs. Additionally, we develop an innovative transfer technique for manipulating individual SWNTs of known chiralities (determined by optical spectroscopies) by integrating a polarization imaging technique with the conventional van der Waals (vdW) dry transfer. The unprecedented spatial and structural control of this technique enables a deterministic fabrication and integration of a variety of novel mixed-dimensional vdW heterostructures that were considered unachievable before. Our work significantly extends RBM dataset of SWNT Kataura plot and represents a huge advancement towards controlled fabrication and integration of mixed-dimensional vdW heterostructures encompassing one-dimensional vdW materials.

Nanotube spin defects for omnidirectional magnetic field sensing

Optically addressable spin defects in three-dimensional (3D) crystals and two-dimensional (2D) van der Waals (vdW) materials are revolutionizing nanoscale quantum sensing. Spin defects in one-dimensional (1D) vdW nanotubes will provide unique opportunities due to their small sizes in two dimensions and absence of dangling bonds on side walls. However, optically detected magnetic resonance of localized spin defects in a nanotube has not been observed. Here, we report the observation of single spin color centers in boron nitride nanotubes (BNNTs) at room temperature. Our findings suggest that these BNNT spin defects possess a spin S = 1/2 ground state without an intrinsic quantization axis, leading to orientation-independent magnetic field sensing. We harness this unique feature to observe anisotropic magnetization of a 2D magnet in magnetic fields along orthogonal directions, a challenge for conventional spin S = 1 defects such as diamond nitrogen-vacancy centers. Additionally, we develop a method to deterministically transfer a BNNT onto a cantilever and use it to demonstrate scanning probe magnetometry. Further refinement of our approach will enable atomic scale quantum sensing of magnetic fields in any direction.

Improved Efficiency in WSe2 Solar Cells Using Amorphous InOx Heterocontacts.

van der Waals (vdW) layered materials have been shown to have excellent optoelectronic properties relevant to photovoltaics. Despite their promise, the demonstrated efficiencies of vdW material solar cells remain low and are seldom supported by statistics or spectral quantum efficiency analysis. In this study, we utilize a p-type WSe2 absorber, forming a solar cell with a transparent front InOx electron contact, and a rear Pd reflector/hole contact. We fabricate multiple devices providing statistics for 10 devices with an average 1 sun conversion efficiency above 5%, among which a champion efficiency of 6.37% is achieved. This is the highest AM 1.5G 1 sun efficiency reported for a vdW material solar cell, with a current density supported by external quantum efficiency analysis. This cell is also shown to have near unity quantum efficiency around λ = 600 nm. This work provides support to vdW materials being considered as serious candidates for future thin-film solar cells.

Friends not Foes: Exfoliation of Non-van der Waals Materials.

ConspectusTwo-dimensional materials have been a focus of study for decades, resulting in the development of a library of nanosheets made by a variety of methods. However, many of these atomically thin materials are exfoliated from van der Waals (vdW) compounds, which inherently have weaker bonding between layers in the bulk crystal. Even though there are diverse properties and structures within this class of compounds, it would behoove the community to look beyond these compounds toward the exfoliation of non-vdW compounds as well. A particular class of non-vdW compounds that may be amenable to exfoliation are the ionically bonded layered materials, which are structurally similar to vdW compounds but have alkali ions intercalated between the layers. Although initially they may have been more difficult to exfoliate due to a lack of methodology beyond mechanical exfoliation, many synthesis techniques have been developed that have been used successfully in exfoliating non-vdW materials. In fact, as we will show, in some cases it has even proven to be advantageous to start the exfoliation from a non-vdW compound.The method we will highlight here is chemical exfoliation, which has developed significantly and is better understood mechanistically compared to when it was first conceived. Encompassing many methods, such as acid/base reactions, solvent reactions, and oxidative extractions, chemical exfoliation can be tailored to the delamination of non-vdW materials, which opens up many more possibilities of compounds to study. In addition, beginning with intercalated analogues of vdW materials can even lead to more consistent and higher quality results, overcoming some challenges associated with chemical exfoliation in general. To exemplify this, we will discuss our group's work on the synthesis of a 1T'-WS2 monolayer ink. By starting with K0.5WS2, the exfoliated 1T'-WS2 nanosheets obtained were larger and more uniform in thickness than those from previous syntheses beginning with vdW materials. The crystallinity of the nanosheets was high enough that films made from this ink were superconducting. We will also show how soft chemical methods can be used to make new phases from existing compounds, such as HxCrS2 from NaCrS2. This material was found to have alternating amorphous and crystalline layers. Its biphasic structure improved the material's performance as a battery electrode, enabling reversible Cr redox and faster Na-ion diffusion. From these and other examples, we will see how chemical exfoliation of non-vdW materials compares to other methods, as well as how this technique can be further extended to known compounds that can be deintercalated electrochemically and to quasi-one-dimensional crystals.

Manipulation of Exchange Bias in Two-Dimensional van der Waals Ferromagnet Near Room Temperature.

As a host for exchange bias (EB), van der Waals (vdW) magnetic materials have exhibited intriguing and distinct functionalities from conventional magnetic materials. The EB in most vdW systems is far below room temperature, which poses a challenge for practical applications. Here, by using Kerr microscopy, we demonstrate a record-high blocking temperature that approaches room temperature and a huge positive EB field that nears 2 kOe at 100 K in naturally oxidized two-dimensional (2D) vdW ferromagnetic Fe3GaTe2 nanoflakes. Moreover, we realized a reversible manipulation of both the presence/absence and positive/negative signs of EB via a training magnetic field without multiple field cooling processes. Thus, our study clearly reveals the robust, sizable, and sign-tunable EB in vdW magnetic materials up to near room temperature, thereby establishing Fe3GaTe2 as an emerging room-temperature-operating vdW material and paving the way for designing practical 2D spintronic devices.

(Invited) Deformable Electronics Based on Strain-Engineered Van Der Waals Materials

Materials exhibit drastically different physical and chemical properties as their dimensionality varies. Atomically-thin van der Waals (vdW) materials possess unique mechanical, electrical, optical, and thermal characteristics, originated from quantum confined low-dimensionality and a lack of dangling bond. In particular, strong in-plane covalent bonding and weak interlayer vdW interaction of two-dimensional (2D) vdW materials enable exceptional combination of mechanical properties, such as high Young’s modulus, high in-plane strength, and ultralow bending stiffness at a level of cell membrane. Moreover, mechanical strain can induce modulation of electronic and phononic band structures of 2D vdW materials more effectively due to mechanical resilience and high surface-to-volume ratio. For these reasons, deformation engineering has garnered substantial interest in mechanics, materials and electronics communities as a new tuning knob for emerging phenomena and functionalities of 2D vdW materials. In this talk, I will discuss deformation engineering of 2D vdW materials – (1) active and dynamic deformation strategies, including in-plane interfacial slippage and out-of-plane structural deformation, (2) deformation strain induced phenomena, and (3) emerging device concepts based on deformation engineering of 2D vdW materials.

One-Dimensional Van Der Waals V2Se9 As a Conversion-Type Anode Material for Advanced Li Rechargeable Batteries

Volume expansion of active materials during lithium storage is one of the main issues in Li-ion batteries (LIBs) that should be addressed. In particular, large volume changes and particle cracking are problematic for electrode materials based on conversion reaction, which are considered attractive anode materials with high energy densities. In this study, a one-dimensional van der Waals (1D vdW) material, V2Se9, is proposed as a conversion-type anode material for LIBs. The V2Se9 electrode material has a 1D chain molecular structure with vdW interactions between chains to form a bulk crystal structure. The spaces between the 1D chains act as structural buffers and provide short Li+ diffusion paths that can improve the electrochemical performance of the electrode materials. The V2Se9 electrode exhibits a high reversible capacity of 563.3 mAh g-1 at 100 mA g-1 and a high rate capability of 109.1 mAh g-1 at 3200 mA g-1. The capacity retention of V2Se9 electrodes is 83.72 % after 100 cycles. Synchrotron X-ray diffraction and X-ray absorption spectroscopy reveal the formation of Li2Se and V metal, indicating that Li ion storage occurs via the conversion reaction. The structural stability of V2Se9 is demonstrated using SEM measurements of the discharged and charged V2Se9 electrodes during cycling. These results demonstrate the reaction mechanism of V2Se9 and highlight the potential of 1D vdW materials as high-performance anode materials in LIBs.

(Invited) Two-Dimensional Layered Materials and Their Mixed-Dimensional Hybrids for Electrochemical Applications

Compared with their 3D counterparts, two-dimensional (2D) van der Waals (vdW) materials exhibit quantum confinement where charge carriers are spatially confined at the physical boundaries. Particularly, when mixing 2D materials with other low-dimensional (LD) materials, they exhibit enormous potential in electrochemical energy applications due to the unique properties arising from reduced dimensionality and, more importantly, material integration synergy.In this work, 2D transition metal dichalcogenides (MoS2) and their mixed low-dimensional hybrids (MLDHs) are introduced with an emphasis on the hybrid structure construction using the one-step solvothermal synthesis technique and their electrochemical applications. Fundamental insight into the synergistic effect of the MLDHs integration in terms of active site exposure, surface area enlargement, electrical conductivity improvement, and structural phase engineering for advancing the development of Li-ion batteries (LIBs) and electrocatalytic hydrogen evolution reaction (HER) will also be discussed. Specifically, in the case of LIBs, 2D-based binary hybrids (i.e., MoS2/MXene) deliver a Li-ion storage capacity of 540 mA h g−1 at 100 mA g−1 and a stable cycle performance up to 300 cycles. In sharp contrast, the 2D-based ternary MLDHs (i.e., MoS2/MXene/CNT) exhibit a higher Li-ion storage capacity (1500 mAh g−1 at 100 mA g−1), outstanding cycle stability (up to 300 cycles) and excellent rate capability (500 mAh g−1 at 4000 mA g−1) as an anode of LIBs. In the case of HER, the MLDH heterostructures show an overpotential of 169 mV with a low Tafel slope of 51 mV/dec and can be cycled to 1000 cycles. Leveraging the unique microreactor platform based on the 2D vdW platform, a mechanistic understanding of charge transport dynamics at the electrified electrode and current collector interface will be highlighted.The knowledge gained on how mixed-dimensional physics and chemistry will shed light on the design principle of the electrode materials for electrochemical energy storage and conversion and deepen the understanding of the process-structural-property-performance (PSPP) relationship of the vdW-based hybrid structures.