Mechanical Properties Of Two Dimensional Materials Using Micro/Nanoelectromechanical Resonators

Since the exfoliation of graphene in 2004 by Geim,Novoselov and their colleagues,the rise of graphene has inspired researchers’interest to explore two-dimensional materials.In the past two decades,research on two-dimensional materials has undergone an explosive growth,including condensed matter physics,electronic engineering,materials science,and chemistry.In addition to possessing exceptional optical,electrical,and thermal properties,two-dimensional materials also exhibit a distinctive combination of mechanical properties,namely high in-plane stiffness and strength,along with extremely low bending rigidity.The exceptional properties exhibited by these materials have positioned two-dimensional materials as a unique platform for advancing research in the realm of low-dimensional physics.Furthermore,they have demonstrated immense potential in various applications,including Micro-and Nanoelectromechanical Systems（M/NEMS）,flexible electronics,and strain engineering for precise control over physical properties.Thus far,the majority of investigations pertaining to the mechanical properties of two-dimensional materials have closely followed the work by the Hone research group at Columbia University using nanoindentation on suspended graphene sheet.Nevertheless,it is important to acknowledge that the nanoindentation technique has its limitations.In this dissertation,a different method is used to study the mechanical properties of two-dimensional materials.Different types of two-dimensional material-based micro/nanoelectromechanical resonators were fabricated and their performance was evaluated using optical interference technology.The Young’s modulus and surface tension of the suspended section of the two-dimensional material were successfully determined through the circular drum resonator model.Notably,the anisotropic Young’s modulus of the two-dimensional materials in two perpendicular mechanical axes was accurately extracted using finite element simulation and mode analysis.The results demonstrate a highly promising method for exploring the mechanical properties of two-dimensional materials,complementing the nanoindentation technique.This dissertation consists of the following specific parts:1.In the first study（Chapter 3）,mechanical property of two-dimensional Ti₃C₂T_xcrystals was studied using a circular drum resonator model.By successfully fabricating resonators of different thicknesses and diameters using dry transfer techniques,the Young’s modulus of the Ti₃C₂T_x crystal and the surface tension of the devices were extracted.The resonator’s frequency scaling was studied by measuring its resonant characteristics,and by combining experimental results with theoretical calculations,the Young’s modulus of the two-dimensional Ti₃C₂T_x crystal was independently derived as270～360 GPa,which is consistent with nanoindentation measurements.The initial tension of the devices ranged from 0.05 N/m to 0.4 N/m.Finally,the chapter further demonstrates the electrical tuning of the resonant frequency of the Ti₃C₂T_x resonator and a frequency-shift based Ti₃C₂T_x vacuum gauge with a responsivity of 736%/Torr and a detection range as low as 10^-4 Torr.2.Building upon the successful extraction of the mechanical properties of two-dimensional Ti₃C₂T_x crystals,a second study（Chapter 4）extended this method to explore the mechanical properties of semiconductor WSe₂.Resonators of different thicknesses and diameters were successfully fabricated using dry transfer techniques.The resonator’s frequency scaling was then studied.Combining experimental results with theoretical calculations,the Young’s modulus of WSe₂ was extracted as 130 GPa,with an initial tension ranging from 0.05 N/m to 0.4 N/m.Finally,the A wide frequency tuning range（up to 230%）and a high tuning efficiency（up to 23%/V）.3.In the third study（Chapter 5）,two-dimensional resonators based on non-layered semiconductor materialβ-In₂S₃ were successfully fabricated using water-assisted transfer techniques,and also studied using the aforementioned mechanical property measurement method.β-In₂S₃,prepared by chemical vapor deposition（CVD）on mica substrates,was transferred onto polydimethylsiloxane（PDMS）using water-assisted transfer.Resonators of different diameters and thicknesses were then fabricated using dry transfer techniques.The frequency scaling behavior of the devices was obtained by measuring their amplitude-frequency response using laser interferometry.The experimental data matched the frequency scaling behavior predicted by the circular drum resonator model.The Young’s modulus ofβ-In₂S₃ was extracted as 45 GPa,with an initial tension ranging from0.05 N/m to 0.5 N/m.Meanwhile,the two-dimensionalβ-In₂S₃ resonator prepared in this chapter exhibits an ultra-wide sensing range in six pressure regimes from 10^-3 Torr to atmospheric pressure,and displays a highly linear resonance frequency tuning characteristic with a nonlinearity of 0.71%,the best among two-dimensional NEMS pressure sensors.In addition,through periodic pressure switching experiments and real-time data comparison with a commercial vacuum gauge,the fast response and reproducibility of the two-dimensionalβ-In₂S₃ micro-nano electromechanical resonator in pressure sensing applications are demonstrated.4.In the fourth study（Chapter 6）,the theoretical and experimental aspects of the mechanical anisotropy in two-dimensional Re S₂ crystals were demonstrated by simulating and measuring the multimodal resonant characteristics of Re S₂nanoelectromechanical resonators.Key features reflecting the mechanical anisotropy in Re S₂ two-dimensional crystal materials were investigated through finite element method simulations.Resonators with a diameter of d=10μm and a thickness of t=106.4 nm（in the“plate”region）were fabricated using mechanical exfoliation and dry transfer techniques.By measuring the multimodal resonant response of these resonators,the mechanical anisotropy of the Re S₂ crystal was determined.The experimental data were used to extract the Young’s moduli in two orthogonal directions:E_Yx=127 GPa and E_Yy=201 GPa.Polarization reflection experiments confirmed that the crystal’s b-axis corresponds to the mechanically“soft”axis.In summary,the methods presented in this thesis complement nanoindentation techniques,providing a non-destructive method of extracting mechanical properties of two-dimensional materials.Moreover,this method is highly compatible and independent of the bandgap and crystal structure of two-dimensional materials,providing a promising means for the rapid exploration of the mechanical properties of novel two-dimensional materials.