The pursuit of high quality multifunctional carbon-based composite materials is driven by a myriad of application lined-up for strongest synthetic structure ever produced. There have been extensive studies, over the past decades, incorporating low-dimensional materials such as carbon nanotube and graphene in metal matrix, whereas no rational design route of graphene-metal composites has been elucidated due to the lack of understanding of graphene-metal interacting at multiple length scales. To gain insights in graphene-metal interactions and to build a rational design route of strong graphene-based macroscopic assemblies and composites, we use a model system incorporating high quality monolayer graphene in single crystalline ultra-thin metal matrix via rapid chemical vapor deposition (CVD) processes.
The ultra-thin metal matrix having thickness less than 200 nm has been manufactured by beating and used extensively for gilding sculptures, precious furniture and building exteriors for over 3000 years. Palladium (Pd) leaf is used as a gilding material for its silver color and excellent anti-corrosion properties. We choose Pd leaf as the metal matrix for two additional reasons: (i) its high carbon solubility at elevated temperatures (~ 150-fold higher than that of copper at 1000 C). This helps to achieve high carbon segregation flux for graphene nucleation. And (ii) its high binding energy with graphene layer (~ 2.5-hold higher than that of copper). Graphene-metal interaction gives rich insights of strengthening/toughening mechanisms of graphene in macroscopic composites. In this work, the Pd leaf catalyst is made by hammer beating of a micrometer thick foil. During this process, Pd thickness is reduced to 150 nm while the average grain size exceeds 20 μm. The Pd leaves made by high strain rate beating are stable at synthesis temperature up to 1100 C for 30 s, resisting solid state dewetting owing to their extremely low grain triple junctions density (~ 0.017 µm-1). Mathematical models of low pressure CVD synthesis kinetics on ultrathin metal catalysts guide the development of extremely rapid graphene synthesis conditions, resulting in the formation of high quality uniform graphene monolayer on thin Pd films in less than one minute. Graphene grains growth rate is twice as fast as copper-catalyzed growth. Uniaxial strain testing with Raman spectroscopy reveals the excellent crystallinity of graphene by probing the stress-induced phonon shifts.
Notable enhancement in mechanical properties is observed in the as-grown Pd-Gr thin film composites. The pristine Pd leaves can be readily transferred to a slotted transmission electron microscopy (TEM) grid, then shaped into doubly-clamped freestanding strips via focused ion beam (FIB). We dope the Pd strip surface with monolayer graphene via the proposed rapid CVD synthesis. Using nanoindentation with a wedge indenter on the Pd-Gr strips, we demonstrate the graphene’s ability to significantly stiffen the Pd leaves by over 116 %. Moreover, indentation on a central-cracked Pd-Gr strip enables the fracture toughness measurement of ultra-thin film materials. The Pd-Gr exhibits 289.5 J m-2 fracture energy, which is over 3.7-fold of the pristine Pd. This new graphene-metal composite material could open exciting opportunities in harnessing superb properties of 2D materials.