Exploring The Effect Of Molecular Structure On Electron Transport Based On STM Break Junction Technique

With the progress of modern science and technology,the further miniaturization of integrated circuits based on traditional silicon has been subject to limitations by technology and principle.Therefore,people turn their attention to the bottom-up construction of molecular devices,namely the construction of functional devices at the single molecular scale.Molecular devices include molecular switches,molecular rectifiers,molecular transistors and molecular wires,while their functions are similar to those of macro components.The idea of constructing molecular devices has promoted the progress of experimental measurement technique,such as scanning tunnelling microscopy break junction（STM-BJ）and mechanically controllable break junction（MCBJ）,which has also derived a new research field—molecular electronics.One of the problems to be solved in molecular electronics is the electronic transport mechanism of molecules.At present,the main method is to measure molecular conductance by constructing molecular junction（metal-molecule-metal junction）.However,molecular conductance is affected by many factors,such as electrode material,external environment,molecular structure and so on.The molecular structure is an important research direction,including anchoring groups,substituents,molecular conjugation,molecular length,etc.In the field of molecular electronics,searching for molecules with special functions is the key to the design of molecular devices.Although there have been good research results,molecular electronics still needs a long development process,and molecular structure as a key part of the impact of molecular electron transport still needs a lot of experimental exploration.In this thesis,Au was used as the working electrode to study the molecular electron transport behavior with different structures by scanning tunneling microscopy（STM-BJ）.The main research contents are as follows:1.With Au as the electrode,we explored the influence of metal ions on the conductance of protoporphyrin molecules（TPP）and metalloporphyrin molecules（Zn TPP,Cu TPP,Fe TPP）with BT（dihydrobenzothiophene）as the anchoring group.The effect of metal ions on molecular conductance was verified by I-V（current-voltage）experiments.It is found that the molecular conductance decreases by 5 times after the introduction of metal ion coordination,and the order of conductance is TPP>Cu TPP>Fe TPP>Zn TPP,which was consistent with the results of the theoretical transmission coefficient spectrum.This work contributes to a better understanding of the effect of porphyrin-coordinated metal ions on electron transport,and provides an experimental and theoretical basis for the design of high-performance molecular devices based on metalloporphyrin molecules.2.Using Au as the electrode,the electron transport properties of heterocyclic and homocyclic ferrocene molecules were investigated by STM-BJ technique.Through the conductance measurement and molecular junction distance modulation experiment of heterocycle ferrocene molecule（1b）with pyridine as the anchoring group,heterocycle ferrocene molecule（1a）and homocycle ferrocene molecule（2）with BT as the anchoring group,it is found that heterocycle ferrocene molecule has two sets of conductance values,which is caused by the rotation of two alkyne side arms of the molecule around Cp-Fe-Cp,contributing to the different molecular structure types–stretching configuration or folding configuration.However,the homocycle ferrocene molecule only has a low conductance state,which is attributed to the fact that the two alkyne side arms of the homocycle ferrocene molecule are connected to the same ferrocene ring with only one stretching configuration.Further theoretical calculations show that the transition between the stretching configuration and the folding configuration is about 60°（the dihedral angle between the two alkyne side arms along the central heterocycle ferrocene axis）for molecules 1a and 1b.Our work shows that repeated mechanical elongation and compression of molecular junctions can realize the switching between high and low conductance states for heterocycle ferrocene molecule,which provides excellent conductance switching ratio.The current work provides a new way for the design of high-performance molecular switches.3.With Au as the electrode,STM-BJ was used to explore the effect of concentration on the conductance of molecules with different structure.For conjugated molecules,the single molecule junction conductance increases up to three times with the increasing of molecular concentration from 10^-7M to 10^-3M.Which is likely to form a double molecular junction and produce enhanced quantum interference.For unconjugated molecules,increasing concentration has little effect on molecular conductance.We also used the meta-BT molecules with destructive quantum interference to measure the conductance.At the low concentration of 10^-6M,there was no obvious conductance peak,while there was an obvious conductance peak at 10^-5.21 G₀ at the concentration of10^-3 M.This indicates that the destructive quantum interference is weakened or disappeared,and the increasing concentration enhances the electron transport of molecules.Our work reveals the effect of concentration on electron transport in molecules with conjugated or unconjugated structure.