Measurement of the effect of growth quality and number of layers on the mechanical properties of graphene using a MEMS tensile tester
Creators: name: Cho, Joon Hyong
name: 0000-0002-7329-2556
Publisher: name: The University of Texas at Austin
Year: 2022
Subjects: Multilayer graphene
2-D material
MEMS
Fracture

description: Main goal of research is to measure mechanical properties of graphene grown under different conditions mainly controlling growth substrate and growth pressure of the graphene to control the number of graphene layers grown. Mechanical properties of graphene are measured in many ways, yet conventional mechanical tensile testing of graphene sheets is difficult due to one atomic thickness of graphene and difficulties in handling graphene tensile specimen in nanoscale. I propose to implement Microelectromechanical System (MEMS) based tensile tester to accurately measure the mechanical properties of suspended graphene from strain versus stress curve. Understanding mechanical properties of graphene in nanoscale can be beneficial in many application perspectives. First, mechanically tested graphene on MEMS device can be used as a graphene-based nanoscale device such as precision displacement sensors, load detectors, and electrical filters. One of the major challenges in producing highly accurate graphene-based nanoscale devices is the poor fabrication repeatability of graphene-based nanoscale devices due to small variations in the residual stress and initial tension of the graphene film. This has meant that graphene-based nanoscale devices tend to have large variations in natural frequency and quality factor from device to device. This poor repeatability makes it impossible to use these devices to make accurate, high-precision force and displacement sensors or electromechanical filters. However, by actively controlling the tension on the graphene it is possible both to increase repeatability between devices and to increase the force/mass sensitivity of the nanoelectromechanical resonators produced. Second, in addition to designing devices that can compensate for manufacturing errors in nano-manufactured devices, several methods are proposed to greatly expand the scope and rate at which nanomaterials-based devices can be fabricated. For example, a transfer-free, wafer-scale manufacturing process can be used to produce suspended graphene-based devices such as the graphene-based nanoelectromechanical resonators in the wafer scale. Therefore, for the scope of the research, it is necessary to 1) understand the growth parameters of graphene on thin film which determines the overall quality of graphene, 2) find the methods to control the uniformity and thickness of graphene, 3) successfully implement and test graphene MEMS tensile tester, and 4) analyze and compare the tensile testing results of graphene to previously conducted research. As a result, ideal adhesion layer for platinum thin film and thickness for Cu-Ni composite alloy are investigated for growing defect less, uniform, continuous monolayer graphene. In addition, we have found a critical parameter which can control the number of graphene layers on Cu foil. We obtained graphene layers from mono to 15 layers by controlling only the growth pressure. We also have developed an effective and repeatable graphene transfer process which enables us to transfer small patterned graphene onto graphene MEMS devices. Lastly, we have tested mechanical properties of different number of graphene layers on MEMS tensile tester and obtained critical strain energy release rate of fracture as 145 J/m², 71.3 J/m², and 22 J/m² with ~15 layers, ~10 layers, and ~5 layers of graphene, respectively. Also, gauge factors were calculated according to ~15, ~10, and ~5 graphene layers to be 3.76, 3.09, 12.35, respectively. Detecting strain change in multilayer graphene layer above 5 layers by Raman spectroscopy was challenging since rate of Raman shift was as low as 0.05 cm⁻¹ per percentage of strain change compared to 0.328 cm⁻¹/%ε of monolayer