Development of in situ characterization of two-dimensional materials grown on insulator substrates with spectroscopic photoemission and low energy electron microscopy

Ultrathin two-dimensional (2D) materials offer great potential for next-generation integrated circuit and opto-electronic devices. Chemical vapor deposition (CVD)-grown 2D materials provide a way to mass production industry. However, how to in situ characterize their intrinsic electric/photoelectric properties and carrier dy-namics with electron/photoelectron probes is still a problem due to the interference from the conducting sub-strate. Here, we present a grounding Au grids method to realize in situ characterization of the CVD-grown MoS2 on the insulating thick SiO2 layer covered Si substrate with spectroscopic photoemission and low energy electron microscopy (SPELEEM). Through depositing Au grids afterwards, we have achieved good grounding of MoS2 flakes in the photoemission electron microscopy (PEEM), mirror electron microscopy (MEM), and micro-area low energy electron diffraction (mu-LEED) measurements. We have clarified the false signal caused by stray photo-electrons originated from the Au stripes, and as well as the space charge effects induced by intense photo-emission. We have also confirmed that time-resolved PEEM results are not affected by the stray signal, and adopting a small light spot, both static and time-resolved micro-area photoelectron spectroscopy (mu-PES) can unaffected by space charge effects. Our results provide a reliable way to in situ investigate 2D materials grown insulating substrates by probing photoelectrons or backscattered electrons.