Abstract
We present the electronic and structural properties of monolayer WSe$_{2}$ grown by pulsed-laser deposition on monolayer graphene (MLG) on SiC. The spin splitting in the WSe$_{2}$ valence band at $\overline{\mathrm{K}}$ was $\Delta_\mathrm{SO}=0.469\pm0.008$ eV by angle-resolved photoemission spectroscopy (ARPES). Synchrotron-based grazing-incidence in-plane X-ray diffraction (XRD) revealed the in-plane lattice constant of monolayer WSe$_{2}$ to be $a_\mathrm{WSe_2}=3.2757\pm0.0008 \mathrm{\r{A}}$. This indicates a lattice compression of -0.19 % from bulk WSe$_{2}$. By using experimentally determined graphene lattice constant ($a_\mathrm{MLG}=2.4575\pm0.0007 \mathrm{\r{A}}$), we found that a 3$\times$3 unit cell of the slightly compressed WSe$_{2}$ is perfectly commensurate with a 4$\times$4 graphene lattice with a mismatch below 0.03 %, which could explain why the monolayer WSe$_{2}$ 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 $\Delta_\mathrm{SO}$ 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 WSe$_{2}$ and MLG/SiC and indicate electron transfer from graphene to the WSe$_{2}$ monolayer. As further revealed by atomic force microscopy, the WSe$_{2}$ island size depends on the number of carbon layers on top of the SiC substrate. This suggests that the epitaxy of WSe$_{2}$ 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.
Highlights
INTRODUCTIONDue to time reversal symmetry, the K and K valleys have opposite out-of-plane spin polarizations, and each valley is associated with optical selection rules of opposite chirality as well as opposite signs of Berry curvature [7]
Two-dimensional (2D) transition-metal dichalcogenides (TMDs) MX 2 (M = Mo or W, X = S, Se, or Te) possess outstanding electronic, spin, and optical properties at thicknesses of a few layers and hold great promise for future optoelectronic and spintronic applications [1,2,3,4,5,6]
By using the experimentally determined graphene lattice constant, 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 monolayer graphene (MLG)
Summary
Due to time reversal symmetry, the K and K valleys have opposite out-of-plane spin polarizations, and each valley is associated with optical selection rules of opposite chirality as well as opposite signs of Berry curvature [7] This leads to the valley-contrasting physics of monolayer TMDs, such as optical valley polarization and the valley Hall effect [1,4]. Considering a band gap of 1.95 eV as determined by STS, this suggests a small downshift (∼100 meV) of the WSe2 bands, consistent with an electron transfer to the TMD layer. To unambiguously resolve the issue of charge transfer across the TMD/graphene interface, a comparison of ARPES measurements performed both before and after the creation of the vdW heterostructure could be highly useful. Atomic force microscopy (AFM) further reveals a significant impact of the substrate morphology on the WSe2 island size
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
