Session W7: Focus Session: Graphene Devices - Mechanical Effects

Nanomechanical Field-Effect Magnetometry in Graphene

This presentation will describe our studies of graphene nanomechanical resonators in the quantum Hall (QH) regime. We observe strong magneto-mechanical coupling to de Haas-van Alphen oscillations of magnetization, resulting in oscillatory frequency shifts of up to 1 MHz. This response is over two orders of magnitude larger than the expected response in a ``standard'' torque magnetometry framework. Instead, we find that the electric field-effect modulation of the magnetic energy provides a gradient force without a magnetic field gradient. Modeling of the effect produces excellent agreement with experiment, using only the disorder as a free parameter. We further use this novel mechanism to quantify the many-body exchange interaction of broken-symmetry QH states. This new mechanism may prove to be a useful tool for magnetic studies across low-dimensional materials and in sensing applications.   

High frequency graphene resonators

We study mechanical resonance properties of suspended graphene devices through radio frequency electromechanical~downmixing techniques. Taking advantage of graphene's atomically thin nature, unusually large transconductance, and extremely high Young's Modulus, we have fabricated high-frequency suspended graphene resonators.~Our global-gate devices have resonated at over 500MHz in a low temperature environment with reasonable quality factors. This research will pave the way for high-quality resonators in the GHz regime which will be used in high frequency applications such as ultra high sensitive mass, chemical and charge detectors.   

Optical Detection of Vibrations and Mass Loading of Graphene Mechanical Resonators Compatible with TEM and AFM

We produce arrays of exceptionally clean, suspended graphene mechanical resonators in high-yield using a simple, polymer-free procedure by transferring CVD-graphene to a flexible perforated substrate. The lack of a backing substrate facilitates Transmission Electron Microscopy (TEM) characterization, yet the membranes are still compatible with Atomic Force Microscopy (AFM) studies. We detect mechanical vibrations of resonators through optical interferometry and find excellent agreement with a theoretical model based on the 2D wave equation. We find quality factors of 1.25 $\mu$m diameter circular membranes to be as high as Q $\sim$ 800-1000 at room temperature, and observe lifting of mode degeneracies in square membranes. TEM and AFM studies reveal graphene folding, nanoparticle contamination, holes, tears, and other defects that can lead to the observed degeneracy splitting in square membranes. Controlled mass loading is also explored to suppress certain vibrational modes and tune vibrational frequencies for possible high density archival memory applications. The graphene membrane devices reported here open up a host of new possibilities for correlating TEM and AFM studies of an individual graphene membrane to its performance and properties as a mechanical resonator.   

Graphene xylophone: Mechanical properties of graphene based nano resonators

Mechanical properties of graphene xylophone structure were investigated. The arrays of the graphene ribbon patterns were prepared using nano lithography and suspended graphene structure was realized on the pre-patterned trench by the micro contact transfer printing method. Xylophone like structure was prepared for studying length and thickness dependence of mechanical properties of graphene resonator. The various mechanical behaviors, such as frequency tuning, non-linearity and bistability of single, bi and multi layer graphene structure will be discussed in this presentation. The potential application of our graphene xylophone structure for RF component will be suggested in the end of this presentation.   

Cavity Optomechanics with Graphene Resonators

Optical manipulation of micromechanical and nanomechanical resonators promises control of quantum states of macroscopic systems, among other applications. Because the spring constant of a resonator scales with its mass, there are advantages associated with using the lightest possible membranes as the mechanical elements. Here, we demonstrate that graphene, a one-atom-thick membrane, can be used as the mechanically active part of an optomechanical system. We show that a laser coupled to a Fabry-Perot cavity between a graphene resonator and a reflective backplane can both enhance and damp graphene motion. The enhancement of resonator motion is sufficient to induce self-oscillation, which is useful for applications in sensing and signal processing. These experiments demonstrate that graphene resonators are useful for optomechanical applications and show promise for resonator cooling toward the quantum ground state.   

Ultrathin circular membranes with high quality factors

We have fabricated large ultrathin circular drum resonators from monolayer graphene (up to 90 $\mu $m in diameter) and silicon nitride ($\sim $15 nm, up to 1mm in diameter). Resonant frequency, quality factor (Q) of different modes of these self tensioned graphene drums and high tensile stress ($\sim $ 1GPa) silicon nitride were measured using optical interferometric detection technique. We measured extremely high quality factors (up to 5,000 for graphene and up to 4,000,000 for ultrathin silicon nitride membranes) at room temperature. High quality factors observed in these resonators indicate dissipation mechanisms which differ from the conventional high surface to volume ratio resonators that show very low quality factors. The measured mechanical dissipation (Q$^{-1})$ shows a strong size and modal dependence, possibly indicating the influence of clamping losses in these tensioned membranes. These findings pave the way for identifying optimum size and modes for achieving high Q oscillators for applications in mass sensing and optomechanical coupling experiments which are underway.   

Novel Fabrication Techniques for Wafer-Scale Graphene Drum NanoElectroMechanical Resonators

Graphene NanoElectroMechanical Systems (NEMS) have shown excellent mass sensitivity as well as resonant and oscillatory behaviors that are desirable in mass sensors and active elements in Radio Frequency Integrated Circuit (RFIC) design. Out of many structures proposed for graphene NEMS, it has been recently shown that a drum resonator exhibits higher Q-factor than other structures such as a bar resonator. However, fabricating a large array of drum graphene resonator has been problematic because liquid or gas can be trapped inside the drum. Such issues led to designs with a hole in the center of a drum or with a drainage trench, either at the cost of additional lithography step or lowered Q-factor. Here, we demonstrate two novel fabrication methods that are free of the trapping without any compromise in additional lithography step or Q-factor degradation. In one method, wafer scale graphene is dry-stamped on prefabricated leads, holes and local gates. In the other method, an resist strip with a circular hole at the center holds graphene underneath. I will discuss direct electrical readout and characterization of devices using these two methods. These drum structures may provide a practical way to achieve wafer scale high Q graphene NEMS.   

Graphene NanoElectroMechanical Oscillator

Graphene based NanoElectroMechanical Systems (NEMS) working in Radio Frequency (RF) regime possess considerable advantages own to the remarkable electrical and mechanical properties of this atomic thin material. Here we demonstrated self-sustained nanoelectromechanical oscillator made from graphene. Our recent developed transduction scheme enables the direct conversion from mechanical motion to electrical domain, then subsequently being fed back to the system as excitation. The absence of extra RF actuation and the stable performance shows the possibility of practical application of graphene oscillator, for example, as signal filters, mass sensors or timing devices.   

Uniaxially Strained Graphene Resonators

Strained graphene allows one to explore the connection between the mechanical and electrical properties of graphene. Exploring this interplay between the mechanical and electrical properties in graphene may enable tuning graphene's mechanical and electrical properties as well as opening up new exotic electronic states. We have developed a technique to fabricate uniaxial strained graphene transistors and mechanical resonators with strains as high as 0.5{\%}. We demonstrate how strain perturbs both the mechanical and electrical properties of graphene, highlighted by the strain quenching of flexural phonons.   

Local Optical Probe of Motion and Stress in a NEMS

Nanoelectromechanical systems (NEMSs) are emerging nanoscale elements at the crossroads between mechanics, optics and electronics, with significant potential for actuation and sensing applications. The reduction of dimensions compared to their micronic counterparts brings new effects including sensitivity to very low mass, resonant frequencies in the radiofrequency range, mechanical non-linearities and observation of quantum mechanical effects. An important issue of NEMS is the understanding of fundamental physical properties conditioning dissipation mechanisms, known to limit mechanical quality factors and to induce aging due to material degradation. There is a need for detection methods tailored for these systems which allow probing motion and stress at the nanometer scale. Here, we show a non-invasive local optical probe for the quantitative measurement of motion and stress within a multilayer graphene NEMS provided by a combination of Fizeau interferences, Raman spectroscopy and electrostatically actuated mirror. Interferometry provides a calibrated measurement of the motion, resulting from an actuation ranging from a quasi-static load up to the mechanical resonance while Raman spectroscopy allows a purely spectral detection of mechanical resonance at the nanoscale. Such spectroscopic detection reveals the coupling between a strained nano-resonator and the energy of an inelastically scattered photon, and thus offers a new approach for optomechanics.   

Nonlinear Damping Mechanism in Mechanical Graphene Resonators.

Based on a continuum mechanical model for single-layer graphene\footnote{J. Atalaya, A. Isacsson, and J. M. Kinaret, Nano Letters 8, 4196 (2008).} we propose and analyze a microscopic mechanism for dissipation in nano-electromechanical graphene resonators. We find that coupling between flexural modes and in-plane phonons leads to linear and nonlinear damping of out-of-plane vibrations. By tuning external parameters, such as static gate voltage, one can cross over from a linear to a nonlinear-damping dominated regime. We discuss how the effective quality factor depends on parameters such as temperature, and compare our results with recent experiments.   

Graphene Nanoelectromechanical Systems are Unique

Graphene Nanoelectromechanical systems (GNEMS) with their light mass show a lot of interesting novel physics effects compared to conventional Nanoelectromechanical systems (NEMS). Superior gate tunability of the order of $>$10MHz/V at room temperature as well as high quality factors of $\sim$10$^5$ at mK temperatures for the fundamental mode have already been obtained. Studying their high frequency modes, we have observed for the first time avoided crossing between graphene modes as well as between graphene modes and their suspended contacts. In addition we have studied the atypical variation of the mode dispersion versus gating resulting from tensioning and electrostatic coupling to the gate. In particular, we measure an additional frequency shift near 0Vg which could be due to a strong electromechanical coupling near the Dirac Point.   

Current-induced forces in graphene-based nanoelectromechanical systems

Transport currents have distinct effects on the vibrational dynamics of nanoelectromechanical systems. Recently, we have developed a comprehensive scattering-matrix approach to treat out-of-equilibrium current-induced forces [c.f. Phys. Rev. Lett. 107, 036804 (2011)]. We apply our method to the vibrational dynamics of a suspended graphene membrane, paying special attention to the different coupling mechanisms between Dirac fermions and flexural modes in graphene.