Interfacial Interactions in van der Waals Heterostructures of MoS2 and Graphene

Helicity-Resolved Vibrational Coupling in Twist WS2/WSe2 Heterostructures

Helicity-resolved Raman spectra can provide an intricate view into lattice structural details. Through the analysis of peak positions, intensities, and circular polarized Raman signals, a wealth of information about chiral structure arrangement within the moiré superlattice, interlayer interaction strength, polarizability change in chemical bond, and beyond can be unveiled. However, the relationship between the circular polarization of high-frequency Raman and twist angle is still not clear. Here, we utilize helicity-resolved Raman spectroscopy to explore the interlayer interactions and the effect of the moiré superlattice in WS2/WSe2 heterostructures. For the out-of-plane Raman mode A1g of WS2 (A1g and 1E2g of WSe2), its intensity is significantly enhanced (suppressed) in WS2/WSe2 heterostructures when θ is less than 10° or greater than 50°. This observation could be attributed to the large polarizability changes in both W-S and W-Se covalent bonds. The circular polarization of 2LA(M) in WSe2 of the WS2/WSe2 heterostructure (θ < 10° or θ > 50°) is significantly enhanced compared to that of 2LA(M) in the monolayer WSe2. We deduce that the circular polarization of the Raman mode correlates with the proportion of high-symmetry area within a supercell of the moiré lattice. Our findings improve the understanding of twist-angle-modulated Raman modes in TMD heterostructures.

Interlayer bond polarizability model for interlayer phonons in van der Waals heterostructures

Raman scattering provides essential insights into phonons, electronic structures and electron-phonon coupling within solids through the intensity of Raman peaks, which cannot be easily quantified using the classical bond polarizability model. The interlayer bond polarizability model (IBPM) had been developed to understand the Raman intensity of layer-breathing modes (LBMs) in two-dimensional materials. However, the quantitative understanding of the LBM intensity of a van der Waals heterostructure (vdWH) remains challenging. Here, in polynary vdWHs comprising twisted multilayer graphene (tMLG), MoS2 and hBN, we observed a series of LBMs, whose intensity is markedly dependent on the excitation energy and twist angle of the tMLG constituent. An improved IBPM is proposed to quantitatively understand the Raman intensity of LBMs in the tMLG-based vdWHs, including the emergence or absence of a specific LBM when the excitation energy is resonant with the electronic states of tMLG or MoS2 constituents. This work underscores the significant potential of the improved IBPM in accurately understanding and predicting the intensity profile of LBM in polynary vdWHs, even for the case of Raman scattering with excitation energies selectively resonant with the electronic states of the corresponding specific constituents.

Enhanced Layer-Breathing Modes in van der Waals Heterostructures Based on Twisted Bilayer Graphene

The characterization of interlayer coupling in two-dimensional van der Waals heterostructures (vdWHs) is essential to understand their quantum behaviors and structural functionalities. Interlayer shear and layer-breathing (LB) phonons carry rich information on interlayer interaction, but they are usually too weak to be detected via standard Raman spectroscopy due to the weak electron-phonon coupling (EPC). Here, we report a universal strategy to enhance LB modes of vdWHs based on twisted bilayer graphene (tBLG). In both tBLG/hBN and tBLG/MoS₂ vdWHs, the resonantly excited electrons in tBLG can strongly couple to LB phonons extended over the entire layers in the vdWHs, whose resonance condition is tunable by the twist angle of tBLG. In vdWHs containing twisted graphene layers with multiple twisted interfaces, the EPC of LB phonons coming from the collective LB vibrations of entire heterostructure layers can be tuned by resonant excitation of programmable van Hove singularities according to each twisted interface. The universality and tunability of enhanced LB phonons by tBLG make it a promising method to investigate EPC and interlayer interaction in related vdWHs.

Emergent second-harmonic generation in van der Waals heterostructure of bilayer MoS2 and monolayer graphene

Van der Waals (vdW) stacking of two-dimensional (2D) materials to create artificial structures has enabled remarkable discoveries and novel properties in fundamental physics. Here, we report that vdW stacking of centrosymmetric 2D materials, e.g., bilayer MoS₂ (2LM) and monolayer graphene (1LG), could support remarkable second-harmonic generation (SHG). The required centrosymmetry breaking for second-order hyperpolarizability arises from the interlayer charge transfer between 2LM and 1LG and the imbalanced charge distribution in 2LM, which are verified by first-principles calculations, Raman spectroscopy, and polarization-resolved SHG. The strength of SHG from 2LM/1LG is of the same order of magnitude as that from the monolayer MoS₂, which is well recognized with strong second-order nonlinearity. The emergent SHG reveals that the interlayer charge transfer can effectively modify the symmetry and nonlinear optical properties of 2D heterostructures. It also indicates the great opportunity of SHG spectroscopy for characterizing interlayer coupling in vdW heterostructures.

Two Dimensional Heterostructures for Optoelectronics: Current Status and Future Perspective

Researchers have found various families of two-dimensional (2D) materials and associated heterostructures through detailed theoretical work and experimental efforts. Such primitive studies provide a framework to investigate novel physical/chemical characteristics and technological aspects from micro to nano and pico scale. Two-dimensional van der Waals (vdW) materials and their heterostructures can be obtained to enable high-frequency broadband through a sophisticated combination of stacking order, orientation, and interlayer interactions. These heterostructures have been the focus of much recent research due to their potential applications in optoelectronics. Growing the layers of one kind of 2D material over the other, controlling absorption spectra via external bias, and external doping proposes an additional degree of freedom to modulate the properties of such materials. This mini review focuses on current state-of-the-art material design, manufacturing techniques, and strategies to design novel heterostructures. In addition to a discussion of fabrication techniques, it includes a comprehensive analysis of the electrical and optical properties of vdW heterostructures (vdWHs), particularly emphasizing the energy-band alignment. In the following sections, we discuss specific optoelectronic devices, such as light-emitting diodes (LEDs), photovoltaics, acoustic cavities, and biomedical photodetectors. Furthermore, this also includes a discussion of four different 2D-based photodetector configurations according to their stacking order. Moreover, we discuss the challenges that remain to be addressed in order to realize the full potential of these materials for optoelectronics applications. Finally, as future perspectives, we present some key directions and express our subjective assessment of upcoming trends in the field.

van der Waals Semiconductor Empowered Vertical Color Sensor

Biomimetic artificial vision is receiving significant attention nowadays, particularly for the development of neuromorphic electronic devices, artificial intelligence, and microrobotics. Nevertheless, color recognition, the most critical vision function, is missed in the current research due to the difficulty of downscaling of the prevailing color sensing devices. Conventional color sensors typically adopt a lateral color sensing channel layout and consume a large amount of physical space, whereas compact designs suffer from an unsatisfactory color detection accuracy. In this work, we report a van der Waals semiconductor-empowered vertical color sensing structure with the emphasis on compact device profile and precise color recognition capability. More attractive, we endow color sensor hardware with the function of chromatic aberration correction, which can simplify the design of an optical lens system and, in turn, further downscales the artificial vision systems. Also, the dimension of a multiple pixel prototype device in our study confirms the scalability and practical potentials of our developed device architecture toward the above applications.

Magnesene: a theoretical prediction of a metallic, fast, high-capacity, and reversible anode material for sodium-ion batteries

Sodium-ion batteries (SIBs) have attracted great attention owing to their low cost and inherent safety. High-performance anode materials for SIBs should possess intrinsically metallic characteristic and be composed of non-toxic, earth abundant, and lightweight elements. We predict a two-dimensional Mg material (named magnesene) to be an excellent anode material, which can meet these design requirements. It is demonstrated to be stable in terms of the cohesive energy, phonon spectrum, ab initio molecular dynamics simulation, and elastic constants. The magnesene monolayer exhibits good SIB performances, including a high storage capacity of 551.3 mA h g^-1, low diffusion energy barrier (0.16-0.19 eV), low open-circuit voltage (0.71-0.82 V), and small volume change (4.7%). Moreover, graphene or h-BN on top of magnesene could serve as a protective cover to preserve the performances of pristine magnesene, such as metallicity, strong Na adsorption capability, and fast ionic mobility. These intriguing theoretical findings make magnesene a promising anode material for SIBs.

Developing Graphene-Based Moiré Heterostructures for Twistronics

Graphene-based moiré heterostructures are strongly correlated materials, and they are considered to be an effective platform to investigate the challenges of condensed matter physics. This is due to the distinct electronic properties that are unique to moiré superlattices and peculiar band structures. The increasing research on strongly correlated physics via graphene-based moiré heterostructures, especially unconventional superconductors, greatly promotes the development of condensed matter physics. Herein, the preparation methods of graphene-based moiré heterostructures on both in situ growth and assembling monolayer 2D materials are discussed. Methods to improve the quality of graphene and optimize the transfer process are presented to mitigate the limitations of low-quality graphene and damage caused by the transfer process during the fabrication of graphene-based moiré heterostructures. Then, the topological properties in various graphene-based moiré heterostructures are reviewed. Furthermore, recent advances regarding the factors that influence physical performances via a changing twist angle, the exertion of strain, and regulation of the dielectric environment are presented. Moreover, various unique physical properties in graphene-based moiré heterostructures are demonstrated. Finally, the challenges faced during the preparation and characterization of graphene-based moiré heterostructures are discussed. An outlook for the further development of moiré heterostructures is also presented.

Physical and Chemical Sensors on the Basis of Laser-Induced Graphene: Mechanisms, Applications, and Perspectives

Laser-induced graphene (LIG) is produced rapidly by directly irradiating carbonaceous precursors, and it naturally exhibits as a three-dimensional porous structure. Due to advantages such as simple preparation, time-saving, environmental friendliness, low cost, and expanding categories of raw materials, LIG and its derivatives have achieved broad applications in sensors. This has been witnessed in various fields such as wearable devices, disease diagnosis, intelligent robots, and pollution detection. However, despite LIG sensors having demonstrated an excellent capability to monitor physical and chemical parameters, the systematic review of synthesis, sensing mechanisms, and applications of them combined with comparison against other preparation approaches of graphene is still lacking. Here, graphene-based sensors for physical, biological, and chemical detection are reviewed first, followed by the introduction of general preparation methods for the laser-induced method to yield graphene. The preparation and advantages of LIG, sensing mechanisms, and the properties of different types of emerging LIG-based sensors are comprehensively reviewed. Finally, possible solutions to the problems and challenges of preparing LIG and LIG-based sensors are proposed. This review may serve as a detailed reference to guide the development of LIG-based sensors that possess properties for future smart sensors in health care, environmental protection, and industrial production.

Coherent control of interlayer vibrations in Bi2Se3 van der Waals thin-films

Interlayer vibrations with discrete quantized modes in two-dimensional (2D) materials can be excited by ultrafast light due to the inherent low dimensionality and van der Waals force as a restoring force. Controlling such interlayer vibrations in layered materials, which are closely related to fundamental nanomechanical interactions and thermal transport, in spatial- and time-domain provides an in-depth understanding of condensed matters and potential applications for advanced phononic and photonics devices. The manipulation of interlayer vibrational modes has been implemented in a spatial domain through material design to develop novel optoelectronic and phononic devices with various 2D materials, but such control in a time domain is still lacking. We present an all-optical method for controlling the interlayer vibrations in a highly precise manner with Bi₂Se₃ as a promising optoelectronic and thermoelasticity material in layered structures using a coherently controlled pump and probe scheme. The observed thickness-dependent fast interlayer breathing modes and substrate-induced slow interfacial modes can be exactly explained by a modified linear chain model including coupling effect with substrate. In addition, the results of coherent control experiments also agree with the simulation results based on the interference of interlayer vibrations. This investigation is universally applicable for diverse 2D materials and provides insight into the interlayer vibration-related dynamics and novel device implementation based on an ultrafast timescale interlayer-spacing modulation scheme.

Shear and Breathing Modes of Layered Materials

Layered materials (LMs), such as graphite, hexagonal boron nitride, and transition-metal dichalcogenides, are at the center of an ever-increasing research effort, due to their scientific and technological relevance. Raman and infrared spectroscopies are accurate, non-destructive approaches to determine a wide range of properties, including the number of layers, N, and the strength of the interlayer interactions. We present a general approach to predict the complete spectroscopic fan diagrams, i.e., the relations between frequencies and N for the optically active shear and layer-breathing modes of any multilayer comprising N ≥ 2 identical layers. In order to achieve this, we combine a description of the normal modes in terms of a one-dimensional mechanical model, with symmetry arguments that describe the evolution of the point group as a function of N. Group theory is then used to identify which modes are Raman- and/or infrared-active, and to provide diagrams of the optically active modes for any stack composed of identical layers. We implement the method and algorithms in an open-source tool to assist researchers in the prediction and interpretation of such diagrams. Our work will underpin future efforts on Raman and infrared characterization of known, and yet not investigated, LMs.

Intrinsic effect of interfacial coupling on the high-frequency intralayer modes in twisted multilayer MoTe2

The interfacial coupling at the interface makes the van der Waals heterostructures (vdWHs) exhibit many unique properties that cannot be realized in its constituents. Such a study usually starts with a twisted stack of two flakes exfoliated from the same layered materials to form twisted multilayers, in which the impact of interfacial coupling on the low-frequency interlayer modes had been well understood. However, it is not clear how interfacial coupling affects the high-frequency intralayer modes of twisted multilayers. Herein, we perform high-resolution resonance Raman spectroscopy of the high-frequency intralayer modes in twisted multilayer MoTe2 (tMLM). All the Davydov entities of the out-of-plane intralayer mode are observed and distinguished at 4 K. It is found that the out-of-plane intralayer modes in tMLM are sensitive to its interfacial layer-breathing coupling so that the out-of-plane intralayer modes in tMLM do not show a direct relationship with those of the two constituents. However, the case is quite different for the in-plane intralayer modes in tMLM, whose spectral profile can be fitted by those of the corresponding modes of its constituents. This indicates that the in-plane intralayer modes are localized within the constituents in tMLM because of its negligible interfacial shear coupling at the interface. All the results can be well understood using the vdW model in which only the nearest neighbor interlayer/interfacial interaction is taken into account. This work directly builds the relationship between the Davydov splitting of the high-frequency intralayer vibrations and the low-frequency interlayer vibrations in tMLM, which can be further extended to other twisted materials and the related vdWHs.

Type-II Interface Band Alignment in the vdW PbI2-MoSe2 Heterostructure

Energy band alignments at heterostructure interfaces play key roles in device performance, especially between two-dimensional atomically thin materials. Herein, van der Waals PbI₂-MoSe₂ heterostructures fabricated by in situ PbI₂ deposition on monolayer MoSe₂ are comprehensively studied using scanning tunneling microscopy/spectroscopy, atomic force microscopy, photoemission spectroscopy, and Raman and photoluminescence (PL) spectroscopy. PbI₂ grows on MoSe₂ in a quasi layer-by-layer epitaxial mode. A type-II interface band alignment is proposed between PbI₂ and MoSe₂ with the conduction band minimum (valence band maximum) located at PbI₂ (MoSe₂), which is confirmed by first-principles calculations and the existence of interfacial excitons revealed using temperature-dependent PL. Our findings provide a scalable method to fabricate PbI₂-MoSe₂ heterostructures and new insights into the electronic structures for future device design.

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.

Heterostructures Based on 2D Materials: A Versatile Platform for Efficient Catalysis

The unique structural and electronic properties of 2D materials, including the metal and metal-free ones, have prompted intense exploration in the search for new catalysts. The construction of different heterostructures based on 2D materials offers great opportunities for boosting the catalytic activity in electo(photo)chemical reactions. Particularly, the merits resulting from the synergism of the constituent components and the fascinating properties at the interface are tremendously interesting. This scenario has now become the state-of-the-art point in the development of active catalysts for assisting energy conversion reactions including water splitting and CO₂ reduction. Here, starting from the theoretical background of the fundamental concepts, the progressive developments in the design and applications of heterostructures based on 2D materials are traced. Furthermore, a personal perspective on the exploration of 2D heterostructures for further potential application in catalysis is offered.

High-Frequency Elastic Coupling at the Interface of van der Waals Nanolayers Imaged by Picosecond Ultrasonics

Although the topography of van de Waals (vdW) layers and heterostructures can be imaged by scanning probe microscopy, high-frequency interface elastic properties are more difficult to assess. These can influence the stability, reliability, and performance of electronic devices that require uniform layers and interfaces. Here, we use picosecond ultrasonics to image these properties in vdW layers and heterostructures based on well-known exfoliable materials, i.e., InSe, hBN, and graphene. We reveal a strong, uniform elastic coupling between vdW layers over a wide range of frequencies of up to tens of gigahertz (GHz) and in-plane areas of 100 μm². In contrast, the vdW layers can be weakly coupled to their supporting substrate, behaving effectively as free-standing membranes. Our data and analysis demonstrate that picosecond ultrasonics offers opportunities to probe the high-frequency elastic coupling of vdW nanolayers and image both "perfect" and "broken" interfaces between different materials over a wide frequency range, as required for future scientific and technological developments.

Ultrahigh thermal isolation across heterogeneously layered two-dimensional materials

Heterogeneous integration of nanomaterials has enabled advanced electronics and photonics applications. However, similar progress has been challenging for thermal applications, in part due to shorter wavelengths of heat carriers (phonons) compared to electrons and photons. Here, we demonstrate unusually high thermal isolation across ultrathin heterostructures, achieved by layering atomically thin two-dimensional (2D) materials. We realize artificial stacks of monolayer graphene, MoS₂, and WSe₂ with thermal resistance greater than 100 times thicker SiO₂ and effective thermal conductivity lower than air at room temperature. Using Raman thermometry, we simultaneously identify the thermal resistance between any 2D monolayers in the stack. Ultrahigh thermal isolation is achieved through the mismatch in mass density and phonon density of states between the 2D layers. These thermal metamaterials are an example in the emerging field of phononics and could find applications where ultrathin thermal insulation is desired, in thermal energy harvesting, or for routing heat in ultracompact geometries.

MoS2-Decorated Laser-Induced Graphene for a Highly Sensitive, Hysteresis-free, and Reliable Piezoresistive Strain Sensor

Advancement of sensing systems, soft robotics, and point-of-care testing requires the development of highly efficient, scalable, and cost-effective physical sensors with competitive and attractive features such as high sensitivity, reliability, and preferably reversible sensing behaviors. This study reports a highly sensitive and reliable piezoresistive strain sensor fabricated by one-step carbonization of the MoS₂-coated polyimide film to obtain MoS₂-decorated laser-induced graphene. The resulting three-dimensional porous graphene nanoflakes decorated with MoS₂ exhibit stable electrical properties yielding a reliable output for longer strain/release cycles. The sensor demonstrates high sensitivity (i.e., gauge factor, GF ≈1242), is hysteresis-free (∼2.75%), and has a wide working range (up to 37.5%), ultralow detection limit (0.025%), fast relaxation time (∼0.17 s), and a highly stable and reproducible response over multiple test cycles (>12 000) with excellent switching response. Owing to the outstanding performances of the sensor, it is possible to successfully detect various subtle movements ranging from phonation, eye-blinking, and wrist pulse to large human-motion-induced deformations.

Cross-dimensional electron-phonon coupling in van der Waals heterostructures

The electron-phonon coupling (EPC) in a material is at the frontier of the fundamental research, underlying many quantum behaviors. van der Waals heterostructures (vdWHs) provide an ideal platform to reveal the intrinsic interaction between their electrons and phonons. In particular, the flexible van der Waals stacking of different atomic crystals leads to multiple opportunities to engineer the interlayer phonon modes for EPC. Here, in hBN/WS₂ vdWH, we report the strong cross-dimensional coupling between the layer-breathing phonons well extended over tens to hundreds of layer thick vdWH and the electrons localized within the few-layer WS₂ constituent. The strength of such cross-dimensional EPC can be well reproduced by a microscopic picture through the mediation by the interfacial coupling and also the interlayer bond polarizability model in vdWHs. The study on cross-dimensional EPC paves the way to manipulate the interaction between electrons and phonons in various vdWHs by interfacial engineering for possible interesting physical phenomena.

The strength of electron-phonon coupling can be directly probed by Raman spectroscopy. Here, the authors use low-frequency Raman spectroscopy to unveil the existence of a strong cross-dimensional coupling between the bulk-like layer-breathing phonons in an hBN/WS2 heterostructure and the electrons localized within its few-layer WS2 constituent.

2D/2D Heterojunctions for Catalysis

2D layered materials with atomic thickness have attracted extensive research interest due to their unique physicochemical and electronic properties, which are usually very different from those of their bulk counterparts. Heterojunctions or heterostructures based on ultrathin 2D materials have attracted increasing attention due to the integrated merits of 2D ultrathin components and the heterojunction effect on the separation and transfer of charges, resulting in important potential values for catalytic applications. Furthermore, 2D/2D heterostructures with face-to-face contact are believed to be a preferable dimensionality design due to their large interface area, which would contribute to enhanced heterojunction effect. Here, the cutting-edge research progress in 2D/2D heterojunctions and heterostructures is highlighted with a specific emphasis on synthetic strategies, reaction mechanism, and applications in catalysis (photocatalysis, electrocatalysis, and organic synthesis). Finally, the key issues and development perspectives in the applications of 2D/2D layered heterojunctions and heterostructures in catalysis are also discussed.

Band Structure Engineering of Interfacial Semiconductors Based on Atomically Thin Lead Iodide Crystals

To explore new constituents in two-dimensional (2D) materials and to combine their best in van der Waals heterostructures is in great demand as being a unique platform to discover new physical phenomena and to design novel functionalities in interface-based devices. Herein, PbI₂ crystals as thin as a few layers are synthesized, particularly through a facile low-temperature solution approach with crystals of large size, regular shape, different thicknesses, and high yields. As a prototypical demonstration of band engineering of PbI₂ -based interfacial semiconductors, PbI₂ crystals are assembled with several transition metal dichalcogenide monolayers. The photoluminescence of MoS₂ is enhanced in MoS₂ /PbI₂ stacks, while a dramatic photoluminescence quenching of  WS₂ and WSe₂ is revealed in WS₂ /PbI₂ and WSe₂ /PbI₂ stacks. This is attributed to the effective heterojunction formation between PbI₂ and these monolayers; type I band alignment in MoS₂ /PbI₂ stacks, where fast-transferred charge carriers accumulate in MoS₂ with high emission efficiency, results in photoluminescence enhancement, and type II in WS₂ /PbI₂ and WSe₂ /PbI₂ stacks, with separated electrons and holes suitable for light harvesting, results in photoluminescence quenching. The results demonstrate that MoS₂ , WS₂ , and WSe₂ monolayers with similar electronic structures show completely distinct light-matter interactions when interfacing with PbI₂ , providing unprecedented capabilities to engineer the device performance of 2D heterostructures.

Interlayer interaction on twisted interface in incommensurate stacking MoS2: A Raman spectroscopy study

In the present work, properties of the interlayer breathing modes in twisted MoS₂ samples with different structures are carefully examined. The distribution of peak position of the breathing modes is mainly dependent on overall layer numbers, and is only slightly affected by specific structure of the sample. Linear chain model calculation shows quantitatively a markedly decrease in interlayer force on the twisted interface, and slight difference in Raman frequency may mean considerable difference in interlayer force. When one or both of the component layers get too thick, no new breathing modes can be found, indicating that rigidity of the component may affect the coupling efficiency. Our work demonstrates that low-frequency Raman spectroscopy can act as an effective indirect strategy for the detection of interlayer interaction, which is a good complement to existing methods such as AFM technologies.

Interface Characterization and Control of 2D Materials and Heterostructures

2D materials and heterostructures have attracted significant attention for a variety of nanoelectronic and optoelectronic applications. At the atomically thin limit, the material characteristics and functionalities are dominated by surface chemistry and interface coupling. Therefore, methods for comprehensively characterizing and precisely controlling surfaces and interfaces are required to realize the full technological potential of 2D materials. Here, the surface and interface properties that govern the performance of 2D materials are introduced. Then the experimental approaches that resolve surface and interface phenomena down to the atomic scale, as well as strategies that allow tuning and optimization of interfacial interactions in van der Waals heterostructures, are systematically reviewed. Finally, a future outlook that delineates the remaining challenges and opportunities for 2D material interface characterization and control is presented.

Moiré Phonons in Twisted Bilayer MoS2

The material choice, layer thickness, and twist angle widely enrich the family of van der Waals heterostructures (vdWHs), providing multiple degrees of freedom to engineer their optical and electronic properties. The moiré patterns in vdWHs create a periodic potential for electrons and excitons to yield many interesting phenomena, such as Hofstadter butterfly spectrum and moiré excitons. Here, in the as-grown/transferred twisted bilayer MoS₂ (tBLMs), one of the simplest prototypes of vdWHs, we show that the periodic potentials of moiré patterns also modify the properties of phonons of its monolayer MoS₂ constituent to generate Raman modes related to moiré phonons. These Raman modes correspond to zone-center phonons in tBLMs, which are folded from the off-center phonons in monolayer MoS₂. However, the folded phonons related to crystallographic superlattices are not observed in the Raman spectra. By varying the twist angle, the moiré phonons of tBLM can be exploited to map the phonon dispersions of the monolayer constituent. The lattice dynamics of the moiré phonons are modulated by the patterned interlayer coupling resulting from periodic potential of moiré patterns, as confirmed by density functional theory calculations. The Raman intensity related to moiré phonons in all tBLMs are strongly enhanced when the excitation energy approaches the C exciton energy. This study can be extended to various vdWHs to deeply understand their Raman spectra, moiré phonons, lattice dynamics, excitonic effects, and interlayer coupling.

Raman spectroscopy of graphene-based materials and its applications in related devices

Graphene-based materials exhibit remarkable electronic, optical, and mechanical properties, which has resulted in both high scientific interest and huge potential for a variety of applications. Furthermore, the family of graphene-based materials is growing because of developments in preparation methods. Raman spectroscopy is a versatile tool to identify and characterize the chemical and physical properties of these materials, both at the laboratory and mass-production scale. This technique is so important that most of the papers published concerning these materials contain at least one Raman spectrum. Thus, here, we systematically review the developments in Raman spectroscopy of graphene-based materials from both fundamental research and practical (i.e., device applications) perspectives. We describe the essential Raman scattering processes of the entire first- and second-order modes in intrinsic graphene. Furthermore, the shear, layer-breathing, G and 2D modes of multilayer graphene with different stacking orders are discussed. Techniques to determine the number of graphene layers, to probe resonance Raman spectra of monolayer and multilayer graphenes and to obtain Raman images of graphene-based materials are also presented. The extensive capabilities of Raman spectroscopy for the investigation of the fundamental properties of graphene under external perturbations are described, which have also been extended to other graphene-based materials, such as graphene quantum dots, carbon dots, graphene oxide, nanoribbons, chemical vapor deposition-grown and SiC epitaxially grown graphene flakes, composites, and graphene-based van der Waals heterostructures. These fundamental properties have been used to probe the states, effects, and mechanisms of graphene materials present in the related heterostructures and devices. We hope that this review will be beneficial in all the aspects of graphene investigations, from basic research to material synthesis and device applications.

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.

Spin transport in graphene/transition metal dichalcogenide heterostructures

This review summarizes the theoretical and experimental studies of spin transport in graphene interfaced with transition metal dichalcogenides, and assesses its potential for future spintronic applications.

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.