Investigation Of The Carrier Density Effect Of Semiconductor Based On Surface-enhanced Raman Scattering

SERS is a powerful,non-destructive,and ultrasensitive spectroscopic technique that is widely used in chemical detection,biological detection,and environmental analysis.As is well known,these unique properties and applications of SERS depend critically on its highly active substrate.Meanwhile,with the rapid development of nanotechnology,SERS active substrate is no longer limited to noble metal nanoparticles,more types of SERS materials and more choices have become possible.At present,semiconductor and semiconductor/metal composites show versatility and SERS activity in both theoretical and practical applications due to their advantages of low cost,good biocompatibility and high stability.Electromagnetic enhancement(EM)and chemical enhancement(CM)are widely accepted mechanism for SERS enhancement.The former is mainly related to LSPR of SERS substrate materials,while the latter is closely related to CT process.However,for most semiconductor nanomaterials,it is difficult to obtain LSPR-induced SERS enhanced in a suitable wavelength range and wide band-gap led to low CT efficiency,which greatly hindered the application and development of SERS substrates for semiconductors.Based on this problem,we tried to change the LSPR absorption and band gap structure of semiconductor materials by regulating the free carrier density,and utilized SERS technology for monitoring.Based on this assumption,we constructed a series of semiconductor/probe molecular composite systems to adjust the free carrier density of semiconductor materials by changing the semiconductor concentration of composite,constructing semiconductor/metal heterostructure and modifying the surface state by Li+adsorption.The free carrier density effect of semiconductor nanoparticles was studied by observing the SERS spectra changes of probe molecules.This study is of great significance to further understand the free carrier density effect of semiconductor nanomaterials and promote the modification design of SERS substrates for non-noble metals.Based on the above ideas,the thesis is mainly composed of the following parts:1.Modulating carrier density of the(Ag)x(MoO3)y system to enhance SERS:Localized surface plasmon resonance contributionHerein,we fabricated ordered(Ag)x(MoO3)y substrates on selfassembled monodisperse polystyrene colloidal microspheres(PSCM)arrays using a magnetron sputtering system under Ar plasma bombardment,where the sputtering power of Ag is fixed at 5 W and the power of MoO3 is 50,70 and 90 W,respectively.In this process,the Ar plasma with high-energy will activate oxygen atoms of MoO3,resulting in the oxygen atom of the bulk diffusing to its surface.Thus,MoO3 will produce a mass of oxygen defects.X-ray photoelectron spectroscopy(XPS)can prove that the system does contain oxygen defects.Meanwhile,we control the carrier density and adjust the LSPR absorption of the(Ag)x(MoO3)y composite by changing the MoO3 content.In addition,we discussed how the controllable carrier of(Ag)x(MoO3)y composite influences the LSPR based on SERS test and UV-VisNIR absorption spectra.We were surprised to find that the LSPR absorption wavelength can be easily tuned from 950 to 735 nm by changing the sputtering power of MoO3 of the(Ag)x(MoO3)y composite.This shows that LSPR can be precisely adjusted by increasing the semiconductor content and even the carrier density.In the end,the carrier density was measured by Hall effect to investigate the SERS intensity change caused by EM,and obtain the relationship between the two.The development of some substrate materials with LSPR effect has farreaching significance for the research and practical application of SERS.2.Metal-Semiconductor Plasmonic Resonance Coupling:Surface-enhanced Raman Scattering(SERS)studiesAlbeit intense research on plasmon-induced charge transfer within metal/semiconductor heterostructures,previous studies all focus on the surface plasmonic resonance(SPR)of noble metals only.Herein and for the first time,we observe and take into account the plasmonic coupling between SPR of both noble metal and semiconductor nanostructures.W18O49/Ag heterostructure composed of metallic Ag nanoparticles(Ag NPs)and semiconducting W18O49 nanowires(W18O49 NWs)is designed and fabricated,which exhibits a broad and strong SPR absorption in the visible wavelength range.This SPR band is attributed to the SPR coupling between the SPR of both Ag NPs and W18O49 NWs.SERS is then used to reveal the interactions between the metal SPR,semiconductor SPR and the heterostructure’s CT process,demonstrating that such coupled SPR enhanced the heterostructure’s internal CT and SERS signals.Finally,we proposed a new coupled-plasmon-induced charge transfer mechanism to interpret the improved CT efficiency between SERS substrate and molecules.The work provides insight for further studies on plasmonic effects and interfacial charge transfer in metal/semiconductor heterostructures.3.New pathway for SERS activity enhancement in semiconductors:Li+adsorption modulates photo-induced charge transfer efficiencyWhile the Lithium ion(Li+)modification strategy is widely adopted in tuning the band gap of semiconductors in dye-sensitized solar cells(DSSCs),such strategy is yet to be applied to semiconductor-based surface enhanced Raman scattering(SERS),which is also a photo-induced charge-transfer(PICT)process.Here,we propose a new DSSCs-inspired modification method for TiO2 NPs:through the adsorption of Li+,the surface states of TiO2 NPs are more abundant,so that the SERS intensity and charge transfer of the adsorbed molecule(MBA)are significantly enhanced.Through Transmission Electron Microscope(TEM)and Xray diffraction(XRD)tests,it was found that the adsorption of Li+on the surface of TiO2 NPs did not change the crystal phase of TiO2 NPs.Based on UV-Vis absorption spectra and CV curves,similar to DSSCs systems,it is found that the CB edge and Ess of TiO2 NPs shift downward with increasing Li+concentration.At the same time,TiO2 NPs band gap contracted,and when Li+concentration was 10-3 M,the band gap gap decreased to the minimum value of 2.92 eV,which was conducive to CT interaction between molecules and SERS substrates.The EF can reach to 104,1-2 degrees higher than previously reported pure semiconductors.This is the first time to use the DSSCs-inspired Li+adsorption strategy on TiO2 NPs as SERS substrate to investigate its SERS enhancement effect,which may provide new ideas for the development of semiconductor nanomaterials in the research of SERS substrates.4.A new strategy for improving quantitative detection of SERS:using CH3NH3PbBr3 as a substrate to narrow the FWHM of adsorbed molecular spectrumSpectra in SERS are always accompanied by a continuous emission known as"background",which complicates the analysis with a poor signal-to-noise ratio,especially for qualitative and quantitative detection.Here,The SERS spectra of CH3NH3PbBr3 substrates exhibit significant specificity with ultra-narrow FWHM.The significantly increased SNR of the probe molecules indicates that the substrate helps to improve the quantitative analysis capability of the SERS technology and alleviate the background-related problems.For the CH3NH3PbBr3 substrate,good linearity(R2=0.9591)in the range of 1.0×10-7-1.0×10-3 M was obtained and the limit of detection(LOD)is 7.88×10-6 M.The reason for this excellent performance is that the introduction of organic molecules splits the 3D CH3NH3PbBr3 substrate to form a 2D perovskite that fixes the molecular orientation through chemical interactions.The CH3NH3PbBr3 has the advantage of maintaining the conformation and orientation consistency of adsorbed molecules.Meanwhile,compared with the commonly used noble metal(Au,Ag)and semiconductor(ZnO,TiO2)SERS substrates,the CH3NH3PbBr3 substrate has obvious superiorities in both spectral peak intensity and peak shape.Furthermore,the CH3NH3PbBr3 substrate possesses specific recognition ability that has the same enhancement effect for a series of pyridine-containing probe molecules.Finally,the enhancement mechanism is discussed according to the CT contribution.As a promising substrate for SERS technology,CH3NH3PbBr3 will provide more possibilities for quantitative and multiplex detection in the future.