Thermal relaxation rate constants of
Defects in single-wall carbon nanotubes (SWCNTs) have a great influence on their physical properties. In real SWCNTs, various types of defects such as vacancies, Stone–Wales defects, adatoms, or H–C complex are contained as shown in Figure 1. Such defects can be introduced at the stage of SWCNT growth or later on during device or composite production. They can be also created deliberately by chemical treatment or by irradiation with electron, ion, or laser light. Understanding the properties of such defects in SWCNTs is important for improving SWCNT growth methods, tailoring their physical properties, and controlling the irradiation-induced damages.
The irradiation with electron, ion, or laser light has been widely used for the studies not only on the properties of defects but also on the modification of CNTs. The nice reviews on electron and ion irradiation-induced effects in CNTs have been already published by Krasheninnikov
Resonant Raman spectroscopy is one of the most powerful tools for characterizing structural and electronic properties of SWCNTs . In the Raman spectra, defect-induced phonon mode so-called D band is observed at around 1350 cm−1. The intensity of the D band can be enhanced as the number of defects is increased in the SWCNT. Therefore, the D band has been used for the assessment of imperfection of SWCNTs and the understanding of the properties of their defects. Further finding Raman bands associated with defects can lead the Raman spectroscopy to a more effective tool for the characterization of defects.
This chapter presents our recent studies [8-11]on the characterization of laser-induced defects and modification in SWCNTs by Raman spectroscopy. This chapter consists of four parts as mentioned below: (1) Thermal relaxation of laser-induced defects in SWCNTs, (2) Phonon control in metallic SWCNTs by laser–induced defects, (3) Fine structure of D band related to laser-induced defects in SWCNTs, and (4) Formation of
2. Raman spectra of SWCNTs
The resonant Raman spectra of SWCNTs include two main features: a radial breathing mode (RBM) observed in the range of 50‒350 cm-1 and a tangential mode (the so-called G band) observed in the range of 1450–1650 cm-1 .
The RBM is a signature for the presence of SWCNTs, and is observed as a peak or a multi-peak feature. In the RBM, as suggested by its name, all the C atoms are vibrating in the radial direction with the same phase, as if the tube are breathing. The atomic motion does not break the tube symmetry, that is, the RBM is a totally symmetric (
A very important characteristic is the RBM frequency (
with values for the parameters
The G band (where the notation G comes from graphite) is related to the in plane C‒C bond stretching mode in graphite and graphene. The G band is the Raman signature for all the sp2 carbon materials, and is observed as a peak or multi-peak feature. The G band in SWCNTs is a more complex spectral feature. Due to the folding of graphene sheet into the SWCNT and the symmetry breaking effects associated with the nanotube curvature, G band splits into G+ and G−, which are related to atomic vibrations preferencially along (LO) and perpendicular (TO) to the tube (folding) axis, respectively, for semiconducting SWCNT. For metallic tubes, electron-phonon coupling softens the LO modes, so that G+ and G− are actually with TO and LO modes, respectively.
The G- peak for metallic tubes is fitted by asymmetric and broad Breit-Wigner-Fano (BWF) line:
Actually, due to the symmetry breaking effects associated with the nanotube curvature, G band in SWCNT generates up to six Raman-allowed G-band peaks corresponding to two totally symmetric
In addtion, in the Raman spectra in SWCNTs, defect-induced phonon mode so-called D band is often observed at around 1350 cm−1 .The D band is a Raman signature of disorder in sp2 carbons materials. The intensity of the D band can be enhanced as the number of defects is increased in the SWCNT. The D band has been used for the assessment of imperfection of SWCNTs and the understanding of the properties of their defects.
3. Thermal relaxation of laser-induced defects in SWCNTs
3.1. Laser irradiation for SWCNTs synthesized by electric arc-discharge method
SWCNTs synthesized by an electric arc-discharge method were used for laser irradition experiments. As-grown SWCNTs were purified by heating at 350°C for 90 min in air. A suspension of purified SWNTs in ethanol was prepared by ultrasonication. By drop-coating and air-drying the suspension, a SWCNT thin film was formed on a quartz substrate. The SWCNT film was irradiated with a 248 nm (~5.0 eV) pulsed KrF excimer laser in air. The irradiation fluence was approximately 3 J/(cm2·pulse). The irradiation pulse number was selected to be only one because more than two pulses led to the breakdown of the SWCNTs.
3.2. Change in D band by laser irradiaton and thermal annealing
Figure 2 shows D and G bands in the Raman spectra for non-irradiated SWCNTs, irradiated SWNTs with a 248 nm pulsed excimer laser of 3 J/(cm2·pulse) in air, and annealed SWCNTs at 673 K in a vacuum of 1 Pa for 240 min after the laser irradiation. The inset shows the close up of the D band. All of the spectra were normalized to the maximum intensity of the G band. Note that the Raman excitation was provided with a 532 nm (~2.33 eV) of a Nd:YVO4 laser where the laser power level in a focal spot of 1 μm in diameter on the sample was kept below 0.1 mW to prevent overheating the sample.
It is found that the D band intensity significantly changes by laser irradiation and thermal annealing, while the spectral feature of the G band remains almost unchanged. For more clarifying the change in D band intensity, the relative intensity of the D band main peak at 1346 cm−1 to the G band main peak at 1593 cm−1 was defined as
Let us consider the formation process of the laser-induced defects in SWCNTs. The knock-on energy of carbon atom into the direction perpendicular to the tube surface for an isolated SWCNT with a diameter over 1 nm is estimated to be 15–17 eV . This energy is much higher than the irradiation energy of 248 nm (~5.0 eV) used in this experiment. This means that the formation of the laser-induced defects in SWCNTs would not be due to the physical knock-on phenomena. However, the increase of D band intensity related to the formation of defects clearly occurs for SWCNTs irradiated in air. The degree of the increase of D band intensity is much higher than those for SWCNTs irradiated in vacuum and Ar atmosphere. Therefore, the formation of the laser-induced defects in SWCNTs can be attributed to the chemical reaction with O2 and H2O in air by laser heating.
3.3. Analysis of recovery of D band by themal annealing
To examine the thermal relaxation of the laser-induced defects in SWCNTs, the time evolution of the relative intensity of D band in the irradiated samples at various annealing temperatures from 296 to 698 K was measured in the range of annealing times of 0 to 240 min. The typical annealing time evolution of the relative intensity of the D band at 573, 673, and 698 K are shown in Figure 3. Note that the
As shown in Fig. 3, the
As the previous analysis of the thermal relaxation kinetics of defects for graphite , the dependence of the
|296||< 10-6||< 10-6|
|623||1.5 ×10-3||5.6 ×10-6|
|648||2.9 ×10-3||1.6 ×10-5|
|673||2.4 ×10-3||2.2 ×10-5|
|698||3.5 ×10-3||2.0 ×10-5|
3.4. Activation energies of thermal relaxation of laser-induced defects
Arrhenius plots of thermal relaxation rate constants of
On the other hand, two such relaxation processes have also been reported for graphite irradiated with He+ ions . According to the report, the fast and slow relaxation processes correspond to the vacancy-interstitial recombination and vacancy migration in the graphene plane, respectively, which have the activation energies of 0.89 eV and 1.8 eV. This suggests that the slow relaxation process corresponds to the vacancy migration in both SWCNT and graphite. The activation energy of 0.7 eV of the vacancy migration for SWCNTs as deterimined experimentally above is much smaller than 1.8 eV for graphite. This smaller value for SWCNTs would be due to the curvature effect of nanotube that breaks the trigonal symmetry of a perfect graphene sheet. It is expected that, due to the curvature effect of nanotube, the activation energy of the vacancy-interstitial recombination for SWCNTs is also smaller than 0.89 eV for graphite. Thus, it is suggested that the fast relaxation process with the activation energy of 0.4 eV as determined in this experiment corresponds to the vacancy-interstitial recombination in SWCNTs.
In summary, laser-induced defects in SWCNTs can be introduced by the irradiation with a 248 nm pulsed excimer laser. The formation of defects might be related to thermal oxidation and burning by laser heating. Such laser-induced defects are thermally relaxed with two processes with fast and slow rates. The two relaxation processes show the strong temperature dependence. The activation energies of the fast and slow relaxation processes are determined to be 0.4 and 0.7 eV, respectively. These processes can correspond to vacancy-interstitial recombination and vacancy migration along the tube axis. Such relaxation processes with fast and slow rates for SWCNTs are similar to those for graphite irradiated with He+ ions. However, their activation energies for SWCNTs are smaller than those for graphite. The smaller activation energies for SWCNTs would be due to the effect of curvature of nanotube.
4. Phonon control in metallic SWCNTs by laser–induced defects
4.1. Change in G band for metallic nanotubes by laser irradiation and thermal annealing
SWCNT samples after laser irradiation and themal annealing were prepared by similar procedure as described in 3.1. Figure 5 show D and G bands in Raman spectra for (a) pristine SWCNTs, (b) laser-irradiated SWCNT swith a 248 nm (~5.0 eV) pulsed KrF excimer laser of approximately 5 J/(cm2 ·pulse), and (c) annealed SWNTs at 400 °C in a vacuum of ~9 Pa for 60 min after the irradiation. Note that the spectra excitation was provided with a 632.8 nm (~1.96 eV) of a He–Ne laser where the laser power level in a focal spot of 1 μm in diameter on the sample was kept below 0.1 mW to prevent overheating the samples.
D band was fitted with one Lorentzian line. On the other hand, G band was fitted with two Lorentzian lines and one asymmetric line that is Breit–Wigner–Fano (BWF) line  as given by Eq. (2). The values of the fitting parameters are listed in Table 2. In the G band for pristine SWCNTs in Fig. 5(a), one Lorentzian line at 1593 cm−1 corresponds to G+ peaks for semiconducting SWCNTs and metallic SWCNTs. Note that G+ peak associated with semiconducting SWCNTs is assumed to overlap with that with metallic ones. The other Lorentzian line at 1567 cm−1 corresponds to G− peak associated with semiconducting SWCNTs. The asymmetric and broad line at 1546 cm−1 corresponds to G− peak associated with metallic SWCNTs, which is largely downshifted, relative to G+ one. These spectral components of the G band suggest that both semiconducting and metallic SWCNTs are resonant in the Raman spectrum of pristine SWCNTs taken with 1.96 eV in Fig. 5(a).
The D band for pristine SWCNTs was fitted with one Lorentzian curve at 1323 cm−1. It is known that the intensity of the D band increases as the number of defects in SWCNTs is increased. The intensity ratio of D/G+ peaks is often used as a measure of the defect density in SWNTs. The relative intensity of the D band for pristine SWCNTs is 0.12. This value is quite small, comparable to those of high quality SWCNTs as reported so far. Thus, pristine SWCNTs used in this experiment have high quality or quite low defect density.
A significant change in the Raman spectrum was observed for laser-irradiated SWCNTs, as shown in Fig. 5(b). The intensity of the D band increased with the laser irradiation. Note that the frequency and linewidth remained almost unchanged even after the irradiation. These results mean that some specific defects were introduced in SWCNTs by the laser irradiation. The formation of defects might be related to thermal oxidation and burning by laser heating as discussed in 3.2. Moreover, it should be noticed that not only D band but also G band were affected by the laser irradiation. Especially, a significant change was observed for G− peak associated with metallic tubes. The frequency of the G− peak was upshifted by 8 cm−1. Correspondingly, the linewidth of the G− peak was reduced by 25%. In addition, the intensity ratio of the G−/G+ peak was reduced by 25%. On the other hand, the frequency, linewidth, relative intensity of G− peak associated with semiconducting SWCNTs remained almost unchanged even after the laser irradiation. Such behavior in the G− peak associated with semiconducting SWCNTs is consistent with that in the G− peak for the same SWCNTs taken with 2.33 eV in which only semiconducting SWNTs are resonant. Thus, the laser-induced defects significantly affect G− peak associated not with semiconducting SWCNTs but metallic ones.
The G− peak associated with metallic SWCNTs is due to the electron-phonon coupling as described in 2 [18,19]. The upshift of the frequency, the narrowing of the linewidth, and the reduction in the relative intensity for the G− peak associated with metallic SWCNTs as seen in Fig. 5(b) imply the breaking of the electron-phonon coupling. Moreover, it should be noticed that D and G bands for the irradiated SWNTs recover the original ones after annealing in a vacuum of ~9 Pa at a sample temperature of 400 °C for 60 min, as shown in Fig. 5(c). As described in 3.4, the laser-induced defects such as vacancies can be annihilated by vacancy-interstitial recombination and vacancy migration to the nanotube end due to the thermal annealing . Such annihilation of vacancies can be responsible for the recovery of the Raman spectral profile. Thus, the electron-phonon coupling can be reversibly controlled by the generation and annihilation of specific defects due to laser irradiation and thermal annealing.
4.2. Change in RBM for metallic nanotubes by laser irradiation and themal annealing
The change corresponding to that in D and G bands in Fig. 5 was also observed for radial breathing modes (RBMs) at the range of 150–200 cm−1 in Raman spectra for pristine SWNTs, laser-irradiated SWCNTs, and annealed SWCNTs after the irradiation, taken with
The RBM peaks associated with metallic SWCNTs changed after the irradiation, as shown in Fig. 6(b). Especially, the most intense RBM peak at 173 cm−1 drastically decreased with the irradiation. On the other hand, no significant change was observed for only RBM peak at 157 cm−1 associated with semiconducting SWCNTs. These results mean that the resonant off for the Raman excitation of 1.96 eV occurs for metallic SWCNTs. This also suggests that the change in the electronic structure for metallic SWCNTs occurs due to the laser-induced defects.
Vacancy defects can cause a bandgap opening in metallic SWCNTs due to the breaking of the symmetry [23,24]. Such metal-semiconductor transition has been also experimentally demonstrated by the measurements of electrical properties for metallic SWCNTs with the introduction of defects [25,26]. Therefore, the change in the electronic structure with the bandgap opening can be responsible for the resonant off for metallic SWCNTs. Such change in the electronic structure for metallic SWCNTs due to the laser-induced defects is also consistent with the change in the corresponding G− band, i.e., the breaking of the electron-phonon coupling, as discussed in 4.1.
Moreover, as seen in Fig. 6(c), the thermal annealing also leads to the recovery of the RBMs to original ones, as D and G band in Fig. 5. This recovery can be also explained by that in the electronic structure due to the thermal annihilation of laser-induced defects such as vacancies as discussed in 3.4.
In summary, laser-induced defects influence not only D band but also G− peak associated with metallic SWCNTs, which is attributed to the electron-phonon coupling with Kohn anomaly. The upshift and narrowing of the G− peak occur due to the laser irradiation. The G− peak can recover to the original one due to the thermal annealing. The electron-phonon coupling for metallic SWCNTs can be reversibly controlled by the generation and annihilation of specific defects due to the laser irradiation and thermal annealing.
5. Fine structure of D band related to laser-induced defects in CoMoCAT SWCNTs
5.1. Heating and laser irradiation for CoMoCAT SWCNTs
As-received CoMoCAT SWCNTs (SWeNT® CG 100, SouthWest NanoTechnologies, Inc.) were used for heating and laser irradiation experiments. A suspension of SWCNTs in ethanol was prepared by ultrasonication. By drop-coating and air-drying the suspension, a SWCNT thin film was formed on a quartz substrate. For heating experiments, the film samples were annealed at 350 °C for 90 min in air. For laser irradiations, the samples were irradiated with a 532 nm (~2.33 eV) from a Nd:YVO4 laser for 180 min. The irradiation power level in a focal spot of 1 μm in diameter on the sample was kept at ~20 mW. The heating and laser irradiation experiments were also carried out in a vacuum of∼4.5 Pa and a dynamic vacuum of ∼3.5×10−4 Pa, respectively.
5.2. Change in D band by heating
Figure 7 shows D and G bands in the Raman spectrum for a pristine CoMoCAT SWCNT sample. The corresponding radial breathing modes (RBMs) are also shown in the inset in the figure. Note that the spectrawere taken with
Figure 8 shows
Actually, pristine SWCNT sample contains impurities such as amorphous carbon, water, and C–H complex. The thermal oxidation gives rise to open-end structures of SWNTs or holes in the walls . The extension of the oxidation process can generate C=O, C–O–C, and C–OH . In addition, the corresponding
5.3. Change in D band by laser irradiation
Figure 9 shows
On the other hand, the laser irradiation in a dynamic vacuum of ∼3.5×10−4 Pa leads to the appearance of a new
6. Formation of
trans-polyacetylene from CoMoCAT SWCNTs by laser irradiation
6.1. Laser irradiation for CoMoCAT SWCNTs
As-received CoMoCAT SWCNTs (SWeNT@CG 100, SouthWest NanoTechnologies, Inc.) were used in this experiment. A suspension of SWCNTs in ethanol was prepared by ultrasonication. The suspension was dropped on a clean quartz substrate and allowed to be air-dried at room temperature. The SWCNTs samples prepared in above procedure were used for laser irradiation experiments. The samples, which were exposed to air for less than 1 h before laser irradiation, are called “short air-exposure“ ones. Some of samples were kept in air at room temperature for more than six months before laser irradiation. They are called ‘‘long air-exposure’’ ones.
Laser irradiation experiments were carried out using a micro-Raman systemequipped with mirrors, attenuators, a 100× microscope objective, a holographic notch filter, a single grating spectrometer (1800 1/mm grating), and a charge coupled device detector. In the laser irradiation experiments, all attenuators were removed. A 532 nm (~2.33 eV) from a cw Nd:YVO4 laser was used to irradiate the samples. The laser beam was focused on the sample through the 100× microscope objective, with spot size of 1 μm. The laser power level on the sample was kept at 17.8 mW/μm2. The irradiation time was 1 h.
6.2. Irradition effect for “short air-exposure“ CoMoCAT SWCNTs
Figure 10 shows D and G bands in Raman spectra for a ‘‘short air-exposure’’CoMoCAT SWCNT sample (a) before and (b) after laser irradiation of a 532 nm with 17.8 mW/μm2 for 1 h in air. The corresponding RBMs are also shown in the inset in the figure. Note that the spectra excitation was also provided with
The spectral peaks are fitted with Lorentzian lines. From the RBM frequencies in Fig. 10(a), the mean diameter of SWCNTs is estimated to be 0.8 nm, which corresponds to typical mean diameter of CoMoCAT ones . Note that the diameter is estimated using
After laser irradiation, the increase of relative intensity ofD band is observed as seen in Fig. 10(b). Especially, the lower-frequency component increases. The increase of D band intensity can be attributed to the oxidation and/or hydrogenationof SWCNTs [10,29,30] as discussed in 5. In addition, the RBMs exhibit the decrease of higher-frequency components by laser irradiation. This means that smaller diameter SWCNTs are degraded by thermal oxidation due to laser heating. On the other hand, no significant change in G band is observed as seen in Fig. 10(b).
6.3. Irradiation effect for “long air-exposure“ CoMoCAT SWCNTs
Figure 11 shows D and G bands in Raman spectra for a ‘‘long air-exposure’’CoMoCAT SWCNT sample (a) before and (b) after laser irradiation of a 532 nm with 17.8 mW/μm2 for 1 h in air. The corresponding RBMs are also shown in the inset in the figure.Note that the spectra excitation was also provided with
It should be noted that a significant change in Raman spectra is observed for the irradiated ‘‘long air-exposure’’sample as seen in Fig. 11(b). Namely, new intense peaks appearat 1138 cm-1 and 1514 cm-1 for the irradiated ‘‘long air-exposure’’ sample. These spectral features are quite similar to those of
6.4. Formation of polyacetylene from SWCNTs
In ‘‘long air-exposure’’ samples, a lot of water vapor can be absorbed. To remove the absorbed water, the ‘‘long air-exposure’’samples were annealed at 150 °C for 1 h under a dynamic vacuum of 8.7×10-7 Pa. In order to examine the effect of absorbed water on the formation of
Let us consider the formation process of
This chapter presented the characterization of laser-induced defects in SWCNTs by Raman spectroscopy. The laser irradiation with heating followed by burning can produce defects such as vacancies and interstitials in SWCNTs. These defects greatly influence electronic structures and phonon properties especially in metallic nanotubes. They can also be thermally relaxed by vacancy-interstitial recombination and vacancy migration along the tube axis with activation energy of 0.4 eV and 0.7 eV, respectively. This means that the electronic structures and phonon properties in metallic nanotubes can be reversibly controlled by the generation and annihilation of specific defects due to laser irradiation and thermal annealing. In addition, it was presented that the fine structure of Raman D band can be related to specific defects such as C-H complex on nanotubes and nanotube edges produced by laser irradiation. This can lead the Raman spectroscopy to a more effective tool for the characterization of defects in SWCNTs. Finally, it was presented that the laser irradiation can give rise to the formation of trans-polyacetylene from SWCNTs. The formation process might be related to the cutting of SWCNTs due to the catalytic hydrogenation of carbon atoms with laser heating, although the detailed mechanism is not yet understood. This shows a new use of the laser irradiation for the formation of functional materials from SWCNTs. Thus, the laser irradiation is useful for not only the understanding of the properties of defects in SWCNTs but also the modificaton of SWCNTs, as electron and ion irradiations.
The author acknowledges the contributions to the works presented here by all past and present collaborators, especially Dr. Takashi Uchida, Dr. Hironori Kawamoto, Mr Ken-ichi Kato, Mr. Dongchul Kang, Ms. Mari Hakamatsuka, Ms. Nagisa Hosoya, Mr. Noriaki Nemoto, and emeritus Prof. Kenichi Kojima. The works were supported in part by Strategic Research Projects in Yokohama City University and MEXT/JSPS KAKENHI.
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