Layer-dependent electronic structure of an atomically heavy two-dimensional dichalcogenide

Abstract

We report angle-resolved photoemission spectroscopic measurements of the evolution of the thickness-dependent electronic band structure of the heavy-atom two-dimensional layered, dichalcogenide, tungsten-diselenide (WSe 2 ). Our data, taken on mechanically exfoliated WSe 2 singlecrystals, provide direct evidence for shifting of the valence-band maximum from Γ (multilayer WSe 2 ), to K , (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.Single layers of two-dimensional metal dichalcogenides (TMDCs) such as MoS 2 , have emerged as a new class of non-centrosymmetric direct-bandgap materials with potential photonic and spintronic applications.[ [4] . In addition, ML WSe 2 has been demonstrated to be the first TMDC material possessing ambipolar, i.e., both p-type and n-type conducting behavior, [4][12] thus making it possible to design additional electronic functionality, such as p-n junctions or complementary logic circuits.Despite these intriguing characteristics, measurements of ML WSe 2 have generally been limited to probing of optical and transport properties. [4][5] [6] In this paper, we report thickness-dependent measurements of the surface and electronic structure of exfoliated WSe 2, using low-energy electron microscopy (LEEM), diffraction (LEED), and micrometer-scale angle-resolved photoemission spectroscopy (µ-ARPES) of samples supported on a native-oxide terminated silicon substrate. Our experimental results provide direct evidence for a predicted valence-band maximum (VBM) symmetrypoint change, which leads to an indirect-to-direct bandgap transition. Because TMDCs have a large carrier effective mass and reduced screening in two dimensions, electron-hole interactions are much stronger than in conventional semiconductors. [13][14] [15] Our results allow us to obtain a direct measurement of the hole effective mass. Finally, our measurements allow us to directly infer a lower bound on the energy of the direct band gap. 2Our measurements were performed using the spectroscopic photoemission and low-energy electron microscope (SPE-LEEM) system at the National Synchrotron Light Source (NSLS) beamline U5UA.[16] [17] The spectrometer energy resolution of this instrument was set to 100 meV at 33 eV incident photon energy with a beam spot size of 1 μm in diameter. The momentum resolution is ~0.02 Å -1. Exfoliated WSe 2 samples were transferred to a native-oxide covered Si substrate; prior to measurements, these samples were annealed at 350 o C for ~12 hours under UHV conditions. The layer number of the sample is determined by Raman and photoluminescence spectroscopy.[18] [19] Additional experimental details can be found in the Supplemental Materials section.Sample quality and crystal orientation were examined using both LEEM and µ-LEED (Fig. 1). Diffraction patterns (at a primary electron energy of 48eV) of exfoliated WSe 2 flakes of 1-3ML and bulk are shown in Fig. 2a-d, respectively, and cl...

WSe 2 -contact metal interface chemistry and band alignment under high vacuum and ultra high vacuum deposition conditions

“…Monolayer (bilayer, bulk) WSe 2 exhibits a band gap of 2.2 (1.8, 1.3) ±0.1 eV as measured by STS [44]. Angle-resolved photoemission spectroscopy indicates the VBM of monolayer (bilayer, bulk) WSe 2 reside 1.8 (1.5, 1.1) eV from the Fermi level [72]. Error in photoemission measurements manifests in both the resolution of the spectrometer and also the method by which the VBM is extracted, typically on the order of ±0.03- 0.04 eV [73].…”

Spin- and valley-coupled electronic states in monolayer WSe2 on bilayer graphene

“…[9,10] In contrast, monolayer MoSe 2 grown epitaxially on bilayer graphene [11] and monolayer MoS 2 on HOPG [12] exhibit the band splitting of ∼180 meV. There is no consensus on whether the energy bands at the K (K') point in bulk WSe 2 are spin-polarized or not [13,14].…”

“…It is noted that the energy difference of the top of the holelike bands between the and K points is 0.6 eV in monolayer WSe 2 while it is 0.1 0.4 eV in MoS 2 , MoSe 2 , and WS 2 . [9,10] This suggests that the band gap at the K (K') point in monolayer WSe 2 on bilayer graphene has the strongest "direct-gap" nature among known monolayer transitionmetal dichalcogenides. This may be related to the strong spin-orbit interaction in WSe 2 .…”

“…[3,13,14] It is also noted that the large spin splitting would be a great advantage to realize more effective electronic devices. While angle-resolved photoemission spectroscopy (ARPES) has been applied to both an exfoliated monolayer film and a bulk crystal of WSe 2 [10,13,14], the electronic structure of an epitaxially grown monolayer WSe 2 film remains elusive. It is thus of particular importance to elucidate the basic electronic states of eptaxially grown monolayer WSe 2 to develop electronic devices based with monolayer MX 2 .…”

Growth and electronic structure of epitaxial single-layerWS2on Au(111)

“…Furthermore, the unique spin texture of both conduction and valence band makes SL TMDCs well-suited for studying quantum degrees of freedom such as spin or valley pseudospin or their interactions [15][16][17] . In the case of the tungsten dichalcogenides, much stronger spin-orbit coupling is expected than in the case of the Mo-based analogues, and the properties just enumerated should thus be more stable at room temperature [18][19][20] . Naively, a strong spin-orbit splitting of the bands can be expected to result in an increased band curvature near the top of the valence band and hence to a reduced effective mass; and indeed, WS 2 is predicted to be the best material among all of the TMDCs for a transistor channel, due to its low effective hole mass [21][22][23] .…”