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Study and Fabrication of Electrical & Electromechanical Resonators Based on Nanostructures for Sensing Application

Fardindoost, Somayeh | 2015

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 47081 (48)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Iraji Zad, Azam; Sarvari, Reza; Mohajerzadeh, Shamsoddin
  7. Abstract:
  8. In this thesis we present fabrication, characterization, and experimental results describing electrical actuation and readout of the mechanical vibratory response of PVP, graphene-doped and ZnO fibers by employing electrical actuation. For a fiber resonator with an approximate radius of 850 nm and length of 100 µm, we observed a resonance frequency around 580 kHz with a quality factor (Q) of about 2511 in air at ambient conditions. Through the use of finite element simulations, we show that the reported frequency of resonance is relevant. Also, the measurements were performed in an enclosed chamber with controlled levels of ethanol vapor. The adsorption of ethanol causes a shift in the resonance frequency of the fibers, which can be related to the concentration of ethanol vapor for sensing purposes. We present flexible strain sensors made of graphene flakes fabricated, characterized and analyzed for the electrical actuation and readout of their mechanical vibratory response in strain sensing applications. For a suspended graphene membrane fabricated with an approximate length of 10 µm, a mechanical resonance frequency around 144 MHz with a quality factor (Q) of ~70 in air under ambient conditions was observed. The applied strain can shift the resonance frequency substantially, which is found related to the physical dimension alteration and built-in strain in the graphene flake. Strain sensing was performed using both planar and non-planar surfaces (bending with different radii of curvature) as well as by stretching with different elongation. In the second part, we propose and demonstrate novel stacked layers including two monolayer graphene sheets which are separated by a high dielectric spacer of Si3N4, all rested on both rigid (Si (111)) and flexible (Kapton) substrates. The strain responses of the double patterned and single patterned monolayers on both flexible and rigid substrates are measured wirelessly at radio frequency ranges. The single patterned flexible sensor showed a significantly improved sensitivity of 56 kHz/kgf as compared to 10.6 kHz/kgf for the rigid substrate sensor. Also, the double patterned architecture showed improved sensitivity as compared to the single patterned architecture by demonstrating larger frequency shifts and relatively strong resonances due to better electromagnetic wave coupling. These wireless sensing results indicated that the graphene based strain sensor can be utilized in real life applications
  9. Keywords:
  10. Graphene ; Nanofiber ; Strain Sensor ; Sensory Properties ; Electromechanical Structures ; Ethanol Gas Sensor

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