Raman Shifts in Electron-Irradiated Monolayer MoS2

Cyclic Wear Reliability of 2D Monolayers

Understanding wear, a critical factor impacting the reliability of mechanical systems, is vital for nano-, meso-, and macroscale applications. Due to the complex nature of nanoscale wear, the behavior of nanomaterials such as two-dimensional materials under cyclic wear and their surface damage mechanism is yet unexplored. In this study, we used atomic force microscopy coupled with molecular dynamic simulations to statistically examine the cyclic wear behavior of monolayer graphene, MoS2, and WSe2. We show that graphene displays exceptional durability and lasts over 3000 cycles at 85% of the applied critical normal load before failure, while MoS2 and WSe2 last only 500 cycles on average. Moreover, graphene undergoes catastrophic failure as a result of stress concentration induced by local out-of-plane deformation. In contrast, MoS2 and WSe2 exhibit intermittent failure, characterized by damage initiation at the edge of the wear track and subsequent propagation throughout the entire contact area. In addition to direct implications for MEMS and NEMS industries, this work can also enable the optimization of the use of 2D materials as lubricant additives on a macroscopic level.

Scanning Probe Microscopies for Characterizations of 2D Materials

2D materials are intriguing due to their remarkably thin and flat structure. This unique configuration allows the majority of their constituent atoms to be accessible on the surface, facilitating easier electron tunneling while generating weak surface forces. To decipher the subtle signals inherent in these materials, the application of techniques that offer atomic resolution (horizontal) and sub-Angstrom (z-height vertical) sensitivity is crucial. Scanning probe microscopy (SPM) emerges as the quintessential tool in this regard, owing to its atomic-level spatial precision, ability to detect unitary charges, responsiveness to pico-newton-scale forces, and capability to discern pico-ampere currents. Furthermore, the versatility of SPM to operate under varying environmental conditions, such as different temperatures and in the presence of various gases or liquids, opens up the possibility of studying the stability and reactivity of 2D materials in situ. The characteristic flatness, surface accessibility, ultra-thinness, and weak signal strengths of 2D materials align perfectly with the capabilities of SPM technologies, enabling researchers to uncover the nuanced behaviors and properties of these advanced materials at the nanoscale and even the atomic scale.

Metrology for 2D materials: a perspective review from the international roadmap for devices and systems

The International Roadmap for Devices and Systems (IRDS) predicts the integration of 2D materials into high-volume manufacturing as channel materials within the next decade, primarily in ultra-scaled and low-power devices. While their widespread adoption in advanced chip manufacturing is evolving, the need for diverse characterization methods is clear. This is necessary to assess structural, electrical, compositional, and mechanical properties to control and optimize 2D materials in mass-produced devices. Although the lab-to-fab transition remains nascent and a universal metrology solution is yet to emerge, rapid community progress underscores the potential for significant advancements. This paper reviews current measurement capabilities, identifies gaps in essential metrology for CMOS-compatible 2D materials, and explores fundamental measurement science limitations when applying these techniques in high-volume semiconductor manufacturing.

Defect-induced helicity dependent terahertz emission in Dirac semimetal PtTe2 thin films

Nonlinear transport enabled by symmetry breaking in quantum materials has aroused considerable interest in condensed matter physics and interdisciplinary electronics. However, achieving a nonlinear optical response in centrosymmetric Dirac semimetals via defect engineering has remained a challenge. Here, we observe the helicity dependent terahertz emission in Dirac semimetal PtTe2 thin films via the circular photogalvanic effect under normal incidence. This is activated by a controllable out-of-plane Te-vacancy defect gradient, which we unambiguously evidence with electron ptychography. The defect gradient lowers the symmetry, which not only induces the band spin splitting but also generates the giant Berry curvature dipole responsible for the circular photogalvanic effect. We demonstrate that the THz emission can be manipulated by the Te-vacancy defect concentration. Furthermore, the temperature evolution of the THz emission features a minimum in the THz amplitude due to carrier compensation. Our work provides a universal strategy for symmetry breaking in centrosymmetric Dirac materials for efficient nonlinear transport.

Temperature-Dependent Frictional Behavior of MoS2 in Humid Environments: Insights from Water Molecule Adsorption and DFT Analyses

This study investigates the temperature-dependent frictional behavior of MoS2 in humid environments within the context of a long-standing debate over increased friction due to oxidation processes or molecular adsorption. By combining sliding friction experiments and density functional theory (DFT)-based first-principles simulations, it aims to clarify the role of water molecule adsorption in influencing frictional properties of MoS2, addressing the challenge of identifying interfacial bonding behavior responsible for friction in such conditions. Sliding experiments revealed that magnetron-sputtered MoS2 exhibits a reduction in the coefficient of friction (COF) with an increase in temperature from 25 to 100 °C under 20 and 40% relative humidity. This change in the COF obeys the Arrhenius law, presenting an energy barrier of 0.165 eV, indicative of the temperature-dependent nature of these frictional changes and suggests a consistent frictional mechanism. DFT simulations showed that H2O molecules are adsorbed at MoS2 vacancy defects with adsorption energies ranging from -0.56 to -0.17 eV, which align with the experimentally determined energy barrier. Adsorptive interactions, particularly the formation of stable H···S interfacial hydrogen bonds at defect sites, increase the interlayer adhesion and impede layer shearing. TEM analysis confirms that although MoS2 layers align parallel to the sliding direction in humid conditions, the COF remains at 0.12, as opposed to approximately 0.02 in dry air. This demonstrates that parallel layer alignment does not notably decrease the COF, underscoring humidity's significant role in MoS2's COF values, a result also supported by the Arrhenius analysis. The reversibility of the physisorption process, demonstrated by the recovery of the COF in high-temperature humid environments, suggests its dynamic nature. This study yields fundamental insights into MoS2 interfaces for environments with variable humidity and temperature, crucial for demanding tribological applications.

Selected Area Manipulation of MoS2 via Focused Electron Beam-Induced Etching for Nanoscale Device Editing

We demonstrate direct-write patterning of single and multilayer MoS2 via a focused electron beam-induced etching (FEBIE) process mediated with the XeF2 precursor. MoS2 etching is performed at various currents, areal doses, on different substrates, and characterized using scanning electron and atomic force microscopies as well as Raman and photoluminescence spectroscopies. Scanning transmission electron microscopy reveals a sub-40 nm etching resolution and the progression of point defects and lateral etching of the consequent unsaturated bonds. The results confirm that the electron beam-induced etching process is minimally invasive to the underlying material in comparison to ion beam techniques, which damage the subsurface material. Single-layer MoS2 field-effect transistors are fabricated, and device characteristics are compared for channels that are edited via the selected area etching process. The source-drain current at constant gate and source-drain voltage scale linearly with the edited channel width. Moreover, the mobility of the narrowest channel width decreases, suggesting that backscattered and secondary electrons collaterally affect the periphery of the removed area. Focused electron beam doses on single-layer transistors below the etching threshold were also explored as a means to modify/thin the channel layer. The FEBIE exposures showed demonstrative effects via the transistor transfer characteristics, photoluminescence spectroscopy, and Raman spectroscopy. While strategies to minimize backscattered and secondary electron interactions outside of the scanned regions require further investigation, here, we show that FEBIE is a viable approach for selective nanoscale editing of MoS2 devices.

Engineering Vacancies for the Creation of Antisite Defects in Chemical Vapor Deposition Grown Monolayer MoS2 and WS2 via Proton Irradiation

It is critical to understand the laws of quantum mechanics in transformative technologies for computation and quantum information science applications to enable the ongoing second quantum revolution calls. Recently, spin qubits based on point defects have gained great attention, since these qubits can be initiated, selectively controlled, and read out with high precision at ambient temperature. The major challenge in these systems is controllably generating multiqubit systems while properly coupling the defects. To address this issue, we began by tackling the engineering challenges these systems present and understanding the fundamentals of defects. In this regard, we controllably generate defects in MoS2 and WS2 monolayers and tune their physicochemical properties via proton irradiation. We quantitatively discovered that the proton energy could modulate the defects' density and nature; higher defect densities were seen with lower proton irradiation energies. Three distinct defect types were observed: vacancies, antisites, and adatoms. In particular, the creation and manipulation of antisite defects provides an alternative way to create and pattern spin qubits based on point defects. Our results demonstrate that altering the particle irradiation energy can regulate the formation of defects, which can be utilized to modify the properties of 2D materials and create reliable electronic devices.

Validating the Use of Conductive Atomic Force Microscopy for Defect Quantification in 2D Materials

Defects significantly affect the electronic, chemical, mechanical, and optical properties of two-dimensional (2D) materials. Thus, it is critical to develop a method for convenient and reliable defect quantification. Scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM) possess the required atomic resolution but have practical disadvantages. Here, we benchmark conductive atomic force microscopy (CAFM) by a direct comparison with STM in the characterization of transition metal dichalcogenides (TMDs). The results conclusively demonstrate that CAFM and STM image identical defects, giving results that are equivalent both qualitatively (defect appearance) and quantitatively (defect density). Further, we confirm that CAFM can achieve single-atom resolution, similar to that of STM, on both bulk and monolayer samples. The validation of CAFM as a facile and accurate tool for defect quantification provides a routine and reliable measurement that can complement other standard characterization techniques.

Photoactive Au@MoS2 Micromotors for Dynamic Surface-Enhanced Raman Spectroscopy Sensing

Photophoretic Au@MoS2 micromotors are used as smart mobile substrates for dynamic surface-enhanced Raman spectroscopy (SERS) sensing. The photophoretic capabilities and swarming-like propulsion of the micromotors allow for their schooling and accumulation in the measuring spot, increasing the density of SERS-active gold nanoparticles for Raman mapping and, simultaneously, the preconcentration of the target analyte. The generation of "hot-microflake spots" directly in the Raman irradiation point results in a 15-18-fold enhancement in the detection of crystal violet without the requirement for additional external sources for propulsion. Moreover, the reproducible collective micromotor motion does not depend on the exact position of the laser spot concerning individual micromotors, which greatly simplifies the experimental setup, avoiding the requirements of sophisticated equipment. The strategy was further applied for the detection of malachite green and paraquat with a good signal enhancement. The new on-the-move-based SERS strategy holds great promise for on-site detection with portable instrumentation in a myriad of environmental monitoring and clinical applications.

Disorder-induced bulk photovoltaic effect in a centrosymmetric van der Waals material

Sunlight is widely seen as one of the most abundant forms of renewable energy, with photovoltaic cells based on pn junctions being the most commonly used platform attempting to harness it. Unlike in conventional photovoltaic cells, the bulk photovoltaic effect (BPVE) allows for the generation of photocurrent and photovoltage in a single material without the need to engineer a pn junction and create a built-in electric field, thus offering a solution that can potentially exceed the Shockley-Queisser efficiency limit. However, it requires a material with no inversion symmetry and is therefore absent in centrosymmetric materials. Here, we demonstrate that breaking the inversion symmetry by structural disorder can induce BPVE in ultrathin PtSe2, a centrosymmetric semiconducting van der Waals material. Homogenous illumination of defective PtSe2 by linearly and circularly polarized light results in a photoresponse termed as linear photogalvanic effect (LPGE) and circular photogalvanic effect (CPGE), which is mostly absent in the pristine crystal. First-principles calculations reveal that LPGE originates from Se vacancies that act as asymmetric scattering centers for the photo-generated electron-hole pairs. Our work emphasizes the importance of defects to induce photovoltaic functionality in centrosymmetric materials and shows how the range of materials suitable for light sensing and energy-harvesting applications can be extended.

Spontaneous Decoration of Ultrasmall Pt Nanoparticles on Size-Separated MoS2 Nanosheets

Liquid exfoliation can be considered as a viable approach for the scalable production of 2D materials due to its various benefits, although the polydispersity in the obtained nanosheet size hinders their straightforward incorporation. Size-separation can help alleviate these concerns, however a correlation between nanosheet size and property needs to be established to bring about size-specific applicability. Herein, size-selected aqueous nanosheet dispersions have been obtained via centrifugation-based protocols, and their chemical activity in the spontaneous reduction of chloroplatinic acid is investigated. Growth of ultrasmall Pt nanoparticles was achieved on nanosheet surfaces without a need for reducing agents, and stark differences in the nanoparticle coverage were observed as a function of nanosheet size. Defects in the nanosheets were probed via Raman spectroscopy, and correlated to the observed size-activity. Additionally, the effect of reaction temperature during synthesis was investigated. The electrochemical activity of the ultrasmall Pt nanoparticle decorated MoS2 nanosheets was evaluated for the hydrogen evolution reaction, and enhancement in performance was observed with nanosheet size, and nanoparticle decoration density. These findings shine light on the significance of nanosheet size in controlling spontaneous reduction reactions, and provide a deeper insight to intrinsic properties of liquid exfoliated nanosheets.

Temperature-Dependent Structural and Electrical Properties of Metal-Organic CVD MoS2 Films

Metal-Organic CVD method (MOCVD) allows for deposition of ultrathin 2D transition metal dichalcogenides (TMD) films of electronic quality onto wafer-scale substrates. In this work, the effect of temperature on structure, chemical states, and electronic qualities of the MOCVD MoS2 films were investigated. The results demonstrate that the temperature increase in the range of 650 °C to 950 °C results in non-monotonic average crystallite size variation. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and Raman spectroscopy investigation has established the film crystal structure improvement with temperature increase in this range. At the same time, X-Ray photoelectron spectroscopy (XPS) method allowed to reveal non-stoichiometric phase fraction increase, corresponding to increased sulfur vacancies (VS) concentration from approximately 0.9 at.% to 3.6 at.%. Established dependency between the crystallite domains size and VS concentration suggests that these vacancies are form predominantly at the grain boundaries. The results suggest that an increased Vs concentration and enhanced charge carriers scattering at the grains' boundaries should be the primary reasons of films' resistivity increase from 4 kΩ·cm to 39 kΩ·cm.

Ferromagnetic Interlayer Coupling in CrSBr Crystals Irradiated by Ions

Layered magnetic materials are becoming a major platform for future spin-based applications. Particularly, the air-stable van der Waals compound CrSBr is attracting considerable interest due to its prominent magneto-transport and magneto-optical properties. In this work, we observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr crystals exposed to high-energy, non-magnetic ions. Already at moderate fluences, ion irradiation induces a remanent magnetization with hysteresis adapting to the easy-axis anisotropy of the pristine magnetic order up to a critical temperature of 110 K. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggests that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favors ferromagnetic order between the layers.

Solvent-assisted sulfur vacancy engineering method in MoS2 for a neuromorphic synaptic memristor

Recently, two-dimensional transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) have attracted great attention due to their unique properties. To modulate the electronic properties and structure of TMDs, it is crucial to precisely control chalcogenide vacancies and several methods have already been suggested. However, they have several limitations such as plasma damage by ion bombardment. Herein, we introduced a novel solvent-assisted vacancy engineering (SAVE) method to modulate sulfur vacancies in MoS2. Considering polarity and the Hansen solubility parameter (HSP), three solvents were selected. Sulfur vacancies can be modulated by immersing MoS2 in each solvent, supported by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy analyses. The SAVE method can further expand its application in memory devices representing memristive performance and synaptic behaviors. We represented the charge transport mechanism of sulfur vacancy migration in MoS2. The non-destructive, scalable, and novel SAVE method controlling sulfur vacancies is expected to be a guideline for constructing a vacancy engineering system of TMDs.

Isomer Discrimination via Defect Engineering in Monolayer MoS2

The all-surface nature of two-dimensional (2D) materials renders them highly sensitive to environmental changes, enabling the on-demand tailoring of their physical properties. Transition metal dichalcogenides, such as 2H molybdenum disulfide (MoS2), can be used as a sensory material capable of discriminating molecules possessing a similar structure with a high sensitivity. Among them, the identification of isomers represents an unexplored and challenging case. Here, we demonstrate that chemical functionalization of defect-engineered monolayer MoS2 enables isomer discrimination via a field-effect transistor readout. A multiscale characterization comprising X-ray photoelectron spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, and electrical measurement corroborated by theoretical calculations revealed that monolayer MoS2 exhibits exceptional sensitivity to the differences in the dipolar nature of molecules arising from their chemical structure such as the one in difluorobenzenethiol isomers, allowing their precise recognition. Our findings underscore the potential of 2D materials for molecular discrimination purposes, in particular for the identification of complex isomers.

Towards Low-Temperature CVD Synthesis and Characterization of Mono- or Few-Layer Molybdenum Disulfide

Molybdenum disulfide (MoS2) transistors are a promising alternative for the semiconductor industry due to their large on/off current ratio (>1010), immunity to short-channel effects, and unique switching characteristics. MoS2 has drawn considerable interest due to its intriguing electrical, optical, sensing, and catalytic properties. Monolayer MoS2 is a semiconducting material with a direct band gap of ~1.9 eV, which can be tuned. Commercially, the aim of synthesizing a novel material is to grow high-quality samples over a large area and at a low cost. Although chemical vapor deposition (CVD) growth techniques are associated with a low-cost pathway and large-area material growth, a drawback concerns meeting the high crystalline quality required for nanoelectronic and optoelectronic applications. This research presents a lower-temperature CVD for the repeatable synthesis of large-size mono- or few-layer MoS2 using the direct vapor phase sulfurization of MoO3. The samples grown on Si/SiO2 substrates demonstrate a uniform single-crystalline quality in Raman spectroscopy, photoluminescence (PL), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy. These characterization techniques were targeted to confirm the uniform thickness, stoichiometry, and lattice spacing of the MoS2 layers. The MoS2 crystals were deposited over the entire surface of the sample substrate. With a detailed discussion of the CVD setup and an explanation of the process parameters that influence nucleation and growth, this work opens a new platform for the repeatable synthesis of highly crystalline mono- or few-layer MoS2 suitable for optoelectronic application.

Effect of gamma irradiation on the physical properties of MoS2 monolayer

Two-dimensional transition metal dichalcogenides (2D-TMDs) have been proposed as novel optoelectronic materials for space applications due to their relatively light weight. MoS2 has been shown to have excellent semiconducting and photonic properties. Although the strong interaction of ionizing gamma radiation with bulk materials has been demonstrated, understanding its effect on atomically thin materials has scarcely been investigated. Here, we report the effect of gamma irradiation on the structural and electronic properties of a monolayer of MoS2. We perform Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) studies of MoS2, before and after gamma ray irradiation with varying doses and density functional theory (DFT) calculations. The Raman spectra and XPS results demonstrate that point defects dominate after the gamma irradiation of MoS2. DFT calculations elucidate the electronic properties of MoS2 before and after irradiation. Our work makes several contributions to the field of 2D materials research. First, our study of the electronic density of states and the electronic properties of a MoS2 monolayer irradiated by gamma rays sheds light on the properties of a MoS2 monolayer under gamma irradiation. Second, our study confirms that point defects are formed as a result of gamma irradiation. And third, our DFT calculations qualitatively suggest that the conductivity of the MoS2 monolayer may increase after gamma irradiation due to the creation of additional defect states.

Deciphering Vacancy Defect Evolution of 2D MoS2 for Reliable Transistors

Two-dimensional (2D) MoS2 is an excellent candidate channel material for next-generation integrated circuit (IC) transistors. However, the reliability of MoS2 is of great concern due to the serious threat of vacancy defects, such as sulfur vacancies (VS). Evaluating the impact of vacancy defects on the service reliability of MoS2 transistors is crucial, but it has always been limited by the difficulty in systematically tracking and analyzing the changes and effects of vacancy defects in the service environment. Here, a simulated initiator is established for deciphering the evolution of vacancy defects in MoS2 and their influence on the reliability of transistors. The results indicate that VS below 1.3% are isolated by slow enrichment during initiation. Over 1.3% of VS tend to enrich in pairs and over 3.5% of the enriched VS easily evolve into nanopores. The enriched VS with electron doping in the channel cause the threshold voltage (Vth) negative drift approaching 6 V, while the expanded nanopores initiate the Vth roll-off and punch-through of transistors. Finally, sulfur steam deposition has been proposed to constrain VS enrichment, and reliable MoS2 transistors are constructed. Our research provides a new method for deciphering and identifying the impact of defects.

MoS2 Photoelectrodes for Hydrogen Production: Tuning the S-Vacancy Content in Highly Homogeneous Ultrathin Nanocrystals

Tuning the electrocatalytic properties of MoS2 layers can be achieved through different paths, such as reducing their thickness, creating edges in the MoS2 flakes, and introducing S-vacancies. We combine these three approaches by growing MoS2 electrodes by using a special salt-assisted chemical vapor deposition (CVD) method. This procedure allows the growth of ultrathin MoS2 nanocrystals (1-3 layers thick and a few nanometers wide), as evidenced by atomic force microscopy and scanning tunneling microscopy. This morphology of the MoS2 layers at the nanoscale induces some specific features in the Raman and photoluminescence spectra compared to exfoliated or microcrystalline MoS2 layers. Moreover, the S-vacancy content in the layers can be tuned during CVD growth by using Ar/H2 mixtures as a carrier gas. Detailed optical microtransmittance and microreflectance spectroscopies, micro-Raman, and X-ray photoelectron spectroscopy measurements with sub-millimeter spatial resolution show that the obtained samples present an excellent homogeneity over areas in the cm2 range. The electrochemical and photoelectrochemical properties of these MoS2 layers were investigated using electrodes with relatively large areas (0.8 cm2). The prepared MoS2 cathodes show outstanding Faradaic efficiencies as well as long-term stability in acidic solutions. In addition, we demonstrate that there is an optimal number of S-vacancies to improve the electrochemical and photoelectrochemical performances of MoS2.

3D self-assembled indium sulfide nanoreactor for in-situ surface covalent functionalization: Towards high-performance room-temperature NO2 sensing

Thiol functionalization of two-dimensional (2D) metal sulfides has been demonstrated as an effective approach to enhance the sensing performances. However, most thiol functionalization is realized by multiple-step approaches in liquid medium and depends on the dispersity of 2D materials. Here, we utilize a three-dimensional (3D) In₂S₃ nano-porous structure that self-assembled from 2D components as the nanoreactor, in which the surface-absorbed thiol molecules from the chemical residues of the nanoreactor are used for the in-situ covalent functionalization. Such functionalization is realized by facile heat the nanoreactor at 100 °C, leading to the recombing sulfur vacancies with thiol-terminated groups. The NO₂ sensing performances of such functionalized nanoreactor are investigated at room temperature, in which In₂S₃-100 exhibits a response magnitude of 21.5 towards 10 ppm NO₂ with full reversibility, high selectivity, and excellent repeatability. Such high-performance gas sensors can be attributed to the additional electrons that transferring from the functional group into the host, thus significantly modifying the electronic band structure. This work provides a guideline for the facile in-situ functionalization of metal sulfides and an efficient strategy for the high performances gas sensors without external stimulus.

Gamma-Ray Irradiation Induced Ultrahigh Room-Temperature Ferromagnetism in MoS2 Sputtered Few-Layered Thin Films

Defect engineering is of great interest to the two-dimensional (2D) materials community. If nonmagnetic transition-metal dichalcogenides can possess room-temperature ferromagnetism (RTFM) induced by defects, then they will be ideal for application as spintronic materials and also for studying the relation between electronic and magnetic properties of quantum-confined structures. Thus, in this work, we aimed to study gamma-ray irradiation effects on MoS₂, which is diamagnetic in nature. We found that gamma-ray exposure up to 9 kGy on few-layered (3.5 nm) MoS₂ films induces an ultrahigh saturation magnetization of around 610 emu/cm³ at RT, whereas no significant changes were observed in the structure and magnetism of bulk MoS₂ (40 nm) films even after gamma-ray irradiation. The RTFM in a few-layered gamma-ray irradiated sample is most likely due to the bound magnetic polaron created by the spin interaction of Mo 4d ions with trapped electrons present at sulfur vacancies. In addition, density functional theory (DFT) calculations suggest that the defect containing one Mo and two S vacancies is the dominant defect inducing the RTFM in MoS₂. These DFT results are consistent with Raman, X-ray photoelectron spectroscopy, and ESR spectroscopy results, and they confirm the breakage of Mo and S bonds and the existence of vacancies after gamma-ray irradiation. Overall, this study suggests that the occurrence of magnetism in gamma-ray irradiated MoS₂ few-layered films could be attributed to the synergistic effects of magnetic moments arising from the existence of both Mo and S vacancies as well as lattice distortion of the MoS₂ structure.

Fully Optical in Operando Investigation of Ambient Condition Electrical Switching in MoS2 Nanodevices

MoS₂ nanoswitches have shown superb ultralow switching energies without excessive leakage currents. However, the debate about the origin and volatility of electrical switching is unresolved due to the lack of adequate nanoimaging of devices in operando. Here, three optical techniques are combined to perform the first noninvasive in situ characterization of nanosized MoS₂ devices. This study reveals volatile threshold resistive switching due to the intercalation of metallic atoms from electrodes directly between Mo and S atoms, without the assistance of sulfur vacancies. A "semi-memristive" effect driven by an organic adlayer adjacent to MoS₂ is observed, which suggests that nonvolatility can be achieved by careful interface engineering. These findings provide a crucial understanding of nanoprocess in vertically biased MoS₂ nanosheets, which opens new routes to conscious engineering and optimization of 2D electronics.

Defect MoS Misidentified as MoS2 in Monolayer MoS2 by Scanning Transmission Electron Microscopy: A First-Principles Prediction

The defect types in layered semiconductors can be identified by matching the scanning transmission electron microscopy (STEM) images with the structures from first-principles simulations. In a PVD-grown MoS₂ monolayer, the Mo_S2 antisite (one Mo replaces two S) is recognized as being dominant, because its calculated structure matches the distortive structure in STEM images. Therefore, Mo_S2 has received much attention in MoS₂-related defect engineering. We reveal that Mo_S (one Mo replaces one S) may be mistaken for Mo_S2, because ionized Mo_S also has similar structural distortion and can easily be ionized under electron irradiation. Unfortunately, the radiation-induced ionization and associated structural distortion of Mo_S were overlooked in previous studies. Because the formation energy of Mo_S is much lower than that of Mo_S2, it is more likely to exist as the dominant defect in MoS₂. Our results highlight the necessity of considering the defect ionization and associated structural distortion in STEM identification of defects in layered semiconductors.

Real-Time Investigation of Sulfur Vacancy Generation and Passivation in Monolayer Molybdenum Disulfide via in situ X-ray Photoelectron Spectromicroscopy

Understanding the chemical and electronic properties of point defects in two-dimensional materials, as well as their generation and passivation, is essential for the development of functional systems, spanning from next-generation optoelectronic devices to advanced catalysis. Here, we use synchrotron-based X-ray photoelectron spectroscopy (XPS) with submicron spatial resolution to create sulfur vacancies (SVs) in monolayer MoS₂ and monitor their chemical and electronic properties in situ during the defect creation process. X-ray irradiation leads to the emergence of a distinct Mo 3d spectral feature associated with undercoordinated Mo atoms. Real-time analysis of the evolution of this feature, along with the decrease of S content, reveals predominant monosulfur vacancy generation at low doses and preferential disulfur vacancy generation at high doses. Formation of these defects leads to a shift of the Fermi level toward the valence band (VB) edge, introduction of electronic states within the VB, and formation of lateral pn junctions. These findings are consistent with theoretical predictions that SVs serve as deep acceptors and are not responsible for the ubiquitous n-type conductivity of MoS₂. In addition, we find that these defects are metastable upon short-term exposure to ambient air. By contrast, in situ oxygen exposure during XPS measurements enables passivation of SVs, resulting in partial elimination of undercoordinated Mo sites and reduction of SV-related states near the VB edge. Correlative Raman spectroscopy and photoluminescence measurements confirm our findings of localized SV generation and passivation, thereby demonstrating the connection between chemical, structural, and optoelectronic properties of SVs in MoS₂.

Defect-engineered room temperature negative differential resistance in monolayer MoS2 transistors

The negative differential resistance (NDR) effect has been widely investigated for the development of various electronic devices. Apart from traditional semiconductor-based devices, two-dimensional (2D) transition metal dichalcogenide (TMD)-based field-effect transistors (FETs) have also recently exhibited NDR behavior in several of their heterostructures. However, to observe NDR in the form of monolayer MoS₂, theoretical prediction has revealed that the material should be more profoundly affected by sulfur (S) vacancy defects. In this work, monolayer MoS₂ FETs with a specific amount of S-vacancy defects are fabricated using three approaches, namely chemical treatment (KOH solution), physical treatment (electron beam bombardment), and as-grown MoS₂. Based on systematic studies on the correlation of the S-vacancies with both the device's electron transport characteristics and spectroscopic analysis, the NDR has been clearly observed in the defect-engineered monolayer MoS₂ FETs with an S-vacancy (V_S) amount of ∼5 ± 0.5%. Consequently, stable NDR behavior can be observed at room temperature, and its peak-to-valley ratio can also be effectively modulated via the gate electric field and light intensity. Through these results, it is envisioned that more electronic applications based on defect-engineered layered TMDs will emerge in the near future.

SERS enhancement induced by the Se vacancy defects in ultra-thin hybrid phase SnSex nanosheets

Improving the photo-induced charge transfer (PICT) efficiency by adjusting the energy levels difference between adsorbed probe molecules and substrate materials is a key factor for boosting the surface enhanced Raman scattering (SERS) based on the chemical mechanism (CM). Herein, a new route to improve the SERS activity of two-dimensional (2D) selenium and tin compounds (SnSe_x, 1 ≤ x ≤ 2) by the hybrid phase materials is researched. The physical properties and the energy band structure of SnSe_x were analyzed. The enhanced SERS activity of 2D SnSe_x can be attribute to the coupling of the PICT resonance caused by the defect energy levels induced by Se vacancy and the molecular resonance Raman scattering (RRS). This established a relationship between the physical properties and SERS activity of 2D layered materials. The resonance probe molecule, rhodamine (R6G), which is used to detect the SERS performance of SnSe_x nanosheets. The enhancement factor (EF) of R6G on the optimized SnSe_1.35 nanosheets can be as high as 2.6 × 10⁶, with a detection limit of 10^-10 M. The SERS result of the environmental pollution, thiram, shows that the SnSe_x nanosheets have a practical application in trace SERS detection, without the participation of metal particles. These results demonstrate that, through hybrid phase materials, the SERS sensitivity of 2D layered nanomaterials can be improved. It provides a kind of foreground non-metal SERS substrate in monitoring or detecting and provide a deep insight into the chemical SERS mechanism based on 2D layered materials.

An in-plane supercapacitor obtained by facile template method with high performance Mn-Co sulfide-based oxide electrode

Designing in-plane supercapacitors with high electrode materials selectivity is an indispensable approach to improve electrochemical performance. In this work, a facile template method was employed to fabricate in-plane supercapacitors. This template method could select any electrochemical active materials as electrode materials of in-plane supercapacitors. Hence, a high electrochemical performance material Mn-Co LDO-2S with optimized metal-sulfur bonds proportion and abundant sulfur vacancies was employed as electrode material of symmetrical in-plane supercapacitor (SPS). SPS exhibits excellent electrochemical performance finally, and has considerable area energy density 55.0μWh cm^-2with an area power density of 0.7 mW cm^-2. As a result, introducing sulfur atoms and sulfur vacancies are efficient approaches to improve electrode materials' electrochemical performance, and template method that proposed in this work is a promising approach to widen selectivity of in-plane supercapacitors' electrode materials.

Experimental Realization and Computational Investigations of B2S2 as a New 2D Material with Potential Applications

A new two-dimensional material B₂S₂ has been successfully synthesized for the first time and validated using first-principles calculations, with fundamental properties analyzed in detail. B₂S₂ has a similar structure as transition-metal dichalcogenides (TMDs) such as MoS₂, and the experimentally prepared free-standing B₂S₂ nanosheets show a uniform height profile lower than 1 nm. A thickness-modulated and unique oxidation-level dependent band gap of B₂S₂ is revealed by theoretical calculations, and vibration signatures are determined to offer a practical scheme for the characterization of B₂S₂. It is shown that the functionalized B₂S₂ is able to provide favorable sites for lithium adsorption with low diffusion barriers, and the prepared B₂S₂ shows a wide band photoluminescence response. These findings offer a feasible new and lighter member for the TMD-like 2D material family with potential for various aspects of applications, such as an anode material for Li-ion batteries and electronic and optoelectronic devices.

The first progress of plasma-based transition metal dichalcogenide synthesis: a stable 1T phase and promising applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted attention as polymorphs depending on their phases (1T and 2H) when applying typical synthesis methods. The 2H phase is generally synthesised through chemical vapour deposition (CVD) on a wafer-scale at high temperatures, and many synthesis methods have been reported owing to their thermodynamic stability and semiconductor properties. By contrast, although the 1T phase is meta-stable with an octahedral coordination, thereby limiting the use of synthesis methods, the recent structural advantage in terms of the hydrogen evolution reaction (HER) has been emphasised. Despite this demand, no large-area thin-film synthesis method for 1T-TMDs has been developed. Among several strategies of synthesizing metallic-phase (1T) TMDs, chemical exfoliation (alkali metal intercalation) is a major strategy and others have been used for electron-beam irradiation, laser irradiation, defects, plasma hot electron transfer, and mechanical strain. Therefore, we suggest an innovative synthesis method using plasma-enhanced CVD (PECVD) for both the 1T and 2H phases of TMDs (MoS2 and WS2). Because ions and radicals are accelerated to the substrate within the sheath region, a high-temperature source is not needed for vapour ionisation, and thus the process temperature can be significantly lowered (150 °C). Moreover, a 4-inch wafer-scale of a thin film is an advantage and can be synthesised on arbitrary substrates (SiO2/Si wafer, glassy carbon electrode, Teflon, and polyimide). Furthermore, the PECVD method was applied to TMD-graphene heterostructure films with a graphene-transferred substrate, and for the first time, sequential metal seed layer depositions of W (1 nm) and Mo (1 nm) were sulfurized to MoS2-WS2 vertical heterostructures with Ar + H2S plasma. We considered the prospects and challenges of the new PECVD method in the development of practical applications in next-generation integrated electronics, HER catalysts, and flexible biosensors.

Atomic and structural modifications of two-dimensional transition metal dichalcogenides for various advanced applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs) and their heterostructures have attracted significant interest in both academia and industry because of their unusual physical and chemical properties. They offer numerous applications, such as electronic, optoelectronic, and spintronic devices, in addition to energy storage and conversion. Atomic and structural modifications of van der Waals layered materials are required to achieve unique and versatile properties for advanced applications. This review presents a discussion on the atomic-scale and structural modifications of 2D TMDs and their heterostructures via post-treatment. Atomic-scale modifications such as vacancy generation, substitutional doping, functionalization and repair of 2D TMDs and structural modifications including phase transitions and construction of heterostructures are discussed. Such modifications on the physical and chemical properties of 2D TMDs enable the development of various advanced applications including electronic and optoelectronic devices, sensing, catalysis, nanogenerators, and memory and neuromorphic devices. Finally, the challenges and prospects of various post-treatment techniques and related future advanced applications are addressed.

Fast and efficient electrochemical thinning of ultra-large supported and free-standing MoS2 layers on gold surfaces

Molybdenum disulfide (MoS₂) is a very promising layered material for electrical, optical, and electrochemical applications because of its unique and outstanding properties. To unlock its full potential, among different preparation routes, electrochemistry has gain interest due to its simple, fast, scalable and simple instrumentation. However, obtaining large-area monolayer MoS₂ that will enable the fabrication of novel electronic and electrochemical devices is still challenging. In this work, we reported a simple and fast electrochemical thinning process that results in ultra-large MoS₂ down to monolayer on Au surfaces. The high affinity of MoS₂ by Au surfaces enables the removal of bulk layers while preserving the first layer attached to the electrode. With a proper choice of the applied potential, more than 90% of the bulk regions can be removed from large-area MoS₂ crystals, as confirmed by atomic force microscopy, photoluminescence, and Raman spectroscopy. We further address a set of contributions that are helpful to elucidate the features of MoS₂, namely, the hyphenation of electrochemistry and optical microscopy for real-time observation of the thinning process that was revealed to occur from the edges to the center of the flake, an image treatment to estimate the thinning area and thinning rate, and the preparation of free-standing MoS₂ layers by electrochemically thinning bulk flakes on microhole-structured Ni/Au meshes.

Activation of the MoS2 Basal Plane to Enhance CO Hydrogenation to Methane Activity Through Increasing S Vacancies

The active site of MoS₂ is usually located at the edge of crystalline MoS₂, which has a lower proportion than that from the basal plane, limiting the hydrogenation activity. Therefore, activating the basal plane of MoS₂ is expected to greatly enhance the hydrogenation activity. Herein, we prepared a series of MoS₂ catalysts by acidolysis of ammonium tetrathiomolybdate and subsequently pyrolyzing at high temperature with different atmospheres. Through analysis, we found that the prepared MoS₂ catalysts were curved, which was different from commercial MoS₂. Through X-ray diffraction, transmission electron microscopy, and Raman and X-ray photoelectron spectroscopy characterization, it was found that the MoS₂ catalyst pyrolyzed under a N₂ atmosphere had a larger number of S-vacancies than the MoS₂ catalysts under a H₂ atmosphere. In addition, temperature-programmed reduction results showed that the Mo-S bond energy was decreased with the increasing content of S-vacancies, which might be related to bending. Sulfur-resistant methanation results indicated that the curved MoS₂ exhibited increased CO conversion with the increasing S vacancies. Furthermore, density functional theory calculation was used to simulate the generation of S vacancy and numbers of S vacancies. It was found that with the generation of S vacancy, three unsaturated coordination Mo atoms were exposed around one S vacancy and became new active sites, resulting in enhanced activity. What is more, the higher methanation activity was attributed not only from more S vacancies but also from the decreased activation energy for CO hydrogenation activation.

Understanding Heterogeneities in Quantum Materials

Quantum materials are usually heterogeneous, with structural defects, impurities, surfaces, edges, interfaces, and disorder. These heterogeneities are sometimes viewed as liabilities within conventional systems; however, their electronic and magnetic structures often define and affect the quantum phenomena such as coherence, interaction, entanglement, and topological effects in the host system. Therefore, a critical need is to understand the roles of heterogeneities in order to endow materials with new quantum functions for energy and quantum information science applications. In this article, several representative examples are reviewed on the recent progress in connecting the heterogeneities to the quantum behaviors of real materials. Specifically, three intertwined topic areas are assessed: i) Reveal the structural, electronic, magnetic, vibrational, and optical degrees of freedom of heterogeneities. ii) Understand the effect of heterogeneities on the behaviors of quantum states in host material systems. iii) Control heterogeneities for new quantum functions. This progress is achieved by establishing the atomistic-level structure-property relationships associated with heterogeneities in quantum materials. The understanding of the interactions between electronic, magnetic, photonic, and vibrational states of heterogeneities enables the design of new quantum materials, including topological matter and quantum light emitters based on heterogenous 2D materials.

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

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

Mixed-Dimensional 1D/2D van der Waals Heterojunction Diodes and Transistors in the Atomic Limit

Inverting a semiconducting channel is the basis of all field-effect transistors. In silicon-based metal-oxide-semiconductor field-effect transistors (MOSFETs), a gate dielectric mediates this inversion. Access to inversion layers may be granted by interfacing ultrathin low-dimensional semiconductors in heterojunctions to advance device downscaling. Here we demonstrate that monolayer molybdenum disulfide (MoS₂) can directly invert a single-walled semiconducting carbon nanotube (SWCNT) transistor channel without the need for a gate dielectric. We fabricate and study this atomically thin one-dimensional/two-dimensional (1D/2D) van der Waals heterojunction and employ it as the gate of a 1D heterojunction field-effect transistor (1D-HFET) channel. Gate control is based on modulating the conductance through the channel by forming a lateral p-n junction within the CNT itself. In addition, we observe a region of operation exhibiting a negative static resistance after significant gate tunneling current passes through the junction. Technology computer-aided design (TCAD) simulations confirm the role of minority carrier drift-diffusion in enabling this behavior. The resulting van der Waals transistor architecture thus has the dual characteristics of both field-effect and tunneling transistors, and it advances the downscaling of heterostructures beyond the limits of dangling bonds and epitaxial constraints faced by III-V semiconductors.

Optical Inspection of 2D Materials: From Mechanical Exfoliation to Wafer-Scale Growth and Beyond

Optical inspection is a rapid and non-destructive method for characterizing the properties of two-dimensional (2D) materials. With the aid of optical inspection, in situ and scalable monitoring of the properties of 2D materials can be implemented industrially to advance the development and progress of 2D material-based devices toward mass production. This review discusses the optical inspection techniques that are available to characterize various 2D materials, including graphene, transition metal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), group-III monochalcogenides, black phosphorus (BP), and group-IV monochalcogenides. First, the authors provide an introduction to these 2D materials and the processes commonly used for their fabrication. Then they review several of the important structural properties of 2D materials, and discuss how to characterize them using appropriate optical inspection tools. The authors also describe the challenges and opportunities faced when applying optical inspection to recently developed 2D materials, from mechanically exfoliated to wafer-scale-grown 2D materials. Most importantly, the authors summarize the techniques available for largely and precisely enhancing the optical signals from 2D materials. This comprehensive review of the current status and perspective of future trends for optical inspection of the structural properties of 2D materials will facilitate the development of next-generation 2D material-based devices.

Brønsted acid-enhanced CoMoS catalysts for hydrodeoxygenation reactions

Brønsted solid acids greatly promote the hydrodeoxygenation activity of CoMoS catalysts through weakening Car–O bonds by protonation of the OH group.

A Polarization-Sensitive Self-Powered Photodetector Based on a p-WSe2/TaIrTe4/n-MoS2 van der Waals Heterojunction

Polarization-sensitive photodetection is highly appealing considering its great important applications. However, the inherent in-plane symmetry of a material and the single structure of a detector hinder the further development of polarization detectors with high anisotropic ratios. Herein, we design a p-WSe₂/TaIrTe₄/n-MoS₂ (p-Ta-n) heterojunction. As a type-II Weyl semimetal, TaIrTe₄ with an orthorhombic structure has strong in-plane asymmetry, which is confirmed by angle-resolved polarized Raman spectroscopy and second-harmonic generation. Due to the specific structure of the p-Ta-n junction with two vertical built-in electric fields, the device obtains a broadband self-powered photodetection ranging from visible (405 nm) to telecommunication wavelength (1550 nm) regions. Further, an optimized device containing 50-70 nm-thick layered TaIrTe₄ has been realized. What is more, high-resolution imaging of "T" based on the device with clear borders illustrates excellent stability of the device. Significantly, the photocurrent anisotropic ratio of the p-Ta-n detector can reach 9.1 under 635 nm light, which is more than eight times that of the best known TaIrTe₄-based photodetector reported before. This p-Ta-n junction containing a type-II Weyl fermion semimetal can provide an effective approach toward highly polarization-sensitive and high-performance integrated broadband photodetectors.

Strain-engineering on GeSe: Raman spectroscopy study

Among the IV-VI compounds, GeSe has wide applications in nanoelectronics due to its unique photoelectric properties and adjustable band gap. Even though modulation of its physical characteristics, including the band gap, by an external field will be useful for designing novel devices, experimental work is still rare. Here, we report a detailed anisotropic Raman response of GeSe flakes under uniaxial tension strain. Based on theoretical analysis, the anisotropy of the phonon response is attributed to a change in anisotropic bond length and bond angle under in-plane uniaxial strain. An enhancement in anisotropy and band gap is found due to strain along the ZZ or AC directions. This study shows that strain-engineering is an effective method for controlling the GeSe lattice, and paves the way for modulating the anisotropic electric and optical properties of GeSe.

Radiation damage and abnormal photoluminescence enhancement of multilayer MoS2under neutron irradiation

In this work, neutron irradiation effects on the optical property of multilayer MoS₂have been investigated in depth. Our results display that the intensity of the photoluminescence (PL) spectra of MoS₂flakes tends to slightly decrease after exposed to neutron irradiation with low fluence of 4.0 × 10⁸n/cm². An unexpected improvement of PL intensity, however, is observed when the irradiation fluence accumulates to 3.2 × 10⁹n/cm². Combined with the experimental results and first-principles calculations, neutron irradiation damage effects of multilayer MoS₂are analyzed deeply. Sulfur vacancy (V_S) is found to be responsible for the attenuation of the PL intensity as a major defect. In addition, our results reveal that the adsorbed hydroxyl groups (OH) and oxygen atoms (O) on the surface of MoS₂flakes not only promote the transition from trion excitons to neutral excitons, but also repair theV_Sin MoS₂, both of which contribute to the enhancement of luminescence properties. The detailed evolution process of irradiation-induced defects is discussed to reveal the microscopic mechanism of the significantly difference in luminescence intensity of MoS₂under different irradiation stages. This work has great significance for evaluating the neutron radiation hardness of multilayer MoS₂, which is helpful to enrich the fundamental research on neutron irradiation effects.

Morphology-induced spectral modification of self-assembled WS2 pyramids

Due to their intriguing optical properties, including stable and chiral excitons, two-dimensional transition metal dichalcogenides (2D-TMDs) hold the promise of applications in nanophotonics. Chemical vapor deposition (CVD) techniques offer a platform to fabricate and design nanostructures with diverse geometries. However, the more exotic the grown nanogeometry, the less is known about its optical response. WS₂ nanostructures with geometries ranging from monolayers to hollow pyramids have been created. The hollow pyramids exhibit a strongly reduced photoluminescence with respect to horizontally layered tungsten disulphide, facilitating the study of their clear Raman signal in more detail. Excited resonantly, the hollow pyramids exhibit a great number of higher-order phononic resonances. In contrast to monolayers, the spectral features of the optical response of the pyramids are position dependent. Differences in peak intensity, peak ratio and spectral peak positions reveal local variations in the atomic arrangement of the hollow pyramid crater and sides. The position-dependent optical response of hollow WS₂ pyramids is characterized and attributed to growth-induced nanogeometry. Thereby the first steps are taken towards producing tunable nanophotonic devices with applications ranging from opto-electronics to non-linear optics.

Effect of Point Defects on Electronic Structure of Monolayer GeS

Using density functional theory calculations, atomic and electronic structure of defects in monolayer GeS were investigated by focusing on the effects of vacancies and substitutional atoms. We chose group IV or chalcogen elements as substitutional ones, which substitute for Ge or S in GeS. It was found that the bandgap of GeS with substitutional atoms is close to that of pristine GeS, while the bandgap of GeS with Ge or S vacancies was smaller than that of pristine GeS. In terms of formation energy, monolayer GeS with Ge vacancies is more stable than that with S vacancies, and notably GeS with Ge substituted with Sn is most favorable within the range of chemical potential considered. Defects affect the piezoelectric properties depending on vacancies or substitutional atoms. Especially, GeS with substitutional atoms has almost the same piezoelectric stress coefficients eij as pristine GeS while having lower piezoelectric strain coefficients dij  but still much higher than other 2D materials. It is therefore concluded that Sn can effectively heal Ge vacancy in GeS, keeping high piezoelectric strain coefficients.

Spectroscopic Signatures of Interlayer Coupling in Janus MoSSe/MoS2 Heterostructures

The interlayer coupling in van der Waals heterostructures governs a variety of optical and electronic properties. The intrinsic dipole moment of Janus transition metal dichalcogenides (TMDs) offers a simple and versatile approach to tune the interlayer interactions. In this work, we demonstrate how the van der Waals interlayer coupling and charge transfer of Janus MoSSe/MoS₂ heterobilayers can be tuned by the twist angle and interface composition. Specifically, the Janus heterostructures with a sulfur/sulfur (S/S) interface display stronger interlayer coupling than the heterostructures with a selenium/sulfur (Se/S) interface as shown by the low-frequency Raman modes. The differences in interlayer interactions are explained by the interlayer distance computed by density-functional theory (DFT). More intriguingly, the built-in electric field contributed by the charge density redistribution and interlayer coupling also play important roles in the interfacial charge transfer. Namely, the S/S and Se/S interfaces exhibit different levels of photoluminescence (PL) quenching of MoS₂ A exciton, suggesting enhanced and reduced charge transfer at the S/S and Se/S interface, respectively. Our work demonstrates how the asymmetry of Janus TMDs can be used to tailor the interfacial interactions in van der Waals heterostructures.

MoS2 with Controlled Thickness for Electrocatalytic Hydrogen Evolution

Molybdenum disulfide (MoS₂) has moderate hydrogen adsorption free energy, making it an excellent alternative to replace noble metals as hydrogen evolution reaction (HER) catalysts. The thickness of MoS₂ can affect its energy band structure and interface engineering, which are the avenue way to adjust HER performance. In this work, MoS₂ films with different thicknesses were directly grown on the glassy carbon (GC) substrate by atomic layer deposition (ALD). The thickness of the MoS₂ films can be precisely controlled by regulating the number of ALD cycles. The prepared MoS₂/GC was directly used as the HER catalyst without a binder. The experimental results show that MoS₂ with 200-ALD cycles (the thickness of 14.9 nm) has the best HER performance. Excessive thickness of MoS₂ films not only lead to the aggregation of dense MoS₂ nanosheets, resulting in reduction of active sites, but also lead to the increase of electrical resistance, reducing the electron transfer rate. MoS₂ grown layer by layer on the substrate by ALD technology also significantly improves the bonding force between MoS₂ and the substrate, showing excellent HER stability.

Atomic-layered Pt clusters on S-vacancy rich MoS2-x with high electrocatalytic hydrogen evolution

Developing suitable supports to maximize the atomic utilization efficiency of platinum group metals is of great significance to hydrogen evolution from water splitting. Herein, we report a fully exposed Pt cluster supported on an S-vacancy rich MoS2-x support (Pt/Sv-MoS2-x) by a facile impregnation method. Pt/Sv-MoS2-x exhibits an outstanding electrochemical HER performance with a low overpotential of 26.6 mV at a current density of 10 mA cm-2, a small Tafel slope of 34.8 mV dec-1 and good durability. Most importantly, the mass activity of Pt is an order of magnitude more active than that of commercial Pt/C at an overpotential of 0.08 V. We attribute this exceptional HER catalytic performance to the fact that platinum and Sv-MoS2-x act in synergy to accelerate the reaction kinetics.

Davydov Splitting, Double-Resonance Raman Scattering, and Disorder-Induced Second-Order Processes in Chemical Vapor Deposited MoS2 Thin Films

Atomically thin MoS₂ hosts rich and distinct vibrational spectral features, which are prominent under selective excitation energies near the excitonic transitions. In this work, we have investigated the resonant Raman scattering of the MoS₂ layers of different thicknesses, from monolayer to five-layer samples, measured near resonance with the A excitonic transition. We show that the near-resonance excitation (1.96 eV) resulted in a Davydov splitting of the out-of-plane A-like phonon mode (A_1g) around 406 cm^-1 caused by the weak interlayer interaction. The number of Davydov splitting components (N) equals the number of layers (NL) of the MoS₂, suggesting that it can be used as a thickness indicator. The origin of various Davydov components is understood based on a simple nearest-interlayer interaction. We extend our investigation to identify some acoustic phonon modes associated with characteristic second-order double-resonance Raman and disorder-induced bands.

Modification of thermal transport in few-layer MoS2 by atomic-level defect engineering

Molybdenum disulfide (MoS2) has attracted significant attention due to its good charge carrier mobility, high on/off ratio in field-effect transistors and novel layer-dependent band structure, with potential applications in modern electronic, photovoltaic and valleytronic devices. Despite these advantages, its thermal transport property has often been neglected until recently. In this work, we probe phonon transport in few-layer MoS2 flakes with various point defect concentrations enabled by helium ion (He+) irradiation. For the first time, we experimentally show that Mo-vacancies greatly impede phonon transport compared to S-vacancies, resulting in a larger reduction of thermal conductivity. Furthermore, Raman characterization shows that the in-plane Raman-sensitive peak E2g1 was red-shifted with increasing defect concentration, corresponding to the gradual damage of the in-plane crystalline networks and the gradual reduction in the measured thermal conductivity. Our work provides a practical approach for atomic-level engineering of phonon transport in two-dimensional (2D) layered materials by selectively removing elements, thus holding potential applications in designing thermal devices based on various emerging 2D materials.

Tailoring of Surface Acidic Sites in Co-MoS2 Catalysts for Hydrodeoxygenation Reaction

CoMo sulfides are typical catalysts for selective hydrodeoxygenation (HDO) of phenolics to aromatics which is important in bio-oil upgrading. However, it is still a challenge to promote the intrinsic activity of Co-MoS₂ catalysts. Defect chemistry provides a good option to improve surface reactivity in catalysis. In this work, we report a facile H₂O₂ etching method to tailor the concentration of surface acidic sites. The molar ratio of H₂O₂/MoS₂ can be altered to tune sulfur defects on the MoS₂ surface for stabilizing Co species to form CoMoS active sites. The optimized Co-MoS₂-2 catalyst, with the highest concentration of acidic sites, exhibits 3.4 times higher activity than the Co-MoS₂-0 sample in the HDO of p-cresol to toluene. It is also found the HDO activity shows a linear relationship with the amount of surface acid (both Lewis and Brønsted acid) over the Co-MoS₂-x catalysts. We believe that the understanding of the role of surface acidity would provide new opportunities for the rational design of efficient Co-MoS₂ catalysts.