Evolution of the Valley Position in Bulk Transition-Metal Chalcogenides and their Mono-Layer Limit (original) (raw)
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Inter-Layer Coupling Induced Valence Band Edge Shift in Mono- to Few-Layer MoS2
Scientific reports, 2017
Recent progress in the synthesis of monolayer MoS2, a two-dimensional direct band-gap semiconductor, is paving new pathways toward atomically thin electronics. Despite the large amount of literature, fundamental gaps remain in understanding electronic properties at the nanoscale. Here, we report a study of highly crystalline islands of MoS2 grown via a refined chemical vapor deposition synthesis technique. Using high resolution scanning tunneling microscopy and spectroscopy (STM/STS), photoemission electron microscopy/spectroscopy (PEEM) and μ-ARPES we investigate the electronic properties of MoS2 as a function of the number of layers at the nanoscale and show in-depth how the band gap is affected by a shift of the valence band edge as a function of the layer number. Green's function based electronic structure calculations were carried out in order to shed light on the mechanism underlying the observed bandgap reduction with increasing thickness, and the role of the interfacial ...
Nano letters, 2016
Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), we examine the electronic structure of transition metal dichalcogenide heterostructures (TMDCHs) composed of monolayers of MoS2 and WS2. STS data are obtained for heterostructures of varying stacking configuration as well as the individual monolayers. Analysis of the tunneling spectra includes the influence of finite sample temperature, yield information about the quasi-particle bandgaps, and the band alignment of MoS2 and WS2. We report the band gaps of MoS2 (2.17 ± 0.04 eV) and WS2 (2.39 ± 0.06 eV) in the individual materials and type II band alignment for the heterostructure with an interfacial band gap of 1.45 ± 0.06 eV.
Physical Review Letters, 2013
We report on the evolution of the thickness-dependent electronic band structure of the two-dimensional layered-dichalcogenide molybdenum disulfide (MoS 2 ). Micrometer-scale angle-resolved photoemission spectroscopy of mechanically exfoliated and chemical-vapor-deposition-grown crystals provides direct evidence for the shifting of the valence band maximum from " À to " K, for the case of MoS 2 having more than one layer, to the case of single-layer MoS 2 , as predicted by density functional theory. This evolution of the electronic structure from bulk to few-layer to monolayer MoS 2 had earlier been predicted to arise from quantum confinement. Furthermore, one of the consequences of this progression in the electronic structure is the dramatic increase in the hole effective mass, in going from bulk to monolayer MoS 2 at its Brillouin zone center, which is known as the cause for the decreased carrier mobility of the monolayer form compared to that of bulk MoS 2 .
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.
2D Materials, 2014
We present a detailed study of the e ect of spin-orbit coupling on the band structure of single-layer and bulk transition metal semiconductor dichalcogenides, including explicitly the role of the chalcogen orbitals and their hybridization with the transition metal atoms. To this aim, we generalize the Slater-Koster tightbinding (TB) model presented in Ref. 1 by including the e ect of an atomic spin-orbit coupling on all the atoms. The present framework permits us to study analytically the e ect of the atomic spin-orbit associated with the chalcogen atom. In particular, we present a scenario where, in the case of strong spin-orbit coupling, the spin/orbital/valley entanglement at the minimum of the conduction band at Q can be probed and be of experimental interest in samples with the most common electron-doping reported for this family of compounds.
Giant valley-Zeeman coupling in the surface layer of an intercalated transition metal dichalcogenide
Nature Materials
Spin-valley locking is ubiquitous to transition-metal dichalcogenides (TMDs) with local or global inversion asymmetry, in turn stabilising properties such as Ising superconductivity, and opening routes towards 'valleytronics'. The underlying valley spin splitting is set by spin-orbit coupling, but can be tuned via application of external magnetic fields or through proximity coupling. However, only modest changes have been realised to date. Here, we investigate the electronic structure of the V-intercalated TMD V 1/3 NbS2 using microscopic area spatially-and angle-resolved photoemission spectroscopy. Our measurements and corresponding density-functional theory calculations reveal that the bulk magnetic order induces a giant valley-selective Ising coupling exceeding 50 meV in the surface NbS2 layer, equivalent to application of a ∼ 250 T magnetic field. This is of comparable magnitude to the intrinsic spin-orbit splittings, and indicates how coupling of local magnetic moments to itinerant states of a TMD monolayer provides a powerful route to controlling their valley spin splittings.
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.
Gap states and valley-spin filtering in transition metal dichalcogenide monolayers
Physical Review B, 2020
The magnetically-induced valley-spin filtering in transition metal dichalcogenide monolayers (M X2, where M =Mo, W and X=S, Se, Te) promises new paradigm in information processing. However, the detailed understanding of this effect is still limited, regarding its underlying transport processes. Herein, it is suggested that the filtering mechanism can be greately elucidated by the concept of metal-induced gap states (MIGS), appearing in the electrode-terminated M X2 materials i.e. the referential filter setup. In particular, the gap states are predicted here to mediate valley-and spin-resolved charge transport near the ideal electrode/M X2 interface, and therefore to initiate filtering. It is also argued that the role of MIGS increases when the channel length is diminished, as they begin to govern the overall valley-spin transport in the tunneling regime. In what follows, the presented study yields fundamental scaling trends for the valley-spin selectivity with respect to the intrinsic physics of the filter materials. As a result, it facilitates insight into the analyzed effects and provide design guidelines toward efficient valley-spin filter devices, that base on the discussed materials or other hexagonal monolayers with a broken inversion symmetry. I. INTRODUCTION Most of the present concepts behind the electronic control of information rely on the manipulation of charge flow or spin angular momentum of electrons. However, recent developments in quantum electronics show that it is also possible to address an alternative property of the electron, namely its valley pseudospin [1-6]. In comparison to its charge and spin counterparts, the valley degree of freedom constitute binary index for the low-energy electrons associated with the local conduction band minima (valleys) in the momentum space of a crystal [1]. As a result, it is expected that the valley-based (valleytronic) devices should provide new or improved functionalities in the field of classical and quantum information processing e.g. in terms of low-power valley or hybrid valley-spin logic devices [7-9] as well as complex qubit basis sets [9, 10]. Nonetheless, to efficiently perform valleytronic operations in solid state systems, it is required to have control over the selective population of distinguishable valleys, toward their polarization [3-6]. Moreover, the electrons should occupy polarized valleys long enough to allow logic operations of interest [8, 9]. Given the above background, not all solid state materials that exhibit local energy extrema in the momentum space are well suited for the valley control of information. From among the systems already considered as potential hosts for valleytronics [1, 2, 11-13], the most promising are the two-dimensional (2D) layered crystals with honeycomb structures, due to their strong valley-selective coupling with the external fields [8, 9]. In the family of such 2D systems, currently of particular attention are the group-VIB transition metal dichalcogenide monolayers
Physical Review B, 2017
Tailoring band alignment layer-by-layer using heterojunctions of two-dimensional (2D) semiconductors is an attractive prospect for producing next-generation electronic and optoelectronic devices that are ultra-thin, flexible, and efficient. 2D layers of transition metal dichalcogenides (TMDs) in laboratory devices have already shown favorable characteristics for electronic and optoelectronic applications. Despite these strides, a systematic understanding of how band alignment evolves from monolayer to multilayer structures is still lacking in experimental studies, which hinders development of novel devices based on TMDs. Here we
Atomic-Scale Spectroscopy of Gated Monolayer MoS2
Nano letters, 2016
The electronic properties of semiconducting monolayer transition-metal dichalcogenides can be tuned by electrostatic gate potentials. Here we report gate-tunable imaging and spectroscopy of monolayer MoS2 by atomic-resolution scanning tunneling microscopy/spectroscopy (STM/STS). Our measurements are performed on large-area samples grown by metal-organic chemical vapor deposition (MOCVD) techniques on a silicon oxide substrate. Topographic measurements of defect density indicate a sample quality comparable to single-crystal MoS2. From gate voltage dependent spectroscopic measurements, we determine that in-gap states exist in or near the MoS2 film at a density of 1.3 × 10(12) eV(-1) cm(-2). By combining the single-particle band gap measured by STS with optical measurements, we estimate an exciton binding energy of 230 meV on this substrate, in qualitative agreement with numerical simulation. Grain boundaries are observed in these polycrystalline samples, which are seen to not have str...