Diameter dependent TEM and Raman study of WS₂ nanotubes

Metal dichalcogenide (MX₂) nanotubes and particles, commonly classified as inorganic fullerene-like (IF) materials, represent the inorganic counterparts of carbon nanotubes and fullerenes. They are usually constituted of several MS₂ triple layers, whose curvature is provided by specific defect sites. WS₂ and MoS₂ IF's have recently attracted much interest as dry lubricants and grease additives due to friction coefficients lower than those of platelet based standard mineral oil additives.

In this study WS₂ nanotubes with diameters in the range of 28 nm to 290 nm are studied by electron microscopy and resonant 1st and 2nd order Raman spectroscopy. The Raman response is compared to that of 2H-WS₂ single crystals and microcrystalline 2H-WS₂ powder. With decreasing tube diameter a disorder induced line in the A_1g range, termed D-A_1g line, is increasingly enhanced. For the thinnest tubes it is even stronger than the fundamental A_1g crystal mode. The intensity ratio I (D-A_1g)/ I (A_1g) follows a simple functional relationship. It resembles that of the well known I_D/ I_G ratio in disordered carbon. The enhancement mechanism of the D-A_1g line is discussed in terms of the increasing number of local defects, which is the consequence of the increasing curvature.

[1] M. Krause, M. Viršek, M. Remškar, N. Salacan, N. Fleischer, L. Chen, P. Hatto, A. Kolitsch, W. Möller, "Diameter and Morphology Dependent Raman Signatures of WS₂ Nanostructures", ChemPhysChem, 10, 2221-2225 (2009)