Fabrication and mechanical characterization of graphene based membranes and their use in thermoacoustics
Following the first report on electronic transport measurements of graphene, an atom-thick carbon material, many scientists have devoted effort to understand its fundamental properties. In this work, the mechanical properties of graphene-based materials, including monolayer graphene oxide and chemical vapor deposition (CVD) grown graphene, were determined using membrane structures. Furthermore, a membrane structure was used to demonstrate thermoacoustic sound generation from monolayer graphene. In order to realize the mechanical characterization, reproducible methods to fabricate graphene membranes were developed using dry and wet transfer techniques. A novel dry transfer technique produced graphene-sealed microchambers without trapping liquid inside. An improved wet transfer technique enabled the transfer of graphene onto perforated substrates. Monolayer graphene oxide was mechanically tested using scanning atomic force microscopy (AFM) combined with finite element analysis of the data. The mechanical deformation was measured by scanning AFM tips over the suspended graphene oxide membranes. The Young’s modulus of the membranes was obtained by analyzing the deformation using finite element analysis together with a mapping technique. In addition, membranes with 2 and 3 layers of graphene oxide were identified using transmission electron microscopy and mechanically characterized. Moreover, these same methods were used for measuring mechanical properties of ultra-thin amorphous carbon membranes. Bulge tests, which apply uniform pressure on the suspended membrane, revealed the mechanical behavior of polycrystalline graphene grown on copper foils by chemical vapor deposition. In particular, the effect of grain boundaries on the elastic properties of polycrystalline graphene was studied by correlating its Young’s modulus with the density of grain boundaries within the membranes. It was observed that a large number of grain boundaries softened the graphene membranes. Graphene, along with monolayer hexagonal boron nitride, is the ultimate limit of thin materials. Thus, it is an ideal candidate as a thermoacoustic sound source because of its low heat capacity per unit area. The work presented here provides the first demonstration of thermoacoustic sound generation from large-area monolayer graphene. A fundamental understanding of the influence of the underlying substrates was achieved by comparing the acoustic performance of graphene membranes on various patterned substrates with different porosities.