Conductance of Sidewall-Functionalized Carbon Nanotubes: Universal Dependence on Adsorption Sites

We use density functional theory to study the effect of molecular adsorbates on the conductance of metallic carbon nanotubes (CNT). The five molecules considered (${\mathrm{NO}}_2$, ${\mathrm{NH}}_2$, H, COOH, OH) lead to very similar scattering of the electrons. The adsorption of a single molecule suppresses one of the two available transport channels at the Fermi level while the other is left undisturbed. If more molecules are adsorbed on the same sublattice, the remaining open channel may or may not be blocked, depending on the relative position of the adsorbates. If the relative positions satisfy a simple geometric condition, this channel remains fully open independently of the number of adsorbed molecules.

Figure 1

Top-Left: Transmission curves for a (6,6) carbon single wall nanotube (SWNT) with a single molecule chemisorbed on top of a C atom: COOH (black), H (red), ${\mathrm{NH}}_2$ (green), ${\mathrm{NO}}_2$ (blue) and OH (orange). The transmission curve for the pure nanotube is also shown (black dashed line). Bottom-Left: The same for a (9,0) CNT. Bottom-Right: The same for a (8,2) CNT. Top-Right: Transmission of the different eigenchannels for a (6,6) CNT with a single COOH molecule adsorbed. Notice that channel 1 (solid line) is completely unaffected by the impurity, while channel 2 (dashed line) is completely suppressed in the vicinity of the Fermi level.

Figure 2

Contour surfaces of the two eigenchannels at the Fermi level of a (6,6) CNT with a single hydrogen atom adsorbed on top of a C atom. The states correspond to channels 1 and 2 in the top-right panel of Fig. 1. The position of the hydrogen atom is indicated by the blue arrow. The CNT is shown unrolled for clarity. (a) Fully transmitting eigenchannel, $T=1$. (b) Blocked eigenchannel, $T=0$. Notice that channel 1 has zero weight on all sites fulfilling Eq. (2).

Figure 3

Top: Transmission functions for a (6,6) carbon SWNT with two hydrogen atoms adsorbed at different $A$ sites. The six inequivalent $A-A$ configurations in one unit cell of the CNT are shown. The blue solid lines correspond to the cases where Eq. (2) is fulfilled. The red dashed lines correspond to the cases where Eq. (2) is not fulfilled. The transmission function for the pure CNT is also shown (black dashed line) Middle: The same for a (9,0) CNT. We show the 9 inequivalent $A-A$ configurations in one unit cell of the CNT. Bottom: The same for a (8,2) CNT. We show out 6 of the 14 inequivalent $A-A$ configurations within a CNT unit cell.