Stable and controlled amphoteric doping by encapsulation of organic molecules inside carbon...

ARTICLES

nature materials | VOL 2 | OCTOBER 2003 | www.nature.com/naturematerials 683

SWNTs are wires with molecular-scale diameters (~1 nm),and individual semiconducting SWNTs have been activelyexplored to construct nanotube field-effect transistors (FETs)1.

One advantage of SWNTs is the high carrier mobility2,3, becauseelectrical transport in high-quality nanotubes can be ballistic4,5.Recently, top-gated SWNT-FET with zirconium oxide dielectricsshowed surprisingly high performance6.

For further evolution of molecular electronics, the ability to obtainboth p- and n-type FETs is important for constructing complementaryelectronics that are known to be superior in performance (for example,low power) to devices consisting of unipolar p- or n-type transistors.Key points to p- and n-type SWNTs are air stability, tuning of dopinglevel, and mass production. SWNT-FETs built from as-grown tubes arefound to be unipolar p-type, that is, no electron current flow even atlarge positive gate biases. The transistor action in SWNT-FETs can beunderstood on the basis of transport across a Shottky barrier at themetal–SWNT contact, and the main effect of oxygen exposure is tochange the work function of the metal contact7,8.The production of air-stable n-type transistors is important. Several doping methods havebeen developed for nanotubes, including exposure to gaseousmolecules9,10, annealing in vacuum8 or in inert gas6,11, intercalation ofinorganic materials12,13, electrochemical experiments14 and adsorptionof molecules15,16, although stability in air was not sufficient for the n-type doping. Recently, n-type FETs that are stable in air by polymerfunctionalization have been reported17, however fine tuning of carrierconcentration is still very difficult.

On the other hand, owing to their size and geometry, SWNTs alsoprovide a unique opportunity for nanoscale engineering of novel one-dimensional systems, created by self-assembly of molecules inside theSWNT’s hollow core18,19,20. It has been experimentally shown thatfullerenes and/or endohedral metallofullerenes can be inserted intoSWNTs, forming a peapod-like structure20. The composite nature of

Single-walled carbon nanotubes (SWNTs) have strong

potential for molecular electronics, owing to their unique

structural and electronic properties. However, various

outstanding issues still need to be resolved before SWNT-

based devices can be made. In particular, large-scale,

air-stable and controlled doping is highly desirable. Here we

present a method for integrating organic molecules into

SWNTs that promises to push the performance limit of these

materials for molecular electronics. Reaction of SWNTs with

molecules having large electron affinity and small ionization

energy achieved p- and n-type doping, respectively.

Optical characterization revealed that charge transfer

between SWNTs and molecules starts at certain critical

energies. X-ray diffraction experiments revealed that

molecules are predominantly encapsulated inside SWNTs,

resulting in an improved stability in air. The simplicity of the

synthetic process offers a viable route for the large-scale

production of SWNTs with controlled doping states.

Stable and controlled amphoteric doping byencapsulation of organic molecules insidecarbon nanotubesTAISHI TAKENOBU*1,2, TAKUMI TAKANO1, MASASHI SHIRAISHI3, YOUSUKE MURAKAMI4,MASAFUMI ATA3, HIROMICHI KATAURA5, YOHJI ACHIBA6 AND YOSHIHIRO IWASA*1,2

1Institute for Materials Research,Tohoku University,Sendai,980-8577,Japan2CREST,Japan Science and Technology Corporation,Kawaguchi,332-0012,Japan3Materials Laboratories,SONY Corporation,Yokohama 240-0036,Japan4Global Professional Solutions,SONY Corporation,Yokohama 240-0036,Japan5Department of Physics,Tokyo Metropolitan University,Hachioji,192-0397,Japan6Department of Chemistry,Tokyo Metropolitan University,Hachioji,192-0397,Japan*e-mail: [email protected]; [email protected]

Published online:7 September 2003; doi:10.1038/nmat976

nmat976PRINT 10/9/03 1:48 pm Page 683

© 2003 NaturePublishing Group

© 2003 Nature Publishing Group

ARTICLES

684 nature materials | VOL 2 | OCTOBER 2003 | www.nature.com/naturematerials

peapod materials raises an exciting possibility of a nanoscale materialhaving a tunable structure that can be tailored to a particular electronicfunctionality.One of the notable features is the simplicity of synthesis incomparison with chemical functionalization.The spatial modulation ofthe bandgap has been performed in Gd@C82 (ref. 18) or C60 (ref. 19)encapsulated SWNTs, offering possible applications of thisencapsulation to functionalization of SWNTs. In this article, we reportthe synthesis and characterization of organic/SWNT compounds, anddemonstrate that the charge transfer between SWNTs and organicmolecules are controlled by the ionization energy or the electron affinityof guest organic molecules. As organic molecules predominantlyoccupy the inner space of SWNTs,the doping state is rather stable in air.This hybrid material satisfies the requirements of controllable doping,air stability and simplicity of synthesis that are vital in applications.

SWNTs were manufactured by laser vaporization of carbon rodsdoped with Co/Ni in an atmosphere of argon and subsequentlypurified with H2O2,HCl and NaOH (refs 20,21).The purified SWNTsalready have a sufficient amount of entrance holes for molecules22.SWNTs were reacted with a vapour of organic molecules in a similarmanner to that for the C60-peapod20.Organic molecules (for example,see ref. 23), summarized in Table 1 (see also Fig. 1), were purified bysublimation before reaction with the SWNTs. Purified and decappedSWNTs were loaded in a glass tube with the organic powder samples.This tube was evacuated to 2 ×10–6 torr,and the SWNTs were degassedby heating with a blow torch. Organic molecules were placed in aseparate position in the tube so that they stayed at room temperaturein this process. The tube was then sealed and annealed for twelvehours in a furnace just above the sublimation temperature of theorganic molecules. At this temperature, these molecules are mobileand able to enter the SWNTs. We prepared two kinds of samples, theas-purified bulk samples and thin films. Bulk samples were used forX-ray powder diffraction (XRD) and transmission electron

microscopy (TEM; Hitachi HF-200) characterizations, and the filmsamples were for measurements of Raman spectroscopy (JASCONRS-1000), optical absorption (Nicolet MAGNA-IR 760 andVARIAN Cary 5000), resistivity, X-ray photoemission spectroscopy(XPS; Omicron EA125) and thin-film FETs. SWNT films wereprepared by spraying SWNTs dispersed in ethanol on silicon orquartz plates using an airbrush. All procedures, except for checkingstability in air, were carried out in an atmosphere-controlled argonglove box, inside glass tubes, or in an indium-sealed optical cell toavoid the effect of oxygen.

Table 1 Electron affinity and ionization energy for molecules used in thisstudy.

Molecule Electron affinity Ionization energy (eV) (eV)

Tetrakis(dimethylamino)ethylene (TDAE) 5.36Tetramethyl-tetraselenafulvalene (TMTSF) 6.27Tetrathiafulvalene (TTF) 6.40Pentacene 1.392 6.58Anthracene 0.56 7.363,5-Dinitrobenzonitrile 2.16C60 2.65Tetracyano-p-quinodimethane (TCNQ) 2.80 9.50Tetrafluorotetracyano-p-quinodimethane 3.38(F4TCNQ)

Figure 1 Structures of molecules used in this study.

S

S

S

S

NC

NC

CN

CNF

F

F

F

NC

NC

CN

CN

CH3 CH3

CH3 CH3

H3C

H3C

H3C

H3C

Se

Se

Se

Se

CH3

CH3

N

N

N

N

CH3

CH3

TDAE TMTSF TTF

Pentacene Tetracene Anthracene

C60 TCNQ F4TCNQ




ARTICLES


For structural characterization, synchrotron XRD data werecollected at room temperature on the beam line BL02B2 at the SuperPhoton Ring (SPring-8), Japan.For this purpose,bulk organics/SWNTsamples were sealed in thin, glass capillaries of 0.5 mm outer diameter.Figure 2 shows typical diffraction profiles for pristine and TCNQ-doped SWNT materials. The most obvious difference between dopedand undoped SWNTs is the strong reduction of peak intensity at amomentum transfer Q of ~0.4 Å-1,which is indexed as a (10) reflection.Such behaviour provides evidence for encapsulation of organicmolecules inside the SWNTs, as encountered in several peapodmaterials and gas-adsorbed SWNTs20,22.

Intensity reduction of the (10) peak allows us to estimate the chemicalconcentration ratio of encapsulated organic molecules versus the carbonatoms of the SWNTs. First, the parameters of pristine SWNTs weredetermined so as to reproduce the observed diffraction pattern shown inFig. 2a,taking the gaussian distribution of the tube diameter into account.The diameter of tubes d, the lattice parameter a, and the diameter ofbundles D were determined as d = 13.7 ± 1.3 Å, a = 17.1 ± 1.3 Å, andD=115Å.These parameters were consistent with the TEM observations.Using these parameters, the intensity distribution of the diffractionpattern for the TCNQ/SWNT compound was well accounted for byinserting a 7-Å-diameter rod of uniform charge inside the tubes.For simplicity,we assumed that the electron distribution of doped TCNQmolecules is uniform.From the density of uniform charge,the number ofTCNQ molecules per carbon atom of a SWNT was derived as 0.0071(5);these values are also expressed in the form C140/TCNQ.

From an XPS measurement on the film, we estimated thecomposition of TCNQ molecules from the relative intensity of C(1s)and N(1s) peaks. The analysis using an argon-ion milling techniqueclarified that the surface and inside of the film have the compositionsC28/TCNQ and C92/TCNQ,respectively.It is noted that the latter value isabout two-thirds of that determined by the XRD analysis, indicatingthat TCNQ molecules are predominantly encapsulated inside theSWNTs. The high TCNQ composition measured on the film surfaceindicates TCNQ molecules were also deposited there.

The chemical concentration of C140/TCNQ corresponds to theaverage distance of 10 Å between neighbouring TCNQ molecules.A possible configuration of encapsulated TCNQ molecules inside a(10,10) SWNT is described in Fig. 2b, and is compared to that of C60

peapods. The intermolecular distance is close to the long axis ofTCNQ molecules, indicating that the packing density isconsiderably high. However, a more likely configuration of TCNQmolecules is random, because we did not observe any extra XRDpeaks attributable to the regular one-dimensional stacking ofmolecules, in contrast with the case of well-aligned C60 and C70

peapods19. The intensity reduction of the (10) peak was observed inall organic/SWNT compounds investigated (Fig. 1), and thechemical concentration was 100–150 carbon atoms per molecule.Additional evidence for encapsulation of organic molecules isprovided from the TEM image shown in the inset of Fig. 2a.We found a sharp contrast inside individual SWNTs, as indicated bythe arrows. A possible interpretation is that the dark area is filledwith molecules, whereas the clear area is empty. Additional objectson the SWNT bundle might be ascribed to the adsorbed molecules.The encapsulation of molecules has been imaged directly by high-resolution TEM in C60 (ref. 24) and ortho-carborane molecules25.

Doping properties have been investigated by means of Raman andoptical absorption spectroscopy on film samples. For a standardmaterial of doping, we synthesized potassium(K)-doped SWNT andmeasured Raman and optical absorption spectra, to clarify theevolution of the spectra on doping. Intercalation of potassium intoSWNTs was carried out by heating a sealed ampoule at 180 οC in afurnace. Raman measurements were performed with the excitationwavelength of 632.8 nm. The pristine film samples show well-knowncharacteristic Raman peaks for SWNT; the radial breathing mode(RBM) at ~163 cm–1, and G-band located at ~1,588 cm–1. Both bandschanged drastically with carrier doping as reported previously26,27.First, RBM quickly disappears on doping with potassium. The twobroad components of the G-band (~1,540 and ~1,560 cm–1) also losttheir intensity and the sharp peak of the G band was upshifted to

Inte

nsity

(a.u

.)

)

*

0.4 0.6 0.8 1.0 1.2 1.4

Q (Å–1)

Pristine SWNT

TCNQ-doped SWNT

a

C150/fullerene

C140/TCNQ

9.3 Å

TCNQ

C60b

7.1 Å

Figure 2 XRD pattern,TEM image and a structure model of organic/SWNT materials. a,Observed (dotted lines) and simulated (solid lines) XRD patterns of pristine and TCNQ-reacted SWNT.Q is the momentum transfer. The asterisk indicates a peak that is possibly due to a slight amount of C60-peapod in the sample as an impurity.a.u.= arbitrary units.The inset shows a TEM image of tetracene/SWNTs.The arrows indicate the boundary between an empty region and those filled with tetracene molecules.b,Schematic stackingpatterns of C60 and TCNQ molecules inside (10,10) SWNTs.




ARTICLES


1,600 cm–1 (Fig. 3a).Elemental analysis by XPS gave a C/K ratio of KC27

for this film.Figure 3b displays optical absorption spectra for pristine SWNT

and K-doped SWNT (KC27). Here, both spectra are shown after anidentical background was subtracted from the raw data.The absorptionspectrum of pristine SWNT film is essentially identical to that alreadyreported and analysed28.The peaks at 0.68 and 1.2 eV were attributed toelectronic transitions between pairs of Van Hove singularities insemiconducting SWNTs, from the valence bands to conduction bands,vs

1→cs1,and vs

2→cs2,respectively13.The feature at 1.8 eV was assigned as

the transition between the first pair of singularity vm1→cm

1 in metallicSWNTs. In addition,a Drude-like absorption that is attributable to freecarriers in metallic tubes was observed in the far-infrared region.The spectrum for KC27 showed a drastic suppression of interbandtransitions associated with an enhancement of the new Drudeabsorption in the mid-infrared region expressed by the dashed line inFig. 3b. These spectral changes are attributed to charge-carrier dopingin semiconducting SWNTs.

Figure 4 displays Raman spectra of pristine films and those reactedwith organic molecules. The intensity of RBM in organic/SWNTsamples was strongly dependent on the type of the organic molecules.Inanthracene- or tetracene-doped SWNT (tetracene data not shown), theRBM did not show any notable change, whereas a significant reduction

Figure 3 Optical spectra of pristine SWNTs and K-doped SWNT (KC27). a,Ramanspectra.b,Optical absorption spectra.The dashed line is a Drude absorption componentin K-doped semiconducting SWNTs,determined by a spectral fit.

Inte

nsity

(a.u

.)In

tens

ity (a

.u.)

Inte

nsity

(a.u

.)

Pristine

Anthracene doped

TCNQ doped

TDAE doped

150 200 250 300

Raman shift (cm–1)

Pristine

Anthracene doped

TCNQ doped

TDAE doped

1,4001,200 1,8001,600


Pristine

TCNQ doped

TCNQ

1,200 1,3001,100 1,5001,400


a

b

*

*

c

Figure 4 Raman spectra of pristine SWNT,and anthracene-,TCNQ- and TDAE-doped SWNTs. a,Radial-breathing mode of pristine and doped SWNTs.The excitationwavelength is 632.8 nm.b,G-band of pristine and doped SWNTs.c,Expanded Ramanspectra of pristine SWNT,TCNQ-doped SWNT and TCNQ powder.The asterisk in the spectraof TCNQ-doped SWNT is assigned as the C=C stretching mode of TCNQ1– anions,and thatin TCNQ powder is the same mode of the neutral TCNQ molecule.

a

b

Inte

nsity

(a.u

.)Ab

sorb

ance

(a.u

.)

1,200 1,400

1 1.5 2 2.5 30 0.5

1,600 1,800

Photon energy (eV)


Pristine SWNT

Pristine SWNT

K-doped SWNT (KC27)

K-doped SWNT (KC27)




ARTICLES


of RBM intensity was observed in TCNQ- and TDAE-doped SWNT.The latter intensity reduction resembles that found in K-doped SWNT,being suggestive of the occurrence of charge transfer.Evidence forcharge transfer is also found in the higher wavenumber region.The two broad components in the G-band located at 1,540 cm–1 and1,560 cm–1 exhibit a slight intensity reduction, which is qualitativelysimilar to that in K-doped SWNTs. More importantly, TCNQ/SWNTmaterials showed a tiny peak at 1,390 cm–1 (Fig. 4c).This is attributableto the C=C stretching mode in TCNQ molecules,which is very sensitiveto their valance state. According to the empirical relationship betweenthe peak position and the formal charge29, TCNQ molecules doped inSWNTs are in the TCNQ1– anion state,providing firm evidence for holedoping of SWNTs from TCNQ molecules.

Comparative resistivity measurements on pristine SWNT andTCNQ/SWNT films have been carried out with a four-probe method.To avoid the effect of oxygen, particularly on pristine SWNTs,experiments were made in helium atmosphere and samples weredegassed at 400 K with the aid of vacuum pumping. Resistance ofTCNQ-doped SWNTs was smaller than that of the pristine SWNTsample by approximately a factor of two at room temperature.Similar reduction in resistance was also observed in TDAE-, TTF-,TMTSF- and F4TCNQ doped samples, and, interestingly, such aresistivity decrease was not affected in air except for TDAE/SWNTs,being indicative of stability of the doped states in air.

The insets of Fig. 5a,b display optical absorption spectra for TDAE-and TCNQ-doped SWNT films.The intensity of the vs

1→cs1 absorption

band at 0.68eV decreases on doping.This spectral change,similar to thatin K-doped SWNTs,was also recorded in SWNTs doped with F4TCNQ,TTF and TMTSF, whereas no appreciable change was detected inSWNTs doped with C60, 3,5-dinitrobenzonitrile, anthracene, tetraceneand pentacene. According to the spectrum in K-doped SWNTs, thereduction of peak intensity at 0.68eV provides direct evidence of carrierinjection from organic molecules. The normalized absorption peak ofsemiconducting SWNTs is plotted against the ionization energy and the electron affinity of molecules in Fig. 5, clearly showing that theamphoteric carrier doping is achieved with organic molecules.The results for tetracene are not shown in Fig. 5. To confirm the changeof carrier sign, we made preliminary FET devices of sprayed films ofSWNT random networks. The devices with pristine SWNTs exhibitedp-type operation, whereas the TTF- and TMTSF-doped SWNTs

showed an n-type operation.On the other hand,TCNQ-doped devicesshowed p-type action. Application of the present system to FETs is ofsignificant interest, and improvement of the device characteristics iscurrently in progress.

The reduction of the interband absorption is strongly dependenton the ionization energy and the electron affinity of molecules, andthere is a critical threshold for the charge transfer. Because excessorganic molecules are supplied in the glass tube, the observed dopinglevel is close to the fully doped state. This indicates that the results inFig. 5 should be correlated with the intrinsic properties of organicmolecules and SWNTs: the charge transfer between SWNTs andorganic molecules is controlled by the relative ionization energy and electron affinities of these substances. It is noted that the chargetransfer seems to occur discontinuously at a critical value.For example,although the electron affinities of C60 and TCNQ are very close to each other, charge transfer occurs only in TCNQ/SWNT.This discontinuous behaviour is suggestive of a rather small interactionbetween organic molecules and SWNTs.Otherwise,the onset of chargetransfer should be rather continuous.

The advantage of doping with organic molecules is the finetunability of carrier density at the low doping level.Control of the carrierdensity in the low-density level has been rather difficult in the case ofalkali metals and halogen, which quickly produce highly doped states.The present result,on the other hand, indicates that tuning of the Fermilevel is possible near the peak of the Van Hove singularity in the densityof states. Figure 5c shows absorption spectra of TCNQ/SWNTmaterials, obtained by a dedoping process by heating in vacuum,unambiguously showing that quasi-continuous doping is achieved bychanging the dedoping temperature and time.

Carrier density in organic/TCNQ material was derived from thereduction of the optical absorption intensity shown in the inset ofFig. 5b, using the result of K-doping. First, we made a fit of theabsorption spectra of the pristine and K-doped SWNT (KC27) in Fig. 3to a Drude–Lorentz oscillator model,and quantitatively determined thereduction of the absorption intensity at 0.68 eV (SupplementaryInformation, Fig. S1). This analysis revealed that the 66% reduction ofthe absorption intensity corresponds to the carrier density of 0.037electrons per carbon atom in SWNTs. This relation between intensityreduction and carrier density in KC27 was used as a standard for theestimation of carrier density in organic/SWNT compounds. Using this

Electron affinity (eV)

Abso

rban

ce (a

.u.)

0.4 0.8 1.2

pristineSWNT

pristineSWNT

Photon energy (eV)

Abso

rban

ce (a

.u.)

0.4 0.8 1.2Photon energy (eV)

TDAEdoped

TCNQdoped

TCNQ-doped SWNT

Pristine SWNT

1

0.9

0.8

0.7

0.6

0.55 5.5 6 6.5 7 7.5 8

Ionization energy (eV)

I (dop

ed)/I

(pris

tine)

1

0.9

0.8

0.7

0.6

0.50.5 1 1.5 2 2.5 3 3.5 4 0.5 1 1.5 2

Photon energy (eV)

I (dop

ed)/I

(pris

tine)

0.8

0.4

0.6

0.2

0

Abso

rban

ce (a

.u.)

cba

Figure 5 Relationship between the absorption intensity and ionization energy or electron affinity of molecules in Table 1. a, Intensity ratio versus ionization energy (note that the data for TCNQ was out of range.). b, Intensity ratio versus electron affinity.The insets display absorption spectra of pristine SWNT (dashed lines),TDAE-doped SWNT and TCNQ-doped SWNT (solid lines). c,Absorption spectra of TCNQ-doped SWNT materials, obtained by a dedoping process in vacuum at different dedoping temperatures.From bottom to top the curves are: fully TCNQ-doped SWNT, dedoped at 200 °C, dedoped at 250 °C, dedoped at 300 °C, and pristine SWNT.




ARTICLES


relation, we were able to derive carrier density in semiconductingSWNTs doped with organic molecules. The obtained carrierconcentration at the saturated phase was 0.021 holes per carbon atomand 0.009 electrons per carbon atom in TCNQ and TTF, respectively.The former value is larger by a factor of two to three than the TCNQconcentration determined from the XRD analysis and XPS.This difference is attributed to the contribution from TCNQ depositedon the film surface.The carrier concentration of 0.021 holes per carbonatom in TCNQ/SWNT corresponds to the Fermi level of –0.12 eV basedon a tight-binding band calculation on semiconducting SWNTs withd ≈ 13.5 Å, and this value is changeable as demonstrated by theabsorption spectra in Fig. 5c.

Other crucial points for applicable p- and n-type SWNTs are airstability and large-scale production. As oxygen modifies SWNT to a p-type semiconductor, stable n-type nanotubes are highly desirable formolecular electronics.We have investigated air stability oforganic/SWNTcompounds through measurements of resistance, Raman and opticalabsorption spectra before and after expose to air.Spectra were unchangedfor one week under ambient conditions (Supplementary Information,Fig. S2), except for TDAE/SWNT compounds. The dedoping was notobserved even in n-type compounds at room temperature, providingevidence of air stability of organic/SWNT materials. Encapsulation oforganic molecules in SWNTs is one of the possible reasons for the stabilityas the outer wall of SWNTs will prevent molecules from attack by oxygen.Furthermore, due to the simple synthesis method of these materials, theorganic/SWNT compounds are suitable for mass production.

In summary, we have synthesized SWNTs having encapsulatedorganic molecules, in which amphoteric carriers are injected intoSWNTs. The carrier doping is controlled by the ionization energy andelectronic affinity of the reacted organic molecules. These organic/SWNT materials could be the first nanotube semiconductor thatsatisfies the three requirements for practical application: air stability,controllable doping and simple synthesis and promise to push theperformance limit of SWNT-based devices for molecular electronics.

Received 9 June 2003; accepted 4 August 2003; published 7 September 2003.

References1. Dresselhaus, M. S., Dresselhaus, G. & Avouris, P. (eds.) Carbon Nanotubes (Spring, Berlin, 2001).

2. Franklin, N. R. et al. Integration of suspended carbon nanotube arrays into electronic devices and

electromechanical systems. Appl. Phys. Lett. 81, 913–915 (2002).

3. Fuhrer, M. S., Kim, B. M., Dürkop, T. & Brintlinger, T. High-mobility nanotube transistor memory.

Nano Lett. 2, 755–759 (2002).

4. Liang, W. et al. Fabry-Perot interference in a nanotube electron waveguide. Nature 411, 665–669

(2001).

5. Kong, J. et al. Quantum interference and ballistic transmission in nanotube electron wave-guides.

Phys. Rev. Lett. 87, 106801 (2001).

6. Javey, A. et al. High-κ dielectrics for advanced carbonnanotube transistors and logic gates. Nature

Mater. 1, 241–246 (2002).

7. Heinze, S. et al. Carbon nanotubes as schottky barrier transistors. Phys. Rev. Lett. 89, 106801 (2002).

8. Derycke, V., Martel, R., Appenzeller, J. & Avouris, P. Controlling doping and carrier injection in carbon

nanotube transistors. Appl. Phys. Lett. 80, 2773–2774 (2002).

9. Kong, J. et al. Nanotube molecular wires as chemical sensors. Science 287, 622–625 (2000).

10. Collins, P., Bradley, K., Ishigami, M. & Zettl, A. Extreme oxygen sensitivity of electronic properties of

carbon nanotubes. Science 287, 1801–1804 (2000).

11. Martel. R. et al. Ambipolar electrical transport in semiconducting single-wall carbon nanotubes. Phys.

Rev. Lett. 87, 106801 (2001).

12. Zhou, C., Kong, J., Yenilmez, E. & Dai, H. Modulated chemical doping of individual carbon

nanotubes. Science 290, 1552–1555 (2000).

13. Kazaoui, S., Minami, N., Jacquemin, R., Kataura, H. & Achiba, Y. Amphoteric doping of single-wall

carbon-nanotube thin films as probed by optical absorption spectroscopy. Phys. Rev. B 60,

13339–13342 (1999).

14. Kazaoui, S., Minami, N., Matsuda, N., Kataura, H. & Achiba, Y. Electrochemical tuning of electronic

states in single-wall carbon nanotubes studied by in situ absorption spectroscopy and ac resistance.

Appl. Phys. Lett. 78, 3433–3435 (2001).

15. Jouguelet, E., Mathis, C. & Petit, P. Controlling the electronic properties of single-wall carbon

nanotubes by chemical doping. Chem. Phys. Lett. 318, 561–564 (2000).

16. Kong, J. & Dai, H. Full and modulated chemical gating of individual carbon nanotubes by organic

amine. J. Phys. Chem. B 105, 2890–2893 (2001).

17. Shim, M., Javey, A., Kam, N. W. S. & Dai, H. Polymer functionalization for air-stable n-type carbon

nanotube field-effect transistors. J. Am. Chem. Soc. 123, 11512–11513 (2001).

18. Lee, J. et al. Bandgap modulation of carbon nanotubes by encapsulated metallofullerenes. Nature 415,

1005–1008 (2002).

19. Hornbaker, D. J. et al. Mapping the one-dimensional peapod structures. Science 295, 828–831 (2002).

20. Kataura, H. et al. Optical properties of fullerene and non-fullerene peapods. Appl. Phys. A 74, 1–6

(2002).

21. Shiraishi, M., Takenobu, T., Yamada, A., Ata, M. & Kataura, H. Hydrogen storage in single-walled

carbon nanotube bundles and peapods. Chem. Phys. Lett. 358, 213–218 (2002).

22. Maniwa, Y. et al. Anomaly of x-ray diffraction profile in single-walled carbon nanotubes. Jpn J. Appl.

Phys. 38, L668–L670 (1999).

23. Seki, N. Ionization energies of free molecules and molecular solids. Mol. Cryst. Liq. Cryst. 171,

255–270 (1989).

24. Smith, B. W., Monthioux, M. & Luzzi, D. E. Encapsulated C60 in carbon nanotubes. Nature 396,

323–323 (1998).

25. Morgan, D. A., Sloan, J. & Green, M. L. H. Direct imaging of o-carborane molecules with single walled

carbon nanotubes. Chem. Comm. 20, 2442–2443 (2002).

26. Rao, A. M., Eklund, P. C., Bandow, S., Thess, A. & Smalley, R. E. Evidence for charge transfer in doped

carbon nanotube bundles from Raman scattering. Nature 388, 257–259 (1997).

27. Iwasa, Y. et al. Intercalation processes of single-walled carbon nanotube ropes. New Diam. Front. C.

Tec. 12, 325–330 (2002).

28. Kataura, H. et al. Optical properties of single-wall carbon nanotubes. Synth. Met. 103, 2555–2558

(1999).

29. Matsuzaki, S., Kawata, R. & Toyoda, K. Raman spectra of conducting TCNQ salt; estimation of the

degree of charge transfer from vibrational frequencies. Solid State Commun. 33, 403–405 (1980).

AcknowledgementsThe authors are grateful to R. Maruyama for experimental help, and to T. Hasegawa for his provision of

purified organic molecules. This work has been partly supported by a grant from the MEXT, Japan

(13440110 and 14750019). The synchrotron radiation experiments were performed at SPring-8, Japan,

with the approval of JASRI as Nanotechnology Support Project of The MEXT. (Proposal No. 2002B0210-

ND1-np/BL-No.02B2 and 2003A0323-ND1-np/BL-No.02B2)

Correspondence and requests for materials should be addressed to T.T and Y.I.

Supplementary Information accompanies the paper on www.nature.com/naturematerials

Competing financial interestsThe authors declare that they have no competing financial interests.

nmat976PRINT 10/9/03 1:48 pm 88



Stable and controlled amphoteric doping by encapsulation of organic molecules inside carbon...

Documents

Transcript of Stable and controlled amphoteric doping by encapsulation of organic molecules inside carbon...