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 exfoliatedmonolayer and few-layer nanoplatelets, and nanotubes. The similarities and differences between these nanostructuredmaterials have been examined using a combination of opticalmicroscopy, scanning and high-resolution transmission electronmicroscopy and atomic forcemicroscopy. Photoluminescence andRaman spectroscopy have also been used to distinguish betweenmonolayer and few-layermaterial. Strain induced phonon shifts have been followed from the changes in the positions of the A1g and E2gRaman bands during uniaxial deformation. This has beenmodelled formonolayer using density functional theorywith excellent agreement between themeasured and predicted behaviour. It has been found that as the number of WS2 layers increases for few-layer crystals or nanotubes, the A1gmode hardenswhereas the E2gmode softens. This is believed to be due to theA1gmode, which involves out of plane atomicmovements, being constrained by the increasing number ofWS2 layers whereas easy sliding reduces stress transfer to the individual layers for the E2gmode, involving only in-plane vibrations. Thisfinding has enabled the anomalous phonon shift behaviour in earlier pressuremeasurements onWS2 to be resolved, as well as similar effects in other transitionmetal dichalcogenides, such asmolybdenumdisulfide, to be explained.