Electrochemical Tip-enhanced Raman Spectroscopy Based On Atomic Force Microscopy

Tip-enhanced Raman spectroscopy(TERS)is a technique that combines Raman spectroscopy with scanning probe microscopy(SPM).TERS can simultaneously provide high resolution topographic information and Raman signal down to sub-nanometer level.Since its discovery in 2000,TERS has found wide applications and shown great potential in surface sciences,nanomaterials,and biological systems.However,nowadays TERS enters the bottleneck stage due to the poor tip fabrication and uncontrollable surface state.The tip is the core element in TERS,as it is the only source through which to support the enhancement effect and provide the high spatial resolution.However,TERS suffers and will continue to suffer from the limited availability of TERS tips with a high enhancement,good stability,and high reproducibility.The key issue is that TERS tip must meet the requirements of both SPM and Raman spectroscopy.What's more,different fabrication methods should be developed for each kind of SPM-TERS techniques.But all of them do exist some key problems.On the other hand,most of the previous TERS studies were performed in an am'bient environment,which may suffer from the contamination of the tip and surface by the environment contaminants.Meanwhile,it is difficult to control the surface coverage of the adsorbate and the surface state of sample,which limits to some extend its application in the study of the surface dynamic processes.If we expand TERS to the electrochemical system to develop electrochemical-tip-enhanced Raman spectroscopy(EC-TERS)technique,we will be able to control the electrochemical behavior of the species on the surface by controlling the electrode.Thereby,we will able to obtain the topographic information and Raman signal of the sample under electrochemical process.Therefore,EC-TERS may open a totally new research field for TERS.However,up to now,only four EC-TERS papers were published due to the limitation of tips and instrument.With emphasis on methodology,we made large efforts to solve the key problems in the fabrication of TERS tips.We also focused on the development of EC-TERS instrument.On the basis of the tip and instrument,we performed the EC-TERS study of the electrocatalytic system.The main results and conclusions of this thesis are listed as follows:1.We develop the electrodeposition methods for coating gold and silver over AFM tips to produce plasmonic tips for TERS with a high reproducibility.The radii of metal-coated TERS tips can be rationally controlled by the deposition time and the applied potential.The dependence of the TERS enhancement on the radius of the gold-coated AFM tip was systematically studied.It provides a powerful experiment evidence for the understanding of the TERS mechanism.In addition,the as-prepared AFM-TERS tips show high TERS activity and strong adhesive strength,which improve the mechanical stability and lifetime of the tip.2.Three electroless deposition methods were developed for the deposition of the metal on the non-conductive AFM tip to produce plasmonic tips,which expand the application area of TERS.The radii of the as-prepared TERS tips can be tuned by the deposition time.3.By analyzing the existing technical problems,we developed the EC-AFM-TERS technique based on the top-illumination.We updated the existing instrument and optical path to improve the excitation and collection efficiency.Based on the space configuration of the SPM,we designed the special EC-TERS cell.In addition,the tip holder was encapsulated to prevent from the contact of the electrolyte.Meanwhile,we demonstrated the designed function.4.On the basis of the tip and instrument,we studied the defects of the thin layer MoS2.By doing the TERS line scanning,we found the difference between defect and basal plane.We also performed the EC-TERS study of the electrocatalytic hydrogen evolution reaction(HER)and revealed the evolution of active site during the whole process.