Quenching Synthesis,Properties And Applications Of Nanocrystalline Graphene

Graphene,a two-dimensional honeycomb lattice of carbon atoms,has attracted great interest because of its giant carrier mobility,high transparency,record thermal conductivity,extremely high mechanical strength,and large specific surface area.Unlike the perfect hexagonal structure of the interior of a grain,grain boundary(GB)of graphene is composed of pentagonal,heptagonal and distorted hexagonal rings of carbon atoms,which break the crystal periodicity of graphene and affect its electrical,thermal and mechanical properties.It has been widely demonstrated that nanocrystallization makes materials possess many unusual properties that are absent in their monocrystalline and microcrystalline counterparts.Therefore,understanding the relation between the grain size and properties of graphene at nanometer scale and investigating the novel properties of nanocrystalline graphene(NG)are essentially important not only for fundamental studies but also for technological applications.However,it remains a great challenge to synthesize high-quality NG films.This dissertation studied the preparation of NG films,investigated their electrical and mechanical properties,and explored their application as Raman enhancement substrate.The main results are shown as follows:We developed a novel quenching method for ultrafast synthesis of graphene films within several seconds by rapidly immersing a high-temperature metal foil into liquid carbon source at room temperature.The growth rate is significantly faster than that of the traditional chemical vapor chemical vapor deposition methods,which usually require several minutes even hours to synthesize a film.Moreover,the grain size and number of layers of graphene synthesized by quenching method can be tailored by controlling the chemical composition,thickness and onset temperature of the metal substrate.With onset temperature of 1050 ℃ and 900 ℃,respectively,monolayer NG films with average grain size of 12 nm and 4 nm were prepared by using Pt as substrate and ethanol as carbon source.Both NG samples show well-stitched GBs and high crystallinity within the grains.We systematically studied the electrical and mechanical properties of NG.It was found that both electrical conductivity and mechanical properties are degenerated with decreasing the grain size.The fracture strength shows a pseudo inverse Hall-Petch relation with the grain size.The resistance of NG is unexpectedly large due to the strong coupling between neighboring GBs.In comparison with single-crystal graphene,the strength and Young’ s modulus of 4-nm-grained NG film are decreased by～50%,while the electrical conductivity is remarkably declined by over two orders of magnitude.Moreover,4-nm-grained NG film shows a semiconducting behavior with a bandgap of～50 meV.We further investigated the application of NG as Raman enhancement substrate,and found that NG has better enhancement performance than the defective graphene grown on Au and pristine graphene grown on Cu.