Free-standing electronic character of monolayerMoS2in van der Waals epitaxy

Abstract

We have evaluated as-grown $\mathrm{Mo}{\mathrm{S}}_{2}$ crystals, epitaxially grown on a monocrystalline sapphire by chemical vapor deposition (CVD), with direct electronic band-structure measurements by energy-filtered $k$-space photoelectron emission microscopy performed with a conventional laboratory vacuum ultraviolet He I light source under off-normal illumination. The valence states of the epitaxial $\mathrm{Mo}{\mathrm{S}}_{2}$ were mapped in momentum space down to 7 eV below the Fermi level. Despite the high nucleation density within the imaged area, the CVD $\mathrm{Mo}{\mathrm{S}}_{2}$ possesses an electronic structure similar to the free-standing monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$ single crystal, and it exhibits hole effective masses of $2.41\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}{m}_{0}$, and $0.81\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}{m}_{0}$, respectively, at \ensuremath{\Gamma} and $K$ high-symmetry points that are consistent with the van der Waals epitaxial growth mechanism. This demonstrates the excellent ability of the $\mathrm{Mo}{\mathrm{S}}_{2}$ CVD on sapphire to yield a highly aligned growth of well-stitched grains through epitaxial registry with a strongly preferred crystallographic orientation.