Analysis Of Impacts On The Theoretical Calculation Of Single-molecule Conductance

Molecular junctions are potentially applied as an important electric component for the next generation in the electronic field.Structures and properties of such molecular components have already been theoretically researched.Unfortunately,disparity can be seen in the conductance of the single molecular junctions between the theoretical calculations and experimental measurements,especially when a molecule with a conjugated part is connected between two metal electrodes in the metal-molecule-metal junction,the disparity can be 1-2 orders of magnitude.To explain the cause of such disparity,the impacts on the electron transfer are analyzed from aspects of function,voltage and distance of electrodes in Au-BDMT-Au molecular junction and first-principle density functional theory(DFT)combined with non-equilibrium Green' s function(NEGF)approach is applied during the research.The results are as follows.(1)After the bias voltage are applied into the junction,the lengths of Au-S and S-C bonds varies asymmetrically,affecting the molecular energy spectra and current-voltage curves.Since various empirical fits are used,different functions also lead to different structures together with energy spectra and conductance.(2)As the distance between the electrodes increases,the molecular conductance calculated theoretically decreases exponentially and then becomes flat,which refers to the conductance of the single molecule;the conductance of the flat part is still several times larger than the experimental measurement.The tension of the molecule may become larger when the electrodes become farther or nearer because of the deformation,with lower junction stability.(3)The Fermi energy level of Au electrode in the software of ATK is higher than that experimental measurement,causing HOMO and LUMO of the molecule overestimated;PBE function which is common used may also underestimate the energy gap between HOMO and LUMO.Both of the two reasons cause a higher conductance.This thesis mainly contains four parts as below.The first part contains the background of electronics,research situation and problems both experimental and theoretical,together with the mechanism of electron transfer in metal—molecule-metal junctions to draw forth the significance of this thesis.The second part introduces the theoretical method,calculation basis and software applied.The third part analyzes the results about the work and possible reasons for the disparity from three impacts.The fourth part is the conclusion and prospect.