Application Of Valence Bond Thoery In Electron Transport Through Coulomb Blockade Single Molecule Devices

Single electron transistor (SET) was proposed three decades ago since the minia?turization trend of electronic devices. SET works in coulomb blockade regime, whichallows only one electron to hop/out of the device during its working cycle, thus achievedthe limit of device power consumption. A decade later, benefit from the advances insingle molecule preparation technique, researchers further miniaturized the meso-scopicSET into a single molecule size, made the so-called single molecule single electron tran?sistor possible. The key of single molecule SET is a functional molecule accessible toseveral redox states, while in functioning, molecules transit among their redox statesunder coulomb blockade constraint.Inspired by those progresses, and been aware of the incapability of popular DFT-NEGF theory in describing coulomb blocked regime of transport. This thesis choseValence Bond theory as the first principle method in combine with a Master equationtechnique as the dynamic theory to model the electronic structure and electron trans?port process. A benzene based SET is detailed discussed under this framework, boththe electronic structure and electron transport behaviors are analysed for illustration anda preliminary application. While ionized, benzene molecule encounters Jahn-Teller ef?fect, which would remove the degeneracy of states. The lifted degenerate states couplesdifferently to left/right electrodes, which eventually leads to a negative differential con?ductance (NDC) phenomena. This kind ofNDC mechanism holds some universality andmay guide us to better find a NDC molecule: molecule transits to a low symmetry statewhile redoxed and state degeneracy is lifted meanwhile, which leads to NDC.The second order effect, co-tunneling, is also studied as a complimentary. Thoughsubtle the co-tunneling current is, it posses some very different mechanism from the firstorder processes—coherence. Coherence here may cause interesting single moleculeinterference effects. And it is proposed that the interference may well resist the ther?mal vibration and survive under room temperature. The effectiveness of second orderco-tunneling is compared to the full order NEGF method, which turns out that underthe weak molecule-electrode coupling limit, second order expansion gives fairly goodresults. The comparison well testified the validity of our model under weak couplinglimit.