Academic literature on the topic 'MoS2-NEMS'

Abstract Excellent mechanical properties and the presence of piezoresistivity make single layers of transition metal dichalcogenides (TMDCs) viable candidates for integration in nanoelectromechanical systems (NEMS). We report on the realization of electromechanical resonators based on single-layer MoS2 with both piezoresistive and capacitive transduction schemes. Operating in the ultimate limit of membrane thickness, the resonant frequency of MoS2 resonators is primarily defined by the built-in mechanical tension and is in the very high frequency range. Using electrostatic interaction with a gate electrode, we tune the resonant frequency, allowing for the extraction of resonator parameters such as mass density and built-in strain. Furthermore, we study the origins of nonlinear dynamic response at high driving force. The results shed light on the potential of TMDC-based NEMS for the investigation of nanoscale mechanical effects at the limits of vertical downscaling and applications such as resonators for RF-communications, force and mass sensors.

AbstractDeveloping energy efficient electronics or green electronics is an area that is largely driven by the performance limitations of scaled Si-based CMOS due to the exceptionally high power dissipation and high leakage currents arising in such devices at nanoscale dimensions. It is clear now that Si-based CMOS has been stretched over the past several decades to the point that further miniaturization will make such simple size scaling non-sustainable in the future. New materials and technologies are thus vigorously being explored beyond Si, in order to overcome performance limitations from ultra-miniaturized Si-CMOS. Among these materials, carbon-based nanostructures such as graphene and carbon nanotubes are being considered as viable alternatives to Si-CMOS to enable energy efficient green electronics. Novel architectures for enabling low-power, energy-efficient computation are currently being explored, which include tunneling field-effect-transistors (TFETs), as well as nano-electro-mechanical-systems (NEMS) due to their abrupt ON/OFF transitions, low OFF state currents and high speed operation. In this paper, an overview of carbon nanomaterials is presented and the role they play in enabling energy efficient TFETs and NEMS is also highlighted. Finally, the emergence of a new class of 2D systems beyond graphene is discussed such as MoS2, which may open up new avenues for exploration and enabling applications in electronics.

ABSTRACTThe dynamics of one atom thick h-BN suspended nanoribbons have been obtained by first performing ab-initio calculations of the deformation potential energy and then solving numerically a Langevine type equation to explore their use as energy harvesting devices. Similarly to our previous proposal for a graphene-based harvester1, an applied compressive strain is used to drive the clamped-clamped nanoribbon structure into a bistable regime, where quasi-harmonic vibrations are combined with low frequency swings between the minima of a double-well potential. h-BN, graphene and MoS2 similar structures have been compared in terms of the static response to a compressive strain and of the dynamic evolution induced by an external noisy vibration. Due to its intrinsic piezoelectric response, the mechanical harvester naturally provides an electrical power that is readily available or can be stored by simply contacting the monolayer at its ends. Engineering the induced non-linearity, the proposed device is predicted to harvest an electrical root mean square (rms) power of more than 180 fW when it is excited by a noisy external force characterized by a white Gaussian frequency distribution with an intensity in the order of Frms=5pN.

Micro and nanoelectromechanical systems have shown tremendous potential in applications ranging from sensing to obtaining ultrastable oscillators for timing. They have also opened avenues for fundamental quantum studies and exploring nonlinear dynamics. The advent of CNTs and two-dimensional materials has enabled extreme miniaturization of resonators, allowing mass sensitivities down to a proton limit. This is possible since the mass resolution is proportional to the mass of the resonator itself. The limit of detection is also proportional to the frequency stability of the resonator. This is a measure of the uncertainty associated with the frequency measurement. Frequency stability can be effected either by the measurement noise or noise intrinsic to the device's mechanical response. In this thesis, we have explored the room temperature frequency stability of MoS2 resonators in the linear regime. The work involves the fabrication of local gated MoS2 resonators. The devices are characterized using capacitive actuation and homodyne detection techniques. Allan deviation is used as a tool to measure the frequency stability of MoS2 resonators. We study the effect of actuation drive (both AC and DC) on the resonator's frequency stability and correlate it with the signal to noise ratio of the device. The frequency stability measured in MoS2 resonators corresponds to a mass resolution of few attograms. We further identify the various noise sources present in the system through the slope of Allan deviation plots. Recently, Antonio et al. have demonstrated improved frequency stability due to nonlinear intermodal coupling. Coupled resonators have also been shown to enhance the sensitivity of mass sensors and hold promise for future nanomechanical technologies. The linear and nonlinear coupling between modes and/or resonators has enabled the observation of dynamics similar to optomechanics, such as phonon lasing and state squeezing. Nonlinear coupling enables the transfer of energy between vibrational modes having resonant frequencies far apart. Internal resonance is the most common form of nonlinear coupling mechanism. The necessary condition for mechanical modes to be coupled through internal resonance is that the ratio of resonant frequencies of coupled modes should be close to an integer (n=1,2,3). Previous studies on internal resonance have been restricted to clamped-clamped beams. However, our expriemental understanding of modal coupling through internal resonance is limited as it requires the meticulous design of device parameters to obtain resonant modes that are commensurate. Two-dimensional materials such as graphene and metal dichalcogenides have highly tunable resonant frequencies, enabling internal resonance conditions to be easily satisfied. Moreover, vibrational modes of a two-dimensional resonator are coupled through the intrinsic strain in the membrane. Thus, two-dimensional materials serve as a great platform to understand the dynamics of coupled systems. In this work, we demonstrate strong tunable intermodal coupling due to 2:1 internal resonance in MoS2 drum resonators. The modal peak splitting, a signature of coupling, is observed in the linear regime itself in addition to the nonlinear regime. We show the tunability of this coupling with applied gate bias. The simulations enabled us to qualitatively understand the effect of excitation force, frequency detuning and modal coupling strength on the resonator dynamics. Understanding internal resonance in two-dimensional membranes would enable new possibilities in signal transduction and frequency conversion. It could also help in improving the frequency stability of MoS2 resonators through the intermodal coupling. Coupling between different modes of a resonator is not just limited to two-dimensional materials but has also been reported in MEMS structures like clamped-clamped beams and curved arches. Advanced fabrication techniques have paved the way for a new class of MEMS structures, the piezo-micromachined ultrasonic transducer (pMUT). The majority of pMUTs/diaphragms are designed to operate in a linear dynamic range. But, at larger vibrational amplitudes, the nonlinear effect strongly affects the device dynamics. Careful control of these nonlinearities could pave the way to improved stability in microsensors, such as phase fluctuation reduction, frequency control and in-situ amplification schemes. Thus, it is imperative to understand and tune device nonlinearities. Previously tuning of nonlinearities has been achieved in mechanical resonators using capacitive techniques. But the same has not been demonstrated for piezoelectrically actuated ZnO diaphragms. In this thesis, we present the tuning of nonlinearity through diaphragm curvature in these devices. We calculate the effective nonlinearity through the device's backbone curve response and relate it with the diaphragm curvature. Nonlinearity in these resonators also leads to intermodal coupling and energy exchange between the commensurate vibrational modes. We further demonstrate the transfer of energy from the coupled higher vibrational mode to the fundamental mode of the pMUT. This coupling in the future would enable ultrastable piezo-based oscillators.

The advent of carbon nanotubes (CNTs) and atomically thin membranes such as Graphene and layered transition metal chalcogenides (TMDs) have spurred research in the area of Nanoelectromechanical Systems (NEMS) due to their extraordinary mechanical properties and ultralow mass density. These properties make the resonators extremely responsive to external stimuli. This is particularly important for research from both application and fundamental points of view. Nonlinearities in these devices play a vital role in the dynamics as dimensions are reduced to atomic scale. The primary emphasis of this thesis work is to understand various aspects of nonlinearities and manipulate them to enhance the performance of atomically thin NEMS. We demonstrate all electrical actuation and detection of atomically thin MoS2-NEMS in three distinct actuation-detection schemes. We observe multiple vibrational modes of the device and unlike previously reported work on 2D materials, we are able to drive these devices in the strongly nonlinear regime to observe nonlinear coupling. We observe multiple internal resonances for the first time in atomically thin resonators. The internal resonances are likely to occur in systems with large nonlinear mode couplings leading to the spillover of the energy pumped into one mode to other vibrational modes. We also explicitly demonstrate the existence of multiple frequency-plateaus and tunability of internal resonance frequency from one plateau to another plateau with back gate voltage. We provide a qualitative picture for the presence of narrowly spaced multiple internal resonance frequencies. This ability to strongly couple different modes has implications in applications such as high stability oscillator and sensors over a wide range of drive levels. We demonstrate that 2D material based NEMS devices can exhibit strong nonlinear effects that can significantly affect the performance of the device. A clear understanding of nonlinearities and ability to control and manipulate them to enhance the performance are pivotal for applications of these devices. We report an electrostatic mechanism to control the nonlinearities of an atomically thin NEMS. The exquisite control enables us to demonstrate hardening, softening and mixed nonlinear behaviours in the device. The electrostatic control over nonlinearities is utilized to effectively nullify Duffing nonlinearity in a specific regime to improve the dynamic range by ~20dB at room temperature. A simple 1D stretched-string model predicts that mechanical contribution to the nonlinearities is dominated over capacitive contribution and determines the dynamics of the device. The observed mixed behaviour is a result of cross-coupling between strong quadratic and quartic nonlinearities, an aspect explained by the method of multiple scale analysis. We also implement all electrical actuation and detection techniques to characterize monolayer CVD-MoS2 resonator, which is imperative to study from the perspective of large-scale fabrication of 2D-NEMS. The excellent electrical and mechanical properties observed in exfoliated MoS2 based devices are also observed in devices fabricated using CVD grown MoS2. We perform simulations to capture cancellation voltage of the Duffing nonlinearity for a wide range of in-built strain and length of the resonator for different thickness of the membrane. We further experimentally demonstrate and validate our simulated result on the effect of in-built strain in the resonator on the cancellation voltage of Duffing nonlinearity. The simulation and observed result can serve as a simple guide to design and control nonlinearities and effectively improve the linear dynamic range in these devices for next generation of sensors fabricated using these materials. Linear dissipation in NEMS has been studied extensively both theoretically and experimentally. Nonlinear dissipation in NEMS has been studied rarely through experimentation. Exclusive characterization of nonlinear damping has been proven difficult as it is masked by the stronger quadratic and cubic nonlinearities. However, our ability to cancel out the effective Duffing nonlinearity constant, allow us to study nonlinear damping in these devices efficiently. We strictly maintain the amplitude responses well below the critical amplitude throughout the measurements. The van der Poll Duffing equation is used to model the devices. The nonlinear dissipation is characterized by “dissipation backbone curve” analysis and broadening of resonance width with increasing drive voltage. The results clearly indicate that along with nonlinear dissipation, nonlinear spring constant also contributes to the broadening of resonance width. We observe that “dissipation backbone curve” analysis is a much accurate technique to estimate nonlinear dissipation coefficient (η). Analysis of a high strain device indicates that η might depend on the strain, and its value decreases with in-built strain. We demonstrate that the FM technique is highly nonlinear detection scheme even if we strictly maintain the driving force and frequency response in the linear regime. This also contributes to the broadening of the resonance width and as a result, η can be overestimated

In this this thesis, I have studied dynamics of the two-dimensional (2D) material based NEMS resonators with resonant frequency ranging typically from 10 MHz to 100 MHz. The experiment involved fabrication of the suspended nano-scale devices both with global and local gate architectures. The experiments focused on parametric manipulation of MoS2 drum resonator using electrical actuation and detection schemes. This study demonstrated parametric ampli cation in the NEMS at non-cryogenic temperature and discussed effects of During non-linearity on the parametric gain. Further, multimodal coupling among the mechanical modes in the drum resonator was also demonstrated