Spin splitting and strain in epitaxial monolayer WSe2 on graphene

Abstract

We present the electronic and structural properties of monolayer WSe2 grown by pulsed-laser deposition on monolayer graphene (MLG) on SiC. The spin splitting in the WSe2 valence band at K was ∆SO = 0.469 ± 0.008 eV by angle-resolved photoemission spectroscopy (ARPES). Synchrotron-based grazing-incidence inplane X-ray diffraction (XRD) revealed the in-plane lattice constant of monolayer WSe2 to be aWSe 2 = 3.2757 ± 0.0008Å. This indicates a lattice compression of −0.19% from bulk WSe2. By using experimentally determined graphene lattice constant (aMLG = 2.4575 ± 0.0007Å), we found that a 3×3 unit cell of the slightly compressed WSe2 is perfectly commensurate with a 4×4 graphene lattice with a mismatch below 0.03%, which could explain why the monolayer WSe2 is compressed on MLG. From XRD and first-principles calculations, however, we conclude that the observed size of strain is negligibly small to account for a discrepancy in ∆SO found between exfoliated and epitaxial monolayers in earlier ARPES. In addition, angle-resolved, ultraviolet and X-ray photoelectron spectroscopy shed light on the band alignment between WSe2 and MLG/SiC and indicate electron transfer from graphene to the WSe2 monolayer. As further revealed by atomic force microscopy, the WSe2 island size depends on the number of carbon layers on top of the SiC substrate. This suggests that the epitaxy of WSe2 favors the weak van der Waals interactions with graphene while it is perturbed by the influence of the SiC substrate and its carbon buffer layer. arXiv:1912.04770v1 [cond-mat.mtrl-sci]

Strain and Spin-Orbit Coupling Engineering in Twisted WS2/Graphene Heterobilayer

“…The opposite variation is observed for compressive biaxial strain (see Supplementary Information, Figure S4). The final inferred ∆ SOC variation, +15 meV per 1% in plane tensile strain, is independent of the chosen exchange-correlation functionals (PBE or HSE) and matches the existing theoretical results on WS 2 and WSe 2 [36,38]. However, the variation of ∆ SOC at K is not the only marker of biaxial strain in the WS 2 band structure.…”

“…To support this argument, we now compare our experimental ARPES data with band structure calculations computed using DFT. For monolayer WS 2 , a tensile in-plane biaxial strain will decrease the vertical distance between the upper and lower sublayers of the S atoms, which, in turn, reduces the bandgap and affects the ∆ SOC at K [36,38]. In the DFT calculations, the biaxial strain is applied by modifying the lattice constant of WS 2 , followed by a structural relaxation to obtain the most favorable atomic positions.…”

Indirect to direct band gap crossover in two-dimensional WS2(1−x)Se2x alloys

“…The precise electronic characterization of the distinct regions on the flakes becomes possible by adding spatial resolution to ARPES experiments 33 . The CVD-grown flakes were transferred onto epitaxial graphene on SiC before conducting the nano-XPS/ARPES measurements to avoid charging effects [35][36][37][38] (see the "Methods" section). By integrating the photoemission intensity within a selected energy window around the Se 3d peak, while scanning the sample along two in-plane directions, we can first get morphological information on the sample (see Fig.…”

Quantum interference tuning of spin-orbit coupling in twisted van der Waals trilayers

“…Before discussing our numerical calculations, we note that the results depend on i) the position of the Dirac point of graphene within the band gap E g of the TMDC, and ii) on the value of E g . Theoretical DFT calculations are known to often underestimate E g and they seem to give [37] different results from experiments [61,62] for the energy alignment of graphene's Dirac point with the TMDC bands. Therefore we performed our calculations for two parameter sets, one relying on DFT calculations [63], and one extracted from ARPES measurements [61,62].…”

“…Theoretical DFT calculations are known to often underestimate E g and they seem to give [37] different results from experiments [61,62] for the energy alignment of graphene's Dirac point with the TMDC bands. Therefore we performed our calculations for two parameter sets, one relying on DFT calculations [63], and one extracted from ARPES measurements [61,62]. With the exception of Fig.…”