Pressure-assisted tip-enhanced Raman imagingat a resolution of a few nanometresTaka-aki Yano1*, Prabhat Verma1,2*, Yuika Saito3, Taro Ichimura1 and Satoshi Kawata1,4

Scanning probe microscopy methods1,2 can image samples withextremely high resolutions, opening up a wide range of appli-cations in physics3, chemistry4 and biology5. However, thesepassive techniques, which trace the sample surface softly,give indirect topographic information. Here, we show anactive imaging technique that has the potential to achieve opti-cally a molecular resolution by directly interacting and perturb-ing the sample molecules. This technique makes use of anexternal pressure, applied selectively on a nanometric volumeof the sample through the apex of a sharp nanotip, to obtaina local distortion of only those molecules that are pressurized.The vibrational frequencies of these molecules are distinctlydifferent from those of unpressurized molecules. By sensingthis difference, our active microscopic technique can achieveextremely high resolution. Using an isolated single-walledcarbon nanotube and a two-dimensional adenine nanocrystal,we demonstrate a spatial resolution of 4 nm.

Optical microscopy is a well-established technique for the study ofthe basic properties of materials. However, owing to the diffractionlimit imposed by the wave nature of propagating light, spatial resol-ution in optical imaging is restricted to approximately half theprobing light wavelength (Fig. 1a). Although recent advances6–8

in far-field optical microscopy have improved spatial resolution bya small amount, optical imaging still remains far from providingtrue nanoscale resolution. Near-field scanning optical microscopy(NSOM)9, on the other hand, has emerged as a powerful stepforward in optical microscopy towards a breakthrough in subwave-length imaging. Recent developments have been based on usingnanosized conical metallic tips10,11 as the NSOM probe. Theimaging mechanism in this technique relies on confining the incidentlight in close proximity to the tip apex owing to the localized modesof surface plasmon polaritons (SPPs). When the tip is scanned closeto the sample during the imaging process, the size of the confinedlight (here ‘size’ refers to the volume within which the confinedlight has considerable intensity, but where a sharp boundary cannotbe defined), which is comparable to the size of the tip apex, determinesthe spatial resolution (Fig. 1b). As well as the confinement, resonantexcitation of localized SPPs also provides strong enhancement of thelight field at the tip apex. This enhancement makes it possible toobserve optical signals, such as Raman scattering12–14 and fluor-escence15, that would otherwise be quite weak due to the nanometricvolume of the sample. Recent advances in tip-enhanced Raman spec-troscopy and microscopy have demonstrated high sensitivity down tothe single molecular level16–18 and ultrahigh spatial resolution betterthan 15 nm (refs 19–23). Because the resolution in this technique isdetermined by the size of the tip apex, it would seem reasonable toassume that the resolution could be improved simply by decreasingthe size of the tip apex. The current value of spatial resolution(!10–15 nm) could not, however, be improved further, indicating

that the spatial resolution has already reached a plateau where nofurther improvement seems to be possible. This is probably becausethe field enhancement, which arises from the collective oscillationsof free charge carriers, requires large numbers of metal atoms atthe tip apex. By reducing the size of the tip apex, we therefore alsoreduce the possibility of observing the optical signal. This puts apractical limit on improving the resolution by decreasing the size ofthe tip apex. It is, therefore, necessary to think beyond plasmoniceffects, which are predominantly responsible for the high resolutionin NSOM measurements, so that spatial resolution in opticalimaging can be further improved. One of the possible ways to dothis is by combining a completely different phenomenon in theusual NSOM technique in such a way that the combination improvesthe resultant resolution. We have recently shown that combiningnonlinearity19 or chemical effects24 in regular NSOM techniquescan have an obvious impact on improving resolution. Here, wediscuss an entirely different phenomenon, which, when combinedwith NSOM techniques, provides a dramatic improvement inspatial resolution. Using the same tip that is used for confining thelight in NSOM, we apply a controlled amount of pressure on thesample so that the sample molecules directly under the physicalcontact with the tip apex are mechanically perturbed, and hencehave a different optical response compared to the other unpressurizedmolecules of the sample25,26. It has been recently predicted27 that byadopting an advanced experimental technique, it could be possibleto distinctly sense the optical signal originating from the pressurizedpart of the sample and hence obtain a much higher spatial resolutionin a locally pressurized sample. Indeed, by precisely controlling thecontact area between the sample and the tip, it is possible to ensurethat a single or at most a very few molecules in the sample undergoa mechanical perturbation. By sensing the modified optical responseof these sample molecules, a resolution as high as the molecular levelcan potentially be achieved (Fig. 1c). The optical response of thesample in our proposed technique was measured by means oftip-enhanced Raman scattering (TERS). Because the vibrationalfrequencies of Raman modes depend on the mechanical distortionof the sample, pressure-dependent shifts of Raman modesprovide a perfect tool to detect selectively the distorted samplemolecules. We term this new technique ‘tip-pressurized Ramanmicroscopy’. Isolated single-walled carbon nanotubes (SWNTs) andtwo-dimensional adenine nanocrystals were used as the samplesto demonstrate high spatial resolution in the tip-pressurizedRaman experiments.

After setting the silver-coated tip (apex diameter,!35 nm) on anisolated SWNT, Raman spectra (Fig. 2a) were measured in situ whileincreasing the tip-applied force, gradually, up to 3.3 nN. The twopeaks observed in spectra (i) originate from the tangential C–Cstretching vibrational modes and constitute the so-called G-band.The spectral shape of the G-band in this experiment suggests that

1Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan, 2Department of Frontier Biosciences, Osaka University, Suita, Osaka565-0871, Japan, 3Frontier Research Center, Osaka University, Suita, Osaka 565-0871, Japan, 4Nanophotonics Laboratory, RIKEN, Wako, Saitama 351-0198,Japan. *e-mail: yano@ap.eng.osaka-u.ac.jp; verma@ap.eng.osaka-u.ac.jp

LETTERSPUBLISHED ONLINE: 5 JULY 2009 | DOI: 10.1038/NPHOTON.2009.74

NATURE PHOTONICS | VOL 3 | AUGUST 2009 | www.nature.com/naturephotonics 473

© 2009 Macmillan Publishers Limited. All rights reserved.

the nature of the SWNT is semiconducting. The low- and high-fre-quency peaks of the G-band are indicated as mode A and mode B.With increasing tip-applied force, the spectral shapes of the G-banddrastically changed, including peak shifts and intensity changes forboth modes. The force dependences of the peak shifts are shown inFig. 2b. The frequencies of both modes A and B shift by more than10 cm–1 from their initial positions (measured in the absence offorce). This behaviour of the semiconducting SWNT differs fromour previous demonstration on metallic SWNTs in which mode Bdid not exhibit any recognizable peak shift. This is because thevibrational characteristics of the two modes depend on the chiralityof the SWNT. In the previous case, in which the SWNTs wereachiral, modes A and B could be assigned to pure axial and circum-ferential vibrational modes. In the present case of chiral SWNTs,however, the vibrational directions of these modes are not purelycircumferential or axial, but include components of both28.Therefore, when a chiral SWNT is mechanically perturbed in theradial direction by a uniaxial tip-applied force, both modes exhibit

peak shifts. The large amount of shifts observed here confirm thatthe optical response of the pressurized nanotube, under thepresent conditions, can be experimentally detected and distin-guished. In order to estimate the amount of deformation, wecalculated the uniaxial deformation Dd of the nanotube caused bythe tip-applied force, using Hertz’s contact theory29, which is acommonly used method to calculate the deformation of twoobjects in contact with each other under a certain contact force.Figure 2c shows Dd with respect to the tip-applied force, with theinset showing a schematic of the deformation. In this calculation,the tip apex was considered to be a sphere of diameter 35 nm(confirmed by a scanning electron microscopy image of oursilver-coated tip) and the nanotube was assumed to be a cylinderof diameter 1.2 nm (confirmed by atomic force microscopy(AFM) measurements on our sample), resulting in an ellipticalcontact area, as illustrated in Fig. 2d. As expected, the contact areadepends on the amount of applied force. The calculated values ofcontact area are plotted against applied force in Fig. 2e, showing

1,6201,6001,5801,560

Tip-applied force (nN) Tip-applied force (nN)

Ram

an in

tens

ity (

a.u.

)

Peak

shift

, !"

(cm

!1)

Def

orm

atio

n, !

d (n

m)

!d

Contact area

Tip

Tip

Side view

z

z

y

y

x

x

Top view

SWNT Tip-applied force (nN)

Cont

act a

rea

(nm

2 )

1.0

0.8

0.6

0.4

0.2

0.03.02.52.01.51.00.50.0

0.10

0.08

0.06

0.04

0.02

0.003.02.01.00.0

14

12

10

8

6

4

2

03.02.52.01.5

(iii) 1.2 nN

(ii) 0.9 nN

(iv) 1.5 nN

(v) 1.8 nN

(vi) 2.1 nN

(vii) 2.4 nN

(viii) 2.7 nN

(ix) 3.0 nN

(x) 3.3 nN

Raman shift (cm!1)

(i) 0.6 nN

Mode AMode B

Figure 2 | Slight local deformation in a SWNTdue to tip-applied pressure is sensed by means of Raman spectra. a, TERS spectra of a SWNT in theG-band spectral range, comprising mode A (blue curves) and mode B (red curves), show significant dependence on the tip-applied force. b, Both modes Aand B, the peak positions of which are shown by blue and red dots respectively, exhibit force-dependent shifts. c, Calculated deformation of the SWNTunder tip-applied force. d, The contact area between the tip and the tube forms an elliptical shape. e, The contact area as a function of the tip-applied force.

!

Pressurizing

Metallic tip

Sample

Focused light field

~ !

~"/2

Figure 1 | Extremely high spatial resolution in optical microscopy is achieved by sensing the localized pressure applied by a nanosized tip. a, Imageresolution in optical microscopy is determined by the diffraction-limited focal spot, which is approximately a half of the probing wavelength l. b, Spatialresolution in NSOM is determined by the localized light field, comparable to the diameter f of the tip apex. c, When a sample is pressurized by the tip apex,only a few sample molecules are mechanically deformed. By sensing the optical response of these molecules, a spatial resolution up to the molecular levelcan be achieved.

LETTERS NATURE PHOTONICS DOI: 10.1038/NPHOTON.2009.74

NATURE PHOTONICS | VOL 3 | AUGUST 2009 | www.nature.com/naturephotonics474

© 2009 Macmillan Publishers Limited. All rights reserved.

that the contact area measures only !1 nm2 for the maximumamount of force used in the present study. That gives a clearindication, at least theoretically, that even if the tip apex isconsidered to have a smooth spherical surface with a diameter of35 nm, a spatial resolution as high as 1 nm is possible usingthis technique.

Inspired by the above results, we performed one-dimensionalimaging of an isolated SWNT under the application of preciselycontrolled tip-applied force in tip-pressurized Raman microscopyby raster-scanning the tip along a line crossing the nanotube. Thetip-applied force was maintained at a moderate value of 2.4 nNwhile the peak shifts of mode B in TERS spectra were measuredat various points along the scanning line. A schematic of the tipscanning arrangement, combined with a topographic AFM imageof the sample (an isolated SWNT), is shown in Fig. 3a. The variationin spectral shape as the tip moves across the nanotube can be seen inthe Supplementary Information (Video S1). The data points in (i)and (ii) in Fig. 3b show the peak shift plotted as a function oflateral position, corresponding to scanning the tip along thedotted lines (i) and (ii) in Fig. 3a, respectively. The red curvesshow corresponding best fits to the data points. As the tipapproaches the nanotube, moving at steps of 1 nm, the amount offrequency shift in the Raman mode starts to increase, reaching amaximum value of !10 cm–1 when the tip is exactly at the centreof the nanotube, and then decreasing as the tip moves away fromthe nanotube (Fig. 3b(i)). Similar behaviour of the peak shift canbe seen in curve (ii), confirming the reliability of the measurementof the pressure-dependent optical response. The centres of thefitting profiles in (i) and (ii), which indicate the central position

An isolated SWNT

F = 2.4 nN

(i)(ii)

F

F

F

302520151050Lateral position (nm)

302520151050Lateral position (nm)

302520151050Lateral position (nm)

(i)

(ii)

Peak

shift

of

mod

e B

(cm

!1)

Peak

shift

of

mod

e B

(cm

!1)

Hei

ght (

nm)

FWHM: 22.4 nm1.2

0.8

0.4

0.0

FWHM: 4 nm

10

8

6

4

2

0

10

86

4

2

0

Figure 3 | One-dimensional optical image of a SWNTat extremely highspatial resolution. a, An isolated SWNTwas scanned in steps of 1 nm undera constant tip-applied force of 2.4 nN. b, The line profiles (i) and (ii) showthe peak shifts (black dots) as a function of the tip position measured fromthe SWNT, and corresponding best fits (red curves) for the scans are shownby lines (i) and (ii) in a, respectively. The FWHM of the line profile is 4 nm,which is more than five times smaller than that of the topographic profileshown in c.

200

150

100

50

Ram

an in

tens

ity (a

.u.)

Tip-applied force (nN)1.0 2.0 3.0 4.0

Pushing

Pulling

(i)

(ii)(iii)

Figure 4 | Reversible and irreversible Raman intensity responses determinethe threshold value of the tip-applied force for plastic deformation.a, Raman intensity of the G-band of an SWNT is plotted as a function oftip-applied force. With a maximum force of less than 2.0 nN, the Ramanintensity followed a reversible path, shown by (i), during a push–pull cycle,indicating an elastic deformation as shown schematically in b. When theforce was increased to 4 nN, the Raman intensity followed the pathsequence (i) ! (ii) ! (iii) during the push–pull cycle, indicating a plasticdeformation as illustrated in c.

NATURE PHOTONICS DOI: 10.1038/NPHOTON.2009.74 LETTERS

NATURE PHOTONICS | VOL 3 | AUGUST 2009 | www.nature.com/naturephotonics 475

© 2009 Macmillan Publishers Limited. All rights reserved.

of the nanotube, are shifted by a few nanometres because the scan-ning lines are not exactly perpendicular to the nanotube axis, asillustrated in Fig. 3a. Although the full-width at half-maximum(FWHM) of the fitting in (i) is 4 nm, a clear shift of frequency isobserved when the tip is moved by 1 nm in close vicinity to thenanotube, as observed from the data points. This leads us to concludethat the sensitivity of the optical response here is as high as 1 nm. Thisconclusion is also well supported by theoretical calculation of thecontact area in Fig. 2d, which defines the mechanically perturbedarea of the nanotube. Even the value of 4 nm, obtained from theFWHM of the fitting, is more than five times better than that of thetopographic image (Fig. 3c), which was measured with the same tip(apex diameter, !35 nm). It should also be noted here that becausethe scanning direction was not exactly perpendicular to the nanotubeaxis, the actual FWHM should be even smaller than 4 nm.

When a sample is pressurized by the tip, there is a risk of dama-ging the sample by deforming the sample plastically (irreversibly).We therefore always took special care in keeping the tip-appliedforce within the elastic limit of the nanotube that was examined.As an example, Fig. 4 demonstrates the elastic and plastic defor-mation of a particular nanotube, showing a decrease in intensitywith increasing pressure (due to the outgoing resonance effect).We measured the TERS intensity during a push–pull cycle, inwhich the tip gradually pushed the nanotube until the tip-appliedforce reached a certain value and then the tip was retracted bygradually reducing the force. When the maximum force was lessthan 2 nN, the push–pull cycle followed a reversible path shownby (i), but when the maximum force was more than 2 nN, thepush–pull cycle followed the irreversible path sequence (i) ! (ii)! (iii). The corresponding illustrations of elastic and plasticdeformations are shown in Fig. 4b and c, respectively. Thisexperiment shows an example for determining the threshold valueof the tip-applied force for plastic deformation, which is asample-dependent quality.

Owing to their one-dimensional cage structure, SWNTs act asgood test samples under a tip-applied force. However, to confirmthe versatility of our method, we also carried out measurementson a thin and flat adenine nanocrystal, which served as a two-dimensional sample with much weaker elasticity compared toSWNTs. Figure 5a shows an AFM image of the sample. Wescanned this sample across one of its edges (along the arrow inFig. 5a) under a constant tip-applied force of 0.3 nN, and measuredthe frequency position of the ring-breathing mode (RBM) ofadenine during the scan. We found that the peak position of theRBM shifted from!723 to!730 cm–1 when the tip came into phys-ical contact with the sample during the scanning process. The shiftof 7 cm–1 in RBM was caused by the tip-applied pressure. The lineprofile of this scan is plotted in Fig. 5b, which shows a steep jump ofRBM frequency at the edge of the sample, demonstrating a spatial

resolution of 4 nm, which is the same as that obtained for theSWNT measurements.

In conclusion, we have demonstrated an active imaging tech-nique that makes use of an additional effect of local pressure inNSOM-based microscopy to achieve super-high spatial resolution.By testing it on two different samples, we have confirmed that ourtechnique is applicable for samples having different elastic proper-ties, as well as being capable of demonstrating a similar resolutionfor two-dimensional samples. This new technique of pressure-assisted TERS comprises a breakthrough in nano-imaging andnano-analysis of materials, with the potential for molecular resol-ution in optical microscopy.

MethodsSample preparation.We prepared two types of SWNTs for this study. The first type,which was used in the experiments related to Figs 2 and 3, was synthesized using adirect injection pyrolytic synthesis (DIPS) method30,31. The second type, which wasused for demonstrating the threshold value of the tip-applied force for plasticdeformation in Fig. 4, was synthesized using a high-pressure carbon monoxide(HiPCO) method. Both types were ultrasonicated with 2,2,3,3-tetrafluoro-1-propanol suspension, and spin-casted at 3,000 rpm for 1 min onto glass slips, thendried in a vacuum oven at 120 8C for 2 h. The process of sample preparation resultedin spatially isolated SWNTs on glass slips, although some sections of the samplewere still bundled. We carefully selected spatially isolated single nanotubes that werein a near-resonance condition with the incident laser used in this study. The spatialdispersion of the nanotubes was confirmed with a high-quality AFM image, and theresonance behaviour was confirmed using Raman spectroscopy for both thesemiconducting nanotubes synthesized by DIPS and the metallic nanotubessynthesized by HiPCO.

The two-dimensional nanocrystal was prepared by casting and drying a dilutedsolution (0.05 mM) of adenine molecules on a cover glass. The molecules self-assembled into nanocrystals. We then checked the sample with a high-quality AFMimage, and an isolated, thin and flat two-dimensional nanocrystal was selected forour measurement.

Experimental set-up. We developed a tip-pressurized Raman microscope bycombining an atomic force microscope with an inverted optical microscope. TheAFM cantilever tip, which was made of silicon, was pre-coated with silver using avacuum evaporation method. The size of the tip apex was 35 nm, which wasconfirmed by a scanning electron microscope. The silver-coated AFM tip wasoperated in contact mode, so that its position could be precisely controlled in allthree directions. This operation played an important role in applying a well-controlled pressure to the sample. At the same time, the silver coating of the tiphelped in plasmonic enhancement of the weak Raman scattering under the tip. Tip-pressurized Raman spectra were measured in the backscattering configuration usingan objective with high numerical aperture (magnification, "100; NA, 1.4) and asolid-state continuous-wave laser (l, 532 nm; power, 0.1 mW).

Pressure-assisted TERS measurements of isolated SWNT. An isolated SWNTprovided us with a good and sturdy test sample for demonstrating the capability forhigh spatial resolution in pressure-assisted TERS measurements. This was achievedby one-dimensional imaging in which an isolated SWNT was raster scanned in stepsof 1 nm along a line crossing the nanotube while measuring tip-pressurized Ramanspectra under a constant tip-applied force of 2.4 nN. To prevent the nanotube frombeing dragged with the tip, we deliberately reduced the tip-applied force to 0.1 nNduring sample movement in the scanning process; this was then restored to 2.4 nNduring spectral measurement.

Pressure-assisted TERS measurements of two-dimensional nanocrystal ofadenine. To confirm the versatility of our method, we performed furthermeasurements on an adenine nanocrystal, which is a much softer material thanSWNTs and can serve as a two-dimensional sample. The scanning process for thissample was almost the same as for the SWNT, but with an important difference inthe amount of tip-applied force, which was only 0.3 nN in the present case. This wasnecessary because adenine shows a reasonable amount of shift in Raman mode evenfor a small level of tip-applied force, because of its smaller elastic constants. Thesample was scanned across one of its edges, and the resolution was determined fromthe line profile of the Raman shift at the edge during the scan.

Received 10 October 2008; accepted 12 June 2009;published online 5 July 2009

References1. Binnig, G., Rohrer, H., Gerber, C. &Weibel, E. 7 " 7 reconstruction on Si (111)

resolved in real space. Phys. Rev. Lett. 50, 120–123 (1982).2. Binnig, G., Quate, C. F. & Gerber, C. Atomic force microscope. Phys. Rev. Lett.

56, 930–933 (1986).

Lateral direction, x (nm)

730

728

726

724

6040200

4 nm

50 nm

Adenine nanocrystal

Scan directionx = 0

Ram

an sh

ift (c

m!1

)Figure 5 | High spatial resolution for a two-dimensional nanocrystal ofadenine. a, An AFM image of the nanocrystal selected for TERSmeasurement. The tip was scanned across the sample edge in the directionof the arrow under a tip-applied pressure of 0.3 nN. b, The line profile of theshift in RBM of adenine along the scan direction. The steep edge of the lineprofile shows a spatial resolution of about 4 nm.

LETTERS NATURE PHOTONICS DOI: 10.1038/NPHOTON.2009.74

NATURE PHOTONICS | VOL 3 | AUGUST 2009 | www.nature.com/naturephotonics476

© 2009 Macmillan Publishers Limited. All rights reserved.

3. Eigler, D. M. & Schweizer, E. K. Positioning single atoms with a scanningtunneling microscope. Nature 344, 524–526 (1990).

4. Bensimon, A. et al. Alignment and sensitive detection of DNA by a movinginterface. Science 265, 2096–2098 (1994).

5. Sugimoto, Y. et al. Chemical identification of individual surface atoms by atomicforce microscopy. Nature 446, 64–67 (2007).

6. Willig, K. I., Rizzoli, S. O., Westphal, V., Jahn, R. & Hell, S. W. STED microscopyreveals that synaptotagmin remains clustered after synaptic vesicle exocytosis.Nature 440, 935–939 (2006).

7. Betzig, E. et al. Imaging intracellular fluorescent proteins at nanometerresolution. Science 313, 1642–1645 (2006).

8. Huang, B., Wang, W., Bates, M. & Zhuang, X. Three-dimensional super-resolution imaging by stochastic optical reconstruction microscopy. Science 319,810–813 (2008).

9. Pohl, D. W., Denk, W. & Lanz, M. Optical stethoscopy: image recording withresolution l/20. Appl. Phys. Lett. 44, 651–653 (1984).

10. Zenhausern, F., Oboyle, M. P. & Wickramasinghe, H. K. Apertureless near-fieldoptical microscope. Appl. Phys. Lett. 65, 1623–1625 (1994).

11. Bachelot, R., Gleyzes, P. & Boccara, A. C. Near-field optical microscope based onlocal perturbation of a diffraction spot. Opt. Lett. 20, 1924–1926 (1995).

12. Hayazawa, N., Inouye, Y., Sekkat, Z. & Kawata, S. Metallized tip amplification ofnear-field Raman scattering. Opt. Commun. 183, 333–336 (2000).

13. Stockle, R. M., Suh, Y. D., Deckert, V. & Zenobi, R. Nanoscale chemical analysisby tip-enhanced Raman spectroscopy. Chem. Phys. Lett. 318, 131–136 (2000).

14. Anderson, M. S. Locally enhanced Raman spectroscopy with an atomic forcemicroscope. Appl. Phys. Lett. 76, 3130–3132 (2000).

15. Sanchez, E. J., Novotny, L. & Xie, X. S. Near-field fluorescence microscopy basedon two-photon excitation with metal tips. Phys. Rev. Lett. 82, 4014–4017 (1999).

16. Zhang, W. H., Yeo, B. S., Schmid, T. & Zenobi, R. Single molecule tip-enhancedRaman spectroscopy with silver tips. J. Phys. Chem. C 111, 1733–1738 (2007).

17. Neacsu, C. C., Dreyer, J., Behr, N. & Raschke, M. B. Scanning probe Ramanspectroscopy with single molecule sensitivity. Phys. Rev. B 73, 193406 (2006).

18. Steidtner, J. & Pettinger, B. Tip-enhanced Raman spectroscopy and microscopyon single dye molecules with 15 nm resolution. Phys. Rev. Lett. 100,236101 (2008).

19. Ichimura, T., Hayazawa, N., Hashimoto, M., Inouye, Y. & Kawata, S. Tip-enhanced coherent anti-Stokes Raman scattering for vibrational nano-imaging.Phys. Rev. Lett. 92, 220801 (2004).

20. Hartschuh, A., Sanchez, E. J., Xie, X. S. & Novotny, L. High-resolution near-fieldRaman microscopy of single-walled carbon nanotubes. Phys. Rev. Lett. 90,095503 (2003).

21. Anderson, N., Hartschuh, A., Cronin, S. & Novotny, L. Nanoscale vibrationalanalysis of single-walled carbon nanotubes. J. Am. Chem. Soc. 127,2533–2537 (2005).

22. Hillenbrand, R., Taubner, T. & Keilmann, F. Phonon-enhanced light–matterinteraction at the nanometre scale. Nature 418, 159–162 (2002).

23. Hillenbrand, R. & Keilmann, F. Material-specific mapping ofmetal/semiconductor/dielectric nanosystems at 10-nm resolutionby backscattering near-field optical microscopy. Appl. Phys. Lett. 80,25–27 (2002).

24. Ichimura, T. et al. Temporal fluctuation of tip-enhanced Raman spectra ofadenine molecules. J. Phys. Chem. C 111, 9460–9464 (2007).

25. Verma, P., Yamada, K., Watanabe, H., Inouye, Y. & Kawata, S. Near-field Ramanscattering investigation of tip effects on C60 molecules. Phys. Rev. B 73,045416 (2006).

26. Yano, T., Inouye, Y. & Kawata, S. Nanoscale uniaxial pressure effect of a carbonnanotube bundle on tip-enhanced near field Raman spectra. Nano Lett. 6,1269–1273 (2006).

27. Ichimura, T. et al. Subnanometric near-field Raman investigation in the vicinityof the metallic nanostructure. Phys. Rev. Lett. 102, 186101 (2009).

28. Reich, S., Thomsen, C. & Ordejon, P. Phonon eigenvectors of chiral nanotubes.Phys. Rev. B 64, 195416 (2001).

29. Boresi, A. P. & Sidebottom, O. M. Advanced Mechanics of Materials (John Wiley& Sons, 1985).

30. Saito, T. et al. Size control of metal nanoparticle catalysts for the gas-phasesynthesis of single-walled carbon nanotubes. J. Phys. Chem. B 109,10647–10652 (2005).

31. Saito, T. et al. Supermolecular catalysts for the gas-phase synthesis of single-walled carbon nanotubes. J. Phys. Chem. B 110, 5849–5853 (2006).

AcknowledgementsThe authors would like to thank Y. Inouye (Osaka University, Japan) for fruitfuldiscussions, S. Fujii and F. Futamatsu (Osaka University, Japan) for their valuable helpduring some of the experiments, and T. Saito (Advanced Industrial Science andTechnology, Japan) for supplying the high-quality SWNTs used in the present study.This research was financially supported by the Core Research for Educational Science andTechnology (CREST) project of Japan Science and Technology Corporation.

Additional informationSupplementary information accompanies this paper at www.nature.com/naturephotonics.Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/. Correspondence and requests for materials should be addressedto T.Y. and P.V.

NATURE PHOTONICS DOI: 10.1038/NPHOTON.2009.74 LETTERS

NATURE PHOTONICS | VOL 3 | AUGUST 2009 | www.nature.com/naturephotonics 477

© 2009 Macmillan Publishers Limited. All rights reserved.