N-type and p-type molecular doping on monolayer MoS2

Monolayer MoS₂ has attracted much attention due to its high on/off current ratio, transparency, and suitability for optoelectronic devices. Surface doping by molecular adsorption has proven to be an effective method to facilitate the usage of MoS₂. However, there are no works available to systematically clarify the effects of the adsorption of F₄TCNQ, PTCDA, and tetracene on the electronic and optical properties of the material. Therefore, this work elucidated the problem by using density functional theory calculations. We found that the adsorption of F₄TCNQ and PTCDA turns MoS₂ into a p-type semiconductor, while the tetracene converts MoS₂ into an n-type semiconductor. The occurrence of a new energy level in the conduction band for F₄TCNQ and PTCDA and the valence band for tetracene reduces the bandgap of the monolayer MoS₂. Besides, the MoS₂/F₄TCNQ and MoS₂/PTCDA systems exhibit an auxiliary optical peak at the long wavelengths of 950 and 850 nm, respectively. Contrastingly, the MoS₂/tetracene modifies the optical spectrum of the monolayer MoS₂ only in the ultraviolet region. The findings are in good agreement with the experiments.

Pressure controls electronic and optical properties of monolayer MoS₂ with organic molecular adsorption.     

A vapor-phase-assisted growth route for large-scale uniform deposition of MoS2 monolayer films

In this work a vapor-phase-assisted approach for the synthesis of monolayer MoS₂ is demonstrated, based on the sulfurization of thin MoO_3−x precursor films in an H₂S atmosphere. We discuss the co-existence of various possible growth mechanisms, involving solid–gas and vapor–gas reactions. Different sequences were applied in order to control the growth mechanism and to obtain monolayer films. These variations include the sample temperature and a time delay for the injection of H₂S into the reaction chamber. The optimized combination allows for tuning the process route towards the potentially more favorable vapor–gas reactions, leading to an improved material distribution on the substrate surface. Raman and photoluminescence (PL) spectroscopy confirm the formation of ultrathin MoS₂ films on SiO₂/Si substrates with a narrow thickness distribution in the monolayer range on length scales of a few millimeters. Best results are achieved in a temperature range of 950–1000 °C showing improved uniformity in terms of Raman and PL line shapes. The obtained films exhibit a PL yield similar to mechanically exfoliated monolayer flakes, demonstrating the high optical quality of the prepared layers.

Optimization of the sulfurization process of thin MoO₃ precursor layers, pushing the reaction towards vapor-phase-assisted routes to obtain large-scale, homogeneous monolayer MoS₂.     

Homogeneity and tolerance to heat of monolayer MoS2 on SiO2 and h-BN

We investigated the homogeneity and tolerance to heat of monolayer MoS₂ using photoluminescence (PL) spectroscopy. For MoS₂ on SiO₂, the PL spectra of the basal plane differ from those of the edge, but MoS₂ on hexagonal boron nitride (h-BN) was electron-depleted with a homogeneous PL spectra over the entire area. Annealing at 450 °C rendered MoS₂ on SiO₂ homogeneously electron-depleted over the entire area by creating numerous defects; moreover, annealing at 550 °C and subsequent laser irradiation on the MoS₂ monolayer caused a loss of its inherent crystal structure. On the other hand, monolayer MoS₂ on h-BN was preserved up to 550 °C with its PL spectra not much changed compared with MoS₂ on SiO₂. We performed an experiment to qualitatively compare the binding energies between various layers, and discuss the tolerance of monolayer MoS₂ to heat on the basis of interlayer/interfacial binding energy.

We investigated the homogeneity and tolerance to heat of monolayer MoS₂ using photoluminescence (PL) spectroscopy.     

Bandgap recovery of monolayer MoS2 using defect engineering and chemical doping

Two-dimensional transition metal dichalcogenide materials have created avenues for exciting physics with unique electronic and photonic applications. Among these materials, molybdenum disulfide is the most known due to extensive research in understanding its electronic and optical properties. In this paper, we report on the successful growth and modification of monolayer MoS₂ (1L MoS₂) by controlling carrier concentration and manipulating bandgap in order to improve the efficiency of light emission. Atomic size MoS₂ vacancies were created using a Helium Ion Microscope, then the defect sites were doped with 2,3,5,6-tetrafluro7,7,8,8-tetracyanoquinodimethane (F4TCNQ). The carrier concentration in intrinsic (as-grown) and engineered 1L MoS₂ was calculated using Mass Action model. The results are in a good agreement with Raman and photoluminescence spectroscopy as well as Kelvin probe force microscopy characterizations.

Two-dimensional transition metal dichalcogenide materials have created avenues for exciting physics with unique electronic and photonic applications.     

Enhanced photoresponse of a MoS2 monolayer using an AAO template

Monolayer two-dimensional transition metal dichalcogenides (TMDs) with direct band gaps, such as MoS₂, have received great attention from researchers due to their peculiar band structure and physical properties. However, their extremely small thickness (0.65 nm for MoS₂) results in a critically low light absorption efficiency, thus limiting their optoelectronic applications. To achieve the enhancement of the light–matter interaction in MoS₂, a resonant Al/AAO (anodic alumina oxide template)/MoS₂ trilayer nanocavity structure was designed and implemented in the present study. In such a system, the appropriate change in pore size and pore depth of the AAO template via control of the growth conditions allows one to adjust the thickness and refractive index of the dielectric layer (AAO). This nanocavity structure provides a possible way to regulate the light–matter interaction of MoS₂ film.

Schematic diagram of absorption principle and SEM image of the Al–AAO–MoS₂, and Raman results of Al–AAO–MoS₂ and Si/SiO₂-MoS₂.     

First-principles study of vacancy defects at interfaces between monolayer MoS2 and Au

The performance of MoS₂ based devices is closely related to the quality and defect morphology of the monolayer MoS₂ deposited on metal. First-principles calculations were performed to investigate the vacancy effects of Au–mMoS₂ contact. Four possible S-vacancy and a Mo-vacancy were considered in our calculations. Energetic studies show that S-vacancies are easier to form than Mo-vacancy in Au–mMoS₂ contact, while S-vacancy (hollow site at interface, V_S4) has the lowest formation energy under Mo-rich environments. Electron and charge redistribution analysis of defective Au–mMoS₂ contact indicate that the lower contact resistance and higher electron injection efficiency of defective Au–MoS₂ contact than perfect ones. Notably, the S-vacancy at top layer showed better electronic performance than that at bottom layer of monolayer MoS₂ in the contact. High quality n-type Au–mMoS₂ contact can therefore be expected through defect engineering.

Energetically favorable S(hollow)-vacancy has lower contact resistance and higher electron injection efficiency, resulting in better electronic performance in defective Au–MoS₂ contact.     

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.     

Synergetic photoluminescence enhancement of monolayer MoS2via surface plasmon resonance and defect repair

The weak light-absorption and low quantum yield (QY) in monolayer MoS₂ are great challenges for the applications of this material in practical optoelectronic devices. Here, we report on a synergistic strategy to obtain highly enhanced photoluminescence (PL) of monolayer MoS₂ by simultaneously improving the intensity of the electromagnetic field around MoS₂ and the QY of MoS₂. Self-assembled sub-monolayer Au nanoparticles underneath the monolayer MoS₂ and bis(trifluoromethane)sulfonimide (TFSI) treatment to the MoS₂ surface are used to boost the excitation field and the QY, respectively. An enhancement factor of the PL intensity as high as 280 is achieved. The enhancement mechanisms are analyzed by inspecting the contribution of the PL spectra from A excitons and A⁻ trions under different conditions. Our study takes a further step to developing high-performance optoelectronic devices based on monolayer MoS₂.

A synergistic strategy is reported to obtain a highly enhanced photoluminescence (PL) of monolayer MoS₂ by simultaneously improving the intensity of the electromagnetic field around MoS₂ and the QY of MoS₂.     

Manipulating the Raman scattering rotation via magnetic field in an MoS2 monolayer

Magneto-optical effects, which originate from the interactions between light and magnetism, have provided an important way to characterize magnetic materials and hosted abundant applications, such as light modulators, magnetic field sensors, and high-density data storage. However, such effects are too weak to be detected in non-magnetic materials due to the absence of spin degree of freedom. Here, we demonstrated that applying a perpendicular magnetic field can produce a colossal Raman scattering rotation in non-magnetic MoS₂, for A-mode representing the out-of-plane breathing vibration. Our experimental results show that linearly polarized scattering light is rotated by ∓125°, more apparent than the valley Zeeman splitting effect (∓1.2 meV) under the same experimental conditions (±5 T), near room temperature. A detailed and systematic analysis on the polarization-resolved magnetic field-dependent micro-zone Raman intensity offers a feasible way to manipulate the inelastically scattered light via a magnetic technique. This explored phenomenology and physical mechanism arouse a new ramification of probing burgeoning magneto-optical effects in the field of two-dimensional laminar materials.

Herein, we demonstrated that a perpendicular magnetic field could produce a dramatic scattering rotation for the vibrations in MoS₂ monolayers.     

Electronic and optical properties of monolayer MoS2 under the influence of polyethyleneimine adsorption and pressure

MoS₂ is one of the well-known transition metal dichalcogenides. The moderate bandgap of monolayer MoS₂ is fascinating for the new generation of optoelectronic devices. Unfortunately, MoS₂ is sensitive to gases in the environment causing its original electronic properties to be modified unexpectedly. This problem has been solved by coating MoS₂ with polymers such as polyethyleneimine (PEI). Furthermore, the application of pressure is also an effective method to modify the physical properties of MoS₂. However, the effects of polyethyleneimine and pressure on the electronic and optical properties of monolayer MoS₂ remain unknown. Therefore, we elucidated this matter by using density functional theory calculations. The results showed that the adsorption of the PEI molecule significantly reduces the width of the direct bandgap of the monolayer MoS₂ to 0.55 eV because of the occurrence of the new energy levels in the bandgap region due to the contribution of the N-2p_z state of the PEI molecule. Remarkably, the transition from semiconductor to metal of the monolayer MoS₂ and the MoS₂/PEI system occurs at the tensile pressure of 24.95 and 21.79 GPa, respectively. The bandgap of these systems approaches 0 eV at the corresponding pressures. Importantly, new peaks in the optical spectrum of the clean MoS₂ and MoS₂/PEI appear in the ultraviolet region under compressive pressures and the infrared region under tensile strains.

Pressure modifies both electronic and optical properties; however, PEI adsorption only alters the electronic structure of monolayer MoS₂.     

Epitaxial synthesis of Ni–MoS2/Ti3C2Tx MXene heterostructures for hydrodesulfurization

Hierarchical structures of 2D layered Ti₃C₂T_x MXene hold potential for a range of applications. In this study, catalysts comprising few-layered MoS₂ with Ti₃C₂T_x have been formulated for hydrodesulfurization (HDS). The support Ti₃C₂T_x was derived from MAX phases (Ti₃AlC₂) via a liquid-phase exfoliation process, while MoS₂ was obtained from synthesized aqueous ammonium tetrathiomolybdate (ATM). Furthermore, a series of catalysts with different architectures was synthesized by confinement of ATM and/or the promoter Ni in Ti₃C₂T_x at different mole ratios, through a thermal conversion process. The synthesized MoS₂/Ti₃C₂T_x and Ni–MoS₂/Ti₃C₂T_x catalysts were characterized using X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS), high-resolution transmission electron microscopy (HRTEM), and temperature-programmed reduction (TPR) measurements. The number of MoS₂ layers formed on the Ti₃C₂T_x support was calculated using Raman spectroscopy. The heterostructured few-layered MoS₂/Ti₃C₂T_x catalysts were applied in sulfur removal efficiency experiments involving thiophene. The active MoS₂ sites confined by the Ti₃C₂T_x enhanced hydrogen activation by proton saturation, and the electron charge stabilized the sulfur atom to facilitate hydrogenation reactions, leading to predominant formation of C₄ hydrocarbons. The Ni–MoS₂/Ti₃C₂T_x showed the best activity at a promoter molar ratio of 0.3 when compared to the other catalysts. In particular, it is evident from the results that ATM and Ti₃C₂T_x are potential materials for the in situ fabrication of hierarchical few-layered MoS₂/Ti₃C₂T_x catalysts for enhancing hydrodesulfurization activity in clean fuel production.

A facile and efficient method is adopted to intercalate promoter and few-layered MoS₂ in Ti₃C₂T_x MXene for high hydrodesulfurization activity via an in situ thermal conversion process.     

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.

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.     

A study on the tribological property of MoS2/Ti–MoS2/Si multilayer nanocomposite coating deposited by magnetron sputtering

Pure MoS₂ coatings are easily affected by oxygen and water vapor to form MoO₃ and H₂SO₄ which cause a higher friction coefficient and shorter service life. In this work, five kinds of MoS₂/Ti–MoS₂/Si multilayer nanocomposite coatings have been deposited by using unbalanced magnetron sputtering with different modulation period ratios. The tribological tests and nano-indentation experiments have been carried out in order to study the tribological and mechanical properties of the multilayer nanocomposite coating. The results show that the hardness and internal stress of the multilayer nanocomposite coatings are superior to those of the pure MoS₂ coating. The polycrystalline columnar structures are effectively inhibited and the coating densification increases due to the multilayer nanostructure and the doped elements of Ti and Si. The nanocomposite coating with a modulation period ratio of 100 : 100 shows the lowest friction coefficient and wear rate. The multilayer nanocomposite coatings exhibit excellent tribological property under a heavy constant load. Interfaces in multilayer nanostructure coating is able to hinder the dislocations motion and the crack propagation. The doped elements of Ti and Si with nano-multilayer structure enhances the mechanical and tribological properties of MoS₂ coating. This study provides guidelines for optimizing the mechanical and tribological properties of MoS₂ coating.

Pure MoS₂ coatings are easily affected by oxygen and water vapor to form MoO₃ and H₂SO₄ which cause a higher friction coefficient and shorter service life.     

Controllable growth of MoS2 nanosheets on TiO2 burst nanotubes and their photocatalytic activity

MoS₂ nanosheets were grown on TiO₂ nanotubes by the simple hydrothermal method for the first time. The layer-by-layer growth of MoS₂ nanosheets led to a significant increase in the specific surface area of TiO₂/MoS₂ burst tube composites compared with TiO₂ burst tubes, a significantly enhanced ability to separate photo-induced carriers, and synergistic adsorption and visible light catalytic activity of dye molecules. The maximum adsorption (q_max) of MB was 72.46 mg g⁻¹. In addition, 94.1% of MB could be degraded after 30 minutes of visible light irradiation. The microsurface morphology, structure, chemical composition, element valence and band width of TiO₂/MoS₂ nanocomposites were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS) and photoluminescence spectroscopy (PL). The mechanism of photocatalytic reaction was studied via free radical capture experiments.

MoS₂ nanosheets were grown on TiO₂ nanotubes by the simple hydrothermal method for the first time.     

Negative-capacitance and bulk photovoltaic phenomena in gallium nitride nanorods network

An enhanced self-powered near-ultraviolet photodetection phenomenon was observed in epitaxial gallium nitride (GaN) nanorods network grown on an intermediate layer of N:GaN on a nitridated HfO₂(N:HfO₂)/SiO₂/p-Si substrate. The fabricated Au/GaN/N:GaN/N:HfO₂/Ag heterostructure exhibited a giant change (OFF/ON ratio > 50 without applying any external electrical field) in its conductance when illuminated by a very weak (25 mW cm⁻²) near-UV monochromatic light with a low dark current (nearly 20 nA). The presented near-UV photodetector offers photoresponsivity of ∼2.4 mA W⁻¹ at an applied voltage of 1 V. We observed an optically generated internal open circuit voltage of ∼155 mV and short circuit current ∼430 nA, which can be attributed to the quantum confinement of free charge carriers in the nanorod matrix. Interestingly, it also shows a negative capacitance after near-UV illumination. It has great potential as a self-powered UV photodetector and in metamaterial applications.

An enhanced self-powered near-ultraviolet photodetection phenomenon was observed in epitaxial gallium nitride (GaN) nanorod networks grown on an intermediate layer of N:GaN on a nitridated HfO₂(N:HfO₂)/SiO₂/p-Si substrate.     

Structures and characteristics of atomically thin ZrO2 from monolayer to bilayer and two-dimensional ZrO2–MoS2 heterojunction

The understanding of the structural stability and properties of dielectric materials at the ultrathin level is becoming increasingly important as the size of microelectronic devices decreases. The structures and properties of ultrathin ZrO₂ (monolayer and bilayer) have been investigated by ab initio calculations. The calculation of enthalpies of formation and phonon dispersion demonstrates the stability of both monolayer and bilayer ZrO₂ adopting a honeycomb-like structure similar to 1T-MoS₂. Moreover, the 1T-ZrO₂ monolayer or bilayer may be fabricated by the cleavage from the (111) facet of non-layered cubic ZrO₂. Moreover, the contraction of in-plane lattice constants in monolayer and bilayer ZrO₂ as compared to the corresponding slab in cubic ZrO₂ is consistent with the reported experimental observation. The electronic band gaps calculated from the GW method show that both the monolayer and bilayer ZrO₂ have large band gaps, reaching 7.51 and 6.82 eV, respectively, which are larger than those of all the bulk phases of ZrO₂. The static dielectric constants of both monolayer ZrO₂ (ε_‖ = 33.34, ε_⊥ = 5.58) and bilayer ZrO₂ (ε_‖ = 33.86, ε_⊥ = 8.93) are larger than those of monolayer h-BN (ε_‖ = 6.82, ε_⊥ = 3.29) and a strong correlation between the out-of-plane dielectric constant and the layer thickness in ultrathin ZrO₂ can be observed. Hence, 1T-ZrO₂ is a promising candidate in 2D FETs and heterojunctions due to the high dielectric constant, good thermodynamic stability, and large band gap for applications. The interfacial properties and band edge offset of the ZrO₂–MoS₂ heterojunction are investigated herein, and we show that the electronic states near the VBM and CBM are dominated by the contributions from monolayer MoS₂, and the interface with monolayer ZrO₂ will significantly decrease the band gap of the monolayer MoS₂.

The ultrathin ZrO₂ dielectric layer reveals structural stability in contrast to its bulk form, large band gap and high dielectric constant.     

Effect of interfacial defects on the electronic properties of MoS2 based lateral T–H heterophase junctions

The coexistence of semiconducting (2H) and metallic (1T) phases of MoS₂ monolayers has further pushed their strong potential for applications in the next generation of electronic devices based on two-dimensional lateral heterojunctions. Structural defects have considerable effects on the properties of these 2D devices. In particular, the interfaces of two phases are often imperfect and may contain numerous vacancies created by phase engineering techniques, e.g. under an electron beam. Here, the transport behaviors of the heterojunctions with the existence of point defects are explored by means of first-principles calculations and non-equilibrium Green''s function approach. While vacancies in semiconducting MoS₂ act as scattering centers, their presence at the interface improves the flow of the charge carriers. In the case of V_Mo, the current has been increased by two orders of magnitude in comparison to the perfect device. The enhancement of transmission was explained by changes in the electronic densities at the T–H interface, which open new transport channels for electron conduction.

Our systematic study shows significant improvement in transport properties of MoS₂-based lateral T–H heterophase junctions when interfacial defects are present.

Among the developing family of two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) provide some of the most diverse electronic properties including acting as topological insulators, semiconductors, (semi)metals and superconductors.^1–3 Noticeably, such a difference in the electronic structure of TMDs correlates with their structural configurations, called phases.⁴ Monolayers of MoS₂ in the H-phase, with trigonal prismatic coordination of metal atoms, is a semiconducting material,^5,6 while T-phase with octahedral coordination shows metallic character. The H-phase monolayer is reported to be a promising material for field-effect transistors (FETs) with small-scale channel lengths and negligible current leakage.^5,6Recent experiments have already shown controlled transitions from one phase to another via external stimuli such as electron beam,⁷ ion intercalation,⁸ or laser irradiation.⁹ These phase-engineered 2D materials with minimum variations in atomic structure and uniform stoichiometry not only demonstrate rich physical behavior but also open up new avenues for the design of electronic devices. The fabrication of lateral metallic/semiconducting heterostructures has been suggested as a practical method to minimize the contact resistance at the interface between 2D semiconductors and metal electrodes. In particular, the formation of covalent bonds between the two phases can introduce paths for carriers to travel across the interfaces, thus, the Schottky barrier and contact resistance are reduced.^10–13 It has also been demonstrated that 1T-phase engineered electrodes in MoS₂ based electronic devices would generate ohmic contacts and, as a result, improve electrical characteristics.^12,14Apart from intrinsic defects, the local phase transitions induced by electron beam irradiation may give rise to the formation of point defects, in particular at the interface of the two phases.^15–22 Defects can also be intentionally introduced during the post-growth stage via ion bombardment, plasma treatment, vacuum annealing, or chemical etching.^15–22 Indeed, theoretical and experimental results showed that the presence of sulfur vacancies can decrease the energy difference between the H and T phases and eventually stabilize the 1T phase in MoS₂ monolayer.^23,24 The presence of point defects in semiconducting MoS₂ monolayers leads to the observation of the localized states in their electronic structure, which act as short-ranged scattering centers for charge carriers.^25–28 Hence, defects were found to deteriorate the mobility of the fabricated devices.^29–31 It was also shown that sulfur line vacancies in MoS₂ can behave like pseudo-ballistic wire for electron transport.³²So far, several theoretical studies have reported the transport properties of phase-engineered devices based on TMDs monolayers including MoS₂ based lateral junctions.^{11,12,33–35,35–37} In most of these studies, however, it is assumed that two phases have a perfect crystalline structure and connected via an atomically sharp and defect-free interface.Here, transport properties of devices based on MLs MoS₂, containing various point vacancies and antisites at the interface between metallic and semiconducting phases, are the subject of the present study. Our systematic investigations show significant improvements in the current, as molybdenum vacancy and vacancy complexes are created at the interfaces of two phases. These findings render defect engineering as an efficient route to further improve the performance of the devices based on the lateral heterojunctions formed from TMDs.     

Adsorption performance of M-doped (M = Ti and Cr) gallium nitride nanosheets towards SO2 and NO2: a DFT-D calculation

The structure, adsorption characteristics, electronic properties, and charge transfer of SO₂ and NO₂ molecules on metal-doped gallium nitride nanosheets (M-GaNNSs; M = Ti and Cr) were scrutinized at the Grimme-corrected PBE/double numerical plus polarization (DNP) level of theory. Two types, M_Ga-GaNNSs and M_N-GaNNSs, of doped nanostructures were found. The M_Ga sites are more stable than the M_N sites. The results showed that adsorption of SO₂ and NO₂ molecules on Ti_Ga,N-GaNNSs is energetically more favorable than the corresponding Cr_Ga,N-GaNNSs. The stability order of complexes is energetically predicted to be as NO₂–Ti_Ga-GaNNS > NO₂–Ti_N-GaNNS > SO₂–Ti_Ga-GaNNS > NO₂–Cr_N-GaNNS > SO₂–Ti_N-GaNNS > NO₂–Cr_Ga-GaNNS > SO₂–Cr_N-GaNNS > SO₂–Cr_Ga-GaNNS. The electron population analysis shows that charge is transferred from M_Ga,N-GaNNSs to the adsorbed gases. The Ti_Ga-GaNNS is more sensitive than the other doped nanostructures to NO₂ and SO₂ gases. It is estimated that the sensitivity of Ti_Ga-GaNNS to NO₂ gas is more than to SO₂ gas.

The influences of transition metals (Cr and Ti) doping on the adsorption behavior of SO₂ and NO₂ gases on the metal doped Gallium Nitride Nanosheet (GaNNS) were explored at Grimme-corrected PBE/double numerical plus polarization (DNP) level of theory.     

Anomalous kinetic roughening in growth of MoS2 films under pulsed laser deposition

Growth dynamics of thin films expressed by scaling theory is a useful tool to quantify the statistical properties of the surface morphology of the thin films. To date, the growth mechanism for 2D van der Waals materials has been rarely investigated. In this work, an experimental investigation was carried out to identify the scaling behavior as well as the growth mechanism of 2D MoS₂ thin films, grown on glass substrates by pulsed laser deposition for different deposition time durations, using atomic force microscopy images. The growth of MoS₂ films evolved from layer-by-layer to layer plus island with the increase in deposition time from 20 s to 15 min. The film surface exhibited anisotropic growth dynamics in the vertical and lateral directions where RMS roughness varied with deposition time as w ∼ t^β with the growth exponent β = 0.85 ± 0.11, while the lateral correlation length ξ was ξ = t^1/z with 1/z = 0.49 ± 0.09. The films showed a local roughness exponent α_loc = 0.89 ± 0.01, global roughness exponent α = 1.72 ± 0.14 and spectral roughness exponent α_s = 0.85 ± 0.03, suggesting that the growth of MoS₂ thin films followed intrinsic anomalous scaling behavior (α_s < 1, α_loc = α_s ≠ α). Shadowing owing to conical incoming particle flux distribution towards the substrate during deposition has been attributed to the anomalous growth mode. The optical properties of the films, extracted from ellipsometric analysis, were also correlated with RMS roughness and cluster size variation which unveiled the important role played by surface roughness and film density.

MoS₂ films grown on glass by pulsed laser deposition technique evolve from bilayer to bulk-like structure with time following intrinsic anomalous scaling behaviour caused by shadowing effect during deposition.     

Enhanced sunlight-driven photocatalytic property of Mg-doped ZnO nanocomposites with three-dimensional graphene oxide/MoS2 nanosheet composites

Graphene oxide (GO) has been the focus of attention as it can enhance the photocatalytic activity of semiconductors due to its large specific surface area and remarkable optical and electronic properties. However, the enhancing effect is not ideal because of its easy self-agglomeration and low electronic conductivity. To improve the enhancing effect of GO for ZnO, three-dimensional GO/MoS₂ composite carriers (3D GOM) were prepared by electrostatic interactions and then, Mg-doped ZnO nanoparticles (MZ) were supported on the surface of 3D GOM by utilizing the layer-by-layer assembly method. Compared with GO/Mg-ZnO composite (GOMZ), the resultant three-dimensional GO/MoS₂/Mg-ZnO composite (GOMMZ) exhibited excellent photocatalytic performance due to the effective synergistic effect between GO and MoS₂ sheet, and its degradation rate was nearly 100% within 120 min of exposure to visible light; this degradation rate was nearly 8 times higher than that of the GOMZ composite. Moreover, the introduction of the MoS₂ sheet intensified the photocurrent density of the GOMZ composite and endowed it with optical memory ability.

Graphene oxide (GO) has been the focus of attention as it can enhance the photocatalytic activity of semiconductors due to its large specific surface area and remarkable optical and electronic properties.