Potential visible light absorption in 3-Å-diam carbon nanotubes

We examine the optical properties of the recent observed 3-Å-diam carbon nanotube (CNT), whose chirality is predicted as (2,2), using a time-dependent Hartree-Fock method, in comparisons with other small-diameter CNTs such as (3,1), (2,0), (2,1), (3,3), and (4,4) CNTs. The absorption spectra of the (2,2) and (3,1) CNTs, which are of the closest diameter, are found to be very different. The calculated exciton sizes for those small-diameter CNTs depend only slightly on diameter and are insensitive to chirality for the 3-Å-diam CNTs. The (2,2) CNT could absorb green light, which implies that the small CNT can act as a prototype for the future development of CNT-based optoelectronic applications and for new designs of light absorbers or detectors.

Figure 1

Absorption spectra of the (2,2), (3,3), and (4,4) CNTs with different lengths.

Figure 2

(a) Plot of optical gap via inverse of nanotube length for the (2,2), (3,3), and (4,4) CNTs. The optical gaps at their infinite sizes are obtained by extrapolation. The panel shows the optical gap via nanotube diameter. Experimental data for the (10,10) CNT (triangle) are plotted for reference. The dotted line is a guide for the eyes. (b) Plot of absorption intensity per carbon atom via number of carbon atom for the (2,2), (3,3), and (4,4) CNTs.

Figure 3

The absorption spectra for the (2,0), (2,1), (2,2), (3,3), (4,4), and (3,1) CNTs (left column). The absorption spectra shown in dotted lines for the (2,2), (3,3), (4,4), and (3,1) CNTs are obtained by applying an electric field in the direction normal to the nanotube axis. Those spectra are magnified 10 times. The density matrices shown in the right column correspond to the main excitation (label by asterisk) for each CNT. The legend is the scale for the density matrices.

Figure 4

Plot of exciton size via nanotube diameter for each CNT. The dotted line is a guide for the eyes.