Atomically thin two-dimensional crystals are prospective materials for nanoelectromechanical systems due to their extraordinary mechanical properties [1][2][3][4][5][6][7] (high Young's modulus, elasticity and breaking strength) and low mass. Among this family of 2D materials, graphene is the most studied one so far. Graphene mechanical resonators [8] have been already employed as mass and pressure sensors [9] and provide a platform to study interesting nano-mechanical phenomena such as nonlinear damping [10] .Nevertheless, the lack of a bandgap in graphene may limits its usefulness in certain applications requiring a semiconducting material. Molybdenum disulfide (MoS 2 ), a semiconducting analogue to graphene [11][12][13] , presents excellent mechanical properties similar to graphene [5, 6] in combination to a large intrinsic bandgap [14][15][16][17] . Although multilayered MoS 2 resonators have been recently fabricated by Lee et al. [18] (one device 9 layers thick, the remainder devices 20 to 100 layers thick), single layer MoS 2 mechanical resonators have not been demonstrated so far. Unlike multilayer MoS 2 , which has an indirect bandgap of 1.2 eV, monolayer MoS 2 is a direct bandgap semiconductor (1.8 eV) with potential applications in photodetection [19][20][21][22] , photovoltaics [23] and valleytronics [24][25][26] . Therefore, fabrication of single-layer MoS 2 mechanical resonators is a first and necessary step towards nano-electromechanical systems exploiting the direct bandgap of monolayer MoS 2 . Here, we demonstrate the fabrication of single-layer MoS 2 mechanical resonators. The fabricated resonators have fundamental resonance frequencies in the order of 10 MHz to 30 MHz (depending on their geometry) and their quality factor is about ~55 at room temperature in vacuum. We find that the mechanics of these single-layer resonators lies on the membrane limit (tension dominated) while multilayered MoS 2 resonators can be modeled as circular plates (bending rigidity dominated http://onlinelibrary.wiley.com/doi/10.1002/adma.201303569/abstract have found that direct exfoliation of MoS 2 onto the pre-patterned substrates yield a low density of flakes, and no suspended single-layer MoS 2 devices could be fabricated using this method. We have therefore employed an all-dry transfer technique to deposit the MoS 2 flakes onto the pre-patterned substrates, similar to the one described in [27,28] (see Experimental Section and Supporting Information for a more detailed description of the transfer method). Single-layer devices can be identified at glance by means of optical microscopy [29,30] . Confirmation of the exact layer thickness is performed by combination of atomic force microscopy, Raman spectroscopy [31] and photoluminescence [14][15][16] (see Experimental section and Supporting Information for more details). Figure 1ashows optical images of single layer MoS 2 mechanical resonators. The motion of the drum resonators is detected using an optical interferometer (see the Experimental section and the Supp...

We measure the energy relaxation rate of single-and few-layer molybdenum disulphide (MoS 2 ) nanomechanical resonators by detecting the resonator ring-down. Recent experiments on these devices show a remarkably low quality (Q)-factor when taking spectrum measurements at room temperature. The origin of the low spectral Q-factor is an open question, and it has been proposed that besides dissipative processes, frequency fluctuations contribute significantly to the resonance line-width. The spectral measurements performed thus far however, do not allow one to distinguish these two processes. Here, we use time-domain measurements to quantify the dissipation. We compare the Q-factor obtained from the ring-down measurements to those obtained from the thermal noise spectrum and from the frequency response of the driven device. In few-layer and single-layer MoS 2 resonators the two are in close agreement, which demonstrates that the spectral line-width in MoS 2 membranes at room temperature is limited by dissipation, and that excess spectral broadening plays a negligible role.

Controlling the strain in two-dimensional materials is an interesting avenue to tailor the mechanical properties of nanoelectromechanical systems. Here we demonstrate a technique to fabricate ultrathin tantalum oxide nanomechanical resonators with large stress by laser-oxidation of nano-drumhead resonators made out of tantalum diselenide (TaSe2), a layered 2D material belonging to the metal dichalcogenides. Prior to the study of their mechanical properties with a laser interferometer, we checked the oxidation and crystallinity of the freely-suspended tantalum oxide in a high-resolution electron microscope. We show that the stress of tantalum oxide resonators increase by 140 MPa (with respect to pristine TaSe2 resonators) which causes an enhancement of quality factor (14 times larger) and resonance frequency (9 times larger) of these resonators.allows size reduction. For low-noise operation of nanomechanical systems it is desirable to achieve high quality factors Q at high frequencies. In conventional nanomechanical systems, based on silicon nitride (Si3N4) beams, it has been shown that both f0 and Q can be enhanced by increasing the stress in the beam [1,2]. For this purpose, several methods have been proposed to tune the stress in nanomechanical systems based on 2D materials using temperature, mechanical actuators and gas pressure [3][4][5][6]. For permanent stress modification in polycrystalline graphene, a method for direct bonding between graphene platelets has been proposed [6].

We demonstrate a strong coupling between the flexural vibration modes of a clamped-clamped micromechanical resonator vibrating at low amplitudes. This coupling enables the direct measurement of the frequency response via amplitude-and phase modulation schemes using the fundamental mode as a mechanical detector. In the linear regime, a frequency shift of 0.8 Hz is observed for a mode with a line width of 5.8 Hz in vacuum. The measured response is well-described by the analytical model based on the Euler-Bernoulli beam including tension. Calculations predict an upper limit for the roomtemperature Q-factor of 4:5 Â 10 5 for our top-down fabricated micromechanical beam resonators. Nonlinear interactions between the vibration modes in micro-and nanomechanical resonators have attracted significant interest recently. In extensional structures, such as clamped-clamped bridges, the modes are coupled by the displacement-induced tension, 1 which yields a quadratic relation between the resonance frequency of the mode considered and the amplitudes of the other modes. In singly clamped cantilevers the displacement-induced tension is absent; here, the inextensionality condition couples the horizontal and vertical displacements of all modes, resulting in qualitatively similar dynamics. 2-5 Several applications and consequences have been put forward based on the modal interactions, such as enhancement of the dynamic range, 1 modification of the resonator damping by employing phonon-phonon cavities, 6,7 frequency stabilization, 8 the study of relaxation mechanisms, 9 and linear frequency conversion. 10 In thermal equilibrium, via the modal interactions the displacement fluctuations in one mode give rise to frequency fluctuations in the other modes, thus broadening their spectral line. In recent theoretic work these frequency fluctuations were quantified for a carbon nanotube, 11 yielding a boundary for the experimental Q-factor of such resonators at finite temperature. Experiments on suspended carbon nanotube resonators in the Coulomb blockade regime demonstrate that single-electron-tunneling processes provide a strong electrostatic coupling between the modes, in addition to the mechanical mode coupling. 12,13 While the recent experimental work has focused on the mode coupling in strongly driven resonators exhibiting nonlinear vibrations, in this letter we investigate these interactions in the low-amplitude regime. We demonstrate that the modal interactions play a significant role in the dynamic behavior of a linear resonator, as the vibrations of a weakly driven mode modulate the motion of a second vibration mode. We employ this coupling to perform swept-frequency type measurements of the linear frequency response of a high frequency (target) mode, by measuring the induced amplitude (AM) and phase modulation (PM) in a low frequency (probe) mode which is weakly driven at a fixed frequency. This provides a practical way to measure the frequency response, and it should be contrasted to the scheme presented earlier, 1 where the freq...

We investigate the effect of mechanical strain on the dynamics of thin MoS 2 nanodrum resonators.Using a piezoelectric crystal, compressive and tensile biaxial strain is induced in initially flat and buckled devices. In the flat device, we observe a remarkable strain-dependence of the resonance line width, while the change in the resonance frequency is relatively small. In the buckled device, the strain-dependence of the damping is less pronounced, and a clear hysteresis is observed. The experiment suggests that geometric imperfections, such as microscopic wrinkles, could play a role in the strong dissipation observed in nanoresonators fabricated from 2-D materials.