Spin-orbital effects in metal-dichalcogenide semiconducting monolayers (original) (raw)
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The Recent Progress of Two-Dimensional Transition Metal Dichalcogenides and Their Phase Transition
Crystals
Graphene is attracting much attention in condensed matter physics and material science in the two-dimensional(2D) system due to its special structure, and mechanical and electronic properties. However, the lack of electronic bandgap and uncontrollable phase structure greatly limit its application in semiconductors, such as power conversion devices, optoelectronic devices, transistors, etc. During the past few decades, 2D transition metal dichalcogenides (TMDs) with much more phase structures have attracted intensive research interest in fundamental studies and practical applications for energy storage, as catalysts, and in piezoelectricity, energy harvesting, electronics, optoelectronic, and spintronics. The controllable phase transition also provides another degree of freedom to pave the way for more novel devices. In this review, we introduce the abundant phase structures of 2D-TMDs, including 2H, 1T, 1T’ and charge density waves, and highlight the corresponding attractive propert...
Two-Dimensional Transition Metal Dichalcogenide Alloys: Stability and Electronic Properties
The Journal of Physical Chemistry Letters, 2012
Using density-functional theory calculations, we study the stability and electronic properties of single layers of mixed transition metal dichalcogenides (TMDs), such as MoS 2x Se 2(1−x) , which can be referred to as two-dimensional (2D) random alloys. We demonstrate that mixed MoS 2 /MoSe 2 / MoTe 2 compounds are thermodynamically stable at room temperature, so that such materials can be manufactured using chemical-vapor deposition technique or exfoliated from the bulk mixed materials. By applying the effective band structure approach, we further study the electronic structure of the mixed 2D compounds and show that general features of the band structures are similar to those of their binary constituents. The direct gap in these materials can continuously be tuned, pointing toward possible applications of 2D TMD alloys in photonics. SECTION: Plasmonics, Optical Materials, and Hard Matter I n many bulk ternary semiconductor compounds, such as GaInAs 1 or CdZnTe, 2 the band gap depends continuously on constituent composition, making it possible to tune the electronic and optical properties of these materials for uses in specific applications, such as solar cells, radiation detectors, or gas sensors. 3,4 These compounds are random substitutional alloys without translational long-range order. Recently, several two-dimensional (2D) materials, such as graphene, 5 hexagonal boron-nitride (h-BN), 6 and silica bilayer 7,8 were manufactured. These nanostructures possess many fascinating properties that can be used in various applications. 9 Notably, graphene is a semimetal, while h-BN is a wide gap semiconductor. Taking into account the similarities in the atomic structure of these two materials, the possibilities for creating a mixed BCN system with a gap tunable by component concentration have been envisioned. However, theory 10,11 and experiments 12 have indicated that 2D BCN materials are thermodynamically unstable and that h-BN and graphene tend to segregate. This approach, however, may prove to be successful in 2D transition metal dichalcogenides (TMDs). Most of the bulk TMDs are layered compounds with a common structural formula MeCh 2 , where Me stands for transition metals (Mo, W, Ti, etc.) and Ch for chalcogens (S, Se, Te), similar crystal structure and lattice constants. Like graphene and h-BN, 2D TMDs can be manufactured not only by mechanical 9,13 and chemical 14,15 exfoliation of their layered bulk counterparts, but also directly by chemical vapor deposition (CVD) 16−18 or twostep thermolysis. 19 These 2D materials are expected to have electronic properties varying from metals to wide-gap semiconductors, as their bulk counterparts, 20,21 and excellent mechanical characteristics. 22,23 Several nanoelectronic 13,24 and
We report first principles calculations of the electronic structure of monolayer 1H-MX2 (M = Mo, W; X = S, Se, Te), using the pseudopotential and numerical atomic orbital basis sets based methods within the local density approximation. Electronic band structure and density of states calculations found that the states around the Fermi energy are mainly due to metal d states. From partial density of states we find a strong hybridisation between metal d and chalcogen p states below the Fermi energy. All studied compounds in this work have emerged as new direct band gap semiconductors. The electronic band gap is found to decrease as one goes from sulphides to the tellurides of both Mo and W. Reducing the slab thickness systematically from bulk to monolayers causes a blue shift in the band gap energies, resulting in tunability of the electronic band gap. The magnitudes of the blue shift in the band gap energies are found to be 1.14 eV, 1.16 eV, 0.78 eV, 0.64, 0.57 eV and 0.37 eV for MoS2, WS2, MoSe2, WSe2, MoTe2 and WTe2, respectively, as we go from bulk phase (indirect band gap) to monolayer limit (direct band gap). This tunability in the electronic band gap and transitions from indirect to direct band make these materials potential candidates for the fabrication of optoelectronic devices.
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.
Electron Density and Its Relation with Electronic and Optical Properties in 2D Mo/W Dichalcogenides
Nanomaterials, 2020
Two-dimensional MX2 (M = Mo, W; X = S, Se, Te) homo- and heterostructures have attracted extensive attention in electronics and optoelectronics due to their unique structures and properties. In this work, the layer-dependent electronic and optical properties have been studied by varying layer thickness and stacking order. Based on the quantum theory of atoms in molecules, topological analyses on interatomic interactions of layered MX2 and WX2/MoX2, including bond degree (BD), bond length (BL), and bond angle (BA), have been detailed to probe structure-property relationships. Results show that M-X and X-X bonds are strengthened and weakened in layered MX2 compared to the counterparts in bulks. X-X and M-Se/Te are weakened at compressive strain while strengthened at tensile strain and are more responsive to the former than the latter. Discordant BD variation of individual parts of WX2/MoX2 accounts for exclusively distributed electrons and holes, yielding type-II band offsets. X-X BL ...
Two-dimensional transition metal dichalcogenide hybrid materials for energy applications
Nano Today
Monolayers of transition metal dichalcogenides (TMDs), such as MoS 2 and WS 2 , have recently triggered worldwide research interest due to their remarkable optical and electronic properties. More fascinatingly is the fact that these monolayers could also adopt various morphologies with exposed edges that include triangular, hexagonal or star-shaped clusters, in addition to nanoribbons. Exciting progress has been recently achieved in the synthesis, characterization, device fabrication and functionalization of these monolayer and few-layer TMDs. This article firstly reviews the properties of bulk and monolayer/few-layer TMDs. The "top-down" and "bottom-up" synthesis routes for different TMDs are then summarized. Raman spectroscopy is now becoming a key tool
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.
Due to their outstanding properties for optoelectronic and versatile electronic applications, the atomically thin layers of transition-metal dichalcogenide (TMDC) materials have demonstrated a potential candidacy to succeed its analog silicon-based technology. Hence, the elucidation of the most important features of these materials is indispensable. In this study, we provide a theoretical elucidation of the structural, electronic, elastic, and optical characteristics of TMDCs. The study has been carried out by elucidating the material in its two particular forms, namely, bulk and two-dimensional (2D) layered (monolayer). The theoretical investigation was carried out within the framework of the density functional theory (DFT) method using first-principles calculations. The Perdew−Burke−Ernzerhof (PBE) variant of the generalized gradient approximation (GGA) scheme, as performed in the Quantum Espresso package, is used. Van der Waals density functional effects, involving the nonlocal correlation part from the rVV10 and vdW-DF2 methods, were treated to remedy the lack of the long-range vdW interaction. An illustration of the performance of both rVV10 and vdW-DF2 functionalities, with the popular PBE correlations, is elucidated. The Born stability criterion is employed to assess structural stability. The obtained results reveal an excellent stability of both systems. Furthermore, the theoretical results show that band-gap energy is in excellent agreement with experimental and theoretical data. Pugh's rule suggested that both the bulk and MoS 2-2D layered systems are ductile materials. The refractive indices obtained herein are in good agreement with the available theoretical data. Moreover, the theoretical results obtained with the present approach demonstrate the ductility of both systems, namely, the bulk and the MoS 2-2D layered. The results obtained herein hold promise for structural, elastic, and optical properties and pave the way for potential applications in electronic and optoelectronic devices.
Large spin splitting in the conduction band of transition metal dichalcogenide monolayers
Physical Review B, 2013
We study the conduction band spin splitting that arises in transition metal dichalcogenide (TMD) semiconductor monolayers such as MoS2, MoSe2, WS2 and WSe2 due to the combination of spinorbit coupling and lack of inversion symmetry. Two types of calculation are done. First, density functional theory (DFT) calculations based on plane waves that yield large splittings, between 3 and 30 meV. Second, we derive a tight-binding model, that permits to address the atomic origin of the splitting. The basis set of the model is provided by the maximally localized Wannier orbitals, obtained from the DFT calculation, and formed by 11 atomic-like orbitals corresponding to d and p orbitals of the transition metal (W,Mo) and chalcogenide (S,Se) atoms respectively. In the resulting Hamiltonian we can independently change the atomic spin-orbit coupling constant of the two atomic species at the unit cell, which permits to analyse their contribution to the spin splitting at the high symmetry points. We find that-in contrast to the valence band-both atoms give comparable contributions to the conduction band splittings. Given that these materials are most often n−doped, our findings are important for developments in TMD spintronics.