Properties Of Single-layer MoS2 Research Articles

Observation of robust superlubricity of MoS2 on Au(111) in ultrahigh vacuum

The structural and superlubric properties of single layer MoS2 on Au(111) forming moiré superlattice structures have been investigated by means of ultrahigh vacuum atomic force microscope with bimodal and contact modes. We synthesize epitaxial monolayer MoS2 flakes on the Au(111) surface in ultrahigh vacuum. Using friction force microscopy, atomic friction measurements indicate a superlubric regime between the tip apex and the moiré corrugated MoS2 surface in which the friction force remains at an ultralow value and is independent from normal load. Superlubricity conditions are observed for different loads and velocities which indicates the absence of out-of-plane deformations. We find that the MoS2 layer including the moiré superlattice modulation originating from the natural misfit between MoS2 and the Au(111) substrate is relatively rigid. We also demonstrate a low friction coefficient of the MoS2 surface crossing a single Au(111) step. Our results open up a new avenue for minimizing friction in nanoscale electronic devices and other dry rigid contacts used in aerospace lubrication.

Mechanical behavior of single layer MoS2 sheets with aligned defects under uniaxial tension

Compared with a single defect or randomly distributed defects, aligned defects are widely found or artificially designed in structures to realize various functions. However, the mechanical behavior of transition metal dichalcogenides with aligned defects is still unclear, which restricts the blooming application in novel flexible nanodevices. Herein, we report the strength and fracture properties of single layer MoS2 (SLMoS2) sheets with aligned defects under uniaxial tension by numerical calculation and theoretical modeling. It is found that the increase of the number of defects and adjacent spacing leads to the decrease of critical strain as well as tensile strength of both pristine and kirigami MoS2 sheets. Three types of crack propagation phenomena are discovered, and an effective theoretical model is employed to uncover the underlying mechanism of crack deflection phenomenon in SLMoS2 sheets with aligned defects. These results provide important insights into mechanical behavior of SLMoS2 sheets and should be helpful for potential applications of the new two-dimensional material.

Effect of intrinsic structural defects on mechanical properties of single layer MoS2

In this paper, the mechanical properties of single layer Molybdenum disulfide (SLMoS2) containing point defects, such as vacancies, or extra Sulfur (S) or Molybdenum (Mo) atoms are investigated by molecular dynamics (MD) simulations. It was found that the single S vacancy is most probable as the formation energy is the lowest among all point defects. Other antisite defects having a Mo atom replaced by two S atoms are the least possible ones. The structures of antisite defects are also unstable. The Young’s modulus of planner SLMoS2 with defect is found to be within the scatter margin of pristine MoS2 sheet (without defects). The effect of temperature on each type of defect has been studied and it is found that strength is more affected by vacancies than it is by phase transition.

Modification of the optoelectronic properties of two-dimensional MoS2 crystals by ultraviolet-ozone treatment

We report the modification of the optoelectronic properties of mechanically-exfoliated single layer MoS2 by ultraviolet-ozone exposure. Photoluminescence emission of pristine MoS2 monotonically decreased and eventually quenched as ultraviolet-ozone exposure time increased from 0 to 10 min. The reduction of photoluminescence emission accompanied reduction of Raman modes, suggesting structural degradation in ultraviolet-ozone exposed MoS2. Analysis with X-ray photoelectron spectroscopy revealed that the formation of SO and MoO bonding increases with ultraviolet-ozone exposure time. Measurement of electrical transport properties of MoS2 in a bottom-gate thin-film transistor configuration suggested the presence of insulating MoO3 after ultraviolet-ozone exposure. These results demonstrate that ultraviolet-ozone exposure can significantly influence the optoelectronic properties of single layer MoS2, providing important implications on the application of MoS2 and other two-dimensional materials into optoelectronic devices.

Tunable Doping in Hydrogenated Single Layered Molybdenum Disulfide

Structural defects in the molybdenum disulfide (MoS2) monolayer are widely known for strongly altering its properties. Therefore, a deep understanding of these structural defects and how they affect MoS2 electronic properties is of fundamental importance. Here, we report on the incorporation of atomic hydrogen in monolayered MoS2 to tune its structural defects. We demonstrate that the electronic properties of single layer MoS2 can be tuned from the intrinsic electron (n) to hole (p) doping via controlled exposure to atomic hydrogen at room temperature. Moreover, this hydrogenation process represents a viable technique to completely saturate the sulfur vacancies present in the MoS2 flakes. The successful incorporation of hydrogen in MoS2 leads to the modification of the electronic properties as evidenced by high resolution X-ray photoemission spectroscopy and density functional theory calculations. Micro-Raman spectroscopy and angle resolved photoemission spectroscopy measurements show the high quality of the hydrogenated MoS2 confirming the efficiency of our hydrogenation process. These results demonstrate that the MoS2 hydrogenation could be a significant and efficient way to achieve tunable doping of transition metal dichalcogenides (TMD) materials with non-TMD elements.

A density functional theory study of electronic and magnetic properties of rare earth doped monolayered molybdenum disulphide

The effects of Rare Earth (RE) atoms Sm, Eu, Gd, Tb, and Dy doping on structural, electronic, and magnetic properties of single layer MoS2 were investigated using first principles calculations. The analysis of electronic properties pointed out the appearance of host-impurity hybrid states on the edges of principal bands of the material, which appeared to cause narrowing of its bandgap. The values of total magnetic moment were calculated as 3.3 μB, 8.1 μB, 8.5 μB, 6.8 μB, and 6.4 μB for Sm-, Eu-, Gd-, Tb-, and Dy-doped MoS2, respectively. The underestimation of magnetic moment in the case of Sm doping is found which is assigned to shielding of 4f moments and quenching of angular momentum caused by Sm 4f–S 3p hybridization. On the other hand, the observed overestimation of magnetic moments for Eu and Gd cases is likely to be due to increase in angular momentum due to indirect exchange interaction of highly localized 4f states via 5d or 6s conduction electrons. The findings of this work point out the possibilities of tuning the band gap and magnetic properties of monolayer MoS2 upon RE doping for realization of p-type ferromagnetic semiconductors.

Phase-driven magneto-electrical characteristics of single-layer MoS2.

Magnetism of the MoS2 semiconducting atomic layer was highlighted for its great potential in the applications of spintronics and valleytronics. In this study, we demonstrate an evolution of magneto-electrical properties of single layer MoS2 with the modulation of defect configurations and formation of a partial 1T phase. With Ar treatment, sulfur was depleted within the MoS2 flake leading to a 2H (low-spin) → partial 1T (high-spin) phase transition. The phase transition was accompanied by the development of a ferromagnetic phase. Alternatively, the phase transition could be driven by the desorption of S atoms at the edge of MoS2via O2 treatment while with a different ordering magnitude in magnetism. The edge-sensitive magnetism of the single-layer MoS2 was monitored by magnetic force microscopy and validated by a first-principle calculation with graded-Vs (sulfur vacancy) terminals set at the edge, where band-splitting appeared more prominent with increasing Vs. Treatment with Ar and O2 enabled a dual electrical characteristic of the field effect transistor (FET) that featured linear and saturated responses of different magnitudes in the Ids-Vds curves, whereas the pristine MoS2 FET displayed only a linear electrical dependency. The correlation and tuning of the Vs-1T phase transition would provide a playground for tailoring the phase-driven properties of MoS2 semiconducting atomic layers in spintronic applications.

A Revisit to High Thermoelectric Performance of Single-layer MoS2.

Both electron and phonon transport properties of single layer MoS2 (SLMoS2) are studied. Based on first-principles calculations, the electrical conductivity of SLMoS2 is calculated by Boltzmann equations. The thermal conductivity of SLMoS2 is calculated to be as high as 116.8 Wm−1K−1 by equilibrium molecular dynamics simulations. The predicted value of ZT is as high as 0.11 at 500 K. As the thermal conductivity could be reduced largely by phonon engineering, there should be a high possibility to enhance ZT in the SLMoS2-based materials.

Tuning the opto-electronic properties of MoS2 layer using charge transfer interactions: effect of different donor molecules

We have performed density functional theory calculations to study the effect of adsorption of a set of organic electron donor molecules on single layer MoS2 to find the optimum condition to tune the charge transfer, as well as to find how it changes the electronic properties of single layer MoS2. We have performed our calculations for three sets of organic Lewis bases. We have found that all the molecules are physisorbed on MoS2. Our calculations show that the charge transfer from the molecules to the MoS2 layer is highly dependent upon the inductive effect and HOMO–LUMO gap of the molecules. Furthermore, we show that the charge transfer interaction tunes the electronic and optical property of MoS2 to a significant amount: for example, the band-gap of the system can be changed from 1.8 eV to even a low value of 0.2 eV, making it interesting for different optoelectronic device applications.

Dielectric screening of excitons and trions in single-layer MoS2.

Photoluminescence (PL) properties of single-layer MoS2 are indicated to have strong correlations with the surrounding dielectric environment. Blue shifts of up to 40 meV of exciton or trion PL peaks were observed as a function of the dielectric constant of the environment. These results can be explained by the dielectric screening effect of the Coulomb potential; based on this, a scaling relationship was developed with the extracted electronic band gap and exciton and trion binding energies in good agreement with theoretical estimations. It was also observed that the trion/exciton intensity ratio can be tuned by at least 1 order of magnitude with different dielectric environments. Our findings are helpful to better understand the tightly bound exciton properties in strongly quantum-confined systems and provide a simple approach to the selective and separate generation of excitons or trions with potential applications in excitonic interconnects and valleytronics.

Modulating electronic transport properties of MoS2 field effect transistor by surface overlayers

In situ bottom-gated molybdenum disulfide (MoS2) field effect transistors (FETs) device characterization and in situ ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy measurements were combined to investigate the effect of surface modification layers of C60 and molybdenum trioxide (MoO3) on the electronic properties of single layer MoS2. It is found that C60 decoration keeps MoS2 FET performance intact due to the very weak interfacial interactions, making C60 as an ideal capping layer for MoS2 devices. In contrast, decorating MoO3 on MoS2 induces significant charge transfer at the MoS2/MoO3 interface and largely depletes the electron charge carriers in MoS2 FET devices.