Raman and Photoluminescence Spectroscopic Studies of the Micromechanics of WS2 Nanocomposites

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Abstract

The reinforcement mechanism in WS2-based nanocomposites has been investigated in the project. It started with the deformation analysis on the micromechanical exfoliated WS2 (monolayer and fewlayer) and nanotube WS2 using photoluminescence (monolayer only) and Raman spectroscopies. For monolayer, a massive PL energy shift has been observed caused by uniaxial tensile strain engineered optical bandgap. A new feature in the PL spectra corresponding to the indirect band gap transition (Γ-T) has emerged under moderate uniaxial stress level. In all cases, significant Raman band shifts have been found for both E2g1 and A1g modes due to strain-induced phonon softening. The E2g1 mode undergoes splitting at high strain level, which was taken advantage of to determine the crystalline orientation of exfoliated WS2. A systematic study has been undertaken on the monolayer, bilayer, trilayer and fewlayer WS2 under deformation to determine the optimum the number of layers for reinforcement. An anomalous phonon behavior for the thicker WS2 during deformation has been found and addressed. By following the E2g1 mode and PL energy band, the stress transfer in exfoliated WS2 has been examined in a model composite under uniaxial strain. These behaviors have demonstrated the WS2 can act as reinforcing phase in its nanocomposites and the behavior can be predicted with shear lag theory. The study has also suggested moderated interfacial bonding between the WS2 and polymer matrix and given an implication of the use of WS2 for polymer reinforcement. Finally, the bulk PVA nanocomposites reinforced by liquid exfoliated WS2 were investigated to validate our finding from the model composites. Significant increase in Youngâ€Â™s modulus occurs at a very low WS2 loading. The most effective reinforcement is brought by the incorporation of WS2 at 1.2 wt% fractions. The effective modulus of WS2 in the composites was calculated by the rule of mixture. The interlayer slippage of fewlayer WS2 has been found to be a significant factor that compromises the reinforcing effect.

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Keyword(s)

Photoluminescence; Polymer Nanocomposites; Raman; WS2

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