Preparation Of Surface-Enhanced Raman Scattering Substrates By Metallic Thin Films Deposition On Liquid Surfaces

Metallic thin films are fabricated on silicone oil surfaces by thermal evaporation at room temperature.Gold(Au)and silver(Ag)films are prepared by the deposition of micrometer-sized ramified aggregates,which are constituted of closely packed Ag/Au nanoclusters,with the intervals of less than 10 nm.These films can be used as surface-enhanced Raman scattering(SERS)substrates on account of the unique structures.Due to the abundant existence of“hot spots”,probe molecules can be detected in a very low concentration with high repeatability.This simple and stable method of SERS substrates fabrication provides a potential application in pollutants detection and biological disease diagnosis.The main research contents are as follows:(1)Au/Ag/films with different nominal thicknesses are prepared on liquid surfaces by a thermal evaporation method.The morphology observed by optical microscopy show that the growth of films followed the two-stage growth model.With the increase of nominal thickness,the coverage of the film surface increased continuously,and the morphology gradually transfered from ramified aggregates consists of Au/Ag nanoclusters to continuous films.The microscopic morphology of the film is captured by scanning electron microscopy(SEM),which further exhibited that the Au/Ag branched aggregates are constituted by closely-arranged nanometer-sized Au/Ag nanoparticles.The size of Au nanoparticles is mainly distributed within 12~34 nm,with the mean size of about 20 nm.With the increase of deposition thickness,the mean size of Au nanoparticles decreased slightly.On the other hand,Ag nanoparticles mainly sized within 40~100 nm,with the mean size of 60 nm.(2)The prepared Au/Ag films are used as substrates for SERS measurement to detect the Rhodamine 6G(R6G)probe molecule with various concentrations.The results show that the Au/Ag films prepared on liquid surfaces serve as effective substrates for SERS measurement,and the limit of detection can reach 10~-88 M(Au)and10~-77 M(Ag),respectively.With the increase of the nominal thickness of Au film,the Raman signal is weakened gradually.By analyzing the mapping image of the SERS measurement of Au film,it is indicated that these as-prepared substrates possess high uniformity and stability for SERS detection.(3)Finite-difference Time-domain(FDTD)method is utilized to theoretically analyze the mechanism of Raman signal enhancement.Firstly,a dimer model was established to explore the relationship between the electric field enhancement and the size and separation of nanoparticles.The simulation results show that the smaller the separation of gold or silver dimer,the larger the electric field enhancement will be stimulated.When the separation distance is fixed,the electric field enhancement first increases and then decreases with the increasing size of nanoparticles.The electric field enhancement possesses an optimal particle size,and the optimal size is offset to a larger size with the increasing of particle spacing.The position and size information of Au substrates are mapped to the simulation software to build a FDTD model.The simulation results demonstrate that there is a significant enhancement of the electric field at the particle gap position,which verified the experimental results of SERS measurement.