Graphene Terahertz Devices for Sensing and Communication

Graphene-based terahertz (THz) devices have emerged as promising platforms for a variety of applications, leveraging graphene's unique optoelectronic properties. This review explores recent advancements in utilizing graphene in THz technology, focusing on two main aspects: THz molecular sensing and THz wave modulation. In molecular sensing, the environment-sensitive THz transmission and emission properties of graphene are utilized for enabling molecular adsorption detection and biomolecular sensing. This capability holds significant potential, from the detection of pesticides to DNA at high sensitivity and selectivity. In THz wave modulation, crucial for next-generation wireless communication systems, graphene demonstrates remarkable potential in absorption modulation when gated. Novel device structures, spectroscopic systems, and metasurface architectures have enabled enhanced absorption and wave modulation. Furthermore, techniques such as spatial phase modulation and polarization manipulation have been explored. From sensing to communication, graphene-based THz devices present a wide array of opportunities for future research and development. Finally, advancements in sensing techniques not only enhance biomolecular analysis but also contribute to optimizing graphene's properties for communication by enabling efficient modulation of electromagnetic waves. Conversely, developments in communication strategies inform and enhance sensing capabilities, establishing a mutually beneficial relationship.

Defect Passivation of 2D Semiconductors by Fixating Chemisorbed Oxygen Molecules via h-BN Encapsulations

Hexagonal boron nitride (h-BN) is a key ingredient for various 2D van der Waals heterostructure devices, but the exact role of h-BN encapsulation in relation to the internal defects of 2D semiconductors remains unclear. Here, it is reported that h-BN encapsulation greatly removes the defect-related gap states by stabilizing the chemisorbed oxygen molecules onto the defects of monolayer WS2 crystals. Electron energy loss spectroscopy (EELS) combined with theoretical analysis clearly confirms that the oxygen molecules are chemisorbed onto the defects of WS2 crystals and are fixated by h-BN encapsulation, with excluding a possibility of oxygen molecules trapped in bubbles or wrinkles formed at the interface between WS2 and h-BN. Optical spectroscopic studies show that h-BN encapsulation prevents the desorption of oxygen molecules over various excitation and ambient conditions, resulting in a greatly lowered and stabilized free electron density in monolayer WS2 crystals. This suppresses the exciton annihilation processes by two orders of magnitude compared to that of bare WS2. Furthermore, the valley polarization becomes robust against the various excitation and ambient conditions in the h-BN encapsulated WS2 crystals.

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.

First-principles study of the oxidation susceptibility of WS2, WSe2, and WTe2 monolayers

The environmental stability of two-dimensional (2D) transition metal dichalcogenide monolayers is of great importance for their applications in electronic, photonic, and energy storage devices. In this study, we focus on understanding the susceptibility of WS2, WSe2, and WTe2 monolayers to oxygen exposure in the form of atomic oxygen and O2 and O3 molecules, respectively. Calculations based on the van der Waals-corrected density functional theory predicted that O2 and O3 molecules are weakly adsorbed on these monolayers, although atomic oxygen prefers chemisorption accompanied by a significant charge transfer from the surface to oxygen. In the physisorbed molecular configurations consisting of O2 and O3, the partially oxidized monolayers retain their geometrical and electronic structures. The calculated transition path as the oxygen approaches the surface shows a high-energy barrier for all cases, thus explaining the photo-induced formation of the oxidized configurations in the experiments. Furthermore, oxidizing the WS2 monolayer is predicted to modify its electronic structure, reducing the band gap with increasing oxygen coverage on the surface. Overall, the calculated results predict the resilience of WS2, WSe2, and WTe2 monolayers against oxygen exposure, thus ensuring stability for devices fabricated with these monolayers.

Role of S-Vacancy Concentration in Air Oxidation of WS2 Single Crystals

Semiconducting transition metal dichalcogenides (TMDs) are a class of two-dimensional materials with potential applications in optoelectronics, spintronics, valleytronics, and quantum information processing. Understanding their stability under ambient conditions is critical for determining their in-air processability during device fabrication and for predicting their long-term device performance stability. While the effects of environmental conditions (i.e., oxygen, moisture, and light) on TMD degradation are well-acknowledged, the role of defects in driving their oxidation remains unclear. We conducted a systematic X-ray photoelectron spectroscopy study on WS2 single crystals with different surface S-vacancy concentrations formed via controlled argon sputtering. Oxidation primarily occurred at defect concentrations ≥ 10%, resulting in stoichiometric WO3 formation, while a stable surface was observed at lower concentrations. Theoretical calculations informed us that single S-vacancies do not spontaneously oxidize, while defect pairing at high vacancy concentrations facilitates O2 dissociation and subsequent oxide formation. Our XPS results also point to vacancy-related structural and electrostatic disorder as the main origin for the p-type characteristics that persists even after oxidation. Despite the complex interplay between defects and TMD oxidation processes, our work unveils scientifically informed guidance for working effectively with TMDs.

Full automation of point defect detection in transition metal dichalcogenides through a dual mode deep learning algorithm

Point defects often appear in two-dimensional (2D) materials and are mostly correlated with physical phenomena. The direct visualisation of point defects, followed by statistical inspection, is the most promising way to harness structure-modulated 2D materials. Here, we introduce a deep learning-based platform to identify the point defects in 2H-MoTe2: synergy of unit cell detection and defect classification. These processes demonstrate that segmenting the detected hexagonal cell into two unit cells elaborately cropped the unit cells: further separating a unit cell input into the Te2/Mo column part remarkably increased the defect classification accuracies. The concentrations of identified point defects were 7.16 × 1020 cm2 of Te monovacancies, 4.38 × 1019 cm2 of Te divacancies and 1.46 × 1019 cm2 of Mo monovacancies generated during an exfoliation process for TEM sample-preparation. These revealed defects correspond to the n-type character mainly originating from Te monovacancies, statistically. Our deep learning-oriented platform combined with atomic structural imaging provides the most intuitive and precise way to analyse point defects and, consequently, insight into the defect-property correlation based on deep learning in 2D materials.

Atomic insight into the effects of precursor clusters on monolayer WSe2

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been attracting much attention due to their rich physical and chemical properties. At the end of the chemical vapor deposition growth of 2D TMDCs, the adsorption of excess precursor clusters onto the sample is unavoidable, which will have significant effects on the properties of TMDCs. This is a concern to the academic community. However, the structures of the supported precursor clusters and their effects on the properties of the prepared 2D TMDCs are still poorly understood. Herein, taking monolayer WSe2 as the prototype, we investigated the structure and electronic properties of SeN, WN (N = 1-8), and W8-NSeN (N = 1-7) clusters adsorbed on monolayer WSe2 to gain atomic insight into the precursor cluster adsorption. In contrast to W clusters that tightly bind to the WSe2 surface, Se clusters except for Se1 and Se2 are weakly adsorbed onto WSe2. The interaction between W8-NSeN (N = 1-7) clusters and the WSe2 monolayer decreases with the increase in the Se/W ratio and eventually becomes van der Waals interaction for W1Se7. According to the phase diagram, increasing the Se/W ratio by changing the experimental conditions will increase the ratio of SeN and W1Se7 clusters in the precursor, which can be removed by proper annealing after growth. W clusters induce lots of defect energy levels in the band gap region, while the adsorption of W1Se7 and SeN clusters (N = 3-6, 8) promotes the spatial separation of photo generated carriers at the interface, which is important for optoelectronic applications. Our results indicate that by controlling the Se/W ratio, the interaction between the precursor clusters and WSe2 as well as the electronic properties of the prepared WSe2 monolayer can be effectively tuned, which is significant for the high-quality growth and applications of WSe2.

Photo-oxidative Crack Propagation in Transition Metal Dichalcogenides

Monolayered transition-metal dichalcogenides (TMDs) are easily exposed to air, and their crystal quality can often be degraded via oxidation, leading to poor electronic and optical device performance. The degradation becomes more severe in the presence of defects, grain boundaries, and residues. Here, we report crack propagation in pristine TMD monolayers grown by chemical vapor deposition under ambient conditions and light illumination. Under a high relative humidity (RH) of ∼60% and white light illumination, the cracks appear randomly. Photo-oxidative cracks gradually propagated along the grain boundaries of the TMD monolayers. In contrast, under low RH conditions of ∼2%, cracks were scarcely observed. Crack propagation is predominantly attributed to the accumulation of water underneath the TMD monolayers, which is preferentially absorbed by hygroscopic alkali metal-based precursor residues. Crack propagation is further accelerated by the cyclic process of photo-oxidation in a basic medium, leading to localized tensile strain. We also found that such crack propagation is prevented after the removal of alkali metals via the transfer of the sample to other substrates.

Analysis of p-Type Doping in Graphene Induced by Monolayer-Oxidized TMDs

Doping is one of the most difficult technological challenges for realizing reliable two-dimensional (2D) material-based semiconductor devices, arising from their ultrathinness. Here, we systematically investigate the impact of different types of nonstoichiometric solid MOx (M are W or Mo) dopants obtained by oxidizing transition metal dichalcogenides (TMDs: WSe2 or MoS2) formed on graphene FETs, which results in p-type doping along with disorders. From the results obtained in this study, we were able to suggest an analytical technique to optimize the optimal UV-ozone (UVO) treatment to achieve high p-type doping concentration in graphene FETs (∼2.5 × 1013 cm-2 in this study) without generating defects, mainly by analyzing the time dependency of D and D' peaks measured by Raman spectroscopy. Furthermore, an analysis of the structure of graphene sheets using TEM indicates that WOx plays a better protective role in graphene, compared to MoOx, suggesting that WOx is more effective for preventing the degradation of graphene during UVO treatment. To enhance the practical application aspect of our work, we have fabricated a graphene photodetector by selectively doping the graphene through oxidized TMDs, creating a p-n junction, which resulted in improved photoresponsivity compared to the intrinsic graphene device. Our results offer a practical guideline for the utilization of surface charge transfer doping of graphene toward CMOS applications.

Localized coherent phonon generation in monolayer MoSe2 from ultrafast exciton trapping at shallow traps

We report spectroscopic evidence for the ultrafast trapping of band edge excitons at defects and the subsequent generation of defect-localized coherent phonons (CPs) in monolayer MoSe2. While the photoluminescence measurement provides signals of exciton recombination at both shallow and deep traps, our time-resolved pump-probe spectroscopy on the sub-picosecond time scale detects localized CPs only from the ultrafast exciton trapping at shallow traps. Based on occupation-constrained density functional calculations, we identify the Se vacancy and the oxygen molecule adsorbed on a Se vacancy as the atomistic origins of deep and shallow traps, respectively. Establishing the correlations between the defect-induced ultrafast exciton trapping and the generation of defect-localized CPs, our work could open up new avenues to engineer photoexcited carriers through lattice defects in two-dimensional materials.

Fabrication of p-type 2D single-crystalline transistor arrays with Fermi-level-tuned van der Waals semimetal electrodes

High-performance p-type two-dimensional (2D) transistors are fundamental for 2D nanoelectronics. However, the lack of a reliable method for creating high-quality, large-scale p-type 2D semiconductors and a suitable metallization process represents important challenges that need to be addressed for future developments of the field. Here, we report the fabrication of scalable p-type 2D single-crystalline 2H-MoTe2 transistor arrays with Fermi-level-tuned 1T'-phase semimetal contact electrodes. By transforming polycrystalline 1T'-MoTe2 to 2H polymorph via abnormal grain growth, we fabricated 4-inch 2H-MoTe2 wafers with ultra-large single-crystalline domains and spatially-controlled single-crystalline arrays at a low temperature (~500 °C). Furthermore, we demonstrate on-chip transistors by lithographic patterning and layer-by-layer integration of 1T' semimetals and 2H semiconductors. Work function modulation of 1T'-MoTe2 electrodes was achieved by depositing 3D metal (Au) pads, resulting in minimal contact resistance (~0.7 kΩ·μm) and near-zero Schottky barrier height (~14 meV) of the junction interface, and leading to high on-state current (~7.8 μA/μm) and on/off current ratio (~105) in the 2H-MoTe2 transistors.

Sonication-assisted liquid phase exfoliation of two-dimensional CrTe3 under inert conditions

Liquid phase exfoliation (LPE) has been used for the successful fabrication of nanosheets from a large number of van der Waals materials. While this allows to study fundamental changes of material properties' associated with reduced dimensions, it also changes the chemistry of many materials due to a significant increase of the effective surface area, often accompanied with enhanced reactivity and accelerated oxidation. To prevent material decomposition, LPE and processing in inert atmosphere have been developed, which enables the preparation of pristine nanomaterials, and to systematically study compositional changes over time for different storage conditions. Here, we demonstrate the inert exfoliation of the oxidation-sensitive van der Waals crystal, CrTe3. The pristine nanomaterial was purified and size-selected by centrifugation, nanosheet dimensions in the fractions quantified by atomic force microscopy and studied by Raman, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX) and photo spectroscopic measurements. We find a dependence of the relative intensities of the CrTe3 Raman modes on the propagation direction of the incident light, which prevents a correlation of the Raman spectral profile to the nanosheet dimensions. XPS and EDX reveal that the contribution of surface oxides to the spectra is reduced after exfoliation compared to the bulk material. Further, the decomposition mechanism of the nanosheets was studied by time-dependent extinction measurements after water titration experiments to initially dry solvents, which suggest that water plays a significant role in the material decomposition.

Atomic-Scale Mechanisms of MoS2 Oxidation for Kinetic Control of MoS2/MoO3 Interfaces

Oxidation of transition metal dichalcogenides (TMDs) occurs readily under a variety of conditions. Therefore, understanding the oxidation processes is necessary for successful TMD handling and device fabrication. Here, we investigate atomic-scale oxidation mechanisms of the most widely studied TMD, MoS₂. We find that thermal oxidation results in α-phase crystalline MoO₃ with sharp interfaces, voids, and crystallographic alignment with the underlying MoS₂. Experiments with remote substrates prove that thermal oxidation proceeds via vapor-phase mass transport and redeposition, a challenge to forming thin, conformal films. Oxygen plasma accelerates the kinetics of oxidation relative to the kinetics of mass transport, forming smooth and conformal oxides. The resulting amorphous MoO₃ can be grown with subnanometer to several-nanometer thickness, and we calibrate the oxidation rate for different instruments and process parameters. Our results provide quantitative guidance for managing both the atomic scale structure and thin-film morphology of oxides in the design and processing of TMD devices.

The effects of intercalated environmental gas molecules on carrier dynamics in WSe2/WS2 heterostructures

Effective tuning of carrier dynamics in two-dimensional (2D) materials is significant for multi-scene device applications. Using first-principles and ab initio nonadiabatic molecular dynamics calculations, the kinetics of O₂, H₂O, and N₂ intercalation into 2D WSe₂/WS₂ van der Waals heterostructures and its effect on carrier dynamics have been comprehensively explored. It is found that the O₂ molecule prefers to dissociate into atomic O atoms spontaneously after intercalation of WSe₂/WS₂ heterostructures, whereas H₂O and N₂ molecules remain intact. O₂ intercalation significantly speeds up the electron separation process, while H₂O intercalation largely speeds up the hole separation process. The lifetime of excited carriers can be prolonged by O₂ or H₂O or N₂ intercalations. These intriguing phenomena can be attributed to the effect of interlayer coupling, and the underlying physical mechanism for tuning the carrier dynamics is fully discussed. Our results provide useful guidance for the experimental design of 2D heterostructures for optoelectronic applications in photocatalysts and solar energy cells.

Zinc Single Atom Confinement Effects on Catalysis in 1T-Phase Molybdenum Disulfide

Active sites are atomic sites within catalysts that drive reactions and are essential for catalysis. Spatially confining guest metals within active site microenvironments has been predicted to improve catalytic activity by altering the electronic states of active sites. Using the hydrogen evolution reaction (HER) as a model reaction, we show that intercalating zinc single atoms between layers of 1T-MoS2 (Zn SAs/1T-MoS2) enhances HER performance by decreasing the overpotential, charge transfer resistance, and kinetic barrier. The confined Zn atoms tetrahedrally coordinate to basal sulfur (S) atoms and expand the interlayer spacing of 1T-MoS2 by ∼3.4%. Under confinement, the Zn SAs donate electrons to coordinated S atoms, which lowers the free energy barrier of H* adsorption-desorption and enhances HER kinetics. In this work, which is applicable to all types of catalytic reactions and layered materials, HER performance is enhanced by controlling the coordination geometry and electronic states of transition metals confined within active-site microenvironments.

Utilizing Ultraviolet Photons to Generate Single-Photon Emitters in Semiconductor Monolayers

The understanding and controlled creation of atomic defects in semiconductor transition metal dichalcogenides (TMDs) are highly relevant to their applications in high-performance quantum optics and nanoelectronic devices. Here, we demonstrate a versatile approach in generating single-photon emitters in MoS₂ monolayers using widely attainable UV light. We discover that only defects engendered by UV photons in vacuum exhibit single-photon-emitter characteristics, whereas those created in air lack quantum emission attributes. In combination with theoretical calculations, we assign the defects generated in vacuum to unpassivated sulfur vacancies, whose highly localized midgap states give rise to single-photon emission. In contrast, UV irradiation of the MoS₂ monolayers in air results in oxygen-passivated sulfur vacancies, whose optical properties are likely governed by their pristine band-to-defect band optical transitions. These findings suggest that widely available light sources such as UV light can be utilized for creating quantum photon sources in TMDs.

Nanostructured 2D WS2@PANI nanohybrids for electrochemical energy storage

2D materials are interesting flat nanoplatforms for the implementation of different electrochemical processes, due to the high surface area and tunable electronic properties. 2D transition metal dichalcogenides (TMDs) can be produced through convenient top-down liquid-phase exfoliation (LPE) methods and present capacitive behaviour that can be exploited for energy storage applications. However, in their thermodynamically stable 2H crystalline phase, they present poor electrical conductivity, being this phase a purely semiconducting one. Combination with conducting polymers like polyaniline (PANI), into nanohybrids, can provide better properties for the scope. In this work, we report on the preparation of 2D WS2@PANI hybrid materials in which we exploit the LPE TMD nanoflakes as scaffolds, onto which induce the in-situ aniline polymerization and thus achieve porous architectures, with the help of surfactants and sodium chloride acting as templating agents. We characterize these species for their capacitive behaviour in neutral pH, achieving maximum specific capacitance of 160 F/g at a current density of 1 A/g, demonstrating the attractiveness of similar nanohybrids for future use in low-cost, easy-to-make supercapacitor devices.

Degradation Chemistry and Kinetic Stabilization of Magnetic CrI3

The discovery of the intrinsic magnetic order in single-layer chromium trihalides (CrX₃, X = I, Br, and Cl) has drawn intensive interest due to their potential application in spintronic devices. However, the notorious environmental instability of this class of materials under ambient conditions renders their device fabrication and practical application extremely challenging. Here, we performed a systematic investigation of the degradation chemistry of chromium iodide (CrI₃), the most studied among CrX₃ families, via a joint spectroscopic and microscopic analysis of the structural and composition evolution of bulk and exfoliated nanoflakes in different environments. Unlike other air-sensitive 2D materials, CrI₃ undergoes a pseudo-first-order hydrolysis in the presence of pure water toward the formation of amorphous Cr(OH)₃ and hydrogen iodide (HI) with a rate constant of k_I = 0.63 day^-1 without light. In contrast, a faster pseudo-first-order surface oxidation of CrI₃ occurs in a pure O₂ environment, generating CrO₃ and I₂ with a large rate constant of k_Cr = 4.2 day^-1. Both hydrolysis and surface oxidation of CrI₃ can be accelerated via light irradiation, resulting in its ultrafast degradation in air. The new chemical insights obtained allow for the design of an effective stabilization strategy for CrI₃ with preserved optical and magnetic properties. The use of organic acid solvents (e.g., formic acid) as reversible capping agents ensures that CrI₃ nanoflakes remain stable beyond 1 month due to the effective suppression of both hydrolysis and oxidation of CrI₃.

Borophane Polymorphs

Hydrogenated borophenes─borophanes─have recently been synthesized as a new platform for studying low-dimensional borides, but most of their lattice structures remain unknown. Here, we determine the structures of borophane polymorphs on Ag(111) by performing extensive structural search using the cluster expansion method augmented with first-principles calculations. Our results reveal rich borophane polymorphs whose stability depends on hydrogen pressure. At relatively low hydrogen pressures, borophane structures with rhombic patterns of two-center-two-electron B-H bonds are energetically preferred, in excellent agreement with two experimentally observed phases. In a wider range of hydrogen pressures, the structure with a combination of two-center-two-electron B-H and three-center-two-electron B-H-B bonds is a deep global minimum, rationalizing its experimental prevalence. For all these borophane polymorphs, their hydrogen "skin" raises the energy barriers for oxidation above 1.1 eV, while their work functions can be reduced by more than 0.5 eV through varying the hydrogen coverage.

Anisotropic point defects in rhenium diselenide monolayers

Point defects in 1T″ anisotropic ReSe2 offer many possibilities for defect engineering, which could endow this two-dimensional semiconductor with new functionalities, but have so far received limited attention. Here, we systematically investigate a full spectrum of point defects in ReSe2, including vacancies (VSe1-4), isoelectronic substitutions (OSe1-4 and SSe1-4), and antisite defects (SeRe1-2 and ReSe1-4), by atomic-scale electron microscopy imaging and density functional theory (DFT) calculations. Statistical counting reveals a diverse density of various point defects, which are further elaborated by the formation energy calculations. Se vacancy dynamics was unraveled by in-situ electron beam irradiation. DFT calculations reveal that vacancies at Se sites notably introduce in-gap states, which are largely quenched upon isoelectronic substitutions (O and S), whereas antisite defects introduce localized magnetic moments. These results provide atomic-scale insight of atomic defects in 1T″-ReSe2, paving the way for tuning the electronic structure of anisotropic ReSe2 via defect engineering.

Controlling Carrier Transport in Vertical MoTe2/MoS2 van der Waals Heterostructures

Two-dimensional (2D) transition-metal dichalcogenide (TMDC)-based semiconducting van der Waals (vdW) heterostructures are considered as potential candidates for next-generation nanoelectronics due to their unique and tunable properties. Controlling the carrier type and band alignment in 2D TMDCs and their vdW heterostructures is critical for realizing heterojunctions with the desired performances and functionalities. In this report, controlling the carrier type and band alignment in a vertical MoTe₂/MoS₂ heterojunction is presented via thickness engineering and surface charge transfer doping. A highly rectifying p-n diode and a nonrectifying n-n junction are obtained with different MoTe₂ thicknesses due to their different doping conditions. A vertical tunnel diode is subsequently achieved with a controlled oxygen plasma treatment, which selectively induces degenerate p-type doping to MoTe₂, whereas the intrinsic n-type characteristic of MoS₂ is maintained during the treatment. These techniques to realize multifunctional diodes are universal and applicable to emerging nanoelectronics based on 2D materials.

Atomic-Layer-Deposition-Based 2D Transition Metal Chalcogenides: Synthesis, Modulation, and Applications

Transition metal chalcogenides (TMCs) are a large family of 2D materials with different properties, and are promising candidates for a wide range of applications such as nanoelectronics, sensors, energy conversion, and energy storage. In the research of new materials, the development and investigation of industry-compatible synthesis techniques is of key importance. In this respect, it is important to study 2D TMC materials synthesized by the atomic layer deposition (ALD) technique, which is widely applied in industries. In addition to the synthesis of 2D TMCs, ALD is used to modulate the characteristic of 2D TMCs such as their carrier density and morphology. So far, the improvement of thin film uniformity without oxidation and the synthesis of low-dimensional nanomaterials on 2D TMCs have been the research focus. Herein, the synthesis and modulation of 2D TMCs by ALD is described, and the characteristics of ALD-based TMCs used in nanoelectronics, sensors, and energy applications are discussed.

Defect Engineering of Out-of-Plane Charge Transport in van der Waals Heterostructures for Bi-Direction Photoresponse

Defects are ubiquitous in two-dimensional (2D) transition-metal dichalcogenides (TMDs), generated by the initial growth- or the postprocessing. However, the defects may play negative roles in the photoelectronic properties of TMDs due to the reduction of in-plane transport of carriers. In this work, we demonstrate that the Se-vacancy defects in MoSe₂ side of the van der Waal heterostructure is able to switch direction of out-of-plane charge transport. Photoresponse spectra showed defect density enable modified surface potential of MoSe_2-x, leading to the barrier reverse between graphene and MoSe_2-x and switches of the photoresponse from the negative to the positive. This unexpected property stemmed from appearance of midgap states by defects at heterostructure, as demonstrated by the density functional theory calculation and scanning tunneling microscope results. MoSe_2-0.2/graphene heterostructure has a broadband response ranging from 450 to 1064 nm and exhibits comparable or higher positive responsivity (5.4 × 10³ A/W to -15.3 × 10³ A/W at 632.8 and 5.7 × 10³ A/W to -1.2 × 10³ A/W at 1064 nm) to the negative one of the pristine MoSe₂/graphene. Based on defect-engineered heterostructures, we construct optoelectronic OR and AND logic devices with a broadband operation. Our work elucidates an alternative avenue to tailor the out-of-plane charge transport in TMD-based heterostructure through defects, and potentially invokes applicable utilization for 2D photodetectors and optoelectronic logic gates.

Electronic work function modulation of phosphorene by thermal oxidation

In this study, we evaluate the variation of the work function of phosphorene during thermal oxidation at different temperatures. The ultraviolet photoelectron spectroscopy results show an N-shaped behaviour that is explained by the oxidation process and the dangling-to-interstitial conversion at elevated temperatures. The exfoliation degree and x-ray photoelectron spectroscopy confirm the formation of native oxides in the top-most layer that passivates the material. Ex-situ XPS reveals the full oxidation of monolayers at temperatures higher than 140 °C, but few-layer phosphorene withstands the thermal oxidation even up to 200 °C with slight modifications of the A 2 g/A 1 g and A 2 g/B 2g vibrational mode ratios and a weak fluorescence in the Raman spectra of the heat-treated samples.

The Role of Additives in Suppressing the Degradation of Liquid-Exfoliated WS2 Monolayers

Group VI transition metal dichalcogenides (TMDs) are considered to be chemically widely inert, but recent reports point toward an oxidation of monolayered sheets in ambient conditions, due to defects. To date, the degradation of monolayered TMDs is only studied on individual, substrate-supported nanosheets with varying defect type and concentration, strain, and in an inhomogeneous environment. Here, degradation kinetics of WS₂ nanosheet ensembles in the liquid phase are investigated through photoluminescence measurements, which selectively probe the monolayers. Monolayer-enriched WS₂ dispersions are produced with varying lateral sizes in the two common surfactant stabilizers sodium cholate (SC) and sodium dodecyl sulfate (SDS). Well-defined degradation kinetics are observed, which enable the determination of activation energies of the degradation and decouple photoinduced and thermal degradation. The thermal degradation is slower than the photoinduced degradation and requires higher activation energy. Using SC as surfactant, it is sufficiently suppressed. The photoinduced degradation can be widely prevented through chemical passivation achieved through the addition of cysteine which, on the one hand, coordinates to defects on the nanosheets and, on the other hand, stabilizes oxides on the surface, which shield the nanosheets from further degradation.

Bidirectional doping of two-dimensional thin-layer transition metal dichalcogenides using soft ammonia plasma

Because of suitable band gap and high mobility, two-dimensional transition metal dichalcogenide (TMD) materials are promising in future microelectronic devices. However, controllable p-type and n-type doping of TMDs is still a challenge. Herein, we develop a soft plasma doping concept and demonstrate both n-type and p-type doping for TMDs including MoS₂ and WS₂ through adjusting the plasma working parameters. In particular, p-type doping of MoS₂ can be realized when the radio frequency (RF) power is relatively small and the processing time is short: the off-state current increases from ∼10^-10 A to ∼10^-8 A, the threshold voltage is positively shifted from -26.2 V to 8.3 V, and the mobility increases from 7.05 cm² V^-1 s^-1 to 16.52 cm² V^-1 s^-1. Under a relatively large RF power and long processing time, n-type doping was realized for MoS₂: the threshold voltage was negatively shifted from 6.8 V to -13.3 V and the mobility is reduced from 10.32 cm² V^-1 s^-1 to 3.2 cm² V^-1 s^-1. For the former, suitable plasma treatment can promote the substitution of N elements for S vacancies and lead to p-type doping, thus reducing the defect density and increasing the mobility value. For the latter, due to excessive plasma treatment, more S vacancies will be produced, leading to heavier n-type doping as well as a decrease in mobility. We confirm the results by systematically analyzing the optical, compositional, thickness and structural characteristics of the samples before and after such soft plasma treatments via Raman, photoluminescence (PL), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements. Due to its nondestructive and expandable nature and compatibility with the current microelectronics industry, this potentially generic method may be used as a reliable technology for the development of diverse and functional TMD-based devices.

Rapid Degradation of the Electrical Properties of 2D MoS2 Thin Films under Long-Term Ambient Exposure

The MoS₂ thin film has attracted a lot of attention due to its potential applications in flexible electronics, sensors, catalysis, and heterostructures. Understanding the effect of long-term ambient exposure on the electrical properties of the thin film is important for achieving many overreaching goals of this material. Here, we report for the first time a systematic study of electrical property variation and stability of MoS₂ thin films under ambient exposure of up to a year. The MoS₂ thin films were grown via the sulfurization of 6 nm thick molybdenum films. We found that the resistance of the samples increases by 114% just in 4 weeks and 430% in 4 months and they become fully insulated in a year of ambient exposure. The dual-sweep current-voltage (I-V) characteristic shows hysteretic behavior for a 4-month-old sample which further exhibits pronounced nonlinear I-V curves and hysteretic behavior after 8 months. The X-ray photoelectron spectroscopy measurements show that the MoS₂ thin film gradually oxidizes and 13.1% of MoO₃ and 11.8% oxide of sulfur were formed in 4 months, which further increased to 23.1 and 12.7% in a year, respectively. The oxide of the sulfur peak was not reported in any previous stability studies of exfoliated and chemical vapor deposition-grown MoS₂, suggesting that the origin of this peak is related to the distinct crystallinity of the MoS₂ thin film due to its smaller grain sizes, abundant grain boundaries, and exposed edges. Raman studies show the broadening of E_2g ¹ and A_1g peaks with increasing exposure time, suggesting an increase in the disorder in MoS₂. It is also found that coating the MoS₂ thin film with polymethylmethacrylate can effectively prevent the electrical property degradation, showing only a 6% increase in resistance in 4 months and 40% over a year of ambient exposure.

Thermodynamic Perspective on the Oxidation of Layered Materials and Surface Oxide Amelioration in 2D Devices

Surface oxidation is an unneglectable problem for 2D semiconductors because it hinders the practical application of 2D material-based devices. In this research, the oxidation of layered materials is investigated by a thermodynamic approach to verify their oxidation tendency. It was found that almost all 2D materials are thermodynamically unstable in the presence of oxygen at room temperature. Two potential solutions for surface oxidation are proposed in this work: (i) the conversion of the surface oxides to functional oxides through the deposition of active metals and (ii) the recovery of original 2D materials from the surface oxides by 2D material heterostructure formation with the same chalcogen group. Supported by thermodynamic calculations, both approaches are feasible to ameliorate the surface oxides of 2D materials by the appropriate selection of metals for deposition or 2D materials for heterostructure formation. Thermodynamic data of 64 elements and 75 2D materials are included and compared in this research, which can improve gate insulator or electrode contact material selection in 2D devices to solve the surface oxidation issue. For instance, yttrium and titanium are good candidates for surface oxide conversion, while zirconium and hafnium chalcogenide can trigger the recovery of original 2D materials from their surface oxides. The systematic diagrams presented in this work can serve as a guideline for considering surface oxidation in future device fabrication from various 2D materials.

Oxidative etching of S-vacancy defective MoS2 monolayer upon reaction with O2

The reactions of O2 with S vacancy sites within a MoS2 monolayer were investigated using density functional theory calculations. We considered the following defects: single S vacancy, double S vacancy, two adjacent S vacancies and two S vacancies separated by a sulphur atom. We found that the surface distribution of S vacancy sites plays a key role in determining the surface reactivity towards O2. We observed the desorption of SO2 only for the last vacancy distribution. For the other cases, the surface becomes passivated with very stable structures having O atoms on the original vacancy sites and in some cases an SO group in an adjacent position. The ab initio molecular dynamics simulations showed that the impingement of the O2 molecule on an S vacancy site produces a stable chemisorbed O2 molecule with an upright configuration. The surface reactions initiate after the O2 molecule switches to the lying-down configuration which favours the breakage of the O-O bond and the concurrent formation of S-O bonds. In the most reactive vacancy site configuration, the dissociation of the first O2 molecule produces an SO intermediate which finally leads to desorption of SO2 after oxygen abstraction from the other adjacent O2 molecule. The formation of a MoO3 moiety within the monolayer was also observed in the molecular dynamic simulations at higher oxidation levels.

Identification of Point Defects in Atomically Thin Transition-Metal Dichalcogenide Semiconductors as Active Dopants

Selective doping in semiconductors is essential not only for monolithic integrated circuity fabrications but also for tailoring their properties including electronic, optical, and catalytic activities. Such active dopants are essentially point defects in the host lattice. In atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDCs), the roles of such point defects are particularly critical in addition to their large surface-to-volume ratio, because their bond dissociation energy is relatively weaker, compared to elemental semiconductors. In this Mini Review, we review recent advances in the identifications of diverse point defects in 2D TMDC semiconductors, as active dopants, toward the tunable doping processes, along with the doping methods and mechanisms in literature. In particular, we discuss key issues in identifying such dopants both at the atomic scales and the device scales with selective examples. Fundamental understanding of these point defects can hold promise for tunability doping of atomically thin 2D semiconductor platforms.

Interfacial Doping Effects in Fluoropolymer-Tungsten Diselenide Composites Providing High-Performance P-Type Transistors

In this study, we investigated the p-doping effects of a fluoropolymer, Cytop, on tungsten diselenides (WSe₂). The hole current of the Cytop–WSe₂ field-effect transistor (FET) was boosted by the C–F bonds of Cytop having a strong dipole moment, enabling increased hole accumulation. Analysis of the observed p-doping effects using atomic force microscopy (AFM) and Raman spectroscopy shed light on the doping mechanism. Moreover, Cytop reduces the electrical instability by preventing the adsorption of ambient molecules on the WSe₂ surface. Annealing Cytop deposited on WSe₂ eliminated the possible impurities associated with adsorbates (i.e., moisture and oxygen) that act as traps on the surface of WSe₂. After thermal annealing, the Cytop–WSe₂ FET afforded higher p-type conductivity and reduced hysteresis. The combination of the Cytop–WSe₂ FET with annealing provides a promising method for obtaining high-performance WSe₂ p-type transistors.

Cooling and diffusion characteristics of a hot carrier in the monolayer WS2

The characteristics of a hot carrier distributed in the C excitonic state of the monolayer WS₂ is investigated by exploiting the transient absorption (TA) spectroscopy. The hot carrier cooling lifetime gradually prolongs from 0.58 ps to 2.68 ps with the absorbed photon flux owing to the hot phonon bottleneck effect, as the excitation photon energy is 2.03 eV. Meanwhile, the normalized TA spectra shows that the spectral feature of hot carriers is different from that of normal carriers. Based on the modified Lennard-Jones model, the average distance among hot carriers can be estimated according to the peak shift of TA spectra and the diffusion velocity can also be calculated simultaneously. The hot carrier limits the diffusion of the photo-generated carrier at the initial several picoseconds. These results help people to elucidate the hot carrier dynamics in 2D TMDCs and give guidance on the designing and optimizing the TMDC-based electronic devices of high performance.

Defect Engineering of Two-Dimensional Transition-Metal Dichalcogenides: Applications, Challenges, and Opportunities

Atomic defects, being the most prevalent zero-dimensional topological defects, are ubiquitous in a wide range of 2D transition-metal dichalcogenides (TMDs). They could be intrinsic, formed during the initial sample growth, or created by postprocessing. Despite the majority of TMDs being largely unaffected after losing chalcogen atoms in the outermost layer, a spectrum of properties, including optical, electrical, and chemical properties, can be significantly modulated, and potentially invoke applicable functionalities utilized in many applications. Hence, controlling chalcogen atomic defects provides an alternative avenue for engineering a wide range of physical and chemical properties of 2D TMDs. In this article, we review recent progress on the role of chalcogen atomic defects in engineering 2D TMDs, with a particular focus on device performance improvements. Various approaches for creating chalcogen atomic defects including nonstoichiometric synthesis and postgrowth treatment, together with their characterization and interpretation are systematically overviewed. The tailoring of optical, electrical, and magnetic properties, along with the device performance enhancement in electronic, optoelectronic, chemical sensing, biomedical, and catalytic activity are discussed in detail. Postformation dynamic evolution and repair of chalcogen atomic defects are also introduced. Finally, we offer our perspective on the challenges and opportunities in this field.

Growth Kinetics and Atomistic Mechanisms of Native Oxidation of ZrSxSe2-x and MoS2 Crystals

A thorough understanding of native oxides is essential for designing semiconductor devices. Here, we report a study of the rate and mechanisms of spontaneous oxidation of bulk single crystals of ZrS_xSe_2-x alloys and MoS₂. ZrS_xSe_2-x alloys oxidize rapidly, and the oxidation rate increases with Se content. Oxidation of basal surfaces is initiated by favorable O₂ adsorption and proceeds by a mechanism of Zr-O bond switching, that collapses the van der Waals gaps, and is facilitated by progressive redox transitions of the chalcogen. The rate-limiting process is the formation and out-diffusion of SO₂. In contrast, MoS₂ basal surfaces are stable due to unfavorable oxygen adsorption. Our results provide insight and quantitative guidance for designing and processing semiconductor devices based on ZrS_xSe_2-x and MoS₂ and identify the atomistic-scale mechanisms of bonding and phase transformations in layered materials with competing anions.

Atomic-Scale Edge Morphology, Stability, and Oxidation of Single-Layer 2H-TaS2

Tantalum disulphide belongs to the group of transition metal dichalcogenides (TMDs) and has attracted attention for its unique structural, electronic, and catalytic properties. Herein, we report the edge properties of single-layer 2H-TaS₂ studied by using density functional theory calculations, because the knowledge of the edge morphology, stability, and surface energy is essential for the determination of nanoparticle shapes and understanding the nature of catalytically active sites. We calculate the grand canonical potential of TaS₂ clusters having various edge morphologies to evaluate the edge energies of the Ta-edge and S-edge terminated surfaces. Under S-rich conditions, the most likely shape of TaS₂ is a deformed hexagon dominated by the Ta-edge covered by S monomers, while the triangular shape is preferred under S-poor conditions. Exposed edges of the single-layer TaS₂ are susceptible to oxidation in air because both oxygen adsorption and substitution at the edge are strongly exothermic, -0.96 and -2.20 eV for single O atom, respectively. The XPS calculation shows that specific initial steps of oxidative process (adsorption, vacancy creation, substitution) are unlikely to be distinguished in the XPS spectra due to small shift of respective binding energies, but initial edge oxidation of TaS₂ should be observable by an asymmetry of the Ta 4f doublet towards higher binding energies.

Surface Defect Engineering of MoS2 for Atomic Layer Deposition of TiO2 Films

In this manuscript, we combine experimental and computational approaches to study the atomic layer deposition (ALD) of dielectrics on MoS₂ surfaces for a very common class of ALD precursors, the alkylamines. More specifically, we study the thermal ALD of TiO₂ from TDMAT and H₂O. Depositions on as-produced chemical vapor deposition MoS₂ flakes result in discontinuous films. Surface treatment with mercaptoethanol (ME) does not improve the surface coverage, and DFT calculations show that ME reacts very weakly with the MoS₂ surface. However, creation of sulfur vacancies on the MoS₂ surface using Ar ion beam irradiation results in much improved surface coverage for films with a nominal thickness of 6 nm, and the calculations show that TDMAT reacts moderately with either single or extended sulfur vacancies. ME also reacts with the vacancies, and defect-rich surfaces treated with ME provide an equally good surface for the nucleation of ALD TiO₂ films. The computational studies however reveal that the creation of surface vacancies results in the introduction of gap states that may deteriorate the electronic properties of the stack. Treatment with ME results in the complete removal of the gap states originating from the most commonly found single vacancies and reduces substantially the density of states for double and line vacancies. As a result, we provide a pathway for the deposition of high-quality ALD dielectrics on the MoS₂ surfaces, which is required for the successful integration of these 2D materials in functional devices.

Effect of Adventitious Carbon on Pit Formation of Monolayer MoS2

Forming pits on molybdenum disulfide (MoS₂ ) monolayers is desirable for (opto)electrical, catalytic, and biological applications. Thermal oxidation is a potentially scalable method to generate pits on monolayer MoS₂ , and pits are assumed to preferentially form around undercoordinated sites, such as sulfur vacancies. However, studies on thermal oxidation of MoS₂ monolayers have not considered the effect of adventitious carbon (C) that is ubiquitous and interacts with oxygen at elevated temperatures. Herein, the effect of adventitious C on the pit formation on MoS₂ monolayers during thermal oxidation is studied. The in situ environmental transmission electron microscopy measurements herein show that pit formation is preferentially initiated at the interface between adventitious C nanoparticles and MoS₂ , rather than only sulfur vacancies. Density functional theory (DFT) calculations reveal that the C/MoS₂ interface favors the sequential adsorption of oxygen atoms with facile kinetics. These results illustrate the important role of adventitious C on pit formation on monolayer MoS₂ .

Insight into the Na adsorption on WSe2xS2(1-x)monolayers: a hybrid functional investigation

Monolayer WSe2has aroused strong interest as a semiconducting 2D material. The tuning of its electronic properties is important for expanding the range of applications. In this work, by hybrid density functional theory calculations, we investigate the effect of Na adsorption on the electronic and magnetic properties of stoichiometric and defective WSe2monolayers and their WSe2xS2(1-x)alloys. Our results suggest that Na adsorption on defective WSe2xS2(1-x)monolayers would split and partially fill the empty defect states in the gap region inducing a local magnetic moment of 1μBper alkaline atom. The efficient tuning of the electronic and magnetic properties of WSe2monolayer by Na adsorption combined with the Se vacancy and S alloying would benefit its wide applications.

Mechanism and activity of the oxygen reduction reaction on WTe2 transition metal dichalcogenide with Te vacancy

WTe2 transition metal dichalcogenide is a promising candidate for the cathode of proton-exchange membrane fuel cells. In this paper, we investigated the mechanism and activity of the oxygen reduction reaction on the monolayer of the WTe2 transition metal dichalcogenide with Te vacancy denoted as WTed 2. By using density functional theory calculations, we studied the reaction intermediates on the surface of WTed 2. The Gibbs free energy was calculated to clarify the thermodynamic properties of the reaction. We discovered that the ORR mechanisms are more favorable outside than inside the vacancy. The ORR activity was found to be comparable to that of the well-known transition metal electro-catalysts.

In situ transport measurements reveal source of mobility enhancement of MoS2 and MoTe2 during dielectric deposition

Layered transition metal dichalcogenides (TMDs) and other two-dimensional (2D) materials are promising candidates for enhancing the capabilities of complementary metal-oxide-semiconductor (CMOS) technology. Field-effect transistors (FETs) made with 2D materials often exhibit mobilities below their theoretical limit, and strategies such as encapsulation with dielectrics grown by atomic layer deposition (ALD) have been explored to tune carrier concentration and improve mobility. While molecular adsorbates are known to dope 2D materials and influence charge scattering mechanisms, it is not well understood how ALD reactants affect 2D transistors during growth, motivating in situ or operando studies. Here, we report electrical characterization of MoS₂ and MoTe₂ FETs during ALD of MoO_x. The field effect mobility improves significantly within the first five cycles of ALD growth using Mo(NMe₂)₄ as the metal-organic precursor and H₂O as the oxidant. Analyses of the in situ transconductance at the growth temperature and ex situ variable temperature transconductance measurements indicate that the majority of the mobility enhancement observed at the beginning of dielectric growth is due to screening of charged impurity scattering by the adlayer. Control experiments show that exposure to only H₂O or O₂ induces more modest and reversible electronic changes in MoTe2 FETs, indicating that negligible oxidation of the TMD takes place during the ALD process. Due to the strong influence of the first <2 nm of deposition, when the dielectric adlayer may be discontinuous and still evolving in stoichiometry, this work highlights the need for further assessment of nucleation layers and initial deposition chemistry, which may be more important than the bulk composition of the oxide itself in optimizing performance and reproducibility.

Scanning Probe Lithography Patterning of Monolayer Semiconductors and Application in Quantifying Edge Recombination

Scanning probe lithography is used to directly pattern monolayer transition metal dichalcogenides (TMDs) without the use of a sacrificial resist. Using an atomic-force microscope, a negatively biased tip is brought close to the TMD surface. By inducing a water bridge between the tip and the TMD surface, controllable oxidation is achieved at the sub-100 nm resolution. The oxidized flake is then submerged into water for selective oxide removal which leads to controllable patterning. In addition, by changing the oxidation time, thickness tunable patterning of multilayer TMDs is demonstrated. This resist-less process results in exposed edges, overcoming a barrier in traditional resist-based lithography and dry etch where polymeric byproduct layers are often formed at the edges. By patterning monolayers into geometric patterns of different dimensions and measuring the effective carrier lifetime, the non-radiative recombination velocity due to edge defects is extracted. Using this patterning technique, it is shown that selenide TMDs exhibit lower edge recombination velocity as compared to sulfide TMDs. The utility of scanning probe lithography towards understanding material-dependent edge recombination losses without significantly normalizing edge behaviors due to heavy defect generation, while allowing for eventual exploration of edge passivation schemes is highlighted, which is of profound interest for nanoscale electronics and optoelectronics.

Reversible Photoluminescence Tuning by Defect Passivation via Laser Irradiation on Aged Monolayer MoS2

Atomically thin (1L)-MoS₂ emerged as a direct band gap semiconductor with potential optical applications. The photoluminescence (PL) of 1L-MoS₂ degrades due to aging-related defect formation. The passivation of these defects leads to substantial improvement in optical properties. Here, we report the enhancement of PL on aged 1L-MoS₂ by laser treatment. Using photoluminescence and Raman spectroscopy in a gas-controlled environment, we show that the enhancement is associated with efficient adsorption of oxygen on existing sulfur vacancies preceded by removal of adsorbates from the sample's surface. Oxygen adsorption depletes negative charges, resulting in suppression of trions and improved neutral exciton recombination. The result is a 6- to 8-fold increase in PL emission. The laser treatment in this work does not cause any measurable damage to the sample as verified by Raman spectroscopy, which is important for practical applications. Surprisingly, the observed PL enhancement is reversible by both vacuum and ultrafast femtosecond excitation. While the former approach allows switching a designed micropattern on the sample ON and OFF, the latter provides a controllable mean for accurate PL tuning, which is highly desirable for optoelectronic and gas sensing applications.

Contact Engineering High-Performance n-Type MoTe2 Transistors

Semiconducting MoTe₂ is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains underexplored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe₂ transistors with the highest performance to date, including high saturation current (>400 μA/μm at 80 K and >200 μA/μm at 300 K) and relatively low contact resistance (1.2 to 2 kΩ·μm from 80 to 300 K), achieved with Ag contacts and AlO_x encapsulation. We also investigate other contact metals (Sc, Ti, Cr, Au, Ni, Pt), extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer hexagonal boron nitride between MoTe₂ and Sc. These metal-insulator-semiconductor (MIS) contacts partly depin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.

Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides

Chalcogen vacancies are generally considered to be the most common point defects in transition metal dichalcogenide (TMD) semiconductors because of their low formation energy in vacuum and their frequent observation in transmission electron microscopy studies. Consequently, unexpected optical, transport, and catalytic properties in 2D-TMDs have been attributed to in-gap states associated with chalcogen vacancies, even in the absence of direct experimental evidence. Here, we combine low-temperature non-contact atomic force microscopy, scanning tunneling microscopy and spectroscopy, and state-of-the-art ab initio density functional theory and GW calculations to determine both the atomic structure and electronic properties of an abundant chalcogen-site point defect common to MoSe2 and WS2 monolayers grown by molecular beam epitaxy and chemical vapor deposition, respectively. Surprisingly, we observe no in-gap states. Our results strongly suggest that the common chalcogen defects in the described 2D-TMD semiconductors, measured in vacuum environment after gentle annealing, are oxygen substitutional defects, rather than vacancies.

Unveiling chemical reactivity and oxidation of 1T-phased group VI disulfides

Transition metal dichalcogenides (TMDs) are of particular interest because of their unique electrical and optical properties that evolve from the quantum confinement and surface effects. However, their long-term stability in air is proved to be a main concern for practical applications of the ultrathin materials, especially for TMDs with 1T phased structures. Here, we provide an in-depth understanding of the oxidation and degradation mechanisms of monolayers of group VIB disulfides, including TiS₂, ZrS₂, and HfS₂. As the atomic radius of the transitional metals increases, their air stability significantly decreases and the oxidation mechanisms are quite different from one another. In particular, the oxygen induced oxidations initiated at both the surface vacancy sites and edges of ZrS₂ and HfS₂ are studied, while the oxidation of TiS₂ starts at the edges and water plays a crucial role in the continuous oxidation process. Moreover, the defective sites expose the metals for activation and dissociation of either oxygen or water, causing the breakdown of the systems eventually. Meanwhile, these sites can be used as active centers for specific applications in catalysts and surface functionalized materials.

In-plane Aligned Colloidal 2D WS2 Nanoflakes for Solution-Processable Thin Films with High Planar Conductivity

Two-dimensional transition-metal dichalcolgenides (2D-TMDs) are among the most intriguing materials for next-generation electronic and optoelectronic devices. Albeit still at the embryonic stage, building thin films by manipulating and stacking preformed 2D nanosheets is now emerging as a practical and cost-effective bottom-up paradigm to obtain excellent electrical properties over large areas. Herein, we exploit the ultrathin morphology and outstanding solution stability of 2D WS₂ colloidal nanocrystals to make thin films of TMDs assembled on a millimetre scale by a layer-by-layer deposition approach. We found that a room-temperature surface treatment with a superacid, performed with the precise scope of removing the native insulating surfactants, promotes in-plane assembly of the colloidal WS₂ nanoflakes into stacks parallel to the substrate, along with healing of sulphur vacancies in the lattice that are detrimental to electrical conductivity. The as-obtained 2D WS₂ thin films, characterized by a smooth and compact morphology, feature a high planar conductivity of up to 1 μS, comparable to the values reported for epitaxially grown WS₂ monolayers, and enable photocurrent generation upon light irradiation over a wide range of visible to near-infrared frequencies.

Point Defects and Localized Excitons in 2D WSe2

Identifying the point defects in 2D materials is important for many applications. Recent studies have proposed that W vacancies are the predominant point defect in 2D WSe₂, in contrast to theoretical studies, which predict that chalcogen vacancies are the most likely intrinsic point defects in transition metal dichalcogenide semiconductors. We show using first-principles calculations, scanning tunneling microscopy (STM), and scanning transmission electron microscopy experiments that W vacancies are not present in our CVD-grown 2D WSe₂. We predict that O-passivated Se vacancies (O_Se) and O interstitials (O_ins) are present in 2D WSe₂, because of facile O₂ dissociation at Se vacancies or due to the presence of WO₃ precursors in CVD growth. These defects give STM images in good agreement with experiment. The optical properties of point defects in 2D WSe₂ are important because single-photon emission (SPE) from 2D WSe₂ has been observed experimentally. While strain gradients funnel the exciton in real space, point defects are necessary for the localization of the exciton at length scales that enable photons to be emitted one at a time. Using state-of-the-art GW-Bethe-Salpeter-equation calculations, we predict that only O_ins defects give localized excitons within the energy range of SPE in previous experiments, making them a likely source of previously observed SPE. No other point defects (O_Se, Se vacancies, W vacancies, and Se_W antisites) give localized excitons in the same energy range. Our predictions suggest ways to realize SPE in related 2D materials and point experimentalists toward other energy ranges for SPE in 2D WSe₂.