The Research On Rectifying Performance Of D-σ-A Organic Molecular Devices

Molecular electronic shows the possibility of building the circuit based onmolecule, and is deemed to be the best candidate of furture information technology.As fundamental one of the molecular electronic, molecular rectifier has animportantly theoretical significance and the promising prospect for realisticapplications, which has been a focus of study in the molecular electronics. In thispaper, we perform the first-principles calculations based on the density-functionaltheory combined with nonequilibrim Green’s function method to investigaterectifying performances and transport properties of D-σ-A molecular rectifiers，inorder to build the molecular rectifier with high rectification ratio and stable control.We firstly introduce the formed background of the molecular electronics, thebasic mechanism and the recent development of D-σ-A molecular rectifiers, andtheoretic basis of molecular electronics. On this basis, we design the A-R rectifierbased on D-σ-A molecules to examine the rectifying performances. Our main worksinclude: by carbon chain C7acting as the side group attached to either the left orright side or both sides of the D-σ-A molecule, we investigate theoretically itsrectifying performance. The calculated results show that increasing geometricalasymmetries of a molecule may not be a necessary condition to greatly promoterectifying characteristics, and the suitable side-group engineering might be a keyissue. We also investigate rectifying performances and the electronic transportproperties of the D-σ-A molecular devices with different lengths spacers. Thecalculated results show that the rectifying performances and the negative differentialresistance have a special dependence on the length n of the spacers. The currentrectification ratio of several hundreds is achievable because of the molecular stateslocated asymmetrically with respect to the electrodes. The models show a strikingodd-even effect: RRmax of the models clearly demonstrate a dramatic oscillatingbehavior, even-n models have semiconductor behavior but not show any NDR, andthe odd-n models have metallic behaviors and a remarkable NDR phenomenon. Thepronounced NDR is produced by the depression of the tunneling peak under theapplied bias. However, for the carbon-chain-modified D-π-A molecular devices,almost no rectifying performance can be found. Finally, we investigate a rectifyingperformance and electronic transport properties of four different molecular devicesbased on different end groups for the same D-σ-A molecule. The results show that the end groups can significantly affect the rectifying performance of such molecularrectifiers, because the end groups can influence the coupling effects between themolecule and the electrodes, thus changing the delocalization of molecular orbitals,and further changing their transport properties and rectifying performance. Moreinterestingly, it has been found that the rectifying directions and working mechanismfor all of our studied systems are in disagreement with ones proposed originally byAviram and Ratner. This property can be rationalized through the asymmetric shift ofmolecular levels under biases of different polarities.