Properties Of Monolayer MoS2 Research Articles

Effect of Introducing Defects and Doping on Different Properties of Monolayer MoS2

Herein, the comprehensive study of different properties of undoped MoS2, MoS2 lattice with sulfur (S) and, molybdenum (Mo) vacancy, and MoS2 with substitutional doping of niobium (Nb), vanadium (V), and zinc (Zn) atoms is done. The density functional theory (DFT) is used and the electronic properties like density of states, band structure, electron density, and optical properties like dielectric function, optical conductivity, and refractive index are studied. It is observed that undoped MoS2 monolayer shows direct bandgap semiconductor characteristics with a bandgap of around 1.79 eV. P‐type characteristics are observed for Nb‐, V‐, and Zn‐doped MoS2 lattices. The real part and imaginary parts of all optical parameters along x and z directions for different MoS2 supercells are found to be anisotropic in nature up to a photon energy of almost 11 eV and thereafter they show nearly isotropic nature. Finally, it is found that the obtained properties of MoS2 monolayer as per literature are suitable for next‐generation MOSFET application.

First Principles Study on Properties of Monolayer MoS2 Under Different Strains

First Principles Study on Properties of Monolayer MoS2 Under Different Strains

Effects of photoelectric properties of monolayer MoS<sub>2</sub> under tensile strain

Monolayer molybdenum disulfide is an ideal material for making various micro/nano components and flexible electronic devices. However, the strain of material caused by the environment is a key problem that cannot be avoided in practical applications, and the electronic structure of material will also change with the strain. In this paper, the effect of tensile strain on the photoelectric properties of monolayer MoS<sub>2</sub> is studied based on first principles and tensile tests after wet transfer. The results are obtained as follows. 1) Intrinsic monolayer MoS<sub>2</sub> is a direct bandgap semiconductor with a band gap of 1.68 eV, the highest peak of the absorption coefficient curve is nearly 10.92 eV, and a maximum absorption coefficient is 1.66 × 10<sup>5</sup> cm<sup>–1</sup>. 2) A small tensile strain (1%) will result in the transition from direct to indirect gap for monolayer MoS<sub>2</sub>. With the increase of strain, the feature of the indirect gap can be preserved but the gap decreases linearly. The gap will decrease to 0 eV when the tensile strain is 10%, and the absorption coefficient curve is red-shifted as a whole with strain. 3) The in-plane mode peak and the out-of-plane mode A<sub>1g</sub> peak in Raman spectra are re-dshifted with stretching by tensile test of wet-transferred monolayer MoS<sub>2</sub>, and the difference in peak frequency between the two peaks is maintained at about 18.6 cm<sup>–1</sup>. The strong emission peak of an exciton of monolayer MoS<sub>2</sub> is observed at 1.83 eV of the photoluminescenc spectrum. With the increase of tensile strain, the relative strength of the peak decreases and is linearly re-dshifted, which means that the band gap decreases linearly. It is consistent with the theoretical calculation result.

Spin polarization in monolayer MoS2 in the presence of proximity-induced interactions

When monolayer (ML) MoS2 is placed on a substrate, the proximity-induced interactions such as the Rashba spin-orbit coupling (RSOC) and exchange interaction (EI) can be introduced. Thus, the electronic system can behave like a spintronic device. In this study, we present a theoretical study on how the presence of the RSCO and EI can lead to the band splitting, the lifting of the valley degeneracy and to the spin polarization in [Formula: see text]- and [Formula: see text]-type ML MoS2. We find that the maxima of the in-plane spin orientation in the conduction and valence bands in ML MoS2 depend on the Rashba parameter and the effective Zeeman field factor. At a fixed Rashba parameter, the minima of the split conduction band and the maxima of the split valence band along with the spin polarization in ML MoS2 can be tuned effectively by varying the effective Zeeman field factor. On the basis that the EI can be induced by placing the ML MoS2 on a ferromagnetic substrate or by magnetic doping in ML MoS2, we predict that the interesting spintronic effects can be observed in [Formula: see text]- and [Formula: see text]-type ML MoS2. This work can be helpful to gain an in-depth understanding of the basic physical properties of ML MoS2 for application in advanced electronic and optoelectronic devices.

The modulation of the TiAlSiN coating on the band gap of monolayer MoS2

ABSTRACT MoS2 is a two-dimensional semiconductor nanoelectronic device, the application value and application field of which can be increased based on band-gap modulation. In this study, monolayer MoS2 samples were prepared on four TiAlSiN-coated substrates with different Ti compositions using the mechanical stripping approach. The band-gap variation in monolayer MoS2 was studied using the photoluminescence (PL) spectrum. The modulation in the electronic properties of monolayer MoS2 when the TiAlSiN coating was applied was obtained based on the first-principle calculations. Furthermore, contact models were obtained for three TiAlSiN coatings with different Ti compositions with respect to monolayer MoS2. The state densities of the three crystal structures as well as the differences between the charge density and the state of density at the contact surface were evaluated. After the analysis, the calculated results were observed to agree with the experimental results, presenting an explanation for the modulation mechanism.

Collective Dipole‐Dominated Doping of Monolayer MoS2: Orientation and Magnitude Control via the Supramolecular Approach

AbstractMolecular doping is a powerful, tuneable, and versatile method to modify the electronic properties of 2D transition metal dichalcogenides (TMDCs). While electron transfer is an isotropic process, dipole‐induced doping is a collective phenomenon in which the orientation of the molecular dipoles interfaced to the 2D material is key to modulate and boost this electronic effect, despite it is not yet demonstrated. A novel method toward the molecular functionalization of monolayer MoS2 relying on the molecular self‐assembly of metal phthalocyanine and the orientation‐controlled coordination chemistry of axial ligands is reported here. It is demonstrated that the subtle variation of position and type of functional groups exposed on the pyridinic ligand, yields a molecular dipole with programed magnitude and orientation which is capable to strongly influence the opto‐electronic properties of monolayer MoS2. In particular, experimental results revealed that both p‐ and n‐type doping can be achieved by modulating the charge carrier density up to 4.8 1012 cm−2. Density functional theory calculations showed that the doping mechanism is primarily resulting from the effect of dipole‐induced doping rather than charge transfer. The strategy to dope TMDCs is a highly modulable and robust, and it enables to enrich the functionality of 2D materials‐based devices for high‐performance applications in optoelectronics.

Flexible engineering of light emission in monolayer MoS2 via direct laser writing for multimode optical recording

Direct laser writing has been proved to be capable for effective modulation of optical and electrical properties of various nanomaterials. In this work, we designed a flexible direct laser writing approach to engineer photoluminescence (PL) properties of monolayer MoS2 and present its potential application in optical recording. PL spectra evolution of monolayer MoS2 under continuous-wave laser writing has been explored, where its PL energy can be precisely controlled by changing the writing time. This feature enables a multimode optical recording with spectral contrast in monolayer limit materials for high-capacity data storage technologies. Here, we constructed a proof-of-principle multimode optical recording on monolayer MoS2 based on the PL wavelength division multiplexing scheme and discussed the relevant challenges for practical applications. Our flexible approach with a mask-free feature and high spatial resolution is promising for applications in two-dimensional material-based information storage and optoelectronic devices.

Substrate-induced electronic localization in monolayer MoS2 measured via terahertz spectroscopy.

We study terahertz (THz) optoelectronic properties of monolayer (ML) MoS2 placed on different substrates such as SiO2/Si, sapphire, and quartz. Through the measurements of THz Fourier transform spectroscopy (2.5-6.5THz) and THz time-domain spectroscopy (TDS, 0.2-1.2THz), we find that the real part of optical conductivity increases for ML MoS2 on SiO2/Si and sapphire substrates and decreases for it on quartz with increasing radiation frequency. It is shown that the complex optical conductivity for ML MoS2, obtained from THz TDS measurements, can fit very well to the Drude-Smith formula. Thus, the dependence of optical conductivity of ML MoS2 on different substrates can be understood via a mechanism of electronic localization, and the electron density, relaxation time, and localization factor of the sample can be determined optically. Furthermore, we examine the influence of temperature on these key parameters in ML MoS2 on different substrates. The results obtained from this Letter indicate that THz spectroscopy is a very powerful tool in studying and characterizing ML MoS2-based electronic systems, especially in examining the electronic localization effect which cannot be directly measured in conventional electrical transport experiment. This Letter is relevant to an in-depth understanding of the optoelectronic properties of ML MoS2 and of the proximity effect induced by different substrates.

Probing the Effect of Chemical Dopant Phase on Photoluminescence of Monolayer MoS2 Using in Situ Raman Microspectroscopy

Understanding the role of chemical dopants is crucial to modulating the optoelectronic properties of monolayer (ML) MoS2 and realizing its optoelectronic applications such as photodetectors, switching devices, and ultrathin transistors. Here, the effect of the dopant phase of the same dopants—liquid and gaseous—on the optical properties of ML MoS2 has been investigated using in situ Raman microspectroscopy with an environment-controlled reaction cell. The results show that the gaseous n-type dopant, that is, pyridine, completely quenched the PL intensity of ML MoS2, while liquid pyridine preserved 50% of the original PL intensity attributed to its less effective charge transfer to MoS2 than the gaseous counterpart. Photoemission of the quenched MoS2 could not be recovered with mild annealing in nitrogen and only partially recovered upon long-term exposure to air. The results show that the dopant phase strongly impacts the PL properties of ML MoS2.

Research on the Factors Affecting the Growth of Large-Size Monolayer MoS₂ by APCVD.

The transition-metal chalcogenides (TMDs) are gaining increased attention from many scientists recently. Monolayer MoS2 is an emerging layered TMD material with many excellent physical and electrical properties. It can be widely used in catalysis, transistors, optoelectronics and integrated circuits. Here, the large-sized monolayer MoS2 is grown on the silicon substrate with a 285-nm-thick oxide layer by atmospheric pressure chemical vapor deposition (APCVD) of sulfurized molybdenum trioxide. This method is simple and it does not require vacuum treatment. In addition, the effects of growth conditions, such as sulfur source, molybdenum source, growth temperature, and argon flow rate on the quality and area of MoS2 are further studied systematically. These analysis results help to master the morphology and optical properties of monolayer MoS2. The high quality, excellent performance, and large-size monolayer MoS2 under the optimal growth condition is characterized by optical microscopy, AFM, XPS, photoluminescence, and Raman spectroscopy. The Raman spectrum and PL mapping show that the grown MoS2 is a uniform triangular monolayer with a side length of 100 μm, which can pave the way for the applications of photodetectors and transistors.

Low-temperature photoluminescence emission of monolayer MoS2 on diverse substrates grown by CVD

Monolayer MoS2 films on three different substrates, SiO2/Si, quartz and sapphire are synthesized by the method of chemical vapor deposition (CVD). Through a series of the optical characterization methods, the films are confirmed to be uniform and continuous on micrometer scale with a reliable quality. Low-temperature photoluminescence (PL) properties of monolayer MoS2 on diverse substrates are primarily studied. As the temperature increases, a decrease in PL intensity and a red shift in PL wavelength are observed. This is because the thermal motion of the phonons becomes stronger, which means that the nonradiative recombination increases and the PL intensity decreases. And the reduction of semiconductor band gap contributes to the redshift of PL. Furthermore, the influence of the lattice structure and thermal conductivity of the substrate on the low-temperature PL of monolayer MoS2 is also discussed.

Properties and Potential Applications of Quasi-Two-Dimensional Molybdenum Disulfide for Nanoelectronic Elements

Currently, the prospects for replacing traditional materials with quasi-two-dimensional compounds based on transition metal dichalcogenides are actively being studied. The quasi-two-dimensional molybdenum disulfide MoS2, a semiconductor with a finite band gap, can be used either as a standalone material or as a part of layered heterostructures. When creating nanosized electronics elements based on such ultrathin materials, the application of an atomic layer etching technology is of key importance. In this paper, a brief description of the properties of MoS2 monolayers in comparison with graphene and the monolayers of hexagonal boron nitride is considered. On the basis of the results of computer simulation by means of a DFT (density functional theory) method, effects caused in the MoS2 monolayer by chlorine atoms and molecules widely used in the state-of-the-art of atomic layer etching applied to silicon materials are demonstrated.

Electronic structure and spin/valley transport properties of monolayer MoS2 under the irradiation of the off-resonant circularly polarized light

The new-type monolayer semiconductor material molybdenum disulfide (MoS2) is direct band gap semiconductor with a similar geometrical structure to graphene, and as it owns superior physical features such as spin/valley Hall effect, it should be more excellent than graphene from the viewpoint of device design and applications. The manipulation of the spin and valley transport in MoS2-based device has been an interesting subject in both experimental and theoretical researches. Experimentally, the photoninduced quantum spin and valley Hall effects may result in high on-off speed spin and/or valley switching based on MoS2. Theoretically, the off-resonant electromagnetic field induced Floquet effective energy should modulate effectively the electronic structure, spin/valley Hall conductance as well as the spin/valley polarization of the MoS2, through the virtual photon absorption and/or emission processes. Utilizing the low energy effective Hamilton model from the tight-binding approximation and Kubo linear response theorem, we theoretically investigate the electronic structure and spin/valley transport properties of the monolayer MoS2 under the irradiation of the off-resonant circularly polarized light in the present work. The band gaps around the K and K' point of the Brillouin region for monolayer MoS2 proves to increase linearly and decrease firstly and then increase, respectively with the increase of external off-resonant right-circularly polarized light induced effective coupling energy, and decrease firstly and then increase and increase linearly with the increase of left-circularly polarized light induced effective coupling energy, therefore, the interesting transition of semiconducting-semimetallic-semiconducting may be observable in monolayer MoS2. Furthermore, the spin and valley Hall conductance of the monolayer MoS2 for the case without off-resonant circularly polarized light are 0 and 2e2/h, respectively, and they will convert into -2e2/h and 0 when the absolute value of the off-resonant circularly polarized light induced effective coupling energy is in a range of 0.79-0.87 eV. Finally, the spin polarization for monolayer MoS2 increases up to a largest value and changes from positive to negative and/or negative to positive at the vicinity of the effective coupling energy ±0.79 eV of the off-resonant right/left circularly polarized light, while the valley polarization should increase firstly and then decrease with the off-resonant circularly polarized light, and goes up to 100% in the range of 0.79-0.87 eV of the absolute value for effective coupling energy. Therefore, the external off-resonant circularly polarized electromagnetic field should be an effective means in manipulating the electronic structure, spin/valley Hall conductance and spin/valley polarization of the monolayer MoS2, the two-dimensional MoS2 may be tuned into a brand bandgap material with excellent spin/valley and optoelectrical properties.

Electron dynamics and optical properties modulation of monolayer MoS2 by femtosecond laser pulse: a simulation using time-dependent density functional theory

In this study, we adopted time-dependent density functional theory to investigate the optical properties of monolayer MoS2 and the effect of intense few-cycle femtosecond laser pulses on these properties. The electron dynamics of monolayer MoS2 under few-cycle and multi-cycle laser irradiation were described. The polarization direction of the laser had a marked effect on the energy absorption and electronic excitation of monolayer MoS2 because of anisotropy. Change in the polarization direction of few-cycle pulse changed the absorbed energy by a factor over 4000. Few-cycle pulse showed a higher sensitivity to the electronic property of material than multi-cycle pulse. The modulation of the dielectric properties of the material was observed on the femtosecond time scale. The negative divergence appeared in the real part of the function at low frequencies and photoinduced blue shift occurred due to Burstein-Moss effect. The irradiation of femtosecond laser caused the dielectric response within the infrared region and introduced anisotropy to the in-plane optical properties. Laser-based engineering of optical properties through controlling transient electron dynamics expands the functionality of MoS2 and has potential applications in direction-dependent optoelectronic devices.

First-principles investigation of the effect of charged unit cell on the electronic structure of two-dimensional MoS2

Abstract Density-functional theory has been applied to investigate the effect of charged unit cell on the structural and electronic properties of two-dimensional MoS2 within PBE-GGA. The charge of the unit cell of the monolayer changes from zero to n = ± 4 e with e the absolute value of the elementary electric charge. Variations of the lattice constant, Mo–S bond length, S–Mo–S bond angle, total energy, exchange and correlation contributions, and the Fermi level versus n have been calculated quantitatively, indicating decrease in the stability of the atomic structure of the monolayer with increase in the absolute value of n. It is found that the Fermi level for two-dimensional MoS2 is a function of both the number of electrons in allowed states and the inverse of the volume of the unit cell. The electronic properties of each monolayer have been also calculated via examining the related electronic band structure and density of states. Results broadly support the view that the effect of charged unit cell ( n =+ e to − 4 e ) on the electronic properties of MoS2 monolayer is manifested in the form of semiconductor-to-metal transition in addition to the Fermi level shift. It is also verified that as the negative charge of the unit cell increases from n = − e to − 4 e , there is an ever-increasing trend in the total number of allowed electronic states at the Fermi level, implying a direct correlation between electrical conductivity and the value of n in a way that the more negative the charge of the unit cell, the higher the electrical conductivity of the monolayer.

Large-Scale Synthesis and Systematic Photoluminescence Properties of Monolayer MoS2 on Fused Silica.

Monolayer MoS2, with fascinating mechanical, electrical, and optical properties, has generated enormous scientific curiosity and industrial interest. Controllable and scalable synthesis of monolayer MoS2 on various desired substrates has significant meaning in both basic scientific research and device application. Recent years have witnessed many advances in the direct synthesis of single-crystalline MoS2 flakes or their polycrystalline aggregates on numerous diverse substrates, such as SiO2-Si, mica, sapphire, h-BN, and SrTiO3, etc. In this work, we used the dual-temperature-zone atmospheric-pressure chemical vapor deposition method to directly synthesize large-scale monolayer MoS2 on fused silica, the most ordinary transparent insulating material in daily life. We systematically investigated the photoluminescence (PL) properties of monolayer MoS2 on fused silica and SiO2-Si substrates, which have different thermal conductivity coefficients and thermal expansion coefficients. We found that there exists a stronger strain on monolayer MoS2 grown on fused silica, and the strain becomes more obvious as temperature decreases. Moreover, the monolayer MoS2 grown on fused silica exhibits the unique trait of a fractal shape with tortuous edges and has stronger adsorbability. The monolayer MoS2 grown on fused silica may find application in sensing, energy storage, and transparent optoelectronics, etc.

Charge compressibility and quantum magnetic phase transition inMoS2

We investigate the ground-state properties of monolayer MoS$_2$ incorporating the Coulomb interaction together with a short-range intervalley interaction between charged particles between two valleys within the Hartree-Fock approximation. We consider four variables as independent parameters, namely homogeneous charge (electron or hole) density, averaged dielectric constant, spin degree of freedom and finally the Hubbard repulsion coefficient which originates mostly from $4d$ orbits of Mo atoms. We find the electronic charge compressibility within the mean-field approximation and show that non-monotonic behavior of the compressibility as a function of carrier density which is rather different from those of the two-dimensional electron gas. We also explore a paramagnetic-to-ferromagnetic quantum phase transition for the wide range of the electron density in the parameter space.

The Dilute Magnetic Properties of Monolayer MoS<sub>2</sub> Doped with Transition Metal Fe and VA Atoms

The two-dimensional material MoS2 has attracted a growing attention due to its potential applications in electronic devices in recent years [1,2,3], and the monolayer MoS2 is a direct gap semiconductor with a band gap of 1.8eV [4]. In the existing studies, it has indicated that MoS2 can get an available magnetism with doping transition metal atoms [5], and is expected to be a new generation of diluted magnetic semiconductor (DMS) [6,7]. Moreover, we found that Fe-doped MoS2 could present a strong magnetism but a semimetal characteristic, losing its original semiconductor properties while obtaining magnetism. Therefore, it is necessary to explore some methods to make monolayer MoS2 exhibit both magnetic and semiconductor properties. In this paper, we propose the method of N, Fe atoms co-doping to achieve this objective. The structural, electronic and magnetic properties of MoS2 doped with transition metal Fe and VA atoms have been investigated by first principle calculations based on density functional theory. The 3×3×1 supercell of monolayer MoS2 as a calculation model has been used. The result shows that pure MoS2 has no magnetism, while Fe-doped MoS2 exhibits a good magnetism about 1.849μB but a semimetal characteristic. This is due to that Mo-4d, S-2p, Fe-3d states has a strong coupling around the Femi energy for the introduction of Fe atom, and the Femi energy only pass through the spin-up density of states. For the co-doping with VA atoms and Fe atoms, it is found that the magnetic moment of Fe-N, Fe-P and Fe-As co-doped MoS2 is 0.956μB, 0.775μB, 0.782μB. Moreover, the Fe-N co-doped MoS2 presents semiconductor characteristics, in contrast, Fe-P and Fe-As co-doped MoS2 appear semimetal properties. It indicates that the semimetal characteristic of Fe-doped MoS2 could change into indirect band gap semiconductor due to the introduction of N atom. The band gap is 0.2eV. Our study demonstrate that the method of Fe, N co-doping could make MoS2 have good magnetic and also semiconductor properties at the same time.

Tuning the electronic property of monolayer MoS2 adsorbed on metal Au substrate: a first-principles study

Using first principles calculations within density functional theory, we investigate the electronic property of a single-layer MoS2 adsorbed on Au. All the quantities are calculated using the Vienna ab initio simulation package. Calculations are performed using the projector augmented wave method with the Perdew-Burke-Ernzerhof functional and a kinetic energy cutoff of 400 eV. The atomic plane and its neighboring image are separated by a 15 Å vacuum layer. The k-meshes for the structure relaxation and post analysis are 9×9×1 and 19×19×1, respectively. The spin-orbit coupling is considered in the calculation. The research includes the binding energy, the band structure, density of states (DOS) and electric charge difference density. Three contact modes between MoS2 (0001) and Au (111) are considered. When the atom S layer and the atom Au layer on the contacting interface have the same structure, the minimum binding energy and distance between MoS2 (0001) and Au(111) are 2.2 eV and 2.5 Å respectively. The minimum binding energy confirms that the absorption is unstable. The band structure demonstrates that the MoS2-Au contact nature is of the Schottky-barrier type, and the barrier height is 0.6 eV which is bigger than MoS2-Sc contact. By comparison with other metal contacts such as Ru(0001), Pd(111) and Ir(111), the dependence of the barrier height on the work function difference exhibits a Fermi-level pinning. But the MoS2 is so thin that the Fermi-level pinning must be very small. Maybe there is a metal induced gap state. DOS points out that the Au substrate has no influence on the covalent bond between Mo and S. The influence of the Au substrate is that it shifts the DOS of monolayer MoS2 left on the axis. The change of DOS results in the increases of electron concentration and electric conductivity. Other calculation points out that Ti substrate can excite more electrons. Electric charge density difference demonstrates that there are a few electric charges that transfer on the contact interface. The conducting path of monolayer MoS2 may emerge at the interface between Au and MoS2. In summary, the Au electrode is not the best electrode in the MoS2 device. The Ti electrode can excite more electrons from MoS2. The work function of Sc electrode is close to the affine of MoS2. The Fermi energy level of graphene can be tuned by external voltage. So the Ti, Sc and graphene will be the better electrodes for MoS2 device. Results of this study may provide a theoretical basis for single-layer MoS2 transistor and guidance for its applications.

Electronic and magnetic properties of 3d-metal trioxides superhalogen cluster-doped monolayer MoS2: A first-principles study

Abstract Utilizing first-principle calculations, the structural, electronic, and magnetic properties of monolayer MoS 2 doped with 3d transition-metal (TM) atoms and 3d-metal trioxides (TMO 3 ) superhalogen clusters are investigated. 3d-metal TMO 3 superhalogen cluster-doped monolayers MoS 2 almost have negative formation energies except CoO 3 and NiO 3 doped monolayer MoS 2 , which are much lower than those of 3d TM-doped structures. 3d-metal TMO 3 superhalogen clusters are more easily embedded in monolayer MoS 2 than 3d-metal atoms. MnO 3 , FeO 3 , CoO 3 , and NiO 3 incorporated into monolayer MoS 2 are magnetic, and the total magnetic moments are approximately 1.0, 2.0, 3.0, and 4.0 μB per supercell, respectively. MnO 3 and FeO 3 incorporated into monolayer MoS 2 become semiconductors, whereas CoO 3 and NiO 3 incorporated into monolayer MoS 2 become half-metallic. Our studies demonstrate that the half-metallic ferromagnetic nature of 3d-metal TMO 3 superhalogen clusters-doped monolayer MoS 2 has a great potential for MoS 2 -based spintronic device applications.