Frequency Controlling And Tuning Of Two-dimensional Nanoelectromechanical Resonator

The emergence of two-dimensional（2D）layered materials such as graphene,hexagonal boron nitride（h-BN）,transition metal chalcogenides（TMDCs）,etc.,marks the introduction of two-dimensional（2D）material to the family of low-dimensional materials.Their reduced dimension and rich physical properties lead to great potential in both fundamental research and potential applications,such as the discovery of superconductivity in magic-angle graphene,and the development of 2D transistors and2D resonators.Among the many 2D devices,nanoelectromechanical（NEMS）resonators based on 2D materials can have sizes in certain dimension down to atomic-level thickness,which is far beyond the size limit of the microfabrication process used in making microelectromechanical（MEMS）resonators.It enhances the size effect of micro-nano electromechanical devices,and at the same time provides a powerful experimental platform for the study of atomic-level mechanical vibration and low-dimensional condensed matter physics.In the study of 2D resonators,the resonance frequency scaling and tuning of 2D resonators have always been important aspects.The frequency scaling and tuning of the device are usually determined by the mechanical properties of the 2D material,the device parameter,the fabrication process and other factors,and sometimes the measurement scheme,which will all be discussed in this dissertation.First,a high-efficiency preparation method for 2D nanoelectromechanical resonators is presented,using which2D NEMS resonators based on Ti₃C₂T_x and rhenium sulfide（Re S₂）are successfully fabricated.Next,the characteristics of 2D resonator measurement methods are analyzed,and measurement schemes based on multi-physics coupling are presented.Further next,based on resonance frequency formula of 2D resonator,the choice of parameter space is discussed.Appropriate measurement schemes are then used according to the characteristics of the device,using which the resonant response of the resonator are successfully characterized in the frequency domain.Finally,the frequency scaling for the two 2D resonators is presented,and resonant frequency tuning by changing the gate voltage and air pressure are demonstrated.The research presented in this dissertation provides important guidance for the device application of these 2D resonators based on frequency scaling and tuning.The main research achievements in this dissertation are:1.A facile fabrication method for 2D NEMS resonator.This fabrication method can successfully build 2D NEMS devices covering different thickness which ranges from monolayer,few-layer to multi-layer ones.Further,gold electrodes are deposited on the2D NEMS resonator to enhance the electrical contact between the material and the metal electrode,and ensure that the electrical connection between the 2D resonator and the measurement set-up are successfully established.2.Demonstration of nanomechanical resonators and extraction of mechanical properties of Ti₃C₂T_x materials.Based on calculation and simulation,the effects from Young’s modulus of the device and the tension of the device on the fundamental mode resonant frequency of the device are analyzed.Based on the theoretical calculation and analysis results,the appropriate parameter space（diameter,etc.）for 2D Ti₃C₂T_x NEMS resonators with different diameters and different thicknesses is determined,and device are successfully fabricated and measured.After the frequency domain characterization of the resonant response of all devices,experimental data are used in combination with theoretical analysis to determine the Young’s modulus of the Ti₃C₂T_x material,and the frequency scaling law for the 2D Ti₃C₂T_x NEMS resonator is determined.Finally,based on the device frequency scaling,tuning of the device resonant frequency by changing gate voltage and air pressure is successfully demonstrated.3.Elucidation of the mechanical anisotropy in 2D Re S₂ using resonator measurements.Compared with 2D NEMS resonator with mechanically isotropic material,the mechanically anisotropic devices can exhibit different resonant responses.Based on theoretical calculations,effect of mechanical anisotropy（Young’s modulus）,device parameters（diameter,thickness,etc.）and tension of device on the device frequency is analyzed.2D Re S₂ resonators are then fabricated and the frequency domain characterization of the multi-mode resonant response is performed.By using mechanically anisotropic parameter sweeping and fitting of Re S₂ in FEM software,Young’s modulus of in Re S₂ determined.Based on the resonant response of 2D Re S₂resonators with different diameters and thicknesses,the frequency scaling rule of 2D Re S₂resonators are established and frequency tuning using gate voltage sweeping is realized.