Strain Relaxation of Monolayer WS₂ on Plastic Substrate

Abstract

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.

Oxidation of Monolayer WS2 in Ambient Is a Photoinduced Process

“…arranged with threefold rotational symmetry around the crystal centers (Figure 2e-f), as well as the edges and red-shifted PL regions of the crystals (Figures 1b, 3b, 3c, S10, S11b and Figures 2a, b and d respectively). The same threefold rotationally symmetric pattern is seen in the µ-PL (Figure 2a-b) and has been observed in our group previously, 31 and by other groups. [26][27][28][29][30] Recent work counting individual defects using conductive AFM has demonstrated that this three-fold symmetric region has a higher defect density than surrounding areas of the single crystal.…”

“…Increased PL brightness around the edges of monolayer S-TMDs has also been reported on by others, 6,24,[26][27][28][29][30][31]33 and the effect is attributed to either differences in chemical composition on the edges of the crystal 6,[26][27][28][29][30]33 or to water intercalation at the edges of the S-TMDs. 24,31,34 Figure 2g and 2h show higher-resolution AFM topography and phase images respectively, of the triangular oxidation island feature outlined by the dotted black box in Figure 2f. The AFM topography image in Figure 2g shows that these triangular oxidation islands are not holes (as suggested by the LSCM in Figure 2e) but raised in topography by ≈ 1.14 nm above the surrounding WS2; this is consistent with thicker WOx remaining as the reaction product (see Supporting Information Section 3 for in-depth analysis of oxidation heights).…”

“…It can be seen that the PL intensity on the edges is much brighter than that of the center, as is observed in many other reports. 6,24,[26][27][28][29][30][31] Also, the µ-PL shows a three-fold rotationally symmetric pattern, not only in the intensity map, but also mirrored in the map of the peak photon energy (relating to the location of the exciton) shown in Figure 2b, i.e., the two correlate with each other. 28 Figure 2c shows a representative spectrum from the central region of the crystal, deconvolved as two Voigt 32 spectra identified as the exciton peak at approximately 2.03±0.01 eV and smaller trion peak at 1.97±0.01 eV.…”

“…However, the physical reasons behind the changes in PL intensity [26][27][28][29][30][31] and peak position 28 observed in the PL of WS2 (Figures 2a-d) remain unclear. In particular, some groups have considered strain from lattice mismatch between crystal and substrate to explain changes in PL, but have subsequently ruled out this strain due to the persistence of the threefold symmetric pattern upon transfer of the crystal to another substrate, which is thought to relieve strain in as-grown crystals.…”

Ferroelectric-Driven Exciton and Trion Modulation in Monolayer Molybdenum and Tungsten Diselenides

“…KEYWORDS: ferroelectric polarization, lithium niobite, transition metal dichalcogenides, photoluminescence modulation, electrostatic doping, Direct bandgap monolayer transition metal dichalcogenides (TMDs) [1][2][3][4] have attracted tremendous interest due to their optically controlled valley polarization and coherence, [5][6] giant spin-valley coupling and tightly bound excitonic states. [7][8][9][10] Owing to their atomically thin structure, monolayer TMDs act as semiconductors which can undergo a complete transition from indirect to a direct bandgap, with energy gaps located at the Brillouin zone producing a strong exciton pumping efficiency.…”