Abstract We visualized that semiconducting single wall carbon

nanotubes (SWCNTs) represent metallic property at a point where a few tubes crossed each other, by tip-enhanced Raman scattering (TERS) microscopy.

I. INTRODUCTION Tuning electronic properties of SWCNTs has been an

ultimate goal for scientists who aim to develop nano-electronic devices with a variety of characteristic. A surge of the interests give rise to many methods for modifying electronic properties of SWCNTs such as chemical functionalization [1], encapsulation of organic and non-organic materials inside SWCNTs and structural deformation [2]. Since a prediction of electronic band modulation have been made theoretically, bending SWCNTs by the apex of an atomic force microscope (AFM) tip [3] and bridging SWCNTs over electrodes or other SWCNTs [4], which makes SWCNTs undergo strong deformations at a crossing part, has attracted many researchers. In particular, an interesting configuration to study is crossing over other SWCNTs because it is a easy technique for electronic band modifications by strong radial compressions on SWCNTs [5] and interconnects which induce tunnel barriers. A compression of semiconducting SWCNTs was predicted to be a cause of metallization of SWCNTs attributable to π* - σ* hybridization effect which occurs only near maximum curvature points of a compressed SWCNT [6], which is expected to occur in a pair of crossing tubes. A change in electronic properties at a crossing area was explained by the band gap modifications through a formation of continuum state by a first-principles study [7]. A dramatic transforming from semiconductor to metal SWCNTs by a local pressure was revealed using scanning tunneling spectroscopy (STS) [8]. However, the causes of this metallization, which might be strain, chemical interactions or inter-tube interactions, have not been explained, so there remains considerable debates and stimulate extensive research enthusiasm on this issue. Raman spectroscopy is a suitable tool for paving the way to understand what is happening on SWCNTs because of their ability to detect directly vibrational features of molecule as a 2D imaging [9], enabling us to see what is happening on the sample. Provided that the problem of the diffraction limit and the inefficiency of detecting Raman scattering were resolved, we could understand what causes property-changes at a crossing part in crossed SWCNTs with high resolution.

Tip enhanced Raman scattering (TERS) microscopy is a fascinating tool which can break the diffraction limit and enhance the efficiency of Raman scattering together by utilizing near-field microscopy and localized plasmon [10], which satisfies our demand.

The present paper is devoted to the investigation of vibrational features of semiconducting SWCNTs crossing other semiconducting SWCNTs by TERS microscopy for an optical visualization of the relation between a strain applied on SWCNTs and a metallization with high resolution beyond diffraction limit. Most remarkably, we visualize the metal-semiconducting transition of the semiconducting SWCNTs at crossing point.

II. EXPERIMENTAL

A. Preparation of semiconducting SWCNT In our experiment, semiconducting SWCNTs used for

TERS measurements were purchased by Meijo carbon Inc., with a purity of ∼99%. Individual bundle of this semiconducting SWCNTs with diameters 1.4 nm were ultrasonically dispersed with 1-2 dichrolethane, and spin-casted onto the glass coverslip.

B. Optical setup for tip enhanced Raman microscopy Our experimental setup, based on an inverted optical

microscope combined with a contact-mode AFM, was especially established for TERS microscopy. Silicon cantilever tips used in this measurements were water-oxidized in order to make plasmon resonance shift to the wavelength of light we used before the cantilever was silver-coated up until the end of a diameter reach 15 nm by the vacuum evaporation method. Raman scattering from the sample were excited by solid state laser (λ: 488 nm) which was radially polarized with the use of eight divided half wavepletes so as to efficiently excite localized surface plasmon polariton along tip apex. The enhanced Raman scattering were detected by CCD incorporated into a spectrometer. N.A. of objective lens is 1.4. Exposure time for taking one spectra is 0.5 sec and step size for imaging is 10 nm.

III. TERS ANALYSIS OF METALLIZATION OF SEMICONDUCTING SWCNTS

TERS image of G+ mode for crossed SWCNTs were shown in Fig. 1 (a). The bundle was ranging from 3-5 tubes. The evolution of TERS spectra in G-band region normalized by the peak intensity of G+ mode obtained at different positions along the tube are indicated in Fig.1 (b).

Yoshito Okuno*, Yuika Saito*, Satoshi Kawata*,** and Prabhat Verma*

*Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan

** RIKEN, Wako, Saitama 35-0198, Japan

Tip-enhanced Raman Scattering Study of Metalized-Semiconducting Carbon Nanotube

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By the comparisons of each TERS spectra derived from the position (1)-(6) near the crossing point of SWCNTs, the G band were fitted by 3 symmetric Lorentzian which have respnance peaks at 1565 cm-1, (Green line) 1589 cm-1 (pink) and 1607 cm-1 (blue), respectively . The peak at 1548 cm-1, (Yellow line) was fitted by Breit-Wigner-Fano (BWF), were found over the crossing point as can be seen in Fig.1 (b). From Fig.1 (b), the spectra of G-band at crossing point were totally different with that of G band from the area which are far from the center.

Fig. 1. (a) TERS spectra obtained across the crossed nanotubes and (b) TERS image of crossed carbon nanotubes. (b) (1)-(6) each represents TERS spectra gotten from the place of (1)-(6) in (a). Green, red and blue line shows Lorentzian fit at 1565, 1588 and 1607 cm-1. Arrows indicate the point of intensity increment of some vibrational modes.

Then BWF function used in this study can be simply described as [11].

Origin of the Breit-Wigner-Fano lineshape of the tangential G-band featureof metallic carbon nanotubes

S. D. M. Brown, A. Jorio, and P. CorioDepartment of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307

M. S. DresselhausDepartment of Physics and Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology,

Cambridge, Massachusetts 02139-4307

G. DresselhausFrancis Bitter Magnet Laboratories, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307

R. SaitoDepartment of Electronics-Engineering, University of Electro-Communications, Tokyo 182-8585, Japan

K. KneippTechnical University of Berlin, 10623 Berlin, Germany

and G. R. Harrison Spectroscopy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307�Received 26 April 2000; revised manuscript received 10 October 2000; published 29 March 2001�

A detailed line-shape analysis of the tangential G-band feature attributable to metallic single-walled carbonnanotubes is presented. Only two components are needed to account for the entire G-band feature for metallicnanotubes. The higher-frequency component has a Lorentzian line shape, and the lower one has a Breit-Wigner-Fano �BWF� line shape. Through comparisons of the Raman tangential G-band spectra from threedifferent diameter distributions of carbon nanotubes, we Þnd that both the frequency and linewidth of the BWFcomponent are diameter dependent and show functional forms consistent with theory. The nanotube curvatureis responsible for both the frequency differences between the two components of the characteristic metallicG-band spectrum and the BWF coupling of the lower-frequency component. Surface-enhanced Raman spec-troscopy studies provide supporting evidence that the phonon BWF coupling is to an electronic continuum.

DOI: 10.1103/PhysRevB.63.155414 PACS number�s�: 61.48.�c, 78.30.Na

I. INTRODUCTION

One method for distinguishing between metallic andsemiconducting single-walled carbon nanotubes �SWNTs� inany given sample1,2 is based on the distinct differences in theline shape of the tangential G-band (�1600 cm�1) featurein their Raman spectra. The G-band of semiconducting nano-tubes has been extensively studied, and is well accounted forusing Lorentzian oscillators to describe the six Raman-activemodes,1,2 recently identiÞed by polarization studies of thesymmetries of the various line-shape components.3 Some re-searchers previously Þtted the Raman line shape for metallicSWNTs using Lorentzians4,5 while others used aBreit-Wigner-Fano6,7 �BWF� line shape to Þt the lower-frequency component of the G-band spectrum. There has,however, been no explanation of the mechanism responsiblefor the downshift and broadening of the tangential G band ofmetallic SWNTs relative to semiconducting SWNTs.The analysis presented here conÞrms that only two Ra-

man components are needed to Þt the tangential G band formetallic SWNTs,7 with a Lorentzian line shape describingthe higher-frequency feature and a BWF line describing thelower-frequency feature. Both components are found to ex-hibit predominantly A (A1g) symmetry. The differences intheir peak frequencies are attributed to �1� a difference inforce constant for vibrations along the tube axis �higher force

constant� versus circumferentially �lower force constant�;and �2� an additional downshifting and broadening of thelower-frequency peak due to coupling of the discretephonons to an electronic continuum, resulting in the BWFline shape.The asymmetric BWF line shape,8 described by

I����I0�1�����BWF�/q��2

1������BWF�/��2�1�

�where 1/q is a measure of the interaction of the phonon witha continuum of states, and �BWF is the BWF peak frequencyat maximum intensity I0), has previously been used to Þtsome of the Raman bands of the metallic forms of varioussp2 carbons, such as the �1540 cm�1 feature of metallicSWNTs,6,7 the tangential G-band feature of alkali-metaldoped SWNTs,9 the feature near 1600 cm�1 of carbonaerogels10 and alkali-metal graphite intercalationcompounds,11 as well as the �270 cm�1 feature12 of metallicK3C60 . In contrast, the Raman bands of the semiconductingforms of sp2 carbons �undoped C60 and K6C60 ,12 and semi-conducting SWNTs �Ref. 3�� exhibit Lorentzian line shapes.The inset to Fig. 1 shows the Stokes Raman signal �from

900 to 2000 cm�1) from a sample of SWNTs with diametersdt�1.49�0.20 nm, for laser excitation energies E laser�1.58 and 2.41 eV. The lower-frequency tail of the broad

PHYSICAL REVIEW B, VOLUME 63, 155414

0163-1829/2001/63�15�/155414�8�/$20.00 ©2001 The American Physical Society63 155414-1

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where I0, ωBWF, Γ, and 1/q are intensity, renormalized

frequency, broadening parameter and the asymmetric parameter, respectively . The value 1/q reveals the evidence of electron-phonon coupling because it includes the ratio of interaction of continuum state with discrete states. Each frequency used for the fitting is based on polarization analysis of the line shape [12].

The absolute values of 1/q plotted over the TERS image of the crossed SWCNTs, as shown in Fig.2. The metallization just at the crossing part are successfully visuallized and the distribution of metal-semiconducting transitions are clarified. And the broadening of each spectra obtained from crossing part and scrutinized here showed similar behavior induced by the radial compression which was reported before.

Fig. 2. 3D TERS image with the value of |1/q| for evaluation of metal transition. Planar direction of this image shows x-y topological position of crossed SWCNTs and perpendicular direction to plane shows |1/q| value. The color shows intensity distribution of TERS spectra at peak position of G-band

IV. CONCLUSIONS In summary, we reported the metallzation of SWCNTs

at junction part by the TERS analysis of the G band spectra.We found peak appearing at shoulder of G+ mode and increment of G- mode on junction can be explained by occurrence of radial compression of structure and metallization. This is the first report of optical nanoscale imaging which visualize metallization of semiconducting nanotubes by the strain applied just at crossing part of SWCNTs. Through this work, we got a better understanding that the strong deformation of semiconducting SWCNTs induce metallization. These results imply the possibility for band gap modification and making electronic devices only by crossing two tubes or bending them.

ACKNOWLEDGMENT This work was supported by a grant from the Japan

Science and Technology Agency under a Core Research for Evolutional Science and Technology (CREST) project ‘‘Plasmonic Scanning Analytical Microscopy’’. I want to thank T.Yano, Y.Umeno, S.Irle, Y.Nishimura and S.Yamamoto for helpful discussions.

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(1994). [7] Mario S. C. Mazzoni et al., APPLIED PHYSICS LETTERS, Vol.

76, NUMBER 12, 1561 (2000). [8] L. Vitali et al., Phys. Rev. Lett., Vol. 96, 086804 (2006). [9] A Jorio et al., New Journal of Physics, 5, 139.1–139.17(2003) [10] Hayazawa et al., Chem. Phys. Lett., Vol. 376, pp. 174-180, (2003) [11] V. Klein et al., in Light Scattering in Solids I, edited by M.

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