Electronic and Magneto-Electronic Properties of Nanopatterned and Multilayered Graphene.

摘要

Various approaches to induce a band gap in graphene based structures are theoretically investigated. The band structure and the electron transport of the proposed devices are calculated using semi-empirical extended Huckel theory (EHT) coupled with the nonequilibrium Green's function (NEGF) formalism. We consider a stacked structure of two arm-chair nanoribbons and observe negative differential resistance (NDR) behavior in the simulated current-voltage (I -- V) characteristics. The magnitude of the NDR decreases with an increase of the ribbon width. A 2D nanomesh structure of graphene patterned with a periodic array of nano holes is also investigated. The results suggest that the bandgap opening is a result of quantum confinement. However obtaining a modest bandgap in graphene often comes at the expense of strongly degraded electron mobility with lithographic difficulties. Therefore, an unconventional biasing approach of modulating the I -- V characteristics without inducing any bandgap is studied. In such a scheme, NDR is observed in both single layer and bi-layer graphene field-effect transistors. The NDR is an intrinsic property of graphene resulting from its symmetric band structure.;Experimentally, multiple layers of graphene tend to be misoriented with respect to each other. The effects of magnetic field and interlayer bias on the interlayer electron transport of large misoriented bilayer graphene nanoribbons is calculated. Edge states can result in a large peak in the transmission at the charge neutrality point that is several orders of magnitude larger than the surrounding low-energy transmission. The transmission is consistently asymmetric around the charge neutrality point for all structures with the value differing by up to 3 orders of magnitude within 50 meV on either side of the charge neutrality point. The low-energy states exhibit a high magnetoconductance ratio, and the magnetoconductance ratio tends to increase as the width of the ribbons decrease. The maximum value of magnetoconductance ratio for the 35 nm wide bilayer ribbons at 10T is 15,000%. The effect of the bias on the transmission gives rise to non-linear I -- V characteristics.