Study Of Various CVD Deposited Sp~3/sp~2-carbon Complex Films As Sensitive SERS Substrate

Surface-enhanced Raman scattering(SERS)technique exhibits promising applications in the fields of environmental pollutant detection and biomedical analysis owing to its characteristics of high sensitivity,non-destructive detection and rich structure information.The manipulation of the materials and structure of the SERS substrates is one of the major keys to enhance the SERS performance.For noble metal nanoparticles,strong enhancement of Raman signals of probe molecule adsorbed on noble metal nanomaterials can be achiveved based on the localized surface plasmon resonance effect.However,the instability of the metal materials(surface oxidation and particle agglomeration)limits practical application.Semiconductor materials can be employed for SERS detection in extreme environments due to their good stability and biocompatibility.The SERS enhancement of semiconductor materials mainly derives from the charge transfer process between substrates and probe molecules.However,its enhancement is weak for trace analysis.In this paper,carbon materials with different structures and compositions were prepared by microwave plasma chemical vapor deposition(MPCVD).Also,Au nanoparticles are decorated onto the surface of the carbon materials to enhance sensitivity of the SERS substrate.The SERS performance of the above carbon complicated materials are evaluated by R6G molecules in detail.The main research and conclusions of this paper are as follows:1.Surface modifications of Si nanowires,without the presence of noble metals,were carried out to develop biocompatible SERS substrates.The treatment of hydrogen plasma on as-prepared Si nanowires leads to no enhanced Raman signals for the absorbed R6G molecules.The coatings of nanocrystalline diamond and multilayer graphene enable Raman signals to be enhanced with reduced fluorescent background.The graphene-coated sample exhibits a much lower fluorescent background than the diamond-coated sample at a high concentration of the dye molecules.For the sample with graphene-coated Si nanowires,the minimum detection limit is 10-7 mol/L,and the enhancement factor reaches above 104.Importantly,the Raman enhancement is independent of the number of graphene layers.The Raman signal remains stable after exposure to the atmosphere for one month.2.Tailoring the sp3/sp2 phase compositions and chemical termination in the diamond films were carried out to improve the SERS properties.At higher deposition temperature,the sp2 carbon in nanocrystalline diamond films is evolved into the crystalline graphene nanosheets enclosing the diamond nanosheets,and forming a porous diamond/multilayer graphene nanosheet(D/G)films.The average thicknesses of the diamond and graphite nanosheets are about 5 nm and 6.5 nm,respectively.When the D/G samples are surface modified with different methods,it is found that the sample treated by the modified Hummers method further increases the Raman signals of the probe molecules with a detection limit of 10-8 mol/L and an enhancement factor of 7×106.The Raman and XPS spectra reveal that such SERS enhancement is attributed to the C=O groups formed on the conductive graphite and the presence of the porous structure.Additionally,the oxidized D/G films exhibit excellent SERS stability,recyclability and high mechanical properties.3.Au nanoparticles are decorated on the diamond/multilayer graphene film to improve the SERS performance.The size and spatial distribution of Au nanoparticles on diamond/multilayer graphene nanosheet(D/G)films was optimized by the evaporation and subsequent annealing parameters.For the D/G films,the Au nanoparticles have a discrete distribution.The particle size increases and the particle gap decreases with the increase of the film thickness.In addition,the gold nanoparticles changed from discrete Au nanoparticles to nanospheres with uniform size distribution(28～33 nm)after annealing at high temperatures.The Au-D/G sample annealed at 800 ℃increases the Raman signals of the R6G molecules with a detection limit of 10-13 mol/L.Such enhancement of SERS performance is attributed to both the mechanisms of the charge transfer and localized surface plasmon resonance.The π-π interactions between the R6G molecules and graphene increase the charge transfer efficiency.