Molecular Fluorescence Modulated By Localized Surface Plasmon Of Metal Nanostructures

At the nanoscale,metallic nanostructures exhibit excellent optical and structural properties.Especially in the presence of external electromagnetic fields,the collective response of the free electrons on the surface of the metallic nanostructures forms a coupled resonance with the incident electromagnetic field,resulting in a very localized and enhanced electric field in the vicinity of the metallic nanostructure,which has attracted great interest and led to the introduction of the term "surface plasmon".It has been found that metal nanostructures of different materials,sizes and shapes exhibit an even greater variety of excellent properties,and that there is a very strange symbiosis between plasmon and spectroscopy,which makes it possible to effectively modulate fluorescence radiation.Nowadays,plasmon-enhanced fluorescence has developed into an important member of the family of surface-enhanced spectroscopy techniques and has been used in a wide range of applications,including single-molecule fluorescence imaging,fluorescent semiconductor quantum dots,enhanced photochemistry,photovoltaic devices,biosensors and many others.However,the effect of plasmon on fluorescence is not only enhanced but also quenched,and these two effects exist simultaneously and compete with each other.The key point of which effect is stronger or weaker is the distance between metal nanostructures and fluorescent molecules.In addition,different types of metal nanostructures and different shapes of metal nanoparticles exhibit a variety of localized surface plasmon properties,and better modulation can be achieved when the localized surface plasmon resonance peaks of the metal nanostructures match well with the emission or absorption peaks of the fluorescent molecules.Therefore,we chose three different metal nanostructures,namely gold nanospheres,gold nanobipyramids and the nano-tip of scanning tunneling microscope,and used their localized surface plasmon to modulate the fluorescence of Rhodamine 6G,Meso-Tetraphenylporphyrin and 2-Methylbenzimidazole molecules respectively.Then,the competition between the enhancement and quenching effects of the localized surface plasmon and the interactions between metal nanostructures and fluorescent molecules are investigated.The main researches of the dissertation are as follows:1.The distance dependent of the fluorescence enhancement effect of gold nanospheres on submonolayer Rhodamine 6G molecules was investigated by covering the surface of gold nanospheres with a controlled thickness of Polymethyl Methacrylate films(1.5-21 nm)with the method of spin-coating.The variation of the fluorescence lifetime of Rhodamine 6G molecules and the enhancement factors were elucidated and the theoretical simulations based on the Finite-difference time-domain method have shown the intensity of the localized electric field near the gold nanospheres to decay rapidly with increasing thickness of the organic film.Moreover,an attempt was made to prepare spherical core-shell nanostructures by layer-by-layer self-assembly method,achieving precise control of the shell layer thickness(2-6 nm)and laying the experimental foundation for subsequent distance modulation at sub-nanometer scale.2.The distance dependence of two competing mechanisms in the near-field,the enhancement and quenching effect of localized surface plasmon on molecular fluorescence,was quantitatively investigated.Experimentally,gold nanobipyramids and Meso-Tetraphenylporphyrin molecules were stably linked together by chemical bonding using polyethylene glycol molecular chains,and precise distance modulation between the particles and molecules at a sub-nanometer scale(1.4-4.5 nm)was achieved by varying the length of the polyethylene glycol molecular chains.The intrinsic electron transition mechanism of the Meso-Tetraphenylporphyrin molecules was elucidated that the fluorescent intensity increases firstly and then decreases with increasing distance using the density matrix equation.The effects of different distance control methods and the size of the gold nanobipyramids on the molecular fluorescence were also investigated.3.The interactions between the localized surface plasmon of metal nanotip and 2-Methylbenzimidazole molecules have been studied using an optically linked lowtemperature ultra-high vacuum scanning tunneling microscope system.The selfassembly behavior of 2-Methylbenzimidazole molecules on the Au(111)surface was revealed.The electronic state properties of the molecules under different bias conditions,molecule-molecule hydrogen bonding and molecule-substrate coordination interactions were also elucidated.The experimental results demonstrate that in a highly localized nano-cavity,the localized surface plasmon of the tip has no effect on the molecular fluorescence under photoluminescent conditions,whereas the fluorescent molecules can modulate the localized surface plasmon resonance peak of the tip under electroluminescent conditions.Furthermore,the effects of different functional groups on the properties of the benzimidazole molecule were investigated by comparing the self-assembly behaviour and luminescence properties of 2-Methylbenzimidazole molecules with those of 2-(2-Hydroxyphenyl)-1H-benzimidazole molecules.