Electronic structure of straight semiconductor-semiconductor carbon nanotube junctions

We calculate the scanning tunneling microscopy images and the scanning tunneling spectroscopy of straight semiconductor-semiconductor single-wall carbon nanotube junctions with two different diameters. Two kinds of localized states associated with topological defects, one donorlike and the other acceptorlike, arise in the energy gap of the junctions, whose levels strongly depend on the number of topological defects and their spatial arrangement. We find that defects on one side of the tube surface influence the electronic distribution on the other (back) side sufficiently strongly that they may be detected by the scanning of the opposite side as well.

Figure 1

Atomic model and calculated STS image of the (14,0)-(19,0) SWNT junction. Only the central part of the simulated system of 42 nm in length is shown. (a) Atomic model of the junction with one pentagon and one heptagon defect 1.23 nm apart. The solid line is the scanning line of the tip to obtain the simulated STS data below. (b) STS image of the junction. The abscissa is the position along the tube axis and the ordiante is the sample bias voltage. Localized states in the gap are indicated by arrows (i) and (ii). The arrow (iii) denotes the state originating from the (19,0) valence band and decaying into the (14,0) SWNT gap across the defective region. The arrow (iv) shows the opposite behavior to (iii). (c) Density profiles along the solid line in (a) for individual states indicated by arrows (i)–(iv) in (b).

Figure 2

Calculated topographic images of the (14,0)-(19,0) junction in the solid rectangular region at different tip-sample bias voltages. All figures presented here are STM images on the opposite side of the defects (dark atoms in the atomic model denotes the defects on the back side). (a) Atomic model of the junction. (b) STM images of the junction at different bias voltages.

Figure 3

Calculated STS image of the (12,1)-(18,2) junction. (a) Atomic model of the junction. (b) STS image of the junction.

Figure 4

Calculated topographic images of the (12,1)-(18,2) junction. (a) Atomic model of the junction. (b) Topographic images of the junction at different bias voltages.

Figure 5

Four differenct defect configurations of the (15,2)-(19,3) junction as explained in the text. (a) $h\text{−}p$, (b) $h\text{−}php$, (c) $hph\text{−}p$, and (d) $hph\text{−}php$.

Figure 6

Calculated STS and topographic images of the (15,2)-(19,3) junction with the $h\text{−}php$ defect. (a) Atomic model of the junction. (b) STS image of the junction. (c) Topographic images on the back side of the defects at different bias voltages.

Figure 7

Calculated STS and topographic images of the (15,2)-(19,3) junction with the $hph\text{−}php$ defect. (a) Atomic model of the junction. (b) STS image of the junction. (c) Topographic images of the junction on the back side of the defects at different bias voltages.

Figure 8

Calculated STS image of the (15,2)-(19,3) junction with the $hph\text{−}p$ defect. (a) Atomic model of the junction. (b) STS image of the junction. The arrow (i) indicates the localized state and the arrows (ii) and (iii) indicate the valence band edge states of the (19,3) SWNT and the (15,2) SWNT, respectively. An acceptorlike localized state is mixed with the state (ii). (c) Density profiles along the tube of the individual states indicated by arrows (i)–(iii) in (b).