Research Of Plasmon Resonance "Hot Spots" And The Enhancement Mechanism

Surface-enhanced Raman scattering(SERS)is considered as a powerful analytical tool in biomedicine,environmental science,materials,catalysis,energy,etc.because of its extraordinarily high surface sensitivity,which can unveil chemical structures and reaction processes at the molecular level.The extraordinarily strong enhancement effect originates from surface plasmons(SPs),generated by collective oscillations of electrons induced by incident light.However,it is a long-standing challenge to control the precise location of hot spots in nanostructures because of their small spatial volumes.In order to solve this problem and improve the sensitivity of SERS and achieve the purpose of controllable adjustment of "hot spot",it is necessary to have in-depth research on "hot spot" and explore the mechanism.Generally,the surface plasmon resonance modes of metal nanoparticles can be classified into electric mode and magnetic mode.In general,most researches focus on the electric mode of plasmon and ignore the effect of magnetic mode.Several investigations on near-field properties have theoretically demonstrated that the introduction of plasmon-induced magnetic resonance(PIMR)helps generate magnetic and electric "hot spots",prompting the construction of simultaneous electric and magnetic near-field platforms.It is expected to further enhance the enhancement ability of "hot spot" by using plasmon-induced magnetic resonance.In addition,in some special situation,such as the electrochemical environment,the electron distribution on the electrode surface will also influence the distribution of electromagnetic field.Our group had found that,at the very negative potential,the water molecules adsorbed on metal electrodes such as Au,Ag had abnormally increased SERS signals.It is of great significance to research the distribution of the electron on the electrode at very negative potential.This paper mainly aimed for exploration of the mechanism of the surface enhanced Raman spectroscopy.Firstly,we studied the controllable transfer of "hot spots" in the three dimensional system.In order to further improve the enhancement of "hot spots",we then developed plasmon-induced magnetic resonance enhanced Raman spectroscopy.Finally,for electrochemical system,we studies the special Raman enhancement at extremely negative potential.The main research contents of this paper are as follows:1.We fabricated a highly sensitive and uniform 3D hot spots matrix by assembling Au@probe@SiO2 closely-packed nanoparticle monolayer films onto a substrate via a consecutive layer on layer deposition method.Three types of Au@probe@SiO2 with different probe molecules(MBA,NT,and DTNB)were utilized to identify the exact location of hot spots in the 3D trilayered structure.Two different excitation lasers(633 and 785 nm)were used to control the precise locations of hot spots and the experimental results were consistent with numerical simulation results.The transfer of hot spots between different layers was demonstrated to depend on the wavelength of the excitation laser.It is promising to realize the transfer of hot spots at the nanoscale and this approach provides a convenient plat-form to probe the location of hot spots in SERS substrates in future,which will have profound implications in both surface analysis and surface plasmonic.2.We have built a simple and practical PIMR-based platform made of the metallic nanosphere and the metallic film.Single particle dark-field scattering characterizations and in-depth theoretical analyses for the proposed system have revealed a magneticbased Fano resonance as a result of interactions of electric and magnetic dipolar modes,of which the latter behaves as a dark magnetic mode appearing near the Fano dip.Our Raman experimental results have further disclosed that PIMR significantly enhances the SERS response of probing molecules due to large electric-field enhancements generated by the PIMR.High capabilities of PIMR on electric and magnetic near-field enhancements originate from(a)low electromagnetic radiative losses and(b)efficient coupling between PIMR and both electric-and magnetic-field components of the incident light.Notably,via the correlation between the dark-field and Raman spectra,we have uncovered the rarely reported connection of near-field peaks to scattering valleys.Our findings of PIMR serve as a new insight for surface enhanced Raman spectroscopy and non-Raman spectroscopies such as Fluorescence and Infrared absorption.Furthermore,design of the built large-scale plasmonic platform based on PIMR can be applied to magnetic-based optical metamaterials.3.We designed a molecule ruler with several different functional groups to detect the length of electron spillover on the electrode surface at extreme negative potential.The spatial distribution of electron on electrode surface under extreme negative potential was detected,and the mechanism of Raman spectroscopy on electrode surface at extreme negative potential was further understood.