Changyao Chen

Performance of monolayer graphene nanomechanical resonators with electrical readout

The enormous stiffness and low density of graphene make it an ideal material for nanoelectromechanical applications. Here, we demonstrate the fabrication and electrical readout of monolayer graphene resonators, and test their response to changes in mass and temperature. The devices show resonances in the megahertz range, and the strong dependence of resonant frequency on applied gate voltage can be fitted to a membrane model to yield the mass density and built-in strain of the graphene. Following the removal and addition of mass, changes in both density and strain are observed, indicating that adsorbates impart tension to the graphene. On cooling, the frequency increases, and the shift rate can be used to measure the unusual negative thermal expansion coefficient of graphene. The quality factor increases with decreasing temperature, reaching approximately 1 x 10(4) at 5 K. By establishing many of the basic attributes of monolayer graphene resonators, the groundwork for applications of these devices, including high-sensitivity mass detectors, is put in place.

Phonon softening and crystallographic orientation of strained graphene studied by Raman spectroscopy

We present a systematic study of the Raman spectra of optical phonons in graphene monolayers under tunable uniaxial tensile stress. Both the G and 2D bands exhibit significant red shifts. The G band splits into 2 distinct subbands (G+, G−) because of the strain-induced symmetry breaking. Raman scattering from the G+ and G− bands shows a distinctive polarization dependence that reflects the angle between the axis of the stress and the underlying graphene crystal axes. Polarized Raman spectroscopy therefore constitutes a purely optical method for the determination of the crystallographic orientation of graphene.

Graphene mechanical oscillators with tunable frequency

Oscillators, which produce continuous periodic signals from direct current power, are central to modern communications systems, with versatile applications including timing references and frequency modulators. However, conventional oscillators typically consist of macroscopic mechanical resonators such as quartz crystals, which require excessive off-chip space. Here, we report oscillators built on micrometre-size, atomically thin graphene nanomechanical resonators, whose frequencies can be electrostatically tuned by as much as 14%. Self-sustaining mechanical motion is generated and transduced at room temperature in these oscillators using simple electrical circuitry. The prototype graphene voltage-controlled oscillators exhibit frequency stability and a modulation bandwidth sufficient for the modulation of radiofrequency carrier signals. As a demonstration, we use a graphene oscillator as the active element for frequency-modulated signal generation and achieve efficient audio signal transmission.

Radio frequency electrical transduction of graphene mechanical resonators

We report radio frequency (rf) electrical readout of graphene mechanical resonators. The mechanical motion is actuated and detected directly by using a vector network analyzer, employing a local gate to minimize parasitic capacitance. A resist-free doubly clamped sample with resonant frequency ∼34 MHz, quality factor ∼10 000 at 77 K, and signal-to-background ratio of over 20 dB is demonstrated. In addition to being over two orders of magnitude faster than the electrical rf mixing method, this technique paves the way for use of graphene in rf devices such as filters and oscillators.

Graphene nanoelectromechanical systems

Graphene possesses a combination of properties that make it extremely well suited for use in nanoelectromechanical systems (NEMS). Its exceptional mechanical properties include high stiffness and low mass, which lead to high resonant frequencies; and ultrahigh strength, which allows for strain tuning of frequency over a wide range. Its optical properties and high electronic mobility enable robust optical and electrical transduction, while its chemical inertness enables atomically thin devices. This paper reviews the basic properties of graphene NEMS, and recent work toward exploring device properties, readout techniques, and applications.

Electrically integrated SU-8 clamped graphene drum resonators for strain engineering

Graphene mechanical resonators are the ultimate two-dimensional nanoelectromechanical systems (NEMS) with applications in sensing and signal processing. While initial devices have shown promising results, an ideal graphene NEMS resonator should be strain engineered, clamped at the edge without trapping gas underneath, and electrically integratable. In this letter, we demonstrate fabrication and direct electrical measurement of circular SU-8 polymer-clamped chemical vapor deposition (CVD) graphene drum resonators. The clamping increases device yield and responsivity, while providing a cleaner resonance spectrum from eliminated edge modes. Furthermore, this resonator is highly strained, indicating its potential in strain engineering for performance enhancement.Graphene mechanical resonators are the ultimate two-dimensional nanoelectromechanical systems (NEMS) with applications in sensing and signal processing. While initial devices have shown promising results, an ideal graphene NEMS resonator should be strain engineered, clamped at the edge without trapping gas underneath, and electrically integratable. In this Letter, we demonstrate fabrication and direct electrical measurement of circular SU-8 polymer-clamped chemical vapor deposition graphene drum resonators. The clamping increases device yield and responsivity, while providing a cleaner resonance spectrum from eliminated edge modes. Furthermore, the clamping induces a large strain in the resonator, increasing its resonant frequency.

Modulation of mechanical resonance by chemical potential oscillation in graphene

By coupling to electrons in the quantum Hall regime, the mechanical response of graphene resonators is modulated by changes in the chemical potential.

NEMS applications of graphene

Graphene, which consists of a single atomic sheet of graphene, possesses high electronic mobility, and excellent mechanical properties, making it an ideal candidate for nanoelectromechanical (NEMS) applications, including sensing and signal processing. Toward these applications, we have measured the mechanical properties of graphene sheets and demonstrated fabrication and electrical readout of monolayer graphene NEMS that show vibrational resonances in the MHz range. The dependence of the resonant frequency on applied gate voltage yields the mass density and built-in strain. Upon addition of mass, we observe changes in both the density and the strain, indicating that adsorbates impart tension to the graphene. The quality factor increases monotonically with decreasing temperature and reaches ∼104 at 5 K.

Stress-enhanced chemical vapor deposited graphene NEMS RF resonators

In this work we present room-temperature measurements of graphene nanoelectromechanical resonators (GN-ERs) demonstrating quality factors (Qs) greater than 200 at resonance. A nominal resonant frequency (fo) of 200 MHz is attained by applying strain to the suspended graphene using an SU-8 polymer clamp. Additionally, the device fo can be tuned by more than 5% by application of a DC gate bias on the order of 5V. Chemical vapor deposited (CVD) graphene is used to demonstrate the scalability of the process.

Raman Spectroscopy of Graphene under Uniaxial Strain

Polarized Raman spectroscopy was performed on single-layer graphene under uniaxial strain. The G- mode softens and splits into two components that exhibit distinct polarization properties related to the orientation of the graphene lattice.