Probing Local Strain at MX₂–Metal Boundaries with Surface Plasmon-Enhanced Raman Scattering

Abstract

Interactions between metal and atomically thin two-dimensional (2D) materials can exhibit interesting physical behaviors that are of both fundamental interests and technological importance. In addition to forming a metal–semiconductor Schottky junction that is critical for electrical transport, metal deposited on 2D layered materials can also generate a local mechanical strain. We investigate the local strain at the boundaries between metal (Ag, Au) nanoparticles and MX2 (M = Mo, W; X = S) layers by exploiting the strong local field enhancement at the boundary in surface plasmon-enhanced Raman scattering (SERS). We show that the local mechanical strain splits both the in-plane vibration mode E2g(1) and the out-of-plane vibration mode A1g in monolayer MoS2, and activates the in-plane mode E1g that is normally forbidden in backscattering Raman process. In comparison, the effects of mechanical strain in thicker MoS2 layers are significantly weaker. We also observe that photoluminescence from the indirect bandgap transition (when the number of layers is ≥2) is quenched with the metal deposition, while a softened and broadened shoulder peak emerges close to the original direct-bandgap transition because of the mechanical strain. The strain at metal–MX2 boundaries, which locally modifies the electronic and phonon structures of MX2, can have important effects on electrical transport through the metal–MX2 contact.

Sulfur vacancy-induced reversible doping of transition metal disulfides via hydrazine treatment

“…The A1g peak at 418 cm -1 corresponds to the S-S atom out-of-plane vibration mode and is sensitive to the charging effect, whereas the E 2g 1 peak is associated with the W-S atom in-plane vibration mode and is sensitive to the strain field. [23][24][25][26] To verify the doping of WS2 flakes with hydrazine, we examined the shifts of the E 2g 1 and A1g peaks in the Raman spectra ( Fig. 1d and S1).…”

Probing thermal expansion coefficients of monolayers using surface enhanced Raman scattering

“…However, interestingly, it has been experimentally observed that the degree mechanical strain is largely independent of the NP size. 34 Notably strain from lattice mismatch is not expected to be a signicant contributing factor to within Ag NP lms grown by e-beam. 36 However, during temperature dependent measurements strain can also be generated due to the mismatch in the TECs of the materials.…”

“…However, this enhancement is also accompanied by a slight red-shi of the E 0 (G) mode and additional peaks appear, caused by a strain of approximately 1% in the WS 2 induced by the NPs. 32,34 The modication of the Raman spectra by the Ag NP over-layer can be can be understood considering two contributing factors; strain and electric eld enhancement. First considering strain, the application strain can frustrate the lattice symmetry, leading to peak splitting in the Raman spectra.…”

“…Thus the scattering process is enhanced by a factor of (E local /E 0 ) 4 , where E 0 is the strength of the incident E-eld and E local is the strength of the total local electric eld in the presence of the metallic NPs. 34,38,39 For example, the intensity of the 2LA(M) mode is enhanced by a factor of 3. Using nite-difference time-domain (FDTD) methods, we simulated the electrical eld distribution of an area of monolayer WS 2 containing several 5 nm nominal thickness Ag NPs (Fig.…”

Interfacing 2D Semiconductors with Functional Oxides: Fundamentals, Properties, and Applications

“…Interface interactions between 2D semiconductors and substrates must be considered. Besides the traditional dielectric screening effect, several new effects of oxide substrates, including charge transfer, energy transfer, and strain, have emerged and may separately or co-functionally dominate the properties of overlayer 2D semiconductors [4,5].…”