Study On Preparation And Surface Enhanced Raman Scattering Effect Of Bilayer Graphene

Recently, study on two-dimensional materials such as graphene is a hot topic in the condensed matter physics field. Graphene is constructed by sp2 hybridized carbon atoms with perfect hexagonal structure arranged in a honeycomb lattice, which exhibits lots of unique physical properties, i.e. excellent electrical transport properties with mobility of～2000000 cm2·V-1·s-1, ultrahigh optical transmittance of visible light (97.7%), large ultimate tensile strength (42 N·m-1), very high thermal conductivity (5x103 W·m-1·K-1) and large surface area (2630 m3·g-1). Therefore, it has emerged widely potential applications in the fields of photoelectric devices, flexible transparent electrodes, supercapacitor devices, etc. In 2004, Geim, A. K. et al. firstly prepared mono-and few-layer graphene by mechanical exfoliation from highly oriented pyrolytic graphite. This approch can produce the graphene with best quality, however, its size is only a few tens of micrometres. So, it becomes a big challenge to prepare large-scale graphene in graphene research.In the past few years, some methods have been developed to prepare large-scale graphene. Among them, chemical vapor deposition is regarded as a promising approach for the preparation of large-scale graphene. Based on low pressure chemical vapor deposition using CH4 as carbon source, one has already prepared monolayer graphene with large-scale and high-quality on Cu foils, whose growth is followed by a "self-limited" growth mechanism. In spite of excellent electrical transprot properties, the zero bandgap of the monolayer graphene hinders its actual applications. In contrast, bilayer graphene is now drawing great interesting due to its tunable band structure. Its bandgap can be tuned up to 250 meV by applying an external electric field. Therefore, scalable preparation of bilayer graphene with high-quality is important for its applications. In addition, the bilayer graphene can be used as a favorable platform for surface-enhanced Raman scattering (SERS), which is poential for ultrasensitive analysis with molecular selectivity. How to imporve the SERS of graphene is important for its applications.In this thesis, we mainly study the preparation of poly- or single-crystal bilayer graphene and the enhancement of Raman scattering effect. Based on the structural characterizations, electrically and optically measurements, we explore the mechanism of growth and SERS of the graphene. The main contents and results of this thesis are presented as follows.We develop a nonisothermal atmospheric pressure chemical vapor deposition method to break down the "self-limited" growth mechanism. By using this method, we build a nonequilibrium steady state in the gas phase along the furnace tube, and succefully prepare large-scale high-quality AB-stacked bilayer graphene polycrystal films. We characterize the growth features with Raman spectra, SEM, TEM, etc. The effects of the cooling rate, CH4 flow rate and growth temperature are systematically investigated to seek the growth law of the bilayer graphene. The results show that slower cooling rate and higher CH4 flow rate will cause the production of multilayer domain, while fast cooling rate and lower growth temperature lead no growth of graphene. Accordingly, we establish a phase diagram for the growth of bilayer graphene. According to this phase diagram, we obtained the optimized parameters for preparing the large-scale and high-quality bilayer graphene. In addition, we find that bilayer graphene prefers to stack deviating from the AB-stacked geometry instead of the "inverted wedding cake" geometry. Accordingly, we propose the growth mechanism which is related to the surface catalysis and seed growth.We develop a novel method to control the bilayer graphene nucleation density under the atmospheric pressure chemical vapor deposition (CVD) condition, realizing the growth of single crystal bilayer graphene with a laterial size up to 1.3 mm, which is the largest so far. We perform an integrated strategy, i.e. pre-annealing the copper foil in pure argon environment and then making thermal treatment in H2 environment for a long time, to dramatically reduce the nucleation density of graphene domains during CVD growth. Using this method, the growth pattern like "inverted wedding cake" can be avoided effectively, and both upper and lower layers of the bilayer graphene grow simultaneously. We study the effects of the cooling rate and CH4 flow rate on the growth of bilayer graphene, and find that slower cooling rate and CH4 flow rate can decrease the size of the single-crystal. The measurement results by SEM, TEM, SAED and Raman spectra confirm that the as-grown bilayer graphene is synchronous growth, independent of the top layer of graphene. Finally, we proposed a new growth mechanism, namely, a "drawer" growth model from the surface adsorption-diffusion combining with bulk phase segregation.We prepare two kinds of composite films consisting of bilayer graphene and Ag nanoparticles (NPs) on the SiO2/Si wafers, and obtain great enhancement of surface-enhanced Raman scattering (SERS) in the composite films. The Ag NPs are prepared using a polyol process. We investigate the effects of the structures and annealing time on the SERS of the composite films. We find that, in the compsite film (Sample B) where the Ag-NPs is on the surface of graphene, the Ag-NPs are easy to slide during annealing, causing a large amount of Ag-NPs to aggregate on the surface of graphene and form bigger polymers. Although Sample B gains a 67-fold enhancement for the G peak, its 2D peak dissappears completely. The reason lie in the fact that during annealing a large amount of structural defects are induced into the graphene and give rise to a great change in two-phonon nonelastic scattering around the 2D peak. Differently, for the composite film (Sample A) in which the Ag-NPs are underneath the graphene, it keeps prefect structure and obtains enhancement factors up to 49-fold for the G peak and 21-fold for the 2D peak. We also perform electrical measurements and find the charge transfer from Ag-NPs to graphene. The subsequent absorption spectra measurements show a red-shift and a slight broadening for the plasmonic resonance peak of the Ag-NPs, indicative of strong interaction between the Ag-NPs and graphene. Accordingly, we propose a possible mechanism of Raman enhancement for the present composite films, i.e. a joint effect from the charge transfer and electromagnetic coupling between the Ag-NPs and graphene, which is closely correlated with the near-field plasmon coupling of Ag-NPs combining with the graphene.