The Electronic Transport Properties Studies Of Graphene Artificial Structures

Graphene, a single layer of carbons, is found to exist as a free-standing form and exhibits many unusual and intriguing physical, chemical and mechanical properties. In the past few years, graphene which has caused researchers’extensive attention is becoming the leading edge and hot topic in various relative fields. In this paper, electronic transport properties of some graphene artificial structures have been simulated and computed, which could provide theoretical guidance for their applications in nano—electronic devices. The major contents and important results are given as follows:(1) The effects of boron nitride (BN) nanodot on the electronic and transport properties of armchair graphene nanoribbons are studied by first-principles calculations. The results show that the band structure of the graphene superlattice strongly depends on the geometric shape and size of BN nanodot, as well as the concentration of nanodot. The conduction bands and valence bands near the Fermi level are nearly symmetric, which is induced by electron-hole symmetry. When B and N atoms in the graphene superlattices with triangular BN nanodot are exchanged, the valance bands and conduction bands are inverted with respect to the Fermi level due to electron-hole symmetry. In addition, the hybridization ofπorbitals from C and redundant B atoms or N atoms leads to a localized band appearing near the Fermi level. Our results also show a series of resonant peaks appearing in the conductance. This strongly depends on the distance of the two BN nanodots and on the shape of BN nanodot. Controlling these parameters could be possible to modulate the electronic response of the systems.(2) The current-voltage (I-V) characteristics and the transmission spectrums of zigzag graphene nanoribbon with different spin-configurations are investigated by using the first-principles calculations. It is shown that the current-voltage curves and the transmission spectrums strongly depend on the spin-configurations of the two sides of the ribbon. For the spin-parallel configuration structure, the curve is linear under lower bias voltage; for the spin-antiparallel configuration structure, there is a strong spin-polarization-dependent transmission which implies the ribbon can be used as a spin filter; while for other spin-configurations structures, the curve has the characteristic of semiconductor. It is found that there is a large magneto-resistance (MR) when the bias voltage is small. The impurity in the central scattering region significantly influences the spin-dependent current and the spin filter efficiency, which may lead the large MR to disappear.(3) We perform first-principles calculation to investigate the spin-dependent electronic transport in edge-defect junction, which constructed by removing carbon atoms from the edges of ZGNR. The H-terminated and bare edge-defect junctions have been considered. The results show that the existence of edge-defect changs the electronic transport behavior from the spin degenerate case of perfect to highly spin polarized at the Fermi level. The electronic LDOS isosurface calculations help the understanding of the transport results. These devices could generate spin-polarized currents under bias. Especially, the bare edge-defect junction has higher spin filter efficiency regardless of the external bias. The study of ZGNR edge-defect junction benefits graphene integrated circuit engineering that might be realized by ultrafine GNR fabrication technologies in the future and can be useful in novel spintronics applications.(4) We have studied the eigenenergy and the resonant tunneling modes of the quantum well consisting of graphene superlattices with modulated periodical potentials. The eigenenergy equation is presented in terms of the transfer matrix elements. The results show that the number of eigenmodes is equal to the number of periods in the well region. If there are N quantum wells, the inter-well coupling leads to the resonant modes N-fold splitting. The locations of the resonant modes shift toward the high energy with increasing the incident angle, and the width of resonant peaks becomes very narrow due to the strong confinement effect at large incident angle. The behaviors of the tunneling can be controlled by changing the superlattice potential. The results could be helpful for the design of electronic filter of graphene devices.