We report systematic optical studies of WS2 and WSe2 monolayers and multilayers. The efficiency of second harmonic generation shows a dramatic even-odd oscillation with the number of layers, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show the crossover from an indirect band gap semiconductor at multilayers to a direct-gap one at monolayers. A hot luminescence peak (B) is observed at ~0.4 eV above the prominent band edge peak (A) in all samples. The magnitude of A-B splitting is independent of the number of layers and coincides with the spin-valley coupling strength in monolayers. Ab initio calculations show that this thickness independent splitting pattern is a direct consequence of the giant spin-valley coupling which fully suppresses interlayer hopping at valence band edge near K points because of the sign change of the spin-valley coupling from layer to layer in the 2H stacking order.
The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71 ± 0.01 eV around K valley in the Brillouin zone.
We report the observation of anomalously robust valley polarization and valley coherence in bilayer WS 2 . The polarization of the photoluminescence from bilayer WS 2 follows that of the excitation source with both circular and linear polarization, and remains even at room temperature. The near-unity circular polarization of the luminescence reveals the coupling of spin, layer, and valley degree of freedom in bilayer system, and the linearly polarized photoluminescence manifests quantum coherence between the two inequivalent band extrema in momentum space, namely, the valley quantum coherence in atomically thin bilayer WS 2 . This observation provides insight into quantum manipulation in atomically thin semiconductors.valleytronics | spin-valley coupling | valley quantum control T ungsten sulfide WS 2 , part of the family of group VI transition metal dichalcogenides (TMDCs), is a layered compound with buckled hexagonal lattice. As WS 2 thins to atomically thin layers, WS 2 films undergo a transition from indirect gap in bulk form to direct gap at monolayer level with the band edge located at energy-degenerate valleys (K, K′) at the corners of the Brillouin zone (1-3). Like the case of its sister compound, monolayer MoS 2 , the valley degree of freedom of monolayer WS 2 could be presumably addressed through nonzero but contrasting Berry curvatures and orbital magnetic moments that arise from the lack of spatial inversion symmetry at monolayers (3, 4). The valley polarization could be realized by the control of the polarization of optical field through valley-selective interband optical selection rules at K and K′ valleys as illustrated in Fig. 1A (4-6). In monolayer WS 2 , both the top of the valence bands and the bottom of the conduction bands are constructed primarily by the d orbits of tungsten atoms, which are remarkably shaped by spin-orbit coupling (SOC). The giant spin-orbit coupling splits the valence bands around the K (K′) valley by 0.4 eV, and the conduction band is nearly spin degenerated (7). As a result of time-reversal symmetry, the spin splitting has opposite signs at the K and K′ valleys. Namely, the Kramer's doublet jK↑i and jK′↓i is separated from the other doublet jK′↑i and jK↓i by the SOC splitting of 0.4 eV. The spin and valley are strongly coupled at K (K′) valleys, and this coupling significantly suppresses spin and valley relaxations as both spin and valley indices have to be changed simultaneously.In addition to the spin and valley degrees of freedom, in bilayer WS 2 there exists an extra index: layer polarization that indicates the carriers' location, either up-layer or down-layer. Bilayer WS 2 follows the Bernal packing order and the spatial inversion symmetry is recovered: each layer is 180°in plane rotation of the other with the tungsten atoms of a given layer sitting exactly on top of the S atoms of the other layer. The layer rotation symmetry switches K and K′ valleys, but leaves the spin unchanged, which results in a sign change for the spin-valley coupling from layer to layer (Fi...
Atomically thin MoS2 crystals have been recognized as a quasi-2D semiconductor with remarkable physical properties. This letter reports our Raman scattering measurements on multilayer and monolayer MoS2, especially in the low-frequency range (<50 cm −1 ). We find two low-frequency Raman modes with contrasting thickness dependence. With increasing the number of MoS2 layers, one shows a significant increase in frequency while the other decreases following a 1/N (N denotes the number of unit layers) trend. With the aid of first-principles calculations we assign the former as the shear mode E 2 2g . The latter is distinguished as the compression vibrational mode, similar to the surface vibration of other epitaxial thin films. The opposite evolution of the two modes with thickness demonstrates novel vibrational modes in atomically thin crystal as well as a new and more precise way to characterize thickness of atomically thin MoS2 films. In addition, we observe a broad feature around 38 cm −1 ( 5 meV) which is visible only under near-resonance excitation and pinned at the fixed energy independent of thickness. We interpret the feature as an electronic Raman scattering associated with the spin-orbit coupling induced splitting in conduction band at K points in their Brillouin zone.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.