Study On The Optical Properties Of Noble Metal Nanostructures And Enhancement Of Optical Effects Utilizing Surface Plasmons

Surface plasmons can be applied to biosensors, imaging, solar cells, and optoelectronic devices in nanoscale due to their distinct characteristcs, such as surface bounded effect and localized near-field enhancement. The spectral properties of metal nanostructures, especially the noble metal nanostructures, can actually reflect the optical properties of surface plasmons localized on them. Therefore, the research on optical properties of noble metal nanostructures is of great interest.This thesis provides a comprehensive review on the history, characteristics and application of surface plasmons, along with the research progress of the optical properties of the noble metal nanostructures. Investigations are carried out on optical properties and application of silver and gold nanostructures of novelty fabricated by electron beam lithography. A series of important conclusions and innovative results were obtained.We report on the instability of silver nanoblocks under atmospheric conditions. The localized surface plasmon resonance band of the silver nanoblocks shows a red shift, broadening, and damping with increasing storage time in air. The spectral change of silver nanoblocks is considered to be due to sulfidation of silver and structural breakage of silver nanoblock based on scanning electron microscopy observation and numerical simulation. The effect of aspect ratio of silver nanoblocks on the spectral change of the nanoengineered silver blocks is also discussed.In order to solve the instable problem of silver nanostructures, we designed and fabricated Ag/Au bilayer nanoblocks. The gold layer is on the top of the silver layer. The spectral properties of the bilayer nanostructures were studied. Compared with Ag monolayer nanoblocks, Ag/Au bilayer nanorods show broader localized surface plasmon resonance band. In addition, the longitudinal mode and transverse mode localized surface plasmon bands show blue-shift and red-shift, respectively. The maximum near-field intensity enhancement of longitudinal mode of the Ag/Au nanoblocks is smaller as half as that of the Ag monolayer nanoblocks. Shape-induced modification of Ag/Au bilayer nanorods on the spectral properties was also discussed.As the gold is expensive, we coated the titanium, which is cheaper than gold and own excellent resistance to corrosion, on the top of silver instead of gold. Apart from the optical properties of the bilayer nanorods, we further investigated the influence of the titanium thickness on the protective action. We found that silver nanoblocks with titanium coating on the top did not show apparent spectral and morphological change even after storage for 2-3 months under atmospheric conditions. It clearly demonstrates that coating titanium on the top of the silver nanostructures is an effective way to prevent them and stabilize their plasmonic spectral properties.We also studied the optical properties of gold nano bowtie structures mainly by numerical simulation. The influence of sharpness and gaps of nano bowtie upon the localized surface plasmon resonance wavelength, extinction coefficient, as well as the localized near-field enhancement has also been investigated and the experimental extinction spectra of the gold nano bowtie structures fabricated by electron beam lithography almost agreed with simulation result. This investigation may help us to design plasmonic sub-wavelength gold bowtie structure with desired spectral properties or largest localized near-field enhancement.The photoluminescence and Raman scattering enhanced by the localized surface plasmons was also investigated. We have found that the photoluminescence intensity of Tb3+ions doped in the glass ceramics (60SiO2-20Al2O3-20CaF2-0.3Tb3+-20Yb3+, in mol%) measured from the area with silver nanoblocks is enhanced by 1.6 times. Considering the overlap of localized surface plasmon resonance frequency with the optical excitation and emission frequencies of Tb3+ions and the simulation result of localized near-field enhancement, we suggest that the photoluminescence enhancement is mainly due to the localized near-fied enhancement and coupling of the 7F6 to 5D4 transition dipole of Tb3+ions with localized surface plasmons localized on the silver nanoblocks near the Tb3+ions. In addition, we found that the Raman scattering signal of crystal violet on the gold nanodisk structures was enhanced under the excitation of 785 nm. However, we couldn't observe the enhancement of Raman scattering under the excitation of 514 nm. As the localized surface plasmon resonance peak matches well with the excitation wavelength, and the simulation result shows that the localized near-field enhancement factor is large, we ascribed the enhancement of Raman scattering to the localized near-field enhancement and the coupling of the excitation light with the localized surface plasmons localized on the gold nanodisks near the crystal violet molecules.