Electromechanics in MoS2 and WS2: nanotubes vs. monolayers

Abstract

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 signals of the in-plane and out-of-plane lattice vibrations depend significantly and linearly on the strain, showing that Raman spectroscopy is an excellent tool to determine the strain of the individual nanotubes and hence monitor the progress of nanoelectromechanical experiments 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.

Electron transport in MoWSeS monolayers in the presence of an external electric field

“…For example, by applying a small mechanical strain of about 1% to the MoS 2 monolayer, the band gap shifts from direct to indirect, and for larger deformations a semiconductor-metal transition occurs. [12][13][14][15][16] By means of further theoretical studies it has been reported that applying an external electric field to a rippled MoS 2 monolayer 17 or an armchair MoS 2 nanoribbon 18 reduces the band gap and causes severe changes in the electronic structure. Ramasubramaniam and co-workers 19 have studied the effect of the perpendicular external electric field applied to TX 2 bilayers.…”

Pressure-induced semiconducting to metallic transition in multilayered molybdenum disulphide

“…Within this class of 2D materials are transition metal dichalcogenides (TMDs), such as molybdenum disulphide (MoS 2 ), which have been increasingly explored to access a wealth of phenomena including opto-electronics 5 , valleytronics 6 , spintronics 7 and coupled electro-mechanics 8 . All of these phenomena arise from the complex thickness-dependent physical and electronic structures of the multilayered materials, in which the coupled electro-mechanics can be regarded as straintronics.…”

Electrical Properties of Multilayered Molybdenumdi Sulphide Single Crystal

“…Room temperature electrical resistivity and hall effect measurements as a function of pressure are reported in p-type MOS2 and n-type MOS2 and MOTe2 [17]. The Electrical conductivity and Hall coefficient perpendicular to the c-axis of hexagonal MOS2,MoSe2 and MOTe2 were Measured over the temperature ranges 120 to 1170 K are also reported [18].Considerable literature exists on the electrical properties of the semiconducting transition metal dichalcogenide layer crystals MX2 , where M is Mo and X is S, Se and Te [4,5,[7][8][9]. The Electronic bands in the fundamental gap region of the layered semiconductors MOS2 and MOSe2 have been studied [4,5] and band structures have also been studied [19].…”

“…Within this class of 2D materials are transition metal dichalcogenides (TMDs), such as molybdenum disulphide (MoS2), which have beenincreasingly explored toaccess a wealth of phenomena including optoelectronics [5], valleytronics [6], spintronics [7]and coupled electromechanics [8]. All of these phenomena arise from the complex thickness-dependent physical and electronic structures of the multilayered materials, in which the coupled electro-mechanics can be regarded as straintronics.…”