Selective Aggregation of Single-Walled Carbon Nanotubes Using the Large Optical Field Gradient of a Focused Laser Beam

We demonstrate the selective aggregation of single-walled carbon nanotubes by photon forces, using the large optical field gradient of a laser focused through a high numerical aperture objective lens. The nanotubes, dispersed in an aqueous solution with a surfactant, are detected via Raman scattering from the confocal volume of the optical trap. By using a visible-light laser for both trapping and detection, the dynamics of the radial breathing mode signal taken at short intervals shows an increase of a single breathing mode over time, indicating the increase in the density of only one species of tube in the focal volume. This result represents a significant step toward the development of techniques for the arbitrary manipulation and sorting of nanotubes by optical fields.

Figure 1

The experimental setup for Raman trapping experiments. A He:Ne laser (633 nm) was used for both laser trapping and Raman excitation.

Figure 2

Calibration of the Raman scattering intensity for radial breathing modes (RBMs) of metallic (asterisks) and semiconducting (dots) tubes, using various concentrations of SWCNT solution. Intensities are normalized to the full concentration of $\mathrm{SWCNT}+\mathrm{\text{surfactant}}$ solution after dispersion. Inset shows a typical radial breathing mode RBM signal, with RBMs identified at $196{\mathrm{cm}}^{-1}$ ($A$), $255{\mathrm{cm}}^{-1}$ ($B$), $260{\mathrm{cm}}^{-1}$ ($C$), and $285{\mathrm{cm}}^{-1}$ ($D$).

Figure 3

The radial breathing mode dynamics of SWCNTs dispersed in solution, as measured by Raman scattering. This ensemble of 300 spectra was taken at intervals of 2 s, over a period of 10 min. Inset shows the dynamics over time of the signal intensities of the four peaks $A$, $B$, $C$, and $D$.

Figure 4

Calculated dielectric function of the 6 chiralities of SWCNT that are observable in our experiment. The upper figures [(a) and (b)] show the real and imaginary parts of the dielectric function, respectively, for the semiconducting chiralities [7 5], [7 6], and [10 3]. Lower figures [(c) and (d)] show the real and imaginary parts of the dielectric function of the metallic species with chiral indices [14 2], [13 4], [12 6], and [11,8]. Guides for the eye show the 633 nm laser line.