Abstract
Graphene has been widely used in the form of micro-flakes to fabricate composite materials with enhanced mechanical properties. Due to the small size of the inclusions and their random orientation within the matrix, the superior mechanical properties of graphene cannot be fully exploited. Recently, attempts have been made to fabricate nanolaminate composites by interleaving large sheets of chemical vapor deposition (CVD) monolayer graphene between thin layers of polymer matrices. However, CVD graphene is inevitably accompanied by wrinkles that are formed in the synthesis process, and it remains unknown how the wrinkles affect the mechanical properties of graphene. Here, we employ Brillouin light spectroscopy to study the elastic moduli of CVD graphene by probing graphene/poly(methylmethacrylate) hybrid Bragg stacks at zero strain. We find the Young’s and shear moduli of the CVD graphene, which has wrinkles in the form of sharp elevations with a height of about 6 nm and a fullwidth at half maximum (FWHM) of ca. 30 nm, to be 680 ± 16 and 290 ± 10 GPa, respectively, with the former being about 30% lower than that of exfoliated, flat graphene. This work sheds light on the elastic properties of CVD graphene and provides a method that can be extended to studying the wrinkle-induced softening effect in other two-dimensional materials.
Highlights
The unique hexagonal sp2 C-C bonds endow graphene with remarkable physical properties, including extremely high thermal conductivity [1], electron mobility [2], and intrinsic strength [3]
3.2 Elastic modulus of chemical vapor deposition (CVD) graphene from Brillouin light spectroscopy (BLS)
The relatively stable sound velocity of graphene allows us to measure it by probing the Gr/poly(methyl methacrylate) (PMMA) stacks
Summary
The unique hexagonal sp C-C bonds endow graphene with remarkable physical properties, including extremely high thermal conductivity [1], electron mobility [2], and intrinsic strength [3]. For the mechanical properties, exfoliated suspended graphene has a Young’s modulus of 1 ± 0.1 TPa, and a breaking strength of 42 N/m [3]. Due to the thermal expansion mismatch between graphene and the copper substrate, severe biaxial stresses are developed on CVD graphene as it cooled from approximately 1000 oC down to ambient temperature. These stresses are relaxed by the formation of a network of folds that form a mosaic structure as seen in many AFM images [17, 18]. All of these can inevitably introduce out-of-plane wrinkles [19,20,21,22]
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