Stacking-Dependent Interlayer Coupling in Trilayer MoS₂ with Broken Inversion Symmetry

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

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

Polytypic Two-Dimensional FeAs with High Anisotropy

In the realm of two-dimensional (2D) crystal growth, the chemical composition often determines the thermodynamically favored crystallographic structures. This relationship poses a challenge in synthesizing novel 2D crystals without altering their chemical elements, resulting in the rarity of achieving specific crystallographic symmetries or lattice parameters. We present 2D polymorphic FeAs crystals that completely differ from bulk orthorhombic FeAs (Pnma), differing in the stacking sequence, i.e., polytypes. Preparing polytypic FeAs outlines a strategy for independently controlling each symmetry operator, which includes the mirror plane for 2Q-FeAs (I4/mmm) and the glide plane for 1Q-FeAs (P4/nmm). As such, compared to bulk FeAs, polytypic 2D FeAs shows highly anisotropic properties such as electrical conductivity, Young's modulus, and friction coefficient. This work represents a concept of expanding 2D crystal libraries with a given chemical composition but various crystal symmetries.

Optical Enhancement of Indirect Bandgap 2D Transition Metal Dichalcogenides for Multi-Functional Optoelectronic Sensors

The unique electrical and optical properties of transition metal dichalcogenides (TMDs) make them attractive nanomaterials for optoelectronic applications, especially optical sensors. However, the optical characteristics of these materials are dependent on the number of layers. Monolayer TMDs have a direct bandgap that provides higher photoresponsivity compared to multilayer TMDs with an indirect bandgap. Nevertheless, multilayer TMDs are more appropriate for various photodetection applications due to their high carrier density, broad spectral response from UV to near-infrared, and ease of large-scale synthesis. Therefore, this review focuses on the modification of the optical properties of devices based on indirect bandgap TMDs and their emerging applications. Several successful developments in optical devices are examined, including band structure engineering, device structure optimization, and heterostructures. Furthermore, it introduces cutting-edge techniques and future directions for optoelectronic devices based on multilayer TMDs.

Inversion Symmetry and Exotic Interlayer Exciton Behavior in Twisted Trilayer MoS2 Produced by Vapor Deposition

Two-dimensional materials (2DMs) that are stacked vertically with a certain twist angle provide new degrees of freedom for designing novel physical properties. Twist-related properties of homogeneous bilayer and heterogeneous bilayer 2DMs, such as excitons and phonons, have been described in many pioneering works. However, twist-related properties of homogeneous trilayer 2DMs have been rarely reported. In this work, trilayer MoS₂ with the twisted angle of 12° by optimized vapor deposition rather than the conventional mechanical stacking method was successfully fabricated. The inversion symmetry of trilayer MoS₂ is changed by twist. Phonons and excitons produced by twist have an enormous influence on the interlayer interaction of trilayer MoS₂, making trilayer MoS₂ appear to have exotic optical properties. Compared with monolayer MoS₂, the phonon vibration and photoluminescence intensity of trilayer MoS₂ with one-interlayer-twisted are significantly improved, and the second harmonic generation response in the non-twist region of trilayer MoS₂ is ∼3 times that of monolayer MoS₂. In addition, interlayer coupling, inversion symmetry, and exciton behavior of the twist region show regional differences. This work provides a new way for designing twist and exploring the influence of twist on the structures of 2DMs with few layers.

PVD growth of spiral pyramid-shaped WS2 on SiO2/Si driven by screw dislocations

Atomically thin layered transition metal dichalcogenides (TMDs), such as MoS₂ and WS₂, have been getting much attention recently due to their interesting electronic and optoelectronic properties. Especially, spiral TMDs provide a variety of candidates for examining the light-matter interaction resulting from the broken inversion symmetry, as well as the potential new utilization in functional optoelectronic, electromagnetic and nanoelectronics devices. To realize their potential device applications, it is desirable to achieve controlled growth of these layered nanomaterials with a tunable stacking. Here, we demonstrate the Physical Vapor Deposition (PVD) growth of spiral pyramid-shaped WS₂ with ∼200  μ m in size and the interesting optical properties via AFM and Raman spectroscopy. By controlling the precursors concentration and changing the initial nucleation rates in PVD growth, WS₂ in different nanoarchitectures can be obtained. We discuss the growth mechanism for these spiral-patterned WS₂ nanostructures based on the screw dislocations. This study provides a simple, scalable approach of screw dislocation-driven (SDD) growth of distinct TMD nanostructures with varying morphologies, and stacking.

Pulsed Carrier Gas Assisted High-Quality Synthetic 3R-Phase Sword-like MoS2: A Versatile Optoelectronic Material

Synthesizing a material with the desired polymorphic phase in a chemical vapor deposition (CVD) process requires a delicate balance among various thermodynamic variables. Here, we present a methodology to synthesize rhombohedral (3R)-phase MoS₂ in a well-defined sword-like geometry having lengths up to 120 μm, uniform width of 2-3 μm and thickness of 3-7 nm by controlling the carrier gas flow dynamics from continuous mode to pulsed mode during the CVD growth process. Characteristic signatures such as high degree of circular dichroism (∼58% at 100 K), distinct evolution of low-frequency Raman peaks and increasing intensity of second harmonic signals with increasing number of layers conclusively establish the 3R-phase of the material. A high value (∼844 pm/V) of second-order susceptibility for few-layer-thick MoS₂ swords signifies the potential of MoS₂ to serve as an atomically thin nonlinear medium. A field effect mobility of 40 cm²/V-s and I_on/I_off ratio of ∼10⁶ further confirm the electronic-grade standard of this 3R-phase MoS₂. These findings are significant for the development of emerging quantum electronic devices utilizing valley-based physics and nonlinear optical phenomena in layered materials.

Rational Design of WSe2 /WS2 /WSe2 Dual Junction Phototransistor Incorporating High Responsivity and Detectivity

The excellent semiconducting properties and ultrathin morphological characteristics allow van der Waals (vdW) heterostructures based on 2D materials to be promising channel materials for the next-generation optoelectronic devices, especially in photodetectors. Although various 2D heterostructure-based photodetectors have been developed, the unavoidable trade-off between responsivity and detectivity remains a critical issue for these devices. Here, an ingenious phototransistor based on WSe₂ /WS₂ /WSe₂ dual-vdW heterostructures is constructed, performing both high responsivity and detectivity. In the charge neutrality point (gate voltage of -15 V and bias voltage of 1 V), this device demonstrates a pronounced photosensitivity, accompanying with high detectivity of 1.9 × 10¹⁴  Jones, high responsivity of 35.4 A W^-1 , and fast rise/fall time of 3.2/2.5 ms at 405 nm with power density of 60 µW cm^-2 . Density functional theory calculations, energy band profiles, and optoelectronic characteristics jointly verify that the high performance is ascribed to the distinctive device design, which not only facilitates the separation of photogenerated carriers but also produces a strong photogating effect. As a feasible application, an automotive radar system is demonstrated, proving that the device has considerable potential for application in vehicle intelligent assisted driving.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Controllable Preparation of 2D Vertical van der Waals Heterostructures and Superlattices for Functional Applications

2D van der Waals heterostructures (vdWHs) and superlattices (SLs) with exotic physical properties and applications for new devices have attracted immense interest. Compared to conventionally bonded heterostructures, the dangling-bond-free surface of 2D layered materials allows for the feasible integration of various materials to produce vdWHs without the requirements of lattice matching and processing compatibility. The quality of interfaces in artificially stacked vdWHs/vdWSLs and scalability of production remain among the major challenges in the field of 2D materials. Fortunately, bottom-up methods exhibit relatively high controllability and flexibility. The growth parameters, such as the temperature, precursors, substrate, and carrier gas, can be carefully and comprehensively controlled to produce high-quality interfaces and wafer-scale products of vdWHs/vdWSLs. This review focuses on three types of bottom-up methods for the assembly of vdWHs and vdWSLs with atomically clean and electronically sharp interfaces: chemical/physical vapor deposition, metal-organic chemical vapor deposition, and ultrahigh vacuum growth. These methods can intuitively illustrate the great flexibility and controllability of bottom-up methods for the preparation of vdWHs/vdWSLs. The latest progress in vdWHs and vdWSLs, related physical phenomena, and (opto)electronic devices are summarized. Finally, the authors discuss current challenges and future perspectives in the synthesis and application of vdWHs and vdWSLs.

Commensurate Stacking Phase Transitions in an Intercalated Transition Metal Dichalcogenide

Intercalation and stacking-order modulation are two active ways in manipulating the interlayer interaction of transition metal dichalcogenides (TMDCs), which lead to a variety of emergent phases and allow for engineering material properties. Herein, the growth of Pb-intercalated TMDCs-Pb(Ta_1+x Se₂ )₂ , the first 124-phase, is reported. Pb(Ta_1+x Se₂ )₂ exhibits a unique two-step first-order structural phase transition at around 230 K. The transitions are solely associated with the stacking degree of freedom, evolving from a high-temperature (high-T) phase with ABC stacking and R3m symmetry to an intermediate phase with AB stacking and P3m1, and finally to a low-temperature (low-T) phase again with R3msymmetry, but with ACB stacking. Each step involves a rigid slide of building blocks by a vector [1/3, 2/3, 0]. Intriguingly, gigantic lattice contractions occur at the transitions on warming. At low-T, bulk superconductivity with T_c  ≈ 1.8 K is observed. The underlying physics of the structural phase transitions are discussed from first-principle calculations. The symmetry analysis reveals topological nodal lines in the band structure. The results demonstrate the possibility of realizing higher-order metal-intercalated phases of TMDCs and advance the knowledge of polymorphic transitions, and may inspire stacking-order engineering in TMDCs and beyond.

Direct Detection of Inhomogeneity in CVD-Grown 2D TMD Materials via K-Means Clustering Raman Analysis

It is known that complex growth environments often induce inhomogeneity in two-dimensional (2D) materials and significantly restrict their applications. In this paper, we proposed an efficient method to analyze the inhomogeneity of 2D materials by combination of Raman spectroscopy and unsupervised k-means clustering analysis. Taking advantage of k-means analysis, it can provide not only the characteristic Raman spectrum for each cluster but also the cluster spatial maps. It has been demonstrated that inhomogeneities and their spatial distributions are simultaneously revealed in all CVD-grown MoS₂, WS₂ and WSe₂ samples. Uniform p-type doping and varied tensile strain were found in polycrystalline monolayer MoS₂ from the grain boundary and edges to the grain center (single crystal). The bilayer MoS₂ with AA and AB stacking are shown to have relatively uniform p-doping but a gradual increase of compressive strain from center to the periphery. Irregular distribution of 2LA(M)/E2g1 mode in WS₂ and E2g1 mode in WSe₂ is revealed due to defect and strain, respectively. All the inhomogeneity could be directly characterized in color-coded Raman imaging with correlated characteristic spectra. Moreover, the influence of strain and doping in the MoS₂ can be well decoupled and be spatially verified by correlating with the clustered maps. Our k-means clustering Raman analysis can dramatically simplify the inhomogeneity analysis for large Raman data in 2D materials, paving the way towards direct evaluation for high quality 2D materials.

Electronic Structure and Stacking Arrangement of Tungsten Disulfide at the Gold Contact

There is an intensive effort to control the nature of attractive interactions between ultrathin semiconductors and metals and to understand its impact on the electronic properties at the junction. Here, we present a photoelectron spectroscopy study on the interface between WS₂ films and gold, with a focus on the occupied electronic states near the Brillouin zone center (i.e., the Γ point). To delineate the spectra of WS₂ supported on crystalline Au from the suspended WS₂, we employ a microscopy approach and a tailored sample structure, in which the WS₂/Au junction forms a semi-epitaxial relationship and is adjacent to suspended WS₂ regions. The photoelectron spectra, as a function of WS₂ thickness, display the expected splitting of the highest occupied states at the Γ point. In multilayer WS₂, we discovered variations in the electronic states that spatially align with the crystalline grains of underlying Au. Corroborated by density functional theory calculations, we attribute the electronic structure variations to stacking variations within the WS₂ films. We propose that strong interactions exerted by Au grains cause slippage of the interfacing WS₂ layer with respect to the rest of the WS₂ film. Our findings illustrate that the electronic properties of transition metal dichalcogenides, and more generally 2D layered materials, are physically altered by the interactions with the interfacing materials, in addition to the electron screening and defects that have been widely considered.

Recent Developments in van der Waals Antiferromagnetic 2D Materials: Synthesis, Characterization, and Device Implementation

Magnetism in two dimensions is one of the most intriguing and alluring phenomena in condensed matter physics. Atomically thin 2D materials have emerged as a promising platform for exploring magnetic properties, leading to the development of essential technologies such as supercomputing and data storage. Arising from spin and charge dynamics in elementary particles, magnetism has also unraveled promising advances in spintronic devices and spin-dependent optoelectronics and photonics. Recently, antiferromagnetism in 2D materials has received extensive attention, leading to significant advances in their understanding and emerging applications; such materials have zero net magnetic moment yet are internally magnetic. Several theoretical and experimental approaches have been proposed to probe, characterize, and modulate the magnetic states efficiently in such systems. This Review presents the latest developments and current status for tuning the magnetic properties in distinct 2D van der Waals antiferromagnets. Various state-of-the-art optical techniques deployed to investigate magnetic textures and dynamics are discussed. Furthermore, device concepts based on antiferromagnetic spintronics are scrutinized. We conclude with remarks on related challenges and technological outlook in this rapidly expanding field.

Wafer-Scale Uniform Growth of an Atomically Thin MoS2 Film with Controlled Layer Numbers by Metal-Organic Chemical Vapor Deposition

The growth control of a molybdenum disulfide (MoS₂) thin film, including the number of layers, growth rate, and electrical property modulation, remains a challenge. In this study, we synthesized MoS₂ thin films using the metal-organic chemical vapor deposition (MOCVD) method with a 2 inch wafer scale and achieved high thickness uniformity according to the positions on the substrate. In addition, we successfully controlled the number of MoS₂ layers to range from one to five, with a growth rate of 10 min per layer. The layer-dependent optical and electrical properties were characterized by photoluminescence, Raman spectroscopy, differential reflectance spectroscopy, and field effect transistors. To guide the growth of MoS₂, we summarized the relation between the growth aspects and the precursor control in the form of a growth map. Reference to this growth map enabled control of the growth rate, domain density, and domain size according to the application purposes. Finally, we confirmed the electrical performance of MOCVD-grown MoS₂ with five layers under a high-κ dielectric environment, which exhibited an on/off current ratio of 10^∼6 and a maximum field effect mobility of 8.6 cm² V^-1 s^-1.

Layer-dependent interface reconstruction and strain modulation in twisted WSe2

Twistronics has emerged as one of the most attractive playgrounds for manipulating the interfacial structures and electronic properties of two-dimensional materials. However, the layer-dependent lattice reconstruction and resulted strain distribution in marginally twisted transition metal dichalcogenides still remain elusive. Here we report a systematic study by both electron diffraction quantification and atomic-resolution imaging on the interface reconstruction of twisted WSe₂, which shows a strong dependence on the constituent layer numbers and twist angles. The competition between the interlayer interaction, which varies with local atomic configurations, and the intralayer elastic deformation, related to the layer thickness, leads to rich superlattice motifs and strain modulation patterns, i.e. triangular for odd and kagome-like textures for even layer numbers, against the rigid stacking moiré model. The strain effects of small twist angles are further demonstrated by electrical transport measurements, manifesting intriguing conducting states at low temperatures beyond the flat band features of large twist angles. Our work not only provides a comprehensive understanding of layer-dependent twist structures, but also may shed light on the future design of twistronic devices.

Strategies, Status, and Challenges in Wafer Scale Single Crystalline Two-Dimensional Materials Synthesis

The successful exfoliation of graphene has given a tremendous boost to research on various two-dimensional (2D) materials in the last 15 years. Different from traditional thin films, a 2D material is composed of one to a few atomic layers. While atoms within a layer are chemically bonded, interactions between layers are generally weak van der Waals (vdW) interactions. Due to their particular dimensionality, 2D materials exhibit special electronic, magnetic, mechanical, and thermal properties, not found in their 3D counterparts, and therefore they have great potential in various applications, such as 2D materials-based devices. To fully realize their large-scale practical applications, especially in devices, wafer scale single crystalline (WSSC) 2D materials are indispensable. In this review, we present a detailed overview on strategies toward the synthesis of WSSC 2D materials while highlighting the recent progress on WSSC graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenide (TMDC) synthesis. The challenges that need to be addressed in future studies have also been described. In general, there have been two distinct routes to synthesize WSSC 2D materials: (i) allowing only one nucleus on a wafer scale substrate to be formed and developed into a large single crystal and (ii) seamlessly stitching a large number of unidirectionally aligned 2D islands on a wafer scale substrate, which is generally single crystalline. Currently, the synthesis of WSSC graphene has been realized by both routes, and WSSC hBN and MoS₂ have been synthesized by route (ii). On the other hand, the growth of other WSSC 2D materials and WSSC multilayer 2D materials still remains a big challenge. In the last section, we wrap up this review by summarizing the future challenges and opportunities in the synthesis of various WSSC 2D materials.

Controllable Epitaxial Growth of Large-Area MoS2 /WS2 Vertical Heterostructures by Confined-Space Chemical Vapor Deposition

The controllable large-area growth of single-crystal vertical heterostructures based on 2D transition metal dichalcogenides (TMDs) remains a challenge. Here, large-area vertical MoS₂ /WS₂ heterostructures are synthesized using single-step confined-space chemical vapor epitaxy. The heterostructures can evolve into two different kinds by switching the H₂ flow on and off: MoS₂ /WS₂ heterostructures with multiple WS₂ domains can be achieved without introducing the H₂ flow due to the numerous nucleation centers on the bottom MoS₂ monolayer during the transition stage between the MoS₂ and WS₂ monolayer growth. In contrast, isolated MoS₂ /WS₂ heterostructures with single WS₂ domain can be obtained with introducing the H₂ flow due to the reduced nucleation centers on the bottom MoS₂ monolayer arising from the hydrogen etching effect. Both the two kinds of the vertical MoS₂ /WS₂ heterostructures feature high quality. The photodetectors based on the isolated MoS₂ /WS₂ heterostructures exhibit a high responsivity of 68 mA W^-1 and a short response time of 35 ms. This single-step chemical vapor epitaxy can be used to synthesize vertical MoS₂ /WS₂ heterostructures with high production efficiency. The new epitaxial growth approach may open new pathways to fabricate large-area heterostructures made of different 2D TMDs monolayers of interest to electronics, optoelectronics, and other applications.

Study of local anodic oxidation regimes in MoSe2

Scanning probe microscopy is widely known not only as a well-established research method but also as a set of techniques enabling precise surface modification. One such technique is local anodic oxidation (LAO). In this study, we investigate the LAO of MoSe₂ transferred on an Au/Si substrate, focusing specifically on the dependence of the height and diameter of oxidized dots on the applied voltage and time of exposure at various humidities. Depending on the humidity, two different oxidation regimes were identified. The first, at a relative humidity (RH) of 60%-65%, leads to in-plane isotropic oxidation. For this regime, we analyze the dependence of the size of oxidized dots on the oxidation parameters and modify the classical equation of oxidation kinetics to account for the properties of MoSe₂ and its oxide. In this regime, patterns with a maximum spatial resolution of 10 nm were formed on the MoSe₂ surface. The second is the in-plane anisotropic oxidation regime that arises at a RH of 40%-50%. In this regime, oxidation leads to the formation of triangles oxidized inside the zigzag edges. Based on the mutual orientation of zigzag and armchair directions in successive oxidized layers, the stacking type and phase of MoSe₂ flakes were determined. These results allow LAO to be considered not only as an ultra-high-resolution nanolithography method, but also as a method for investigating the crystal structure of materials with strong intrinsic anisotropy, such as transition metal dichalcogenides.

Interface Kinetics Assisted Barrier Removal in Large Area 2D-WS2 Growth to Facilitate Mass Scale Device Production

Growth of monolayer WS2 of domain size beyond few microns is a challenge even today; and it is still restricted to traditional exfoliation techniques, with no control over the dimension. Here, we present the synthesis of mono- to few layer WS2 film of centimeter2 size on graphene-oxide (GO) coated Si/SiO2 substrate using the chemical vapor deposition CVD technique. Although the individual size of WS2 crystallites is found smaller, the joining of grain boundaries due to sp 2-bonded carbon nanostructures (~3-6 nm) in GO to reduced graphene-oxide (RGO) transformed film, facilitates the expansion of domain size in continuous fashion resulting in full coverage of the substrate. Another factor, equally important for expanding the domain boundary, is surface roughness of RGO film. This is confirmed by conducting WS2 growth on Si wafer marked with few scratches on polished surface. Interestingly, WS2 growth was observed in and around the rough surface irrespective of whether polished or unpolished. More the roughness is, better the yield in crystalline WS2 flakes. Raman mapping ascertains the uniform mono-to-few layer growth over the entire substrate, and it is reaffirmed by photoluminescence, AFM and HRTEM. This study may open up a new approach for growth of large area WS2 film for device application. We have also demonstrated the potential of the developed film for photodetector application, where the cycling response of the detector is highly repetitive with negligible drift.

Probing stacking configurations in a few layered MoS2 by low frequency Raman spectroscopy

Novel two-dimensional (2D) layered materials, such as MoS₂, have recently gained a significant traction, chiefly due to their tunable electronic and optical properties. A major attribute that affects the tunability is the number of layers in the system. Another important, but often overlooked aspect is the stacking configuration between the layers, which can modify their electro-optic properties through changes in internal symmetries and interlayer interactions. This demands a thorough understanding of interlayer stacking configurations of these materials before they can be used in devices. Here, we investigate the spatial distribution of various stacking configurations and variations in interlayer interactions in few-layered MoS₂ flakes probed through the low-frequency Raman spectroscopy, which we establish as a versatile imaging tool for this purpose. Some interesting anomalies in MoS₂ layer stacking, which we propose to be caused by defects, wrinkles or twist between the layers, are also reported here. These types of anomalies, which can severely affect the properties of these materials can be detected through low-frequency Raman imaging. Our findings provide useful insights for understanding various structure-dependent properties of 2D materials that could be of great importance for the development of future electro-optic devices, quantum devices and energy harvesting systems.

Electric Field Tuning of Interlayer Coupling in Noncentrosymmetric 3R-MoS2 with an Electric Double Layer Interface

Interlayer coupling in two-dimensional (2D) layered materials plays an important role in controlling their properties. 2H- and 3R-MoS₂ with different stacking orders and the resulting interlayer coupling have been recently discovered to have different band structures and a contrast behavior in valley physics. However, the role of carrier doping in interlayer coupling in 2D materials remains elusive. Here, based on the electric double layer interface, we demonstrated the experimental observation of carrier doping-enhanced interlayer coupling in 3R-MoS₂. A remarkable tuning of interlayer Raman modes can be observed by changing the stacking sequence and carrier doping near their monolayer limit. The modulated interlayer vibration modes originated from the interlayer coupling show a doping-induced blue shift and are supposed to be associated with the interlayer coupling enhancement, which is further verified using our first-principles calculations. Such an electrical control of interlayer coupling of layered materials in an electrical gating geometry provides a new degree of freedom to modify the physical properties in 2D materials.

Stacking Domains and Dislocation Networks in Marginally Twisted Bilayers of Transition Metal Dichalcogenides

We apply a multiscale modeling approach to study lattice reconstruction in marginally twisted bilayers of transition metal dichalcogenides (TMD). For this, we develop density functional theory parametrized interpolation formulae for interlayer adhesion energies of MoSe_{2}, WSe_{2}, MoS_{2}, and WS_{2}, combine those with elasticity theory, and analyze the bilayer lattice relaxation into mesoscale domain structures. Paying particular attention to the inversion asymmetry of TMD monolayers, we show that 3R and 2H stacking domains, separated by a network of dislocations develop for twist angles θ^{∘}<θ_{P}^{∘}∼2.5° and θ^{∘}<θ_{AP}^{∘}∼1° for, respectively, bilayers with parallel (P) and antiparallel (AP) orientation of the monolayer unit cells and suggest how the domain structures would manifest itself in local probe scanning of marginally twisted P and AP bilayers.

Precise control of the interlayer twist angle in large scale MoS2 homostructures

Twist angle between adjacent layers of two-dimensional (2D) layered materials provides an exotic degree of freedom to enable various fascinating phenomena, which opens a research direction—twistronics. To realize the practical applications of twistronics, it is of the utmost importance to control the interlayer twist angle on large scales. In this work, we report the precise control of interlayer twist angle in centimeter-scale stacked multilayer MoS₂ homostructures via the combination of wafer-scale highly-oriented monolayer MoS₂ growth techniques and a water-assisted transfer method. We confirm that the twist angle can continuously change the indirect bandgap of centimeter-scale stacked multilayer MoS₂ homostructures, which is indicated by the photoluminescence peak shift. Furthermore, we demonstrate that the stack structure can affect the electrical properties of MoS₂ homostructures, where 30° twist angle yields higher electron mobility. Our work provides a firm basis for the development of twistronics.

Interlayer twist angle between vertically stacked 2D material layers can trigger exciting fundamental physics. Here, the authors report precise control of interlayer twist angle of stacked centimeter scale multilayer MoS₂ homostructures that enables continuous change in their indirect bandgap, Moiré phonons and electrical properties.

Robust Sample Preparation of Large-Area In- and Out-of-Plane Cross Sections of Layered Materials with Ultramicrotomy

Layered materials (LMs) such as graphene or MoS2 have attracted a great deal of interest recently. These materials offer unique functionalities due to their structural anisotropy characterized by weak van der Waals bonds along the out-of-plane axis and covalent bonds in the in-plane direction. A central requirement to access the structural information on complex nanostructures built upon LMs is to control the relative orientation of each sample prior to their inspection, e.g., with transmission electron microscopy (TEM). However, developing sample preparation methods that result in large inspection areas and ensure full control over the sample orientation while avoiding damage during the transfer to the TEM grid is challenging. Here, we demonstrate the feasibility of deploying ultramicrotomy for the preparation of LM samples in TEM analyses. We show how ultramicrotomy leads to the reproducible large-scale production of both in-plane and out-of-plane cross sections, with bulk vertically oriented MoS2 and WS2 nanosheets as a proof of concept. The robustness of the prepared samples is subsequently verified by their characterization by means of both high-resolution TEM and Raman spectroscopy measurements. Our approach is fully general and should find applications for a wide range of materials as well as of techniques beyond TEM, thus paving the way to the systematic large-area mass-production of cross-sectional specimens for structural and compositional studies.

Direct bilayer growth: a new growth principle for a novel WSe2 homo-junction and bilayer WSe2 growth

Homo-junction and multi-layer structures of transition metal chalcogenide (TMD) materials provide great flexibility for band-structure engineering and designing photoelectric devices. However, the knowledge of van der Waals epitaxy growth limits the development of these heterostructures. Herein, we employed the chemical vapor deposition (CVD) growth strategy to synthesize novel WSe₂ homo-junction samples with a triangular monolayer in the center and three AA stacking bilayer flakes connected to the vertexes of the monolayer. The emitted photon energy from the bilayer near the junction showed a blueshift in energy of up to 24 meV compared with bare bilayer WSe₂, confirming the charge transfer effect from monolayer to bilayer WSe₂. Further growth studies revealed the shape evolution from WSe₂ homo-junction to bilayer. The whole homo-junction formation and evolution process cannot be explained by the traditional layer-by-layer growth mechanism. Instead, a direct bilayer growth approach is proposed to explain the bilayer formation and evolution at the vertexes of the bottom layer of WSe₂. These findings suggest that the growth of bilayer TMDs is more complex than our previous understanding. This work presents deepens insight into van der Waals epitaxy growth, and thus is valuable for guiding the fabrication of novel homo-junctions for both fundamental science and optoelectronic applications.

Atomic-Scale Fabrication of In-Plane Heterojunctions of Few-Layer MoS2 via In Situ Scanning Transmission Electron Microscopy

Layered MoS₂ is a prospective candidate for use in energy harvesting, valleytronics, and nanoelectronics. Its properties strongly related to its stacking configuration and the number of layers. Due to its atomically thin nature, understanding the atomic-level and structural modifications of 2D transition metal dichalcogenides is still underdeveloped, particularly the spatial control and selective precision. Therefore, the development of nanofabrication techniques is essential. Here, an atomic-scale approach used to sculpt 2D few-layer MoS₂ into lateral heterojunctions via in situ scanning/transmission electron microscopy (STEM/TEM) is developed. The dynamic evolution is tracked using ultrafast and high-resolution filming equipment. The assembly behaviors inherent to few-layer 2D-materials are observed during the process and included the following: scrolling, folding, etching, and restructuring. Atomic resolution STEM is employed to identify the layer variation and stacking sequence for this new 2D-architecture. Subsequent energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy analyses are performed to corroborate the elemental distribution. This sculpting technique that is established allows for the formation of sub-10 nm features, produces diverse nanostructures, and preserves the crystallinity of the material. The lateral heterointerfaces created in this study also pave the way for the design of quantum-relevant geometries, flexible optoelectronics, and energy storage devices.

Analyzing the microstructure and related properties of 2D materials by transmission electron microscopy

Two-dimensional materials such as transition metal dichalcogenide and graphene are of great interest due to their intriguing electronic and optical properties such as metal-insulator transition based on structural variation. Accordingly, detailed analyses of structural tunability with transmission electron microscopy have become increasingly important for understanding atomic configurations. This review presents a few analyses that can be applied to two-dimensional materials using transmission electron microscopy.

Giant Valley Coherence at Room Temperature in 3R WS2 with Broken Inversion Symmetry

Breaking the space-time symmetries in materials can markedly influence their electronic and optical properties. In 3R-stacked transition metal dichalcogenides, the explicitly broken inversion symmetry enables valley-contrasting Berry curvature and quantization of electronic angular momentum, providing an unprecedented platform for valleytronics. Here, we study the valley coherence of 3R WS₂ large single-crystal with thicknesses ranging from monolayer to octalayer at room temperature. Our measurements demonstrate that both A and B excitons possess robust and thickness-independent valley coherence. The valley coherence of direct A (B) excitons can reach 0.742 (0.653) with excitation conditions on resonance with it. Such giant and thickness-independent valley coherence of large single-crystal 3R WS₂ at room temperature would provide a firm foundation for quantum manipulation of the valley degree of freedom and practical application of valleytronics.

Electrocatalysis on Edge-Rich Spiral WS2 for Hydrogen Evolution

Transition metal dichalcogenides (TMDs) exhibit promising catalytic properties for hydrogen generation, and several approaches including defect engineering have been shown to increase the active catalytic sites. Despite preliminary understandings in defect engineering, insights on the role of various types of defects in TMDs for hydrogen evolution catalysis are limited. Screw dislocation-driven (SDD) growth is a line defect and yields fascinating spiral and pyramidal morphologies for TMDs with a large number of edge sites, resulting in very interesting electronic and catalytic properties. The role of dislocation lines and edge sites of these spiral structures on their hydrogen evolution catalytic properties is unexplored. Here we show that the large number of active edge sites connected together by dislocation lines in the vertical direction for a spiral WS₂ domain results in exceptional catalytic properties toward hydrogen evolution reaction. A micro-electrochemical cell fabricated by photo- and electron beam-lithography processes is used to study the electrocatalytic activity of a single spiral WS₂ domain, controllably grown by chemical vapor deposition. Conductive atomic force microscopy studies show improved vertical conduction for the spiral domain, which is compared with monolayer and mechanically exfoliated thick WS₂ flakes. The obtained results are interesting and shed light on the role of SDD line defects, which contribute to large number of edge sites without compromising the vertical electrical conduction, on the electrocatalytic properties of TMDs for hydrogen evolution.

Effect of layer and stacking sequence in simultaneously grown 2H and 3R WS2 atomic layers

In two-dimensional layered materials, layer number and stacking order have strong effects on the optical and electronic properties. Tungsten disulfide (WS₂) crystal, as one important member among transition metal dichalcogenides, has been usually prepared in a layered 2H prototype structure with space group P6₃/mmc ([Formula: see text]) in spite of many other expected ones such as 3R. Here, we report simultaneous growth of 2H and 3R stacked multilayer (ML) WS₂ crystals in large scale by chemical vapor deposition and effects of layer number and stacking order on optical and electronic properties. As revealed in Raman and photoluminescence (PL) measurements, with an increase in layer number, 2H and 3R stacked ML WS₂ crystals show similar variation of PL and Raman peaks in position and intensity. Compared to 2H stacked ML WS₂, however, 3R stacked one always exhibits the larger red (blue) shift of Raman [Formula: see text] (A_1g) peak and the appearance of PL A, B and I peaks at lower energies. Thereby, PL and Raman features depend on not only layer number but also stacking order. In addition, circularly polarized luminescence from two prototype WS₂ crystals under circularly polarized excitation has also been investigated, showing obvious spin or valley polarization of these CVD-grown multilayer WS₂ crystals.

Two-Dimensional van der Waals Materials with Aligned In-Plane Polarization and Large Piezoelectric Effect for Self-Powered Piezoelectric Sensors

Piezoelectric two-dimensional (2D) van der Waals (vdWs) materials are highly desirable for applications in miniaturized and flexible/wearable devices. However, the reverse-polarization between adjacent layers in current 2D layered materials results in decreasing their in-plane piezoelectric coefficients with layer number, which limits their practical applications. Here, we report a class of 2D layered materials with an identical orientation of in-plane polarization. Their piezoelectric coefficients (e₂₂) increase with layer number, thereby allowing for the fabrication of flexible piezotronic devices with large piezoelectric responsivity and excellent mechanical durability. The piezoelectric outputs can reach up to 0.363 V for a 7-layer α-In₂Se₃ device, with a current responsivity of 598.1 pA for 1% strain, which is 1 order of magnitude higher than the values of the reported 2D piezoelectrics. The self-powered piezoelectric sensors made of these newly developed 2D layered materials have been successfully used for real-time health monitoring, proving their suitability for the fabrication of flexible piezotronic devices due to their large piezoelectric responses and excellent mechanical durability.

Anomalous lattice vibrations of CVD-grown monolayer MoS2 probed using linear polarized excitation light

A novel physical phenomenon and advanced application have been explored in 2D low-dimensional van der Waals layered materials due to their reduced in-plane symmetry. The light-matter interaction is observed upon rapid characterization of the 2D material's crystal orientation. Here, the effects of the sample's rotation angle and the incident light's linear polarization angle on the Raman scattering of chemical vapor deposition (CVD)-grown monolayer MoS₂ were investigated. The results show that the crystal orientation of monolayer MoS₂ can be distinguished by analyzing the intensity ratio and frequency difference of its two dominant Raman vibration modes. In addition, an increase in the incident light's power intensity causes the Raman peaks to red shift due to the photothermal effect. Strikingly, it was found that, with an increase in the incident linear polarization angle, the out-of-plane A_1g phonon mode red shifts, while the in-plane E_2g¹ phonon mode blue shifts. The frequency difference consequently decreases from 19.5 cm^-1 to 17.4 cm^-1. The anomalous lattice vibrations of monolayer MoS₂ originate from the built-in strain introduced by the SiO₂/Si substrate. This work paves the way for the investigation and characterization of 2D MoS₂, providing further understanding of the light-matter interaction in 2D materials, which is beneficial for advanced studies on anisotropic MoS₂ based electronic and photoelectric information technologies and sensing applications.

Intrinsic Van Der Waals Magnetic Materials from Bulk to the 2D Limit: New Frontiers of Spintronics

2D van der Waals (vdW) magnets, which present intrinsic ferromagnetic/antiferromagnetic ground states at finite temperatures down to atomic-layer thicknesses, open a new horizon in materials science and enable the potential development of new spin-related applications. The layered structure of vdW magnets facilitates their atomic-layer cleavability and magnetic anisotropy, which counteracts spin fluctuations, thereby providing an ideal platform for theoretically and experimentally exploring magnetic phase transitions in the 2D limit. With reduced dimensions, the susceptibility of 2D magnets to a large variety of external stimuli also makes them more promising than their bulk counterpart in various device applications. Here, the current status of characterization and tuning of the magnetic properties of 2D vdW magnets, particularly the atomic-layer thickness, is presented. Various state-of-the-art optical and electrical techniques have been applied to reveal the magnetic states of 2D vdW magnets. Other emerging 2D vdW magnets and future perspectives on the stacking strategy are also given; it is believed that they will excite more intensive research and provide unprecedented opportunities in the field of spintronics.

Atomic structural catalogue of defects and vertical stacking in 2H/3R mixed polytype multilayer WS2 pyramids

We examine the atomic structure of chemical vapour deposition grown multilayer WS2 pyramids using aberration corrected annular dark field scanning transmission electron microscopy coupled with an in situ heating holder. The stacking orders and specific types of defects after partial degradation by S and W atomic loss at high temperature are resolved layer-by-layer. Our study of an individual WS2 pyramid with at least six layers, reveals a mixed 2H and 3R polytype stacking. Etching occurred both top and bottom of the WS2 pyramid, which aids in determining the exact vertical layer stacking configurations in the thicker regions. We provide an extensive catalogue of the contrast profiles associated with defects in WS2 as a function of layer number and stacking type, as imaged using ADF-STEM. These results provide extensive details about the identification of a wide range of defects in S2 layers, and the unique ADF-STEM contrast patterns that arise from complex multilayer stacking.

Healing of Planar Defects in 2D Materials via Grain Boundary Sliding

Understanding the mechanisms and kinetics of defect annihilations, particularly at the atomic scale, is important for the preparation of high-quality crystals for realizing the full potential of 2D transition metal dichalcogenides (TMDCs) in electronics and quantum photonics. Herein, by performing in situ annealing experiments in an atomic resolution scanning transmission electron microscope, it is found that stacking faults and rotational disorders in multilayered 2D crystals can be healed by grain boundary (GB) sliding, which works like a "wiper blade" to correct all metastable phases into thermodynamically stable phases along its trace. The driving force for GB sliding is the gain in interlayer binding energy as the more stable phase grows at the expanse of the metastable ones. Density functional theory calculations show that the correction of 2D stacking faults is triggered by the ejection of Mo atoms in mirror twin boundaries, followed by the collective migrations of 1D GB. The study highlights the role of the often-neglected interlayer interactions for defect repair in 2D materials and shows that exploiting these interactions has significant potential for obtaining large-scale defect-free 2D films.

Intrinsic Electric Field-Induced Properties in Janus MoSSe van der Waals Structures

Focusing on two-dimensional (2D) Janus MoSSe monolayers, we show that simultaneously existing in-plane and out-of-plane intrinsic electric fields cause Zeeman- and Rashba-type spin splitting, respectively. In MoSSe van der Waals (vdW) structures, intrinsic electric field results in a large interlayer band offset. Therefore, large interlayer band offset, being the driving force for interlayer excitons, endows ultralong lifetimes to excitons and might dissociate excitons into free carriers. In comparison to its parent structure (i.e., MoS₂), MoSSe vdW structures are rather appealing for new concepts in light-electricity interconversion. In addition, the Rashba effects could be tuned by changing the interlayer distances due to the competition between the intralayer and interlayer electric field. Due to the large band offset, valley polarization relaxation is markedly reduced, promising enhanced valley polarization and ultralong valley lifetimes. As a result, MoSSe vdW structures harbor strong valley-contrasting physics, making them competitive systems to their parent structures.

Transition metal dichalcogenides bilayer single crystals by reverse-flow chemical vapor epitaxy

Epitaxial growth of atomically thin two-dimensional crystals such as transition metal dichalcogenides remains challenging, especially for producing large-size transition metal dichalcogenides bilayer crystals featuring high density of states, carrier mobility and stability at room temperature. Here we achieve in epitaxial growth of the second monolayer from the first monolayer by reverse-flow chemical vapor epitaxy and produce high-quality, large-size transition metal dichalcogenides bilayer crystals with high yield, control, and reliability. Customized temperature profiles and reverse gas flow help activate the first layer without introducing new nucleation centers leading to near-defect-free epitaxial growth of the second layer from the existing nucleation centers. A series of bilayer crystals including MoS₂ and WS₂, ternary Mo_1−xW_xS₂ and quaternary Mo_1−xW_xS_2(1−y)Se_2y are synthesized with variable structural configurations and tunable electronic and optical properties. The robust, potentially universal approach for the synthesis of large-size transition metal dichalcogenides bilayer single crystals is highly-promising for fundamental studies and technological applications.

Epitaxial growth of the two-dimensionally thin material flakes with effective layer number and shape calls for precise control over temperature and carrier gas. Here, authors report controlled epitaxial growth of the second layer vertically for MoS2, WS2, MoWS and MoWSSe compounds by reverse hydrogen gas flow chemical vapor epitaxy.

Hybrid Three-Dimensional Spiral WSe2 Plasmonic Structures for Highly Efficient Second-Order Nonlinear Parametric Processes

Two-dimensional (2D) layered materials, with large second-order nonlinear susceptibility, are currently growing as an ideal candidate for fulfilling tunable nanoscale coherent light through the second-order nonlinear optical parametric processes. However, the atomic thickness of 2D layered materials leads to poor field confinement and weak light-matter interaction at nanoscale, resulting in low nonlinear conversion efficiency. Here, hybrid three-dimensional (3D) spiral WSe₂ plasmonic structures are fabricated for highly efficient second harmonic generation (SHG) and sum-frequency generation (SFG) based on the enhanced light-matter interaction in hybrid plasmonic structures. The 3D spiral WSe₂, with AA lattice stacking, exhibits efficient SH radiation due to the constructive interference of nonlinear polarization between the neighboring atomic layers. Thus, extremely high external SHG conversion efficiency (about 2.437×10⁻⁵) is achieved. Moreover, the ease of phase-matching condition combined with the enhanced light-matter interaction in hybrid plasmonic structure brings about efficient SHG and SFG simultaneously. These results would provide enlightenment for the construction of typical structures for efficient nonlinear processes.

Differentiating Polymorphs in Molybdenum Disulfide via Electron Microscopy

The presence of rich polymorphs and stacking polytypes in molybdenum disulfide (MoS₂ ) endows it with a diverse range of electrical, catalytic, optical, and magnetic properties. This has stimulated a lot of interest in the unique properties associated with each polymorph. Most techniques used for polymorph identification in MoS₂ are macroscopic techniques that sample averaged properties due to their limited spatial resolution. A reliable way of differentiating the atomic structure of different polymorphs is needed in order to understand their growth dynamics and establish the correlation between structure and properties. Herein, the use of electron microscopy for identifying the atomic structures of several important polymorphs in MoS₂ , some of which are the subjects of mistaken assignment in the literature, is discussed. In particular, scanning transmission electron microscopy-annular dark field imaging has emerged as the most effective and reliable approach for identifying the different phases in MoS₂ and other 2D materials because its images can be directly correlated to the atomic structures. Examples of the identification of polymorphs grown under different conditions in molecular beam epitaxy or chemical vapor deposition, for example, 3R, 1T, 1T'-phases, and 1T'-edges, are presented, including their atomic structures, fascinating properties, growth methods, and corresponding thermodynamic stabilities.

Thickness and Stacking Sequence Determination of Exfoliated Dichalcogenides (1T-TaS2, 2H-MoS2) Using Scanning Transmission Electron Microscopy

Layered transition metal dichalcogenides (TMDs) have attracted interest due to their promise for future electronic and optoelectronic technologies. As one approaches the two-dimensional (2D) limit, thickness and local topology can greatly influence the macroscopic properties of a material. To understand the unique behavior of TMDs it is therefore important to identify the number of atomic layers and their stacking in a sample. The goal of this work is to extract the thickness and stacking sequence of TMDs directly by matching experimentally recorded high-angle annular dark-field scanning transmission electron microscope images and convergent-beam electron diffraction (CBED) patterns to quantum mechanical, multislice scattering simulations. Advantageously, CBED approaches do not require a resolved lattice in real space and are capable of neglecting the thickness contribution of amorphous surface layers. Here we demonstrate the crystal thickness can be determined from CBED in exfoliated 1T-TaS2 and 2H-MoS2 to within a single layer for ultrathin ≲9 layers and ±1 atomic layer (or better) in thicker specimens while also revealing information about stacking order-even when the crystal structure is unresolved in real space.

Stacking change in MoS2 bilayers induced by interstitial Mo impurities

We use a theoretical approach to reveal the electronic and structural properties of molybdenum impurities between MoS₂ bilayers. We find that interstitial Mo impurities are able to reverse the well-known stability order of the pristine bilayer, because the most stable form of stacking changes from AA’ (undoped) into AB’ (doped). The occurrence of Mo impurities in different positions shows their split electronic levels in the energy gap, following octahedral and tetrahedral crystal fields. The energy stability is related to the accommodation of Mo impurities compacted in hollow sites between layers. Other less stable configurations for Mo dopants have larger interlayer distances and band gaps than those for the most stable stacking. Our findings suggest possible applications such as exciton trapping in layers around impurities, and the control of bilayer stacking by Mo impurities in the growth process.

Two-Dimensional Pyramid-like WS2 Layered Structures for Highly Efficient Edge Second-Harmonic Generation

Two-dimensional (2D) layered materials, with large second-order nonlinear susceptibility, have received much scientific interest due to their potential applications in nonlinear optical devices. However, the atomic thickness of 2D layered materials leads to poor field confinement and weak light-matter interaction at the nanoscale, resulting in low nonlinear conversion efficiency. Here, 2D pyramid-like multilayer (P-multilayer) layered structures are fabricated for efficient edge second-harmonic generation (SHG) based on the enhanced light-matter interaction in whispering-gallery mode (WGM) cavities. The P-multilayer 2D layered materials, where the basal planes shrink gradually from the bottom to the top layers, exhibit efficient edge SH radiation due to the partial destructive interference of nonlinear polarizations between the neighboring atomic layers. Moreover, the well-defined 2D plate-like triangle morphology of P-multilayer WS₂ forms a WGM resonance cavity, which results in enhanced light-matter interaction and thus the enhancement of edge SHG, which can be further enhanced by hybridizing the WGM mode with plasmonics. These results provide enlightenment for the construction of specific structures for efficient nonlinear processes.

Etchant-free transfer of 2D nanostructures

The effective use of the van der Waals 2D materials relies on their successful transfer from the growth substrate onto other target substrates in the form of large films or flakes. In particular, it is important to transfer such atomically thin samples to various substrates with minimal sample damage and exposure to etchants and chemicals to realize their applications. Here we develop a universal transfer method that not only is free of reactive etchants, but also can maintain the film morphology intact with no tears and cracks. We show a variety of different 2D crystals and thin films with various sizes and thicknesses transferred from different substrates, i.e. metal-organic chemical vapor deposition-grown WSe₂ coalesced thin films on sapphire and mechanically exfoliated 2D crystalline flakes on titanium nitride. Further examination by transmission electron microscopy indicates successful transfer of all the samples. This study presents a universal and etchant-free transfer method that can be used to transfer a variety of 2D crystals and other nanostructures from/to various substrates.

Atomic structure of defects and dopants in 2D layered transition metal dichalcogenides

Transmission electron microscopy can directly image the detailed atomic structure of layered transition metal dichalcogenides, revealing defects and dopants.

Step-like band alignment and stacking-dependent band splitting in trilayer TMD heterostructures

We propose a kind of trilayer TMD heterostructure with step-like band alignment, and the effects of interlayer coupling, strain and SOC are also discussed.

Spotting the differences in two-dimensional materials – the Raman scattering perspective

This review discusses the Raman spectroscopic characterization of 2D materials with a focus on the “differences” from primitive 2D materials.

Structural stability and magnetic-exchange coupling in Mn-doped monolayer/bilayer MoS2

Ferromagnetic (FM) two-dimensional (2D) transition metal dichalcogenides (TMDs) have potential applications in modern electronics and spintronics and doping of TMDs with transition metals can enhance the magnetic characteristics.

Low-Frequency Shear and Layer-Breathing Modes in Raman Scattering of Two-Dimensional Materials

Ever since the isolation of single-layer graphene in 2004, two-dimensional layered structures have been among the most extensively studied classes of materials. To date, the pool of two-dimensional materials (2DMs) continues to grow at an accelerated pace and already covers an extensive range of fascinating and technologically relevant properties. An array of experimental techniques have been developed and used to characterize and understand these properties. In particular, Raman spectroscopy has proven to be a key experimental technique, thanks to its capability to identify minute structural and electronic effects in nondestructive measurements. While high-frequency (HF) intralayer Raman modes have been extensively employed for 2DMs, recent experimental and theoretical progress has demonstrated that low-frequency (LF) interlayer Raman modes are more effective at determining layer numbers and stacking configurations and provide a unique opportunity to study interlayer coupling. These advantages are due to 2DMs' unique interlayer vibration patterns where each layer behaves as an almost rigidly moving object with restoring forces corresponding to weak interlayer interactions. Compared to HF Raman modes, the relatively small attention originally devoted to LF Raman modes is largely due to their weaker signal and their proximity to the strong Rayleigh line background, which previously made their detection challenging. Recent progress in Raman spectroscopy with technical and hardware upgrades now makes it possible to probe LF modes with a standard single-stage Raman system and has proven crucial to characterize and understand properties of 2DMs. Here, we present a comprehensive and forward-looking review on the current status of exploiting LF Raman modes of 2DMs from both experimental and theoretical perspectives, revealing the fundamental physics and technological significance of LF Raman modes in advancing the field of 2DMs. We review a broad array of materials, with varying thickness and stacking configurations, discuss the effect of in-plane anisotropy, and present a generalized linear chain model and interlayer bond polarizability model to rationalize the experimental findings. We also discuss the instrumental improvements of Raman spectroscopy to enhance and separate LF Raman signals from the Rayleigh line. Finally, we highlight the opportunities and challenges ahead in this fast-developing field.

Robust Stacking-Independent Ultrafast Charge Transfer in MoS2/WS2 Bilayers

Van der Waals-coupled two-dimensional (2D) heterostructures have attracted great attention recently due to their high potential in the next-generation photodetectors and solar cells. The understanding of charge-transfer process between adjacent atomic layers is the key to design optimal devices as it directly determines the fundamental response speed and photon-electron conversion efficiency. However, general belief and theoretical studies have shown that the charge transfer behavior depends sensitively on interlayer configurations, which is difficult to control accurately, bringing great uncertainties in device designing. Here we investigate the ultrafast dynamics of interlayer charge transfer in a prototype heterostructure, the MoS₂/WS₂ bilayer with various stacking configurations, by optical two-color ultrafast pump-probe spectroscopy. Surprisingly, we found that the charge transfer is robust against varying interlayer twist angles and interlayer coupling strength, in time scale of ∼90 fs. Our observation, together with atomic-resolved transmission electron characterization and time-dependent density functional theory simulations, reveals that the robust ultrafast charge transfer is attributed to the heterogeneous interlayer stretching/sliding, which provides additional channels for efficient charge transfer previously unknown. Our results elucidate the origin of transfer rate robustness against interlayer stacking configurations in optical devices based on 2D heterostructures, facilitating their applications in ultrafast and high-efficient optoelectronic and photovoltaic devices in the near future.