Preparation Of Novel Semiconductor SERS-active Substrates And Their Surface Coupled Resonance Effect Study

Surface-enhanced Raman scattering?SERS?is a fast,sensitive,and non-destructive detection technology,and has be widely used in many fields,such as drug detection,environmental science,biomedicine,etc.The development of SERS technology is inseparable from the exploration of novel SERS-active substrate materials.As the earliest developed noble metal SERS-active substrates gradually expose many disadvantages,semiconductors can make up for their shortcomings and greatly broaden the application SERS by virtue of their rich variety,adjustable band structure,good chemical stability and biocompatibility,etc.The excellent SERS performance of noble metal substrates is mainly attributed to electromagnetic enhancement?EM?based on the strong localized surface plasmon resonance?LSPR?.However,due to weak chemical enhancement?CM?,semiconductor substrates have the disadvantage of the weaker SERS performance.Therefore,the focus of this work is to develop novel semiconductor SERS-active substrate materials by combining the CM with EM together,and synergistically achieve the coupled resonance effects of three resonances,including?1?electromagnetic resonance?including LSPR and Mie resonance?,?2?charge transfer?CT?resonance,as well as?3?molecular resonance,to obtain the ultra-sensitive semiconductor SERS-active substrates.The hydrogenation technology not only can promote the CT transition between TiO₂ and probe molecules,but also help to increase the carrier concentration to achieve LSPR in the visible light region,and thus the EM and CM are expected to be cooperatively realized on TiO₂.In this work,the hydrothermal method and solid-state hydrogenation technology were used to obtain the hydrogenated TiO₂ SERS-active substrate,which was formed by covering the surface of the flexible Timesh with randomly oriented hydrogenated TiO₂ nanowires?NWs?.By adjusting the hydrogenation time,the TiO₂ NWs substrate hydrogenated for 3 hours exhibited the best SERS performance.The substrate showed a very low limit of detection(10^-7 M)for rhodamine 6G?R6G?,methyl violet?MV?and methylene blue?Me B?molecules,and its enhancement factor?EF?reached 1.2×10⁶,which was three orders of magnitude higher than that of unhydrogenated TiO₂ NWs.This sensitive SERS performance should be contributed to the collaborative contributions of the following three aspects:?1?the easier CT process between TiO₂ and probe molecules due to the introduction of defect state energy level,?2?the LSPR in the visible light region due to the increased carrier concentration,and?3?the strong light-substrate coupling at the cross regions among TiO₂ NWs.In order to obtain a more sensitive Nb₂O₅ SERS-active substrate,the unique amorphous nanoflower structure is applied to the novel Nb₂O₅ to promote the electromagnetic field enhancement and the CT process.In this work,a novel two-step hydrothermal method was used to obtain Nb₂O₅ nanoflowers?NFs?.Through controlling the Si content added in the second-step hydrothermal process,the Nb₂O₅NFs were grown with a diameter of 262 nm and formed by the wrinkled amorphous Nb₂O₅ nanosheets.The Nb₂O₅ NFs substrate exhibited the best morphology-dependent SERS performance with the detection limit of 10^-7M for crystal violet?CV?molecules under the 532 nm incident laser.The sensitive SERS performance should be attributed to the synergistic contribution of the following three aspects:?1?the more probe molecules adsorption promoted by the large specific surface area of the nanoflower morphology,?2?the strong CT resonance in the amorphous Nb₂O₅ NFs and CV molecular system near the incident light,as well as?3?the 10² electromagnetic field enhancement on the surface of Nb₂O₅ NFs.In order to improve the performance of the novel Ta₂O₅ SERS-active substrate,the energy band structure of Ta₂O₅ is regulated through the quantum size design to achieve the energy level matching between Ta₂O₅ and probe molecules,and thus obtaining a sensitive Ta₂O₅ SERS-active substrate.In this work,an improved hydrothermal method was used to synthesize the Ta₂O₅ superstructure with a diameter of about 29 nm,which was composed of crystalline Ta₂O₅ quantum dots with an average particle size of 2.39nm.The Ta₂O₅ superstructure exhibited a sensitive detection limit of 10^-7 M for MV and Me B detection under the irradiation of 532 nm laser.There are three important aspects,including?1?the more probed molecules adsorption due to the huge specific surface area of quantum dots,?2?the energy level matching and the easier CT process between the Ta₂O₅superstructure and MV,and?3?the intense MV molecular resonance near the incident laser,jointly endowing the Ta₂O₅ superstructure a sensitive SERS detection ability.In order to achieve the optimal SERS performance of Ta₂O₅,the"coupled resonance"strategy was firstly proposed to achieve three quasi-resonant Raman effect(?_laser??_mol??_CT??_EM),that is the cooperative resonance of?1?molecular resonance,?2?molecular-semiconductor CT resonance,as well as?3?electromagnetic resonance near the incident laser,on the surface of Ta₂O₅ through energy band engineering.In this work,Ta₂O₅ nanorods were synthesized by a simple hydrothermal method,and the 15%-Mo-Ta₂O₅ NRs substrate with the best SERS performance was obtained by controlling the doping amount of Mo of 15%?molar ratio of Mo to Ta?.The substrate exhibited ultra-sensitive SERS performance with an EF of 2.2×10⁷ and a limit of detection as low as9×10^-9 M for detecting MV molecules.Through the first-principle calculation and the finite-difference time-domain calculation,it can be demonstrated that there are three aspects,including?1?MV molecular resonance,?2?CT resonance between MV molecules and 15%-Mo-Ta₂O₅ NRs,and?3?the EM resonance at the"gap"and"tip"positions of anisotropic Ta₂O₅ NRs,working together to achieve the optimal SERS performance of 15%-Mo-Ta₂O₅ NRs substrate.