Abstract
Transition-metal dichalcogenides (TMDs) nanostructures including nanotubes and monolayers have attracted great interests in materials science, chemistry to condensed matter physics. We present an interesting study of the vibration modes in multi-walled tungsten sulfide (WS2) nanotubes prepared via sulfurizing tungsten oxide (WO3) nanowires which are investigated by confocal micro-Raman spectroscopy. The inter-layer vibration mode of WS2 nanotubes, A1g, is found to be sensitive to the diameter and curvature strain, while the in-plane vibration mode, E12g, is not. A1g mode frequency shows a redshift by 2.5 cm−1 for the multi-layered nanotubes with small outer-diameters, which is an outcome of the competition between the Van der Waals force stiffening and the curvature strain softening. We also show that the Raman peak intensity ratio is significantly different between the 1–2 wall layered nanotubes and monolayer flat sheets.
Highlights
Stimulated by the great success of carbon-based nanostructures including nanotubes, graphene and nanoparticles, transition-metal dichalcogenides (TMDs) nanostructures from nanotubes to monolayers have attracted a great deal of interest from fields, such as materials science, chemistry, and condensed matter physics in recent years[1,2,3,4,5,6]
In the multi-walled Transition-metal dichalcogenides (TMDs) nanotubes, the curvature strain induced by bending stress and the inter-layer Van der Waals force shall jointly play a key role in determining their mechanical properties, which has been proved in carbon
To the best of our knowledge, there have been very few reports investigating the influence of curvature strain and Van der Waals force on the vibration modes of multi-walled WS2 nanotubes
Summary
Transition-metal dichalcogenides (TMDs) nanostructures including nanotubes and monolayers have attracted great interests in materials science, chemistry to condensed matter physics. A recent theoretical study shows that Raman signals of the in-plane and out-of-plane (or inter-layer used in the present article) lattice vibration modes depend significantly and linearly on the strain in TMD nanotubes, and concludes that Raman spectroscopy is an excellent tool to determine the strain of the TMD nanotubes and monitor the progress of nanoelectromechanical experiments[14] Due to their high stretchability, two-dimensional crystals demonstrate the potential in controlling their optical and electrical properties by means of strain engineering[15], such as physically bending or elongating a flexible substrate[16,17], piezoelectric stretching and thermal expansion of the substrate[18,19], and controlled wrinkling[20,21]. The joint effects of curvature strain and Van der Waals force on the nanomechanical properties of multi-walled WS2 nanotubes can be precisely controlled to some extent
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