Study On Properties And Mechanism Of Surface-enhanced Raman Scattering Based On W₁₈O₄₉ Substrate

Surface Enhanced Raman Scattering?SERS?has attracted tremendous attention as an ultrasensitive detection technique.Because of the surface plasmon resonance,noble metal?gold,silver?is often used as substrate in SERS research.However,the metal SERS substrate materials have several disadvantages such as susceptible to oxidation,poor biocompatibility and difficult for mass production.Compared with metal substrates,semiconductor SERS substrates have many advantages such as greater chemical stability,good biocompatibility and low price.It is significant to study the SERS enhancement mechanism of semiconductor materials and explore the general method that can effectively improve the semiconductor SERS performance,which can promote the development and practical application of SERS technology.Tungsten oxide materials,especially nonstoichiometric W₁₈O₄₉,have rich surface state and the impurity level,which is very suitable to study the chemical enhancement mechanism of semiconductor SERS substrates,so we can find effective method to enhance its SERS performance.In this paper,W₁₈O₄₉ was studied as the main research object,and the influence of oxygen vacancy doping and carrier doping on performance of SERS substrates was studied.The first chapter introduces the research background and significance of the research.The second chapter introduces the experimental characterization method and the theoretical calculation method.In third chapter,we studied the influence of carrier doping and oxygen vacancy doping on the electronic structure of tungsten oxide with the first principle calculation.Hybrid density functional HSE06 method was used to study the electronic structure of hexagonal WO₃,and the band gap was evaluated as 2.09 eV.The influence of carrier doping on WO₃ electron structure was also investigated.Results showed that the location of WO₃ was significantly reduced in the case of electron doping,and the up shift of the valence band edge was obtained in the case of hole doping.Then the influence of oxygen vacancies was studied.Result showed that the oxygen vacancy introduced impurity level in the forbidden band,which caused increase electron transition probability in semiconductor.In chapter 4,the calculated result of oxygen vacancy doping was verified by experiments.The non-stoichiometric material W₁₈O₄₉,which has rich oxygen vacancy,was synthesized by hydrothermal method,and then SERS signal of R6G was observed in this substrate.For comparison study,WO₃,which has no oxygen vacancy,was acquired by annealing the W₁₈O₄₉ sample in air.No clear R6G molecule signals were discerned on WO₃ substrate.This result showed the important role of oxygen vacancy in improving the SERS performance of semiconductor substrate.Enhancement factor of W₁₈O₄₉ substrate was found to be increasing with the lower concentration of R6G,and finally the detection limit of R6G concentration reached 10^-7M,which showed application prospect of the semiconductor material used for SERS substrate in trace detection.In chapter 5,experimental verification of the calculated result of electron doping was presented.A similar amount of Ag and Pt nanoparticles is loaded onto the as-prepared W₁₈O₄₉ nanowires via a photoreduction technique at room temperature.Under illumination at the SPR frequency of noble metal Ag,“hot electrons”injected from the metal nanoparticle to CB and trapped states?e.g.oxygen defect states Vo?of the semiconductor W₁₈O₄₉,so realized the purpose of the electron doping.It was found that the SERS enhancement factor of W₁₈O₄₉ which loaded with Ag nanopaticles was 1.75 times higher than the unloaded W₁₈O₄₉.In addition,the surface of W₁₈O₄₉ was modified by the organic molecule 4-ATP which contained the electron-donating group.4-ATP could increase the surface charge concentration of W₁₈O₄₉ and realized the electron doping by adsorbing on the surface of W₁₈O₄₉.The result was similar to the metal loaded substrate,the modified SERS substrate performance was significantly improved,its enhancement factor was about1.5 times compared with original.Experiment results show that increasing the charge concentration on the semiconductor surface can improve the performance of SERS substrates.In the end,we proved that the oxygen vacancy doping and electronic were two effective ways to improve the enhancement factor of W₁₈O₄₉9 with the calculated and experimental results.It is promising to improve performance of semiconductor SERS substrates with these ways.