Indirect Bandgap Puddles in Monolayer MoS2 by Substrate-Induced Local Strain

Impact of Interface and Surface Oxide Defects on WS2 Electronic Properties from First Principles

The industrial-scale growth of dielectrics on top of a 2D material transistor channel without deterioration of its transport characteristics remains challenging today. Here, we investigate the origin of the performance degradation issue by constructing several atomistic interface models between a WS2 monolayer and an amorphous Al2O3 or HfO2 thin film. We then computed their properties using first-principles methods. We show that, while it is in principle possible to achieve a van der Waals interface between these materials, surface defects (e.g., undercoordinated metal atoms at the surface) are detrimental since they create localized states close to the bottom of the conduction band of WS2. Even in their absence, the inhomogeneity of the surface topology creates a nonuniform potential that is felt by charge carriers in WS2. While surface defects can potentially be kept under control with an appropriate oxide choice, the surface inhomogeneity appears to act as a bottleneck, limiting the performance of WS2 as a transistor channel and, in general, for all 2D materials.

Effective Strain Engineering of 2D Materials via Metal Deposition

2D materials, especially their monolayers, have garnered significant attention due to their unique electrical, optical and mechanical properties. Strain engineering is an effective way to modulate these properties. However, challenges remain in preventing slip or decoupling between the 2D material and the substrate due to the inherent weak van der Waals interactions. In this study, metal films are employed to apply strain to 2D materials. The high surface energy of metals helps to provide higher interaction forces, thereby improving strain transfer efficiency. Biaxial compressive and uniaxial tensile strain can be applied to monolayer MoS2, with the highest modulation rate of 542 and 161.7 meV/%, respectively, as characterized by photoluminescence (PL) spectra. Furthermore, this new approach can be broader to other 2D materials, such as WS2 or WSe2, allowing for precise control over strain manipulation. The work introduces a promising new approach for efficient and controllable strain engineering of 2D materials.

Nanoscale Identification of Local Strain Effect on TMD Catalysis

Strain engineering plays a crucial role in activating the basal plane of the TMD catalysts. However, experimental evidence linking strain strength to activity and distinguishing effects of compressive and tensile strain remains elusive due to the absence of high-resolution in situ correlation techniques. Here, we utilize nanobubble imaging by on-chip total-internal reflection microscopy to visualize active sites on the basal plane of strained MoS2 during hydrogen evolution reaction and atomic force microscopy to correlatively capture the nanoscale morphology and strain maps. By integrating the activity, morphology, and strain maps into comprehensive statistical analyses, we elucidate the strain effect on local activity at both multiprotrusion and (sub)single-protrusion levels. Our findings demonstrate that strain effectively activates sulfur vacancies on the basal plane, with tensile strain significantly enhancing local activity compared to compressive strain. Furthermore, we observe a time-dependent propagation of activity from high-activity to low-activity regions within single protrusions. This work clarifies the interplay between structural morphology and catalytic activity and provides new guidelines for the rational design of optimal TMD catalysts.

Low-Defect-Density Monolayer MoS2 Wafer by Oxygen-Assisted Growth-Repair Strategy

Atomic chalcogen vacancy is the most commonly observed defect category in two dimensional (2D) transition-metal dichalcogenides, which can be detrimental to the intrinsic properties and device performance. Here a low-defect density, high-uniform, wafer-scale single crystal epitaxial technology by in situ oxygen-incorporated "growth-repair" strategy is reported. For the first time, the oxygen-repairing efficiency on MoS2 monolayers at atomic scale is quantitatively evaluated. The sulfur defect density is greatly reduced from (2.71 ± 0.65) × 1013 down to (4.28 ± 0.27) × 1012 cm-2, which is one order of magnitude lower than reported as-grown MoS2. Such prominent defect deduction is owing to the kinetically more favorable configuration of oxygen substitution and an increase in sulfur vacancy formation energy around oxygen-incorporated sites by the first-principle calculations. Furthermore, the sulfur vacancies induced donor defect states is largely eliminated confirmed by quenched defect-related emission. The devices exhibit improved carrier mobility by more than three times up to 65.2 cm2 V-1 s-1 and lower Schottky barrier height reduced by half (less than 20 meV), originating from the suppressed Fermi-level pinning effect from disorder-induced gap state. The work provides an effective route toward engineering the intrinsic defect density and electronic states through modulating synthesis kinetics of 2D materials.

Charged Black-Hole-Like Electronic Structure Driven by Geometric Potential of 2D Semiconductors

One of the exotic expectations in the 2D curved spacetime is the geometric potential from the curvature of the 2D space, still possessing unsolved fundamental questions through Dirac quantization. The atomically thin 2D materials are promising for the realization of the geometric potential, but the geometric potential in 2D materials is not identified experimentally. Here, the curvature-induced ring-patterned bound states are observed in structurally deformed 2D semiconductors and formulated the modified geometric potential for the curvature effect, which demonstrates the ring-shape bound states with angular momentum. The formulated modified geometric potential is analogous to the effective potential of a rotating charged black hole. Density functional theory and tight-binding calculations are performed, which quantitatively agree well with the results of the modified geometric potential. The modified geometric potential is described by modified Gaussian and mean curvatures, corresponding to the curvature-induced changes in spin-orbit interaction and band gap, respectively. Even for complex structural deformation, the geometric potential solves the complexity, which aligns well with experimental results. The understanding of the modified geometric potential provides us with an intuitive clue for quantum transport and a key factor for new quantum applications such as valleytronics, spintronics, and straintronics in 2D semiconductors.

Chemical passivation of 2D transition metal dichalcogenides: strategies, mechanisms, and prospects for optoelectronic applications

The interest in obtaining high-quality monolayer transition metal dichalcogenides (TMDs) for optoelectronic device applications has been growing dramatically. However, the prevalence of defects and unwanted doping in these materials remain challenges, as they both limit optical properties and device performance. Surface chemical treatments of monolayer TMDs have been effective in improving their photoluminescence yield and charge transport properties. In this scenario, a systematic understanding of the underlying mechanism of chemical treatments will lead to a rational design of passivation strategies in future research, ultimately taking a step toward practical optoelectronic applications. We will therefore describe in this mini-review the strategies, progress, mechanisms, and prospects of chemical treatments to passivate and improve the optoelectronic properties of TMDs.

Room-temperature quantum emission from interface excitons in mixed-dimensional heterostructures

The development of van der Waals heterostructures has introduced unconventional phenomena that emerge at atomically precise interfaces. For example, interlayer excitons in two-dimensional transition metal dichalcogenides show intriguing optical properties at low temperatures. Here we report on room-temperature observation of interface excitons in mixed-dimensional heterostructures consisting of two-dimensional tungsten diselenide and one-dimensional carbon nanotubes. Bright emission peaks originating from the interface are identified, spanning a broad energy range within the telecommunication wavelengths. The effect of band alignment is investigated by systematically varying the nanotube bandgap, and we assign the new peaks to interface excitons as they only appear in type-II heterostructures. Room-temperature localization of low-energy interface excitons is indicated by extended lifetimes as well as small excitation saturation powers, and photon correlation measurements confirm antibunching. With mixed-dimensional van der Waals heterostructures where band alignment can be engineered, new opportunities for quantum photonics are envisioned.

Charged Exciton Generation by Curvature-Induced Band Gap Fluctuations in Structurally Disordered Two-Dimensional Semiconductors

Curvature is a general factor for various two-dimensional (2D) materials due to their flexibility, which is not yet fully unveiled to control their physical properties. In particular, the effect of structural disorder with random curvature formation on excitons in 2D semiconductors is not fully understood. Here, the correlation between structural disorder and exciton formation in monolayer MoS2 on SiO2 was investigated by using photoluminescence (PL) and Raman spectroscopy. We found that the curvature-induced charge localization along with band gap fluctuations aid the formation of the localized charged excitons (such as trions). In the substrate-supported region, the trion population is enhanced by a localized charge due to the microscopic random bending strain, while the trion is suppressed in the suspended region which exhibits negligible bending strain, anomalously even though the dielectric screening effect is lower than that of the supported region. The redistribution of each exciton by the bending strain leads to a huge variation (∼100-fold) in PL intensity between the supported and suspended regions, which cannot be fully comprehended by external potential disorders such as a random distribution of charged impurities. The peak position of PL in MoS2/SiO2 is inversely proportional to the Raman peak position of E12g, indicating that the bending strain is correlated with PL. The supported regions exhibit an indirect portion that was not shown in the suspended regions or atomically flat substrates. The understanding of the structural disorder effect on excitons provides a fundamental path for optoelectronics and strain engineering of 2D semiconductors.

Hexagonal Boron Nitride Slab Waveguides for Enhanced Spectroscopy of Encapsulated 2D Materials

The layered insulator hexagonal boron nitride (hBN) is a critical substrate that brings out the exceptional intrinsic properties of two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs). In this work, the authors demonstrate how hBN slabs tuned to the correct thickness act as optical waveguides, enabling direct optical coupling of light emission from encapsulated layers into waveguide modes. Molybdenum selenide (MoSe2 ) and tungsten selenide (WSe2 ) are integrated within hBN-based waveguides and demonstrate direct coupling of photoluminescence emitted by in-plane and out-of-plane transition dipoles (bright and dark excitons) to slab waveguide modes. Fourier plane imaging of waveguided photoluminescence from MoSe2 demonstrates that dry etched hBN edges are an effective out-coupler of waveguided light without the need for oil-immersion optics. Gated photoluminescence of WSe2 demonstrates the ability of hBN waveguides to collect light emitted by out-of-plane dark excitons.Numerical simulations explore the parameters of dipole placement and slab thickness, elucidating the critical design parameters and serving as a guide for novel devices implementing hBN slab waveguides. The results provide a direct route for waveguide-based interrogation of layered materials, as well as a way to integrate layered materials into future photonic devices at arbitrary positions whilst maintaining their intrinsic properties.

Investigation of Interface Interactions Between Monolayer MoS2 and Metals: Implications on Strain and Surface Roughness

Achieving a low contact resistance has been an important issue in the design of two-dimensional (2D) semiconductor-metal interfaces. The metal contact resistance is dominated by interfacial interactions. Here, we systematically investigate 2D semiconductor-metal interfaces formed by transferring monolayer MoS2 onto prefabricated metal surfaces, such as Au and Pd, using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and Raman spectroscopy. In contrast to the MoS2/HOPG interface, the interfaces of MoS2/Au and MoS2/Pd feature the formation of weak covalent bonds. The XPS spectra reveal distinct peak positions for S-Au and S-Pd, indicating a higher doping concentration at the S-Au interface. This difference is a key factor in understanding the electronic interactions at the metal-MoS2 interfaces. Additionally, we observe that the metal surface roughness is a critical determinant of the adhesion behavior of transferred monolayer MoS2, resulting in different strains and doping concentrations. The strain on transferred MoS2 increases with an increase in substrate roughness. However, the strain is released when the roughness of metal surface surpasses a certain threshold. The dependence of the contact material and the influence of the substrate roughness on the contact interface provide critical information for improving 2D semiconductor-metal contacts and device performance.

Enhanced HER Efficiency of Monolayer MoS2 via S Vacancies and Nano-Cones Array Induced Strain Engineering

Molybdenum disulfide (MoS2 ) has gained significant attention as a promising catalyst for hydrogen evolution reaction (HER). The catalytic performance of MoS2 can be enhanced by either altering its structure or regulating external conditions. In this study, a novel approach combining the introduction of sulfur vacancy (VS ) and biaxial tensile strain to create more active sites and modulate the band structure of monolayer MoS2 is proposed. To achieve the desired strain level, nano-cones (NCs) array substrates facilely fabricated by dip-pen nanolithography (DPN) are employed. The magnitude of the applied tensile strain can be finely tuned via adjusting the height of the NCs. Furthermore, on-chip electrochemical devices are constructed based on artificial structures, enabling precise optimization of HER performance of MoS2 through the synergistic effect of VS and strain. Combined with the d-band theory, it reveals that the HER properties of VS -MoS2 are highly dependent on the degree of tensile strain. This study presents a promising avenue for the design and preparation of high-performance 2D catalysts for energy conversion and storage applications.

Local Nanostrain Engineering of Monolayer MoS2 Using Atomic Force Microscopy-Based Thermomechanical Nanoindentation

Strain engineering in two-dimensional materials (2DMs) has important application potential for electronic and optoelectronic devices. However, achieving precise spatial control, adjustable sizing, and permanent strain with nanoscale resolution remains challenging. Herein, a thermomechanical nanoindentation method is introduced, inspired by skin edema caused by mosquito bites, which can induce localized nanostrain and bandgap modulation in monolayer molybdenum disulfide (MoS2) transferred onto a poly(methyl methacrylate) film utilizing a heated atomic force microscopy nanotip. Via adjustment of the machining parameters, the strains of MoS2 are manipulated, achieving an average strain of ≤2.6% on the ring-shaped expansion structure. The local bandgap of MoS2 is spatially modulated using three types of nanostructures. Among them, the nanopit has the largest range of bandgap regulation, with a substantial change of 56 meV. These findings demonstrate the capability of the proposed method to create controllable and reproducible nanostrains in 2DMs.

Ultrafast Electronic Relaxation Dynamics of Atomically Thin MoS2 Is Accelerated by Wrinkling

Strain engineering is an attractive approach for tuning the local optoelectronic properties of transition metal dichalcogenides (TMDs). While strain has been shown to affect the nanosecond carrier recombination dynamics of TMDs, its influence on the sub-picosecond electronic relaxation dynamics is still unexplored. Here, we employ a combination of time-resolved photoemission electron microscopy (TR-PEEM) and nonadiabatic ab initio molecular dynamics (NAMD) to investigate the ultrafast dynamics of wrinkled multilayer (ML) MoS2 comprising 17 layers. Following 2.41 eV photoexcitation, electronic relaxation at the Γ valley occurs with a time constant of 97 ± 2 fs for wrinkled ML-MoS2 and 120 ± 2 fs for flat ML-MoS2. NAMD shows that wrinkling permits larger amplitude motions of MoS2 layers, relaxes electron-phonon coupling selection rules, perturbs chemical bonding, and increases the electronic density of states. As a result, the nonadiabatic coupling grows and electronic relaxation becomes faster compared to flat ML-MoS2. Our study suggests that the sub-picosecond electronic relaxation dynamics of TMDs is amenable to strain engineering and that applications which require long-lived hot carriers, such as hot-electron-driven light harvesting and photocatalysis, should employ wrinkle-free TMDs.

High optical spin-filtering in antiferromagnetic stanene nanoribbons induced by band bending and uniaxial strain

Non-equilibrium spin-polarized transport properties of antiferromagnetic stanene nanoribbons are theoretically studied under the combining effect of a normal electric field and linearly polarized irradiation based on the tight-binding model at room temperature. Due to the existence of spin-orbit coupling in stanene lattice, applying normal electric field leads to splitting of band degeneracy of spin-resolved energy levels in conduction and valence bands. Furthermore, unequivalent absorption of the polarized photons at two valleys which is attributed to an antiferromagnetic exchange field results in unequal spin-polarized photocurrent for spin-up and spin-down components. Interestingly, in the presence of band bending which has been induced by edge potentials, an allowable quantum efficiency occurs over a wider wavelength region of the incident light. It is especially important that the variation of an exchange magnetic field generates spin semi-conducting behavior in the bended band structure. Moreover, it is shown that optical spin-filtering effect is obtained under the simultaneous effect of uniaxial strain and narrow edge potential.

Localised strain and doping of 2D materials

There is a growing interest in 2D materials-based devices as the replacement for established materials, such as silicon and metal oxides in microelectronics and sensing, respectively. However, the atomically thin nature of 2D materials makes them susceptible to slight variations caused by their immediate environment, inducing doping and strain, which can vary between, and even microscopically within, devices. One of the misapprehensions for using 2D materials is the consideration of unanimous intrinsic properties over different support surfaces. The interfacial interaction, intrinsic structural disorder and external strain modulate the properties of 2D materials and govern the device performance. The understanding, measurement and control of these factors are thus one of the significant challenges for the adoption of 2D materials in industrial electronics, sensing, and polymer composites. This topical review provides a comprehensive overview of the effect of strain-induced lattice deformation and its relationship with physical and electronic properties. Using the example of graphene and MoS₂ (as the prototypical 2D semiconductor), we rationalise the importance of scanning probe techniques and Raman spectroscopy to elucidate strain and doping in 2D materials. These effects can be directly and accurately characterised through Raman shifts in a non-destructive manner. A generalised model has been presented that deconvolutes the intertwined relationship between strain and doping in graphene and MoS₂ that could apply to other members of the 2D materials family. The emerging field of straintronics is presented, where the controlled application of strain over 2D materials induces tuneable physical and electronic properties. These perspectives highlight practical considerations for strain engineering and related microelectromechanical applications.

Structural-disorder-driven critical quantum fluctuation and localization in two-dimensional semiconductors

Quantum fluctuations of wavefunctions in disorder-driven quantum phase transitions (QPT) exhibit criticality, as evidenced by their multifractality and power law behavior. However, understanding the metal-insulator transition (MIT) as a continuous QPT in a disordered system has been challenging due to fundamental issues such as the lack of an apparent order parameter and its dynamical nature. Here, we elucidate the universal mechanism underlying the structural-disorder-driven MIT in 2D semiconductors through autocorrelation and multifractality of quantum fluctuations. The structural disorder causes curvature-induced band gap fluctuations, leading to charge localization and formation of band tails near band edges. As doping level increases, the localization-delocalization transition occurs when states above a critical energy become uniform due to unusual band bending by localized charge. Furthermore, curvature induces local variations in spin-orbit interactions, resulting in non-uniform ferromagnetic domains. Our findings demonstrate that the structural disorder in 2D materials is essential to understanding the intricate phenomena associated with localization-delocalization transition, charge percolation, and spin glass with both topological and magnetic disorders.

Ultrafast Charge Transfer 2D MoS2 /Organic Heterojunction for Sensitive Photodetector

The 2D MoS₂ with superior optoelectronic properties such as high charge mobility and broadband photoresponse has attracted broad research interests in photodetectors (PD). However, due to the atomic thin layer of 2D MoS₂ , its pure photodetectors usually suffer from inevitable drawbacks such as large dark current, and intrinsically slow response time. Herein, a new organic material BTP-4F with high mobility is successfully stacked with 2D MoS₂ film to form an integrated 2D MoS₂ /organic P-N heterojunction, facilitating efficient charge transfer as well as significantly suppressed dark current. As a result, the as-obtained 2D MoS₂ /organic (PD) has exhibited excellent response and fast response time of 332/274 µs. The analysis validated photogenerated electron transition from this monolayer MoS₂ to subsequent BTP-4F film, whereas the transited electron is originated from the A^- exciton of 2D MoS₂ by temperature-dependent photoluminescent analysis. The ultrafast charge transfer time of ≈0.24 ps measured by time-resolved transient absorption spectrum is beneficial for efficient electron-hole pair separation, greatly contributing to the obtained fast photoresponse time of 332/274 µs. This work can open a promising window to acquire low-cost and high-speed (PD).

Synergistically Designed Dual Interfaces to Enhance the Electrochemical Performance of MoO2 /MoS2 in Na- and Li-Ion Batteries

It is indispensable to develop and design high capacity, high rate performance, long cycling life, and low-cost electrodes materials for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). Herein, MoO₂ /MoS₂ /C, with dual heterogeneous interfaces, is designed to induce a built-in electric field, which has been proved by experiments and theoretical calculation can accelerate electrochemical reaction kinetics and generate interfacial interactions to strengthen structural stability. The carbon foam serves as a conductive frame to assist the movement of electrons/ions, as well as forms heterogeneous interfaces with MoO₂ /MoS₂ through CS and CO bonds, maintaining structural integrity and enhancing electronic transport. Thanks to these unique characteristics, the MoO₂ /MoS₂ /C renders a significantly enhanced electrochemical performance (324 mAh g^-1 at 1 A g^-1 after 1000 cycles for SIB and 500 mAh g^-1 at 1 A g^-1 after 500 cycles for LIBs). The current work presents a simple, useful and cost-effective route to design high-quality electrodes via interfacial engineering.

In-situ atomic level observation of the strain response of graphene lattice

Strain is inevitable in two-dimensional (2D) materials, regardless of whether the film is suspended or supported. However, the direct measurement of strain response at the atomic scale is challenging due to the difficulties of maintaining both flexibility and mechanical stability at low temperature under UHV conditions. In this work, we have implemented a compact nanoindentation system with a size of [Formula: see text] 160 mm[Formula: see text] [Formula: see text] 5.2 mm in a scanning tunneling microscope (STM) sample holder, which enables the reversible control of strain and gate electric field. A combination of gearbox and piezoelectric actuator allowed us to modulate the depth of the indentation continuously with nanometer precision. The 2D materials were transferred onto the polyimide film. Pd clamp was used to enhance the strain transfer from the polyimide from to the 2D layers. Using this unique technique, strain response of graphene lattice were observed at atomic precision. In the relaxed graphene, strain is induced mainly by local curvature. However, in the strained graphene with tented structure, the lattice parameters become more sensitive to the indentor height change and stretching strain is increased additionally. Moreover, the gate controllability is confirmed by measuring the dependence of the STM tip height on gate voltage.

Uniaxial compressions induced complementarity and anisotropic behaviors in CuVP2S6

Uniaxial compressions in layered materials can change their electronic structures and properties. In this work, a bimetallic compound CuVP₂S₆is simulated by using Density Functional Theory (DFT) in the presence of uniaxial compressions. Our results clearly show vertical compressions could lead to anisotropic behaviors, which include the compression effect caused by interlayer compression and the anisotropy of intralayer stretching. The vertical compressions change the V-S bonds and the P-S bonds respectively in AA and AB structures. The complementarity between intralayer stretching and interlayer compression could also result in adjustable bandgaps and degeneracy breakdown of V atoms. Results from the electron localization function analysis demonstrate that the free electrons of AA and AB structures tend to delocalize, and ionic features in V-S bonds could be weakened with increasing vertical compressions. Moreover, the two internal binding energies of AA and AB structures and the charge density difference analysis show that the anisotropy in the intralayer stretch and the charge transfer between metal atoms and S atoms increases gradually.

Recent Progress of Metal Sulfide Photocatalysts for Solar Energy Conversion

Artificial photosynthetic solar-to-chemical cycles enable an entire environment to operate in a more complex, yet effective, way to perform natural photosynthesis. However, such artificial systems suffer from a lack of well-established photocatalysts with the ability to harvest the solar spectrum and rich catalytic active-site density. Benefiting from extensive experimental and theoretical investigations, this bottleneck may be overcome by devising a photocatalytic platform based on metal sulfides with predominant electronic, physical, and chemical properties. These tunable properties can endow them with abundant active sites, favorable light utilization, and expedited charge transportation for solar-to-chemical conversion. Here, it is described how some vital lessons extracted from previous investigations are employed to promote the further development of metal sulfides for artificial photosynthesis, including water splitting, CO₂ reduction, N₂ reduction, and pollutant removal. Their functions, properties, synthetic strategies, emerging issues, design principles, and intrinsic functional mechanisms for photocatalytic redox reactions are discussed in detail. Finally, the associated challenges and prospects for the utilization of metal sulfides are highlighted and future development trends in photocatalysis are envisioned.

Exciton-dominant photoluminescence of MoS2 by a functionalized substrate

Transition metal dichalcogenides (TMDs) have been considered as promising candidates for transparent and flexible optoelectronic devices owing to their large exciton binding energy and strong light-matter interaction. However, monolayer (1L) TMDs exhibited different intensities and spectra of photoluminescence (PL), and the characteristics of their electronic devices also differed in each study. This has been explained in terms of various defects in TMDs, such as vacancies and grain boundaries, and their surroundings, such as dielectric screening and charged impurities, which lead to non-radiative recombination of trions, low quantum yield (QY), and unexpected doping. However, it should be noted that the surface conditions of the substrate are also a critical factor in determining the properties of TMDs located on the substrate. Here, we demonstrate that the optical and electrical properties of 1L MoS₂ are strongly influenced by the functionalized substrate. The PL of 1L MoS₂ placed on the oxygen plasma-treated SiO₂ substrate was highly p-doped owing to the functional groups of -OH on SiO₂, resulting in a strong enhancement of PL by approximately 20 times. The PL QY of 1L MoS₂ on plasma-treated SiO₂ substrate increased by one order of magnitude. Surprisingly, the observed PL spectra show the suppression of non-radiative recombination by trions, thus the exciton-dominant PL led to a prolonged lifetime of MoS₂ on the plasma-treated substrate. The MoS₂ field-effect transistors fabricated on plasma-treated SiO₂ also exhibited a large hysteresis in the transfer curve owing to charge trapping of the functional groups. Our study demonstrates that the functional groups on the substrate strongly affect the characteristics of 1L MoS₂, which provides clues as to why MoS₂ exfoliated on various substrates always exhibited different properties in previous studies.

Gate-Tuned Gas Molecule Sensitivity of a Two-Dimensional Semiconductor

In this work, we develop a gate-tunable gas sensor based on a MoS2/hBN heterostructure field effect transistor. Through experimental measurements and numerical simulations, we systematically reveal a principle that relates the concentration of the target gas and sensing signals (ΔI/I0) as a function of gate bias. Because a linear relationship between ΔI/I0 and the gas concentration guarantees reliable sensor operation, the optimal gate bias condition for linearity was investigated. Taking NO2 and NH3 as target molecules, it is clarified that the bias condition greatly depends on the electron accepting/donating nature of the gas. The effects of the bandgap and polarity of the transition metal dichalcogenides (TMDC) channel are also discussed. In order to achieve linearly increasing signals that are stable with respect to the gas concentration, a sufficiently large VBG within VBG > 0 is required. We expect this work will shed light on a way to precisely design reliable semiconducting gas sensors based on the characteristics of TMDC and target gas molecules.

Atomically Thin TaSe2 Film as a High-Performance Substrate for Surface-Enhanced Raman Scattering

An atomically thin TaSe₂ sample, approximately containing two to three layers of TaSe₂ nanosheets with a diameter of 2.5 cm is prepared here for the first time and applied on the detection of various Raman-active molecules. It achieves a limit of detection of 10^-10  m for rhodamine 6G molecules. The excellent surface-enhanced Raman scattering (SERS) performance and underlying mechanism of TaSe₂ are revealed using spectrum analysis and density functional theory. The large adsorption energy and the abundance of filled electrons close to the Fermi level are found to play important roles in the chemical enhancement mechanism. Moreover, the TaSe₂ film enables highly sensitive detection of bilirubin in serum and urine samples, highlighting the potential of using 2D SERS substrates for applications in clinical diagnosis, for example, in the diagnosis of jaundice caused by excess bilirubin in newborn children.

Exploring the Thickness-Dependence of the Properties of Layered Gallium Sulfide

Group III layered monochalcogenide gallium sulfide, GaS, is one of the latest additions to the two-dimensional (2D) materials family, and of particular interest for visible-UV optoelectronic applications due to its wide bandgap energy in the range 2.35–3.05 eV going from bulk to monolayer. Interestingly, when going to the few-layer regime, changes in the electronic structure occur, resulting in a change in the properties of the material. Therefore, a systematic study on the thickness dependence of the different properties of GaS is needed. Here, we analyze mechanically exfoliated GaS layers transferred to glass substrates. Specifically, we report the dependence of the Raman spectra, photoluminescence, optical transmittance, resistivity, and work function on the thickness of GaS. Those findings can be used as guidance in designing devices based on GaS.

Acoustically Driven Stark Effect in Transition Metal Dichalcogenide Monolayers

The Stark effect is one of the most efficient mechanisms to manipulate many-body states in nanostructured systems. In mono- and few-layer transition metal dichalcogenides, it has been successfully induced by optical and electric field means. Here, we tune the optical emission energies and dissociate excitonic states in MoSe₂ monolayers employing the 220 MHz in-plane piezoelectric field carried by surface acoustic waves. We transfer the monolayers to high dielectric constant piezoelectric substrates, where the neutral exciton binding energy is reduced, allowing us to efficiently quench (above 90%) and red-shift the excitonic optical emissions. A model for the acoustically induced Stark effect yields neutral exciton and trion in-plane polarizabilities of 530 and 630 × 10^-5 meV/(kV/cm)², respectively, which are considerably larger than those reported for monolayers encapsulated in hexagonal boron nitride. Large in-plane polarizabilities are an attractive ingredient to manipulate and modulate multiexciton interactions in two-dimensional semiconductor nanostructures for optoelectronic applications.

Synthesis of High-Performance Monolayer Molybdenum Disulfide at Low Temperature

The large-area synthesis of high-quality MoS₂ plays an important role in realizing industrial applications of optoelectronics, nanoelectronics, and flexible devices. However, current techniques for chemical vapor deposition (CVD)-grown MoS₂ require a high synthetic temperature and a transfer process, which limits its utilization in device fabrications. Here, the direct synthesis of high-quality monolayer MoS₂ with the domain size up to 120 µm by metal-organic CVD (MOCVD) at a temperature of 320 °C is reported. Owing to the low-substrate temperature, the MOCVD-grown MoS₂ exhibits low impurity doping and nearly unstrained properties on the growth substrate, demonstrating enhanced electronic performance with high electron mobility of 68.3 cm² V^-1 s^-1 at room temperature. In addition, by tuning the precursor ratio, a better understanding of the MoS₂ growth process via a geometric model of the MoS₂ flake shape, is developed, which can provide further guidance for the synthesis of 2D materials.

Effective Hamiltonian for silicene under arbitrary strain from multi-orbital basis

A tight-binding (TB) Hamiltonian is derived for strained silicene from a multi-orbital basis. The derivation is based on the Slater-Koster coupling parameters between different orbitals across the silicene lattice and takes into account arbitrary distortion of the lattice under strain, as well as the first and second-order spin-orbit interactions (SOI). The breaking of the lattice symmetry reveals additional SOI terms which were previously neglected. As an exemplary application, we apply the linearized low-energy TB Hamiltonian to model the current-induced spin accumulation in strained silicene coupled to an in-plane magnetization. The interplay between symmetry-breaking and the additional SOI terms induces an out-of-plane spin accumulation. This spin accumulation remains unbalanced after summing over the Fermi surfaces of the occupied bands and the two valleys, and can thus be utilized for spin torque switching.

Subatomic Distortion of Surface Monolayer Lattice Visualized by Moiré Pattern

Mapping of the local lattice distortion is required to understand the details of the chemical and physical properties of thin films. However, the resolution by the direct microscopic methods was insufficient to detect the local distortion. Here, we have demonstrated that the local distortion of a monatomic film on a substrate is estimated with high resolution using the moiré pattern of the topographic scanning tunneling microscopy image. The analysis focused on the apparently low centers of the moiré pattern of the hexagonal iron nitride monolayer on the Cu(111) substrate. The local distortion was visualized by estimating the displacement of the experimentally detected center position from the ideal position that is extracted from the adjacent center positions. The map of the local distortion revealed that the subsurface impurities are preferentially located on the largely distorted areas. This approach is widely applicable to other thin films on the substrates.

Maskless Device Fabrication and Laser-Induced Doping in MoS2 Field Effect Transistors Using a Thermally Activated Cyclic Polyphthalaldehyde Resist

We present a novel maskless device fabrication technique for rapid prototyping of two-dimensional (2D)-based electronic materials. The technique is based on a thermally activated and self-developed cyclic polyphthalaldehyde (c-PPA) resist using a commercial Raman system and 532 nm laser illumination. Following the successful customization of electrodes to form field effect transistors based on MoS₂ monolayers, the laser-induced electronic doping of areas beneath the metal contacts that were exposed during lithography was investigated using both surface potential mapping and device characterization. An effective change in the doping level was introduced depending on the laser intensity, i.e., low laser powers resulted in p-doping, while high laser powers resulted in n-doping. Fabricated devices present a low contact resistance down to 10 kΩ·μm at a back-gate voltage of V_G = 80 V, which is attributed to the laser-induced n-type doping at the metal contact regions.

Thermomechanical Nanostraining of Two-Dimensional Materials

Local bandgap tuning in two-dimensional (2D) materials is of significant importance for electronic and optoelectronic devices but achieving controllable and reproducible strain engineering at the nanoscale remains a challenge. Here, we report on thermomechanical nanoindentation with a scanning probe to create strain nanopatterns in 2D transition metal dichalcogenides and graphene, enabling arbitrary patterns with a modulated bandgap at a spatial resolution down to 20 nm. The 2D material is in contact via van der Waals interactions with a thin polymer layer underneath that deforms due to the heat and indentation force from the heated probe. Specifically, we demonstrate that the local bandgap of molybdenum disulfide (MoS2) is spatially modulated up to 10% and is tunable up to 180 meV in magnitude at a linear rate of about -70 meV per percent of strain. The technique provides a versatile tool for investigating the localized strain engineering of 2D materials with nanometer-scale resolution.

Tailoring Domain Morphology in Monolayer NbSe2 and WxNb1-xSe2 Heterostructure

Domain morphology plays a pivotal role not only for the synthesis of high-quality 2D transition metal dichalcogenides (TMDs) but also for the further unveiling of related physical and chemical properties, yet little has been divulged to date, especially for metallic TMDs. In addition, solid precursor as a transition metal source has been conventionally introduced for the synthesis of TMDs, which leads to an inhomogeneous distribution of local domains with the substrate position, making it difficult to obtain a reliable film. Here, we tailor the domain morphologies of metallic NbSe₂ and NbSe₂/WSe₂ heterostructures using liquid-precursor chemical vapor deposition (CVD). We find that triangular, hexagonal, tripod-like, and herringbone-like NbSe₂ flakes are constructed through control of growth temperature and promoter and precursor concentration. Liquid-precursor CVD ensures domain morphologies that are highly reproducible over repeated growth and uniform along the gas-flow direction. A domain coverage of ∼80% is achieved at a high precursor concentration, starting with tripod-like NbSe₂ domain and evolving to the herringbone fractal. Furthermore, mixing liquid W and Nb precursors results in sea-urchin-like heterostructure domains with long-branch-shaped NbSe₂ at low temperature, whereas protruded hexagonal heterostructure domains grow at high temperature. Our liquid precursor approach provides a shortcut for tailoring the domain morphologies of metallic TMDs as well as metal/semiconductor heterostructures.

Strain-Engineered Anisotropic Optical and Electrical Properties in 2D Chiral-Chain Tellurium

Atomically thin materials, leveraging their low-dimensional geometries and superior mechanical properties, are amenable to exquisite strain manipulation with a broad tunability inaccessible to bulk or thin-film materials. Such capability offers unexplored possibilities for probing intriguing physics and materials science in the 2D limit as well as enabling unprecedented device applications. Here, the strain-engineered anisotropic optical and electrical properties in solution-grown, sub-millimeter-size 2D Te are systematically investigated through designing and introducing a controlled buckled geometry in its intriguing chiral-chain lattice. The observed Raman spectra reveal anisotropic lattice vibrations under the corresponding straining conditions. The feasibility of using buckled 2D Te for ultrastretchable strain sensors with a high gauge factor (≈380) is further explored. 2D Te is an emerging material boasting attractive characteristics for electronics, sensors, quantum devices, and optoelectronics. The results suggest the potential of 2D Te as a promising candidate for designing and implementing flexible and stretchable devices with strain-engineered functionalities.

Unveiling Valley Lifetimes of Free Charge Carriers in Monolayer WSe2

We report on nanosecond-long, gate-dependent valley lifetimes of free charge carriers in monolayer WSe₂, unambiguously identified by the combination of time-resolved Kerr rotation and electrical transport measurements. While the valley polarization increases when tuning the Fermi level into the conduction or valence band, there is a strong decrease of the respective valley lifetime consistent with both electron-phonon and spin-orbit scattering. The longest lifetimes are seen for spin-polarized bound excitons in the band gap region. We explain our findings via two distinct, Fermi-level-dependent scattering channels of optically excited, valley-polarized bright trions either via dark or bound states. By electrostatic gating we demonstrate that the transition-metal dichalcogenide WSe₂ can be tuned to be either an ideal host for long-lived localized spin states or allow for nanosecond valley lifetimes of free charge carriers (>10 ns).

Group delay time and Hartman effect in strained Weyl semimetals

The group delay time was theoretically studied in Weyl semimetals (WSMs) in the presence of strain. The Hartman effect, where the delay time for tunneling through a barrier tends to a constant for large barrier thickness, can be observed in WSMs when the incident angles [Formula: see text] and [Formula: see text], and the unidirectional strain tensor u ₃₃ and shear strain tensor u ₃₂, are larger than some critical values. We show that the Hartman effect is strongly dependent on the strength of the unidirectional strain tensor u ₃₃ and the ratio of the shear strain tensor [Formula: see text]. We also found that tensile and compressive strains have different effects on the group delay time and the transmission probability T in WSMs. Our study shows the possibility of modulating the group delay time and the Hartman effect in strained WSMs.

Local engineering of topological phase in monolayer MoS2

Monolayer transition metal dichalcogenides (TMDCs) with the 1T' structure are a new class of large-gap two-dimensional (2D) topological insulators, hosting topologically protected conduction channels on the edges. However, the 1T' phase is metastable compared to the 2H phase for most of 2D TMDCs, among which the 1T' phase is least favored in monolayer MoS₂. Here we report a clean and controllable technique to locally induce nanometer-sized 1T' phase in monolayer 2H-MoS₂ via a weak Argon-plasma treatment, resulting in topological phase boundaries of high density. We found that the stabilization of 1T' phase arises from the concerted effects of S vacancies and the tensile strain. Scanning tunneling spectroscopy (STS) clearly reveals a spin-orbit band gap (~60 meV) and topologically protected in-gap states residing at the 1T'-2H phase boundary, which are corroborated by density-functional theory (DFT) calculations. The strategy developed in this work can be generalized to a large variety of TMDCs materials, with potentials to realize scalable electronics and spintronics with low dissipation.

Strain Engineering of 2D Materials: Issues and Opportunities at the Interface

Triggered by the growing needs of developing semiconductor devices at ever-decreasing scales, strain engineering of 2D materials has recently seen a surge of interest. The goal of this principle is to exploit mechanical strain to tune the electronic and photonic performance of 2D materials and to ultimately achieve high-performance 2D-material-based devices. Although strain engineering has been well studied for traditional semiconductor materials and is now routinely used in their manufacturing, recent experiments on strain engineering of 2D materials have shown new opportunities for fundamental physics and exciting applications, along with new challenges, due to the atomic nature of 2D materials. Here, recent advances in the application of mechanical strain into 2D materials are reviewed. These developments are categorized by the deformation modes of the 2D material-substrate system: in-plane mode and out-of-plane mode. Recent state-of-the-art characterization of the interface mechanics for these 2D material-substrate systems is also summarized. These advances highlight how the strain or strain-coupled applications of 2D materials rely on the interfacial properties, essentially shear and adhesion, and finally offer direct guidelines for deterministic design of mechanical strains into 2D materials for ultrathin semiconductor applications.

Probing Effective Out-of-Plane Piezoelectricity in van der Waals Layered Materials Induced by Flexoelectricity

Many van der Waals layered 2D materials, such as h-BN, transition metal dichalcogenides (TMDs), and group-III monochalcogenides, have been predicted to possess piezoelectric and mechanically flexible natures, which greatly motivates potential applications in piezotronic devices and nanogenerators. However, only intrinsic in-plane piezoelectricity exists in these 2D materials and the piezoelectric effect is confined in odd-layers of TMDs. The present work is intent on combining the free-standing design and piezoresponse force microscopy techniques to obtain and directly quantify the effective out-of-plane electromechanical coupling induced by strain gradient on atomically thin MoS₂ and InSe flakes. Conspicuous piezoresponse and the measured piezoelectric coefficient with respect to the number of layers or thickness are systematically illustrated for both MoS₂ and InSe flakes. Note that the promising effective piezoelectric coefficient (d^eff ₃₃ ) of about 21.9 pm V^-1 is observed on few-layered InSe. The out-of-plane piezoresponse arises from the net dipole moment along the normal direction of the curvature membrane induced by strain gradient. This work not only provides a feasible and flexible method to acquire and quantify the out-of-plane electromechanical coupling on van der Waals layered materials, but also paves the way to understand and tune the flexoelectric effect of 2D systems.

Dielectric disorder in two-dimensional materials

Understanding and controlling disorder is key to nanotechnology and materials science. Traditionally, disorder is attributed to local fluctuations of inherent material properties such as chemical and structural composition, doping or strain. Here, we present a fundamentally new source of disorder in nanoscale systems that is based entirely on the local changes of the Coulomb interaction due to fluctuations of the external dielectric environment. Using two-dimensional semiconductors as prototypes, we experimentally monitor dielectric disorder by probing the statistics and correlations of the exciton resonances, and theoretically analyse the influence of external screening and phonon scattering. Even moderate fluctuations of the dielectric environment are shown to induce large variations of the bandgap and exciton binding energies up to the 100 meV range, often making it a dominant source of inhomogeneities. As a consequence, dielectric disorder has strong implications for both the optical and transport properties of nanoscale materials and their heterostructures.

Large-Area and Strain-Reduced Two-Dimensional Molybdenum Disulfide Monolayer Emitters on a Three-Dimensional Substrate

Atomically thin membranes of two-dimensional (2-D) transition-metal dichalcogenides (TMDCs) have distinct emission properties, which can be utilized for realizing ultrathin optoelectronic integrated systems in the future. Growing a large-area and strain-reduced monolayer 2-D material on a three-dimensional (3-D) substrate with microstructures or nanostructures is a crucial technique because the electronic band structure of TMDC atomic layers is strongly affected by the number of stacked layers and strain. In this study, a large-area and strain-reduced MoS₂ monolayer was fabricated on a 3-D substrate through a two-step growth procedure. The material characteristics and optical properties of monolayer TMDCs fabricated on the nonplanar substrate were examined. The growth of monolayer MoS₂ on a cone-shaped sapphire substrate effectively reduced the tensile strain induced by the substrate by decreasing the thermal expansion mismatch between the 2-D material and the substrate. Monolayer MoS₂ grown on the nonplanar substrate exhibited uniform strain reduction and luminescence intensity. The fabrication of monolayer MoS₂ on a nonplanar substrate increased the light extraction efficiency. In the future, large-area and strain-reduced 2-D TMDC materials grown on a nonplanar substrate can be employed as novel light-emitting devices for applications in lighting, communication, and displays for the development of ultrathin optoelectronic integrated systems.

Microscopic Imaging of the Stress Tensor in Diamond Using in Situ Quantum Sensors

The precise measurement of mechanical stress at the nanoscale is of fundamental and technological importance. In principle, all six independent variables of the stress tensor, which describe the direction and magnitude of compression/tension and shear stress in a solid, can be exploited to tune or enhance the properties of materials and devices. However, existing techniques to probe the local stress are generally incapable of measuring the entire stress tensor. Here, we make use of an ensemble of atomic-sized in situ strain sensors in diamond (nitrogen-vacancy defects) to achieve spatial mapping of the full stress tensor, with a submicrometer spatial resolution and a sensitivity of the order of 1 MPa (10 MPa) for the shear (axial) stress components. To illustrate the effectiveness and versatility of the technique, we apply it to a broad range of experimental situations, including mapping the stress induced by localized implantation damage, nanoindents, and scratches. In addition, we observe surprisingly large stress contributions from functional electronic devices fabricated on the diamond and also demonstrate sensitivity to deformations of materials in contact with the diamond. Our technique could enable in situ measurements of the mechanical response of diamond nanostructures under various stimuli, with potential applications in strain engineering for diamond-based quantum technologies and in nanomechanical sensing for on-chip mass spectroscopy.

Single-Crystalline Monolayer Graphene Wafer on Dielectric Substrate of SiON without Metal Catalysts

Many scientific and engineering efforts have been made to realize graphene electronics by fully utilizing intrinsic properties of ideal graphene for last decades. The most technical huddles come from the absence of wafer-scale graphene with a single-crystallinity on dielectric substrates. Here, we report an epitaxial growth of single-crystalline monolayer graphene directly on a single-crystalline dielectric SiON-SiC(0001) with a full coverage via epitaxial chemical vapor deposition (CVD) without metal catalyst. The dielectric surface of SiON provides atomically flat and chemically inert interface by passivation of dangling bonds, which keeps intrinsic properties of graphene. Atomic structures with a clean interface, full coverage of single-crystalline monolayer, and the epitaxy of graphene on SiON were confirmed macroscopically by mapping low energy electron diffraction (LEED) and Raman spectroscopy, and atomically by scanning tunneling microscopy (STM). Both of measured and calculated local density of states (LDOS) exhibit a symmetric and sharp Dirac cone with a Dirac point located at a Fermi level. Our method provides a route to utilize a single-crystalline dielectric substrate for ideal graphene growth for future applications.

Disorder in van der Waals heterostructures of 2D materials

Realizing the full potential of any materials system requires understanding and controlling disorder, which can obscure intrinsic properties and hinder device performance. Here we examine both intrinsic and extrinsic disorder in two-dimensional (2D) materials, in particular graphene and transition metal dichalcogenides (TMDs). Minimizing disorder is crucial for realizing desired properties in 2D materials and improving device performance and repeatability for practical applications. We discuss the progress in disorder control for graphene and TMDs, as well as in van der Waals heterostructures realized by combining these materials with hexagonal boron nitride. Furthermore, we showcase how atomic defects or disorder can also be harnessed to provide useful electronic, optical, chemical and magnetic functions.

Baseline correction method based on doubly reweighted penalized least squares

The spectrum acquired on the optical instrument usually contains the pure spectrum and undesirable components such as baseline and random noise. However, the intensity of the baseline, which seriously submerges the spectrum, is the primary limitation of spectral applications. Thus, baseline correction has become one of the most significant challenges for spectral applications. In this paper, we propose a doubly reweighted penalized least squares method to estimate the baseline. This method utilizes the first-order derivative of the original spectrum and established spectrum as a constraint of similarity. Meanwhile, the doubly reweighted strategy achieves a better effort. Considering the drawbacks of the weighting rules for the adaptive iteratively reweighted penalized least squares method, we adapt a boosted weighting rule based on the softsign function, which performs well when the spectrum contains high noise. The simulated results confirm that the proposed method yields better outcomes. The proposed method can be applied to Raman and near-infrared spectra as well, and the result shows that it can estimate various kinds of baselines effectively.

Crested two-dimensional transistors

Two-dimensional transition metal dichalcogenide (TMD) materials, albeit promising candidates for applications in electronics and optoelectronics^1-3, are still limited by their low electrical mobility under ambient conditions. Efforts to improve device performance through a variety of routes, such as modification of contact metals⁴ and gate dielectrics^5-9 or encapsulation in hexagonal boron nitride¹⁰, have yielded limited success at room temperature. Here, we report a large increase in the performance of TMD field-effect transistors operating under ambient conditions, achieved by engineering the substrate's surface morphology. For MoS₂ transistors fabricated on crested substrates, we observed an almost two orders of magnitude increase in carrier mobility compared to standard devices, as well as very high saturation currents. The mechanical strain in TMDs has been predicted to boost carrier mobility¹¹, and has been shown to influence the local bandgap^12,13 and quantum emission properties¹⁴ of TMDs. With comprehensive investigation of different dielectric environments and morphologies, we demonstrate that the substrate's increased corrugation, with its resulting strain field, is the dominant factor driving performance enhancement. This strategy is universally valid for other semiconducting TMD materials, either p-doped or n-doped, opening them up for applications in heterogeneous integrated electronics.

Unveiling exceptionally robust valley contrast in AA- and AB-stacked bilayer WS2

Valleytronics is a particularly interesting field that employs the valley degree of freedom for information manipulation. The fascinating prospects for realizing valleytronic devices have inspired persistent efforts towards exploring material systems with robust valley polarization. Monolayer transition metal dichalcogenides (TMDs) obey the well-known valley-dependent selection rule as a result of their inversion asymmetry. However, for inversion-symmetric bilayer tungsten-based TMDs, highly selective valley polarization has been surprisingly observed and is not yet fully understood. Here we systematically study the origin of the anomalously high valley polarization in bilayer WS₂ by temperature-dependent polarization-resolved photoluminescence measurements. It is found that acoustic phonons play a critical role in the valley polarization of bilayer WS₂. For some WS₂ bilayers with relatively small intensity ratios of indirect to direct bandgap emission, acoustic phonons could remarkably assist the intervalley scattering process and smear the valley contrast. On the other hand, in other bilayers, which show obvious indirect band gap emission, the indirect optical transition process depletes the phonon mode at the Λ point dramatically and results in anomalously robust valley polarization in bilayer WS₂. These results help recognize the crucial role of electron-phonon coupling in intervalley relaxation in bilayer WS₂ and provide new insights into the future design of valleytronic devices based on two-dimensional TMDs.

Layer-controlled epitaxy of 2D semiconductors: bridging nanoscale phenomena to wafer-scale uniformity

The rapid cadence of MOSFET scaling is stimulating the development of new technologies and accelerating the introduction of new semiconducting materials as silicon alternative. In this context, 2D materials with a unique layered structure have attracted tremendous interest in recent years, mainly motivated by their ultra-thin body nature and unique optoelectronic and mechanical properties. The development of scalable synthesis techniques is obviously a fundamental step towards the development of a manufacturable technology. Metal-organic chemical vapor deposition has recently been used for the synthesis of large area TMDs, however, an important milestone still needs to be achieved: the ability to precisely control the number of layers and surface uniformity at the nano-to micro-length scale to obtain an atomically flat, self-passivated surface. In this work, we explore various fundamental aspects involved in the chemical vapor deposition process and we provide important insights on the layer-dependence of epitaxial MoS₂ film's structural properties. Based on these observations, we propose an original method to achieve a layer-controlled epitaxy of wafer-scale TMDs.