Monitoring Local Strain Vector in Atomic-Layered MoSe₂ by Second-Harmonic Generation

Abstract

Strain serves as a powerful freedom to effectively, reversibly, and continuously engineer the physical and chemical properties of two-dimensional (2D) materials, such as bandgap, phase diagram, and reaction activity. Although there is a high demand for full characterization of the strain vector at local points, it is still very challenging to measure the local strain amplitude and its direction. Here, we report a novel approach to monitor the local strain vector in 2D molybdenum diselenide (MoSe) by polarization-dependent optical second-harmonic generation (SHG). The strain amplitude can be evaluated from the SHG intensity in a sensitive way (-49% relative change per 1% strain); while the strain direction can be directly indicated by the evolution of polarization-dependent SHG pattern. In addition, we employ this technique to investigate the interlayer locking effect in 2H MoSe bilayers when the bottom layer is under stretching but the top layer is free. Our observation, combined with ab initio calculations, demonstrates that the noncovalent interlayer interaction in 2H MoSe bilayers is strong enough to transfer the strain of at least 1.4% between the bottom and top layers to prevent interlayer sliding. Our results establish that SHG is an effective approach for in situ, sensitive, and noninvasive measurement of local strain vector in noncentrosymmetric 2D materials.

Nonlinear optics of two‐dimensional transition metal dichalcogenides

“…Mechanic strain is an effective way to alter the electronic and optical properties by deforming the crystal lattice . Recently, strain has been proved as an effective method to engineer the NLO . For instance, a monolayer MoS 2 under a strain is no longer belongs to D 3 h space group and the χ (2) tensor has more than one independent components.…”

“…Liang et al demonstrated that the strain can not only change the angle dependence pattern, but also change the total SHG intensity. They reported a ~49% intensity change for 1% strain . In addition, SHG modulation of bilayer MoSe 2 was realized by applying a strain on the underneath monolayer.…”

“…It was reported that TMDs could sustain a stain larger than 10% due to the excellent mechanic properties, indicating the promising applications in stretchable and flexible nonlinear devices. In addition, SHG can serve as a powerful technique to probe the magnitude and direction of strain by measuring the polarization‐dependent SHG . Recently, Liang et al reported that THG could serve as a universal method to image the strain tensor because THG does not require the absence of inversion symmetry …”

“…155 Recently, strain has been proved as an effective method to engineer the NLO. 78,153,156,157 For instance, a monolayer MoS 2 under a strain is no longer belongs to D 3h space group F I G U R E 5 A, Charge-induced SHG of bilayer WSe 2 . 42 Copyright 2015, American Chemical Society.…”

“…They reported a~49% intensity change for 1% strain. 157 In addition, SHG modulation of bilayer MoSe 2 was realized by applying a strain on the underneath monolayer. Mennel et al compared the behavior of four types of TMDs (MoS 2 , MoSe 2 , WS 2 , and WSe 2 ) and found that WSe 2 was most insensitive to the strain.…”