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Defect healing and interlayer exciton in a CVD grown MoS2/MoSe2/MoS2 heterostructure - Alessandro Surrente

Laboratoire National des Champs Magnétiques Intenses, CNRS, UGA-INSA-UPS, Grenoble and Toulouse

LPS amphi moyen

Chemical vapour deposition (CVD) is a scalable growth technique, which is extremely promising for the fabrication of large area transition metal dichalcogenide (TMD) monolayers for optoelectronic applications. Unfortunately, CVD grown TMDs are generally characterized by poor optical quality with broad photoluminescence (PL) emission dominated by defect bound excitons at cryogenic temperatures. In our talk, we demonstrate that a simple fabrication approach, based on double-sided encapsulation of a CVD-grown MoSe2 monolayer via flake transfer of CVD-grown MoS2 flakes completely suppresses the defect- related emission, restoring the excellent optical quality of the sandwiched MoSe2 layer. The low temperature PL emission of the encapsulated MoSe2 monolayer exhibits a line width comparable to that of mechanically exfoliated MoSe2. This is consistent with a defect healing scenario, suggested by density functional theory, which predicts that it is energetically favourable for the more reactive MoS2 to donate a chalcogen atom to fill Se vacancies in the encapsulated MoSe2 [1].

The MoS2/MoSe2 heterostructure has a type-II band alignment. The charge transfer process after the initial photoexcitation is spin conserving, allowing us to observe for the first time a significant valley polarization of the MoSe2 emission while exciting in resonance with the A exciton of MoS2 [1]. The interlayer nature of the low energy peak observed at 1.4eV is confirmed by its sublinear dependence on the excitation power, the slow recombination dynamics with characteristic lifetimes of nanoseconds, and the non-monotonic temperature dependence of the peak intensity. Intriguingly, the interlayer exciton emission is counter- polarized with respect to the circularly polarized excitation [2]. The interlayer exciton emission in magnetic fields up to 28T shows a giant valley Zeeman splitting, yielding an effective g-factor of -13.1, which suggests a stacking angle of 60° between the optically active monolayers [3]. The large Zeeman splitting allows us to manipulate the valley polarization, which can be tuned controllably up to almost 100%. The magnetic field dependence of the valley polarization can be explained using a simple rate equation model, which accounts for intervalley scattering processes such as valley depolarization via electron-hole exchange interaction and one-phonon spin-lattice relaxation [4].

[1] Surrente, et al, Nano Letters 17, 4130 (2017)

[2] Baranowski, Surrente, et al, Nano Letters 17, 6360 (2017)
[3] Nagler, et al, Nature Communications 8, 1551 (2018)

[4] Surrente, et al, Nano Letters 18, 3994 (2018)


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