Research On Fluorescence Enhancement And SERS Induced By Plasmonic Coupling Between Ag Nano-cubes And Ag Film

The coupling in the metallic nano-structures and its applications have aroused wide interesting and investigations. Now, the study of coupling among surface plasmon polaiton (SPP) has emerged as an active and important area of research. Two kinds of coupling structure are proposed in this thesis made of Ag nano-cubes and Ag film.One is the coupling between the localized surface plasmons (LSP) and LSP(LSP- LSP) of Ag nano-cubes and the other is the coupling between the LSP and the SPP ( LSP- SPP) based on Ag nano-cubes and Ag film. At first, we numerical analyze the optical properties of Ag nano-cubes, such as, the influence of size and dielectric environment on the characteristic of electric field distribution of the coupling structures respectively. Secondly, we experimental study the enhanced optical property (enhanced fluorescence and SERS) of the coupling structures through the fluorescence and Raman spectrum. At last, we investigate the physical mechanism of these optical phenomena. Our work has potential significance to develop low cost, easy to manufacture and highly sensitive sensor or optical chip, which can be used in the fluorescence imaging, detection of biological molecules, and detection of environmental pollutants and so on etc.The details of this thesis are listed following:1. We successfully synthesize the single crystal of Ag nano-cubes with the character of regular morphology, uniform size and good monodispersibility by the chemical oxidation and reduction methods. The side size of Ag nano-cube is about 96 nm. We also prepare the Ag film with uniform surface and controllable thickness.2. According to the coupling structure, we numerically study the spectroscopy characteristics and electric field distribution of a single Ag nano-cube with DDA(Discret dipole time domain) and electric field distribution of transverse or vertical coupling between Ag nano-cubes with FEM (Finite element method) respectively. The electric field distribution of Ag nano-cubes-Ag film coupling structure is studied with FDTD (Finite difference time domain) method. Specially we have investigate the influence of the spacer between the Ag nano-cubes and Ag film. Numerical calculation results show that the thickness of spacer between Ag nano-cube and Ag film is critical to on the electric distribution of the coupling structure. The electric distribution and surface plasmon resonance (SPR) of the coupling structure could be tuned through controlling the thickness of the spacer. All the numerical investigation provide theoretical guidance for the following experiments.3. We experimentally investigate the plasmonics enhanced fluorescence based on the Ag nano-cubes and its coupling structures or its coupling with Ag film. At first, we study the fluorescence intensity change with the distance between the Ag nano-cubes and the fluorescence molecules.Secondly, we use the transverse or vertical LSP-LSP coupling to enhance fluorescence of dye molecules Rhodamine B (RhB) doped in PMMA film. The fluorescence enhancement factor (EF) is 56 for the Ag nano-cubes transverse coupling structure and 60 for the Ag nano-cubes vertical coupling structure respectively. For the Ag nano-cubes-Ag film coupling structure, experimental results show that the EF is sensitive to the thickness of the spacer and Ag film. The largest enhancement factor obtained is 900 with the RhB doped PMMA film of 10nm thickness and 78nm thick Ag film.4. Based on the SPP coupling-enhanced optical effects, we have performed the SERS experiments through designing and optimizing the coupling structure. The LSP-SPP substrate based on Ag nano-cubes and Ag film can produce very strong Raman signals because of their strong coupling effect and the Raman signal can be detected from R6G molecules with solution concentration down to 10-13M. The SERS enhancement factor for the Raman band 1650cm-1 of R6G is about 109. The Ag nano-cubes -Ag film substrate can also be used to detect other molecules such as 4-ATP. It can detect the 4-ATP molecules with concentrations of 10-12M. The results show that the coupling structure can be used for simple, low cost and widely useful Raman substrates.The innovations of this thesis are shown as following:1. Two kinds of coupling structure are proposed based on the coupling effects among Ag nano-cubes and Ag film. The selected Ag nano-cubes have regular shape with 8 sharp corners and 12 edges. Because of the"hot"effect, the electromagnetic field can be focalized at the corners or the edges which can produce very large enhanced localized electromagnetic field. The regular planar structure is conducive to form a monodisperse and uniform layer on glass or Ag film substrate.2. The spacer between the metallic nano-particles and film has important influence on the coupling property among the LSP and SPP. We use the PMMA as the spacer, which is easy to form a layer of good quality and its'thickness is easy to be controlled the optical characteristics of the coupling structures can be uned simply by change the thickness of the PMMA. The fluorescence enhanced factor is 60 for the LSP- LSP structure and 900 for the LSP- SPP structure respectively.3. We take advantage of the adsorption of R6G molecules on Ag nano-cubes, the surface enhanced Raman (SERS) substrate is designed successfully with the coupling structure. The Raman signals from the probe molecule of R6G are enhanced effectively. The lowest detectable concentration of R6G was 10-13M.