Strain-induced phonon shifts in tungsten disulfide nanoplatelets and nanotubes

The relationship between structure and properties has been followed for different nanoscale forms of tungsten disulfide (2H-WS₂) namely exfoliated monolayer and few-layer nanoplatelets, and nanotubes. The similarities and differences between these nanostructured materials have been examined using a combination of optical microscopy, scanning and high-resolution transmission electron microscopy and atomic force microscopy. Photoluminescence and Raman spectroscopy have also been used to distinguish between monolayer and few-layer material. Strain induced phonon shifts have been followed from the changes in the positions of the A_1g and E_2g¹ Raman bands during uniaxial deformation. This has been modelled for monolayer using density functional theory with excellent agreement between the measured and predicted behaviour. It has been found that as the number of WS₂ layers increases for few-layer crystals or nanotubes, the A_1g mode hardens whereas the E_2g¹ mode softens. This is believed to be due to the A_1g mode, which involves out of plane atomic movements, being constrained by the increasing number of WS₂ layers whereas easy sliding reduces stress transfer to the individual layers for the E_2g¹ mode, involving only in-plane vibrations. This finding has enabled the anomalous phonon shift behaviour in earlier pressure measurements on WS₂ to be resolved, as well as similar effects in other transition metal dichalcogenides, such as molybdenum disulfide, to be explained.