Effects of Interlayer Coupling and Band Offset on Second Harmonic Generation in Vertical MoS2/MoS2(1- x)Se2 x Structures

Chinh Tam Le; Jungcheol Kim; Farman Ullah; Anh Duc Nguyen; Thao Nhi Nguyen Tran; Tuan-Em Le; Koo-Hyun Chung; Hyeonsik Cheong; Joon I Jang; Yong Soo Kim

doi:10.1021/acsnano.9b09901

Effects of Interlayer Coupling and Band Offset on Second Harmonic Generation in Vertical MoS₂/MoS_{2(1- x)}Se_{2 x} Structures

ACS Nano. 2020 Apr 28;14(4):4366-4373. doi: 10.1021/acsnano.9b09901. Epub 2020 Mar 31.

Authors

Affiliations

¹ Department of Physics and Energy Harvest Storage Research Center, University of Ulsan, Ulsan 44610, South Korea.
² Department of Physics, Sogang University, Seoul 04107, South Korea.
³ School of Mechanical Engineering, University of Ulsan, Ulsan 44610, South Korea.

PMID: 32212675
DOI: 10.1021/acsnano.9b09901

Abstract

Noncentrosymmetric monolayers (MLs) of transition metal dichalcogenides (TMDCs) and their 3R-type vertical stacks provide an ideal platform for studying atomic-scale nonlinear light-matter interaction in terms of second harmonic generation (SHG). Unlike the case of MLs, SHG from artificial stacks can be nontrivially affected by interlayer coupling and band offset between the constituent MLs, where the latter occurs for band-gap-engineered vertical heterostructures (VHs). In order to study these effects, we produced different sets of 3R-type homobilayers (homo-BLs) and heterobilayers (hetero-BLs) composed of MoS₂ and its ternary alloy MoS_2(1-x)Se_2x. We first investigated the impact of interlayer coupling on the SHG response across the A- and B-exciton resonances in the MoS₂ homo-BLs. The coupling strength was varied by preparing (i) decoupled BLs (SiO₂ intercalated), (ii) weakly coupled BLs (dry transferred), and (iii) strongly coupled BLs (postannealed) and monitored by photoluminescence, Raman, and reflectance difference spectroscopy, and atomic force microscopy. Unlike the decoupled BL, SHG in the coupled BLs cannot be explained by the simple square law in thickness due to coupling-induced band modification. The impact of exciton-resonance offset on SHG was also investigated in the hetero-BLs by controlling the Se concentration in MoS_2xSe_2(x-1). Although these VHs can significantly broaden the spectral range for efficient SHG by vertically superposing distinct resonances of the constituent MLs, coherent reinforcement of SHG cannot be achieved basically because of the π/2 phase difference between the on-resonance SHG field in one ML and the off-resonance SHG field in the other ML. Upon postannealing, however, the overlapping resonance regime exhibited unexpectedly high SHG enhancement. This may arise from the formation of the strong resonance when the VHs approach ideal 3R-type hetero-BLs. Our approach may be utilized for fully exploiting the TMDC VHs for highly efficient broadband SHG applications.

Keywords: interlayer coupling; monolayer alloys; resonantly enhanced second harmonic generation; transition metal dichalcogenides; vertical heterostructures.