Anomalous Doping Effects on Charge Transport in Graphene Nanoribbons

We present first-principles calculations of quantum transport in chemically doped graphene nanoribbons with a width of up to 4 nm. The presence of boron and nitrogen impurities is shown to yield resonant backscattering, whose features are strongly dependent on the symmetry and the width of the ribbon, as well as the position of the dopants. Full suppression of backscattering is obtained on the $\pi -{\pi }^{*}$ plateau when the impurity preserves the mirror symmetry of armchair ribbons. Further, an unusual acceptor-donor transition is observed in zigzag ribbons. These unconventional doping effects could be used to design novel types of switching devices.

Figure 1

Left panel: Conductance of the 35-aGNR as a function of energy for different B positions. The dashed lines correspond to conductance for the undoped case. Insets: same as in main frame for two selected nitrogen dopant positions [at the edge (E) and off-center (B)] for the 20-aGNR. Right panel: Unit cell of the 35-aGNR showing the considered dopant positions. The carbon atoms are shown in a lighter color (grey online) and the passivating H atoms as smaller and darker atoms (pink online); the darker (colored online) atoms with labels A–E represent B or N atoms, each label (and color online) referring to the corresponding labelled (colored online) solid line conductance curve.

Figure 2

Conductance as a function of the energy (left panel) and band structure (right panel) of the 35-aGNR [35]. Dashed lines correspond to the undoped ribbon; solid lines correspond to the case of B at the center. The solid blue (dashed red) arrow shows the first (second) band below the charge neutrality point (CNP) for the doped case.

Figure 3

(a–c) 2-dimensional projection of the wave functions at the $\Gamma$ point associated with the first occupied band below the CNP [solid (blue online) arrow in Fig. 2], for a 35-aGNR with B at the center (a), no dopant (b), and B off-center (c). (d–f) Same as in (a–c) for the second occupied band below the CNP [dashed (red online) arrow in Fig. 2]. The position of the B impurity is marked as a dark atom (green online). The darker (blue online) or lighter (yellow online) color corresponds to the opposite sign of the wave function. Dashed lines show the ribbon axis (located exactly in the middle of the ribbon).

Figure 4

Conductance as a function of the energy for the 12-zGNR (left panel) and 20-zGNR (right panel) with a single B dopant placed at the ribbon edge (E, top panel) and off-center (B, bottom panel). The dashed lines correspond to the undoped case. The labelled darker (red online) or lighter (light blue online) colored atoms in the unit cell show the position of the dopant for each ribbon.