Theoretical Study on the Electronic States and Magnetism Properties of Monolayer MoS2 and Their Modulation by Doping/Defects/Strain

單層MoS2電子態及磁性的摻雜/空位/應變調控效應的理論研究

Student thesis: Doctoral Thesis

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Award date7 Aug 2018

Abstract

The successful exfoliation of graphene in 2004 has expedited the development of two dimensional (2D) materials such as boron nitride (BN), silicene, phosphorene, layered metal oxides, as well as transition metal dichalcogenides (TMDs). Owing to the quantum confinement effect, the 2D materials exhibit novel physical properties and have great potential in many application fields such as electronics, solar cell and energy harvesting. TMDs have received much attention because of their versatile properties spanning from semiconducting to superconducting, from magnetic to nonmagnetic, depending on the coordination environment and the number of d electrons in transition metal (TM). Among the different types of TMDs, monolayer MoS2 has been extensively and intensively studied due to the scientific indication that it is a very promising candidate material for the next generation field effect transistors, optoelectronics, and solar cell and so on. It is the prerequisite for design and application of MoS2-related devices to understand the electronic characteristics of monolayer MoS2 and predict its performance under external modulations. In the thesis, the effects of strain, doping and adsorption on the thermodynamics, electronic and magnetic properties of monolayer MoS2 have been investigated by first-principles calculation. The main contents and results are included as following:

(1) The first-principles calculation is done to study the electronic states and phonon dispersion of monolayer MoS2 under strain. It is found that the band gap decreases with the strain, which is ascribed to the strain-sensitive π bond-like interaction between interlayers. The frequency of the A'1 mode of MoS2 decreases significantly under biaxial strain, but it changes little even when the uniaxial strain is up to 20%. Interestingly, the degenerate E' mode is split into two modes, labeled as and , under uniaxial strain. The lattice vibration modes are significantly softened under strain and some new vibration modes are induced as a result of lowered crystal symmetry. This will affect the thermal and optical properties of monolayer MoS2. The strain-tunable effects on the electronic and magnetic properties of monolayer MoS2 open up a new opportunity for 2D crystals to be applied in flexible electronics and spin-electronics.

(2) Doping is an effective way to modulate the properties of 2D materials for new applications. The C- and O-doped monolayer MoS2 are still nonmagnetic semiconductor, but the H-, B-, N- and F-doped MoS2 are magnetic and show a total magnetic moment of 1.0 μB. The transition metal (TM) dopants induce magnetism in the monolayer MoS2 and the magnetic moment is related to the total number of valence electrons in the TM. In the case of co-doping case, in which the S and Mo atoms are substituted by the NM and TM atoms, respectively, spin polarization depends on the total number of valence electrons in the TM and NM dopants. 100% spin polarization occurs if the total number of the outer electrons is odd, but does not if it is even.

(3) The doping effects with different vanadium (V) concentrations on the electronic and magnetic properties of monolayer MoS2 are investigated by first-principles calculation. The doped V produces antiferromagnetic (AFM) or ferromagnetic (FM) states depending on the separation between V dopants. When the separation between V dopants is smaller than 6.38 Å and the maximum dopant concentration is 25%, the superexchange interaction between V atoms is stronger than the double exchange interaction between V 3d orbitals and Mo 4d orbitals, resulting in the AFM state in monolayer MoS2. However, the double exchange interaction between the V and Mo atoms becomes stronger than the superexchange interaction between V atoms if the separation between V dopants is larger than 9.57 Å. Consequently, the FM state is observed from the V-MoS2 and 100% spin polarization takes place if the separation between V atoms is further increased to 12.76 Å at a dopant concentration of 6.25%. The results suggest potential applications of monolayer MoS2 as diluted magnetic semiconductors (DMS) in spintronics.

(4) First-principles calculation is conducted to study the electronic and magnetic states of Mn-doped monolayer MoS2 under lattice strain. The Mn-doped MoS2 exhibits half-metallic characteristics in which the majority spin channel exhibits metallic feature but there is a bandgap in the minority spin channel. The FM state and the total magnetic moment of 1 B are always maintained for the larger supercells of monolayer MoS2 with only one doped Mn, no matter under tensile or compressive strain. However, the FM state will be enhanced by the tensile strain if two Mo atoms are substituted by Mn atoms in the monolayer MoS2. The magnetic moment is increased by 0.50 μB per unit cell at a tensile strain of 7%. Nevertheless, the Mn-doped MoS2 is changed into metallic and antiferromagnetic (AFM) under compressive strain. This is because the spin polarization of Mn 3d orbitals disappears gradually and the super-exchange interaction between Mn atoms increases gradually with compressive strain. The results suggest that the strain-sensitive electronic and magnetic properties of Mn-doped MoS2 offer great opportunities for the application in future electronic and spintronic devices.

(5) The effect of small gas molecules (H, O, CO, NH3 and NO2) on the electronic states of the monolayer MoS2 and N-doped one (N-MoS2) are investigated. According to the charge density difference, the interactions between gas molecules and N-MoS2 are stronger than that between the gas molecules and MoS2. So the sensitivity of MoS2 to gas molecule is improved by introducing N dopant.

In brief, first-principles calculation is conducted to study the effects of strain, doping and adsorption on the electronic states and magnetic properties of monolayer MoS2. The results provide us a further insight into the electronic and magnetic properties of monolayer MoS2, which will facilitate the applications of MoS2-based electronic devices in the future.

    Research areas

  • Monolayer MoS2, Electronic states, Magnetism, Strain, Doping