Energy Gaps In Graphene Nanoribbons Research Articles

Graphene and boron nitride nanoribbons with multiple doping: an ab initio study

Nanoribbons are strong candidates for use in future nanoelectronics devices due to reduced dimensionality end fascinating properties. Despite their rich properties, they have some limitations such as the wide bandgap and no magnetic order of boron nitride nanoribbons and the absence/small energy gap of graphene nanoribbons. The chemical doping of nanoribbons is an interesting topic since the structural, electronic, magnetic, and quantum transport properties could be significantly altered depending on the species of the dopant, the location of the dopants in the structure and their concentration. Here, we study the chemical doping of armchair and zigzag graphene/boron nitride nanoribbons using density functional theory. We investigate the structural, electronic and magnetic properties of these systems. We find that the armchair edge could reduce its energy by establishing a double bond between the outer carbon atoms. Chemical doping with boron and nitrogen atoms in graphene nanoribbons act as a p-type and n-type dopant, which introduce impurity states close to the valence and conduction bands, increasing/opening the energy gap tuning both the nanoelectronic and the nanomagnetic properties. In boron nitride nanoribbons the controlled chemical doping shows itself an important tool to reduce the wide energy gap and to introduce magnetism in these systems. The zigzag graphene nanoribbons, originally, present an imbalance between spin up and spin down, such as their edges belong to different sublattices. However, the doping, in different sublattices, due to the same type of atom, produces a balance between spin up and spin down, resulting in a null polarization.

Energy gap of novel edge-defected graphene nanoribbons

Herein, the effects of width and boundary defects on the energy gap of graphene nanoribbons (GNRs) have been explored and theoretically investigated by means of semi-empirical atomic basis Extended Hückel method. Due to the existence of boundary defects, the energy gap of GNRs is mainly determined by the width of graphene nanoribbons for armchair graphene nanoribbons (AGNRs) or zigzag graphene nanoribbons (ZGNRs). Interestingly, the energy gap of AGNRs with a 120° V-type defect displays the monotone decreasing tendency when the width reaches to 2 nm, while the energy gap of intrinsic AGNRs is oscillatory. At the same time, the energy gap of U-type defected ZGNRs is opened, which differs from the zero energy gap characteristics of the intrinsic zigzag graphene. Furthermore, the size of energy gap of the defected AGNRs and ZGNRs with the same width is proved to be very close. Calculation results demonstrate that the energy gap of GNRs is just inversely proportional to the width and has little to do with the crystallographic direction. All the findings above provide a basis for energy gap engineering with different edge defects in GNRs and signify promising prospects in graphene-based semiconductor electronic devices.

Conductance gap induced by orbital symmetry mismatch in inhomogeneous hydrogen-terminated zigzag graphene nanoribbons

Using the non-equilibrium Greens function method in combination with the density functional theory, we investigate the electronic transport properties of zigzag graphene nanoribbons (zGNRs) passivated with Hydrogen atoms. The coexistence of sp2-edges and sp3-edges in zGNRs can induce quite stable conductance gaps in a large range from 0 to 3.5eV, which can even occur in a wide zGNR with a width up to 10nm. We found that the orbital symmetry mismatch between the sections with sp2 and sp3 edges is responsible for the totally suppressed conductivity of the edge states, and the gap size is determined by the minimal energy difference between the second highest valence band and the second lowest conduction band of the corresponding sp2-edged ribbons. These findings provide a very attractive design and fabrication strategy for controlling the energy gap of graphene nanoribbons.

Erratum: Energy Gaps in Graphene Nanoribbons [Phys. Rev. Lett.97, 216803 (2006)

Erratum: Energy Gaps in Graphene Nanoribbons [Phys. Rev. Lett.<b>97</b>, 216803 (2006)

Energy Gaps in Graphene Nanoribbons

Based on a first-principles approach, we present scaling rules for the band gaps of graphene nanoribbons (GNRs) as a function of their widths. The GNRs considered have either armchair or zigzag shaped edges on both sides with hydrogen passivation. Both varieties of ribbons are shown to have band gaps. This differs from the results of simple tight-binding calculations or solutions of the Dirac's equation based on them. Our ab initio calculations show that the origin of energy gaps for GNRs with armchair shaped edges arises from both quantum confinement and the crucial effect of the edges. For GNRs with zigzag shaped edges, gaps appear because of a staggered sublattice potential on the hexagonal lattice due to edge magnetization. The rich gap structure for ribbons with armchair shaped edges is further obtained analytically including edge effects. These results reproduce our ab initio calculation results very well.