Endohedral terthiophene in zigzag carbon nanotubes: Density functional calculations

The inclusion and encapsulation of terthiophene (T3) molecules inside zigzag single-walled carbon nanotubes (CNT’s) is addressed by density functional calculations. We consider the T3 molecule inside six semiconducting CNT’s with diameters ranging approximately from 8 to $13\mathrm{\AA }$. Our results show that the T3 inclusion process is exothermic for CNT’s with diameters larger than $9\mathrm{\AA }$. The highest energy gain is found to be of $2\mathrm{eV}$, decreasing as the CNT diameter increases. This notable effect of stabilization is attributed to the positively charged CNT inner space, as induced by its curvature, which is able to accommodate the neutral T3 molecule. The band structure of the T3@CNT system shows that T3 preserves its electronic identity inside the CNT’s, superimposing their molecular orbitals onto the empty CNT band structure without hybridization. Our results predict that the electronic states added by the T3 molecules would give rise to optical effects and nonradiative relaxation from excited states.

Figure 1

(Color online) Atomic geometry of the T3 molecule $\left({\mathrm{C}}_{12}{\mathrm{H}}_6{\mathrm{S}}_3\right)$ in the gas phase. The three thiophene units adopt a nonplanar configuration (lower figure) characterized by the dihedral angle S-C-C-S $\left({\theta }_{\mathrm{D}}\right)$.

Figure 2

(Color online) Energy profile during the axial insertion of the T3 molecule into the (13,0) CNT by both clean and hydrogenate entrances. The inset shows the T3 molecule entering inside the (13,0) CNT by the clean entrance.

Figure 3

(Color online) Energy gain as a function of the CNT diameters for the T3 molecule inside the ($n$,0) CNT’s, with $n=10–16$. The zero energy corresponds to T3 in the gas phase plus the empty CNT. The inset shows the unit cell of T3@(13,0).

Figure 4

(Color online) Atomic geometry of the T3 molecule inside carbon nanotubes of different diameters. (a) T3@(10,0), (b) T3@(12,0), and (c) T3@(13,0). The black dot indicates the nanotube centers.

Figure 5

Density of states (DOS) of empty CNT’s (13,0), (14,0), and (16,0) (upper panels) and DOS of the same CNT’s filled with T3 molecules (lower panel). The arrows indicate the HOMO-LUMO energy width of T3.