Electron Density and Its Relation with Electronic and Optical Properties in 2D Mo/W Dichalcogenides (original) (raw)
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Layer-dependent electronic structure of an atomically heavy two-dimensional dichalcogenide
Physical Review B, 2015
We report angle-resolved photoemission spectroscopic measurements of the evolution of the thicknessdependent electronic band structure of the atomically heavy two-dimensional layered dichalcogenide, tungsten diselenide (WSe 2). Our data, taken on mechanically exfoliated WSe 2 single crystals, provide direct evidence for shifting of the valence-band maximum from¯ (multilayer WSe 2) toK (single-layer WSe 2). Further, our measurements also set a lower bound on the energy of the direct band gap and provide direct measurement of the hole effective mass.
Physical Review B, 2012
Using the first-principles calculations, we explore the electronic structures of 2H-MX 2 (M = Mo, W; X = S, Se, Te). When the number of layers reduces to a single layer, the indirect gap of bulk becomes a direct gap with larger gap and the band curvatures are found to lead to the drastic changes of effective masses. On the other hand, when the strain is applied on the single layer, the direct gap becomes an indirect gap and the effective masses vary. Especially, the tensile strain reduces the gap energy and effective masses while the compressive strain enhances them. Furthermore, the much larger tensile stress leads to become metallic.
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.
Structure–Property Relationships in Transition Metal Dichalcogenide Bilayers under Biaxial Strains
Nanomaterials, 2021
This paper reports a Density Functional Theory (DFT) investigation of the electron density and optoelectronic properties of two-dimensional (2D) MX2 (M = Mo, W and X = S, Se, Te) subjected to biaxial strains. Upon strains ranging from −4% (compressive strain) to +4% (tensile strain), MX2 bilayers keep the same bandgap type but undergo a non-symmetrical evolution of bandgap energies and corresponding effective masses of charge carriers (m*). Despite a consistency regarding the electronic properties of Mo- and WX2 for a given X, the strain-induced bandgap shrinkage and m* lowering are strong enough to alter the strain-free sequence MTe2, MSe2, MS2, thus tailoring the photovoltaic properties, which are found to be direction dependent. Based on the quantum theory of atoms in molecules, the bond degree (BD) at the bond critical points was determined. Under strain, the X-X BD decreases linearly as X atomic number increases. However, the kinetic energy per electron G/ρ at the bond critical...
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.
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.
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...
Inorganics, 2022
An attempt was made, using computational methods, to understand whether the intermolecular interactions in the dimers of molybdenum dichalcogenides MoCh2 (Ch = chalcogen, element of group 16, especially S, Se and Te) and similar mixed-chalcogenide derivatives resemble the room temperature experimentally observed interactions in the interfacial regions of molybdenites and their other mixed-chalcogen derivatives. To this end, MP2(Full)/def2-TVZPPD level electronic structure calculations on nine dimer systems, including (MoCh2)2 and (MoChCh′2)2 (Ch, Ch′ = S, Se and Te), were carried out not only to demonstrate the energetic stability of these systems in the gas phase, but also to reproduce the intermolecular geometrical properties that resemble the interfacial geometries of 2D layered MoCh2 systems reported in the crystalline phase. Among the six DFT functionals (single and double hybrids) benchmarked against MP2(full), it was found that the double hybrid functional B2PLYPD3 has some a...
Lateral heterostructures and one-dimensional interfaces in 2D transition metal dichalcogenides
Journal of Physics: Condensed Matter, 2019
The growth and exfoliation of two-dimensional (2D) materials have led to the creation of edges and novel interfacial states at the juncture between crystals with different composition or phases. These hybrid heterostructures (HSs) can be built as vertical van der Waals stacks, resulting in a 2D interface, or as stitched adjacent monolayer crystals, resulting in one-dimensional (1D) interfaces. Although most attention has been focused on vertical HSs, increasing theoretical and experimental interest in 1D interfaces is evident. In-plane interfacial states between different 2D materials inherit properties from both crystals, giving rise to robust states with unique 1D non-parabolic dispersion and strong spinorbit effects. With such unique characteristics, these states provide an exciting platform for realizing 1D physics. Here, we review and discuss advances in 1D heterojunctions, with emphasis on theoretical approaches for describing those between semiconducting transition metal dichalcogenides M X 2 (with M =Mo, W and X= S, Se, Te), and how the interfacial states can be characterized and utilized. We also address how the interfaces depend on edge geometries (such as zigzag and armchair) or strain, as lattice parameters differ across the interface, and how these features affect excitonic/optical response. This review is intended to serve as a resource for promoting theoretical and experimental studies in this rapidly evolving field.