Contact Transparency of Nanotube-Molecule-Nanotube Junctions

The transparency of contacts between conjugated molecules and metallic single-walled carbon nanotubes is investigated using a single-particle Green’s function method which combines a Landauer approach with ab initio density functional theory. We find that the overall conjugation required for good contact transparency is broken by connecting through a six-member ring on the tube. Full conjugation achieved by an all-carbon contact through a five-member ring leads to near perfect contact transparency for different conjugated molecular bridges.

Figure 1

Optimized structures of the (5,5)CNT-benzenediamide-CNT junction in (a) perpendicular and (b) parallel configurations, along with (c) their transmission functions. The two configurations have almost the same energy. The device region is indicated by the dashed frames; the equilibrium conductance is in the legend in units of $G_0=2e^2/h$. Note the poor transmission over a wide window around $E_F$.

Figure 2

Structures with all-carbon connections from 6-member rings of (5,5) CNTs to (a) ${\mathrm{C}}_6{\mathrm{H}}_4$, (b) two units of polypyrrole (PPR), and (c) polythiophene (PTP), as well as (d) their transmission functions. Only the perpendicular configuration is shown as it is much more stable. The transparency is poor in all three cases.

Figure 3

Structures with all-carbon connections from 5-member rings of (5,5) CNTs to (a) ${\mathrm{C}}_6{\mathrm{H}}_4$, (b) polypyrrole (PPR), and (c) polythiophene (PTP). Only the much more stable parallel configuration is shown. (d), (e) Transmission function of charge neutral benzene for the two spin configurations. (f) Transmission functions of the three systems under doping. Note the excellent transparency near $E_F$. $T(E)$ for an amide-linked benzene system is also shown in (f).

Figure 4

Connections to zigzag CNT ends. Benzene connected to (a) diagonally cut (5,5) tubes, and (b) (9,0) tubes. Their transmission functions are given in (c) together with that of a perfect (9,0) tube (dotted line).

Figure 5

Optimized structures for (a) straight and (b) tilted junctions of (5,5)-C-OPE3-C-(5,5). (c) Band structure of the uniform OPE chain. (d) Transmission functions of the doped junctions. (e) Local density of states in the energy window [$-0.5\mathrm{eV}$, 0.5 eV] for the junction in (a). Note that the peak around $T(E_F)$ decays when the molecule gets longer.