CARBON NANOTUBES

Experimental studies of the optical properties of single-walled carbon nanotubes (SWCNT) have recently generated considerable experimental and theoretical interest. Smalley et. al. discovered empirical rules that allow the type of tube, i.e. the (n,m) numbers that characterize a nanotube, to be derived from optical spectra. There is growing agreement that the optical excitations correspond to bound electron-hole pairs, or excitons, but the details of these exciton states are still under debate. Our goal is to better understand the nature of these excitations in different types of semiconducting tubes. Here, we explore the electron-hole pair effects by comparing the electronic spectra of tubes at three levels of theory, using periodic boundary conditions: Huckel, Pariser-Parr-Pople (PPP) and Intermediate Neglect of Differential Overlap (INDO). The current results can be downloaded from here . The predicted INDO band structure of an (8, 0) semiconducting tube is similar to that of Louie's DFT results[2]. However, significant differences are present in the lower energy bands. The INDO band gap of 2.9 eV is also substantially lower than the 1.75 eV gap obtained from DFT. The absorption spectrum obtained from INDO/SCI calculations including the 2-fold degenerate highest-energy valence and the 2-fold degenerate lowest-energy conduction band shows dark excitons at low energy, in agreement with Mazumdar's results[3]. This confirms that these dark states are present even when periodic boundary conditions are used. Comparison of the INDO results with Huckel and PPP theory will allow us to explore the nature of the exciton binding and the effects of the tube's curvature.

1. S.M. Bachilo, M. S. Strano, C. Kittrell, R.H. Hauge, R. E. Smalley, R. B. Weisman, Science, 2002, 2361-236
2. C.D. Spataru, S. Ismail-Beigi, L.X. Benedict, S. G. Louie, Phys. Rev. Lett., 2004, 92, 077402
3. H. Zhao, S. Muzimdar, Phys. Rev. Lett., 2004, 93, 157402