SPE-LEEM Studies on the Surface and Electronic Structure of 2-D Transition Metal Dichalcogenides*

In this work, we studied the surface and electronic structure of monolayer and few-layer exfoliated MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$, as well as chemical-vapor-deposition (CVD) grown MoS$_{\mathrm{2}}$, using Spectroscopic Photoemission and Low Energy Electron Microscope (SPE-LEEM). LEEM measurements reveal that, unlike exfoliated MoS$_{\mathrm{2}}$, CVD-grown MoS$_{\mathrm{2}}$ exhibits grain-boundary alterations due to surface strain. However, LEEM and micro-probe low energy electron diffraction show that the quality of CVD-grown MoS$_{\mathrm{2}}$ is comparable to that of exfoliated MoS$_{\mathrm{2}}$. Micrometer-scale angle-resolved photoemission spectroscopy (ARPES) measurement on exfoliated MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$ single-crystals provides direct evidence for the shifting of the valence band maximum from $\Gamma $ to K, when the layer number is thinned down to one, as predicted by density functional theory. Our measurements of the k-space resolved electronic structure allow for further comparison with other theoretical predictions and with transport measurements.