vdW Materials Research Articles

Proton-fluence dependent magnetic properties of exfoliable quasi-2D van der Waals Cr2Si2Te6 magnet

The discovery of long-range magnetic ordering in atomically thin materials catapulted the van der Waals (vdW) family of compounds into an unprecedented popularity, leading to potentially important technological applications in magnetic storage and magneto-transport devices, as well as photoelectric sensors. With the potential for the use of vdW materials in space exploration technologies it is critical to understand how the properties of such materials are affected by ionizing proton irradiation. Owing to their robust intra-layer stability and sensitivity to external perturbations, these materials also provide excellent opportunities for studying proton irradiation as a non-destructive tool for controlling their magnetic properties. Specifically, the exfoliable Cr2Si2Te6 (CST) is a ferromagnetic semiconductor with the Curie temperature (T C) of ∼32 K. Here, we have investigated the magnetic properties of CST upon proton irradiation as a function of fluence (1 × 1015, 5 × 1015, 1 × 1016, 5 × 1016, and 1 × 1018 H+/cm−2) by employing variable-temperature, variable-field magnetization measurements, and detail how the magnetization, magnetic anisotropy vary as a function of proton fluence across the magnetic phase transition. While the T C remains constant as a function of proton fluence, we observed that the saturation magnetization and magnetic anisotropy diverge at the proton fluence of 5 × 1016 H+/cm−2, which is prominent in the ferromagnetic phase, in particular.This work demonstrates that proton irradiation is a feasible method for modifying the magnetic properties and local magnetic interactions of vdWs crystals, which represents a significant step forward in the design of future spintronic and magneto-electronic applications.

Defect and doping properties of sliding ferroelectric γ-InSe for photovoltaic applications

Layered van der Waals (vdw) materials have been proposed as light-absorbing materials for photovoltaic applications. InSe is a layered vdw semiconductor with ultra-high carrier mobility, strong charge transfer ability, super deformability, thermoelectricity, and optoelectronic properties. Its γ phase, or γ-InSe, was greatly stabilized by doping recently, which also exhibits sliding ferroelectricity. In this study, we propose that γ-phase InSe (γ-InSe), which was recently synthesized in a high-quality bulk phase, could be an excellent light-absorbing material candidate. Based on the first-principles simulations, bulk γ-InSe is found to possess suitable bandgap, decent absorption, and low effective mass. The investigation of defect properties reveals the major defect types, defect levels, and deep-level defects that could possibly harm the efficiency, and the deep-level defects can be significantly suppressed under Se-rich conditions. In addition, γ-InSe is intrinsically n-type, which can be tuned into weak p-type by Zn and Cd doping. We also identify the defect types of Y and Bi doping, which have been experimentally used to adjust the mechanical property of γ-InSe, and find that Y interstices could play an important role in improving the stiffness of γ-InSe. Our study provides theoretical insights for photovoltaic and other photoelectronic applications based on this interesting ferroelectric layered vdw material.

Hierarchical assembly of peptoids on MoS2

Bio-macromolecular assembly forms intricate scaffolds in living systems that allow them to biomineralize bone, enamel, and shell. Such bioinspired approaches have also been widely used in biohybrid materials synthesis. Recently, researchers have demonstrated functional bioelectronic structures by assembling biopolymers, including proteins and peptides, on van der Waals (VdW) materials for sensors and energy storage and harvesting. However, the hierarchical assembly of biopolymers on VdW materials has yet to be fully documented, because modulating the assembly architectures by controlling the energy landscape along with environmental stimuli, in which various assembly phases occur, remains challenging. In this study, we focused on peptoids, biomimetic polymers with properties similar to peptides but offering advantages such as greater side chain diversity, lower complexity due to elimination of backbone-backbone hydrogen binding, lower synthesis costs, and improved stability. Specifically, we designed two short peptoid sequences capable of assembling on MoS2. Our findings reveal diverse self-assembled phases, including monolayer hybrid films with high crystallinity, vesicle-like nanoparticles, lamellae, and multi-layer ribbons. We elucidated the assembly processes of these states and confirmed the occurrence of phase transitions between them by using in situ atomic force microscopy (AFM). The results highlight the critical roles of peptoid-peptoid, peptoid-solvent, and peptoid-MoS2 interactions. These insights significantly advance the understanding needed to design hierarchical biopolymer architectures at VdW material-solvent interfaces with potential implications for enhancing the performance of bioelectronic devices based on 2D vdW materials.

Van der Waals quaternary oxides for tunable low-loss anisotropic polaritonics.

The discovery of ultraconfined polaritons with extreme anisotropy in a number of van der Waals (vdW) materials has unlocked new prospects for nanophotonic and optoelectronic applications. However, the range of suitable materials for specific applications remains limited. Here we introduce tellurite molybdenum quaternary oxides-which possess non-centrosymmetric crystal structures and extraordinary nonlinear optical properties-as a highly promising vdW family of materials for tunable low-loss anisotropic polaritonics. By employing chemical flux growth and exfoliation techniques, we successfully fabricate high-quality vdW layers of various compounds, including MgTeMoO6, ZnTeMoO6, MnTeMoO6 and CdTeMoO6. We show that these quaternary vdW oxides possess two distinct types of in-plane anisotropic polaritons: slab-confined and edge-confined modes. By leveraging metal cation substitutions, we establish a systematic strategy to finely tune the in-plane polariton propagation, resulting in the selective emergence of circular, elliptical or hyperbolic polariton dispersion, accompanied by ultraslow group velocities (0.0003c) and long lifetimes (5 ps). Moreover, Reststrahlen bands of these quaternary oxides naturally overlap that of α-MoO3, providing opportunities for integration. As an example, we demonstrate that combining α-MoO3 (an in-plane hyperbolic material) with CdTeMoO6 (an in-plane isotropic material) in a heterostructure facilitates collimated, diffractionless polariton propagation. Quaternary oxides expand the family of anisotropic vdW polaritons considerably, and with it, the range of nanophotonics applications that can be envisioned.

New twisted van der Waals fabrication method based on strongly adhesive polymer

Observations of emergent quantum phases in twisted bilayer graphene prompted a flurry of activities in van der Waals (vdW) materials beyond graphene. Most current twisted experiments use a so-called tear-and-stack method using a polymer called polypropylene carbonate (PPC). However, despite the clear advantage of the current PPC tear-and-stack method, there are also technical limitations, mainly a limited number of vdW materials that can be studied using this PPC-based method. This technical bottleneck has been preventing further development of the exciting field beyond a few available vdW samples. To overcome this challenge and facilitate future expansion, we developed a new tear-and-stack method using a strongly adhesive polycaprolactone. With similar angular accuracy, our technology allows fabrication without a capping layer, facilitating surface analysis and ensuring inherently clean interfaces and low operating temperatures. More importantly, it can be applied to many other vdW materials that have remained inaccessible with the PPC-based method. We present our results on twist homostructures made with a wide choice of vdW materials—from two well-studied vdW materials (graphene and MoS2) to the first-ever demonstrations of other vdW materials (NbSe2, NiPS3, and Fe3GeTe2). Therefore, our new technique will help expand moiré physics beyond few selected vdW materials and open up more exciting developments.

Proposal for All-Electrical Skyrmion Detection in van der Waals Tunnel Junctions.

A major challenge for magnetic skyrmions in atomically thin van der Waals (vdW) materials is reliable skyrmion detection. Here, based on rigorous first-principles calculations, we show that all-electrical skyrmion detection is feasible in two-dimensional vdW magnets via scanning tunneling microscopy (STM) and in planar tunnel junctions. We use the nonequilibrium Green's function method for quantum transport in planar junctions, including self-energy due to electrodes and working conditions, going beyond the standard Tersoff-Hamann approximation. We obtain a very large tunneling anisotropic magnetoresistance (TAMR) around the Fermi energy for a graphite/Fe3GeTe2/germanene/graphite vdW tunnel junction. For atomic-scale skyrmions, the noncollinear magnetoresistance (NCMR) reaches giant values. We trace the origin of the NCMR to spin mixing between spin-up and -down states of pz and dz2 character at the surface atoms. Both TAMR and NCMR are drastically enhanced in tunnel junctions with respect to STM geometry due to orbital symmetry matching at the interface.

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.

Tailoring Contacts for High-Performance 1D Ta2Pt3S8 Field-Effect Transistors.

Materials with van der Waals (vdW) unit structures rely on weak interunit vdW forces, facilitating physical separation and advancing nanomaterial research with remarkable electrical properties. Recently, there has been growing interest in one-dimensional (1D) vdW materials, celebrated for their advantageous properties, characterized by reduced dimensionality and the absence of dangling bonds. In this context, we synthesize Ta2Pt3S8, a 1D vdW material, and assess its suitability for field-effect transistor (FET) applications. Spectroscopic analysis and electrical characterization confirmed that the band gap and work function of Ta2Pt3S8 are 1.18 and 4.77 eV, respectively. Leveraging various electrode materials, we fabricated n-type FETs based on Ta2Pt3S8 and identified Cr as the optimal electrode, exhibiting a high mobility of 57 cm2 V-1 s-1. In addition, we analyzed the electron transport mechanism in n-type FETs by investigating Schottky barrier height, Schottky barrier tunneling width, and contact resistance. Furthermore, we successfully fabricated p-type operating Ta2Pt3S8 FETs using a molybdenum trioxide (MoO3) layer as a high work function contact electrode. Finally, we achieved Ta2Pt3S8 nanowire rectifying diodes by creating a p-n junction with asymmetric contact electrodes of Cr and MoO3, demonstrating an ideality factor of 1.06. These findings highlight the electronic properties of Ta2Pt3S8, positioning it as a promising 1D vdW material for future nanoelectronics and functional vdW-based device applications.

Thickness-Tunable Zoology of Magnetic Spin Textures Observed in Fe5GeTe2.

The family of two-dimensional (2D) van der Waals (vdW) materials provides a playground for tuning structural and magnetic interactions to create a wide variety of spin textures. Of particular interest is the ferromagnetic compound Fe5GeTe2 that we show displays a range of complex spin textures as well as complex crystal structures. Here, using a high-brailliance laboratory X-ray source, we show that the majority (1 × 1) Fe5GeTe2 (FGT5) phase exhibits a structure that was previously considered as being centrosymmetric but rather lacks inversion symmetry. In addition, FGT5 exhibits a minority phase that exhibits a long-range ordered (√3 × √3)-R30° superstructure. This superstructure is highly interesting in that it is innately 2D without any lattice periodicity perpendicular to the vdW layers, and furthermore, the superstructure is a result of ordered Te vacancies in one of the topmost layers of the FGT5 sheets rather than being a result of vertical Fe ordering as earlier suggested. We show, from direct real-space magnetic imaging, evidence for three distinct magnetic ground states in lamellae of FGT5 that are stabilized with increasing lamella thickness, namely, a multidomain state, a stripe phase, and an unusual fractal state. In the stripe phase we also observe unconventional type-I and type-II bubbles where the spin texture in the central region of the bubbles is nonuniform, unlike conventional bubbles. In addition, we find a bobber or a cocoon-like spin texture in thick (∼170 μm) FGT5 that emerges from the fractal state in the presence of a magnetic field. Among all the 2D vdW magnets we have thus demonstrated that FGT5 hosts perhaps the richest variety of magnetic phases that, thereby, make it a highly interesting platform for the subtle tuning of magnetic interactions.

Magnetic Skyrmions above Room Temperature in a van der Waals Ferromagnet Fe3GaTe2.

2D van der Waals (vdW) ferromagnetic crystals are a promising platform for innovative spintronic devices based on magnetic skyrmions, thanks to their high flexibility and atomic thickness stability. However, room-temperature skyrmion-hosting vdW materials are scarce, which poses a challenge for practical applications. In this study, a chemical vapor transport (CVT) approach is employed to synthesize Fe3GaTe2 crystals and room-temperature Néel skyrmions are observed in Fe3GaTe2 nanoflakes above 58nm in thickness through in situ Lorentz transmission electron microscopy (L-TEM). Upon an optimized field cooling procedure, zero-field hexagonal skyrmion lattices are successfully generated in nanoflakes with an extended thickness range (30-180nm). Significantly, these skyrmion lattices remain stable up to 355 K, setting a new record for the highest temperature at which skyrmions can be hosted. The research establishes Fe3GaTe2 as an emerging above-room-temperature skyrmion-hosting vdW material, holding great promise for future spintronics.

Spin dynamics of room temperature van der Waals (vdW) ferromagnets and their usage in microwave devices

Quasi-two-dimensional van der Waals (vdW) materials exhibiting room-temperature (RT) long-range ferromagnetic nature have emerged as a significant research field to explore fundamental condensed matter physics due to their intriguing physical properties. These vdW materials enable a futuristic platform for implementing novel spintronics devices. Here, we examined the spin dynamics of polycrystalline Fe5GeTe2 and Fe4.8Co0.2GeTe2 vdW materials using ferromagnetic resonance (FMR) spectroscopy. Vibrating Sample Magnetometer (VSM) study reveals that both materials have a soft ferromagnetic character at room temperature. From room temperature FMR measurements, the effective magnetization of Fe5GeTe2 and Fe4.8Co0.2GeTe2 derived ∼0.54 ± 0.056 and 0.50 ± 0.017 kOe, respectively. These results are consistent with reported VSM data. Fe5GeTe2 and Fe4.8Co0.2GeTe2 exhibit broad FMR linewidths of 0.697 ± 0.036 and 0.748 ± 0.056 kOe, respectively, which can be due to inhomogeneous line broadening. Besides its intrinsic contribution to linewidth, it is also affected by extrinsic Gilbert damping (αext). The value of αext is influenced by conflicting intra-band and inter-band electronic transitions, according to Modified Kambersky's theory. Furthermore, the effective Gilbert damping constant (α) obtained is 0.0513 ± 0.0046 for Fe5GeTe2 and 0.0526 ± 0.0031 for Fe4.8Co0.2GeTe2 at RT. Additionally, we developed microwave signal processing devices using these materials and evaluated their functionality both as a microwave band-reject filter and an adjustable phase shifter. The stop-band response was studied across the 5 to 25 GHz frequency range under an applied magnetic field as high as 7 kOe. For these flip-chip-based devices, attenuation is −5 dB/cm for the Fe5GeTe2-based filter and −3.2 dB/cm on sample Fe4.8Co0.2GeTe2 at 6.95 and 5.37 kOe, respectively. The same micro-strip filter was used as a tunable phase shifter in the off-resonance region. The optimal differential phase shift studied for Fe5GeTe2 and Fe4.8Co0.2GeTe2-based phase shifters in the high-frequency region (22 GHz for Fe5GeTe2 and 18 GHz for Fe4.8Co0.2GeTe2) is 23°/cm and 14°/cm, respectively, at high magnetic fields. These versatile devices find integration across a wide spectrum of applications, such as phased-array antennas, radar systems, and wireless communication systems, offering their benefits to diverse fields.

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.

4D‐STEM Nanoscale Strain Analysis in van der Waals Materials: Advancing beyond Planar Configurations

Achieving nanoscale strain fields mapping in intricate van der Waals (vdW) nanostructures, like twisted flakes and nanorods, presents several challenges due to their complex geometry, small size, and sensitivity limitations. Understanding these strain fields is pivotal as they significantly influence the optoelectronic properties of vdW materials, playing a crucial role in a plethora of applications ranging from nanoelectronics to nanophotonics. Here, a novel approach for achieving a nanoscale‐resolved mapping of strain fields across entire micron‐sized vdW nanostructures using four‐dimensional (4D) scanning transmission electron microscopy (STEM) imaging equipped with an electron microscope pixel array detector (EMPAD) is presented. This technique extends the capabilities of STEM‐based strain mapping by means of the exit‐wave power cepstrum method incorporating automated peak tracking and K‐means clustering algorithms. This approach is validated on two representative vdW nanostructures: a two‐dimensional (2D) MoS2 thin twisted flakes and a one‐dimensional (1D) MoO3/MoS2 nanorod heterostructure. Beyond just vdW materials, the versatile methodology offers broader applicability for strain‐field analysis in various low‐dimensional nanostructured materials. This advances the understanding of the intricate relationship between nanoscale strain patterns and their consequent optoelectronic properties.

All-Solution-Processed High-Performance MoS2 Thin-Film Transistors with a Quasi-2D Perovskite Oxide Dielectric.

Assembling solution-processed van der Waals (vdW) materials into thin films holds great promise for constructing large-scale, high-performance thin-film electronics, especially at low temperatures. While transition metal dichalcogenide thin films assembled in solution have shown potential as channel materials, fully solution-processed vdW electronics have not been achieved due to the absence of suitable dielectric materials and high-temperature processing. In this work, we report on all-solution-processedvdW thin-film transistors (TFTs) comprising molybdenum disulfides (MoS2) as the channel and Dion-Jacobson-phase perovskite oxides as the high-permittivity dielectric. The constituent layers are prepared as colloidal solutions through electrochemical exfoliation of bulk crystals, followed by sequential assembly into a semiconductor/dielectric heterostructure for TFT construction. Notably, all fabrication processes are carried out at temperatures below 250 °C. The fabricated MoS2 TFTs exhibit excellent device characteristics, including high mobility (>10 cm2 V-1 s-1) and an on/off ratio exceeding 106. Additionally, the use of a high-k dielectric allows for operation at low voltage (∼5 V) and leakage current (∼10-11 A), enabling low power consumption. Our demonstration of the low-temperature fabrication of high-performance TFTs presents a cost-effective and scalable approach for heterointegrated thin-film electronics.

Controllable van der Waals gaps by water adsorption.

Van der Waals (vdW) gaps with ångström-scale heights can confine molecules or ions to an ultimately small scale, providing an alternative way to tune material properties and explore microscopic phenomena. Modulation of the height of vdW gaps between two-dimensional (2D) materials is challenging due to the vdW interaction. Here we report a general approach to control the vdW gap by preadsorption of water molecules on the material surface. By controlling the saturation vapour pressure of water vapour, we can precisely control the adsorption level of water molecules and vary the height of the vdW gaps of MoS2 homojunctions from 5.5 Å to 53.6 Å. This technique can be further applied to other homo- and heterojunctions, constructing controlled vdW gaps in 2D artificial superlattices and in 2D/3D and 3D/3D heterojunctions. Engineering the vdW gap has great practical potential to modulate the device performance, as evidenced by the vdW-gap-dependent diode characteristics of the MoS2/gap/MoS2 junction. Our work introduces a general strategy of molecular preadsorption that can extend to various precursors, creating more tunability and variability in vdW material systems.

<i>In-situ</i> strain engineering and applications of van der Waals materials

Van der Waals (vdW) materials have attracted extensive research interest in the field of strain engineering due to their unique structure and excellent performance. By changing the atomic lattice and electronic structure, strain can modulate the novel physical properties of vdW materials and generate new quantum states, ultimately realize high-performance electronic devices based on new principles. In this paper, we first comprehensively review various experimental strategies of inducing <i>in-situ</i> strain, which include the bending deformation of flexible substrates, mechanical stretching of microelectromechanical systems and electrodeformation of piezoelectric substrates. Then, we outline the recent research progresses of <i>in-situ</i> strain-modulated magnetism, superconductivity and topological properties in vdW materials, as well as the development of strain-related device applications, such as intelligent strain sensors and strain-programmable probabilistic computing. Finally, we examine the current challenges and provide insights into potential opportunities in the field of strain engineering.

Efficient Optical Control of Magnon Dynamics in van der Waals Ferromagnets

Optical control of magnons in two-dimensional (2D) materials promises new functionalities for spintronics and magnonics in atomically thin devices. Here, we report control of magnon dynamics, using laser polarization, in a ferromagnetic van der Waals (vdW) material, Fe 3.6 Co 1.4 GeTe 2 . The magnon amplitude, frequency, and lifetime are controlled and monitored by time-resolved pump-probe spectroscopy. We show substantial (over 25%) and continuous modulation of magnon dynamics as a function of incident laser polarization. Our results suggest that the modification of the effective demagnetization field and magnetic anisotropy by the pump laser pulses with different polarizations is due to anisotropic optical absorption. This implies that pump laser pulses modify the local spin environment, which enables the launch of magnons with tunable dynamics. Our first-principles calculations confirm the anisotropic optical absorption of different crystal orientations. Our findings suggest a new route for the development of opto-spintronic or opto-magnonic devices.

Li-ion intercalation-driven control of two-dimensional magnetism in van der Waals FePS3 bilayers.

Manipulating two-dimensional (2D) magnetism in layered van der Waals (vdW) materials like FePS3 (FPS), with its wide-ranging applications in flexible spintronic devices, poses a persistent challenge. Through first-principles calculations, we have achieved reversible ferrimagnetic (FiM, FePS3 bilayer) ↔ antiferromagnetic (AFM, 1Li-intercalated FePS3 bilayer) ↔ ferromagnetic (FM, 2Li-intercalated FePS3 bilayer) phase transitions by using a Li-ion intercalation method. Intercalated Li ions significantly enhance the Fe-3d and S-3p hybridization and reduce the Fe-Fe, Li-Fe, Li-S, and Li-P bond lengths. The manipulation of 2D magnetism in Li-intercalated FPS bilayers can be attributed to the charge transfer between two FPS monolayers mediated by Li ions. Moreover, this study offers insights into the underlying physical mechanisms that govern the variations of electronic structures, 2D magnetism, magnetic anisotropy energy, and exchange couplings. Our reversible Li-ion intercalation permits straightforward de-intercalation using a two-step route, thereby reinstating the initial magnetic order of the FPS bilayer. Our purpose-designed FPS bilayer with different Li concentrations and robust exchange coupling not only enriches the Li-intercalation physics in the FPS system but also offers a general pathway for manipulating 2D magnetism in Fe-based vdW trisulfides.

Twisting two-dimensional iron sulfide layers into coincident site superlattices via intercalation chemistry.

Layered van der Waals (vdW) materials are susceptible not only to various stacking polymorphs through translations but also twisted structures due to rotations between layers. Here, we study the influence of such layer-to-layer twisting through the intercalation of ethylenediamine (EDA) molecules into tetragonal iron sulfide (Mackinawite FeS). Selected area electron diffraction patterns of intercalated FeS display reflections corresponding to multiple square lattices with a fixed angle between them, contrary to a single square lattice seen in the unintercalated phase. The observed twist angles of 49.13° and 22.98° result from a superstructure formation well described by the coincident site lattice (CSL) theory. According to the CSL theory, these measured twist angles lead to the formation of larger coincident site supercells. We build these CSL models for FeS using crystallographic group-subgroup transformations and find simulated electron diffraction patterns from the model to agree with the experimentally measured data.

Wafer scale growth of single crystal two-dimensional van der Waals materials.

Two-dimensional (2D) van der Waals (vdW) materials, including graphene, hexagonal boron nitride (hBN), and metal dichalcogenides (MCs), form the basis of modern electronics and optoelectronics due to their unique electronic structure, chemical activity, and mechanical strength. Despite many proof-of-concept demonstrations so far, to fully realize their large-scale practical applications, especially in devices, wafer-scale single crystal atomically thin highly uniform films are indispensable. In this minireview, we present an overview on the strategies and highlight recent significant advances toward the synthesis of wafer-scale single crystal graphene, hBN, and MC 2D thin films. Currently, there are five distinct routes to synthesize wafer-scale single crystal 2D vdW thin films: (i) nucleation-controlled growth by suppressing the nucleation density, (ii) unidirectional alignment of multiple epitaxial nuclei and their seamless coalescence, (iii) self-collimation of randomly oriented grains on a molten metal, (iv) surface diffusion and epitaxial self-planarization and (v) seed-mediated 2D vertical epitaxy. Finally, the challenges that need to be addressed in future studies have also been described.