Theoretical Study On The Influence Of External Force And Electric Field On The Electrical Transport Properties Of Molecular Junctions

Due to the influence of quantum effect,the size of traditional silicon-based devices is difficult to reach the level of single molecule,so the bottom-up method to prepare single molecule devices has attracted the attention of many researchers.Researchers have developed various experimental methods for preparing molecular devices.In the experiment,the molecular configuration,the molecular element composition,the number of connected molecules,the contact configuration between the molecule and the electrode and other factors have great uncertainty.However,these factors can be determined by using surface-enhanced Raman spectroscopy（SERS）,inelastic electron tunneling spectroscopy（IETS）,infrared spectroscopy（IR）,X-ray photoelectron spectroscopy（XPS）and other measuring techniques.In order to further understand the properties of molecular devices,density functional theory was used to simulate the SERS spectrum of OPE-3 molecules,and combined with the experimental Raman spectrum,non-equilibrium Green’s function was used to calculate the electrical transport properties of the molecular devices.The first chapter introduces the development and research status of molecular electronics.Firstly,the experimental preparation and measurement techniques of molecular electronics,including mechanical controlled splitting,scanning tunneling microscopy,atomic force microscopy and surface-enhanced Raman spectroscopy,are briefly introduced.Secondly,it summarizes the common functional molecular devices,including diodes,rectifiers,molecular switches and transistors,and introduces some factors that affect the electrical transport properties of molecular devices,such as electrode materials,anchoring groups,connecting molecules and other external factors.The second chapter briefly introduces some basic theories involved in theoretical calculation.Firstly,Schrodinger equation,theoretical approximation and density functional theory are introduced in quantum chemistry.Secondly,we introduce the non-equilibrium Green’s function and its calculation process for the molecular junction transport properties.Then we introduce the method of solving the Raman spectral intensity based on the theory of Raman scattering and quantum chemistry describing the scattering process.Finally,the application and function of Gaussian,Multiwfn and ATK software packages used in calculation are introduced.In the third chapter,the OPE-3 molecular model was used to investigate the reasons for the new Raman vibration of OPE-3 molecular junction near 1400 cm^-1.The effects of different electric charges,electric fields and the deformation of molecular geometry on the Raman spectra of OPE-3 molecules were calculated.Finally,it is found that molecular charge is the cause of the new Raman vibration in the experiment.That is,when the molecular charge is-1,the vibration peaks and relative intensities of Raman spectra are consistent with those of the simulation and experiment.In order to objectively evaluate the potential of OPE-3 molecular junction as a future semiconductor,the effect of molecular length on Raman spectra was investigated,and it was found that molecular length had little effect on Raman spectra.Then,the effects of the deformation of OPE-3 molecular geometry,external forces（stretching/compression）,different electrode materials and electron-absorbing and electron-donating groups on the electron transport capacity of molecular devices were investigated.The results show that with the increase of the dihedral Angle of molecular rotation,the electron transmission ability of the molecular device decreases gradually until the rotation Angle is 90°,the device almost completely loses the conductivity.Under the action of external force（tension/compression）,the electrode distance changes,and the molecular length also expands and shorens,which leads to the reduction of the electron transport capacity of the molecular device.The electron transmission capacity of the device decreases with the change of Au,Ag and Cu electrode materials.For the OPE-3 molecule,the addition of both electron-donating and electron-sucking groups reduces the electron transport capacity of the molecular device.For e electron-absorbing groups,the conductivity of the connecting group at 1position is greater than that of the connecting group at 2 position.In Chapter 4,12 amine-terminating molecules were randomly selected from the chemical universe database GDP-17,and their electrical transport properties after forming molecular devices with graphene electrodes were investigated.The results show that only M4,M5 and M6 have good rectification effect,especially M4,whose rectification ratio is as high as 1.3×10²³ at 3.2 V.The remaining molecular devices have some ability of electron transport,but their current-voltage curves are quite volatile and lack practicality.