Torsional Resonators Based on Inorganic Nanotubes Yiftach Divon, † Roi Levi, † Jonathan Garel, † Dmitri Golberg, ∥ Reshef Tenne, † Assaf Ya’akobovitz, § and Ernesto Joselevich* ,† † Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel ∥ International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan § Department of Mechanical Engineering, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel * S Supporting Information ABSTRACT: We study for the first time the resonant torsional behaviors of inorganic nanotubes, specifically tungsten disulfide (WS 2 ) and boron nitride (BN) nanotubes, and compare them to that of carbon nanotubes. We have found WS 2 nanotubes to have the highest quality factor (Q) and torsional resonance frequency, followed by BN nanotubes and carbon nanotubes. Dynamic and static torsional spring constants of the various nanotubes were found to be different, especially in the case of WS 2 , possibly due to a velocity- dependent intershell friction. These results indicate that inorganic nanotubes are promising building blocks for high- Q nanoelectromechanical systems (NEMS). KEYWORDS: Nanotube, nanoelectromechanical systems (NEMS), torsion, oscillator, nanomechanics, inorganic nanotubes I norganic nanotubes, first reported in 1992, 1 are increasingly attracting interest as the rolled-up version of noncarbon 2D materials, and potential building blocks for nanotechnology. 2 What is their potential for nanoelectromechanical systems (NEMS)? Carbon nanotubes (CNTs) have long been regarded as attractive building blocks for NEMS owing to their outstanding mechanical and electrical properties, as well as their unique electromechanical coupling. 3 In particular, tor- sional electromechanical systems could be used as the basis for gyroscopes for navigation of ultraminiaturized unmanned aerial vehicles (UAVs), 4 and for various chemical and biological sensors. 5 Extensive work has been done with respect to CNT- based torsional devices: fabrication, 6 characterization of tor- sional 7 and electromechanical properties in single-walled CNTs 8 (SWCNTs) and multiwalled CNTs 9,10 (MWCNTs), and creation of MWCNT and SWCNT torsional resona- tors. 11,12 One of the most critical factors determining the sensitivity of resonant NEMS is their quality factor−a dimensionless parameter corresponding to the ratio between the stored and dissipated energy per cycle. Namely, the higher the quality factor, the less energy gets dissipated during one oscillation cycle. Internal friction, interlayer coupling, crystallo- graphic structure, and chemical composition can play a critical role in determining the torsional behavior of nanotubes and specifically their quality factor (Q). WS 2 nanotubes (WS 2 NTs) are a promising material owing to their significant electro- mechanical response, 13 stick−slip torsional behavior, 14 and high current-carrying capacity. 15 Boron nitride NTs (BNNTs), with their ultrahigh torsional stiffness and torsional strength, 16 and their carbon-doped version, BCNNTs, which have shown a significant electromechanical response, 17 seem very promising as well. Thus, these properties and the aspects influencing the quality factor have motivated us to examine inorganic nanotubes (INTs) as potential building blocks for torsional devices. Here we demonstrate the first torsional resonators based on inorganic nanotubes and study the effect of the NT material on the torsional resonator properties, in ambient conditions and in vacuum. INTs exhibit higher torsional resonance frequencies and quality factors, extending the available material toolbox for torsional NEMS devices. This work further demonstrates that INTs are promising building blocks for NEMS in general and torsional NEMS in particular. The torsional resonators used in this work are quite similar to devices previously used in our group to study the torsional properties of CNTs, 9,10 BNNTs, 16 and WS 2 NTs, 13,14 except for an intentional broken symmetry that enables their electrostatic actuation. A torsional resonator (Figure 1) consists of a suspended nanotube (MWCNT, BNNT or WS 2 NT) clamped between metallic pads at its ends, with a suspended pedal attached to its top. The pedal is off-centered with respect to the nanotube, so that each end of the pedal stands at a different distance from the nanotube (Figures 1b,c). The resonators were fabricated using electron-beam lithography, Received: July 20, 2016 Revised: December 16, 2016 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A DOI: 10.1021/acs.nanolett.6b03012 Nano Lett. XXXX, XXX, XXX−XXX