The Study Of Surface Enhanced Raman Scattering Based On Semiconductor Heterojunction

As a powerful analysis technique of molecular detection,surface-enhanced Raman spectroscopy（SERS）is capable even at the single-molecule level.Due to its wide application in diverse areas such as biological sensing,optoelectronics and catalysis chemistry,SERS has shown a promising prospect and attracted great attention of scholars.Initially,the SERS active substrates were mainly made of substrates sprayed with Au,Ag or other noble metals.The local electromagnetic field is magnified in virtue of the localized surface plasmon resonance when the light illuminates and meanwhile,the Raman scattering is enhanced accordingly.Although quite high SERS signals exist at any position on the substrate,the contribution made by particles of different shapes and sizes varies considerably,which largely restricts its development.Almost 40 years after Fleischmann et al.discovered SERS,a growing body of research showed that many semiconductors exhibit SERS activity,opening the way to Raman enhancement.More importantly,semiconductor materials made it possible to obtain reliable,stable and uniform SERS signals without complex nano-fabrication technology.Besides,the lower cost as well as better biocompatibility compared with noble metals also facilitate its practical applications ranging from surface science to biological detection.It is generally recognized that the photo-induced charge transfer（PICT）between the semiconductor and adsorbed molecules enlarges the scattering cross sections,which in turn amplify the Raman signals.The EFs based on this chemical mechanism are commonly as low as 10-10³.at the early stage.By contrast,some special SERS-active semiconductor substrates could achieve EFs even comparable to electromagnetic mechanism enhancement-dominated system by boosting PICT process,recent study found.It follows that both the massive exciton resonances and high density of states of substrates contribute to the noble metal-comparable SERS enhancement.Previous study has demonstrated that there is a wide application prospect for semiconductor heterojunctions because of a high photocatalytic activity associated with large-scaled electron-hole pair separation and efficient charge transference,and the coupling of two-dimensional materials can also effectively improve the Raman performance of the substrates.Inspired by this,we suppose that heterojunctions could be also utilized in the field of Raman enhancement.On the one hand,their internal properties allow the existence of abundant DOS near Fermi energy level and large-scaled electron-hole pair separation,which increases the probability of electron transition between the substrate and molecules adsorbed on it.On the other hand,more excitons are possible to achieve resonance transitions.Both the two factors may work together to boost the Raman enhancing performance.Based on this point,we propose the concept of heterojunction-enhanced Raman scattering.Moreover,considering the high oxygen vacancy concentration of W₁₈O₄₉ and substantial numbers of electron-hole pairs that are easily excited by visible laser on monolayer MoS₂,we designed a vertical W₁₈O₄₉/monolayer MoS₂ heterojunction with nanometer thickness and confirmed its giant HERS both experimentally and theoretically.In addition,this innovative research approach extends the study of SERS from the material surfaces to the interior of substrate,providing more possibility for further research.