Effects of Interlayer Coupling and Band Offset on Second Harmonic Generation in Vertical MoS₂/MoS_2(1–x)Se_2x Structures

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 MoS 2 and its ternary alloy MoS 2(1−x) Se 2x . We first investigated the impact of interlayer coupling on the SHG response across the Aand B-exciton resonances in the MoS 2 homo-BLs. The coupling strength was varied by preparing (i) decoupled BLs (SiO 2 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 2x Se 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.

Thermal Treatment of Polyvinyl Alcohol for Coupling MoS2 and TiO2 Nanotube Arrays toward Enhancing Photoelectrochemical Water Splitting Performance

“…TiO 2 nanotube arrays (TNAs) are often modified to extend the photoresponse range by combining with a noble metal, creating self-defects, and coupling with other semiconductors to improve the material working efficiency [6,7]. Especially, TNAs modified by MoS 2 demonstrated superior performance in the water-splitting process due to their suitable redox for both HER and OER [8][9][10]. There are many routes to modify TNAs by MoS 2 that demonstrated promising results, i.e., the hydrothermal method [11,12], physical vapor deposition [13], and chemical vapor deposition [14].…”

Harmonic Generation in Low‐Dimensional Materials

“…It was proved both experimentally and theoretically that when the twisted angle between the two monolayer WS 2 stack becomes 30°, it is possible to suppress the SHG signal of one layer whereas the signal is maximum of the other layer. Le et al [200] investigated the effect of bandgap offset and interlayer coupling on the SHG in the vertical homo (MoS 2 /MoS 2 ) and hetero (MoS 2 /MoS 2(1-x) Se 2x ) structures. The samples were produced using chemical vapor deposition (CVD) method and the tilt angle between the monolayers was accurately controlled by micromanipulators.…”

“…The thickness variation law is only applicable to the decoupled bilayer region, whereas the SHG from the other two regions cannot be explained. [200] Several SHG investigations on the homo and hetero structures of graphene have been carried out. Unlike graphene mono or bilayer, graphene tri-layer has two crystallographic structures by nature, ABA (Bernal) and ABC (rhombohedral) (Figure 7g).…”

“…Le et al. [ 200 ] investigated the effect of bandgap offset and interlayer coupling on the SHG in the vertical homo (MoS 2 /MoS 2 ) and hetero (MoS 2 /MoS 2(1‐x) Se 2x ) structures. The samples were produced using chemical vapor deposition (CVD) method and the tilt angle between the monolayers was accurately controlled by micromanipulators.…”

Growth of two-dimensional Janus MoSSe by a single in situ process without initial or follow-up treatments

“…This unique characteristic of 2D Janus TMDCs leads to their novel phenomena, such as Rashba spin splitting, piezoelectric polarization, second-harmonic generation, catalytic effects, and long carrier recombination [1][2][3][4][5][6][7] , which are very promising for their application in sensors, actuators, piezo/thermal-electric devices, solar cells, ion batteries, and other electromechanical devices [8][9][10][11] . The asymmetry of Janus TMDCs has also been used to tailor the van der Waals (vdW) interlayer coupling and charge transfer in vdW heterostructures [12][13][14][15] ; such tailoring of Janus MoSSe/MoS 2 heterobilayers was performed by tuning the twist angle and interface composition 16 . The energy of graphene plasmons in vdW Janus MoSSe/graphene heterostructures was engineered up to 0.5 eV by varying the number of Janus MoSSe layers 17 .…”