High-Resolution Near-Field Raman Microscopy of Single-Walled Carbon Nanotubes

We present near-field Raman spectroscopy and imaging of single isolated single-walled carbon nanotubes with a spatial resolution of $\approx 25\mathrm{n}\mathrm{m}$. The near-field origin of the image contrast is confirmed by the measured dependence of the Raman scattering signal on tip-sample distance and the unique polarization properties. The method is used to study local variations in the Raman spectrum along a single single-walled carbon nanotube.

Figure 1

Confocal Raman images of SWNT bundles produced by arc discharge on glass acquired by detecting the intensity of the $G^{\prime }$ band ($\nu =2615{\mathrm{c}\mathrm{m}}^{-1}$) while raster scanning the sample (without metal tip, scan area $15\times 15\mu \mathrm{m}$) for vertical (a) and horizontal polarization (b) of the laser excitation at 633 nm. The arrows indicate tubes which are oriented completely perpendicular to one polarization direction demonstrating the polarization contrast.

Figure 2

Simultaneous near-field Raman image (a) and topographic image (b) of SWNTs grown by CVD on glass. Scan area $1\times 1\mu \mathrm{m}$. The Raman image is acquired by detecting the intensity of the $G^{\prime }$ band upon laser excitation at 633 nm. No Raman scattering signal is detected from humidity related circular features present in the topographic image (see text). (c) Cross section taken along the indicated dashed line in the Raman image. (d) Cross section taken along the indicated dashed line in the topographic image. The height of individual tubes is $\approx 1.4\mathrm{n}\mathrm{m}$. Vertical units are photon counts per second for (c) and nanometer for (d).

Figure 3

(a) Dependence of the Raman scattering strength of the $G^{\prime }$ band ($I$) on the longitudinal separation ($\Delta z$) between a single SWNT and the tip. The solid line is an exponential fit with a decay length of 11 nm. The signal is normalized with the far-field signal. (b) Scanning electron micrograph of a sharp silver tip fabricated by focused ion beam milling.

Figure 4

(a) Three-dimensional topographic image of a SWNT on glass grown by arc discharge. The three bumps are presumably enclosed $\mathrm{N}\mathrm{i}/\mathrm{Y}$ catalyst particles and indicate the initial point of growth. (b) Near-field Raman spectra detected at the marked positions 1 to 4 in (a). The distance between positions 2 and 3 is 35 nm. The spectra are offset for clarity.