Paired Gap States in a Semiconducting Carbon Nanotube: Deep and Shallow Levels

Several paired, localized gap states were observed in semiconducting single-wall carbon nanotubes using spatially resolved scanning tunneling spectroscopy. A pair of gap states is found far from the band edges, forming deep levels, while the other pair is located near the band edges, forming shallow levels. With the help of a first-principles study, the former is explained by a vacancy-adatom complex while the latter is explained by a pentagon-heptagon structure. Our experimental observation indicates that the presence of the gap states provides a means to perform local band-gap engineering as well as doping without impurity substitution.

Figure 1

(a) A STM image with a CNT. The image size is $60\times 39\mathrm{nm}$. (b) A derivative STM image. This is zoomed from the rectangle area of (a). The image length is 13.5 nm. (c) The spatially resolved LDOS map (STS data) for the CNT (a). On the basis of the STS data, the CNT is divided into regions A, B, C, and D, respectively. The labels I, II, III, and IV are the energy levels for gap states.

Figure 2

Sampled STS data, from regions A to D of Fig. 1c. The curves from B, C, and D are shifted for clearer presentation. The arrows indicate gap states.

Figure 3

Horizontal line profiles of Fig. 1c. The profile I is at 0.12 eV, II is at 0.24 eV, III is at 0.02 eV, and IV is at 0.48 eV. The profiles of II, III, and IV are shifted upward for clearer presentation.

Figure 4

Atomic model and band structure of semiconducting CNTs with defects. (a) Atomic model of a vacancy-adatom complex on the (17,0) CNT, (b) band structure corresponding to the (17,0) CNT with the vacancy-adatom complex in (a).