Dimerization-Initiated Preferential Formation of Coronene-Based

Jun 19, 2012 - Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan. J. Phys...
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Dimerization-Initiated Preferential Formation of Coronene-Based Graphene Nanoribbons in Carbon Nanotubes Miho Fujihara,† Yasumitsu Miyata,† Ryo Kitaura,† Yoshifumi Nishimura,† Cristopher Camacho,† Stephan Irle,† Yoko Iizumi,‡ Toshiya Okazaki,‡ and Hisanori Shinohara*,† †

Department of Chemistry, Nagoya University & Institute for Advanced Research, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan

‡

S Supporting Information *

ABSTRACT: We have investigated the growth mechanism of coronenederived graphene nanoribbons (GNRs) using two different precursors: coronene and a dimer form of coronene, so-called dicoronylene (C48H20). For both of the precursors, the formation of nanoribbon-like materials inside carbon nanotubes (CNTs) was confirmed by transmission electron microscope observations. Experimental and theoretical Raman analysis reveals that the samples also encapsulated dicoronylene and linearly condensed other coronene oligomers, which can be regarded as analogues to GNRs. Interestingly, it was found that the present doping condition of coronene yields dicoronylene prior to encapsulation due to the thermal dimerization of coronene. These results indicate that the dimerization before the encapsulation drives the preferential formation of the coronene-based GNRs within CNTs.

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INTRODUCTION The structure-selective and efficient synthesis of graphene nanoribbons (GNRs) is one of the most important challenges for the realization of graphene-based electronics, optics, and spintronics because of their structure-dependent band gaps and magnetism.1−5 Until now, many groups have reported various synthesis methods such as lithographic techniques,1,6−8 thermal polymerization of organic molecules arranged on metal surface,9 sonication of graphite (chemical exfoliation),10 and unzipping of carbon nanotubes (CNTs).11−14 Recently, it was also reported that GNRs were synthesized from sulfurcontaining organic molecules and polycyclic aromatic hydrocarbons (PAHs) within CNTs.15,16 In particular, the PAHbased method may provide a very simple and scalable way for the production of structure-controlled GNRs because the molecular configuration of precursor within nanotubes could greatly affect the width and edge structure of the nanoribbons formed. Although some promising results have been reported, the successful implementation of the PAH-based method presents a unresolved but intriguing issue in the GNR growth process and the stable structure of PAHs inside CNTs. Apart from the GNR growth of PAHs, it has been reported that the PAHs such as coronene can form an entirely different structure like coaxially stacked coronene columns in CNTs.17 The coronene columns are stabilized by intercoronene π−π interactions and coronene−nanotube interactions without forming any covalent bonds. Interestingly, both of the coronene-derived GNRs and the stacked columns were obtained through the vapor-phase doping of coronene into open-ended single-wall CNTs (SWCNTs) with an average diameter of 1.5 nm on quite © 2012 American Chemical Society

similar conditions as follows. The GNRs were grown during the doping of coronene at 450−470 °C in argon at 1 × 105 Pa, for 1−12 h, whereas the stacked columns were obtained after the doping at approximately 450 °C for 24 h after the sealing of coronene with CNTs in a quartz tube under vacuum (ca. 1 × 10−4 Pa). The origin of this structural diversity, however, still remains unclear in spite of a key issue for the selective and efficient synthesis of GNRs. To clarify this striking structural diversity, we have focused our attention on the doping process of coronene into CNTs. During the doping, coronene has been vacuum sublimed through the thermal annealing at 450−470 °C. It is known that coronene is partially converted into its dimeric form, dicoronylene (C48H20 = 596), upon annealing at 530−550 °C for 1−2 h.18 Although the previous doping has been carried out at lower temperature than 530 °C, long time annealing and/or higher loading of coronene could induce the dimerization of coronene at lower temperature. Considering its large size, we expect that the dicoronylene in nanotubes does not stack on the molecular plane like coronene, but rather parallel to the CNT sidewalls to produce the reported GNRs.15,16 The codoping of coronene and dicoronylene, therefore, may lead to the preferential growth of GNRs rather than the stacked coronene columns. In this study, we report a novel GNR growth through the vapor-phase doping of coronene or dicoronylene into CNTs. After the doping of each precursor, the samples were Received: April 18, 2012 Revised: June 1, 2012 Published: June 19, 2012 15141

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Figure 1. Photographs of (a) the glass ampule after the vapor-phase doping of coronene and (b) the dicoronylene crystal. (c) Mass spectrum of the dicoronylene crystal. Inset: An enlarged spectrum of (c) for 593−601 m/z.

the lowest-lying singlet electronic excitation energies and oscillator strengths were carried out with the CAM-B3LYP functional in conjunction with Ahlrich’s def-SV(P) basis set19 for ground state optimized molecular geometries using the GAMESS quantum chemical package.20 Raman spectra were computed at the BLYP/6-31G(d,p) level of theory,21−24 since previous studies have reported that the BLYP exchangecorrelation functional reproduces the experimentally observed frequencies in good agreement without multiplying any scaling factors.25−29 The equilibrium geometries for coronene (D6h) and its oligomers (D2h) (C24nH12+8(n−1), n = 1−5) have been optimized in the Raman calculations by imposing symmetry constraints as implemented in the Gaussian 09 program.30 Harmonic vibrational frequencies and Raman activities have been computed following the standard procedures, assuming 25 °C and a laser wavelength of 1064 nm. The simulated Raman spectra were created by convoluting calculated Raman activity line peaks with Gaussian functions of 3 cm−1 for the fixed full width at half-maximum (fwhm).

characterized by using high-resolution transmission electron microscope (HRTEM) observations and Raman spectroscopy. HRTEM observations and Raman analysis revealed that the doping of both of these two precursors results in the encapsulation of almost the same GNRs, i.e., linearly condensed coronene oligomers and dicoronylene in the CNTs. The present results indicate that the preferential growth of GNRs from coronene is assisted by the encapsulation of dimerized coronene, which is formed prior to encapsulation during thermal annealing.

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EXPERIMENTAL SECTION Sample Preparation. Purified SWCNTs produced by arc discharge (Meijo, Type-SO) were used as the starting material. To open nanotube cap, a sample (2−3 mg) was annealed in air at 430 °C for 10 min. The samples were sealed with coronene (15−20 mg, 95.0%, Tokyo chemical industry co., LTD) or dicoronylene (10 mg, 99.5%, kentax UHV equipment) in a Pyrex glass ampule under vacuum (1 × 10−4 Pa). The ampules of coronene and dicoronylene were annealed at 450 and 500 °C for 24 h in an electric furnace. After the annealing, the samples were washed with 1,2,4-trichlorobenzene and annealed again at 300 °C for 10 min under vacuum (1 × 10−4 Pa) to remove excess coronene and dicoronylene onto CNT surface. HRTEM Observation. HRTEM observation was performed by a JEM-2100F (JEOL). The instrument was operated at 80 kV accelerating voltage at room temperature under high vacuum (10−5 Pa). The samples were dispersed in methanol by sonication. The dispersions were dropped onto copper grids coated with thin carbon film. The grids were heated in vacuum to remove excess methanol and then observed by HRTEM. HRTEM images were recorded by a charge-coupled device (CCD) camera (MSC794 1k × 1k, Gatan). Exposure time was typically 1 s. Raman Spectroscopy. Visible Raman spectra were measured using a HR-800 single monochromator (Horiba Jobin Yvon) equipped with a CCD detector. The samples were excited by an Ar ion laser at 488 nm (2.54 eV) and a He−Ne laser at 633 nm (1.96 eV). Fourier-transform Raman spectra were recorded using a NXR FT-Raman Spectrometer (Nicolet) with a Ge detector. The samples were excited by using a Nd:YAG laser at 1064 nm (1.17 eV). Computational Methods. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of

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RESULTS AND DISCUSSION The coronene and dicoronylene were encapsulated into singlewall CNTs by the vapor-phase doping as reported previously.16 Under the present preparation conditions, after the thermal annealing of coronene, we found red and needle-shaped crystals in addition to yellow coronene powder in the glass ampule used for the reaction (Figure 1a, b). A mass spectrum of the crystal shows a series of peaks for 596−598 m/z as shown in Figure 1c. These peaks agree well with the isotopic distribution of dicoronylene (C48H20 = 596). Although such dimers form during the annealing of coronene at 530−550 °C for 1−2 h,31 the present results show that coronene can dimerize even at a temperature of 450 °C. This indicates that dicoronylene can be encapsulated in CNTs during the present vapor-phase doping. Here, we will focus our attention on the structure identification of the products obtained from coronene and dicoronylene on the present condition. Figures 2a and S1a, b show HRTEM images of the coronenederived GNRs in CNTs. These images present a double line which is parallel to the tube axis within the CNT. Such a double line was observed for nearly 90% of the doped CNTs, which indicates a high filling ratio of the present products. Even though the precise length evaluation is difficult from such partial images of CNTs, the observed line has a length 15142

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Figure 2. Typical HRTEM images of the CNTs after the vapor-phase doping of (a) coronene and (b) dicoronylene.

distribution from a few nanometers to more than 10 nm. Almost the same image was observed from the dicoronylenedoped CNTs (Figures 2b and S1c, d). This implies that similar products were obtained from coronene and dicoronylene in the present study. Importantly, we have never found single-line images instead of the double line for more than 100 CNTs TEM observation. Considering the Raman analysis given below, we assign the observed double lines to a pair of edges of the GNRs as shown in the Talyzin’s images. The difference in the images between the present and previous studies is probably due to the low TEM resolution without an aberration corrector. To obtain information as to whether or not the products correspond to the Talyzin’s GNRs, we measured the resonance Raman spectra of the present samples obtained from coronene and dicoronylene. The two samples show similar characteristic Raman peaks for 1150−1650 cm−1, which are much different from those of pristine CNTs as shown in Figure S2. It is noteworthy that the spectra recorded by using a 488 nm excitation laser (Figure 3a) coincide with the Raman spectra of GNRs obtained at a 514 nm excitation in the previous report,16 indicating that the present doping process produces the same products. Interestingly, the present Raman spectra show strong resonance effect as shown in Figure 3. Although each spectrum has a series of similar peaks for 1150−1650 cm−1, there are some critical differences in peak position and relative intensity depending on excitation wavelengths. This difference suggests that two or more products are resonantly excited for each excitation wavelength. The Raman spectra at 488 nm excitation derive from dicoronylene encapsulated in CNTs. Figure 4 presents the Raman spectra of the dicoronylene and coronene powders and the coronene-doped CNTs. The two spectra of dicoronylene and the coronene-doped CNTs conform with each other for 1150−1550 cm−1. The peaks of dicoronylene for 1550−1650 cm−1 are not observed for the CNT sample because of the high-intensity peaks of the G-band of CNTs.32 It is difficult to record the Raman spectra of dicoronylene and coronene powders using the 488 and 633 nm lasers due to their strong photoluminescence (PL). For the doped CNTs, the PL is suppressed because of the energy transfer from the encapsulated molecules to the outer CNTs. The resonance of

Figure 3. Resonance Raman spectra of the CNTs after the vaporphase doping of coronene (black) and dicoronylene (red). The spectra were obtained using (a) 488 nm and (b) 633 nm excitation lasers.

Figure 4. Raman spectra of coronene and dicoronylene powder and the CNTs after the vapor-phase doping of coronene. Laser wavelengths for excitation are 488 nm for the doped CNTs and 1064 nm for coronene and dicoronylene powders. The peaks indicated by asterisks originate from the G-band of CNTs.

dicoronylene is consistent with the presence of the absorption bands of shorter than 570 nm for the film sample (Figure S3) and 520 nm in 1,2,4-trichlobenzene31 and the fact that no significant change in absorption spectra was observed for molecules inside and outside CNTs.33,34 These results clearly indicate that the present coronene doping leads finally to the encapsulation of dicoronylene in the CNTs. The spectra for the 633 nm excitation are, however, attributed to the other coronene oligomers. Note that the Raman intensity of dicoronylene should become weak due to an off-resonance condition at 633 nm because of almost no absorbance at this wavelength for both the film and solution samples. It is well-known that the gap between occupied and virtual orbitals and the concomitant lowest excitation energy of π-conjugated molecules decrease as their conjugation length increases. To clarify the structure−property relationships in 15143

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relatively small peak around 1170 cm−1 and one strong peak near 1200 cm−1 in the region between 1150 and 1250 cm−1. The latter shows a red-shift of about 25 cm−1 from dicoronylene to the coronene pentamer, in which the relative intensity increases as the length increases. Second, the oligomers possess a prominent triple peak in the range from 1300 to 1350 cm−1. While the peak around 1315 cm−1 increases in intensity with the increase of length, the peak around 1330 cm−1 decreases. The tetramer and the pentamer, thus, show mostly only double-peak feature in this region. The reduction of this peak from dicoronylene to the trimer is drastic. Finally, one prominent peak and a few weak or medium intensity peaks are located between 1450 and 1580 cm−1. The prominent peak is again red-shifted similar to that observed in the peak around 1200 cm−1. As described above, one can observe a clear difference in the peak profile for 1300−1350 cm−1 between dicoronylene and the other oligomers. For dicoronylene, the calculated triplepeak profile agrees well with the spectra at 488 nm. For the other oligomers, the predicted double-peak feature is consistent with the spectra measure at 633 nm excitation. It is noteworthy that the low-energy Raman spectra at 488 and 633 nm are also well reproduced by the calculated spectra of dicoronylene and oligomers, respectively (see Figure S5). Furthermore, the resonance behavior of oligomers at wavelengths longer than 488 nm can be explained by the extension of conjugation. Both experimental and theoretical Raman results represent the presence of linearly condensed coronene oligomers inside CNTs in the present doping condition. The presence of such coronene oligomers provides important insights into the formation process of the GNRs and the stacked columns. Only the coronene encapsulation presumably leads to the formation of the stacked columns.17 The present results indicate that the column formation is disturbed by the coencapsulation of dicoronylene and coronene. Instead, such encapsulated dicoronylene and coronene stack on the graphene plane of CNTs, followed by a mutual coalesce during the annealing process. This coalescence process leads to the growth of coronene oligomers which can be regarded as an analogue of GNRs. The present GNRs are composed of a framework analogous to linearly condensed coronene oligomers. We, therefore, conclude that the dimerization before the doping provides the preferential formation of the coronene-based GNRs within CNTs as illustrated in Figure 6.

linearly condensed coronene oligomers, we have performed theoretical calculations of their lowest-lying electronic π−π* excitation energies possessing large oscillator strengths and of Raman vibrational spectra as shown in Figure 5a. The results

Figure 5. (a) Structure and (b) calculated Raman spectra of coronene, dicoronylene, trimer, tetramer, and pentamer.

show that the lowest-lying excitation energies decrease with increasing system size (red-shift) due to the increasing πconjugation length (Table 1). This implies that the coronene oligomers should be in resonance with much longer excitation wavelength than 488 nm. Table 1. Calculated Excitation Energies of the Lowest-Lying Bright Excited Singlet States of Coronene, Dicoronylene, and Coronene Trimer and Tetramer at the CAM-B3LYP/ def-SV(P) Level of Theory for Optimized Ground State Geometries energy/eV coronene dicoronylene trimer tetramer

4.69 2.96 2.60 2.42

Figure 5b displays the calculated Raman spectra of coronene and coronene oligomers for 1100−1650 cm−1. (The spectra for 100−1700 cm−1 are also shown in Figure S4a, b.) For the coronene monomer, our theoretical results agree well with the previous quantum chemical computations.25,26 Instead of a prominent single peak in the theoretical spectrum, two intense peaks around 1340 cm−1 are observed in the experimental spectra due to a Fermi resonance.35,36 For the coronene oligomers, several new peaks can be seen in addition to the signatures observed for the coronene. First, there is one

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CONCLUSION We have reported the growth of GNRs and coronene oligomers in CNTs from coronene and dicoronylene. HRTEM observations and Raman analysis reveal that the doping of

Figure 6. Proposed growth mechanism of the coronene-derived GNRs within CNTs. First, coronene molecules partially transform to dicoronylene during the vapor-phase doping. The codoping of dicoronylene and coronene leads to the preferential formation of coronene oligomers, an analogue of GNRs, through thermal coalescence rather than the stacked coronene columns. 15144

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coronene and dicoronylene result in the encapsulation of the GNRs, linearly condensed coronene oligomers, and dicoronylene in the CNTs. The preferential growth of GNRs from coronene is assisted by the encapsulation of dimerized coronene which is, in turn, formed during the thermal annealing, providing an important basis for selective synthesis of molecular-derived GNRs in CNTs. The stacked coronene columns, in contrast, reselectively obtained under the condition where the dimerization of coronene is suppressed during the vapor-phase doping.17 It is important, therefore, to control the loading amount of coronene, pressure in a reaction ampule, and sublimation temperature to selectively synthesize the encapsulates. We are now working on the extraction of GNRs from CNTs as demonstrated for the inner shells of double-wall CNTs.37

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ASSOCIATED CONTENT

S Supporting Information *

HRTEM images of the SWCNTs after the vapor-phase doping of coronene and dicoronylene; Raman spectra of pristine SWCNTs; optical absorption spectrum of the film of dicoronylene; calculated Raman spectra of coronene and coronene oligomers; low-energy Raman spectra of the CNTs after the vapor-phase doping of coronene. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank S. Okada of Tsukuba University for fruitful discussion and M. Komatsu of Thermo Fisher Scientific Inc. for the measurement of FT−Raman spectra. This work has been supported by the Grant-in-Aid for Specific Area Research (no. 19084008) on Carbon Nanotube Nano-Electronics and for Scientific Research S (no. 22225001) of Education, Culture, Sports, Science and Technology (MEXT) of Japan. Y.M. acknowledges the financial support by a Grant-in-Aid for Young Scientist (B) (no. 24750180) from the Japan Society for the Promotion of Science.

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