Interactions between Individual Carbon Nanotubes Studied by Rayleigh Scattering Spectroscopy

The electronic properties of single-walled carbon nanotubes (SWNTs) are altered by intertube coupling whenever bundles are formed. These effects are examined experimentally by applying Rayleigh scattering spectroscopy to probe the optical transitions of given individual SWNTs in their isolated and bundled forms. The transition energies of SWNTs are observed to undergo redshifts of tens of meVs upon bundling with other SWNTs. These intertube coupling effects can be understood as arising from the mutual dielectric screening of SWNTs in a bundle.

Figure 1

Rayleigh spectra at different positions in a nanotube $Y$ junction. (a) Scanning electron micrograph (SEM) image of the $Y$ junction where two nanotube structures ($A$ and $B$) merge together ($A+B$). The positions of the nanotubes have been highlighted with white lines. (b) High-resolution SEM image of the junction area indicated by the circle in (a). (c) Rayleigh spectra of structure $A$ (black dashed curve), structure $B$ (blue dotted curve), and the combination $A+B$ (red solid curve). Structure $A$ is an isolated semiconducting nanotube with diameter of $\sim 1.9\mathrm{nm}$; $B$ is a small bundle of nanotubes. The composite $A+B$ structure exhibits features of both constituents, with peaks arising from structures $A$ and $B$ marked correspondingly. The resonances in SWNT $A$ are redshifted by 35 and 47 meV in the $A+B$ structure. The redshifts for transitions in structure $B$ are smaller.

Figure 2

Rayleigh spectra of a SWNT and a two-tube bundle containing this SWNT. (a) SEM image of the nanotube structure, with nanotubes highlighted by white lines. The arrow indicates the position of the transition between the two-tube bundle $A+B$ and the individual tube $A$. (b) Detailed SEM image of the circled area showing the formation of the two-tube bundle at the edge of the slit. (c) Rayleigh spectra of the individual semiconducting SWNT $A$ of 1.8-nm diameter (black curve) and the two-tube bundle $A+B$ (red curve). The dotted lines are generated using the model for the Rayleigh spectra discussed in the text. Redshifts of 29 and 48 meV for the SWNT transitions at 1.897 and 2.196 eV, respectively, arise from the SWNT’s interaction with the adjacent SWNT. No changes in the optical transition linewidths are observed.

Figure 3

Rayleigh spectra of four separate nanotube structures similar to that of Fig. 2a. Both Rayleigh scattering of individual SWNTs (black solid curves) and bundles containing these tubes (red dashed curves) are displayed. The resonances of the SWNTs are present in the bundled structures (vertical lines), but at redshifted energies.