NANO LETTERS
Double-Wall Carbon Nanotubes Doped with Different Br2 Doping Levels: A Resonance Raman Study
2008 Vol. 8, No. 12 4168-4172
Gustavo M. do Nascimento,*,† Taige Hou,‡ Yoong Ahm Kim,§ Hiroyuki Muramatsu,§ Takuya Hayashi,§ Morinobu Endo,§ Noboru Akuzawa,| and Mildred S. Dresselhaus*,†,‡ Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts, Department of Physics, Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts, Faculty of Engineering, Shinshu UniVersity, Wakasato, Nagano-shi, Japan, and Department of Chemical Science and Engineering, Tokyo National College of Technology, Tokyo, Japan Received June 4, 2008; Revised Manuscript Received September 22, 2008
ABSTRACT This report focuses on the effects of different Br2 doping levels on the radial breathing modes of “double-wall carbon nanotube (DWNT) buckypaper”. The resonance Raman profile of the Br2 bands are shown for different DWNT configurations with different Br2 doping levels. Near the maximum intensity of the resonance Raman profile, mainly the Br2 molecules adsorbed on the DWNT surface contribute strongly to the observed ωBr-Br Raman signal.
Carbon nanotubes are materials whose properties arise from their special geometry and electronic structure. Nowadays, there is an increase in interest in double-wall carbon nanotubes (DWNTs) for both the elucidation of their properties and their possible applications for cylindrical molecular capacitors, GHz oscillators, nanocomposites, field emission sources, nanotube bicables, electronic devices, and other applications.1,2 DWNTs have only two concentric tubes and the diameters and chiral angles of the outer tubes are often similar to those of commonly observed SWNTs, so that a direct comparison can be made to determine the effect of the intertube interactions. In addition, the DWNTs are more stable mechanically than SWNTs and are a prototype system for studying the intermolecular interactions between concentric cylindrical carbon tubes. Nowadays it is possible to prepare very high quality DWNT samples containing a very small amount (wt %) of SWNT constituents.2 The doping of carbon nanotubes with electron donors and electron acceptors is now a very active research field, and much effort has been directed toward understanding and controlling the electronic properties of SWNTs, DWNTs, and * Corresponding author. † Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology (MIT). ‡ Department of Physics, Massachusetts Institute of Technology (MIT). § Shinshu University. | Tokyo National College of Technology. 10.1021/nl801605u CCC: $40.75 Published on Web 10/29/2008
2008 American Chemical Society
MWNTs.3-6 By doping nanotubes, it is possible to control and tune their electronic properties. In addition, these doped systems also open up the opportunity for studying the basic physical properties of nanotubes in a controlled way. In this paper, the resonance Raman technique is used to study high purity bundled DWNTs doped to different Br2 levels. The radial breathing mode region is analyzed considering the effect of different Br2 concentrations on the DWNT spectra as well on the Br2 stretching mode itself. Special attention was given to distinguish between the behavior of the S/M and M/S outer/inner semiconducting (S) and metallic (M) tube configurations. In this study, we have used highly pure and crystalline DWNT buckypaper, as previously reported.1 The bromination of the DWNT buckypaper was performed by reacting the DWNTs with Br2 (Aldrich, ACS reagent, used as received) vapor inside a NMR quartz tube, and the quartz tube was capped with Teflon in order to maintain the Br2 saturated atmosphere during the Raman experiments. After 1 h of bromination, no further change was observed in the Raman spectra of the DWNTs (not shown). This regime was considered as the saturation condition. For preparing samples with lower Br2 levels than the saturation condition, two samples previously saturated with Br2 were exposed to the ambient air for 24 h and 1 week, respectively. Afterward, the samples were encapsulated again inside the NMR quartz
tube. The concentration (wt %) of Br2 in the DWNT samples was measured through XPS data, obtained under high resolution conditions. Three doped samples were prepared, and the obtained bromine concentrations were 31, 11, and 2% by weight of Br2. The quantification of Br2 amount (wt %) in doped DWNTs samples was done through the analysis of the Br 3d line at 68.00 eV using the Kratos processing software. All Raman spectra for the DWNT samples were measured at room temperature, and the samples were analyzed inside a sealed NMR quartz tube in order to maintain the Br2 atmosphere. After 48 h of doping with Br2 vapors, the spectra of the DWNT samples, into the sealed quartz tubes, were acquired. The Raman spectra at 514.5 nm (2.41 eV, Ar+ laser) and 785.0 nm (1.58 eV, solid state laser) were taken with the Kaiser Hololab 5000R Raman Spectrometer and Microprobe with a backscattering geometry and using a 50× objective. The Raman spectra at 760 nm (1.63 eV) and 720 nm (1.73 eV) (Ti-sapphire laser pumped by argon laser), 600 nm (2.07 eV), 593.3 nm (2.09 eV), and 576.8 nm (2.15 eV) (rhodamine 6G dye laser), 647.1 nm (1.92 eV) and 676.0 nm (1.84 eV) (Kr+ laser), and 532.0 nm (2.33 eV, solid state laser) were performed in the backscattering geometry in a homemade spectrometer system. The laser line was focused on the sample using a 50× objective. The power incident on the sample was kept lower than 2 mW to avoid heating effects. Different acquisition times between 5 and 30 s were used for each sample in an attempt to optimize the signalto-noise ratio of the Raman spectra. To avoid problems related to inhomogeneities in the sample, all spectra were taken on five different spots for each sample (see the Supporting Information). The XPS measurements were done in a Kratos AXIS Ultra Imaging X-ray Photoelectron Spectrometer by using characteristic KR radiation from an Al anode to excite the samples and a 100 mm mean radius hemispherical analyzer operating with a constant passage of energy at 44 eV. A small quantity of each sample was pressed between two stainless steel plates to form a thin conglomerate fixed to the sample holder with double-faced conducting tape. The analyses were done at a base pressure of 5 × 10-9 mbar, and correction was made for charging effects by shifting the spectra, so that the C 1s line was at 284.6 eV. Figure 1 shows the Raman spectra (just the radial breathing mode region, RBM) of pristine and DWNTs doped with different Br2 levels obtained with laser excitation energies of Elaser ) 1.58, 1.63, and 2.33 eV. According to the Kataura plot,7 it is expected that the spectra excited with Elaser ) 1.58 and 1.63 eV will be in resonance with semiconducting inner (ES22) and metallic outer tubes (EM 11). For Elaser ) 1.58 eV, only one strong band at ca. 267 cm-1 is observed (see Figure 1). This band can be correlated with the ES22 transition (see Figure 1 in ref 7) of inner semiconducting nanotubes (10,2) of the 2n + m ) 22 family. The weak RBM band at 310 cm-1 can be attributed to the (9,1) tube and correlated with the ES22 transition from family 19. A similar behavior was observed previously by Souza Filho et al.7 No major change was observed for the RBM spectra of the doped DWNTs in Nano Lett., Vol. 8, No. 12, 2008
Figure 1. Resonance Raman spectra at laser lines at 785.0 nm (1.58 eV), 760.0 nm (1.63 eV), and 532.0 nm (2.33 eV) of buckypaper DWNT samples doped with different Br2 levels. The amount of Br2 in the DWNT samples is given in the figure as wt %.
relation to the spectra for the undoped DWNTs for Elaser ) 1.58 eV. It is important to emphasize that owing to instrumental limitations (notch filter cutoff) is was not possible to measure the RBM band below 190 cm-1 for Elaser ) 1.58 eV. For Elaser ) 1.63 eV, DWNTs are also in resonance with the M/S configuration, where the outer tube is metallic and the inner tube is semiconducting. It is interesting to note that, even though Elaser at 1.63 eV is very close to 1.58 eV, the spectrum is very different due to the sharp resonance window of the DWNTs. The RBM bands at ca. 167, 175, and 190 cm-1 seem to be due to outer metallic tubes resonant with the EM 11 excitonic transition. These Raman bands are quenched due to the doping with Br2. At 1.63 eV, the band at 310 cm-1 (tube (9,1)) is much stronger than that at 1.58 eV and we can see the (9,1) tube getting in better resonance with an increase of Br2. In contrast, the RBM bands (near 250 and 265 cm-1), associated with inner semiconducting tubes, increase in intensity as a function of doping level (see Figure 1). These bands can be associated with the (9,4) and (10,2) tubes in resonance with the ES22 transition. Considering for the spectra taken at Elaser ) 1.58 eV, the tube (10,2) shows no change with Br2 doping, we suggest that Br2 doping results in an increased contribution from more than one tube in family 22 to spectral intensity from 250 to 270 cm-1, namely, that tubes (9,4) and (8,6) both come into better resonance with Elaser ) 1.63 eV. Souza Filho et al.8 showed that, for DWNTs treated with H2SO4, the RBM bands associated with metallic tubes reduce their intensities after H2SO4 treatment, independent of whether the configuration of the DWNTs was S/M or M/S. In contrast, they found that the RBM bands associated with semiconducting tubes are only slightly changed. In our case 4169
Figure 2. (A) Plot of the intensity ratio (integrated area) of the Br2 bands (first and second ωBr-Br bands) normalized to the G+ band. The G+ band was obtained by decomposing the G band into four Lorentzian bands, and the integrated area of the band component near 1580 cm-1 was used in the normalization because its integrated intensity remained unchanged with Br2 doping. Data in the figure are given for the four indicated Br2 dopant concentrations in wt %. (B) Plot of the maximum of intensity ratio (Imax(Br2)/IG+) observed for the first- and second-order bands in part A as a function of the Br2 concentration (wt %).
at least in the M/S configuration, the Raman spectra for the semiconducting inner nanotubes are also changed by the higher concentration of Br2 doping. The RBM spectrum at 2.33 eV for pristine DWNTs clearly shows the presence of two groups of ωRBM frequencies that are related to the inner (higher frequency) and outer (lower frequency) tubes, with the S/M configuration dominating. According to the Kataura plot for the electronic transition energies,7 phonons with Elaser ) 2.33 eV are in resonance S with EM 11 for the inner tubes and E33 for the outer ones, which are metallic and semiconducting, respectively. This spectrum for pristine DWNTs in Figure 1 has two intense bands, one at ca. 270 cm-1 and the other at ca. 158 cm-1, and these RBM bands can be assigned, by using the Kataura plot, to S EM 11 and E33, respectively. It is clearly observed that the intensity of the two most intense bands in the pristine spectrum decreases with the increasing doping level. The decrease is more evident for the 270 cm-1 band, related to metallic inner tubes. A similar behavior was also observed by Souza Filho et al. under weak bromine intercalation conditions.7 The Br-Br vibrational mode of the Br2 dopant appears as a feature superimposed on the RBM spectra shown for 4170
Elaser ) 2.33 eV in Figure 1. Both the first order and second order of the ωBr-Br line can be seen in Figure 1. The band due to the stretching of the Br-Br bond can be clearly observed at ca. 231 cm-1 for all doped samples. Figure 2A shows the normalized integrated intensity or area of the ωBr-Br bands as a function of the laser line energy for different doped samples. The area of the ωBr-Br bands was normalized by the G+ band. This G+ band was obtained by decomposing the G band into four Lorentzian bands, and the integrated area of the band component near 1580 cm-1 (G+ band) was used in the normalization. The resonance Raman profile (Figure 2A, first- and second-order bands) shows that the maximum intensity of the ωBr-Br band occurs at energies higher than 2.1 eV for the DWNTs doped with 2 and 5 wt % Br2 and shifts to ca. 2.1 eV for DWNTs doped with 11 and 31 wt % Br2. Figure 2B shows that the intensity of the Br2 bands (first and second order) increases very rapidly until a Br2 concentration of 11 wt % and remains practically constant in going between 11 and 31%, the saturation limit of DWNTs doped with Br2. In the case of the sample doped up to saturation (31 wt %), the Br2 band dominates the Raman spectra in the RBM region from 1.92 to 2.33 eV. This is an important result because it reveals that, in the saturation limit, the resonance window for the dopant Br2 stretching mode is much broader than previously observed7 for DWNTs weakly doped with Br2. In previous studies,7 the spectra of the samples were acquired with the samples exposed to air, and under this condition, it is not possible to maintain a sample under Br2 saturation conditions. However, in the limit of low Br2 concentration, the Br2 bands were only observed close to 2.33 eV. One possible explanation for the broadening of the resonance window of Br2 for the saturated DWNT samples can be attributed to an inhomogeneous DWNT environment (different diameters, chiralities, and configurations of S and M tubes) that changes the electronic density of states for Br2. In addition, electrostatic interactions between close-lying bromine molecules are likely occurring in the highly doped DWNT samples and this latter effect is expected to be responsible for the broadening of the resonance Raman profile with increasing Br2 concentration. In fact, the XPS spectra (not shown) of doped samples exhibit two main components from 69.80 to 67.00 eV observed in the Br 3d XPS band. The first peak can be attributed to physisorbed Br2, and the second peak in the XPS spectra is identified with Br- ions and to bound Cn/Br2 surface complexes, which are formed by charge transfer to the bromine molecule, acting as an electron acceptor.10-12 A small component is also observed at ca. 71.00 eV that can be assigned to bromine covalently bonded to sp2 and sp3 carbon atoms. Thus, mainly physisorbed Br2 and/or chemically adsorbed (surface complexes) contributes for the ωBr-Br Raman bands. In order to give some insight into the interactions between Br2 molecules and/or Br2 molecules with DWNTs, we plot in Figure 3 the values of the ωBr-Br mode frequencies (fundamental- first, and harmonics- second and third) for different laser energies and for DWNTs samples with different Br2 doping levels. A dependence of the ωBr-Br mode Nano Lett., Vol. 8, No. 12, 2008
Figure 3. Plot of the Raman shift values (fwhm) of the ωBr-Br modes (first-, second-, and third-order bands), observed for different Br2DWNT doped samples, as a function of laser energy (eV). The amount of Br2 in the DWNT samples is given in the figure as wt %. For comparison purposes, the ωBr-Br frequencies are listed in a table in the figure with the values of the Br2 stretching band (first band) for different physical situations (gas, liquid, and solid) and also adsorbed on different carbon materials (C60, HOPG, SWNTs, and DWNTs). The values of the laser lines corresponding to the ωBr-Br values are also shown in the table. For the Br2-SWNT and Br2-DWNT samples, the listed value is an average of ωBr-Br obtained over the energy range given in the table.
M/S tube configuration is dominant, where the chargetransfer process is easier, due to the presence of metallic outside tubes. Thus, it seems reasonable that ωBr-Br assumes the minimum value in this energy region. Considering an instrumental error of 2-3 cm-1, it is not possible to correlate the amount of bromine in a DWNT sample with the ωBr-Br values for the same laser line. It could be expected that the ωBr-Br values would increase for more highly doped samples, due to the increase of the Br2-Br2 interactions, such as what is observed for Br2 in condensed phases (see the table inside Figure 3). However, the values for ωBr-Br decrease for all samples as we move away from the maximum of the Br2 electronic resonance. Figure 4. Plot of the line width of the Br2 bands (first and second ωBr-Br bands) as a function of laser energy (eV) observed for different Br2-DWNT samples each with a different Br2 concentration given in wt %.
frequency on laser excitation energy is clearly observed for the ωBr-Br mode (first-order line). For instance, the ωBr-Br value shifts from 232 cm-1 (for Elaser ) 1.73 or 2.41 eV) to 222 cm-1 (for Elaser in the range from 1.92 to 2.1 eV) for bromine doped DWNTs and that this general dependence is almost independent of the Br2 concentration. It is interesting to note that, for all samples, the ωBr-Br value decreases as the intensity of the mode increases and has a minimum frequency for Elaser in the range from 1.92 to 2.1 eV (see Figure 2A). For the samples doped with 2 and 5% Br2, the minimum in Figure 3 is near 2.1 eV, while, for samples doped with 11 and 31%, the minimum is near 1.92 eV. It is important to notice that, from the range 1.92-2.1 eV, the Nano Lett., Vol. 8, No. 12, 2008
One possible explanation for this behavior arises from the strength of the charge transfer between the Br2 molecules and the carbon surfaces. It is reasonable to consider that the frequency of the ωBr-Br mode exhibits low values when the charge transfer with the carbon surface is higher. The table inside Figure 3 shows that the ωBr-Br value is very dependent on the type of the carbon surface used in the adsorption or intercalation of the Br2 molecule, owing to the different reactivity of electrons for each carbon surface. The lowest value is observed for the Br2-DWNT samples, if only the values near the peak intensity of the Br2 resonance are considered. Thus, near the maximum of the resonance Raman profile, mainly the Br2 molecules with highest interaction with the DWNT surface are in resonance and contribute to the ωBr-Br value. The dependence of ωBr-Br on the laser energy is not easily observed for the harmonics (second- and third-order bands), since they are much broader than the firstorder band (the second and third have a line width of 90 4171
cm-1, while the first order has a line width of 30 cm-1, as shown in Figure 4). This work reports on a detailed study of the Raman spectra of double-wall carbon nanotube (DWNT) buckypaper samples. The effects of Br2 doping at different doping levels on the electronic and vibrational properties of the DWNTs are analyzed through the resonance Raman data at 10 laser lines. The frequencies of the radial breathing mode of metallic and semiconducting DWNTs are almost independent of whether the DWNTs are doped with Br2 or not doped, and independent of the bromine doping concentration, but the intensities change strongly with increasing Br2 concentration, especially for metallic tubes. The ωBr-Br Raman bands are however dependent on Elaser, and in the vicinity of the maximum of the electronic resonance energy (from 1.92 to 2.1 eV), the ωBr-Br Raman bands assume their minimum value. In this range, the M/S tube configuration is dominant. Thus, it seems reasonable that ωBr-Br assumes its minimum value in this energy region due to an easier charge transfer to metallic outer tubes. Acknowledgment. G.M.d.N. acknowledges CNPq (Brazilian agency) for his postdoctoral fellowship (No 202479/ 2006-4). The MIT authors acknowledge financial support from the (NSF-Grant No. DMR 07-04197). M.E. acknowledges the support from the CLUSTER project (the second stage) and Specially Promoted Research (No. 19002007) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Supporting Information Available: Resonance Raman spectra of pristine buckypaper DWNT and DWNT sample
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doped with 31 wt % Br2 taken at five different spots. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Endo, M.; Hayashi, T.; Muramatsua, H.; Kim, Y. A.; Terrones, H.; Terrones, M.; Dresselhaus, M. S. Nano Lett. 2004, 4, 1451. (2) Endo, M.; Muramatsu, H.; Hayashi, T.; Kim, Y. A.; Terrones, M.; Dresselhaus, M. S. Nature 2005, 433, 476. (3) Bandow, S.; Chen, G.; Sumanasekera, G. U.; Gupta, R.; Yudasaka, M.; Iijima, S.; Eklund, P. C. Phys. ReV. B 2002, 66, 075416. (4) Duclaux, L. Carbon 2002, 1751, 717. (5) Fischer, J. E. Acc. Chem. Res. 2002, 35, 1079. (6) Cambedouzou, J.; Sauvajol, J. L.; Rahmani, A.; Flahaut, E.; Peigney, A.; Laurent, C. Phys. ReV. B 2004, 69, 235422. (7) Souza Filho, A. G.; Endo, M.; Muramatsu, H.; Hayashi, T.; Kim, Y. A.; Barros, E. B.; Akuzawa, N.; Samsonidze, Ge. G.; Saito, R.; Dresselhaus, M. S. Phys. ReV. B 2006, 73, 235413. (8) Barros, E. B.; Son, H.; Samsonidze, Ge. G.; Souza Filho, A. G.; Saito, R.; Kim, Y. A.; Muramatsu, H.; Hayashi, T.; Endo, M.; Kong, J.; Dresselhaus, M. S. Phys. ReV. B 2007, 76, 045425. (9) Samsonidze, Ge. G.; Saito, R.; Kobayashi, N.; Gru¨neis, A.; Jiang, J.; Jorio, A.; Chou, S. G.; Dresselhaus, G.; Dresselhaus, M. S. Appl. Phys. Lett. 2004, 85, 5703. (10) Papirer, E.; Lacroix, R.; Donnet, J.-B.; Nanse, G.; Fioux, P. Carbon 1994, 32, 1341. (11) Tobias, H.; Soffer, A. Carbon 1985, 23, 281. (12) Maire, J.; Mering, J. Chemistry and Physics of Carbon; Marcel Dekker: New York, 1970. (13) Herzberg, G. Spectra of Diatomic Molecules; Van Nostrand: New York, 1948. (14) Frank Shall, C., III. J. Chem. Educ. 1981, 58, 343. (15) Stammereich, H.; Forneris, R. J. Chem. Phys. 1954, 22, 1624. (16) Cahill, J. E.; Leroi, G. E. J. Chem. Phys. 1969, 51, 4514. (17) Suzuki, M.; Yokoyama, T.; Ito, M. J. Chem. Phys. 1969, 51, 1929. (18) Huong, P. V. Solid State Commun. 1993, 88, 23. (19) Eklund, P. C.; Kambe, N.; Dresselhaus, M. S.; Dresselhaus, G. Phys. ReV. B 1978, 18, 7069.
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Nano Lett., Vol. 8, No. 12, 2008
