Investigation Of The Relationship Between The Coupling Strength And Electrical Transport Properties Of Single-molecule Junctions

Molecular electronic devices are one of the potential technical pathways to further extend Moore’s Law.The concept of bottom-up provides a new perspective for improving circuit integration and miniaturization of electronic components.Meanwhile,the unique physical and chemical properties of molecules provide diversified realization methods for functionalized molecular devices,such as external field(electric,optical,thermal,magnetic,etc.)or pH,proton transfer,solvation effects,etc.The essence of such strategies is to vary the electronic structure of junction thus modulating the electrical transport properties.As for the electrical transport properties of junctions,the coupling interaction between electrodes and molecules in molecular junctions and intramolecular coupling interaction between the internal structures of molecules(hereinafter referred to as molecular junction coupling)are unignored factors to limit the functional expression of molecular devices.After～20 years of development,the methodology of single-molecule devices has made great progresses,however,the method of adjusting the coupling of molecular junctions to achieve the maximum expression of functionalization is not sufficient.Exploring the coupling between each modules of junction is especially important for understanding the electrical transport properties of molecular devices and designing high-performance molecular devices.Inspired by this,in this paper,the works were conducted employing the self-built scanning tunneling microscope break-junction(STM-BJ)device in two aspects:molecule-electrode interface coupling and molecular module coupling.The following is the main research and conclusions of this paper:1.The coupling between molecule and electrode:In order to obtain high coupling strength anchor groups,we have synthesized and explored the single-molecule conductance of air-stable NHCs(N-heterocyclic carbene)compounds of group IB metals(Cu,Ag,Au)with double NHCs ligands and dibenzofulvene backbone.Our work demonstrates a novel strategy to fabricate single-molecule devices with double NHC-anchors using photoinduced transmetalation reaction of air-stable silver NHC compounds.We investigate their charge transport properties using STM-BJ technique.We found the double NHCs-anchoring junction exhibit ultrahigh conductance,which is～2900%higher than the leakage current within a 0.5 nm distance.The combination of current-voltage(Ⅰ-Ⅴ)analysis and Density functional theory(DFT)calculation reveals that the strong electronic coupling and small level difference lead to resonance transport at low bias voltage and ultra-high conductance.2.The coupling between molecular modules:we investigated lactam derivatives with different ring substituents to explore the effect of intramolecular hydrogen bond in twisting structure.The experiments illustrate that the regulation of intramolecular hydrogen bond can improve the coupling between molecular modules,thereby improving electronic structure of molecular junctions to elaborately adjust the charge transport through junctions.The experiments that switching solvent confirmed this strategy can be further used to construct functional devices.Combined with DFT calculations,twisting structure results in the localization of electronic structure of junctions,which impedes the charge transport,thus generating a transition between through-bond and through-space,in accord with flicker noise analysis.Meanwhile,intramolecular hydrogen bonds do not offer additional transmission pathway but only enhances the planarization of the molecular junction.Our works offer a novel strategy for designing reversible switches to control the charge transport properties of junction via the modulation of intramolecular coupling.