wileyonlinelibrary.comThird, monolayer TMDs show piezoelectricity due to the broken inversion symmetry. [5,6] As a result, monolayer TMDs indicate significant potential for flexible optoelectronics, [7,8] piezotronics, [5,6] mechanically enhanced nanocomposites, [9] and smart materials for strain sensing. [10] In these applications, strain is inevitably a critical object requiring comprehensive understanding.So far, substantial efforts have been spent on the study of strain engineering to the band structure of TMDs by using bending, [11][12][13][14][15] high-pressure compression, [16][17][18][19][20][21][22][23][24] and tensile elongation. [25] Their results indicate strain is an effective way to tune the bandgap from direct to indirect, generate the redshifts of trion and exciton peaks, and change the material conductivity from semiconducting to metallic. [26] However, an important question of how strain relaxes inside TMDs is still open.Herein, we investigate the strain relaxation of monolayer WS 2 triangular crystals deposited on polydimethylsiloxane (PDMS) substrate. The uniaxial tensile strain applied to the WS 2 crystals is transferred from PDMS substrate when the substrate is elongated by a loading frame. We observe that the trion and exciton PL peaks undergo a redshift when the substrate strain is increased from 0 to 0.16. However, the redshifts stop when the substrate strain is further increased from 0.16 to 0.32. This is caused by the strain relaxation in WS 2 through Strain-dependent electrical and optical properties of atomically thin transition metal dichalcogenides may be useful in sensing applications. However, the question of how strain relaxes in atomically thin materials remains not well understood. Herein, the strain relaxation of triangular WS 2 deposited on polydimethylsiloxane substrate is investigated. The photoluminescence of trions (X -) and excitons (X 0 ) undergoes linear redshifts of ≈20 meV when the substrate tensile strain increases from 0 to 0.16. However, when the substrate strain further increases from 0.16 to 0.32, the redshifts cease due to strain relaxation in WS 2 . The strain relaxation occurs through formation of wrinkles in the WS 2 crystal. The pattern of wrinkles is found to be dependent on the relative angle between an edge of the triangular WS 2 crystal and tensile strain direction. Finite element simulations of the strain distribution inside the WS 2 crystals explain the experimental observations.

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