Fabrication Of Ag Nanostructures And Their Application In SERS

When molecules adsorb on special metal surface, their Raman intensities can be increased by a factor of 104~106 compared with their normal spectra observed. This phenomenon is generally known as surface enhanced Raman scattering(SERS). Due to its high sensitivity and ability to provide useful vibration information, SERS spectroscopy has been extensively used in chemistry, biology systems and etc. The microscopic structures of SERS substrates are commonly on the nanometer scale which can produce huge enhancement. Up to now, it has been well understood that SERS is tightly relative with nanotechnology, and the enhancement ability of the substrate could be improved through optimizing the configuration of the nanoscale structures. The commonly used SERS substrates, such as electrochemical ORC electrode, metallic colloidal solution, chemically deposited films, and acid etched metal films, are often dissatisfactory in reproducibility and uniformity. Consequently, extending substrate materials and preparing new SERS substrates with high reproducibility and uniformity have been the crucial aspect in the development of SERS. Our study is outlined as follows:Firstly, we devoted our attempt to extending SERS substrates to silver oxide species because of their applicability in various fields such as zinc-silver oxide batteries, heterogeneous catalysis, and high performance materials science. Silver oxide film was electrochemically deposited on the ITO electrode from a silver ammonia solution. SEM, XRD, XPS, and Raman spectroscopy were used to characterize the as-deposited film. The oxide was determined to be AgO. The SERS spectrum of 4-mercaptobenzoic acid was recorded on the AgO substrate. SEM image of the focus spot suggested metallic Ag generates by photolysis of AgO. The development of Raman spectra of 4-mercaptobenzoic acid with the irradiation time of laser indicates that AgO was gradually decomposed under this condition. SERS spectra of 4-aminothiophenol and benzoic acid were also obtained on the AgO substrate. Through analyzing SERS spectra, we have known some information of above molecules which contains adsorption mode, orientation behavior and enhaced mechanism. Enhanced factor of 4-mercaptopyridine was estimated to be 1×103 and the influencing factors were evaluated.Secondly, we focused on developing methods for the preparation of ordered template, at the same time, we also investigated their application in the fabrication of nanostructures and the application of these structures in SERS.We developed a LB-based method to fabricate large area monolayer of polystyrene particle. Polystyrene ( PS ) latex with different diameters was synthesized by emulsion polymerization. PS latex was first dried at a constant temperature and the obtained solid was re-dispersed in the mixture of ethanol and water(1:1, v/v). A film was obtained when we spreaded the PS dispersion on the surface of water. The film on the surface of water was transferred onto a support by the Langmuir-Blodgett technique. SEM image indicated that the film was the monolayer of PS particle. Uv-vis spectroscopy was also used to character the film. To evaluate the surface wettability of the PS particle, a binary colloidal monolayer consisted of both large and small particles were fabricated by the LB technique. The surface wettability was determined by observing the position of the small particles with respect to the large ones.The morphology of the monolayer of PS particles could be adjusted from random packing to hexagonally closed packing under different transfer pressure. Moreover, controlled crack can be introduced into the monolayer under suitable pressure. Porous PS film was fabricated by sintering the monolayer at temperature above its glass-transition temperature(Tg). The shape of the pore was adjusted from strip-like to the circular with increasing the sintering time. To fabricate arrays of Ag nanowire, Ag film was first evaporated onto the PS film with the strip-like pores. The Ag nanowire arrays were obtained after removing the template.Arrays of triangle Ag nanoparticles were fabricated by using monolayer templates with different diameters. SERS spectra of R6G were obtained on the arrays of Ag nanoparticles. The SERS spectrum of R6G recorded on the array of 600 nm PS particle templated-Ag nanoparticle was quantitatively compared with the one recorded on the Ag island. The extra enhancement was attributed to the contribution of the particle shape.We have prepared highly ordered porous anodic alumina(PAA)through the two-step oxidation. The anodic oxidation was curried out on both surfaces of the aluminum sheet. The PAA membrane was detached by a two-step electrochemical method. The corrosion behavior of PAA in H3PO4 was studied and we also observed the sequent occurrence of the pore widening, the touch of pores, the dissolution of pore wall and the appearance of Al2O3 nanaowires.The Ag-PAA substrate was fabricated by a simple chemical deposition process. Ag nanoparticles were produced by reducing Ag(NH3) +2 with formaldehyde. FE-SEM images showed that the density and size of Ag nanoparticles can be adjusted through a deposition cycle including two independent steps and a uniformly dispersed Ag nanoparticles-PAA surface can be obtained after the optimum cycle number. The suggested method can also achieve the deposition of Ag nanoparticles merely inside the pores on the PAA surface without the deposition deep into the pores of the template. The extinction efficiency of Ag-PAA substrate was increasingly augmented with the increase of the density and size of Ag naoparticles. SERS spectra and SERS mapping of 4-ATP obtained on the optimal substrate showed a highly enhanced Raman spectra and good homogeneity.By combining PAA template with magnetron sputtering technique, we fabricated Ag nanopillar and nanodot inside the pores of PAA template. On the one hand, PAA templates with different pore diameters were used for the preparation of Ag nanodot or nanopillar. On the other hand, through changing the angle between PAA plane and the sample holder, the length of Ag nanopillars was changed. SERS spectra of 4-aminothiophenol were obtained on the nanostructured Ag substrate and the influencing factor was discussed.