Abstract
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 A1g and 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 A1g mode hardens whereas the mode softens. This is believed to be due to the A1g 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 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.
Highlights
Tungsten disulfide (WS2) Transition metal dichalcogenides (TMDs) have been used widely as solid lubricants [1] and chemical catalysts [2] for many years
Structure and morphology Nanoplatelets produced by the tape exfoliation normally have a range of number of layers and require the aid of optical microscopy and atomic force microscopy (AFM) to locate and identify them [65, 66]
The calculated values of the Raman band shift rates determined from the Grüneisen parameters for the A1g and E2g1 modes in the monolayer are listed in table 1
Summary
Tungsten disulfide (WS2) Transition metal dichalcogenides (TMDs) have been used widely as solid lubricants [1] and chemical catalysts [2] for many years. Grown rapidly over recent years with the explosion of research into 2D materials This is because TMDs, unlike graphene, have a tuneable bandgap, a transition from an indirect to a direct bandgap with a reducing number of layers [3, 4] and strong spin–orbit coupling arising from broken inversion symmetry [5]. Most research has focused upon the limited number of stable materials that includes WS2, molybdenum disulfide (MoS2), WSe2 and MoSe2 Among this group of materials, WS2 has drawn particular attention as a result of its unique optical [7], thermal [8] and electronic [9] properties
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