Quantum Transport For Graphene-Based Nanostructures And Its Manipulation

With the development of nanotechnique and the exploitation of new material, the various mesoscopic-nano structures have been fabricated, such as quantum dot(QD) and quantum wire, graphene and its nanoribbons. Due to their potential applications in every aspect of our national defence and lives, the investigation of quantum transport in the mesoscopic-nano systems have become one of the hot topics in the condense matter physics. The thesis has mainly investigated the electronic transport and its manipulation for the graphene-based hybrid systems.The thesis is divided into six chapters as follow:In chapter1, the fabrication and basic properties of mesoscopic structures concerned by this thesis, the previous main research about electronic transport in some interesting mesoscopic systems, and our investigations including methods are introduced simply.In chapter2, we investigate the electronic state around the lattice site of armchair-edged graphene nanoribbons(AGNRs), and probe it by way of the scan-ning tunneling microscopy(STM) current with a semi-infinite quantum-wire STM tip. It is shown that the semiconducting AGNRs possess only semiconducting electronic states while metallic and semiconducting electronic states coexist in the metallic ones where the semiconducting electronic states occupy the3jth (j is positive integer) carbon chains from the edge (the1st chain), and other chains are occupied by metallic electronic states. The interlinked sites on the (3j-1)th and (3j-2)th chains construct a plane channel for the low energy electrons, and the3jth chain embedded in two channels ensures their independence of electronic transport. Interestingly, when the STM tip closes to carbon sites with semicon-ducting electronic state, a wide zero STM current fiat with a threshold voltage Vc appears while it disappears for the lattice sites with metallic electronic state. As for the very wide AGNRs, i.e., graphene steets, the semimetallic and semiconduct-ing electronic states also coexist in the edge region about7nm width. Surprisedly, a visible zero STM current flat with a small Vc always appears for alll carbon sites. Moreover, When the STM tip moves away from the edge to the center, the Vc decreases, and stabilizes lastly. Additionally, the line shape of voltage-current in the AGNR system is wrinkled while it is smooth in the graphene sheet system. Based on above, it is possible to measure the width and electrical conductivity of AGNR samples by way of the STM technology.In chapter3, we investigate the electronic state around the lattice site of zigzag-edged graphene nanoribbons(ZGNRs), and probe it in the same STM tech-nology as Chapter2. It is shown that electrons around the Fermi level are not only localized on the edge sites but also on the sites of the odd number chains (2j-1, j is positive integer) from its edge (the1st chain), more nearly and more strongly. However, they are active on the even number chains. These characteristics can be displayed by the STM current. When the STM tip closes to the sites at the odd number chain, the STM current is very small nearly to zero at the low bias voltage V(<Vc). The threshold voltage Vc is maximal at the edge chain, and de-creases rapidly from the edge chain to the center ones. Interestingly, a large STM current with very low V is seen on even number chains, more nearly to edge, more clearly. Thus, we concluded that only the even order number carbon chains near the edge provide good conducting channels. Additionally, based on the different STM characteristics between AGNRs and ZGNRs, one can distinguish them using STM technology.In chapter4, we study theoretically the manipulation of electronic transport through a strongly correlated QD-reservoir(QDR) system by a graphene. It is found that when the Dirac energy of graphene is fixed the Fermi energy(ε=0) of the QDR system, the graphene closes to the QD more nearly, Kondo correlation between the QD and electrodes becomes more active. Thus it is possible that the Kondo effect is experimentally observed at high voltage bias in the graphene-QDR system. Moreover, when the Dirac energy moves far away εF, the Kondo effect of the QDR system is suppressed and even quenched by the strengthening Kondo correlation between the QD and graphene. In this case, the graphene-QDR system acts as a field effect transistor because the current of the QDR system can be adjusted easily by the graphene.In chapter5, we theoretically investigate the manipulation of electronic trans-port through a QDR system by a small mesoscopic AB ring. It is shown that the mesoscopic AB ring side-coupled to QDR system just likes an artificial molecule with many energy levels, providing many disturbed ways for electron to tunnel the QD from the source electrode to drain one. Furthermore, these ways can be chosen by magnetic flux φ of the AB ring,thus, tunable fano effects with many energy levels appear in the ring-QDR system. Interestingly, for the even parity ring-QDR system, there always has one disturb way fixed at the Fermi energy (εF) of the QDR system, adjusting φ can choose its "on" or "off". Once in "on", the Kondo effect in the QDR system is quenched fully by the Fano effect at εF in the ring-QDR system, which leads to Kondo channel at εF, from the source electrode to drain one via QD, closing for electron transport. Contrarily, the Kondo channel is open. Thus the magnetic flux quantum switch effect is realized easily in the even parity ring-QDR system. However, in the odd parity ring-QDR system, it only appears in the case of sufficiently strong ring-QD coupling tc. With the increase of tc, the superposition of many Fano interferences produces the Kondo-Dicke effect in the ring-QDR system, inducing the appearance of zero different conductance wells and stepped current in the QDR system.In chapter6, we summary our work and draw an outlook of the future inves-tigation prospect of this topic.