退火法制备石墨烯-银纳米粒子及其增强拉曼实验

针对目前SERS基底上金属颗粒制备过程中存在的分布不均匀、易氧化和稳定性差等缺点，通过热蒸镀和高温退火获得分布均匀的SERS基底；同时结合石墨烯优良的光学性能、化学惰性、荧光猝灭以及本身的SERS增强等优点，制备了稳定的石墨烯-银纳米颗粒（GE/AgNPs）复合结构SERS基底。通过GE/AgNPs复合结构的拉曼光谱稳定性试验证明了石墨烯在GE/AgNPs结构中起到隔绝银纳米颗粒与空气直接接触及催化氧化银脱氧的作用，有利于SERS基底的时间稳定性。(1) 石墨烯、Ag纳米颗粒及其复合结构的制备。首先采用热蒸镀和高温退火的方法使Ag纳米颗粒均匀地沉积在SiO₂/Si基底上，再采用化学气相沉积法在Cu箔上制备少层石墨烯，并用湿法转移法将石墨烯转移到目标基底上，并实验研究了以不同的退火顺序对GE/AgNPs基底造成的影响。(2) 石墨烯、Ag纳米颗粒及其复合基底的表征。分别采用光学显微镜、扫描电子显微镜和拉曼光谱进行表征，得到转移后的纯石墨烯较完整地覆盖在SiO₂/Si基底上面，表面比较平整，但在少数地方仍然存在褶皱和杂质；SEM表征结果表明对于不同制备流程的GE/AgNPs复合结构上的Ag纳米颗粒基本呈球形。基本符合Ostwald熟化理论，通过对退火温度和时间的控制能获得平均粒径在40~60 nm的银颗粒，且分布较均匀。此外，在不同退火顺序中，石墨烯的加入对银纳米颗粒的扩散形成扩散势垒，从而出现较大的不规则的颗粒。(3) 基底稳定性试验和仿真分析。通过基底本身的Raman mapping测试，分析了石墨烯拉曼特征峰峰值和半高宽的变化，得知基底对石墨烯本身的拉曼增强效果主要来源于银纳米颗粒间的电磁场增强。同时采用浓度为10-6 mol·L-1的罗丹明6G (R6G)水溶液作为探针分子，对转移了石墨烯的GE/AgNPs复合基底和未转移石墨烯的Ag纳米颗粒基底进行了SERS稳定性实验。结果表明GE/AgNPs复合基底在1~33 d内衰减较缓慢，30 d后仍能探测到拉曼信号约为原来信号的35.1%~40.6%；而纯Ag基底上随着Ag纳米颗粒在空气中迅速氧化，基底的SERS性能显著下降，在30 d后只有原来信号的5.9%~11.3%。此外，通过实验得到覆盖了石墨烯之后的增强因子约为6.05×105。最后采用时域有限差分算法（FDTD）计算了复合结构的电磁场分布和理论增强因子，其理论增强因子可以达到5.7×105。实验和仿真结果的差异，主要是源于石墨烯的化学增强作用。

Enhanced Raman Experiments of Graphene-Ag Nanoparticles Prepared with Annealing Method

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.