Theoretical Study On The Vibrationally-Resolved Conductance Spectra And STM Images Of Two Polycyclic Aromatic Hydrocarbon Molecules

Scanning tunneling microscope(STM)is capable of measuring the electronic properties of molecular systems at the single-molecule level.In particular,by introducing a decoupling layer,the interaction between the molecule and the metal substrate can be significantly reduced which enables the detection of vibrationally-resolved molecular conductance spectra.This makes STM a powerful tool for the chemical identification at the singlemolecule level and the real-space investigation of the electron-vibration coupling effect of molecular systems.In this thesis,two types of decoupled polycyclic aromatic hydrocarbon molecules,namely 5,10,15,20-tetraphenylbibenzo [5,6] indeno [1,2,3-cd:1’,2’,3’-lm](DBP)and 5,6,11,12-tetraphenyl(Rubrene),were studied.Based on first-principles calculations,the vibrationally-resolved differential conductance spectra and the STM images associated with the frontier orbitals of the two molecules are systematically studied,which enables us to explain some interesting experimental findings.The main work of this thesis consists of the following two parts:Firstly,theoretical calculations were performed to study the variation of the vibrational characteristics of the conductance spectra and STM images associated with different frontier orbitals of the DBP molecule.The stable structure and vibrational information of free DBP molecule were obtained based on three commonly used hybrid density functionals,B3 LYP,CAM-B3 LYP and B97XD.The calculation results show that the four out-of-plane phenyl groups of DBP molecules are perpendicular to the molecule plane in free space.On this basis,the vibrationally-resolved conductance spectra were calculated and compared with the experimental results.All three functionals were found to describe nicely the vibrational fine structures in the conductance spectra of the DBP molecule,but the results given by CAM-B3 LYP and B97XD were in relatively better agreement with the experimental results.Based on the calculation results,the vibrational modes involved in the charge transport process and the strength of their corresponding electron-vibration coupling are determined.It was found that the differences of the spectral shape of the conductance spectra associated with the different frontier orbitals mainly originate from the changes in the electron-vibration coupling characteristics for the corresponding electronic transitions.In addition,the STM images corresponding to the frontier molecular orbitals were calculated based on the Bardeen approximation.It was found that the changes in the spatial distribution of the molecular orbital wave functions was the main reason for the variation of the STM images.Meanwhile,the STM images were also calculated when the angle of the out-of-plane phenyl groups of DBP molecules were changed.It was found that better agreement with the experimental images were obtained when the phenyl groups were rotated by 45°,indicating that the out-of-plane phenyl groups of DBP molecules might be rotated after adsorption.Secondly,the vibrationally-resolved conductance spectra and STM images of the Rubrene molecule were theoretically studied.Based on the experimental results,it was found that the commonly used PBE and B3 LYP functionals can describe the conductance properties quite well.Moreover,the agreement between the calculation and the experimental results can be further improved with the long-range corrected CAM-B3 LYP and B97XD functionals.In addition,it was found that the size of the basis set used in the calculations did not significantly affect the results,and the commonly used 6-31G(d,p)basis set was sufficient to describe the conductance properties of the Rubrene molecule.The Duschinsky rotation effect and the frequency change of different charge states of the molecules were found to have significant influence on the conductance spectra,which indicates that such effects is important for the correct description of the charge transport process of such spatially distorted complex molecular systems.Simulations of STM images of the Rubrene molecule indicate that the highest occupied molecular orbital and the lowest unoccupied molecular orbital have similar spatial distributions with two discrete bright spots.The main difference between the two sets of STM images is the different spacing between the bright spots and the presence of rotation with respect to each other.