Light-induced accumulation of single-wall carbon nanotubes dispersed in aqueous solution

We present evidence that, when laser light was tightly focused into aqueous suspension of mono-dispersed single-wall carbon nanotubes (SWCNTs), density of nanotubes was locally increased at the focus spot of light. We prepared mono-dispersed HiPco SWCNTs in an aqueous surfactant solution by sonication and following ultracentrifugation. We built a confocal Raman microscope system equipped with a 633 nm He-Ne laser, and launched the laser light into SWCNTs suspension filled in a glass micro-cell by a high numerical aperture objective lens (N.A.=1.35). We monitored temporal change in Raman spectrum of SWCNTs excited by the laser light. We clearly observed significant intensity increase of a particular radial breathing mode, however the increase of the Raman signal did not last permanently, rather showed transient response. This result implies that SWCNTs with particular chirality were selectively accumulated by optical gradient force of Raman-probing laser light. We discuss the independent behavior of the radial breathing modes, with respect to the wavelength of the laser light and the chirality of corresponding SWCNTs.