Quasiparticle electronic structure of 1T’-MoS2 within GW approximation (original) (raw)
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Quasiparticle band structure calculation of monolayer, bilayer, and bulk MoS_{2}
Physical Review B, 2012
Quasiparticle self-consistent GW calculations of the band structures and related effective-mass parameters are carried out for bulk, monolayer, and bilayer MoS 2. Including excitonic effects within the Mott-Wannier theory, quantitative agreement is obtained between the A, B excitons, measured by absorption [Phys. Rev. Lett. 105, 136805 (2010)], and the calculated exciton gap energies at K. The A-B splitting arises from the valence-band splitting which in the monolayer is entirely due to spin-orbit coupling and leads to spin-split states, while in the bilayer it is a combined effect of interlayer and spin-orbit coupling.
Monolayer transition metal dichalcogenides are promising materials for photoelectronic devices. Among them, molybdenum disulphide (MoS 2 ) and tungsten disulphide (WS 2 ) are some of the best candidates due to their favorable band gap values and band edge alignments. Here, various perturbative corrections to the DFT electronic structure, e.g. GW, spin-orbit coupling, as well as many-body excitonic and trionic effects are considered, and accurate band gaps as a function of homogeneous biaxial strain in these materials are calculated. All of these corrections are shown to be of comparable magnitudes and need to be included in order to obtain an accurate electronic structure. The strain at which the direct-to-indirect gap transition occurs is calculated. After considering all contributions, the direct to indirect gap transition strain is predicted to be at 2.7% in MoS 2 and 3.9% in WS 2 . These values are generally higher than the previously reported theoretical values.
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...
Electronic properties of the MoS2-WS2 heterojunction
arXiv preprint arXiv:1212.0111, 2012
We study the electronic structure of a heterojunction made of two monolayers of MoS2 and WS2. Our first-principles density functional calculations show that, unlike in the homogeneous bilayers, the heterojunction has an optically active band-gap, smaller than the ones of MoS2 and WS2 single layers. We find that that the optically active states of the maximum valence and minimum conduction bands are localized on opposite monolayers, and thus the lowest energy electron-holes pairs are spatially separated. Our findings portrait the MoS2−WS2 bilayer as a prototypical example for band-gap engineering of atomically thin two-dimensional semiconducting heterostructures.
One-dimensional electronic instabilities at the edges of MoS2
Physical Review B
The one-dimensional (1D) metallic states that appear at the zigzag edges of semiconducting two-dimensional transition metal dichalcogenides (TMDCs) result from the intrinsic electric polarization in these materials, which for D 3h symmetry is a topological invariant. These 1D states are susceptible to electronic and structural perturbations that triple the period along the edge. In this paper we study possible spin-density waves (SDWs) and charge-density waves (CDWs) at the zigzag edges of MoS 2 , using first-principles density-functional theory calculations. Depending on the detailed structures and termination of the edges, we observe either combined SDW/CDWs or pure CDWs, along with structural distortions. In all cases the driving force is the opening of a band gap at the edge. The analysis should hold for all group VI TMDCs with the same basic structure as MoS 2 .
Physical Review Materials
Owing to unique electronic, excitonic, and valleytronic properties, atomically thin transition metal dichalcogenides are becoming a promising two-dimensional (2D) semiconductor system for diverse electronic and optoelectronic applications. In an ideal 2D semiconductor, efficient carrier transport is very difficult because of lacking free charge carriers. Doping is necessary for electrically driven device applications based on such 2D semiconductors, which requires investigation of electronic structure changes induced by dopants. Therefore probing correlations between localized electronic states and doping is important. Here, we address the electronic nature of broad bound exciton bands and their origins in exfoliated monolayer (1L) WS 2 and MoS 2 through monitoring low-temperature photoluminescence and manipulating electrostatic doping. The dominant bound excitons in 1L WS 2 vary from donor to acceptor bound excitons with the switching from nto p-type doping. In 1L MoS 2 , two localized emission bands appear which are assigned to neutral and ionized donor bound excitons, respectively. The deep donor and acceptor states play critical roles in the observed bound exciton bands, indicating the presence of strongly localized excitons in such 2D semiconductors.
We calculate from first principles the electronic structure and optical properties of a number of transition metal dichalcogenide (TMD) bilayer heterostructures consisting of MoS 2 layers sandwiched with WS 2 , MoSe 2 , MoTe 2 , BN, or graphene sheets. Contrary to previous works, the systems are constructed in such a way that the unstrained lattice constants of the constituent incommensurate monolayers are retained. We find strong interaction between the -point states in all TMD/TMD heterostructures, which can lead to an indirect gap. On the other hand, states near the K point remain as in the monolayers. When TMDs are paired with BN or graphene layers, the interaction around the -point is negligible, and the electronic structure resembles that of two independent monolayers. Calculations of optical properties of the MoS 2 /WS 2 system show that, even when the valenceand conduction-band edges are located in different layers, the mixing of optical transitions is minimal, and the optical characteristics of the monolayers are largely retained in these heterostructures. The intensity of interlayer transitions is found to be negligibly small, a discouraging result for engineering the optical gap of TMDs by heterostructuring.
Band structure and orbital character of monolayer MoS2 with eleven-band tight-binding model
Superlattices and Microstructures, 2018
In this paper, based on a tight-binding (TB) model, first we present the calculations of eigenvalues as band structure and then present the eigenvectors as probability amplitude for finding electron in atomic orbitals for monolayer MoS 2 in the first Brillouin zone. In these calculations we are considering hopping processes between the nearest-neighbor Mo-S, the next nearest-neighbor in-plan Mo-Mo, and the next nearest-neighbor in-plan and out-of-plan S-S atoms in a three-atom based unit cell of two-dimensional rhombic MoS 2. The hopping integrals have been solved in terms of Slater-Koster and crystal field parameters. These parameters are calculated by comparing TB model with the density function theory (DFT) in the high-symmetry k-points (i.e. the K-and Γ-points). In our TB model all the 4d Mo orbitals and the 3p S orbitals are considered and detailed analysis of the orbital character of each energy level at the main high-symmetry points of the Brillouin zone is described. In comparison with DFT calculations, our results of TB model show a very good agreement for bands near the Fermi level. However for other bands which are far from the Fermi level, some discrepancies between our TB model and DFT calculations are observed. Upon the accuracy of Slater-Koster and crystal field parameters, on the contrary of DFT, our model provide enough accuracy to calculate all allowed transitions between energy bands that are very crucial for investigating the linear and nonlinear optical properties of monolayer MoS 2. 1. Introduction With introducing the graphene as a two-dimensional (2D) material, extensive investigations on its properties, such as structural and optical, have been started [1-5].
Large Excitonic Effects in the Optical Properties of Monolayer MoS2
The band structure and absorption spectrum of monolayer MoS2 is calculated using the G0W0 approximation and the Bethe-Salpeter equation (BSE), respectively. We find that the so-called A and B peaks in the absorption spectrum arise from strongly bound excitons (0.7-0.8 eV) localized in distinct regions of the Brillouin zone and not from a split valence band as commonly assumed. Furthermore, we find the minimum band gap to be of the indirect type. This seems to conflict with recent experimental results showing stong luminescence in this material. However, our results indicate that the luminescence is a consequence of the large binding energy of the lowest exciton which stabilizes it against thermal relaxation. PACS numbers: 71.20.Nr, 78.20.Bh, 78.60.Lc Nanostructured forms of the semi-conductor MoS 2 have recieved much attention due to their potential as catalysts for desulferization of crude oil and more recently for (photo)-electrochemical hydrogen evolution . Bulk MoS 2 is composed of two-dimensional sheets held together by weak van der Waals forces and individual sheets can be isolated by exfoliation techniques similar to those used to produce graphene . Single layers of MoS 2 therefore comprise highly interesting twodimensional systems with a finite band gap and have recently been proposed for nano-electronics applications .
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.