Study On The Mechanisms Of Quantum Control Of Electronic Transport In Carbon-Based Molecular System And Its Application

In this thesis, we have performed molecular dynamics simulations combined with first-principles theory and nonequilibrium Green’s function approaches to systematically study the quantum control of electron transport in molecular systems. For exploring the quantum control mechanism and method, the effects of molecular configuration, hydro-gen bond, the π-π stack of molecules, the length of molecular chain, the phase inter-ference of electrons in ring-type molecular circuits, the coupling between molecule and electrodes, the length of carbon nanotubes (CNTs) based heterojunction, the roughness of broken carbon nanotubes (BCNTs), the topological detail of the contact between BCNTs electrodes and molecules, the conformation of the molecule in the junctions, the separator strip of hydrogenated graphene nanoribbons(GNRs), the width and connection length of zigzag/armchair GNRs based heterojunctions, and the chemical modification of the edge of chiral GNRs on the electronic transport properties of molecular devices are studied par-ticularly. Strong negative differential resistance (NDR) behaviors and high-performance electric rectify effects have been observed. Molecular switches with high on/off ratio, molecular rectifications with high performance, and spin-depending molecular devices with strong current polarization have been designed. Some important issues in present molecular electronics, such as the impedance of molecular side-chain, the feasibility of DNA-sequencing electrically, and the mechanism of enhancing the electronic transport properties of graphene by functional modification have also been discussed preliminarily in the present thesis.We investigated the electronic transport behavior of metal-bimolecule-metal junction based on single walled CNTs (8,0), and found some interesting physical effect. We found that the inter-molecular interaction can induce the molecular orbitals splitting, which shows a strong dependence on the inter-tube distance and the rotational angle. The orbital splitting can create some additional orbitals, which offer new electron transport channels and contribute to the conductance of the junction. Especially, with the additional orbitals being cross to the Fermi level, the junction can behave like metallic. We propose that bi-molecular system can show quite different electric behaviors by controlling the interaction between the two molecules.The electronic transport properties of the heterojunction constructed by single walled carbon nanotubes with different diameters have been studied. The first principles results show the length and size of heterojunction play crucial roles in controlling the transport gap of the system. Moreover, a faint NDR behavior is observed in certain heterojunctions, we find that a transport channel has been suppressed at certain bias is the underlying phys-ical reason for the observed NDR. Based on our above findings, a bi-heterojunction device with perfect performance has been designed. Due to the position and the conductivity of the electronic transport channels are mainly determined by the coupling between the π systems of the two heterojunctions, we propose that by modifying the coupling between the two heterojunctions, one can control the transport behavior and NDR effect of the systems.Applying molecular dynamical simulations combining the first principle electron structure theory and non-equilibrium green’s function transport calculations, we studied the transport behaviors of CNTs-molecular junction using BCNTs as electrodes. Dur-ing the molecular dynamical simulations, we find that the CNT prefer to break along its chiral angle and form irregular broken ends. The edges of the BCNTs ends mainly have zigzag feature, armchair type and dangling bonds of carbon atoms also appear in the zigzag arrangement randomly. We also find that the roughness of the edges of the broken ends, especially its apex structure, determined the contacting topology between CNTs and molecule. Statistical results show that the molecule prefers to tilted connect to CNTs through polygonal contacts. Non-equilibrium green’s function transport calcu-lations results show that the CNTs-molecular junctions are metallic, which independs on the chiral angle of metallic CNTs electrodes. While the performance of their electronic transport are sensitive to the contact angle of molecular bridge. Hence, by controlling the position of CNTs electrodes, one can operate the molecular conducting switch. The first principle calculations show that the on/off ratio is perfect and the energy required to operate the conductance switch is very low.Applying density functional theory combining non-equilibrium green’s function method, we investigated the possibility for identifying the DNA base pairs through electrical tech-nologies. Results show that the experimentally observed the different decay of the current of DNA base pairs is not due to the different number of hydrogen bonds in different base pairs, but their different stacking structures that plays an important role in determining the transport properties of metal-DNA junctions. So, we think that one key for identifying DNA base pair through electrical technologies is to find an effective technique to make sure the same DNA base pair has the same conformation in the metal-DNA junctions.We have designed zigzag/armchair GNRs heterojunction (zGNR/aGNR) by chemi-cally connecting a zGNR and an aGNR together, and investigated its electronic transport behaviors. Results show that the coupling between the energy bands of zGNR and aGNR, especially the coupling between the frontier molecular orbitals of the two GNRs,is of decisive importance in determining the transport behavior of the heterojunction. Depend-ing on its wide, the junction can be either metallic or semiconducting, and its zero-bias conductance shows a well-defined oscillation behavior. We find that the rectifying is an intrinsic property of the zGNR/aGNR heterojunction, the rectifying direction can also be tuned by the width of the junction, and the rectification ratio can be tuned by the con-nection length between zGNR and aGNR. Our results show that any methods which can enhance the asymmetry of the transmission spectra between holes and electrons could be used to improve the rectification behavior, a high rectification ratio up to104can be achieved in "T"-shaped semi-conducting zGNR/aGNR heterojunctions.We have investigated the electromagnetic transport behaviors of the GNRs with nar-row hydrogenation-strips. We find that a narrow hydrogenated-strip can be of a perfect separator effect; each hydrogenation separator can introduce two conducting edge-like states into the subbands around the Fermi level, which can greatly enhance the conduc-tance of the system. We also find that the system is still spin polarized. Although the band gap of zGNRs cannot be opened by zigzag hydrogenated-strip, the gap of aGNRs can be easily modified by the armchair hydrogenated-strip, and the heterojunction constructed by an aGNR and a hydrogenated aGNR has a striking rectification behavior with very high rectification ratio. Moreover, we find that the hydrogenation separators can screen the im-pact of rough edges, which makes rough-edge zGNRs behave like smooth-edge zGNRs. Due to it is still a considerable challenge to produce narrow GNRs with smooth edges in large scale, our findings could be very useful for designing electromagnetic devices based on the hydrogenation of graphene nanoribbons.Ideal ZGNRs have magnetic edge state while ideal AGNRs do not. By applying first principles, we have studied the electronic structures and spin-polarizations of chi-ral GNRs with (2,1)-edge structures. Rich electromagnetic properties are found in this type of GNRs. The band gap of edge-passivated (2,1)-GNR decreases rapidly with the width increase, while bias-sensitive edge states are found at the Fermi level of the edge-unpassivated ribbons which can lead to a spin-selective transport and a strong NDR be-havior. The chiral GNRs can be either metallic or semi-conducting, depending on the substituted-position and the configuration of the substitution with nitrogen atoms at the edges, and non-marginalized spin-polarization states can be found in the system with some special substitution configuration, while rich spin-polarization patterns can be found in the system when the edges are dihydrogenated. Our findings offer a new way of de- signing GNRs-based electronic and magnetic devices.Furthermore, we have studied the electronic phase interference in molecular junc-tions. We have discussed the underlaying physical mechanism of the length effect of acetylene side-chain and investigated the impedance effect of side-chain. We have derived a function to describe the relationship between the phase-difference of the two backbones and the length of the side-chain in a molecular ring-type junction. We proclaim that the electronic phase in molecular circuit can be tuned chemically, which is useful for predict-ing the interference in molecular circuits and designing the molecule based information devices.