Electronic Properties Of MoS2 Research Articles

Structural and electronic properties of MoS2 and MoSe2 monolayers grown by chemical vapor deposition on Au(111).

The exceptional electronic and photonic properties of the monolayers of transition metal dichalcogenides including the spin-orbit splitting of the valence and conduction bands at the K points of the Brillouin zone make them promising for novel applications in electronics, photonics and optoelectronics. Scalable growth of these materials and understanding of their interaction with the substrate is crucial for these applications. Here we report the growth of MoS2 and MoSe2 monolayers on Au(111) by chemical vapor deposition at ambient pressure as well as the analysis of their structural and electronic properties down to the atomic scale. To this aim, we apply ultrahigh vacuum surface sensitive techniques including scanning tunneling microscopy and spectroscopy, low-energy electron diffraction, X-ray and angle-resolved ultraviolet photoelectron spectroscopy in combination with Raman spectroscopy at ambient conditions. We demonstrate the growth of high-quality epitaxial single crystalline MoS2 and MoSe2 monolayers on Au(111) and show the impact of annealing on the monolayer/substrate interaction. Thus, as-grown and moderately annealed (<100 °C) MoSe2 monolayers are decoupled from the substrate by excess Se atoms, whereas annealing at higher temperatures (>250 °C) results in their strong coupling with the substrate caused by desorption of the excess Se. The MoS2 monolayers are strongly coupled to the substrate and the interaction remains almost unchanged even after annealing up to 450 °C.

Enhanced UV Absorption By 2D MoS2 Nanoparticles

MoS2 is one of the promising 2D materials that caught the interest of many research fields[1], [2] due to their size-dependent tunable bandgap and attractive magnetic, optical, and electrical properties[3]. Furthermore, recently there has been a growing interest in utilizing MoS2 for solar cell applications that demonstrated measurable device enhancements[4]–[6]. Hence, there is a great interest in understanding its potential for solar energy harvesting. In this study, we show a simple method to deposit a 2D layer of MoS2 nanoparticles (NPs) on top of Aluminum-doped Zinc oxide (AZO) layer (transparent conductive oxide) and investigate its spectral response and potential for application in optoelectronic systems.A thin film of 80 nm AZO layer was grown on a 4-inch quartz wafer using thermal Atomic Layer Deposition (ALD) with a 1:19 ratio which has shown good electrical and optical qualities for solar cell applications[7]. We deposited the MoS2 by spin-coating it on the AZO/quartz wafers for 40 sec at 1000 rpm. Incremental coating is carried on by dispersing seven layers with 500 μL of MoS2 in each step using a precise pipet to a cumulative dispersion volume of 3500 μL. The prepared samples were characterized using a UV-Vis-NIR spectrometer (Perkin Elmer Lambda) across a wide range of wavelengths (250-1200 nm) by measuring both transmittance and reflectance and calculating absorbance. Furthermore, the base AZO/quartz and quartz background signal were measured before spin-coating as reference.The obtained data shows a high absorbance effect due to MoS2 NPs at low wavelengths (<400 nm), where it peaks around 340 nm with an approximate absorbance of ~6.7%. Upon further examination, we notice that this behavior is not linear across the whole spectrum and instead is a function of (i) wavelength and (ii) MoS2 quantity which could be partially due to the quantum confinement effect of several layers of stacked 3D MoS2 nanoparticles[8]. This phenomenon could open the possibility of utilizing this material for low-wavelength filters or UV sensing applications[9]. Also, it can potentially be utilized for quantum down-conversion[10] of high-energy photons to re-emit photons at lower energies in order to enhance solar cells’ efficiencies and reduce thermal burden; however, further investigation is needed.[1] P. Zhou, C. Chen, X. Wang, B. Hu, and H. San, “2-Dimentional photoconductive MoS2 nanosheets using in surface acoustic wave resonators for ultraviolet light sensing,” Sensors and Actuators A: Physical, vol. 271, pp. 389–397, Mar. 2018, doi: 10.1016/j.sna.2017.12.007.[2] H. Dong et al., “Fluorescent MoS 2 Quantum Dots: Ultrasonic Preparation, Up-Conversion and Down-Conversion Bioimaging, and Photodynamic Therapy,” ACS Appl. Mater. Interfaces, vol. 8, no. 5, pp. 3107–3114, Feb. 2016, doi: 10.1021/acsami.5b10459.[3] K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically Thin MoS 2 : A New Direct-Gap Semiconductor,” Phys. Rev. Lett., vol. 105, no. 13, p. 136805, Sep. 2010, doi: 10.1103/PhysRevLett.105.136805.[4] Y. Tsuboi et al., “Enhanced photovoltaic performances of graphene/Si solar cells by insertion of a MoS 2 thin film,” Nanoscale, vol. 7, no. 34, pp. 14476–14482, 2015, doi: 10.1039/C5NR03046C.[5] N. A. Abd Malek et al., “Ultra-thin MoS2 nanosheet for electron transport layer of perovskite solar cells,” Optical Materials, vol. 104, p. 109933, Jun. 2020, doi: 10.1016/j.optmat.2020.109933.[6] Y.-J. Huang, H.-C. Chen, H.-K. Lin, and K.-H. Wei, “Doping ZnO Electron Transport Layers with MoS 2 Nanosheets Enhances the Efficiency of Polymer Solar Cells,” ACS Appl. Mater. Interfaces, vol. 10, no. 23, pp. 20196–20204, Jun. 2018, doi: 10.1021/acsami.8b06413.[7] S. Abdul Hadi, G. Dushaq, and A. Nayfeh, “Effect of atomic layer deposited Al 2 O 3 :ZnO alloys on thin-film silicon photovoltaic devices,” Journal of Applied Physics, vol. 122, no. 24, p. 245103, Dec. 2017, doi: 10.1063/1.4990871.[8] T. Li and G. Galli, “Electronic Properties of MoS 2 Nanoparticles,” J. Phys. Chem. C, vol. 111, no. 44, pp. 16192–16196, Nov. 2007, doi: 10.1021/jp075424v.[9] Z. Lou et al., “High-performance MoS_2/Si heterojunction broadband photodetectors from deep ultraviolet to near infrared,” Opt. Lett., vol. 42, no. 17, p. 3335, Sep. 2017, doi: 10.1364/OL.42.003335.[10] A. P. Sunitha, P. Praveen, M. K. Jayaraj, and K. J. Saji, “Upconversion and downconversion photoluminescence and optical limiting in colloidal MoS2 nanostructures prepared by ultrasonication,” Optical Materials, vol. 85, pp. 61–70, Nov. 2018, doi: 10.1016/j.optmat.2018.08.038.[11] Y. Wu et al., “Monolithic integration of MoS 2 -based visible detectors and GaN-based UV detectors,” Photon. Res., vol. 7, no. 10, p. 1127, Oct. 2019, doi: 10.1364/PRJ.7.001127. Figure 1

Effect of low-energy ion-beam irradiation on the structure and electron transport of monolayer MoS2

The presence of vacancies in two-dimensional (2D) materials, such as monolayer MoS2 (1L-MoS2), significantly affects their structural and electronic properties. In this study, 1L-MoS2 was irradiated with an Ar+ ion beam to introduce vacancies in a controlled manner. A complementary study that combines multiple evaluations was performed to verify the role of vacancies in the structural and electronic properties of MoS2. Raman spectroscopy and conductivity measurements reveal that the S vacancies, introduced into the MoS2 lattice prior to Mo vacancies at the lower dose region, cause electron-doping. The E’ lattice vibration mode was more sensitive to vacancies than the A’1 mode. According to photoluminescence studies, irradiating Ar+ at a lower dose of less than 3.8 × 1013 cm−2 does not change the band gap and only introduces mid-gap states, which is consistent with the smaller and larger influences of vacancies in carrier scattering and doping, respectively, in the electrical conductivity.

Investigating structural, electronic, magnetic, and optical properties of Co-doped and Co-X (X = Fe, Mn) co-doped MoS2 for optoelectronic applications.

We employ first-principle calculations to investigate structural, electronic, magnetic, and optical properties of cobalt and Co-X (X = Fe, Mn) co-doped MoS2. Result demonstrates that pure MoS2 is nonmagnetic, while Co and Co-Fe/Mn co-doping brings magnetism into MoS2 with magnetic moment values of 0 [Formula: see text], 2.022 [Formula: see text], 3.906 [Formula: see text], and 3.643 [Formula: see text] respectively. d states of dopants and p-d hybridization bring significant improvements in electronic properties of MoS2. Novelty of current work lies not only in origin of magnetism in the proposed materials but also in absorption spectra which show blueshift. We notice reduction in optical band gap with Co and Co-Fe/Mn co-doping. Enhanced absorption and conductivity with decrease in reflectivity illustrate potential uses of these materials for revolutionizing future of optoelectronics, spintronics, magneto-optics, and photonics devices. Moreover, crossroads of MoS2 and allied materials may further explore new avenues in sensing, artificial intelligence, and miniaturization of existing technology.

Electronic properties of MoS2/Be2C van der Waals heterostructure: Effect of Bi-axil strain and vertical electric field

In this paper, the structural and electronic properties of MoS 2 /Be 2 C heterostructure and their dependence on the biaxial strain and external electric field were investigated by first-principle calculations. The band alignment of the MoS 2 /Be 2 C heterostructure is type II. As the tensile strain is increase to 4%, the band gap of MoS 2 /Be 2 C heterostructure decreases from 0.28 to 0.19 eV. At the same time, an indirect to direct band gap transition occurs. With further increasing the tensile strain, a type II to type III transition occurs for tensile strain larger than 6%. Compressive strain induced a transition from type II to type III band alignment. The MoS 2 /Be 2 C heterostructure can remain the type II band alignment, but the band gap decreases and increases for the positive and negative electric field applied. Thus, the electronic properties of MoS 2 /Be 2 C vdW heterostructures can by tuned by the biaxial strain and external electric field. • The structural and electronic properties of MoS 2 /Be 2 C heterostructure were investigated by first-principle calculations. • As the tensile strain increases to 4%, an indirect to direct band gap transition occurs. • This heterostructure remains a type II band alignment, but the band gap decreases with increasing the electric field.

Electronic properties of intrinsic vacancies in single-layer CaF2 and its heterostructure with monolayer MoS2

Exploring gate insulator materials for 2D transistors and their defect properties is of importance for device performance optimization. In this work, the structural and electronic properties of intrinsic vacancies in the CaF2 single layer and its heterostructures with monolayer MoS2 are investigated from first-principles calculations. VCa introduces a shallow defect level close to the VBM, whereas VF introduces a deep level below the CBM. In both cases, spin polarization is observed. Overall, VF has a relatively lower formation energy than VCa, except for the extreme Ca-rich case. Thus, VF should be dominant in CaF2. The band offset between CaF2 and MoS2 is determined to be type-I, with large offsets at both the conduction band and valence band. With the presence of vacancies in CaF2, the type-I band offset is preserved. The electron or hole on the defect states will transfer from CaF2 to MoS2 due to the large band offset, and spin polarization vanishes. Nevertheless, there are no defect states inside the gap or around the band edge of MoS2, and the electronic properties of MoS2 are almost intact. Compared with h-BN that has a small valence band offset with MoS2 and could introduce in-gap defect states, CaF2 can be a good candidate to serve as the dielectric layer of MoS2-based transistors.

Substitutional doping at S site of MoS2/G heterostructure: The influence on voltage-current and electronic characteristics

The effect of substitutional doping at S site on the electronic properties of MoS 2 /Graphene (MoS 2 /G) was studied by First-principles calculations. Through the analysis on the energy band and density of states of MoS 2 /Graphene, it was found that the S vacancy in MoS 2 monolayer caused the Dirac point shifted, and produced the defect energy levels at the Dirac point, which hindered the carrier transmission. Periods II-IV elements were doped at the S vacancy to improve the electronic properties and performance of MoS 2 /G. The calculated results show that the charge transfer of MoS 2 /G was greatly enhanced by the doping of N, P and As atoms. The current-voltage characteristic of N, P and As-doped G/MoS 2 /MoS 2 /G structure has higher drive-current and stable OFF-state current which can reduce device current consumption. For O-doped and Se-doped MoS 2 /G heterostructures , there is no impurity level introduced, and the Schottky barrier height increase (decrease) with the increase (decrease) of the biaxial strain, meantime the p-type Schottky contact , n-type Schottky contact and Ohmic contact characteristic can be converted mutually by adjusting biaxial strain or vertical external force. Our findings will be of great guiding significance for the designing of interface devices and explorations on nano-electronics. • The passing carriers were trapped by S vacancy in MoS 2 /G heterostrcture. • The current-voltage characteristics of N-doped, P-doped and As-doped of G/MoS 2 /MoS 2 /G were improved. • The contact type of O-doped, Se-doped and undoped MoS 2 /G can be converted by adjusting external force.

Efficient utilization of light energy based on MoS&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-XS (X=Al, B, Ga) heterostructures

<p indent=0mm>In recent years, two-dimensional transition metal sulfide materials have received widespread attention due to their unique physical and chemical properties, and have been successfully applied in many fields such as microelectronic devices, photovoltaic technology and hydrogen evolution catalysis. However, high recombination rate, low mobility, and low cycle stability all limit the further improvement of the related properties of this kind of material to varying degrees. In our work, two-dimensional materials are composited to optimize the band structure and physical/chemical properties. This kind of approach has been proved to be feasible in many experiments. Not long ago, Demirci et al. studied the III-VI two-dimensional single-layer hexagonal structure MX (M = B, Al, Ga, In; X = O, S, Se, Te). A series of analyses show that all the above binary compounds are structurally stable. They believe that the III-VI group of binary monolayer materials can be widely used for high-performance optoelectronic devices. Based on density functional theory, this work proposes a new heterostructure form of MoS₂-XS (X=Al, B, Ga). Both MoS₂ and XS (M = B, Al, Ga) have the same hexagonal crystal structure (P-6m2), and there are reasons to believe that the MoS₂-XS heterogeneous structures will be fabricated under certain experimental condition. Therefore, it is meaningful to understand how the single-layer MoS₂ and XS form a stable two-dimensional heterostructure. At present, MoS₂-XS based structural characteristics and related physical/chemical properties of the heterostructures have not been reported. Therefore, the purpose of this research is to grasp the basic characteristics and potential application fields of the MoS₂-XS as far as possible. Based on the periodic boundary conditions of the two-dimensional structure, the establishment of a heterostructure requires us to strictly control the mismatch between MoS₂ and XS. We have obtained three highly reliable heterostructures, which maintain the lowest binding energies as well as the smallest mismatches after a series of preliminary calculations. The VBM and CBM of four pristine materials are composed of S atomic states to varying degrees, especially MoS₂ and AlS. Almost all of their band edges are derived from S atoms, which will greatly enhance the recombination of carriers. Therefore, the electrons and holes that can be actually used in the reaction are significantly reduced. How to suppress the recombination of carriers will be one of main goals in this work. The electronic properties of MoS₂-XS will be significantly affected by external strain. We conducted a detailed analysis and discussion on how the strain affects the band gap and band edges. From variable interlayer spacing, electric field to strain, MoS₂-AlS exhibits good catalytic activity, and has an ideal type-II band alignment. Therefore, we believe that MoS₂-AlS is the most ideal choice in terms of photocatalytic water splitting. It is proved that MoS₂-BS is unsuitable for the application of water splitting, but it has excellent efficiency in solar cell applications and its PCE can be as high as ~20.4%. It is worth noting that this value is very close to or even better than that of the reported heterostructure solar cells. In short, we believe that this work provides a meaningful reference for how to efficiently use light energy in two-dimensional composite materials.

Facile synthesis of vacancy-induced 2H-MoS2 nanosheets and defect investigation for supercapacitor application.

In this paper, a 2D molybdenum disulfide (MoS2) nanosheet is prepared via a one-step hydrothermal method as electrode material for supercapacitors. Meanwhile, a series of MoS2−x nanostructures with sulfur vacancies have been successfully obtained in an Ar/H2 mixed atmosphere at different annealing temperatures. The prepared materials were characterized by XRD, HR-TEM, Raman and XPS to identify their morphology and crystal properties. MoS2−x assembled by interconnected nanosheets (MoS2−x-700) provides a maximum specific capacitance of 143.12 F g−1 at a current density of 1.0 A g−1 with 87.1% of initial capacitance reserved after 5000 cycles. The outstanding performance of the annealed MoS2−x nanosheets in sodium storage is mainly attributed to the synergistic effect of the unique interconnected structure and the abundant active vacancy generated by the sulfur vacancies. Atomic models of sulfur vacancy defects on the basal plane, Mo-edge and S-edge were established and the electronic properties of MoS2−x were further evaluated assisted by first principles theory. DFT calculation results show that sulfur vacancy defects can provide additional empty states near the Fermi level and induce unpaired electrons, thus increasing the carrier density and improving electrical conductivity. Our findings in this work provide experimental and theoretical evidence of improving the electrochemical performance of 2H-MoS2 nanosheets by annealing treatment.

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 VMo, 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.

Effect of Ultraviolet Irradiation on Photo-Physical and Surface Electronic Properties of MoS₂.

In this study, exfoliation of molybdenum disulfide (MoS₂) using the chemical exfoliation method was successfully achieved via probe sonication followed by centrifugation. The observed ultravioletvisible (UV-Vis) spectra of the MoS₂ dispersions indicated the presence of a few layers of MoS₂. The morphological, structural, optical and surface electronic properties before and after UV light irradiation were investigated by the technique of X-ray diffraction, the Raman spectroscopic measurements, transmission electron microscopy (TEM), and the scanning Kelvin probe microscopy (SKPM). It was observed that after UV irradiation, the Fermi level moves towards the valence band.

Effect of Cu concentration and dopant site on the band gap of MoS2: A DFT study

Density functional theory (DFT) calculations have been performed to investigate the structural and electronic properties of molybdenum disulfide (MoS2) by doping a varying concentration of Cu as impurity. Additionally, substitutional site importance has also been elaborated. The results indicate that Cu doping can significantly modify the structural and electronic properties of MoS2. Replacement of single S atom with Cu atom reduces the band-gap to 1.04 eV relative to 1.95 eV of the pristine material. Adopting different arrangements of Cu atoms employing substitutional doping technique we computed a variety of band-gap values varying from 0.14 to 1.95 eV. The PDOS calculations allow us to speak that foreign impurity Cu forms good chemistry with host MoS2. Thus with variant concentration of impurity, band-gap of MoS2 can be tuned to a certain desired level making 2D MoS2 a suitable material for such electronic applications where band-gap is required similar to semiconducting materials.

Strain, electric-field and functionalization induced widely tunable electronic properties in MoS2/BC3, /C3N and / van der Waals heterostructures

In this paper, the effect of BC3, C3N and substrates on the atomic and electronic properties of MoS2 were systematically investigated using first-principles calculations. Our results show that the MoS2/BC3 and MoS2/C3N4 heterostructures are direct semiconductors with band gaps of 0.4 and 1.74 eV, respectively, while MoS2/C3N is a metal. Furthermore, the influence of strain and electric field on the electronic structure of these van der Waals heterostructures is investigated. The MoS2/BC3 heterostructure, for strains larger than −4%, transforms it into a metal where the metallic character is maintained for strains larger than −6%. The band gap decreases with increasing strain to 0.35 eV (at +2%), while for strain (>+6%) a direct-indirect band gap transition is predicted to occur. For the MoS2/C3N heterostructure, the metallic character persists for all strains considered. On applying an electric field, the electronic properties of MoS2/C3N4 are modified and its band gap decreases as the electric field increases. Interestingly, the band gap reaches 30 meV at +0.8 V/Å, and with increase above +0.8 V/Å, a semiconductor-to-metal transition occurs. Furthermore, we investigated effects of semi- and full-hydrogenation of MoS2/C3N and we found that it leads to a metallic and semiconducting character, respectively.

Impact of transverse and vertical gate electric field on vibrational and electronic properties of MoS2

Selectivity of the electric field direction plays a vital role in modulating the phonon characteristics as well as electrical properties in low-dimensional materials. A comprehensive study on the effects of the direction-dependent electric field on MoS2 sample is reported herewith. The field-induced changes in the phonon characteristics and electronic band structure have been systematically investigated based on field responsive Raman and photoluminescence measurements. The atomistic insights obtained from density functional theory calculations have been correlated with the experimental observations to elucidate the underlying mechanism. The applied transverse electric field is found to be significantly more efficacious than the electric field applied vertically in altering the phonon signatures and bandgap in MoS2, where the electrostrictive response is found to arise from the field-induced alteration in metal–chalcogen interatomic bonds.

Metallic edge states in zig-zag vertically-oriented MoS2 nanowalls

The remarkable properties of layered materials such as MoS2 strongly depend on their dimensionality. Beyond manipulating their dimensions, it has been predicted that the electronic properties of MoS2 can also be tailored by carefully selecting the type of edge sites exposed. However, achieving full control over the type of exposed edge sites while simultaneously modifying the dimensionality of the nanostructures is highly challenging. Here we adopt a top-down approach based on focus ion beam in order to selectively pattern the exposed edge sites. This strategy allows us to select either the armchair (AC) or the zig-zag (ZZ) edges in the MoS2 nanostructures, as confirmed by high-resolution transmission electron microscopy measurements. The edge-type dependence of the local electronic properties in these MoS2 nanostructures is studied by means of electron energy-loss spectroscopy measurements. This way, we demonstrate that the ZZ-MoS2 nanostructures exhibit clear fingerprints of their predicted metallic character. Our results pave the way towards novel approaches for the design and fabrication of more complex nanostructures based on MoS2 and related layered materials for applications in fields such as electronics, optoelectronics, photovoltaics, and photocatalysts.

Improvement of the arsine adsorption by doping on monolayer MoS2

The improvement of the arsine adsorption on monolayer MoS2 by doping has been investigated by first-principles calculation. The impurity atoms Si, P, and Cl, have been introduced to substitute S atoms to form an [Formula: see text]- or [Formula: see text]-type system. The electronic properties of MoS2 with dopants P and Cl are insensitive to the adsorption of AsH3. The Si-MoS2 with adsorbed AsH3 configuration has the largest adsorption energy, the lowest adsorbed height, and the most effective charge transfer. It is indicated that the properties of MoS2 can be improved by doping for detecting AsH3 molecules.

Strong Variation of Electronic Properties of MoS2 and WS2 Nanotubes in the Presence of External Electric Fields

Transition-metal dichalcogenides attracted a huge international research focus from the point of two-dimensional materials. These materials exist also as nanotubes; however, they have been mostly studied for their lubricant properties. Despite their interesting electronic properties, quite similar to their 2D counterparts, nanotubes remain much less explored. Like in 2D materials, electronic properties of nanotubes can be strongly modulated by external means such as strain or electric field. Here, we report on the effect of external electric fields on the electronic properties of MoS2 and WS2 nanotubes using density functional theory. We show that the electric field induces a strong polarization in these nanotubes, which results in a nearly linear decrease of the band gaps with the field strength and eventually in a semiconductor-metal transition. In particular for large tube diameters, this transition can occur for field strengths between 1–2 V·nm–1. This is an order of magnitude weaker than fields requi...

DFT study of structural and electronic properties of MoS2(1−x)Se2x alloy (x = 0.25)

First-principles calculations have been performed to study the structural features of the monolayer MoS2(1-x)Se2x (x = 0.25) alloy and its electronic properties. We studied the effects of the relative positions of Se atoms in a real monolayer alloy. It was demonstrated that the distribution of the Se atoms between the top and bottom chalcogen planes was most energetically favorable. For a more probable distribution of Se atoms, a MoS2(1-x)Se2x (x = 0.25) monolayer alloy is a direct semiconductor with a fundamental band gap equal to 2.35 eV (calculated with the GVJ-2e method). We also evaluated the optical band gap of the alloy at 77 K (1.86 eV) and at room temperature (1.80 eV), which was in good agreement with the experimentally measured band gap of 1.79 eV.

First principles study on 2H-1T' transition in MoS2 with copper.

The electronic properties of MoS2 are strongly controlled by the structure, providing a route to their modulation. We report, based on first principles calculations, that the adsorption of metal atom Cu on the surface can induce the phase transition of MoS2 from the semiconducting 2H to the metallic 1T' phase. Cu adsorption results in effective n-type doping of MoS2 by charge transfer from Cu in the case of the 1T' phase. This is distinct from the behavior in the 2H phase, where Cu does not donate any charge, and it is also distinct from alkali metal adsorption, where charge is donated to both 2H and 1T' MoS2. Charge donation to the 1T' phase by Cu stabilizes it with respect to the 2H structure and importantly, it also reduces the energy barrier between the 2H and 1T' structures. This difference reflects the higher electronegativity of Cu, which also indicates that Cu-modified MoS2 can be expected to be less chemically reactive than MoS2 with alkali metal adatoms. The main atomic mechanism of the structural transition is the gliding of S atoms on the upper surface. Finally, we report the energetics of the 2H to 1T' transition with several other adatoms, Ag, Au, Ni, Pt and Pd, but none of them are as effective as Cu in inducing the transition.

Structural and electronic properties of MoS2, WS2, and WS2/MoS2 heterostructures encapsulated with hexagonal boron nitride monolayers

In this study, we investigate the structural and electronic properties of MoS2, WS2, and WS2/MoS2 structures encapsulated within hexagonal boron nitride (h-BN) monolayers with first-principles calculations based on density functional theory by using the recently developed non-local van der Waals density functional (rvv10). We find that the heterostructures are thermodynamically stable with the interlayer distance ranging from 3.425 Å to 3.625 Å implying van der Waals type interaction between the layers. Except for the WS2/h-BN heterostructure which exhibits direct band gap character with the value of 1.920 eV at the K point, all proposed heterostructures show indirect band gap behavior from the valence band maximum at the Γ point to the conduction band minimum at the K point with values varying from 0.907 eV to 1.710 eV. More importantly, it is found that h-BN is an excellent candidate for the protection of intrinsic properties of MoS2, WS2, and WS2/MoS2 structures.