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

Achim Hartschuh

Erik J. Sánchez

J. Wessel

Lukas Novotny

X. Sunney Xie

English

Hartschuh, Achim

Sánchez, Erik J.

Xie, X. Sunney

Novotny, Lukas

Universität München: Elektronischen Publikationen

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

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We present near-field Raman spectroscopy and imaging of single isolated single-walled carbon nanotubes with a spatial resolution of approximately 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

PDXScholar (Portland State University)

High-resolution near-field Raman microscopy of single-walled carbon nanotubes

Open Access LMU ( Ludwig-Maximilians-Univ. München)

P H Y S I C A L R E V I E W L E T T E R S week ending7 MARCH 2003VOLUME 90, NUMBER 9High-Resolution Near-Field Raman Microscopy of Single-Walled Carbon NanotubesAchim Hartschuh,1 Erik J. Sa´nchez,2 X. Sunney Xie,3 and Lukas Novotny11The Institute of Optics, University of Rochester, Rochester, New York 146272Department of Physics, Portland State University, Portland, Oregon 972073Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138(Received 9 August 2002; published 4 March 2003)095503-1We present near-field Raman spectroscopy and imaging of single isolated single-walled carbonnanotubes with a spatial resolution of  25 nm. The near-field origin of the image contrast is confirmedby the measured dependence of the Raman scattering signal on tip-sample distance and the uniquepolarization properties. The method is used to study local variations in the Raman spectrum along asingle single-walled carbon nanotube.DOI: 10.1103/PhysRevLett.90.095503 PACS numbers: 61.46.+w, 07.79.–v, 78.30.Na, 78.67.Chcharacteristic Raman bands. Raman scattering onSWNTs has been studied intensively in the literaturepolarized along the tip axis [17]. To achieve this conditionin our on-axis illumination scheme, we displace the tipRecent rapid advances in nanotechnology and nano-science are largely due to our newly acquired ability tomeasure and manipulate individual structures on thenanoscale. Among the new methods are scanning probetechniques, optical tweezers, and high-resolution electronmicroscopes. Recently, a near-field optical technique hasbeen demonstrated which allows spectroscopic measure-ments with 20 nm spatial resolution [1]. The methodmakes use of the strongly enhanced electric field closeto a sharp metal tip under laser illumination and relieson the detection of two-photon excited fluorescence.However, fluorescence imaging requires a high fluores-cence quantum yield of the system studied or artificiallabeling with fluorophores. Furthermore, fluorescencequenching by the metal tip competes with the local fieldenhancement effect and therefore limits the general ap-plicability. On the other hand, Raman scattering probesthe unique vibrational spectrum of the sample and di-rectly reflects its chemical composition and molecularstructure. A main drawback of Raman scattering is theextremely low scattering cross section which is typically14 orders of magnitude smaller than the cross sectionof fluorescence. Surface enhanced Raman scattering(SERS), induced by nanometer-sized metal structures,has been shown to provide enormous enhancement factorsof up to 1015 allowing for Raman spectroscopy even onthe single molecule level [2,3]. Controlling SERS with asharp metal tip which is raster scanned over a samplesurface has been proposed [1,4], and near-field Ramanenhancement has been experimentally demonstrated[5–9]. Here, we show the chemical specificity of thisnear-field technique and demonstrate an unprecedentedspatial resolution.Single-walled carbon nanotubes (SWNTs) have beenthe focus of intense interest due to a large variety ofpotential nanotechnological applications. The uniqueproperties of SWNTs arise from their particular one-dimensional structure which is directly linked to the0031-9007=03=90(9)=095503(4)$20.00 (see, e.g., Refs. [10–13]) and Raman enhancements ofup to 1012 have been reported for tubes in contact withfractal silver colloidal clusters [14]. In this Letter, near-field Raman imaging of SWNTs is demonstrated using asharp silver tip as a probe. We show, for the first time, thatsingle isolated SWNTs can be detected optically with aspatial resolution better than 30 nm. This high-resolutioncapability is applied to resolve local variations in theRaman spectrum along a single SWNT which would behidden in far-field measurements. The near-field origin ofthe Raman enhancement is proven by tip-sample distancemeasurements.Our experimental setup is based on an inverted opticalmicroscope with an x; y scan stage for raster scanning atransparent sample with SWNTs. A laser beam ( 633 nm, 30–100 W) is reflected by a dichroic beamsplitter and focused by a high numerical aperture (NA)objective (1.4NA) on the sample surface. A sharp silvertip is positioned near the focus of the beam and main-tained above the sample surface at a distance of  1 nmby means of a sensitive shear-force feedback mechanism[15]. Raman scattered light is collected with the sameobjective, transmitted by the beam splitter and filtered bya long pass filter. The signal is detected either by acombination of a spectrograph and a thermoelectricallycooled charged coupled device or by a narrow band passfilter (FWHM  10 nm) centered at 760 nm (or 700 nm)followed by a single-photon counting avalanche photo-diode. A near-field Raman image is established by rasterscanning the sample and continuously recording the scat-tered Raman signal. Sharp silver tips with a radius of10–15 nm are produced by electrochemical etching fol-lowed by focused ion beam (FIB) milling using a dualbeam FIB system (FEI Strata-235) and utilizing a similarprocedure as used for STM/AFM (scanning tunnelingmicroscopy/atomic force microscopy) tip sharpening [16].To establish a strong field enhancement at the tip, theelectric field of the exciting laser beam needs to be2003 The American Physical Society 095503-1d(a) (b)P H Y S I C A L R E V I E W L E T T E R S week ending7 MARCH 2003VOLUME 90, NUMBER 9from the center of the beam in the direction of polar-ization into one of the two longitudinal field lobes char-acteristic for strongly focused Gaussian beams [18]. Thestrength of this longitudinal field increases with increas-ing NA of the focusing lens, and in our configuration, isonly 5 times weaker than the transverse field strength atthe center of the focus.Two different types of SWNT samples were studied.The first sample consists of nearly monodisperse SWNTsgrown by chemical vapor deposition (CVD) on SiO2substrates. The substrates were first cleaned by solventwashing then heated to 700 C in a tube furnace for30 min while being exposed to air. 50 l of Ferritinsolution [Sigma-Aldrich, diluted in deionized (DI) water,1:10 ratio (Ferritin:H2O)] was applied to the clean sub-strate which was left for 30 s then rinsed with DI water,then placed in the tube furnace using growth parameterssimilar to commonly known SWNT growth methods[19]. The material for the second sample was purchasedfrom BuckyUSA and consists of purified SWNTs with adiameter distribution from 1.0–1.8 nm, produced by arcdischarge using Ni=Y catalyst particles. The material wasdispersed in dichlorethane, sonicated for 1 h and spin caston a microscope cover glass. AFM measurements wereperformed to determine sample coverage and to distin-guish spatially isolated tubes from nanotube bundles.The far-field Raman spectra of the SWNT samples (notshown) are dominated by the sharp tangential stretchingmode at 1596 cm1 (G band) and a broader band around2615 cm1 (G0 band) which arises from an overtoneof the disorder induced mode around 1310 cm1 [20].Figure 1 shows confocal far-field Raman images ofSWNT bundles produced by arc discharge which wererecorded by integrating the signal intensity of the G0 band(760 nm band pass filter, see above) for vertical (a) andhorizontal (b) polarization of the incident laser beam.(a) (b)FIG. 1 (color online). Confocal Raman images of SWNTbundles produced by arc discharge on glass acquired by detect-ing the intensity of the G0 band (  2615 cm1) while rasterscanning the sample (without metal tip, scan area 15 15 m)for vertical (a) and horizontal polarization (b) of the laserexcitation at 633 nm. The arrows indicate tubes which areoriented completely perpendicular to one polarization direc-tion demonstrating the polarization contrast.095503-2Similar Raman images are obtained by detectingthe intensity of the G band using the 700 nm bandpass filter. The Raman images clearly demonstratethat tubes oriented completely perpendicular to the re-spective light polarization do not produce any signal (seearrows in Fig. 1). This polarization contrast is a conse-quence of the polarization properties of the G and G0Raman bands reported for G in the literature [11,12,21].AFM measurements of the same area reveal that thediameter of the nanotube bundles varies between5–15 nm which corresponds to 10–100 individual nano-tubes per bundle.Confocal Raman imaging of SWNTs grown by CVDwas not possible within feasible acquisition times becausethe sample consists mostly of separated individualSWNTs for which the Raman scattering strength ismuch weaker. However, as shown in Fig. 2(a) for CVDgrown SWNTs, the contrast and resolution of Ramanimages can be greatly increased by positioning a sharpsilver tip near the laser focus. The simultaneously re-corded topographic image is presented in Fig. 2(b). Thelower portions [Figs. 2(c) and 2(d)] show cross sectionstaken along the dashed white lines. Besides SWNTs, thetopographic image shows a large number of small circularfeatures with a height of  2 nm which are due to con-densating water and can be removed by sample heatingto 70 C. Comparison of the widths (FWHM) of SWNTs23 nmposition [nm]29 nmposition [nm]c(c)0 200 400 600 800 100002k4k6k8k (d)height [nm]0 6photons [kcts/s]0 80 100 200012FIG. 2 (color online). Simultaneous near-field Raman image(a) and topographic image (b) of SWNTs grown by CVD onglass. Scan area 1 1 m. The Raman image is acquired bydetecting the intensity of the G0 band upon laser excitation at633 nm. No Raman scattering signal is detected from humidityrelated circular features present in the topographic image (seetext). (c) Cross section taken along the indicated dashed line inthe Raman image. (d) Cross section taken along the indicateddashed line in the topographic image. The height of individualtubes is  1:4 nm. Vertical units are photon counts per secondfor (c) and nanometer for (d).095503-2∆z (nm)(a)1.31.21.1I(∆z)/I20 40 6001(b)500 nmFIG. 3. (a) Dependence of the Raman scattering strength ofthe G0 band (I) on the longitudinal separation (z) between asingle SWNT and the tip. The solid line is an exponential fitwith a decay length of 11 nm. The signal is normalized with thefar-field signal. (b) Scanning electron micrograph of a sharpsilver tip fabricated by focused ion beam milling.P H Y S I C A L R E V I E W L E T T E R S week ending7 MARCH 2003VOLUME 90, NUMBER 9in the topographic and Raman image indicate that theresolution of the Raman image is better than the resolu-tion of the topographic image (25 vs 30 nm). This highspatial resolution is unprecedented and clearly indicatesthe near-field origin of the Raman signal. The widths ofthe optical features and the topographical features resultfrom the convolution of the tip shape with the objectstudied and are therefore limited by the finite size ofthe tip apex. Both images are closely correlated and theSWNTs are clearly identified. However, the small topo-graphic features formed by condensation do not show upin the Raman image which proves the chemical specific-ity of the method. The diameter of SWNTs grown byCVD can be controlled by growth parameters. Thesample used in Fig. 2 consists mostly of SWNTs with adiameter of  1:4 nm which is verified by the topo-graphic data in Figs. 2(b) and 2(d) and also by indepen-dent AFM measurements in the absence of watercondensation. Thus, we observe near-field Raman scatter-ing from spatially separated single SWNTs. It is impor-tant to notice that vertically and horizontally orientedSWNTs are observed in the Raman image with similarsignal intensity even though the incident laser light isoriented vertically. This observation is in strong contrastto the polarization contrast observed in the far-field mea-surements shown in Fig. 1. Accordingly, the polarizationof the enhanced field has to be radially symmetric withrespect to the tip axis. This finding is in agreement withtheoretical calculations [17] and confirms the uniqueproperties of the near field.The optical resolution apparent in Fig. 2 shows that theenhanced field is laterally confined to the size of the metaltip. To demonstrate the confinement of the enhanced fieldsin longitudinal direction, the tip is positioned above aSWNTand the Raman scattering strength is recorded as afunction of tip-sample distance z. The result, shown inFig. 3, has been fitted with an exponential curve andnormalized with the Raman scattering strength in theabsence of the tip, i.e., with the far-field Raman signal.For distances z < 30 nm the increase of the signalclearly indicates a distance dependent near-field contri-bution with a decay length of 11 nm. This longitudinalconfinement is in agreement with the lateral confinementobserved in the near-field Raman images and consistentwith the tip radius of 10–15 nm as determined by scan-ning electron microscopy (SEM) [c.f. Fig. 3(b)]. All thetips used in our experiments were fabricated by the sameprocedure and rendered similar SEM images. However,only about 30% to 40% of the tips produced a measurablesignal enhancement. The reason for this behavior is thesubject of ongoing studies. The highest Raman enhance-ment factors observed so far were estimated to be around103. These numbers follow from the detected ratio ofnear-field to far-field signals and from a comparison ofthe different volumes probed by the near field and the farfield.095503-3To demonstrate the high-resolution spectroscopic ca-pabilities of the here presented near-field technique weinvestigate the Raman spectrum along a SWNT with adiameter of 1.7 nm grown by arc discharge. Figure 4(a)shows the topography near the end of a SWNT producedby this technique. Three bumps with a height of  5 nmcan be recognized on top of the SWNT. Because of theirsize, these bumps are presumably Ni=Y catalyst particlesindicating the initial point of growth [22]. The labels 1 to4 in Fig. 4(a) mark the positions at which Raman spectrawere taken in the presence of the metal tip. At the verybeginning of the SWNT (positions 1 and 2 ) the G band at1596 cm1 (FWHM  17 cm1) dominates the spectraand is significantly stronger than the G0 band at2619 cm1 (FWHM  35 cm1) with an amplitude ratioG=G0 of 1:3 [see Fig. 4(b)]. However, moving along theSWNT to position 3 and further to position 4 (separatedby 35 and 85 nm, respectively) the amplitude of the Gband decreases with respect to the G0 band and the ratioG=G0 reduces to  0:7. Simultaneously, the shape of theG0 band changes and its center frequency is shifted to2610 cm1 whereas the G band remains at 1596 cm1.For 1 and 2, the G0 band has a Lorentzian shape while for3 and 4 two Lorentzian peaks are needed to model itsshape. Raman spectra recorded beyond point 4 in Fig. 4(a)remain unchanged.Variations in the Raman spectrum reflect changes inthe molecular structure which can have several originssuch as external stress, due to the catalyst particles, orlocal defects in the tube structure. The observed spectralvariations between 1 and 3 can also be explained by achange of the tube structure, described by n;m !n0; m0, which modifies the electronic state energies of095503-3G’4312 2004006008001000 Gphoton counts1300 1500 1700 1900 2500 2600 2700Raman shift [cm-1](b)43210.69 µm0.69 µm1.7 nm(a)FIG. 4 (color online). (a) Three-dimensional topographicimage of a SWNT on glass grown by arc discharge. The threebumps are presumably enclosed Ni=Y catalyst particles andindicate the initial point of growth. (b) Near-field Ramanspectra detected at the marked positions 1 to 4 in (a). Thedistance between positions 2 and 3 is 35 nm. The spectra areoffset for clarity.P H Y S I C A L R E V I E W L E T T E R S week ending7 MARCH 2003VOLUME 90, NUMBER 9the tube and therefore the resonance enhancement of theRaman bands at a fixed excitation energy. A decrease ofthe resonance enhancement between positions 2 and 3would lead to a lower G band amplitude and could causea splitting of the G0 band into a double peak [20].Recent experiments have shown that huge surface en-hancement of Raman signals can be accompanied byfluctuating white-light emission [3,23]. According toMichaels et al. [23], huge SERS enhancement and strongwhite-light emission requires chemisorption of theRaman scatterer to the metal surface. Since we maintaina well-defined separation (1 nm) between tip andsample in our experiments, we do not expect a significantwhite-light contribution. Moreover, white-light emissionis observed for Raman enhancement factors more than10 orders of magnitude stronger than the enhancementsdetermined in our experiments. Indeed, our near-fieldRaman spectra show no indication for a significant broad-band background [see Fig. 4(b)]. To exclude any contri-butions to the image contrast arising from backgroundsignals, we recorded the full spectrum for each pixel andintegrated the intensity of the G0 band and the G bandwhile subtracting the background. The resulting imageswere virtually identical to those shown in Fig. 2(a). Wealso note that by comparing the near-field and far-field095503-4Raman spectra of SWNTs, we find, within the noise limit,no indication for a gradient-field Raman effect discussedby Ayars et al. [24].We have demonstrated a method for nanoscale Ramanspectroscopy with sub 30 nm spatial resolution. Thismethod extends our abilities to characterize weakly fluo-rescent structures on the nanoscale. Near-field Ramanspectroscopy combined with simultaneous AFM mea-surements appears to be extremely useful for investiga-tions of carbon nanotubes. The method holds promise forthe experimental investigation of special structural nano-tube features, such as kinks, intertube junctions, or eventubes loaded with C60 molecules.The authors wish to thank M. Beversluis andA. Bouhelier for stimulating discussions. Assistance onthe CVD of SWNTs by Dr. T. Rueckes and Dr. B. Segal ofNantero (Woburn, MA) is gratefully acknowledged. Thisresearch was supported by the NSF through GrantNo. BES-0086368.[1] E. J. Sa´nchez, L. Novotny, and X. S. Xie, Phys. Rev. Lett.82, 4014 (1999).[2] K. Kneipp et al., Phys. Rev. Lett. 78, 1667 (1997).[3] S. Nie and S. R. Emory, Science 275, 1102 (1997).[4] J. Wessel, J. Opt. 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High-Resolution Near-Field Raman Microscopy of Single-Walled Carbon Nanotubes

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