High Quality Factor Mechanical Resonators Based on WSe₂ Monolayers

Abstract

Suspended monolayer transition metal dichalcogenides (TMD) are membranes that combine ultralow mass and exceptional optical properties, making them intriguing materials for opto-mechanical applications. However, the low measured quality factor of TMD resonators has been a roadblock so far. Here, we report an ultrasensitive optical readout of monolayer TMD resonators that allows us to reveal their mechanical properties at cryogenic temperatures. We find that the quality factor of monolayer WSe2 resonators greatly increases below room temperature, reaching values as high as 1.6 × 104 at liquid nitrogen temperature and 4.7 × 104 at liquid helium temperature. This surpasses the quality factor of monolayer graphene resonators with similar surface areas. Upon cooling the resonator, the resonant frequency increases significantly due to the thermal contraction of the WSe2 lattice. These measurements allow us to experimentally study the thermal expansion coefficient of WSe2 monolayers for the first time. High Q-factors are also found in resonators based on MoS2 and MoSe2 monolayers. The high quality-factor found in this work opens new possibilities for coupling mechanical vibrational states to two-dimensional excitons, valley pseudospins, and single quantum emitters and for quantum opto-mechanical experiments based on the Casimir interaction.

Mechanical characterization and cleaning of CVD single-layer h-BN resonators

“…It is unclear if there remains some polymer residue, although the tensioning trend when cooling has been reported in graphene and several transition metal dichalcogenide membranes. [46][47][48] However, this trend should be explained differently for 2D materials with a negative expansion coefficient like h-BN or graphene, 49 and for that reason we propose an alternative tensioning effect caused by the sidewall adhesion, which should balance out or increase the tension when cooling certain 2D membranes, and explains our results and those on graphene drumheads. Other stronger cleaning routes for h-BN are available, 50,51 although not all of them might be compatible with suspended single-layer structures.…”

Energy Dissipation Pathways in Few-Layer MoS2 Nanoelectromechanical Systems

“…A pulsed blue diode laser ( λ  = 412 nm) modulated at frequency, f , thermoelastically excites the resonator into resonance while the reflected light intensity of a red (λ  = 633 nm) helium-neon laser is monitored to detect the resonator motion. In all instances, care is taken to avoid absorptive heating and dynamical photothermal back-action from the lasers, by limiting the amount of laser power delivered to the sample 5 . All measurements are carried out in vacuum (pressure ~10 −6  Torr), minimizing energy dissipation due to gas friction (see the Supplementary Information).…”

High Performance, Low Cost Electromechanical Systems Based on Electrostatically Actuated TiS2 Belts

“…Atomic‐thin layers of novel 2D materials have attracted tremendous attention mainly because of their electrical, optical and magnetic properties; however recently, scientific research has expanded beyond the electronic and optical properties to explore the mechanics of 2D layered materials . Due to small thickness, ultralow mass, ultrahigh Young's modulus, and high crystallinity of 2D materials, they have attracted the attention of the field of mechanical systems as high frequency and high quality factor resonators …”

“…TMDs have also presented excellent mechanical properties as well as graphene . Yet a big challenge for semiconducting TMD and graphene based mechanical resonators is the low quality factor Q (Q ≤ 800) at room temperature and ambient pressure . Moreover, electrical mixing techniques are difficult to apply on semiconducting TMD resonators because of their high electrical resistance which complicates integration of the resonator with CMOS circuitry and electrical measurements …”

“…The layer number, geometry, the driving/read‐out scheme and experimental conditions are included to facilitate the comparison. There are resonators with very high Q factors; yet, they need to operate at low temperatures or low vacuum to reach optimum performance, for example, at low temperature (3.5 K), the Q factor of WSe 2 is around 47000 although it decreases to 166 at temperatures around 300 K . Moreover, complex measurement setups are needed for those experimental conditions.…”