Monolayer graphene exhibits exceptional electronic and mechanical properties, making it a very promising material for nanoelectromechanical devices. Here, we conclusively demonstrate the piezoresistive effect in graphene in a nanoelectromechanical membrane configuration that provides direct electrical readout of pressure to strain transduction. This makes it highly relevant for an important class of nanoelectromechanical system (NEMS) transducers. This demonstration is consistent with our simulations and previously reported gauge factors and simulation values. The membrane in our experiment acts as a strain gauge independent of crystallographic orientation and allows for aggressive size scalability. When compared with conventional pressure sensors, the sensors have orders of magnitude higher sensitivity per unit area.

/pdf/electromechanical-piezoresistive-sensing-in-suspended-2vb2f7a7ci.pdf

Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes

Optomechanical systems couple light stored inside an optical cavity to the motion of a mechanical mode. Recent experiments have demonstrated setups, such as photonic crystal structures, that in principle allow one to confine several optical and vibrational modes on a single chip. Here we start to investigate the collective nonlinear dynamics in arrays of coupled optomechanical cells. We show that such ``optomechanical arrays'' can display synchronization, and that they can be described by an effective Kuramoto-type model.

Collective dynamics in optomechanical arrays.

This review provides a summary of the work completed to date on the nonlinear dynamics of resonant micro- and nanoelectromechanical systems (MEMS/NEMS). This research area, which has been active for approximately a decade, involves the study of nonlinear behaviors arising in small scale, vibratory, mechanical devices that are typically integrated with electronics for use in signal processing, actuation, and sensing applications. The inherent nature of these devices, which includes low damping, desired resonant operation, and the presence of nonlinear potential fields, sets an ideal stage for the appearance of nonlinear behavior, and this allows engineers to beneficially leverage nonlinear dynamics in the course of device design. This work provides an overview of the fundamental research on nonlinear behaviors arising in micro/nanoresonators, including direct and parametric resonances, parametric amplification, impacts, selfexcited oscillations, and collective behaviors, such as localization and synchronization, which arise in coupled resonator arrays. In addition, the work describes the active exploitation of nonlinear dynamics in the development of resonant mass sensors, inertial sensors, and electromechanical signal processing systems. The paper closes with some brief remarks about important ongoing developments in the field.

/pdf/nonlinear-dynamics-and-its-applications-in-micro-and-2d8rp19sh4.pdf

Nonlinear Dynamics and Its Applications in Micro- and Nanoresonators

Summary form only given. The field of optomechanics [1] seeks to explore the interaction between light and mechanical motion. Optomechanical system are typically composed of a single mechanical and a single optical mode interacting via radiation pressure: Ĥint = -ħg0 â†â(b + b†), where â/b are the photon/phonon operators. In this talk, we will introduce arrays of optomechanical cells, and discuss our first theoretical results on the nonlinear dynamics of such a setup [2,3].First we have studied the classical nonlinear dynamics of optomechanical arrays. For blue-detuned laser drive, a Hopf bifurcation towards self-sustained mechanical oscillations takes place. For static disorder of the frequencies in the array, we have shown that there can be a transition towards phase-locking. The slow dynamics of the mechanical oscillation phase field is described by a specific modification of the Kuramoto equation known in synchronization physics: in the simplest case δ φ = -δΩ - Ksin(2δφ), for the phase difference δφ between two cells with frequency difference δΩ, corresponding to a particle sliding down in a tilted washboard potential.In a second step, we have turned towards the quantum dynamics of arrays without static disorder [3]. There, the effects of the fundamental quantum noise can lead to phase diffusion. Upon increasing the coupling between cells, we observe a transition between incoherent mechanical oscillations and a collective phase-coherent mechanical state. To study the driven-dissipative dynamics, we employ a mean-field approach based on the Lindblad master equation, as well as semiclassical Langevin equations. We will also discuss the prospects of observing this non-equilibrium dynamics in an experimental implementation based on currently available setups. Very promising candidates in this regard are optomechanical crystal setups, where defects in photonic crystal structures are used to generate co-localized optical and mechanical modes in a two-dimensional geometry [4].

/pdf/collective-dynamics-in-optomechanical-arrays-4y23pzx4cm.pdf

Collective dynamics in optomechanical arrays

By virtue of their low mass and stiffness, atomically thin mechanical resonators are attractive candidates for use in optomechanics. Here, we demonstrate photothermal back-action in a graphene mechanical resonator comprising one end of a Fabry–Perot cavity. As a demonstration of the utility of this effect, we show that a continuous wave laser can be used to cool a graphene vibrational mode or to power a graphene-based tunable frequency oscillator. Owing to graphene’s high thermal conductivity and optical absorption, photothermal optomechanics is efficient in graphene and could ultimately enable laser cooling to the quantum ground state or applications such as photonic signal processing.

https://www.lassp.cornell.edu/lassp_data/mceuen/homepage/Publications/nl302036x.pdf

Photothermal Self-Oscillation and Laser Cooling of Graphene Optomechanical Systems

We report the first-ever use of graphene as a piezoresistive element in a MEMS device, and report on a giant piezoresistivity in the graphene films used on our devices Owing to the layer nature and the effective confinement of electrons in two-dimensions, applied strain in the graphene films is likely to create large changes in conductivity as the electrons are forces to traverse larger potential wells along increased bond-lengths or result in modulation of current along grain boundaries and multiple graphene layers In this paper we report a very high piezoresistive gauge factor of 18×104 This gauge factor is orders of magnitude higher than that of most piezoresistive materials used in MEMS This high value of piezoresistivity could be transformative in reinterpretation of the use of piezoresistivity for various hybrid transduction approaches

Graphene has ultra high piezoresistive gauge factor

Plasticity based approach for failure modelling of unreinforced masonry

The quality factor of an oscillator is inversely proportional to the damping and is a measure of the width of its amplitude vs. forcing frequency response. For sensing and signal processing applications of microelectromechanical systems (MEMS) oscillators, the quality factor (henceforth called Q) affects the sensitivity and performance of such devices. As MEMS vibrates (resonates) some of its vibrational energy is transmitted to the substrate upon which the MEMS is fabricated. A large component of this energy is carried away as surface acoustic waves (SAW). We demonstrate a design that improves the Q of resonant MEMS oscillators by up to 4times by reflecting surface wave energy back to the MEMS. Wave reflection occurs at trenches fabricated in a circle around the MEMS. The trench creates a "mesa" that provides partial mechanical isolation to the MEMS. The loss of energy due to SAW increases almost exponentially with frequency, with a corresponding decrease in Q. Hence the demonstrated design would become even more useful with the increasing need for higher frequency resonators. The mesa structure presented here is a simple idea which can be easily integrated into existing fabrication procedures and can be used for commercial purposes.

Anchor Loss Reduction in Resonant MEMS using MESA Structures

Hybrid inorganic–organic lead halide perovskites have attracted a significant research interest in the last 10 years due to their broad-area applications in optoelectronic devices such as solar cells, lasers, photodetectors, and light-emitting diodes (LEDs). Fundamental understanding of the charge transportation, defect density, and activation energy is very important for the further progress of this class of semiconductors. Here, we shed light on the interpretation of resistance, capacitance, defect density, and activation energy levels in single-crystalline methylammonium lead iodide (MAPbI₃). In particular, the impedance response of the MAPbI₃ crystal as a function of applied bias and temperature (under both increasing and decreasing temperature cycles) is studied for the first time. From the detailed bias- and temperature-dependent studies, we found that the low-frequency capacitance values are influenced by ion density and mobility. Consequently, single-crystalline MAPbI₃ depicts an activation energy of 0.53–0.54 eV with an exceptionally low electronic trap density of 0.96 × 10¹⁰ cm–³. The present study illustrates that the net electrochemical impedance spectra are due to ionic capacitance coupled to a resistance. The associated resistance is related to the conductivity of the perovskite crystal. These findings are helpful to understand the fundamental electrical properties of the MAPbI₃ single crystal, which could be useful for the further advancement of perovskite single-crystal-based applications.

Manoj Pandey

Papers

Graphene has ultra high piezoresistive gauge factor

Plasticity based approach for failure modelling of unreinforced masonry

Anchor Loss Reduction in Resonant MEMS using MESA Structures

Interpretation of Resistance, Capacitance, Defect Density, and Activation Energy Levels in Single-Crystalline MAPbI₃

Dynamic compressive behaviour of auxetic and non-auxetic hexagonal honeycombs with entrapped gas