A Study On Ag Nanoparticles And Graphene Hybrid Structures As Surface-Enhanced Raman Scattering Substrates

Surface-enhanced Raman scattering（SERS）is highly sensitive to the interaction of molecules and the surface of metal nanoparticles.This analytical approach has overcame the shortcomings of traditional Raman scattering spectroscopy: low sensitivity,susceptibility to interference fluorescence background.SERS was proved to show broad applications in different fields,including chemistry,food safety and medical diagnostics due to its accuracy and operability.The former study suggested that some metals such as gold（Au）,silver（Ag）,copper（Cu）in the SERS substrate exhibited good Raman enhancement effect,but their oxidation in air,greatly reduced the Raman enhancement effect.As a semi-metallic material,graphene also has a Raman enhancement effect,but with low intensity and sensitivity is poor.Under certain kind of conditions,graphene and metal nanoparticles can form into composite materials with good stability.This hybrid material have attracted massive attention in SERS substrate study.Based on the properties of graphene and noble metals as well as on the preparation requirements of SERS substrate,we fabricated the graphene-Ag nanoparticles hybrid structures.The preparation method,characterization and SERS enhancement mechanism were studied,in order to develop a new SERS substrate with large enhancement factor.The dissertation mainly includes:（1）The fabrication of graphene-Ag nanoparticles hybrid structures.Ag nanoparticles with good uniformity were thermal deposited on silicon wafer,which has a good uniformity.Large-area graphene was prepared via chemical vapor deposition（CVD）,and the transferred through wet transfer method with protective medium（PMMA）.（2）The optimization of graphene-Ag nanoparticles hybrid structures.The morphologies and distributions of graphene and Ag nanoparticles on the substrates were observed with optical microscope and SEM.To obtain spherical nanoparticles with better uniformity,the time and temperature during annealing were altered.By changing the order of annealing,different SERS substrates with various structures were prepared.（3）The Raman enhancement of graphene-Ag nanoparticles hybrid structures.The Raman mapping of three different structures of graphene-Ag nanoparticles were done,which showed that the substrates had good uniformity.Simulation showed that charge transfer occurred between the particles and the graphene contact surface,leading to the n-type doping of graphene;the electromagnetic enhancement of the Ag nanoparticles helped to achieve Raman enhancement in the whole hybrid structures.By using "vector decomposition method",the main reasons（strain and doping）for the peak shift of each sample were analyzed.After 33 days of substrates fabrication,the Raman signal intensity of graphene-protected substrate was 40.6% of the new prepared-sample(at 614 cm^-1).This value fell down to 5.9% for substrate without the cover of graphene;This result shows that the graphene could protect Ag nanoparticles by slowing down the oxidation rate of Ag nanoparticles and thus may improve the stability of the substrate.