| Surface-enhanced Raman scattering(SERS) was first discovered on a rough Ag electrode surface by Fleischmann in 1974. It has shown tremendous potential for many applications, especially in the fields of life science, food safety, environmental monitoring, military science, etc. With the development of nanophotonics and surface plasmons, the mechanism of SERS is constantly improving. Relying on the nanofabrication and laser technology, SERS substrates with high sensitivity and reproducibility can be fabricated, thus effectively solving the problems of traditional Raman spectrum in surface science and trace analysis, including weak Raman signal, low detection sensitivity, susceptiblity to the interference of fluorescence, etc. SERS technology has rapidly become one of the most sensitive detection techniques of in-situ spectrum of surface species.Graphene is a two-dimensional nanomaterial with a monolayer of carbon atoms. It has become a research hotspot in the fields of basic physics and advanced functional materials since it was successfully fabricated by Geim et al. using micromechanical cleavage in 2004. Graphene has such characteristics as high transmittance in the visible band, extremely low surface roughness and strong chemical inertness. It also has graphene-enhanced Raman scattering(GERS) and fluorescence quenching effect, which was first discovered by Xie et al. in 2009. Based on the characteristics above, graphene is widely used in the field of SERS.Research results indicate that the enhancement effects of the Raman spectra of molecules in SERS system mainly include electromagnetic(EM) and chemical(CM) enhancement among which EM enhancement is dominant. Common SERS substrates with rough surfaces of Au, Ag, Cu, etc. can generate strong EM enhancement, so that strong Raman scattering signals of molecules adsorbed on the surface of substrate can be excited. Unfortunately, noble metals can be easily oxidized under ambient condition, which could suppress the EM enhancement. Therefore the stability of SERS substrates is poor, which limits their long-term use in molecule detection. Graphene based SERS substrates can overcome the problem of the instability of noble metals, but its sensitivity is low as it only has CM enhancement. Thus it cannot be used in the detection of trace elements.The graphene-metal hybrid structure obtained by combining noble metals and graphene can effectively solve the problems of low stability and sensitivity on SERS substrates. But the fabrication process to obtain SERS substrates with high sensitivity and reproducibility is complex and the cost is high. Besides, the optical and SERS properties of hybrid structures are not clear. Therefore, we develop SERS substrates structured with hybrid Ag nanoparticles and graphene, to improve the enhancement factor, reproducibility, stability as well as to reduce the complexity of the fabrication process and cost. SERS substrates with five different structures, namely Ag nanoparticles(AgNPs), graphene(GE), Ag nanoparticles-graphene(AgNPs-GE), graphene-Ag nanoparticles(GE-AgNPs) and graphene-Ag nanoparticles-graphene(GE-AgNPs-GE) have been studied from the aspects of preparation method, morphologies characterization, optical properties, SERS properties and enhancement mechanism. The main research contents are listed below:(1) The preparation, deposition and optical properties of Ag nanoparticles have been studied. Ag nanoparticles were prepared by chemical reduction method which is simple, environmental-friendly and low-cost. A method of centrifugation, dissolution and ultrasound has been used to optimize the size and reduce the agglomeration of Ag nanoparticles. The coupling agent has been used to improve the uniformity of Ag nanoparticles adsorbed on the target substrates. Annealing treatment has been used to reduce the Raman background noise which is caused by the organic residues on the surface of Ag nanoparticles. The Raman intensities of Rhodamine 6G(R6G) at 1363 cm-1 with concentration of 10-8 mol/L increased by 0.291 after annealing treatment at 450 °C. The morphologies, distributions and optical properties of Ag nanoparticles have been studied experimentally. The optical properties of Ag nanoparticles in different size, media environment and excitation wavelength have been studied theoretically.(2) The preparation, transfer and optical properties of graphene have been studied. Few-layer graphene with large area was grown by chemical vapor deposition(CVD) and wet transfer based on polymethylmethacrylate(PMMA) was used to transfer graphene. The morphologies, layer number and optical properties of graphene have been studied experimentally. The influence of the temperature, carrier concentration/mobility and layer number on the optical properties of graphene have been studied theoretically.(3) The fabrication method, optical properties and SERS effects of the hybrid structures of Ag nanoparticles and graphene have been studied. The properties of surface plasmon resonance(SPR), Raman enhancement and Raman shift have been studied experimentally. The reproducibility of SERS substrates was evaluated by Raman mapping, and the values of relative standard deviation(RSD) in 7 × 7 ?m2 were all less than 13%. The enhancement mechanism of graphene Raman signal was studied by numerical simulations, which show that the main factor of the enhacement is EM enhancement. Charge transfer was happened in the Ag/graphene interface and the electric field is coupled into the graphene layer, and finally lead to the enhancement of graphene Raman signal. The Raman shift of graphene was quantitatively analysed using “vector decomposition method”.(4) The enhancement effect, stability, recyclability of hybrid structures of Ag nanoparticles and graphene on the application of probe molecules' detection have been studied. The experimental results indicate that the detection limit of SERS substrates structured with hybrid Ag nanoparticles and graphene(10-9~10-10 mol/L) is lower than that of AgNPs without graphene(>10-10 mol/L), but graphene can effectively protect Ag nanoparticles against aerobic oxidation, thus improving the stability of SERS substrates. Actually, the stability of GE-AgNPs-GE structure is the best(the Raman intensity decreased by only 32.41% at 1363 cm-1 after 60-day of exposure, compared to a 92.58% decrease in the substrate of AgNPs). The SERS substrates can be reused after being washed with sodium borohydride(NaBH4) solution, but the Raman enhancement decreased by 20%~30%. The influences of size, interparticle gap and combination on the EM enhancement have been analyzed by theoretical simulations. The EM enhancement properties of SERS substrates with hybrid structures have also been studied. |
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