Studing Molecular Adsorption Configuration And Electron Transport At Single Molecule Level Based On STM-BJ Technique

Molecular adsorption on the surface plays an important role in charge and mass transfer in chemical and biological processes such as sensors,catalysis,energy collection and storage,biochemistry and molecular electrons.In the process of heterogeneous catalysis,the adsorption of molecules is often the first step,or even the decisive step,so that the study of molecular adsorption on the surface is very important for understanding many chemical,physical and biological processes.In addition,it is also an important process for the construction of a new concept of functional nanodevices.However,it is not easy to study the chemical properties of the solid/liquid interface at the molecular level,which is hampered by the need to distinguish between the few atoms at the interface and the large number of atoms present in the two volamental phases involved.To overcome this difficulty,many modern surface analysis techniques were developed at the end of the 20th century.By using these techniques,such as infrared absorption spectroscopy,UV-Vis absorption spectroscopy,Raman spectroscopy and X-ray electron spectroscopy,major advances have been made in understanding the chemistry that occurs on solid surfaces.However,most of these techniques rely on the use of particles such as electrons,ions or atoms,which work best in a vacuum so that new methods are needed to study solid/liquid interfaces at the molecular level.In recent years,a single-molecule break junction technique based on molecular electronics research has attracted enormous attention.As a powerful surface analysis technique,it can build a molecular bridge between two nanometer electrodes to form molecule-metal-molecule junction.Therefore,we applyed the STM-BJ techniques to systematically studied the effects of the adsorption of small organic molecules on the adsorption configuration and interfacial electron effects of pyridyl molecules on Au（111）,and revealed the relationship between the adsorption configuration of pyridyl at solid/liquid interface and the interfacial electronic structure.Besides,we studied the effects of electrochemical gating on the adsorption configuration and interfacial electronic effect of BPY on Au（111）by ECSTM-BJ technique,and explained the relationship between the adsorption configuration of BPY at the solid/liquid interface and the interfacial electronic structure under electrochemical gating.Finally,in addition to pyridyl molecules,we also studied the electrochemical gating of ferrocene molecules.The main research contents and conclusions are listed as follows:1.We took STM-BJ techniques to investigate the impacts of solvent modification on molecule-metal adsorptions.The results show two conductance states arise from two-dominated configurations of 4,4’-bipyridine（4,4’-BPY）molecular junctions,which are throughσ-bond of its nitrogen atom with a vertical/tilted orientation（σ-binding）andπ-bonded of its aromatic ring with flat orientation（π-binding）on Au（111）.Furthermore,by combining cyclic voltammetry test and in situ Raman spectroscopy analysis,a fine modulation of molecular adsorption geometry is achieved through tuning the interfacial electronic structure with a hybrid solvent of ILs and H₂O.2.We took ECSTM-BJ techniques,and combine the electrode-molecular level arrangement and the theoretical adsorption model to investigate the impacts of electrochemical gating on the adsorption configuration and interfacial electronic effect of BPY molecule on Au（111）.The results showed that the regulation of the adsorption configuration was caused by the change of the adsorption behavior of BMI⁺on Au（111）by electrochemical potential.On the surface of Au（111）with positive charge,due to the effect of charge repulsion,BMI⁺moved away from Au（111）,which reduced the steric hindrance of BPY adsorption on Au（111）,and making the adsorption configuration of BPY molecule change from vertical to tilte.3.We took ECSTM-BJ techniques to investigate the impacts of electrochemical gating on Ferrocene-based molecules with REDOX active centers,and achieved an about 224 times on/off ratio.Combine with CV experiments and electrochemical level regulation schematic diagram,we find that,near the redox potential off Fc⁺/Fc,the gap of the Fermi level of electrodes and the HOMO of molecule decreases,lead to the changes of electron transport mechanism and the electron transport efficiency increases.