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

Abstract

We present near-field Raman spectroscopy and imaging of single isolated single-walled carbon nanotubes with a spatial resolution of 25 nm. 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. DOI: 10.1103/PhysRevLett.90.095503 PACS numbers: 61.46.+w, 07.79.-v, 78.30.Na, 78.67.Ch Recent rapid advances in nanotechnology and nanoscience are largely due to our newly acquired ability to measure and manipulate individual structures on the nanoscale. Among the new methods are scanning probe techniques, optical tweezers, and high-resolution electron microscopes. Recently, a near-field optical technique has been demonstrated which allows spectroscopic measurements with 20 nm spatial resolution [1]. The method makes use of the strongly enhanced electric field close to a sharp metal tip under laser illumination and relies on the detection of two-photon excited fluorescence. However, fluorescence imaging requires a high fluorescence quantum yield of the system studied or artificial labeling with fluorophores. Furthermore, fluorescence quenching by the metal tip competes with the local field enhancement effect and therefore limits the general applicability. On the other hand, Raman scattering probes the unique vibrational spectrum of the sample and directly reflects its chemical composition and molecular structure. A main drawback of Raman scattering is the extremely low scattering cross section which is typically 14 orders of magnitude smaller than the cross section of fluorescence. Surface enhanced Raman scattering (SERS), induced by nanometer-sized metal structures, has been shown to provide enormous enhancement factors of up to 10 15 allowing for Raman spectroscopy even on the single molecule level [2,3]. Controlling SERS with a sharp metal tip which is raster scanned over a sample surface has been proposed [1,4], and near-field Raman enhancement has been experimentally demonstrated [5][6][7][8][9]. Here, we show the chemical specificity of this near-field technique and demonstrate an unprecedented spatial resolution.Single-walled carbon nanotubes (SWNTs) have been the focus of intense interest due to a large variety of potential nanotechnological applications. The unique properties of SWNTs arise from their particular onedimensional structure which is directly linked to the characteristic Raman bands. Raman scattering on SWNTs has been studied intensively in the literature (see, e.g., Refs. [10 -13]) and Raman enhancements of up to 10 12 have been reported for tubes in contact with fractal silver colloidal clusters [14]. In this Letter, nearfield Raman imaging of SWNTs is demonstrated using a sharp silver tip as a probe. We show, for the first time, that single isolated SWNTs can be detected optically with a spatial resolution better than 30 nm. This high-resolution capability is ap...

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