Search

Strain engineering is one of the most effective approaches to manipulate the physical state of materials, control their electronic properties, and enable crucial functionalities. Because of their rich phase diagrams arising from competing ground states, quantum materials are an ideal playground for on-demand material control, and can be used to develop emergent technologies, such as adaptive electronics or neuromorphic computing. It was recently suggested that complex oxides could bring unprecedented functionalities to the field of nanomechanics, but the possibility of precisely controlling the stress state of materials is so far lacking. Here we demonstrate the wide and reversible manipulation of the stress state of single-crystal WO3 by strain engineering controlled by catalytic hydrogenation. Progressive incorporation of hydrogen in freestanding ultra-thin structures determines large variations of their mechanical resonance frequencies and induces static deformation. Our results demonstrate hydrogen doping as a new paradigm to reversibly manipulate the mechanical properties of nanodevices based on materials control., 6 pages, 4 figures, 9 supplementary sections

Strain engineering is one of the most effective approaches to manipulate the physical state of materials, control their electronic properties, and enable crucial functionalities. Because of their rich phase diagrams arising from competing ground states, quantum materials are an ideal playground for on-demand material control and can be used to develop emergent technologies, such as adaptive electronics or neuromorphic computing. It was recently suggested that complex oxides could bring unprecedented functionalities to the field of nanomechanics, but the possibility of precisely controlling the stress state of materials is so far lacking. Here, we demonstrate the wide and reversible manipulation of the stress state of single-crystal WO

Controlling the strain in two-dimensional (2D) 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 the laser oxidation of nano-drumhead resonators composed of tantalum diselenide (TaSe2), a layered 2D material belonging to the metal dichalcogenides. Before the study of their mechanical properties with a laser interferometer, we verified the oxidation and crystallinity of the freely suspended tantalum oxide using high-resolution electron microscopy. We demonstrate that the stress of tantalum oxide resonators increases by 140 MPa (with respect to pristine TaSe2 resonators), which causes an enhancement in the quality factor (14 times larger) and resonance frequency (9 times larger) of these resonators.

Relaxation oscillators consist of periodic variations of a physical quantity triggered by a static excitation. They are a typical consequence of nonlinear dynamics and can be observed in a variety of systems. VO

Membranes of suspended two-dimensional materials show a large variability in mechanical properties, in part due to static and dynamic wrinkles. As a consequence, experiments typically show a multitude of nanomechanical resonance peaks, which makes an unambiguous identification of the vibrational modes difficult. Here, we probe the motion of graphene nanodrum resonators with spatial resolution using a phase-sensitive interferometer. By simultaneously visualizing the local phase and amplitude of the driven motion, we show that unexplained spectral features represent split degenerate modes. When taking these into account, the resonance frequencies up to the eighth vibrational mode agree with theory. The corresponding displacement profiles however, are remarkably different from theory, as small imperfections increasingly deform the nodal lines for the higher modes. The Brownian motion, which is used to calibrate the local displacement, exhibits a similar mode pattern. The experiments clarify the complicated dynamic behaviour of suspended two-dimensional materials, which is crucial for reproducible fabrication and applications.

Fluidic energy losses have significantly hindered the wide spread adoption of M/NEMS resonators especially in sensing applications. We apply an external feedback loop to amplify the motion of a microbridge resonator with an integrated piezoelectric actuator. The thermal motion of the device is amplified by positive velocity-proportional feedback, resulting in sustained oscillations. The Q-factor increases with the loop gain by up to a factor of 20 when compared to the open-loop system.

We fabricated arrays of cantilever sensors suspended deep inside fluid wells. To eliminate critical alignment and assembling steps during encapsulation of the cantilevers, these 3-D MEMS devices are realized in a single-side, single-wafer process by employing photolithography on bulk micromachined substrates. Silicon nitride cantilevers with varying geometries are suspended up to [email protected] below the wafer surface, and a structured gold layer is patterned on top. After calibrating the cantilevers in an optical lever setup, the formation of a DNA monolayer on the cantilever surface is detected.

The nonlinear interactions between flexural and torsional modes of a microcantilever are experimentally studied. The coupling is demonstrated by measuring the frequency response of one mode, which is sensitive to the motion of another resonance mode. The flexural-flexural, torsional-torsional and flexural-torsional modes are coupled due to nonlinearities, which affect the dynamics at high vibration amplitudes and cause the resonance frequency of one mode to depend on the amplitude of the other modes. We also investigate the nonlinear dynamics of torsional modes, which cause a frequency stiffening of the response. By simultaneously driving another torsional mode in the nonlinear regime, the nonlinear response is tuned from stiffening to weakening. By balancing the positive and negative cubic nonlinearities a linear response is obtained for the strongly driven system. The nonlinear modal interactions play an important role in the dynamics of multi-mode scanning probe microscopes, 17 pages, 6 figures

Reference EPFL-REVIEW-189767 URL: http://www.phantomsnet.net/Foundation/Enano_newsletter24.php Record created on 2013-10-04, modified on 2017-12-07

We demonstrate thermo-mechanical relaxation oscillations in a strongly driven quartz crystal. Dynamic bifurcation leads to two stable oscillation states with a distinct electrical impedance. Slow Joule-heating, which shifts the susceptibility of the crystal, provides a feedback that leads to thermally-induced oscillations, in which the amplitude of the crystal is modulated by a relaxation cycle. The frequency of the relaxation cycle is roughly a million times lower than the resonance frequency of the crystal, and it can be adjusted by the detuning from the critical point for dynamic bifurcation. The experimental observations are reproduced by a simple model that takes into account the slow dynamics of the system., Comment: Main text: 8 pages, 4 figures. Supplementary information: 4 pages, 3 figures

A method is described to integrate particle filters inside a silicon wafer by creating pores in a recessed membrane. The filters are created in either a silicon or a silicon nitride membrane located 100@mm below the wafer surface. The fabrication process involves anisotropic etching of front- and back-side of a (100) silicon wafer, deposition of a silicon nitride layer, spraycoating of photoresist and reactive ion etching of the pores. The filters may serve as a well or reaction chamber at the same time. We have patterned pore diameters in the 10-50@mm range in 100@mm recessed membranes that are suitable for application in stacked systems such as in an integrated inkjet nozzle.

Anisotropic wet etching on the front and backside of a (100) silicon wafer is applied to fabricate free standing crystalline membranes oriented in the (111) plane, The effective area of the membranes is in the order of several tenths of a millimeter, whereas thickness ranges from several hundreds of microns down to less than a micron. The fabrication process requires two masks and a wet etch. Cantilevers with (111) orientation were milled from the membranes by means of a Focused Ion Beam. We propose the application of (111) oriented silicon membranes as filtration elements in a microfluidic system. Cantilevers oriented in {111} planes can be applied as (bio-) chemical sensor structures that can be integrated in the medium channel.

Cantilevers play an important role as linear transducers in nanoscience, with nanomechanical detection of mass and stress as a clear example. We performed gas sensing experiments with a standard functionalized cantilever driven strongly into the regime of nonlinear oscillations. We compared the cantilever response to the selective adsorption of ethanol vapour in the nonlinear regime, to the ones obtained in the conventional linear static and dynamic sensing modes. In the nonlinearly driven cantilever, the adsorption and desorption-induced frequency shifts were enhanced by over a factor of three compared to resonant sensing with the same mode in the linear regime. This demonstrates a route towards gas detectors that exploit nonlinearity to enhance the responsivity, which can be implemented with standard cantilever devices.

In physical systems, decoherence can arise from both dissipative and dephasing processes. In mechanical resonators, the driven frequency response measures a combination of both, while time domain techniques such as ringdown measurements can separate the two. Here, we report the first observation of the mechanical ringdown of a carbon nanotube mechanical resonator. Comparing the mechanical quality factor obtained from frequency- and time-domain measurements, we find a spectral quality factor four times smaller than that measured in ringdown, demonstrating dephasing-induced decoherence of the nanomechanical motion. This decoherence is seen to arise at high driving amplitudes, pointing to a non-linear dephasing mechanism. Our results highlight the importance of time-domain techniques for understanding dissipation in nano-mechanical resonators, and the relevance of decoherence mechanisms in nanotube mechanics.

Impact of fabrication technology on resonant behaviour of LPCVD silicon nitride cantilevers is studied using an optical deflection setup. Two different release techniques are compared, i.e., isotropic dry etching in an SF"6 plasma and wet etching in KOH. The silicon nitride thickness varies between 100 and 680nm. Dependency of resonance frequencies on length and undercut is studied for both release methods using Euler-Bernoulli beam theory. The wet release method introduces a negligible undercut compared to the dry method. We find that the undercut effectively makes the cantilever longer by a value @DL, which depends on the undercut distance but which is independent on cantilever length for the samples studied. We have also extracted the Young's modulus from the resonance frequency vs. length dependency and compared the values of both release methods. The wet release method consistently shows a lower Young's modulus.

We measure the energy relaxation rate of single- and few-layer molybdenum disulphide ($\mathrm{MoS_2}$) 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 $\mathrm{MoS_2}$ resonators the two are in close agreement, which demonstrates that the spectral line-width in $\mathrm{MoS_2}$ membranes at room temperature is limited by dissipation, and that excess spectral broadening plays a negligible role., Comment: 13 pages, 5 figures

We study the dynamics of a nonlinear electromechanical oscillator with delayed feedback. Compared to their linear counterparts, we find that the dynamics is dramatically different. The well-known Barkhausen stability criterion ceases to exist, and two modes of operation emerge: one characterized by hysteresis in combination with a bistable frequency and amplitude; the other, by self-stabilization of the oscillation frequency and amplitude. The observed features are captured by a model based on a Duffing equation with delayed force feedback. Nonlinear oscillators with delayed force feedback are exemplary for a large class of dynamic systems.

The cantilever is a prototype of a highly compliant mechanical system and has an instrumental role in nanotechnology, enabling surface microscopy, and ultrasensitive force and mass measurements. Here we report fluctuation-induced transitions between two stable states of a strongly driven microcantilever. Geometric nonlinearity gives rise to an amplitude-dependent resonance frequency and bifurcation occurs beyond a critical point. The cantilever response to a weak parametric modulation is amplified by white noise, resulting in an optimum signal-to-noise ratio at finite noise intensity. This stochastic switching suggests new detection schemes for cantilever-based instrumentation, where the detection of weak signals is mediated by the fluctuating environment. For ultrafloppy, cantilevers with nanometer-scale dimensions operating at room temperature--a new transduction paradigm emerges that is based on probability distributions and mimics nature.

We demonstrate 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 $\mathrm{0.8\,Hz}$ is observed for a mode with a line width of $\mathrm{5.8\,Hz}$ 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 room-temperature Q-factor of $\mathrm{4.5\times10^5}$ for our top-down fabricated micromechanical beam resonators., 9 pages, 2 figures

The non-linear interaction between two mechanical resonances of the same freely suspended carbon nanotube resonator is studied. We find that in the Coulomb blockade regime, the non-linear modal interaction is dominated by single-electron-tunneling processes, and that the mode-coupling parameter can be tuned with the gate voltage, allowing both mode softening and mode stiffening behavior. This is in striking contrast to tension-induced mode coupling in strings, where the coupling parameter is positive and gives rise to a stiffening of the mode. The strength of the mode coupling in carbon nanotubes in the Coulomb blockade regime is observed to be six orders of magnitude larger than the mechanical mode coupling in micromechanical resonators., 8 pages, 4 figures

The interactions between parametrically and directly driven vibration modes of a clamped-clamped beam resonator are studied. An integrated piezoelectric transducer is used for direct and parametric excitation. First, the parametric amplification and oscillation of a single mode are analyzed by the power and phase dependence below and above the threshold for parametric oscillation. Then, the motion of a parametrically driven mode is detected by the induced change in resonance frequency in another mode of the same resonator. The resonance frequency shift is the result of the nonlinear coupling between the modes by the displacement-induced tension in the beam. These nonlinear modal interactions result in the quadratic relation between the resonance frequency of one mode and the amplitude of another mode. The amplitude of a parametrically oscillating mode depends on the square root of the pump frequency. Combining these dependencies yields a linear relation between the resonance frequency of the directly driven mode and the frequency of the parametrically oscillating mode., 5 pages, 4 figures

We demonstrate that a flexural resonance mode of a clamped-clamped beam resonator can be measured by using another resonance mode of the same resonator as a detector. The detector mode and the mode to be detected are coupled through the beam displacement. Displacement of the resonance mode to be measured introduces tension in the beam, which gives rise to an upwards shift in the resonance frequency of the detector mode. We experimentally show that this mechanism can be used to detect the first and second resonance mode of a silicon beam resonator. The modal interaction can also be used to increase the dynamic range of the resonator.

Micro- and nanomechanical resonators are used in a variety of sensing applications. We investigate the dynamics of clamped-clamped micromechanical string resonators immersed in viscous fluids. The resonators are driven using a magneto-motive technique and an optical lever is used to detect their motion. The resonance frequencies and Q-factors of the odd flexural resonance modes are determined up to the 9th mode. Using liquids with varying density and viscosity, the inertial and viscous effects are discriminated by measuring the resonance frequencies and Q-factors.

A theoretical and experimental investigation is presented on the intermodal coupling between the flexural vibration modes of a single clamped-clamped beam. Nonlinear coupling allows an arbitrary flexural mode to be used as a self-detector for the amplitude of another mode, presenting a method to measure the energy stored in a specific resonance mode. Experimentally observed complex nonlinear dynamics of the coupled modes are quantitatively captured by a model which couples the modes via the beam extension; the same mechanism is responsible for the well-known Duffing nonlinearity in clamped-clamped beams., 5 pages, 3 figures

We investigate the nonlinear dynamics of microcantilevers. We demonstrate mechanical stiffening of the frequency response at large amplitudes, originating from the geometric nonlinearity. At strong driving the cantilever amplitude is bistable. We map the bistable regime as a function of drive frequency and amplitude, and suggest several applications for the bistable microcantilever, of which a mechanical memory is demonstrated.

We demonstrate magnetomotive drive and detection of doubly clamped string resonators in water. A compact 1.9 T permanent magnet is used to detect the fundamental and higher flexural modes of $\mathrm{200 \mu m}$ long resonators. Good agreement is found between the magnetomotive measurements and optical measurements performed on the same resonator. The magnetomotive detection scheme can be used to simultaneously drive and detect multiple sensors or scanning probes in viscous fluids without alignment of detector beams., Comment: 4 pages, 3 figures

The effective Young’s modulus of silicon nitride cantilevers is determined for thicknesses in the range of 20–684 nm by measuring resonance frequencies from thermal noise spectra. A significant deviation from the bulk value is observed for cantilevers thinner than 150 nm. To explain the observations we have compared the thickness dependence of the effective Young’s modulus for the first and second flexural resonance mode and measured the static curvature profiles of the cantilevers. We conclude that surface stress cannot explain the observed behavior. A surface elasticity model fits the experimental data consistently.

We have used double clamped beams to implement a mechanical memory. Compressive stress is generated by resistive heating of the beams and beyond the buckling limit the bistable regime is accessed. Bits are written by applying lateral electrostatic forces. The state of the beam is read out by measuring the capacitance between beam and electrodes. Two ways to implement a mechanical memory are discussed: compensation of initial beam imperfections and snap through of the postbuckled beam. Although significant relaxation effects are observed, both methods prove reliable over thousands of write cycles.

We present a systematic investigation of the dynamic properties of silicon nitride cantilevers in air. The thermal noise spectra of cantilevers have been measured using a home-made optical deflection setup. Torsional and flexural resonances up to the seventh mode are observed. The dependence of resonance frequencies on the dimensions and mode number is studied in detail. It is found that undercut increases the effective length of the cantilever by a value ?L, which depends on the undercut distance and the resonance mode shape, but not on the cantilever length. Finite element modelling confirms these experimental findings. A simple model is suggested for the shape of the undercut region, which agrees well with experimental findings. Using this model, the undercut cantilever can be approximated by a stepped beam, where the clamp distance depends on the underetch duration and the mode shape.

We present a method for efficient spectral readout of mechanical resonator arrays in dissipative environments. Magnetomotive drive and detection is used to drive double clamped resonators in the nonlinear regime. Resonators with almost identical resonance frequencies can be tracked individually by sweeping the drive power. Measurements are performed at room temperature and atmospheric pressure. These conditions enable application in high throughput resonant sensor arrays., Comment: 4 pages, 4 figures

Nanocrystalline diamond (NCD) is a promising material for electronic and mechanical micro- and nanodevices. Here we introduce a versatile pick-up and drop technique that makes it possible to investigate the electrical, optical and mechanical properties of as-grown NCD films. Using this technique, NCD nanosheets, as thin as 55 nm, can be picked-up from a growth substrate and positioned on another substrate. As a proof of concept, electronic devices and mechanical resonators are fabricated and their properties are characterized. In addition, the versatility of the method is further explored by transferring NCD nanosheets onto an optical fibre, which allows measuring its optical absorption. Finally, we show that NCD nanosheets can also be transferred onto 2D crystals, such as MoS2, to fabricate heterostructures. Pick-up and drop transfer enables the fabrication of a variety of NCD-based devices without requiring lithography or wet processing., Comment: 15 pages, 6 figures

The deposition of three wires on the surface of a microcantilever enables both electrothermal and electromagnetic actuation of bending and torsional displacements. Four bending and four torsion modes of a 2700mum times 500mum times 30mum beam were excited electro-magnetically, and two bending and one torsion resonance were excited electrothermally

The dynamic response to white Gaussian noise of a bistable non-linear vibration energy harvester based on the repulsive electrostatic interaction between a microcantilever and an electrode has been theoretically studied. The cantilever-electrode system can be brought from a linear regime characterized by a quadratic potential, when cantilever is far from the electrode, to a non-linear bistable regime characterized by a quartic potential, when both elements are close enough. This distance parameter, which is commonly used to tune bistability, is unusually used here also to inject the energy to the system in the form of displacement noise. Thus, the widening and shifting to the low-frequency region of the response spectrum as well as the enhancement of the rms out-of-plane vibration of the cantilever are both demonstrated through this parametrically-induced bistability.

We demonstrate the coupling between the fundamental and second flexural mode of a microcantilever. A mechanical analogue of cavity-optomechanics is then employed, where the mechanical cavity is formed by the second vibrational mode of the same cantilever, coupled to the fundamental mode via the geometric nonlinearity. By exciting the cantilever at the sum and difference frequencies between fundamental and second flexural mode, the motion of the fundamental mode of the cantilever is amplified and damped. This concept makes it possible to enhance or suppress the Q-factor over a wide range., Comment: 9 pages, 3 figures

We fabricated clamped–clamped resonators by anisotropic etching of silicon-on-insulator wafers with a (1 1 0)-oriented top layer. The geometry of the resonator and its clamping points is confined by silicon (1 1 1) planes. The slow etch rate of these planes yields low surface roughness and enables accurate control over the resonator dimensions, and thus over its resonance frequencies. Lateral resonators were fabricated and characterized, and the measured resonance frequencies match closely with the calculated values for stress-free silicon beams. The resonance frequency can be tuned by electrostatic forces exerted via gate electrodes. The described method enables the fabrication of high-aspect-ratio silicon resonators without the need for deep reactive ion etching.

In the past years, the use of highly sensitive silicon microelectromechanical cantilevers has been proposed as a tool to characterize the spin-crossover phenomenon by employing fast optical readout of the motion. In this work, Fe II -based spin-crossover nanoparticles of the well-known [Fe(Htrz) 2 (trz)](BF 4 ) complex wrapped with thin silica shells of different sizes will be studied by means of silicon microresonators. The silica shell will enhance its chemical stability, whereas the low thickness will allow a proper mechanical coupling between the cantilever and the spin-crossover core. To maximize the sensing of the spin-crossover phenomena, different cantilever geometries and flexural modes were employed. In addition, the experimental observations were also compared with COMSOL numerical simulations, which are in close agreement with them. The probe of spin-crossover phenomena with micro- and nanoelectromechanical actuators offers the possibility of preparing smart sensing memory devices near/above room temperature.

We explore the dynamics of a graphene nanomechanical oscillator coupled to a reference oscillator. Circular graphene drums are forced into self-oscillation, at a frequency fosc, by means of photothermal feedback induced by illuminating the drum with a continuous-wave red laser beam. Synchronization to a reference signal, at a frequency fsync, is achieved by shining a power-modulated blue laser onto the structure. We investigate two regimes of synchronization as a function of both detuning and signal strength for direct (fsync = fosc) and parametric locking (fsync = 2fosc). We detect a regime of phase resonance, where the phase of the oscillator behaves as an underdamped second-order system, with the natural frequency of the phase resonance showing a clear power-law dependence on the locking signal strength. The phase resonance is qualitatively reproduced using a forced van der Pol-Duffing-Mathieu equation., Comment: 11 pages, 4 figures, 25 references