Theses Doctoral

# Graphene NanoElectroMechanical Resonators and Oscillators

Chen, Changyao

Made of only one sheet of carbon atoms, graphene is the thinnest yet strongest material ever exist. Since its discovery in 2004, graphene has attracted tremendous research effort worldwide. Guaranteed by the superior electrical and excellent mechanical properties, graphene is the ideal building block for NanoElectroMechanical Systems (NEMS). In the first parts of the thesis, I will discuss the fabrications and measurements of typical graphene NEMS resonators, including doubly clamped and fully clamped graphene mechanical resonators. I have developed a electrical readout technique by using graphene as frequency mixer, demonstrated resonant frequencies in range from 30 to 200 MHz. Furthermore, I developed the advanced fabrications to achieve local gate structure, which led to the real-time resonant frequency detection under resonant channel transistor (RCT) scheme. Such real-time detection improve the measurement speed by 2 orders of magnitude compared to frequency mixing technique, and is critical for practical applications. Finally, I employed active balanced bridge technique in order to reduce overall electrical parasitics, and demonstrated pure capacitive transduction of graphene NEMS resonators. Characterizations of graphene NEMS resonators properties are followed, including resonant frequency and quality factor ($Q$) tuning with tension, mass and temperatures. A simple continuum mechanics model was constructed to understand the frequency tuning behavior, and it agrees with experimental data extremely well. In the following parts of the thesis, I will discuss the behavior of graphene mechanical resonators in applied magnetic field, {i.e.} in Quantum Hall (QH) regime. The couplings between mechanical motion and electronic band structure turned out to be a direct probe for thermodynamic quantities, {i.e.}, chemical potential and compressibility. For a clean graphene resonators, with quality factors of $1 \times 10^4$, it underwent resonant frequency oscillations as applied magnetic field increases. The chemical potential of graphene shifts smoothly within each LL, causing the resonant frequency to change in an explicit pattern. Between LLs, the finite compressibility caused the resonant frequency changing dramatically. The overall oscillations of resonant frequency with the applied magnetic field could be fitted with only disorder potential as free parameter. Compared with conventional electronic transport technique, such mechanical measurements proven to be a more direct and powerful tool, which we used o study the properties of graphene's ground states in broken symmetry states. In the last part this thesis, I will present the study of graphene NEMS oscillators with positive feedback loop. The demonstrated oscillators are self-sustained (without external radio frequency, RF, stimulus), and the oscillation frequencies can be controlled by tension{i.e.}, (applied gate voltage). I also carefully studied the influence of feedback gain and phase, as well as linewidth compression as function of temperature.