Graphene-Assisted Antioxidation of Tungsten Disulfide Monolayers: Substrate and Electric-Field Effect

Mechanism of local electric oxidation on two-dimensional MoS2 for resistive memory application

The manipulation and mechanism of two-dimensional (2D) transition metal dichalcogenides (TMDs) by external electric field are significant to the photoelectric properties. Herein, the 2D MoS2 nanosheets were oxidized to form MoS2-MoO3 local heterojunctions by an electric field, applied in multistable memristors for the proposal of NanoQR code. A modified thermal oxidation model was derived to reveal the mechanism of local electric oxidation on 2D MoS2. From current-voltage curves, the barrier height of the MoS2 device showed an increase of 0.39 eV due to local oxidation after applying voltage for 480 s. Based on density-functional theory, the increase of barrier height was calculated as 0.38 eV between MoS2-MoS2 and MoS2-MoO3 supercells. The 2D MoS2-MoO3 local heterojunctions were further applied as multistable memory storage at the nanoscale. The findings suggest a novel strategy for controlling local electric oxidation on 2D TMDs to manipulate the properties for the application of photoelectric memory nanodevices.

Noninvasive Photodelamination of van der Waals Semiconductors for High-Performance Electronics

Atomically thin 2D van der Waals semiconductors are promising candidate materials for post-silicon electronics. However, it remains challenging to attain completely uniform monolayer semiconductor wafers free of over-grown islands. Here, the observation of the energy-funneling effect and ambient photodelamination phenomenon in inhomogeneous few-layer WS₂ flakes under low-illumination fluencies down to several nW µm^-2 and its potential as a noninvasive atomic-layer etching strategy for selectively stripping the local excessive overlying islands are reported. Photoluminescent tracking on the photoetching traces reveals relatively fast etching rates of around 0.3-0.8 µm min^-1 at varied temperatures and an activation energy of 1.7 eV. By using crystallographic and electronic characterization, the noninvasive nature of the low-power photodelamination and the highly preserved lattice quality are also confirmed in the as-etched monolayer products, featuring a comparable density of atomic defects (≈4.2 × 10¹³ cm^-2 ) to pristine flakes and a high electron mobility of up to 80 cm² V^-1 s^-1 at room temperature. This approach opens a noninvasive postetching route for thickness uniformity management in 2D van der Waals semiconductor wafers for electronic applications.

2D Materials-based Electrochemical Triboelectric Nanogenerators

The integration of 2D materials in triboelectric nanogenerators (TENGs) is known to increase the mechanical-to-electrical power conversion efficiency. 2D materials are used in TENGs with multiple roles as triboelectric material, charge-trapping fillers, or as electrodes. Here, novel TENGs based on few-layers graphene (FLG) electrodes and stable gel electrolytes composed of liquid phase exfoliated 2D-transition metal dichalcogenides and polyvinyl alcohol are developed. TENGs embedding FLG and gel composites show competitive open-circuit voltage (≈ 300 V), instant peak power (530 mW m^-2 ), and stability (> 11 months). These values correspond to a seven-fold higher electrical output compared to TENGs embedding bare FLG electrodes. It is demonstrated that such a significant improvement depends on the high electrical double-layer capacitance (EDLC) of FLG electrodes functionalized with the gel composites. The wet encapsulation of the TENGs is shown to be an effective strategy to increase their power output further highlighting the EDLC role. It is also shown that the EDLC is dependent upon the transition metal (W vs Mo) rather than the relative abundance of 1T or 2H phases. Overall, this work lays down the roots for novel sustainable electrochemical-(e)-TENGs developed exploiting strategies typically used in electrochemical capacitors.

Oxidation and Degradation of WS2 Monolayers Grown by NaCl-Assisted Chemical Vapor Deposition: Mechanism and Prevention

The preservation of two-dimensional WS₂ in the environment is a concern for researchers. In addition to water vapor and oxygen, the latest research points out that degradation is directly related to light absorption. Based on the selection rules of nonlinear optics, two-photon absorption is dipole forbidden in the exciton 1s states, but second-harmonic generation (SHG) is allowed with virtual transitions. According to this mechanism, we proved that SHG is an optical detection method with non-photooxidative damage and energy characteristics. With this detection method, we can explore the oxidation and degradation mechanisms of WS₂ grown by NaCl-assisted chemical vapor deposition in its original state. The WS₂ monolayers that use NaCl to assist in growth have undergone different degradation processes, starting to oxidize from random positions in the triangular flake. We use a photocatalytic reaction to explain the photo-induced degradation mechanism with sulfur vacancies. It was further found that WS₂ grown with NaCl assistance is hydrolyzed in a dark and high-humidity environment, which does not occur in pure WS₂. Finally, we demonstrated that changing the direction of the sapphire substrate relative to the gas flow direction to grow NaCl-assisted WS₂ can greatly improve its stability in the ambient atmosphere, even when exposed to light. The optimal geometric structures and ground state energies are investigated by the density functional theory-based calculations. According to the orientation and symmetry of NaCl-assisted WS₂, we can expect that it will have a better growth quality when the gas flow direction is perpendicular to the [112̄0] direction of the sapphire substrate. This contributes to the nucleation and subsequent growth of NaCl-assisted WS₂. This research provides a more stable optical inspection method than other established methods and greatly improves the operational stability of NaCl-assisted WS₂ under environmental conditions.

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.

Atomic layer deposited Al2O3passivation layer for few-layer WS2field effect transistors

We have investigated the effect of an Al₂O₃passivation layer on the performance of few-layer WS₂FETs. While the performance of WS₂FETs is often limited by a substantial decrease in carrier mobility owing to charged impurities and a Schottky barrier between the WS₂and metal electrodes, the introduction of an Al₂O₃overlayer by atomic layer deposition (ALD) suppressed the influence of charged impurities by high-κdielectric screening effect and reduced the effective Schottky barrier height. We argue that n-doping of WS₂, induced by positive fixed charges formed at Al₂O₃/WS₂interface during the ALD process, is responsible for the reduction of the effective Schottky barrier height in the devices. In addition, the Al₂O₃passivation layer protected the device from oxidation, and maintained stable electrical performance of the WS₂FETs over 57 d. Thus, the ALD of Al₂O₃overlayer provides a facile method to enhance the performance of WS₂FETs and to ensure ambient stability.

Stabilization of Chemical-Vapor-Deposition-Grown WS2 Monolayers at Elevated Temperature with Hexagonal Boron Nitride Encapsulation

Chemical vapor deposition (CVD)-grown flakes of high-quality monolayers of WS₂ can be stabilized at elevated temperatures by encapsulation with several layer hexagonal boron nitride (h-BN), but to different degrees in the presence of ambient air, flowing N₂, and flowing forming gas (95% N₂, 5% H₂). The best passivation of WS₂ at elevated temperature occurs for h-BN-covered samples with flowing N₂ (after heating to 873 K), as judged by optical microscopy and photoluminescence (PL) intensity after a heating/cooling cycle. Stability is worse for uncovered samples, but best with flowing forming gas. PL from trions, in addition to that from excitons, is seen for covered WS₂ only for forming gas, during cooling below ∼323 K; the trion has an estimated binding energy of ∼28 meV. It might occur because of doping level changes caused by charge defect generation by H₂ molecules diffusing between the h-BN and the SiO₂/Si substrate. The decomposition of uncovered WS₂ flakes in air suggests a dissociation and chemisorption energy barrier of O₂ on the WS₂ surface of ∼1.6 eV. Fitting the high-temperature PL intensities in air gives a binding energy of a free exciton of ∼229 meV.

Mechanical Properties of Atomically Thin Tungsten Dichalcogenides: WS2, WSe2, and WTe2

Two-dimensional (2D) tungsten disulfide (WS₂), tungsten diselenide (WSe₂), and tungsten ditelluride (WTe₂) draw increasing attention due to their attractive properties deriving from the heavy tungsten and chalcogenide atoms, but their mechanical properties are still mostly unknown. Here, we determine the intrinsic and air-aged mechanical properties of mono-, bi-, and trilayer (1-3L) WS₂, WSe₂, and WTe₂ using a complementary suite of experiments and theoretical calculations. High-quality 1L WS₂ has the highest Young's modulus (302.4 ± 24.1 GPa) and strength (47.0 ± 8.6 GPa) of the entire family, overpassing those of 1L WSe₂ (258.6 ± 38.3 and 38.0 ± 6.0 GPa, respectively) and WTe₂ (149.1 ± 9.4 and 6.4 ± 3.3 GPa, respectively). However, the elasticity and strength of WS₂ decrease most dramatically with increased thickness among the three materials. We interpret the phenomenon by the different tendencies for interlayer sliding in an equilibrium state and under in-plane strain and out-of-plane compression conditions in the indentation process, revealed by the finite element method and density functional theory calculations including van der Waals interactions. We also demonstrate that the mechanical properties of the high-quality 1-3L WS₂ and WSe₂ are largely stable in air for up to 20 weeks. Intriguingly, the 1-3L WSe₂ shows increased modulus and strength values with aging in the air. This is ascribed to oxygen doping, which reinforces the structure. The present study will facilitate the design and use of 2D tungsten dichalcogenides in applications such as strain engineering and flexible field-effect transistors.

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.

A Non-Volatile Memory Based on NbOx/NbSe2 Van der Waals Heterostructures

Two-dimensional (2D) van der Waals (vdW) layered transition metal dichalcogenides (TMDs) materials have been receiving a huge interest due to atomically thin thickness, excellent optoelectronic properties, and free dangling bonds. Especially the metallic TMDs, such as MoTe2 (1T’ phase), NbS2, or NbSe2, have shown fascinating physical properties through various applications, such as superconductor and charge density wave. However, carrier transport of metallic TMDs would be degraded due to the poor stability in ambient conditions. To date, achieving both high device performance and long-term stability is still a huge challenge. Thus, an alternative way to develop both unavoidable native oxide and metallic TMDs is under consideration for new era research. In this respect, 2D metallic TMD materials have attracted high attention due to their great potential in neuromorphic-based devices with metal-insulator-metal structures, making it possible to produce scalable, flexible, and transparent memory devices. Herein, we experimentally demonstrated a synthesized metallic NbSe2 by a chemical vapor deposition method with a highly uniform, good shape distribution and layer controller ranging from 2–10 layers. Together, for the first time, we proposed the NbOx/NbSe2 heterostructure memristor device based on the native NbOx oxide on the interface of multi-layer NbSe2 flakes. The ultra-thin native NbOx oxide of 3 nm was formed after a period of oxidation time under air condition, which acts as a memristive surface in the Au-NbOx-Au lateral memristor device, in which oxygen vacancies form a conductive filament. Our NbOx/NbSe2 hetero-tructured memristor exhibits a stable memory window, a low-resistance-state/high-resistance-state ratio of 20, and stable endurance properties over 20 cycles at a low working voltage of 1 V. Furthermore, by the retention property test, non-volatile characteristics were confirmed after over 3000 s in our best data. Through a systematic study of the NbOx/NbSe2 heterostructured memristor device, this report will open new opportunities for next-generation memory devices application.

Oxidation-boosted charge trapping in ultra-sensitive van der Waals materials for artificial synaptic features

Exploitation of the oxidation behaviour in an environmentally sensitive semiconductor is significant to modulate its electronic properties and develop unique applications. Here, we demonstrate a native oxidation-inspired InSe field-effect transistor as an artificial synapse in device level that benefits from the boosted charge trapping under ambient conditions. A thin InO_x layer is confirmed under the InSe channel, which can serve as an effective charge trapping layer for information storage. The dynamic characteristic measurement is further performed to reveal the corresponding uniform charge trapping and releasing process, which coincides with its surface-effect-governed carrier fluctuations. As a result, the oxide-decorated InSe device exhibits nonvolatile memory characteristics with flexible programming/erasing operations. Furthermore, an InSe-based artificial synapse is implemented to emulate the essential synaptic functions. The pattern recognition capability of the designed artificial neural network is believed to provide an excellent paradigm for ultra-sensitive van der Waals materials to develop electric-modulated neuromorphic computation architectures.

Developing efficient memory and artificial synaptic systems based on environmentally sensitive van der Waals materials remains a challenge. Here, the authors present a native oxidation-inspired InSe field-effect transistor that benefits from a boosted charge trapping behavior under ambient conditions.

Enabling room temperature ferromagnetism in monolayer MoS2 via in situ iron-doping

Two-dimensional semiconductors, including transition metal dichalcogenides, are of interest in electronics and photonics but remain nonmagnetic in their intrinsic form. Previous efforts to form two-dimensional dilute magnetic semiconductors utilized extrinsic doping techniques or bulk crystal growth, detrimentally affecting uniformity, scalability, or Curie temperature. Here, we demonstrate an in situ substitutional doping of Fe atoms into MoS2 monolayers in the chemical vapor deposition growth. The iron atoms substitute molybdenum sites in MoS2 crystals, as confirmed by transmission electron microscopy and Raman signatures. We uncover an Fe-related spectral transition of Fe:MoS2 monolayers that appears at 2.28 eV above the pristine bandgap and displays pronounced ferromagnetic hysteresis. The microscopic origin is further corroborated by density functional theory calculations of dipole-allowed transitions in Fe:MoS2. Using spatially integrating magnetization measurements and spatially resolving nitrogen-vacancy center magnetometry, we show that Fe:MoS2 monolayers remain magnetized even at ambient conditions, manifesting ferromagnetism at room temperature.

Ultra-High Sensitive NO2 Gas Sensor Based on Tunable Polarity Transport in CVD-WS2/IGZO p-N Heterojunction

In this work, a thin-film transistor gas sensor based on the p-N heterojunction is fabricated by stacking chemical vapor deposition-grown tungsten disulfide (WS₂) with a sputtered indium-gallium-zinc-oxide (IGZO) film. To the best of our knowledge, the present device has the best NO₂ gas sensor response compared to all the gas sensors based on transition-metal dichalcogenide materials. The gas-sensing response is investigated under different NO₂ concentrations, adopting heterojunction device mode and transistor mode. High sensing response is obtained of p-N diode in the range of 1-300 ppm with values of 230% for 5 ppm and 18 170% for 300 ppm. On the transistor mode, the gas-sensing response can be modulated by the gate bias, and the transistor shows an ultrahigh response after exposure to NO₂, with sensitivity values of 6820% for 5 ppm and 499 400% for 300 ppm. Interestingly, the transistor has a typical ambipolar behavior under dry air, while the transistor becomes p-type as the amount of NO₂ increases. The assembly of these results demonstrates that the WS₂/IGZO device is a promising platform for the NO₂-gas detection, and its gas-modulated transistor properties show a potential application in tunable engineering for two-dimensional material heterojunction-based transistor device.

Oxidation of Monolayer WS2 in Ambient Is a Photoinduced Process

We have studied the ambient air oxidation of chemical vapor deposition (CVD) grown monolayers of the semiconducting transition metal dichalcogenide (S-TMD) WS₂ using optical microscopy, laser scanning confocal microscopy (LSCM), photoluminescence (PL) spectroscopy, and atomic force microscopy (AFM). Monolayer WS₂ exposed to ambient conditions in the presence of light (typical laboratory ambient light for weeks or typical PL spectroscopy map) exhibits damage due to oxidation which can be detected with the LSCM and AFM, though may not be evident in conventional optical microscopy due to poorer contrast and resolution. Additionally, this oxidation was not random and was correlated with "high-symmetry" high intensity edges and red-shifted areas in the PL spectroscopy map, areas thought to contain a higher concentration of sulfur vacancies. In contrast, samples kept in ambient and darkness showed no signs of oxidation for up to 10 months. Low-irradiance/fluence experiments showed that samples subjected to excitation energies at or above the trion excitation energy (532 nm/2.33 eV and 660 nm/1.88 eV) oxidized in as little as 7 days, even for irradiances and fluences 8 and 4 orders of magnitude lower (respectively) than previously reported. No significant oxidation was observed for 760 nm/1.63 eV light exposure, which lies below the trion excitation energy in WS₂. The strong wavelength dependence and apparent lack of irradiance dependence suggests that ambient oxidation of WS₂ is initiated by photon-mediated electronic band transitions, that is, photo-oxidation. These findings have important implications for prior, present, and future studies concerning S-TMDs measured, stored, or manipulated in ambient conditions.

Trion-Induced Distinct Transient Behavior and Stokes Shift in WS2 Monolayers

Understanding the excitonic behavior in two-dimensional transition-metal dichalcogenides (2D TMDs) is of both fundamental interest and critical importance for optoelectronic applications. Here, we investigate the transient excitonic behavior and Stokes shift in WS₂ monolayers on both sapphire and glass substrates. Trion formation was confirmed as the origin of the distinct photoluminescence (PL) emission and Stokes shift in WS₂ monolayers. Moreover, the transient studies demonstrate faster recombination of both the exciton and the short-lived trion on the glass substrate as compared to that on the sapphire substrate, owing to the heavier n-doping and greater number of defects introduced by the glass substrate. In addition, a long-lived trion species attributed to the intervalley triplet trion was observed on the glass substrate, with a lifetime on the nanosecond time scale. These findings offer a comprehensive understanding of the excitonic behavior and Stokes shift in WS₂ monolayers and will lay the foundation for further fundamental investigations in the field.

Nanophase graphene frameworks

Nanophase graphene frameworks (NGFs) assembled by interconnected domains have massive interfaces, where the interfacial interaction and the compact architectures drastically elevate the durability of graphene towards physical and chemical destruction. The excellent electrical conductivity of the NGFs can be perfectly maintained even after 1500 friction cycles or 3 h flame treatment.

Isotope Effect in Bilayer WSe2

Isotopes of an element have the same electron number but differ in neutron number and atomic mass. However, due to the thickness-dependent properties in MX₂ (M = Mo, W; X = S, Se, Te) transition metal dichalcogenides (TMDs), the isotopic effect in atomically thin TMDs still remains unclear especially for phonon-assisted indirect excitonic transitions. Here, we report the first observation of the isotope effect on the electronic and vibrational properties of a TMD material, using naturally abundant ^NAW^NASe₂ and isotopically pure ¹⁸⁶W⁸⁰Se₂ bilayer single crystals over a temperature range of 4.4-300 K. We demonstrate a higher optical band gap energy in ¹⁸⁶W⁸⁰Se₂ than in ^NAW^NASe₂ (3.9 ± 0.7 meV from 4.41 to 300 K), which is surprising as isotopes are neutral impurities. Phonon energies decrease in the isotopically pure crystal due to the atomic mass dependence of harmonic oscillations, with correspondingly longer E_2g and A²_1g phonon lifetimes than in the naturally abundant sample. The change in electronic band gap renormalization energy is postulated as being the dominant mechanism responsible for the change in optical emission spectra.

Giant Mechano-Optoelectronic Effect in an Atomically Thin Semiconductor

Transition metal dichalcogenides (TMDs) are particularly sensitive to mechanical strain because they are capable of experiencing high atomic displacements without nucleating defects to release excess energy. Being promising for photonic applications, it has been shown that as certain phases of layered TMDs MX₂ (M = Mo or W; X = S, Se, or Te) are scaled to a thickness of one monolayer, the photoluminescence response is dramatically enhanced due to the emergence of a direct electronic band gap compared with their multilayer or bulk counterparts, which typically exhibit indirect band gaps. Recently, mechanical strain has also been predicted to enable direct excitonic recombination in these materials, in which large changes in the photoluminescence response will occur during an indirect-to-direct band gap transition brought on by elastic tensile strain. Here, we demonstrate an enhancement of 2 orders of magnitude in the photoluminescence emission intensity in uniaxially strained single crystalline WSe₂ bilayers. Through a theoretical model that includes experimentally relevant system conditions, we determine this amplification to arise from a significant increase in direct excitonic recombination. Adding confidence to the high levels of elastic strain achieved in this report, we observe strain-independent, mode-dependent Grüneisen parameters over the entire range of tensile strain (1-3.59%), which were obtained as 1.149 ± 0.027, 0.307 ± 0.061, and 0.357 ± 0.103 for the E_2g, A_1g, and A²_1g optical phonon modes, respectively. These results can inform the predictive strain-engineered design of other atomically thin indirect semiconductors, in which a decrease in out-of-plane bonding strength may lead to an increase in the strength of strain-coupled optoelectronic effects.

On-surface synthesis: a promising strategy toward the encapsulation of air unstable ultra-thin 2D materials

2D black phosphorus (BP) and transition metal chalcogenides (TMCs) have beneficial electronic, optical, and physical properties at the few-layer limit. However, irreversible degradation of exfoliated or chemical vapor deposition-grown ultrathin BP and TMCs like GaSe via oxidation under ambient conditions limits their applications. Herein, the on-surface growth of an oxidation-resistant 2D thin film of a metal coordination polymer is demonstrated by multiscale simulations. We show that the preparation of such heterostructures can be conducted in solution, in which pristine BP and GaSe present better stability than in an air environment. Our calculations reveal that the interaction between the polymer layer and 2D materials is dominated by van der Waals forces; thus, the electronic properties of pristine BP and GaSe are well preserved. Meanwhile, the isolation from oxygen and water can be achieved by monolayer polymers, due to the nature of their close-packed layers. Our facile strategy for enhancing the environmental stability of ultrathin materials is expected to accelerate efforts to implement 2D materials in electronic and optoelectronic applications.

Reduction in Step Height Variation and Correcting Contrast Inversion in Dynamic AFM of WS2 Monolayers

A model has been developed to account for and prevent the anomalies encountered in topographic images of transition metal dichalcogenide monolayers using dynamic atomic force microscopy (dAFM). The height of WS2 monolayers measured using dAFM appeared to be increased or decreased, resulting from the interactions between the tip and the surface. The hydrophilic SiO2 substrate appeared higher than the weakly hydrophilic WS2 when the tip amplitude was low or at a high set point (high force). Large amplitudes and low set points corrected the step height inversion, but did not recover the true step height. Removing water from the sample resulted in an order of magnitude reduced variation in step height, but the WS2 appeared inverted except at low amplitudes and high set points. Our model explains the varying step heights in dAFM of TMDs as a result of varying tip-sample interactions between the sample and substrate, in the presence or absence of capillaries. To eliminate contrast inversion, high amplitudes can be used to reduce the effect of capillary forces. However, when capillaries are not present, low amplitudes and high set points produce images with proper contrast due to tool operation in the repulsive regime on both materials.