Strain distribution in WS₂ monolayers detected through polarization-resolved second harmonic generation

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Two-dimensional (2D) materials, including graphene and graphene-related materials (GRMs), are vital for next-generation electronics.
• Substrate interactions induce strain in GRMs, altering their electronic properties and enabling the field of straintronics.
• Developing rapid, non-invasive methods to probe strain in large areas of 2D materials is essential for device research.

Purpose of the Study:

• To demonstrate an all-optical imaging technique for mapping strain distribution in 2D material monolayers.
• To investigate strain induced in a WS2 monolayer on a patterned Si/SiO2 substrate using Polarization-resolved Second Harmonic Generation (P-SHG).
• To correlate optical measurements with atomic force microscopy and Raman spectroscopy for strain confirmation.

Main Methods:

• Utilized Polarization-resolved Second Harmonic Generation (P-SHG) optical imaging to detect strain.
• Applied pixel-by-pixel fitting of P-SHG data to generate spatially resolved images of the crystal armchair direction.
• Employed atomic force microscopy (AFM) and Raman mapping for independent verification of strain.

Main Results:

• Successfully identified strain distribution in a single-layer WS2 on a pre-patterned substrate.
• Observed a distinct cross-shaped pattern in armchair direction images in strained regions where WS2 conformed to the substrate topography.
• Confirmed the presence of strain through complementary AFM and Raman spectroscopy analyses.

Conclusions:

• P-SHG imaging serves as an effective, minimally invasive tool for mapping strain in 2D materials over large areas.
• The findings highlight the potential of P-SHG for quality control and development in 2D electronic devices.
• Strain engineering in 2D materials can be precisely characterized using this advanced optical imaging technique.