Defect Healing and Charge Transfer-Mediated Valley Polarization in MoS₂/MoSe₂/MoS₂ Trilayer van der Waals Heterostructures

Abstract

Monolayer transition metal dichalcogenides (TMDC) grown by chemical vapor deposition (CVD) are plagued by a significantly lower optical quality compared to exfoliated TMDC. In this work we show that the optical quality of CVD-grown MoSe 2 is completely recovered if the material is sandwiched in MoS 2 /MoSe 2 /MoS 2 trilayer van der Waals heterostructures. We show by means of density-functional theory that this 1 arXiv:1703.00806v2 [cond-mat.mes-hall] 14 Jun 2017 remarkable and unexpected result is due to defect healing: S atoms of the more reactive MoS 2 layers are donated to heal Se vacancy defects in the middle MoSe 2 layer. In addition, the trilayer structure exhibits a considerable charge-transfer mediated valley polarization of MoSe 2 without the need for resonant excitation. Our fabrication approach, relying solely on simple flake transfer technique, paves the way for the scalable production of large-area TMDC materials with excellent optical quality. KeywordsChemical Vapor Deposition, transition metal dichalcogenides, van der Waals heterostructures, defect healing, charge transfer mediated valley polarization Monolayer transition metal dichalcogenides (TMDC) have a direct bandgap situated in the visible range, which makes them ideal building blocks for novel electronic and optoelectronic devices. [1][2][3][4][5][6][7][8][9][10] The bandgap of monolayer TMDCs occurs at the inequivalent (but degenerate) K and K' points of the hexagonal Brillouin zone. The broken inversion symmetry of a TMDC monolayer combined with the time reversal symmetry imposes opposite magnetic moments at the K and K' valleys. This in turn determines the characteristic circular dichroism exhibited by these materials, wherein each valley can be addressed separately with circularly polarized light of a given helicity. 11-13 Additionally, optical spectra are influenced by the strong spin-orbit coupling, which lifts the degeneracy of band states at the valence band edges, resulting in well-resolved A and B resonances, as observed in reflectivity or absorption spectra. 2,14-16 The interplay of spin-orbit coupling with broken inversion symmetry and time reversal symmetry locks the valley and spin degrees of freedom, making TMDC attractive candidates for valleytronics. 17 The spin-valley index locking along with the large 2 distance in the momentum space between K and K' valleys preserves the valley polarization observed in the degree of circular polarization (DCP) in helicity resolved photoluminescence emission. [18][19][20][21][22] Applications require a scalable fabrication platform providing high-quality large-area monolayer TMDC. Unfortunately, the most promising approach today, namely chemical vapor deposition (CVD) growth 23-27 struggles to compete with exfoliated TMDC in terms of sample quality. Low temperature PL spectroscopy of CVD-grown MoS 2 and MoSe 2 reveals broad emission from defect bound excitons, which is significantly more intense than the free exciton peak [28][29][30] and is related to chalcogen vacancies induced...

Moiré Intralayer Excitons in a MoSe2/MoS2 Heterostructure

“…39,44 The observed significant polarization is surprising considering that we excite far from excitonic transitions of MoSe 2 . 48,[58][59][60] This effect might be related to the encapsulation in hBN because we observe a similar degree of circular polarization in both the heterostructure and monolayer regions, although the exact reason the relatively high degree of polarization after encapsulation requires additional investigation. Finally, we investigate the dynamics of the trion and exciton in MoSe 2 .…”

Probing the Interlayer Exciton Physics in a MoS2/MoSe2/MoS2 van der Waals Heterostructure

“…This is in strong contrast with the co-polarized emission of the A-exciton in MoSe 2 . 33 In addition, under linearly polarized excitation, which simultaneously excites both valleys, I X does not exhibit any detectable degree of circular polarization, which rules out a "built in" imbalance of the valley population 41 or carrier lifetimes as the origin of the observed polarization behavior. The opposite helicity compared to the excitation points to a more complex valley and spin transfer mechanism than that previously invoked to explain the non-trivial dependence of the polarization of inter-layer excitons in gated heterostructures.…”

Two‐dimensional heterostructure promoted infrared photodetection devices

“…Copyright 2014, American Chemical Society. F, Model and optical micrograph of MoS 2 /MoSe 2 /MoS 2 heterostructure via CVD‐grown 2D materials . Copyright 2017, American Chemical Society.…”

“…F, Model and optical micrograph of MoS 2 /MoSe 2 /MoS 2 heterostructure via CVD-grown 2D materials. 24 Copyright 2017, American Chemical Society. CVD, chemical vapor deposition Typically, photodiodes are diodes that consist of a p-n, 49 p-i-n, 50 or Schottky junction.…”