In this thesis, the design, fabrication and characterisation of graphene electromechanical resonators have been presented. Graphene features ultrahigh Young’s modulus and large surface to volume ratio that make it ideal for radio frequency (RF) components, sensors and other micro/nano-electromechanical systems (MEMS/NEMS). A novel batch fabrication process for graphene electromechanical resonators has been developed by using poly-Si film as sacrificial layer. Previously reported fabrication processes of graphene resonators use SiO2 as sacrificial layer only because graphene is visible on 300nm SiO2/Si substrate. However, the wet etching of SiO2 involves HF, which is not compatible with metal connections or SiO2 serving as dielectric or passivation layer in graphene NEMS devices. Moreover, the liquid surface tension during drying after wet etching could damage graphene bridges even critical point drying is used. Therefore, in this work, poly-Si is adopted as the sacrificial material. To facilitate the fabrication of graphene resonators, a poly-Si/SiO2/Si substrate has been designed and optimised to make graphene visible under optical microscope for the first time to the author’s knowledge. Chemical vapour deposition (CVD)-grown monolayer graphene sheet has been transferred onto the optimised poly-Si/SiO2/Si substrate and patterned into strips. Metal electrodes have been deposited by lift-off process to make electrical connections, which is prerequisite for integrating graphene resonator into practical devices. The graphene bridges have been released by etching the poly-Si layer with XeF2 in vapour phase, which completely avoids the capillary force induced damage to the graphene bridges. De-fluorination process has been performed by hydrazine reduction to recover graphene’s conductivity. This fabrication process is scalable for massive production of graphene electromechanical resonators, thus furthering their practical application. One-source current mixing characterisation setup has been constructed to test the graphene resonators. Besides the fundamental peak, the activation and enhancement of the second mode of doubly clamped resonator by electrostatic actuation have been observed for the first time. The second mode amplitude reaches 95% of the fundamental mode, whereas only odd higher modes of small intensity have been reported before in other MEMS/NEMS resonators actuated by electrostatic force or magnetomotive force. The findings in this thesis could lead to substantial increase of the sensitivity of sensors based on the graphene resonators. Modal analysis based on Euler-Bernoulli equation has been performed to understand the mechanism behind the activation and enhancement of the second mode. Finite element analysis agrees very well with experimental results and complies with the theoretical model. Finally, a set of novel alignment marks has been designed, which can be incorporated to process mechanically exfoliated 2D material flakes of micron size and irregular shape with conventional photolithography tools, as have been demonstrated by the successful fabrication of a graphene transistor. This optical alignment technique provides an alternative for prototype device development besides electron beam lithography to prevent electron-induced damage to fragile 2D materials.

Nanoelectromechanical systems (NEMS) have drawn considerable attention towards several sensing applications such as force, spin, charge and mass. These devices due to their smaller size, operate at very high frequencies (MHz - GHz) and have very high quality factors (102 -105). However, the early onset of nonlinearity limits the linear dynamic range of these devices. In this work we investigate the nonlinearities and their effect on the performance of graphene based NEMS. Electromechanical devices based on 2D materials are extremely sensitive to strain. We studied the effect of strain on the performance of single layer Graphene NEMS and show how the strain in Graphene NEMS can be tuned to increase the range of linear operation. Electromechanical properties of the doubly clamped graphene resonators deviates from the flat rectangular plate as the former possesses geometrical imperfections which are sometimes orders of magnitude larger than the thickness of the resonator. Due to these imperfections we report an initial softening behavior, turning to strong hardening nonlinearity for larger vibration amplitude in the back-bone curve. We have also studied the frequency stability of graphene resonators. Frequency stability analysis indicates departure from the nominal frequency of the resonator with time. We have used Allan Variance as a tool to characterize the frequency stability of the device. Frequency stability of graphene resonator is studied in an open loop configuration as a function of temperature and bias voltage. The thesis concludes with a remark on the future work that can be carried out based on the present studies.