Spin splitting and strain in epitaxial monolayer WSe${}_2$ on graphene

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 ${\Delta }_{\mathrm{SO}}=0.469\pm 0.008$ 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 $a_{\mathrm{WS}{\mathrm{e}}_2}=3.2757\pm 0.0008\mathring{\mathrm{A}}$. This indicates a lattice compression of -0.19 % from bulk WSe${}_2$. By using experimentally determined graphene lattice constant ($a_{\mathrm{MLG}}=2.4575\pm 0.0007\mathring{\mathrm{A}}$), 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.

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Cited by in corpus (5)

• Surface charge-transfer doping a quantum-confined silver monolayer beneath epitaxial graphene
• Visualizing giant ferroelectric gating effects in large-scale WSe${}_2$/BiFeO${}_3$ heterostructures
• Spintronics in 2D graphene-based van der Waals heterostructures
• Fermi level depinning via insertion of a graphene buffer layer at the gold-2D tin monoxide contact
• Biaxial strain tuning of excitons in monolayer MoSe${}_2$ by high-temperature physical vapor deposition