Abstract：Considering the defects of uneven distribution, easy oxidation and poor stability during the preparation of metal particles on SERS substrates, we have prepared graphene-silver nanoparticle (GE/AgNPs) composites with uniform distribution using thermal evaporation and high temperature annealing. At the same time, we have investigated their optical and Raman enhancement activities. The Raman spectrum stability test of GE/AgNPs composite structure proves that graphene plays a role in isolating oxygen and catalytic deoxygenation, which is beneficial to the time stability of SERS substrates. (1) The fabrication of graphene-Ag nanoparticles hybrid structure. Firstly, the Ag nanoparticles were uniformly deposited on the SiO2/Si substrate by thermal evaporation and high temperature annealing. Then, the graphene was prepared on the Cu foil by chemical vapor deposition. Finally, the graphene was transferred to the target substrate by a wet transfer method. And the effects of annealing sequence on GE/AgNPs substrates were investigated experimentally. (2) Characterization of graphene, Ag nanoparticles and GE/AgNPs composite substrate. In this paper, optical microscopy, scanning electron microscopy and Raman spectroscopy were used to characterize the properties of samples. The graphene after transfer was completely covered on the SiO2/Si substrate, with a flat surface, but in a few places still with wrinkles and impurities. According to the Ostwald ripening theory, silver particles with an average particle size of 40~60 nm could be obtained by controlling the annealing temperature and time, and the distribution was uniform. In addition, in different annealing sequences, graphene provided a diffusion barrier to the diffusion of silver nanoparticles, resulting in larger irregular particles. (3) Substrate stability test and simulation analysis. Through the Raman mapping test of the substrate itself, the Raman enhancement effect of graphene was mainly due to the enhancement of the electromagnetic field between the silver nanoparticles, and the changes in the peak and FWHM of the graphene Raman characteristic peaks were analyzed. The SERS stability of GE/AgNPs composites and Ag nanoparticle substrate were investigated using rhodamine 6G (R6G) solution with a concentration of 10-6 mol·L-1 as probe molecule. The results showed that the GE/AgNPs composite attenuated slowly from 1 to 33 days, and the Raman signal was still about 35.1%~40.6% of the original signal after 33 days. However, on the pure Ag substrate, nanoparticles oxidized in the airquickly, and the SERS performance decreased significantly, only 5.9%~11.3% after 33 days. In addition, the enhancement factor of the GE/AgNPs composite was about 6.05×105. And the finite difference time domain (FDTD) was used to calculate the electromagnetic field distribution and the theoretical enhancement factor of the composite structure was 5.7×105. The difference between experimental and simulation results was mainly due to the chemical enhancement of graphene.
Key words：Surface-enhanced Raman scattering (SERS); Graphene; Ag nanoparticles; High temperature annealing
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