Structural and conducting properties of metal carbon-nanotube contacts: Extended molecule approximation

The structural and conducting properties of single-wall carbon nanotubes (SWCNs) in embedded as well as in side contact with metal leads are obtained using efficient formalisms based on spin polarized tight-binding formulation incorporating full consideration of $s$, $p$, and $d$ basis sets for carbon and metal atoms. The full structural relaxation of the combined SWCN and metal system is found to be essential for realistic characterization of conductivity. More importantly, convergence with respect to the number of the metal-lead (ML) atoms in contact with the SWCN is found to be even more critical. Our results indicate that in order to maximize device efficiency, one needs to use ML-SWCN systems with a minimal ML-SWCN contact width to SWCN length ratio. If this ratio is large enough, the SWCN cannot be seen independently of its contacts and the ML-SWCN-ML system behaves as a molecular wire. Additionally, the ML-SWCN-ML complex is found to have spin-selective transport properties for certain bias range, making it a promising candidate for use as a spin valve in nanoelectronic devices.

Figure 1

(Color online) Fully relaxed geometry of a (10,0) SWCN in embedded-end-contact configuration in Ni (top panel) used in $I\text{−}V$ calculations. The middle panel is obtained from removing all Ni atoms from the top panel. The bottom panel shows a fully relaxed isolated (10,0) SWCN.

Figure 2

(Color online) Transmission functions for spin-up electrons (upper inset) in units of $e^2∕h$ and $I\text{−}V$ characteristics (current in $\mu \mathrm{A}$) for symmetric bias configurations for a (10,0) SWCN in three embedded-end-contact configurations. In Ni leads consisting of seven (red), five (blue), and three (green) Ni layers. For comparison, the corresponding results for the same nanotube after removing all Ni atoms (Fig. 1, middle panel) are also shown (black curve). The corresponding transmission coefficients are shown by the solid curves in the inset.

Figure 3

(Color online) $I\text{−}V$ characteristics for the (10,0) SWCN in the top panel of Fig. 1 in the symmetric bias and embedded in five Ni layers for electrons with spin up (solid blue) and spin down (dashed blue). For comparison, the total $I\text{−}V$ curve in the asymmetric bias configuration is also shown (upper inset). The transmission functions (in units of $e^2∕h$) for spin-down configurations for three, five, and seven metal layers are shown in the lower inset which also includes the five metal layers case for spin up (broken blue curve) for comparison. The Fermi energy is set to zero.

Figure 4

(Color online) Average electron DOS of the embedded SWCN end-contact system (extended molecule) shown in Fig. 1, top panel. The DOS contribution of Ni atoms in contact with the C atoms of the SWCN is shown in red, while the DOS contribution of C atoms in contact with the Ni atoms is shown in green. The Fermi energy $E_F$ is at zero.

Figure 5

(Color online) Fully relaxed configuration consisting of a (10,0) SWCN in side contact to Ni(001) MLs with each Ni lead consisting of five metal layers in the extended molecule. Both ends of the SWCN were capped to eliminate dangling bonds.

Figure 6

(Color online) $I\text{−}V$ and $T\left(E\right)$ results for the side-contact configuration shown in Fig. 5. $T\left(E\right)$ is expressed in units of $e^2∕h$.

Figure 7

(Color online) Comparison of $I\text{−}V$ curves and $T\left(E\right)$ (inset: in units of $e^2∕h$) for spin-down electrons and for two lengths of (10,0) SWCN embedded in five Ni layers.