Adjustment Of The LSPR Properties Of Silver Nanostructures And Their Graphene-based Nanohybrids And Applications On SERS

Silver nanostructures are important members of plasmonic materials.They possess distinct surface plasmon resonance(SPR)properties in the range of visible and near infrared waveband.The SPR properties of silver nanostructures can be tuned by adjusting their size and shape.Various methods have been developed to control the size and shape of the silver nanostructures,and thus to tune their SPR properties.Photochemical method has been accepted as an available method to tune the SPR properties of silver nanostructures.By adjusting parameters such as excitation wavelength,light intensity,reactant ratio,type of stabilizer,silver nanostructures with different size and shape,as well as different SPR properties can be synthesized.However,rare reports can be found on tune the SPR properties of silver nanostructures by methods such as etching and photoinduced reconstruction and multi-light photochemical growth process.An intense localized electric field(LEF)can be excited around the silver nanostructure when irradiated by the incident light.Both incident light and outgoing optical signals can be enhanced in the range of LEF,which gives birth to the surface-enhanced spectroscopy,such as surface-enhanced Raman scattering(SERS).In this case,normal Raman scattering of the probing molecules can be hugely promoted when they get close to or adsorb directly on the surface of silver nanostructures.However,silver nanostructures always suffered from oxidation,surfuration,or contamination during their usage and storage,which resulted in the decrease of their SERS activities.Moreover,photo-induced damage of the molecules adsorbed directly on the surface of silver nanostructure may mislead the spectral detection results.These problems restrict the development of SERS.One way to solve these problems is to encapsulate the Ag substrate with an inert shell.However,encapsulation always result in a decrease of SERS activities of silver nanostructures.Therefore,how to balance the SERS activities and stabilities of silver nanostructures would be an important research direction in the field of SERS.Based on the aforementioned viewpoints,several researches were carried out in this thesis and are listed as follows:1.Modulation of the shape and their localized surface plasmon resonance(LSPR)properties of silver nanoparticles(Ag NPs)were realized by halide ion etching and subsequent photochemical regrowth.Silver nanotriangles were chemically etched into nanodisks,and were then transformed into nanohexagons,truncated nanotriangles,and nanotriangles with sharp corners by photochemical regrowth process.By changing the excitation wavelength applied in the photochemical regrowth process,the LSPR properties of Ag NPs were tuned in the range from 490 to 800 nm.2.Photochemical growth of silver nanoparticles under the irradiation of mixed light was examined.Monodispersed silver triangular nanoplates were synthesized by mixed light irradiation.By tuning the relative intensity of excitation wavelengths in the mixed light,the localized surface plasmon resonance(LSPR)properties of the silver nanoparticles were modulated and controlled.A cooperative effect of excitation wavelengths on regulating the growth of silver nanoparticles was outlined.3.Ultrathin carbon film protected silver substrate(Ag/C)was prepared via a plasma-enhanced chemical vapor deposition(PECVD)method.The morphological evolution of silver nanostructures underneath,as well as the surface-enhanced Raman scattering(SERS)activity of Ag/C hybrid can be tuned by controlling the deposition time.The stability and reproducibility of the as-prepared hybrid were studied as well.This method can be also applied to other metallic SERS substrates,such as copper and gold.4.Graphene protected silver nanostructures with tunable LSPR properties were synthesized by a controlled annealing process.Two annealing strategies were employed and compared.In the first strategy,silver substrate was annealing treated at 200 oC to tune its SPR property,and was then covered by graphene film and underwent another annealing treatment at 100 oC to form the hybrid structure.In the second strategy,graphene film was covered on top of original silver substrate and was annealing treated at 200 oC to form hybrid structure.The results displayed that the graphene covered on top can influence the annealing behavior of silver substrate underneath.The LSPR properties of silver substrate can be reserved under moderate annealing temperature in the presence of graphene on top.The hybrid structure prepared by the first strategy displayed higher SERS activity due to overlapping of its LSPR peak position and laser line,and the incorporation of electromagnetic enhancement of silver substrate underneath and chemical enhancement of graphene on top.This annealing process can be employed to other nanostructures as well.In conclusion,modulation of the SPR properties of silver nanostructures were realized by methods such as halide ion etching and subsequent photochemical regrowth,and multi-light induced photochemical growth process.Silver/graphene hybrid nanostructures with tunable SPR properties were fabricated by direct deposition process and controlled annealing process.These hybrids displayed promoted SERS activities,stabilities and reproducibilities.The results shown in this thesis may pave ways for the control over the SPR properties of metal nanostructures and applications in the field of plasmon-enhanced spectroscopy.