High optical quality of MoS ₂ monolayers grown by chemical vapor deposition

Abstract

Chemical vapor deposition (CVD) allows growing transition metal dichalcogenides (TMDs) over large surface areas on inexpensive substrates. In this work, we correlate the structural quality of CVD grown MoS 2 monolayers (MLs) on SiO 2 /Si wafers studied by high-resolution transmission electron microscopy (HRTEM) with high optical quality revealed in optical emission and absorption from cryogenic to ambient temperatures. We determine a defect concentration of the order of 10 13 cm −2 for our samples with HRTEM. To have access to the intrinsic optical quality of the MLs, we remove the MLs from the SiO 2 growth substrate and encapsulate them in hBN flakes with low defect density, to reduce the detrimental impact of dielectric disorder. 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 MoS 2 ML photoluminescence is dominated by free excitons and not defects even at low temperature. High optical quality of the samples is further confirmed by the observation of excited exciton states of the Rydberg series. We optically generate valley coherence and valley polarization in our CVD grown MoS 2 layers, showing the possibility for studying spin and valley physics in CVD samples of large surface area. arXiv:1907.03342v1 [cond-mat.mes-hall]

Synthesis of Ultrahigh‐Quality Monolayer Molybdenum Disulfide through In Situ Defect Healing with Thiol Molecules

“…Noteworthily, we have compared the SV density of LCVD MoS 2 with MoS 2 grown by conventional CVD method with solid precursors and the exfoliated MoS 2 (Figure 3f). The SV density of monolayer MoS 2 grown by LCVD is the lowest among all reported CVD‐grown samples, [ 17,32–36 ] and close to the exfoliated one (0.15 nm −2 ; Figure S7, Supporting Information). Besides, it is worth mentioning that all the point defects are mono‐SV, which is different from previous reports of SCVD‐grown MoS 2 where disulfur vacancies are usually observed.…”

“…The coupling constant and the average phonon energy are similar to those obtained in the literature. [ 33,42,47–50 ] The evolution of the linewidth as a function of temperature is shown in Figure 4e and can be phenomenologically approximated by a phonon induced broadening effect (blue dashed line) [ 49,51 ] where γ 0 = 42.6 ± 1.3 meV is the temperature independent inhomogeneous broadening, c 1 = 19 ± 5 μeV K −1 describes the linear increase due to acoustic Γ phonons, c 2 = 87 ± 18 meV is a measure of the strength of exciton–phonon coupling, and 〈ℏω〉 = 25.3 meV is the averaged energy of the relevant phonon, which is obtained by fitting the A exciton energy shift with Equation () for consistency.…”

Guide to optical spectroscopy of layered semiconductors

“…In addition to the intrinsic layer quality, the impact of the underlying substrate can also limit the optical quality. This has been demonstrated by comparing low-temperature optical spectra between TMD monolayers (both CVD and exfoliated) on SiO 2 and encapsulated with hexagonal boron nitride (hBN) 16,55,86 . It is possible to further improve the optical response (in terms of signal strength for a particular transition) by optimizing the thickness of the top Here, transition metal atoms are shown in blue, chalcogen atoms in grey.…”

“…For applications in photonics, such techniques reveal how light-matter coupling is enhanced when layered materials are placed in optical cavities or on resonators 11,12 . Optical spectroscopy can be used as a non-invasive technique for studying lattice structure, interlayer coupling and stacking that complements direct atomic-resolution imaging from electron microscopy [13][14][15][16][17] .…”

Imaging and identification of point defects in PtTe2

“…One such material is PtTe 2 which is a member of the transition metal dichalcogenide (TMD) family 8,13,14 . While PtTe 2 has thus far been less heavily studied than some other members of the TMD family, such as MoS 2 [15][16][17] , it benefits from interesting properties. It has been shown to be a type-II Dirac semimetal 18 with high conductivity 19 , suggesting it may be suitable for applications in spintronics and quantum computing [20][21][22] .…”