NANO LETTERS
Light Modulation of Electronic Transitions in Semiconducting Single Wall Carbon Nanotubes
2004 Vol. 4, No. 8 1529-1533
Rafail F. Khairutdinov,*,† Mikhail E. Itkis,‡ and Robert C. Haddon*,‡ Department of Chemistry & Biochemistry and Center for Nanosensor Technology, UniVersity of Alaska Fairbanks, Fairbanks, Alaska 99775-6160, and Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403 Received March 24, 2004; Revised Manuscript Received April 24, 2004
ABSTRACT UV illumination of semiconducting single wall carbon nanotubes (SWNTs) results in a substantial increase of the intensity of the first interband transition (S11) followed by dark or IR light induced restoration of the initial intensity. The intensity of the S11 transition could also be modulated by light-induced reversible conversion of spiropyran molecules attached to SWNTs from the spiro conformation to its merocyanine form. Both surface-located monomeric and end-located aggregated merocyanines were found in UV illuminated spiropyran-SWNT supramolecular conjugates.
Single wall carbon nanotubes (SWNTs) are widely accepted as a promising material for a new generation of nanoelectronic devices due to their unique shape and electronic properties.1,2 One of the major opportunities for research on SWNTs is in the area of construction of nanometer size sensors.3-7 These devices employ the modification of the electronic properties of SWNTs by chemical doping, highpressure reforming, or adsorption of atoms or groups of atoms.8-15 In the present paper we demonstrate that the electronic characteristics of SWNTs can also be tailored by light. We show that the intensity of interband transitions in chemically functionalized semiconducting SWNTs can be reversibly modified by the light-induced refilling/depletion of valence band states or via photoinduced changes in the polarity of spiropyran dye molecules attached to the SWNT. We emphasize that light provides a convenient and simple tool for controlling the conductivity of semiconducting SWNTs. Preparation and Characterization of Covalently Bonded Spiropyran-SWNTs. The preparation of covalently bonded spiropyran-SWNT (SP-SWNT) is depicted in Figure 1a. In experiments, we have used electric arc produced SWNTs provided by Carbon Solutions, Inc.16 After purification and carboxylation of the SWNT by nitric acid,17,18 the purity of the SWNTs was evaluated to be 90%.19 This treatment is known to produce open-ended SWNTs.20 In the next step (step 1 in Figure 1a), the carboxylated SWNTs were converted to the acid chloride (SWNT-COCl) * Corresponding authors. † University of Alaska Fairbanks. ‡ University of California Riverside. 10.1021/nl049538j CCC: $27.50 Published on Web 07/23/2004
© 2004 American Chemical Society
by using thionyl chloride.20,21 1,3,3-1′-(1-Hydroxyethyl)-3′3′dimethyl-6-nitro-spiro[2H-1-benzopyran-2,2′-indoline] (SPC2H4OH) was synthesized by the literature procedure.22 Attachment of SP-C2H4OH to SWNTs (step 2 in Figure 1a) was achived via linkage to the carbonyl group by heating for 72 h at 110 °C under the argon SWNT-COCl (87 mg) and SP-C2H4OH (260 mg) in 1.5 mL pyridine and 1 mL dimethylformamide. SP-SWNTs were repeatedly washed with cold water, filtered through a membrane (pore size 0.2 µm), and dried in vacuum. The black material was then repeatedly washed with ethanol, CH2Cl2, and DMF, with five min sonication to remove unreacted spiropyran. The IR spectrum of the SP-SWNT measured in the solid state is presented in Figure 1b. It shows a new band at 1732 cm-1 characteristic of the CdO stretch of the ester in functionalized SWNT.23 It also shows rising absorbance increasing with decreasing frequency due to doping introduced in the SWNT by dye attachment.24 SP-SWNTs are slightly soluble in DMF, THF, ethanol, and CHCl3. An AFM image of SP-SWNTs reveals both individual nanotubes and bundles of 2-5 nanotubes 0.4-2 µm in length. SP-SWNTs were dispersed in ethanol or DMF by sonication. Thin films of SP-SWNT were prepared by spraying the dispersions on heated (200 °C DMF or 100 °C ethanol) microscope glass slides.24 Film thickness (0.2-0.5 µm) were limited to the range of optical densities < 1 to avoid significant light scattering. Physical Measurements. Continuous photolysis experiments were performed using a 4 W compact handheld UV lamp UVGL-15 with 365 nm light output or 30 W quartz halogen illuminator with 4 cm water and OG590 glass filters
Figure 1. (a) Synthesis of spiropyran functionalized SWNT. (b) IR spectra of SWNT-COCl, SP-C2H4OH, and SP-SWNT. Spectra of SP and SP-SWNT are shifted vertically with respect to the spectrum of SWNT-COCl. (c) Absorption spectra of SP-C2H4OH solution in DMF after 30 s UV illumination. Vertical column shows time in minutes after UV illumination. Scheme 1.
Ring Opening-closing Isomerization Reactions of Spiropyran
(light transmission at 900 nm > λ > 560 nm). Light intensities were measured using a calibrated PowerMax 500D Laser Power Meter, the effective values obtained were ∼5 × 10-8 einstein/cm2-s for UV illumination and ∼2 × 10-8 einstein/cm2-s for visible light illumination. Optical spectra were recorded using a Nicolet Magna-IR 560 E.S.P. IR spectrometer and Varian Cary 500 double beam scanning UV-vis/NIR spectrophotometer. Film deoxygenation was achieved by blowing Ar for 20 min through the cuvette containing a film. AFM images were taken using Digital Instruments Nanoscope. Photoisomerization of SP-SWNT in Solution. The spiro carbon atom in SP-C2H4OH and structurally related molecules effectively block transannular π-conjugation between the indoline and pyran rings.25,26 Consequently, allowed electronic transitions are relatively high in energy (see Figure 1c). Ultraviolet illumination leads to heterolytic cleavage at the spiro carbon, forming a considerably more polar molecule (MC-C2H4OH) whose π-bonding network extends over the entire molecular framework (Scheme 1).25,26 One manifestation of this increased electronic delocalization is that allowed transitions now appear in the visible spectral region. The ring-opening reaction can be reversed by illumination into the visible band and also occurs by thermal reactions at rates that are dependent upon the spiropyran ring substituents and medium composition.25-28 The nature of the optical changes that occur is illustrated for SP-C2H4OH in DMF in Figure 1c, where the spectra obtained before and after UV illumination and after thermal fading are compared. Slow thermal ring-closing followed first-order decay with an apparent rate constant of k ) 1.9 × 10-3 s-1 at 21 °C. Similar behavior was observed in other organic solvents; the position of the visible band was solvent-sensitive, shifting to higher energies as the polarity increased. This behavior is typical of spiropyrans and has been explained as arising from a reduced 1530
polarity of the electronic excited state in the merocyanine form of the dye.28-30 A bathochromic shift of merocyanine absorption spectra has also been observed for dyes bound on metal and semiconductor surfaces.31,32 Illumination of SP-SWNT solutions results in a similar transformation of the absorption spectra. As with solutions of SP in DMF, UV illumination (365 nm) of a SP-SWNT solution in DMF results in an increase of the absorbance in the visible region characteristic of the merocyanine form of the dye. Merocyanine absorbance fades in the dark with a characteristic half-time of 5.3 min, which is ∼20% faster than that for MC-C2H4OH solution in DMF. The rate of fading increases under visible light illumination. However, the difference absorption spectra of UV illuminated SPC2H4OH and SP-SWNT reveals a significant difference. In the difference absorption spectra of UV illuminated SPSWNT we observed a new band with λmax ≈ 440 nm (see Figure 2a) that faded about two times slower than the characteristic merocyanine absorption bands. An absorption spectrum of MC-SWNT was extracted from spectrum 1 by illuminating the UV preilluminated solution with light of λ > 520 nm, and subtracting spectrum 2 from spectrum 1 (see Figure 2a). The resulting spectrum is shown in Figure 2b (spectrum B). For a comparison, Figure 2b also shows a difference absorption spectrum of SP- C2H4OH solution in DMF. It is seen from Figure 2b that the absorption bands of MC-SWNT are red shifted relative to MC-C2H4OH absorption bands. The shift is about ∼15 meV and ∼80 meV for the long and short wavelength bands of the merocyanine, correspondingly. These shifts in MCSWNT absorption band positions are similar to those reported for merocyanines bound at the surface of bulk materials31,32 and indicate a strong interaction between the merocyanine π-system and the SWNT. Spectrum 2 in Figure 2a is a superposition of the absorption spectra of MC-SWNT and of an unknown species X. The absorption spectrum of X was extracted from the spectrum 2 in Figure 2a by subtracting the MC-SWNT spectrum (spectrum B, Figure 2b) normalized at 566 nm. The resulting spectrum of X is shown in Figure 2c. Formation of a new blue-shifted absorption band in concentrated solution of merocyanines is a well-known phenomenon and occurs because of the H-aggregation of Nano Lett., Vol. 4, No. 8, 2004
Figure 2. (a) Difference absorption spectra of SP-SWNT solution in DMF immediately after 30 s UV illumination (1) and after additional 30 s illumination by light with λ > 520 nm (2). (Inset) Spectra of SP-SWNT in DMF before (lower curve) and after (upper curve) 30 s UV illumination. (b) Difference absorption spectra of UV-illuminated SP solution in DMF (A) and the result of the subtraction of spectrum 2 from spectrum 1 in Figure 2a (B). (c) Difference absorption spectrum of a species X in UV-illuminated SP-SWNT solution in DMF. (Inset) Temporal changes of absorbances of UV-illuminated solution at 570 and 440 nm (solid and open circles, respectively). Solid lines show exponential fits with k ) 1.8 × 10-3 s-1 and 1.0 × 10-3.
Figure 3. (a) IR absorption spectra of purified SWNT film (middle curve), after IR illumination (lower curve), and after UV illumination (upper curve). (b) Absorbance changes at 5400 cm-1 of purified SWNT film following UV (365 nm) illumination, keeping film in the dark, and under IR illumination. Solid lines correspond to the exponential rise and decays with characteristic times of 7 min, 22 min, and 120 min, correspondingly. (c) Absorbance changes at 5400 cm-1 of SP-SWNT film following UV (365 nm), visible (560-900 nm) illumination or keeping film in the dark. The film was preilluminated by UV light during 60 min.
dyes.33,34 Based on the photochromic behavior and on the position of the absorption band, we propose that X is an aggregated form of merocyanine. It is generally assumed that any chemical bonding reactions occur at the open ends of SWNTs where carboxylic groups can be readily derivatized. Recent experiments with coupling chemically functionalized nanotubes with molecular linkers35 and estimations of the amount of functionalized carbon atoms with carbon atoms in the SWNT backbone23 indicate that sidewall functionalization of SWNT can also occur and could play an important role in chemical transformation of SWNTs. Spectral changes in a UV-illuminated solution of SP-SWNT indicate that two types of merocyanines, MC1 and MC2, exist on the surface of SWNTs. MC1 appears at the sidewall of SWNT, where dye molecules are spatially separated from each other there are predominantly monomeric MCs. These merocyanines are responsible for the absorption bands at 398 and 568 nm. MC2 are merocyanines that appear at the ends of SWNTs. Close proximity of merocyanines attached at the open ends of SWNT results in strong mutual interactions and the appearance of the 440 nm band, which is characteristic of aggregated MC. As it is seen from Figure 2c, the fading time of monomeric MC is almost two times smaller than that of aggregated MC. Photoinduced Modulation of Electronic Transitions in SWNT Films. Infrared Bleaching of Interband Transition Intensities. The near-IR absorption spectrum of purified SWNT film is presented in Figure 3a. It shows characteristic Nano Lett., Vol. 4, No. 8, 2004
bands attributed to symmetric transitions between the lowest subbands in semiconducting carbon nanotubes. The ratio of the intensities of the first interband transition (S11) and of the second interband transition (S22) of the film is lower than that of the as-prepared SWNTs. A relative decrease in intensity of the lower energy interband transition is wellknown for chemically modified SWNTs and is due to hole doping introduced by chemical modification.13,19,30,31 We found that interband light illumination of purified SWNT films and films made of acid chloride forms of SWNTs and of SP-SWNTs resulted in ∼10% decrease of S11 band intensity and less than 1% decrease of the intensity of S22 band with a subsequent slow restoration of the band intensities in the dark. For pristine SWNTs, we observed less than 1% decrease of the intensity of S11 band and practically no changes in S22 bands under these conditions. IR illumination of films in an oxygen free atmosphere resulted in a lesser decline of the bands intensities. A schematic picture of light induced and dark processes in semiconductors is shown in Figure 4a. Interband illumination of semiconductors (hνIR) is known to create electrons in the conduction band and leaves behind holes in the valence band.38,39 After fast thermal equilibration with the lattice, they can recombine, either radiatively or nonradiatively. Usually, this charge recombination occurs in a nanosecond or shorter time scale.40 Alternatively, electrons and holes could be captured in deep defect states (DS) in the lattice or on the surface of the semiconductor (dashed line 1), resulting in 1531
Figure 4. Schematic picture of the infrared (a) and ultraviolet (b) light induced transitions (solid arrows) followed by dark processes (dashed arrows 1 and 2) in semiconducting SWNTs. CB and VB are the conduction band and the valence band of SWNTs, respectively; DS are energy levels of defect sites.
long-lived, spatially separated charge carriers.38,39 Eventually, they recombine (dashed line 2), to produce the final electron and hole populations that exist in equilibrium in the dark before photoexcitation. We believe that analogous processes are responsible for IR photobleaching and dark restoration of SWNT bands. Nitric acid used in the purification of SWNTs can partially exfoliate and intercalate carbon nanotube bundles,41 and intercalation of nitric acid into graphite results in hole doping of the graphene sheet.42 It is therefore likely that hole-doping products of nitric acid treatment are responsible for partial IR bleaching of S11 and S22 bands in chemically modified SWNTs. The observed O2 effect on the IR-induced changes of the S11 and S22 band intensities indicates some degree of O2 involvement in this process. UV Enhancement of Interband Transition Intensities. UV illumination of purified SWNT, SWNT-COCl, and SPSWNT films resulted in up to 20% increase in the intensity of the first interband transitions but had little effect on the intensity of the second interband transitions (see Figure 3a). The relative magnitude of the change increased with the extent of hole doping of SWNTs. No UV illumination effect on the intensities of S11 and S22 bands were observed for films prepared from pristine SWNT. For chemically functionalized SWNT films, an effect similar to a UV illumination effect on the intensities of the S11 transition was observed after exposing the films to a vapor of ethylenediamine, a well known electron donor. In contrast, exposing of the films to a bromine vapor, strong oxidizing agent, depressed the intensities of both S11 and S22 transitions. It is likely therefore that UV light illumination results in partial refilling of the valence band by electron transfer from the excited reduced defect sites to the valence band (see dashed line 1 in Figure 4b). In the dark, the intensity of the S11 band declines presumably because of the defect states (DS) refilling. This process is shown in Figure 4b by dashed line 2. Illumination of the film by the infrared light increases the rate of the DS refilling. The rate of the S11 band decay decreases with a decrease of films hole doping. As an example, Figure 3b shows kinetics of the UV-induced rise and dark- or infrared light-induced decay of purified SWNT film absorption at 5400 cm-1. Infrared illumination of films was provided by continuously taking IR spectra in the 4000-12 000 cm-1 range at the maximal incident light intensity of the spectrophotometer. Prolonged UV illumination of films resulted in a decline of the S11 band dark decay rate. Apparently, UV 1532
illumination removes some hole doping species from the surface of SWNTs.24 By measuring the amount of UV light absorbed by the film and taking the absorptivity of the film at 5400 cm-1 ) (1.1 ( 0.8) (mg/mL)-1 cm-1,43 one can obtain from Figure 3a that the quantum efficiency of the S11 band recovery is 0.6 g/einstein. Since carbon weight is 12 g/mol, assuming that one electron from each carbon atom participates in light absorption, one finds that the quantum yield of SWNT conduction band refilling is 0.05. In the SP-SWNT film, UV light is absorbed by metallic and semiconducting SWNTs, as well as by carbon nanoparticles and amorphous carbon. Therefore, the above value 0.05 is the lower limit of the quantum yield. Modulation of SWNT Interband Intensities by Light Induced Ring Opening of CoValently Bonded Dyes. Unlike films made of carboxylated or acid chloride forms of SWNTs, SP-SWNT films revealed reversible changes of S11 band intensity when short UV illumination of UV preilluminated films was followed by visible light illumination in the SP absorption band. UV preillumination of the films was undertaken during ∼60 min in order to minimize the effect of refilling of the valence band by electron transfer from the reduced defect sites. As an example, Figure 3c shows these changes for SP-SWNT film. These reversible changes in the IR absorption spectrum of SP-SWNT film resemble those observed in spiropyran absorption spectra and presumably are due to modulation of SWNT interband transition by ring opening-ring closing of the spiropyran molecules. Strong interaction between the merocyanine and SWNT revealed in the shift of the merocyanine visible absorption bands supports this hypothesis. Alternatively, observed changes of S11 band intensity could be explained by UV induced electron transfer from reduced SP molecules to SWNTs followed by visible light induced re-reduction of SP. However, this explanation seems unreasonable because the reduction potential of spiropyran and its MC form, E° ) -0.98 V (vs NHE),44 is well above the bottom of SWNT valence band (0.3 V vs NHE45), thus making implausible the existence of a reduced form of the dye molecule in SP-SWNTs. In conclusion, results presented in this investigation demonstrate that the electronic characteristics of SWNT could be modified by light. Tailoring of electronic properties of carbon nanotubes by light provides a convenient and simple tool for controlling the conductivity of semiconducting SWNTs. The sensitivity of the SWNT interband transition intensities on the conformation of attached spiropyrans presents an impetus for an exploration of a new type of chemical sensors based on the interaction of an analyte with a host molecule. In these sensors, analyte-induced changes in host molecules attached to carbon nanotubes alter electronic properties of the nanotubes in the same fashion that light-induced changes in spiropyrans modify the intensity of the S11 transition. The sensitivity of the merocyanines absorption maxima to the electronic interactions with SWNTs opens prospects of using merocyanines as a probe for electronic characterNano Lett., Vol. 4, No. 8, 2004
istics of SWNTs. We envision new opportunities for studying the character of SWNT-substrate interaction by exploring changes in spectral and dynamic characteristics of merocyanines caused by carbon nanotubes. The nature of this interaction is important for further development of SWNTbased chemical sensors. Acknowledgment. This research was supported by the Center for Nanosensor Technology UAF under the contract DMEA90-02-C-0266 and grant BES-0322455 from the National Science Foundation (to R.F.K.) and by DOD/ DARPA/DMEA under Award No. DMEA90-02-2-0216 (to R.C.H.). References (1) McEuen, P. L. Nature 1998, 393, 15-16. (2) Dekker, C. Phys. Today 1999, 52, 22-28. (3) Martel, R.; Derycke, V.; Lavoie, C.; Appenzeller, J.; Chan, K. K.; Tersoff, J.; Avouris, Ph. Phys. ReV. Lett. 2001, 87, 256805/1-256805/ 4. (4) Derycke, R. M.; Appenzeller, J.; Avouris, Ph. Appl. Phys. Lett. 2002, 80, 2773-2775. (5) Park, J.; McEuen, P. L. Appl. Phys. Lett. 2001, 79, 1363-1365. (6) Dag, S.; Gulseren, O.; Yildirim, T.; Ciraci, S. Phys. ReV. B 2003, 67, 165424/1-165424/10. (7) Dai, L. Smart Mater. Struct. 2002, 11, 645-651. (8) Kazaoui, S.; Minami, N.; Yamawaki, H.; Aoki, K.; Kataura, H.; Achiba, Y. Phys. ReV. B 2000, 62, 1643-1646. (9) Bockrath, M.; Cobden, D. H.; McEuen, P. L.; Chopra, N. G.; Zettl, A.; Thess, A.; Smalley, R. E. Science 1997, 275, 1922-1925. (10) Zhou, C.; Kong, J.; Yenilmez, E.; Dai, H. Science 2000, 290, 15521555. (11) Kong, J.; Cao, J.; Dai, H.; Anderson, E. Appl. Phys. Lett. 2002, 80, 73-75. (12) Soundarrajan, P.; Patil, A.; Dai, L. J. Vacuum Sci. Technol., A 2003, 21, 1198-1201. (13) Kazaoui, S.; Minami, N.; Kataura, H.; Achiba, Y. Synth. Met. 2001, 121, 1201-1202. (14) Kim, C.; Choi, Y. S.; Lee, S. M.; Park, J. T.; Kim, B.; Lee, Y. H. J. Am. Chem. Soc. 2002, 124, 9906-9911. (15) Kymakis, E.; Amaratunga, G. A. Appl. Phys. Lett. 2002; 80, 112114. (16) http://www.carbonsolution.com. (17) Rinzler, A. G.; Liu, J.; Dai, H.; Nikolaev, P.; Huffman, C. B.; Rodriguez-Macias, F. J.; Boul, P. J.; Lu, A. H.; Heymann, D.; Colbert, D. T.; Lee, R. S.; Fischer, J. E.; Rao, A. M.; Eklund, P. C.; Smalley, R. E. Appl. Phys. A 1998, 67, 29-37. (18) Hu, H.; Zhao, B.; Itkis, M. E.; Haddon, R. C. J. Phys. Chem. B 2003, 107, 13838-13842. (19) Itkis, M. E.; Perea, D. E.; Niyogi, S.; Rickard, S. M.; Hamon, M. A.; Hu, H.; Zhao, B.; Haddon, R. C. Nano Lett. 2003, 3, 309-314.
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