New Insights into Gas-Solid Interactions of NO2/MoS2 Monolayers: a Comparative Study with MoSe2 and MoTe2 Monolayers

Understanding the microscopic electronic structure determines the macroscopic properties of the materials. Sufficient sampling has the same foundational importance in understanding the interactions. The NO2/MoS2 interaction is well known, but there are still many inconsistencies in the basic data, and the source of the NO2 direct dissociation activity has not been revealed. Based on a large-scale sampling density functional theory (DFT) study, the optimal adsorption of the NO2/MoS2 monolayer system is determined. The impurity state on the top of the valence band of the S-vacancy monolayer (MoS2-VS) was determined by cross-analysis of the band structure and density of states, which has been neglected for a long time. This provides a reasonable explanation for the direct dissociation of NO2 on the MoX2 monolayers. Further atomic structure analysis reveals that the impurity state originates from the not-fully occupied valence orbitals. This also corroborates the fact that the Mo material has dissociation activity, while the W material does not. There is no impurity state on the top of the valence band of the X-vacancy WS2 and WSe2 monolayers. Interestingly, NO2 dissociation did not occur in the MoTe2-VTe monolayer. This may be related to the 6s inert electron pair effect of the Te atom. The double-oriented adsorption behavior of NO2is also revealed. In contrast to the MoSe2 and MoTe2 monolayers, NO2-oriented adsorption on the MoS2 perfect monolayer deviates obviously, which is speculated to be related to space limitation and larger electronegativity of the S atom. The oriented adsorption ability of the MoX2 monolayers followed the order MoTe2 (64.4%) > MoSe2 (44.8%) > MoS2 (42.7%), according to the directed proportion. Renewed insights into the adsorption basic data and the understanding of the electronic structure of NO2/MoX2 (X = S, Se, Te) monolayer systems provide a basic understanding of the gas-surface interactions and various future surface-related advanced applications.

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

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

Cluster Formation Effect of Water on Pristine and Defective MoS2 Monolayers

The structure and electronic properties of the molybdenum disulfide (MoS₂) monolayer upon water cluster adsorption are studied using density functional theory and the optical properties are further analyzed with the Bethe-Salpeter equation (BSE). Our results reveal that the water clusters are electron acceptors, and the acceptor tendency tends to increase with the size of the water cluster. The electronic band gap of both pristine and defective MoS₂ is rather insensitive to water cluster adsorbates, as all the clusters are weakly bound to the MoS₂ surface. However, our calculations on the BSE level show that the adsorption of the water cluster can dramatically redshift the optical absorption for both pristine and defective MoS₂ monolayers. The binding energy of the excitons of MoS₂ is greatly enhanced with the increasing size of the water cluster and finally converges to a value of approximately 1.16 eV and 1.09 eV for the pristine and defective MoS₂ monolayers, respectively. This illustrates that the presence of the water cluster could localize the excitons of MoS₂, thereby greatly enhance the excitonic binding energy.

Application of Two-Dimensional Materials towards CMOS-Integrated Gas Sensors

During the last few decades, the microelectronics industry has actively been investigating the potential for the functional integration of semiconductor-based devices beyond digital logic and memory, which includes RF and analog circuits, biochips, and sensors, on the same chip. In the case of gas sensor integration, it is necessary that future devices can be manufactured using a fabrication technology which is also compatible with the processes applied to digital logic transistors. This will likely involve adopting the mature complementary metal oxide semiconductor (CMOS) fabrication technique or a technique which is compatible with CMOS due to the inherent low costs, scalability, and potential for mass production that this technology provides. While chemiresistive semiconductor metal oxide (SMO) gas sensors have been the principal semiconductor-based gas sensor technology investigated in the past, resulting in their eventual commercialization, they need high-temperature operation to provide sufficient energies for the surface chemical reactions essential for the molecular detection of gases in the ambient. Therefore, the integration of a microheater in a MEMS structure is a requirement, which can be quite complex. This is, therefore, undesirable and room temperature, or at least near-room temperature, solutions are readily being investigated and sought after. Room-temperature SMO operation has been achieved using UV illumination, but this further complicates CMOS integration. Recent studies suggest that two-dimensional (2D) materials may offer a solution to this problem since they have a high likelihood for integration with sophisticated CMOS fabrication while also providing a high sensitivity towards a plethora of gases of interest, even at room temperature. This review discusses many types of promising 2D materials which show high potential for integration as channel materials for digital logic field effect transistors (FETs) as well as chemiresistive and FET-based sensing films, due to the presence of a sufficiently wide band gap. This excludes graphene from this review, while recent achievements in gas sensing with graphene oxide, reduced graphene oxide, transition metal dichalcogenides (TMDs), phosphorene, and MXenes are examined.

First-principles modeling of the highly dynamical surface structure of a MoS2 catalyst with S-vacancies

Vacancy sites, e.g., S-vacancies, are essential for the performance of MoS₂ catalysts. As earlier studies have revealed that the size and shape of the S-vacancies may affect the catalytic activity, we have studied the behavior and mobility of such vacancies on MoS₂ using DFT calculations and kinetic Monte-Carlo (kMC) simulations. The diffusion barriers for the S-vacancies are highly dependent on the immediate environment: isolated single S-vacancies are found to be immobile. In contrast, small nS-vacancies formed from n = 2 to 5 neighboring S-vacancies are often highly dynamic systems that can move within a confined area. Large extended nS-vacancies are generally unstable and transform quickly into alternating patterns of S-atoms and vacancy sites. These results illustrate the importance of recognizing MoS₂ (but also other catalysts) as dynamic structures when trying to tune their catalytic performances by introducing specific defect structures.

Predicting Biomolecule Adsorption on MoS2 Nanosheets with High Structural Fidelity

A new force field, MoSu-CHARMM, for the description of bio-interfacial structures at the aqueous MoS2 interface is developed, based on quantum chemical data. The force field describes non-covalent interactions between...

CO2 Capture, Separation and Reduction on Boron-Doped MoS2 , MoSe2 and Heterostructures with Different Doping Densities: A Theoretical Study

Designing high-performance materials for CO₂ capture and conversion is of great significance to reduce the greenhouse effect and alleviate the energy crisis. The strategy of doping is widely used to improve activity and selectivity of the materials. However, it is unclear how the doping densities influence the materials' properties. Herein, we investigated the mechanism of CO₂ capture, separation and conversion on MoS₂ , MoSe₂ and Janus MoSSe monolayers with different boron doping levels using density functional theory (DFT) simulations. The results indicate that CO₂ , H₂ and CH₄ bind weakly to the monolayers without and with single-atom boron doping, rendering these materials unsuitable for CO₂ capture from gas mixtures. In contrast, CO₂ binds strongly to monolayers doped with diatomic boron, whereas H₂ and CH₄ can only form weak interactions with these surfaces. Thus, the monolayers doped with diatomic boron can efficiently capture and separate CO₂ from such gas mixtures. The electronic structure analysis demonstrates that monolayers doped with diatomic doped are more prone to donating electrons to CO₂ than those with single-atom boron doped, leading to activation of CO₂ . The results further indicate that CO₂ can be converted to CH₄ on diatomic boron doped catalysts, and MoSSe is the most efficient of the surfaces studied for CO₂ capture, separation and conversion. In summary, the study provides evidence for the doping density is vital to design materials with particular functions.

Point defects in two-dimensional BeO monolayer: a first-principles study on electronic and magnetic properties

Very recently, the 2D form of BeO monolayer has been successfully fabricated [Hui Zhang et al., ACS Nano, 2021, 15, 2497]. Motivated by these exciting experimental results on 2D layered BeO structures, the effect of atom adsorption, substitutional doping and vacancy defects on the electronic and magnetic properties of a hexagonal BeO monolayer have been systematically investigated employing density functional theory-based first-principles calculations. We found out that BeO monolayer is a semiconductor with an indirect band gap of 5.9 eV. Next, a plethora of atoms (27 in total) were adsorbed on the surface of BeO monolayer to tailor its electronic properties. The bond length, work function, difference in charge and magnetic moment were also calculated for all modifications covering the vacancy defects and substitutional doping. The band gap is also supplied for these changes, showing how these adjustments can provide amazing opportunities in granting a variety of options in band gap engineering and in transforming the BeO monolayer from a semiconductor to a dilute magnetic semiconductor or half-metal in view of different applications. The formation energy of the defects was also computed as an important indicator for the stability of the defected structures, when created in a real experiment. We have theoretically demonstrated several possible approaches to modify the properties of BeO monolayer in a powerful and controllable manner. Thus, we expect to inspire many experimental studies focused on two dimensional BeO growth and property tuning, and exploration for applications in advanced nanoelectronics.

High-Throughput Screening of Atomic Defects in MXenes for CO2 Capture, Activation, and Dissociation

The capture, activation, and dissociation of carbon dioxide (CO₂) is of fundamental interest to overcome the ramifications of the greenhouse effect. In this regard, high-throughput screening of two-dimensional MXenes has been examined using well-resolved first-principles simulations through DFT-D3 dispersion correction. We systematically investigated different types of structural defects to understand their influence on the performance of M₂X-type MXenes. Defect calculations demonstrate that the formation of M₂C(V_MC) and M₂N(V_MN) vacancies require higher energy, while M₂C(V_C) and M₂N(V_N) vacancies are favorable to form during the synthesis of M₂X-type MXenes. The M₂X-type MXenes from group III to VII series show remarkable behavior for active capturing of CO₂, especially group IV (Ti₂X and Zr₂X) MXenes exhibit unprecedentedly high adsorption energies and charge transfer (>2e) from M₂X to CO₂. The potential CO₂ capture, activation, and dissociation abilities of MXenes are emanated from Dewar interactions involving hybridization between π orbitals of CO₂ and metal d-orbitals. Our high-throughput screening demonstrates chemisorption of CO₂ on pure and defective MXenes, followed by dissociation into CO and O species.

Modulation effects of S vacancy and Mo edge on the adsorption and dissociation behaviors of toxic gas (H2S, SO2) molecules on the MoS2 monolayer

This study focuses on the modulation effects of S vacancy and Mo edges on the adsorption and dissociation behaviors of toxic gases (H2S and SO2) on MoS2 by first-principles calculations. Both molecules are found to chemisorb at the S vacancy (SV) and pristine Mo edge and physisorb at the Mo edge with a 50% sulfur coverage (Mo-50 edge). Among them, SO2 has larger adsorption energy than H2S on both S vacancy and pristine Mo edge, which is related to a more electronegative O than S atom and electronically rich for the pristine Mo edge. The defective states of MoS2 induced by SV can be removed by forming Mo-S, Mo-O and Mo-H bonds upon the adsorption of SO2 and the dissociation of H2S, which is applicable in designing MoS2 based nano-electronics devices in the future. The dissociations of H2S and SO2 on pristine Mo edges are found to be more favorable than those on S vacancies due to the catalytically active Mo4+ states at edge sites. H2S is found to dissociate on the Mo-50 edge more easily than SO2. The adsorptions and dissociations of toxic gas on MoS2 with SV or Mo edges suggest MoS2 is a potential candidate in detecting and removal of toxic gases.

Ferromagnetism and intrinsic half-metallicity of two-dimensional MnN monolayer with square-octagonal structure

The MnN monolayer with square-octagonal structure (so-MnN) is explored using density functional calculations. The results show that theso-MnN monolayer is energetically, dynamically, thermally and mechanically stable, and exhibits the ferromagnetism and intrinsic half-metallicity. The total magnetic moment is 16 μ_Bin unit cell (Mn₄N₄). The energy band of spin-up crosses the Fermi energy level (E_F), while the spin-down channel has semiconductor characteristic with a direct band gap of 3.0 eV at Γ-point. By applying the biaxial strain, the band gap in spin-down channel can be tuned, and theso-MnN monolayer still possesses the characteristic of ferromagnetism and intrinsic half-metallicity. Finally, the Curie temperatureT_Cincreases gradually under biaxial strains from 0 to +3%, while theT_Chas a decreasing trend under the biaxial strains from 0 to -3%.

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.

First-principles study of sulfur vacancy concentration effect on the electronic structures and hydrogen evolution reaction of MoS2

Defect engineering has been widely used in experiments to modulate the electrocatalytic properties of molybdenum disulfide (MoS₂). However, the effect of vacancy concentration on the vacancy distribution, electronic properties, and hydrogen evolution reaction (HER) activity remains elusive. Herein, we perform density functional theory (DFT) studies to investigate defective MoS₂ with different numbers of sulfur vacancies. In the case of low S-vacancy concentration, the vacancies prefer to agglomerate rather than being dispersed, while at the higher-vacancy concentration, the combination of local point defect and clustered vacancy chain is preferred. The coupling between S-vacancies leads to decreased band gap and increased Mo-H adsorption strength with increasing vacancy concentration. The optimal HER activity is identified to occur below vacancy concentration of 12.50%. Our work provides an atomic-level understanding about the role of S-vacancies in the HER performance of MoS₂, and offers useful guidelines for the design of defective MoS₂ and other TMDs electrocatalysts.

Force fields for water-surface interaction: is reproduction of the experimental water contact angle enough?

A new protocol based on quantum chemical calculations and molecular dynamics simulations is proposed to revisit water-MoS₂ interfacial force fields (FFs). The accurate reproduction of experimental water contact angles is suggested to be insufficient to ensure reliable FFs for recovering structural properties of the interfacial solvent. As an example, this protocol is used to develop a new set of FF parameters to both capture interfacial structural phenomena at the interface between water and MoS₂ and recover experimental water contact angle data. This approach can be applied to any interface where contact angle data are available.

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.

Remarkably high thermal-driven MoS2 grain boundary migration mobility and its implications on defect healing

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) hold great potential for many important device applications, such as field effect transistors and sensors, which require a robust control of defect type, density, and distribution. However, how to control the defect type, density, and distribution in these materials is still a challenge. In this study, we explore the kinetics and dynamics of four types of grain boundaries (GBs) in monolayer MoS₂, which are composed of S-polar dislocation (S5|7), Mo-polar dislocation (Mo5|7), dislocation-double S vacancy complex (S4|6), and dislocation-double S interstitial complex (S6|8), respectively. Our study shows that these four GBs in monolayer MoS₂ exhibit a great disparity in their migration behavior. More specifically, the S4|6 and S6|8 GBs possess a much higher migration mobility than the S5|7 and Mo5|7 GBs under the same thermal fluctuations or temperature gradient. Interestingly, the S4|6 and S6|8 GBs follow an abnormal relationship with temperature, due to the change in defect configurations with temperature. Our study further shows that the remarkably high mobilities of the S4|6 and Mo6|8 GBs may enable the reactions of GBs, leading to the annihilation and reduction of defect density. In addition, the movement of GBs in MoS₂ under a temperature gradient field can cause defect redistribution, which in turn changes the thermal conductivity. The present study not only deepens our understanding of the dynamic evolution of GBs in TMDs, but also presents new opportunities to engineer GBs for novel electronic applications.

Tuning the binding energy of excitons in the MoS2 monolayer by molecular functionalization and defective engineering

First-principle calculations within many-body perturbation theory are carried out to investigate the influence of the adsorbed molecules and sulfur (S) defects on the electronic and optical properties of the MoS₂ monolayer. The exciton binding energy in the range of 0.05 eV to 1.14 eV is observed as a function of molecular coverage, when NO and 1,3,5-triazin (C₃H₃N₃) are adsorbed on the pristine surface. These results can be explained by the interaction between the exciton and the adsorbed molecule. Furthermore, the combined effect of molecular functionalization and defective doping is studied. Our results show that both the electronic and optical band gaps of the MoS₂ monolayer strongly depend on the molecular species and the defective coverage, and can be tuned up to ∼2 eV. This work demonstrates the great potential of controlling the MoS₂ monolayer's excitonic properties by molecular functionalization and defective engineering.

NO reduction over an Al-embedded MoS2 monolayer: a first-principles study

Converting toxic air pollutants such as nitric oxide (NO) and carbon monoxide (CO) into less harmful gases remains a critical challenge for many industrial technologies. Here, by performing first-principles calculations, we introduce a cheap, stable and novel catalyst for the conversion of NO and CO molecules into N₂O and CO₂ using Al-doped MoS₂ (Al–MoS₂). According to our results, dissociation of NO molecules on Al–MoS₂ has a large energy barrier (3.62 eV), suggesting that it is impossible at ambient temperature. In contrast, the coadsorption of NO molecules to form (NO)₂ moieties is characterized as the first step of the NO reduction process. The formed (NO)₂ is unstable on Al–MoS₂, and hence it is easily decomposed into N₂O molecules, and an oxygen atom is adsorbed onto the Al atom (O_ads). This reaction step is exothermic and needs an activation energy of 0.37 eV to be overcome. Next, the O_ads moiety is removed from the Al atom by a CO molecule, and thereby the Al–MoS₂ catalyst is recovered for the next round of reaction. The side reaction producing NO₂via the reaction of NO with the O_ads moiety cannot proceed on Al–MoS₂ due to its large activation energy.

By performing first-principles calculations, we introduce a stable and novel catalyst for the conversion of NO and CO molecules into N₂O and CO₂ using Al-doped MoS₂.

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.

Atomic Plane-Vacancy Engineering of Transition-Metal Dichalcogenides with Enhanced Hydrogen Evolution Capability

Introducing anion vacancies on two-dimensional transition-metal dichalcogenides (TMDs) would significantly improve their catalytic activity. In this work, we proposed a solid-phase reduction (SPR) strategy to simultaneously achieve efficient exfoliation and controlled generation of chalcogen vacancies on TMDs. Consecutive sulfur vacancies were successfully created on the basal plane of the bulk MoS₂ and WS₂, and their interlamellar distances were distinctly expanded after the SPR treatment (about 16%), which can be conveniently exfoliated by only gentle shaking. The S-vacancy significantly increases the hydrogen-evolution reaction activity of the MoS₂ and WS₂ nanosheets, with overpotential of -238 and -241 mV at 10 mA cm^-2, respectively. We anticipate that our SPR strategy will supply a general platform for the development of TMD-based electrocatalysts for industrial water splitting and hydrogen production in the near future.

The unexpected effect of vacancies and wrinkling on the electronic properties of MoS2 layers

We report a combined experimental/theoretical approach to study the connection of S-vacancies and wrinkling on MoS₂ layers, and how this feature produces significant changes in the electronic structure and reactivity of this 2D material.

Revealing the role of the 1T phase on the adsorption of organic dyes on MoS2 nanosheets

Herein, different phases of MoS₂ nanosheets were synthesized, characterized and tested for dye removal from water. The influence of the MoS₂ phases as well as the 1T concentration on the adsorption performance of organic dyes MO, RhB and MB was deeply investigated. The results revealed that the 1T-rich MoS₂ nanosheets have superior adsorption performance compared to other 2H and 3R phases. The kinetic results of the adsorption process demonstrate that the experimental data followed the pseudo-second order equation. Meanwhile, the adsorption of dyes over the obtained materials was fitted with several isotherm models. The Langmuir model gives the best fitting to the experimental data with maximum a adsorption capacity of 787 mg g⁻¹. The obtained capacity is significantly higher than that of all previous reports for similar MoS₂ materials. Computational studies of the 2H and 1T/2H-MoS₂ phases showed that the structural defects present at the 1T/2H grain boundaries enhance the binding of hydroxide and carboxyl groups to the MoS₂ surface which in turn increase the adsorption properties of the 1T/2H-MoS₂ phase.

The high adsorption capacity of dyes onto the 1T-rich MoS₂ samples is due to the strong binding between the hydroxide/carboxyl groups and the 1T active sites. The capacity can be tuned by controlling the ratio between 1T and 2H phases of MoS₂ nanosheets.

Inorganic molecule (O2, NO) adsorption on nitrogen- and phosphorus-doped MoS2 monolayer using first principle calculations

We performed a systematic study of the adsorption behaviors of O2 and NO gas molecules on pristine MoS2, N-doped, and P-doped MoS2 monolayers via first principle calculations. Our adsorption energy calculations and charge analysis showed that the interactions between the NO and O2 molecules and P-MoS2 system are stronger than that of pristine and N-MoS2. The spin of the absorbed molecule couples differently depending on the type of gas molecule adsorbed on the P- and N-substituted MoS2 monolayer. Meanwhile, the adsorption of O2 molecules leaves N- and P-MoS2 a magnetic semiconductor, whereas the adsorption of an NO molecule turns this system into a nonmagnetic semiconductor, which may provide some helpful information for designing new N- and P-substituted MoS2-based nanoelectronic devices. Therefore, P- and N-MoS2 can be used to distinguish O2 and NO gases using magnetic properties, and P-MoS2-based gas sensors are predicted to be more sensitive to detect NO molecules rather than pristine and N-MoS2 systems.

Atmospheric and Long-term Aging Effects on the Electrical Properties of Variable Thickness WSe2 Transistors

Atmospheric and long-term aging effects on electrical properties of WSe₂ transistors with various thicknesses are examined. Although countless published studies report electrical properties of transition-metal dichalcogenide materials, many are not attentive to testing environment or to age of samples, which we have found significantly impacts results. Our as-fabricated exfoliated WSe₂ pristine devices are predominantly n-type, which is attributed to selenium vacancies. Transfer characteristics of as-fabricated devices measured in air then vacuum reveal physisorbed atmospheric molecules significantly reduced n-type conduction in air. First-principles calculations suggest this short-term reversible atmospheric effect can be attributed primarily to physisorbed H₂O on pristine WSe₂, which is easily removed from the pristine surface in vacuum due to the low adsorption energy. Devices aged in air for over 300 h demonstrate irreversibly increased p-type conduction and decreased n-type conduction. Additionally, they develop an extended time constant for recovery of the atmospheric adsorbents effect. Short-term atmospheric aging (up to approximately 900 h) is attributed to O₂ and H₂O molecules physisorbed to selenium vacancies where electron transfer from the bulk and adsorbed binding energies are higher than the H₂O-pristine WSe₂. The residual/permanent aging component is attributed to electron trapping molecular O₂ and isoelectronic O chemisorption at selenium vacancies, which also passivates the near-conduction band gap state, p-doping the material, with very high binding energy. All effects demonstrated have the expected thickness dependence, namely, thinner devices are more sensitive to atmospheric and long-term aging effects.

Interaction between H2O, N2, CO, NO, NO2 and N2O molecules and a defective WSe2 monolayer

In this study, the interaction between gas molecules, including H₂O, N₂, CO, NO, NO₂ and N₂O, and a WSe₂ monolayer containing an Se vacancy (denoted as V_Se) has been theoretically studied. Theoretical results show that H₂O and N₂ molecules are highly prone to be physisorbed on the V_Se surface. The presence of the Se vacancy can significantly enhance the sensing ability of the WSe₂ monolayer toward H₂O and N₂ molecules. In contrast, CO and NO molecules highly prefer to be molecularly chemisorbed on the V_Se surface with the non-oxygen atom occupying the Se vacancy site. Furthermore, the exposed O atoms of the molecularly chemisorbed CO or NO can react with additional CO or NO molecules, to produce C-doped or N-doped WSe₂ monolayers. The calculated energies suggest that the filling of the CO or NO molecule and the removal of the exposed O atom are both energetically and dynamically favorable. Electronic structure calculations show that the WSe₂ monolayers are p-doped by the CO and NO molecules, as well as the C and N atoms. However, only the NO molecule and N atom doped WSe₂ monolayers exhibit significantly improved electronic structures compared with V_Se. The NO₂ and N₂O molecules will dissociate directly to form an O-doped WSe₂ monolayer, for which the defect levels due to the Se vacancy can be completely removed. The calculated energies suggest that although the dissociation processes for NO₂ and N₂O molecules are highly exothermic, the N₂O dissociation may need to operate at an elevated temperature compared with room temperature, due to its large energy barrier of ∼1 eV.

Au cluster adsorption on perfect and defective MoS2 monolayers: structural and electronic properties

The adsorption of Au_n (n = 1-4) clusters on perfect and defective MoS₂ monolayers is studied using density functional theory. For the pristine MoS₂ monolayer, our results show that the electrons are transferred from the support to the adsorbed Au clusters, thus a p-doping effect is achieved in the pristine MoS₂ monolayer by the Au cluster adsorption, which is in good agreement with the experimental findings. The adsorption of Au clusters can introduce mid-gap states, which modify the electronic and magnetic properties of the systems. The adsorbates containing an odd number of Au atoms can introduce a spin magnetic moment of 1 μ_B into the perfect MoS₂ monolayer, while those systems containing an even number of Au atoms are spin-unpolarized. Two categories of defects, i.e., a single S vacancy and Mo antisite defect with one Mo atom replacing one S atom, are considered for the defective monolayer MoS₂. Compared with the pristine MoS₂ monolayer, the adsorption energies for Au clusters are significantly increased for the MoS₂ monolayer with a single S vacancy, and there are more electrons transferred from the MoS₂ monolayer with an S vacancy to the Au clusters. The mid-gap states and odd-even oscillation magnetic behavior can also be observed when Au clusters are adsorbed on the MoS₂ monolayer with an S vacancy. For those systems of Au clusters on MoS₂ monolayers with Mo antisite defects, the adsorption energies as well as the magnitude and the direction of transferred charge are similar to those for the MoS₂ monolayer with an S vacancy. The spin-polarizations appear in all systems with Mo antisite defects. Our investigations suggest that the electronic and magnetic properties of MoS₂ nanosheets can be effectively modulated by the adsorption of Au clusters.

Controlling the magnetic and optical responses of a MoS2 monolayer by lanthanide substitutional doping: a first-principles study

The absorption spectrum and TDOS of lanthanide doped MoS₂ for the E-field parallel and perpendicular to the xy-plane.