monolayers grown by chemical vapor deposition : impact of dielectric disorder

Chemical vapor deposition (CVD) allows growing transition metal dichalcogenides (TMDs) over large surface areas on inexpensive substrates, but optical transition linewidth of as-grown monolayers (MLs) is usually tens of meV. We correlate the structural quality of CVD grown MoS2 MLs on SiO2/Si wafers studied by high-resolution transmission electron microscopy (HRTEM) with the optical quality revealed in optical emission and absorption from cryogenic to ambient temperatures. With HRTEM we determine a defect concentration of the order of 10¹³/cm² for our samples, comparable to standard exfoliated flakes from bulk crystals. To have access to the intrinsic optical quality of the CVD grown MLs, we remove the MLs from the SiO2 growth substrate and encapsulate them in hBN flakes with low defect density. In encapsluated MLs we show optical transition linewidth of 5 meV at low temperature T=4 K for the free excitons in emission and absorption. This is comparable to the best ML samples obtained by mechanical exfoliation of bulk material. The CVD grown MoS2 ML shows strong photoluminescence that is dominated by neutral excitons and not by defects even at cryogenic temperatures. We study the valley Zeeman effect in magnetic fields up to 9T accessible thanks to the narrow emission linewidth.