One-dimensional electronic instabilities at the edges of MoS2 (original) (raw)
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Physical Review Materials, 2020
Two-dimensional (2D) semiconducting transition metal dichalcogenides such as MoS 2 have attracted extensive research interests for potential applications in optoelectronics, spintronics, photovoltaics, and catalysis. To harness the potential of these materials for electronic devices requires a better understanding of how defects control the carrier concentration, character, and mobility. Utilizing a correction scheme developed by Freysoldt and Neugebauer to ensure the appropriate electrostatic boundary conditions for charged defects in 2D materials, we perform density functional theory calculations to compute formation energies and charge transition levels associated with sulfur vacancies in monolayer and layered bulk MoS 2. We investigate the convergence of these defect properties with respect to vacuum spacing, in-plane supercell dimensions, and different levels of theory. We also analyze the electronic structures of the defects in different charge states to gain insights into the effect of defects on bonding and magnetism. We predict that both vacancy structures undergo a Jahn-Teller distortion, which helps stabilize the sulfur vacancy in the −1 charged state.
4, 2020
Two dimensional (2D) materials are currently gaining a lot of interest due to excellent properties that are different from their bulk structures. Single and few-layered of Transition metal dichalcogenides (TMDCs) have a bandgap that ranges between 1-2 eV, which is used for FET devices or any optoelectronic devices. Within TMDCs, a ton of consideration is focused on Molybdenum Disulfide (MoS2) because of its promising band gap-tuning and transition between direct to indirect bandgap properties relies upon its thickness. The density functional theory (DFT) calculations with different functionals and spin-orbit coupling (SOC) parameters were carried out to study the electronic properties of bulk and monolayer MoS2. The addition of SOC brought about a noteworthy change in the profile of the band energy, explicitly the splitting of the valence band maximum (VBM) into two sub-bands. The indirect bandgap in bulk MoS2 ranges from 1.17- 1.71eV and that of the monolayer bandgap was 1.6 – 1.71...
Physical Review B, 2022
The charge density wave (CDW) states of two-dimensional transition metal dichalcogenides (TMDs) originate from intrinsic couplings between the electronic structures and lattice distortion, inducing interesting physical and chemical properties. The observed TMDs CDW states are mostly nonmagnetic (NM) but with a few ferromagnetic (FM) cases. Physical mechanisms for the formation of FM CDW remain elusive. In this paper, we used density functional theory calculations to study a set of TMDs with magnetic transition metal elements (e.g., V, Cr, and Mn). We found that the FM state can stem from the direct exchange to superexchange transition (e.g., CrX 2) or the MM (M is the metal atom) dimerization (e.g., MnX 2). A crystal structure distortion index is proposed to distinguish the different formation mechanisms of FM CDW states. Interestingly, CrX 2 has both NM and FM CDW states, which is not observed in other TMDs materials. We found that charge (electron or hole) doping could modulate the different formation mechanisms and induce a phase transition between these two CDW states in CrS 2 , leading to significant actuation strain output (i.e., 12.17% and 5.93% along x and y directions, respectively) and drastic change of magnetism, which could enable some multifunctionality applications of TMD materials.
Joint density of states and charge density wave in 2H-structured transition metal dichalcogenides
Journal of Physics and Chemistry of Solids, 2008
The joint density of states of two different 2H-structured transition metal dichalcogenides (TMDs) with and without charge density wave (CDW), Na 0:05 TaS 2 and Cu 0:09 NbS 2 , respectively, are compared. While there is a clear maximum at the 3 Â 3 charge density wavevector for Na 0:05 TaS 2 , the joint density of states for Cu 0:09 NbS 2 does not show such behavior, consistent with the absence of CDW in the system. Our results illustrate that the joint density of states well represents the charge instability in 2D systems.
Layer-dependent anisotropic electronic structure of freestanding quasi-two-dimensional MoS_2
2016
The anisotropy of the electronic transition is a well-known characteristic of low-dimensional transition-metal dichalcogenides, but their layer-thickness dependence has not been properly in- vestigated experimentally until now. Yet, it not only determines the optical properties of these low-dimensional materials, but also holds the key in revealing the underlying character of the elec- tronic states involved. Here we used both angle-resolved electron energy-loss spectroscopy and spectral analysis of angle-integrated spectra to study the evolution of the anisotropic electronic transition involving the low energy valence electrons in the freestanding MoS_2 layers with different thicknesses. We are able to demonstrate that the well-known direct gap at 1.8 eV is only excited by the in-plane polarized field while the out-of-plane polarized optical gap is 2.4±0.2 eV in monolayer MoS_2. This contrasts with the much smaller anisotropic response found for the indirect gap in the few-layer Mo...
Spin-orbital effects in metal-dichalcogenide semiconducting monolayers
Theory. For an accurate reproduction of the electronic structure of transition metal systems, the spin orbit interaction is considered by using fully relativistic pseudopotentials (FRUP). The electronic and spin properties of MX 2 (M = Sc, Cr, Mn, Ni, Mo & W and X = O, S, Se & Te) were obtained with FRUP, compared with the scalar relativistic pseudopotentials (SRUP) and with the available experimental results. Among the differences between FRUP and SRUP calculations are giant splittings of the valence band, substantial band gap reductions and semiconductor to metal or non-magnetic to magnetic "transitions". MoO 2 , MoS 2 , MoSe 2 , MoTe 2 , WO 2 , WS 2 and WSe 2 are proposed as candidates for spintronics, while CrTe 2 , with μ ~ 1.59 μ B , is a magnetic metal to be experimentally explored.
Green's function approach to edge states in transition metal dichalcogenides
Physical Review B, 2016
The semiconducting two-dimensional transition metal dichalcogenides MX 2 show an abundance of onedimensional metallic edges and grain boundaries. Standard techniques for calculating edge states typically model nanoribbons, and require the use of supercells. In this paper, we formulate a Green's function technique for calculating edge states of (semi-)infinite two-dimensional systems with a single well-defined edge or grain boundary. We express Green's functions in terms of Bloch matrices, constructed from the solutions of a quadratic eigenvalue equation. The technique can be applied to any localized basis representation of the Hamiltonian. Here, we use it to calculate edge states of MX 2 monolayers by means of tight-binding models. Aside from the basic zigzag and armchair edges, we study edges with a more general orientation, structurally modifed edges, and grain boundaries. A simple three-band model captures an important part of the edge electronic structures. An 11-band model comprising all valence orbitals of the M and X atoms is required to obtain all edge states with energies in the MX 2 band gap. Here, states of odd symmetry with respect to a mirror plane through the layer of M atoms have a dangling-bond character, and tend to pin the Fermi level.
Group theory analysis of phonons in two-dimensional transition metal dichalcogenides
Physical Review B, 2014
Transition metal dichalcogenides (TMDCs) have emerged as a new two dimensional materials field since the monolayer and few-layer limits show different properties when compared to each other and to their respective bulk materials. For example, in some cases when the bulk material is exfoliated down to a monolayer, an indirect-to-direct band gap in the visible range is observed. The number of layers N (N even or odd) drives changes in space group symmetry that are reflected in the optical properties. The understanding of the space group symmetry as a function of the number of layers is therefore important for the correct interpretation of the experimental data. Here we present a thorough group theory study of the symmetry aspects relevant to optical and spectroscopic analysis, for the most common polytypes of TMDCs, i.e. 2Ha, 2Hc and 1T , as a function of the number of layers. Real space symmetries, the group of the wave vectors, the relevance of inversion symmetry, irreducible representations of the vibrational modes, optical selection rules and Raman tensors are discussed.
Quasiparticle electronic structure of 1T’-MoS2 within GW approximation
Journal of Physics: Conference Series, 2019
Two-dimensional transition metal dichalcogenides, such as MoS2, exhibit several polymorphs, namely semiconducting 1H, metallic 1T, and semi-metallic 1T'. Recent experiment [Xinmao Yin et al., Nat. Commun. 8, 486 (2017)] showed an inverted gap of 0.5 eV and a fundamental gap of 0.1 eV in the absorption spectrum of the semi-metallic 1T'-MoS 2. We carry out first-principles calculations on the electronic band structure of 1T'-MoS2. Since the transition across the fundamental gap occurs at a non-high-symmetry k-point, the choice of k-point sampling is crucial. Our converging result regarding k-point sampling shows that two bands touch at Fermi energy. It indicates the absence of fundamental gap. We report that spin-orbit interaction induces an opening of this fundamental gap of about 0.06 eV, which is smaller than the gap observed in experiment. To see the effects of electron-electron interaction on this fundamental gap, we calculate the quasiparticle electronic band structure within the GW approximation.