2, Friedrich Schiller University Jena, Institute for Physical Chemistry, Jena, , Germany
3, University of Ulm, Ulm, , Germany
4, National Institute for Materials Science, Tsukuba, , Japan
Transition metal dichalcogenides (TMDs) are ideal for exploring fundamental physics and applied optics as they are semiconductors with a direct bandgap in the monolayer (ML) limit. Therefore, it is very important to have access to high quality and also large surface area films. Here we show that chemical vapour deposition (CVD) growth can yield high quality MoS2 monolayers on SiO2/Si substrates.
We show high structural quality of CVD grown MoS2 MLs on SiO2  in 4k x 4k high-resolution transmission electron microscopy (HRTEM) images using the chromatic and spherical aberration-corrected low-voltage TEM instrument operated at 60 kV. We determine a defect concentration of 1013/cm2 in as grown MoS2 MLs. But we observe broad optical transitions in as-grown samples (50 meV FWHM at T = 4K) on SiO2 that are not expected for high structural quality, therefore indicating detrimental ML-substrate interactions. Therefore, we lift off the CVD grown layers from the growth substrate and encapsulate them in hBN flakes, which give us access to the intrinsic optical quality of the MoS2 MLs.
We compare the optical quality of MoS2 MLs in three different structures: CVD grown MoS2 films on SiO2, exfoliated MoS2 ML from bulk MoS2 in exfoliated hBN and most importantly MoS2 ML grown by CVD encapsulated in hBN crystals prepared by mechanical exfoliation. For the latter structure, we show (i) in photoluminescence the neutral A-exciton emission linewidth reduced to 5 meV at T = 4 K, as compare to 50 meV linewidth in the as grown CVD sample and (ii) in absorption well separated optical transitions A:2s stemming from excited states of the A-exciton Rydberg series, indicating comparable quality of our CVD MLs to exfoliated MoS2 material. We optically generate valley coherence and valley polarization in our CVD grown MoS2 layers, showing the possibility for studying spin and valley physics in CVD samples of large surface area.
 A. George, C. Neumann, D. Kaiser, R. Mupparapu, T. Lehnert, U. Hübner, Z. Tang, A. Winter, U. Kaiser, I. Staude, A. Turchanin, J. Phys.: Mater. 2 (2019) 016001