J. Phys. Chem. C 2007, 111, 10347-10352
10347
Recoverable Photoluminescence of Flame-Synthesized Multiwalled Carbon Nanotubes and Its Intensity Enhancement at 240 K Qiaoliang Bao,†,‡ Jun Zhang,‡ Chunxu Pan,*,‡ Jun Li,† Chang Ming Li,*,† Jianfeng Zang,§ and Ding Yuan Tang§ School of Chemical and Biomedical Engineering and Center for AdVanced Bionanosystems, Nanyang Technological UniVersity, Singapore, 639798, Singapore, Department of Physics and Key Laboratory of Acoustic and Photonic Materials and DeVices of Ministry of Education, Wuhan UniVersity, Wuhan, 430072, China, and School of Electrical and Electronic Engineering and Center for AdVanced Bionanosystems, Nanyang Technological UniVersity, Singapore, 639798, Singapore ReceiVed: February 21, 2007; In Final Form: May 17, 2007
The photoluminescence (PL) of multiwalled carbon nanotubes (MWCNTs) synthesized with ethanol flames was investigated at temperatures from 30 to 300 K. Broad-band PL emission in the near-infrared region was observed at temperatures below 240 K. The PL intensity was abruptly boosted up at 240 K, and then quickly became quenched and undetectable as the temperature was further increased. The PL emission of the MWCNTs only appeared in the first cycle of the temperature rising. However, it could be recovered by exposing the sample in an oxygen-abundant environment. We propose that the PL emission of the MWCNTs is resulted from the carbon oxyhydride-like fluorophors bound to the MWCNTs surface.
Introduction Photoluminescence (PL) of carbon nanotubes (CNTs) has attracted a great deal of attention due to the promising applications of CNTs for ultrasmall optical devices.1 Recently, band gap photoluminescence of single-walled carbon nanotubes (SWCNTs) was observed from individually isolated SWCNTs either encapsulated in micelle2,3 or suspended between silicon pillars4 at room temperature. In both cases, the key factor is to prevent the nanotubes from becoming bundled or contacting the substrate. It has become clear that PL of the SWCNTs can be greatly affected by environments and temperature.5,6 Therefore, the temperature-dependent PL from SWCNTs not only has enormous importance in determining the electron-photon interactions,7 but also provides information on band gap temperature dependence8 and influence of environmental factors, i.e., different ambient gases, on their emission.9 Through PL studies new features of CNTs are continually discovered, for example, an abrupt PL transition was identified at high temperature;9 anomalous PL intermittency in temporal evolutions of the PL intensity was observed in specific SWCNTs at room temperature;10 unusual “holes” were detected in scanning PL spectroscopy of freely suspended SWCNTs.11 However, so far most research has been focused on PL of the SWCNTs, and little is known about PL of the multiwalled carbon nanotubes (MWCNTs), especially their low-temperature PL features. In previous reports, Riggs et al.12 first observed strong PL from solubilized MWCNTs and proposed that the PL could be due to the trapping of excitation energy at defect sites. Brennan et al.13 reported a broadband emission from MWCNTs with high purity, and attributed the optical transitions to three-photon absorption and upconverted PL from van Hove singularities. * Corresponding author. E-mails:
[email protected] and ecmli@ntu. edu.sg. † Nanyang Technological University. ‡ Wuhan University. § Nanyang Technological University.
However, the electronic state in MWCNTs is complicated and was not clear until now, and MWCNTs with different structure characteristics or in different environments would be expected to behave in a significantly different way; i.e., solubilized MWCNTs,12 rutheniumdopedMWCNTs,14 acid-treatedMWCNTs15 and well-aligned MWCNTs films16 were found to have variant fluorescent behaviors. In addition, the light emission induced by current,17 tunneling-current,18,19 and field-emission20 from MWCNTs were also observed and different mechanisms were proposed. In this article, the as-grown MWCNTs synthesized in ethanol flames21,22 were used for measurement of the low-temperature PL spectra from 30 to 300 K. Comparing to the regular MWCNTs synthesized using chemical vapor deposition (CVD) method, the present MWCNTs possess low crystallinity and a great deal of defects which may exhibit special physical property on PL emission. Experimental Section The morphology and microstructure of MWCNTs were characterized by using transmission electron microscope (TEM, JEOL JEM 2010, Japan) and high-resolution transmission electron microscope (HRTEM, JEOL JEM 2010 FEF, Japan). The functionalized groups absorbed on the surface of the “asgrown” MWCNTs were identified using a Fourier transform infrared spectroscopy (Nicolet MAGNA-IR 560, FTIR, U.S.) with an attenuated total refraction (ATR) accessory. The fabrication of MWCNTs by using the ethanol flame method was described in the previous papers.21,22 A pulse plating process was used to produce a nanocrystalline Ni layer upon the sampling surface of the substrate, which, then, was taken as the catalysts for synthesizing MWCNTs in ethanol flames. It is confirmed that the element of Fe and/or its compounds promotes growth of solid-cored CNFs, but Ni and/or its compounds favor the growth of graphitic hollow-cored CNTs.23,24 In this work, pure Ni nanocrystalline was electrodeposited on
10.1021/jp071460i CCC: $37.00 © 2007 American Chemical Society Published on Web 06/26/2007
10348 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Bao et al.
the substrate21,22 as the catalyst for the growth of pure MWCNTs. The produced samples were further examed by SEM, TEM and HRTEM (as shown in Figure S1 and S2), indicating there was no solid-cored structure but pure MWCNTs. The sample was stored in drying cabinet. The PL spectrum of the as-grown MWCNTs was measured on a home-built system. The samples were excited by an argon ion laser beam (130 mW) at 488 nm in a vacuum chamber (CdO (1730 cm-1), -C-O (1160 cm-1), and -C-H (2950 cm-1, 2870 cm-1),32,33 existed on the MWCNTs surface before excitation. However, the functional groups were weakened after the excitation at 240 K, which indicated that the functional groups were destroyed or desorbed during excitation of MWCNTs. As expected, the functional groups could be recovered by exposing the MWCNTs in air, as shown by spectrum III in Figure 3c. It is well-known that the PL spectroscopy is an effective method for probing electronic structure of materials. For the semiconducting SWCNTs, it can be explained by the temperature-dependent band gap change6 according to the luminescence mechanism for conventional semiconductors. For the MWCNTs comprising both small and large diameter/both semiconducting and metallic concentric tubes, the electronic state in MWCNTs
10350 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Bao et al.
Figure 4. Optimized atomic structures of CNTs with two atom-vacancies and two kinds of chemical groups (-COOH and -CH2OH): (a) armchair (8, 8) CNT; (c) zigzag (15, 0) CNT. Band structures: (b) armchair (8, 8) CNT; (d) zigzag (15, 0) CNT ((I) Pristine CNT; (II) CNT with one vacancy and -COOH group at the defect site; (III) defective CNT with two atom-vacancies; (IV) functionalized CNT with two atom-vacancies and two kinds of chemical groups (-COOH and -CH2OH). The red dash lines correspond to the Fermi energy.)
is complicated due to the interwall interaction. The excitations related to the inner semiconducting MWCNTs would decay nonradiatively through the fast vibrational manifolds of adjacent metallic tubes, and the outermost tubes with diameter around 20 nm may have the band gap energy as low as 2 kT, which cannot even result in any optical emission.13 However, PL has been observed from flame-synthesized MWCNTs at low temperature. We cannot attribute the optical transitions to multiphoton absorption and broad band PL from van Hove singularities just as Brennan et al. have proposed,13 because in our experiments blue laser was used as excitation source and we obtained featureless optical absorption spectrum from flame synthesized MWCNTs (not shown here). It seems that the present PL might be due to the trapping of excitation energy at defect sites of graphite layers, but the radioactive electron transitions between localized electronic states related to defects or end effects on MWCNTs18 cannot explain the present anomalous PL enhancement and quenching at 240 K. The recovered PL in oxygen-ambient environments, together with the change of FTIR spectrums before and after excitation, provided some clues to explain the mechanism of the observed abnormal PL features. Because CO2 could not recover the PL of MWCNTs, it could conclude that the functional groups (especially > CdO) were not physically but chemically absorbed on the surface of MWCNTs. Experiments and calculations on graphite have shown that O2 molecule dissociatively adsorbs at the vacancy sites of graphite exothermally without an energy barrier and a self-sustaining chain reaction with oxygen can take place.34 The inherent defects of nanotube sidewall were chemically active28 sites and lots of functional groups (CdO, C-O, C-H, O-H) were formed at these sites while exposed in air.27 Gole et al.35 have demonstrated that a
kind of oxyhydride-like fluorophor acting as a surface bound emitter could result in PL. Most recently, Iakoubovskii et al.36 reported that a kind of midgap luminescence centers in SWCNT could be created by ultraviolet illumination. We suggest that similar oxyhydride fluorophors based on these functional groups accumulated at the defect sites of MWCNTs may represent the localized surface centers proposed by Prokes et al.,37 which are responsible for the observed PL. Previously, it was found that sidewall functionalization could greatly affect the conducting properties of SWCNTs.38,39 Heavy functionalization at defect sites will lead to totally different electronic and optical properties of the SWCNT from that of the perfect SWCNT.40 Naturally, there is one question raised: is that the functional groups absorbed on the defect sites changes the MWCNT electronic structure? Generally, it is impracticable to obtain a precise electronic structure of MWCNT by theoretical calculation due to complicated interwall interaction41 and computational limit. However, for MWCNTs, each of its carbon shells can be considered as metallic or semiconducting SWCNTs with different chiralities and the outmost shells where most of the functional groups could be absorbed are generally considered as metallic SWCNTs. Therefore, for simplicity, we build models based on metallic SWCNTs with larger diameter to investigate the physical tendency about how the functional groups change the electronic structure of MWCNTs. Figure 4 illustrates the theoretical calculations based on DFT method. We employed metallic armchair and zigzag SWCNTs to investigate the effect of chemical functionalization on the electronic structure. The chemical groups were focused on carboxyl groups (-COOH) and alcohol groups (-CH2OH), as they were regarded as main prototypes for functionalizaiton after the oxidation of CNTs.42 The results show that the original
Flame-Synthesized Multiwalled Carbon Nanotubes
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10351 Conclusions In conclusion, the MWCNTs grown in ethanol flames provide a novel physical phenomenonsPL emission due to their unique microstructures in special synthesis conditions. That is, (1) a broadband PL emission in the wavelength range of 1200-2400 nm; (2) an abrupt PL intensity enhancement at 240 K and then quenched. The phenomenon is repeatable only when the sample is exposed in oxidizing environments for the reason of the absorbed chemical groups on MWCNT surface. We believe that the phenomenon might have applications in optoelectronics, gas sensor and low temperature indicator.
Figure 5. (a) Schematic model shows functional groups absorbed at the defects area of MWCNT. (b) Schematic diagram of the mechanism for PL (the left part corresponds to the inner layers of MWCNT with perfect structure; the right part corresponds to the outmost layers with high density of defects and functional groups).
π-π* band crossing of either the armchair (8, 8) or the zigzag (15, 0) tube, which is in agreement with Zhao et al.’s results.43 While the carboxyl groups were covalently attached at the sidewall or vacancy defect sites, a band gap opened up between the conduction and valence bands and induced impurity states near the Fermi level, which were also observed in a semiconductor CNT.44 In the case of two atom-vacancies and two chemical groups (-COOH and -CH2OH), the band structures for either the valence or conduction bands are significantly changed by the functionalization in the armchair (8, 8) tube, most importantly, the expanded band gaps were clearly observed in both the armchair and the zigzag tubes. This means that high density of chemical groups combined with more atom-vacancies would greatly change the electronic structure of the SWCNTs, resulting in the totally different electronic and optical characteristics from that of the pristine SWCNTs. It was supposed that the impurity states would act as strong scattering center for exciton around the Fermi level and thus significantly affect the transition between the excited-state and the equilibrium state. That might be why the optical properties of functionalized MWCNTs produced by flame exhibit different characteristics from that of the MWCNTs with high crystallinity. However, the facts might be much more complicated because the vacancy defects may impenetrate several layers of tube (shown in Figure 3b) and there may have several kinds of functional groups absorbed at each defect area. Based on these facts, a simple model was proposed to illustrate the observed PL mechanism, as shown in Figure 5. In our model the localized surface centers have lots of impurity states induced energy levels with much larger band gap than pure MWCNTs, as shown in Figure 5b. This suggests that the observed broad peak be related to a series of induced impurity energy levels of the fluorophors. In the temperature range between 30 and 240 K, the influence of band gap shrinkage was totally counteracted by the localization effect, which resulting in independent PL emission at the temperatures. When measuring at 240 K, most of the excited electrons returned to the impurity energy levels via a nonradiative transition leading to PL enhancement. At the same time, the functionalized carbon fluorophors staying at metastable state became thermally unstable at higher temperature and were destroyed by the laser with high power density during the measurements, leading to quenching of PL. After that, the C dangling bonds and broken >CdO or -C-O could be modified and recovered again in oxidizing atmosphere and then the fluorophors were reorganized resulting in recovery of PL.
Acknowledgment. The authors would like to thank Dr. Xiaohong Tang and Dr. Zongyou Yin for assistance with the photoluminescence measurements. This work was jointly supported by the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (FANEDD No. 200233.) as well as the Center for Advanced Bionanosystems of Nanyang Technological University, Singapore. Supporting Information Available: SEM and HRTEM of MWCNTs, signal from a blank substrate, more control experiments about PL from MWCNTs, and table of temperature for vapor pressure. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Jorio, A.; Saito, R.; Hertel, T.; Weisman, R. B.; Dresselhaus, G.; Dresselhaus, M. S. MRS Bull. 2004, 29, 276. (2) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J. P.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593. (3) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley, R. E.; Weisman, R. B. Science 2002, 298, 2361. (4) Lefebvre, J.; Homma, Y.; Finnie, P. Phys. ReV. Lett. 2003, 90, 217401. (5) Lefebvre, J.; Fraser, J. M.; Homma, Y.; Finnie, P. Appl. Phys., A 2004, 78, 1107. (6) Lefebvre, J.; Finnie, P.; Homma, Y. Phys. ReV. B 2004, 70, 045419. (7) Capaz, R. B.; Spataru, C. D.; Tangney, P.; Cohen, M. L.; Louie, S. G. Phys. ReV. Lett. 2005, 94, 036801. (8) Karaiskaj, D.; Engtrakul, C.; McDonald, T.; Heben, M. J.; Mascarenhas, A. Phys. ReV. Lett. 2006, 96, 106805. (9) Finnie, P.; Homma, Y.; Lefebvre, J. Phys. ReV. Lett. 2005, 94, 247401. (10) Matsuda, K.; Kanemitsu, Y.; Irie, K.; Saiki, T.; Someya, T.; Miyauchi, Y.; Maruyama, S. Appl. Phys. Lett. 2005, 86, 123116. (11) Milkie, D. E.; Staii, C.; Paulson, S.; Hindman, E.; Johnson, A. T.; Kikkawa, J. M. Nano Lett. 2005, 5, 1135. (12) Riggs, J. E.; Guo, Z. X.; Carroll, D. L.; Sun, Y. P. J. Am. Chem. Soc. 2000, 122, 5879. (13) Brennan, M. E.; Coleman, J. N.; Drury, A.; Lahr, B.; Kobayashi, T.; Blau, W. J. Opt. Lett. 2003, 28, 266. (14) Dickey, E. C.; Grimes, C. A.; Jain, M. K.; Ong, K. G.; Qian, D.; Kichambare, P. D.; Andrews, R.; Jacques, D. Appl. Phys. Lett. 2001, 79, 4022. (15) Sun, W. X.; Huang, Z. P.; Zhang, L.; Zhu, J. Spectrosc. Spectral Anal. 2005, 25, 10. (16) Zhang, Y.; Gong, T.; Liu, W. J.; Zhang, X. F.; Chang, J. G.; Wang, K. L.; Wu, D. H. Appl. Phys. Lett. 2005, 87, 173114. (17) Cai, X. Y.; Akita, S.; Nakayama, Y. Thin Solid Films 2004, 46465, 364. (18) Coratger, R.; Salvetat, J. P.; Carladous, A.; Ajustron, F.; Beauvillain, J.; Bonard, J. M.; Forro, L. Eur. Phys. J. 2001, 15, 177. (19) Uemura, T.; Yamaguchi, S.; Akai-Kasaya, M.; Saito, A.; Aono, M.; Kuwahara, Y. Surf. Sci. 2006, 600, L15. (20) Bonard, J. M.; Stockli, T.; Maier, F.; de Heer, W. A.; Chatelain, A.; Salvetat, J. P.; Forro, L. Phys. ReV. Lett. 1998, 81, 1441. (21) Liu, Y. L.; Fu, Q.; Pan, C. X. Carbon 2005, 43, 2264. (22) Bao, Q. L.; Pan, C. X. Nanotechnology 2006, 17, 1016. (23) Liu, Y. L.; Pan, C. X.; Wang, J. B. J. Mater. Sci. 2004, 39, 1091. (24) Pan, C. X.; Liu, Y. L.; Cao, F.; Wang, J. B.; Ren, Y. Y. Micron 2004, 35, 461. (25) Kohn, W.; Sham, L. J. Phys. ReV. 1965, 140, A1133.
10352 J. Phys. Chem. C, Vol. 111, No. 28, 2007 (26) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244. (27) Banks, C. E.; Davies, T. J.; Wildgoose, G. G.; Compton, R. G. Chem. Commun. 2005, 7, 829. (28) Robinson, J. A.; Snow, E. S.; Badescu, S. C.; Reinecke, T. L.; Perkins, F. K. Nano Lett. 2006, 6, 1747. (29) Zhang, J.; Zou, H. L.; Qing, Q.; Yang, Y. L.; Li, Q. W.; Liu, Z. F.; Guo, X. Y.; Du, Z. L. J. Phys. Chem. B 2003, 107, 3712. (30) Strano, M. S.; Huffman, C. B.; Moore, V. C.; O’Connell, M. J.; Haroz, E. H.; Hubbard, J.; Miller, M.; Rialon, K.; Kittrell, C.; Ramesh, S.; Hauge, R. H.; Smalley, R. E. J. Phys. Chem. B 2003, 107, 6979. (31) Dukovic, G.; White, B. E.; Zhou, Z. Y.; Wang, F.; Jockusch, S.; Steigerwald, M. L.; Heinz, T. F.; Friesner, R. A.; Turro, N. J.; Brus, L. E. J. Am. Chem. Soc. 2004, 126, 15269. (32) Wang, X. H.; Liu, H. W.; Jin, Y.; Chen, C. H. J. Phys. Chem. B 2006, 110, 10236. (33) Mawhinney, D. B.; Naumenko, V.; Kuznetsova, A.; Yates, J. T.; Liu, J.; Smalley, R. E. J. Am. Chem. Soc. 2000, 122, 2383.
Bao et al. (34) Lee, S. M.; Lee, Y. H.; Hwang, Y. G.; Hahn, J. R.; Kang, H. Phys. ReV. Lett. 1999, 82, 217. (35) Gole, J. L.; Dudel, F. P.; Grantier, D.; Dixon, D. A. Phys. ReV. B 1997, 56, 2137. (36) Iakoubovskii, K.; Minami, N.; Kim, Y.; Miyashita, K.; Kazaoui, S.; Nalini, B. Appl. Phys. Lett. 2006, 89, 173108. (37) Prokes, S. M.; Glembocki, O. J. Phys. ReV. B 1994, 49, 2238. (38) Park, H.; Zhao, J.; Lu, J. P. Nano Lett. 2006, 6, 916. (39) Kovtyukhova, N. I.; Mallouk, T. E.; Pan, L.; Dickey, E. C. J. Am. Chem. Soc. 2003, 125, 9761. (40) Ma, J.; Guan, S. G.; Lai, C. H. Phys. ReV. B 2006, 74, 205401. (41) D’Yachkov, P. N.; Makaev, D. V. Phys. ReV. B 2006, 74, 155442. (42) Wang, C. C.; Zhou, G.; Liu, H. T.; Wu, J.; Qiu, Y.; Gu, B. L.; Duan, W. H. J. Phys. Chem. B 2006, 110, 10266. (43) Zhao, J.; Park, H.; Han, J.; Lu, J. P. J. Phys. Chem. B 2004, 108, 4227. (44) Wang, C. C.; Zhou, G.; Wu, J.; Gu, B. L.; Duan, W. H. Appl. Phys. Lett. 2006, 89, 173130.
