  The transition-metal dichalcogenides (TMD) MoS2 and WS2 show remarkable
electromechanical properties. Strain modifies the direct band gap into an
indirect one, and substantial strain even induces an semiconductor-metal
transition. Providing strain through mechanical contacts is difficult for TMD
monolayers, but state-of-the-art for TMD nanotubes. We show using
density-functional theory that similar electromechanical properties as in
monolayer and bulk TMDs are found for large diameter TMD single- (SWNT) and
multi-walled nanotubes (MWNTs). The semiconductor-metal transition occurs at
elongations of 16 %. We show that Raman spectroscopy is an excellent tool to
determine the strain of the nanotubes and hence monitor the progress of that
nanoelectromechanical experiment in situ. TMD MWNTs show twice the electric
conductance compared to SWNTs, and each wall of the MWNTs contributes to the
conductance proportional to its diameter.
