Langmuir 2005, 21, 8539-8545
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Controlled Chemical Functionalization of Multiwalled Carbon Nanotubes by Kiloelectronvolt Argon Ion Treatment and Air Exposure De-Quan Yang, Jean-Francois Rochette, and Edward Sacher* Regroupement Que´ be´ cois de Mate´ riaux de Pointe, De´ partement de Ge´ nie Physique, E Ä cole Polytechnique, C.P. 6079, succursale Centre-Ville, Montre´ al, Que´ bec H3C 3A7, Canada Received June 6, 2005. In Final Form: June 30, 2005 The chemical and morphological modifications of multiwalled carbon nanotubes (MWCNTs), by 2 keV Ar+ treatment, have been followed by field emission scanning (FESEM) and high-resolution transmission (HRTEM) electron microscopies and by X-ray photoelectron (XPS) and Raman spectroscopies. Morphological changes were followed, both in situ and on subsequent air exposure, and the data indicate that free radical defects, initially produced under low Ar+ treatment doses (∼1013 ions/cm2), act as the nuclei for the formation of localized asperities that form along the walls of the CNTs. Continued treatment results in their stublike elongation that continues with further treatment, forming extensions under heavy treatment doses. The chemical changes that occur, on reaction with air, reveal that the defects initially created are secondary C atoms, formed when a single bond breaks; further treatment breaks an additional bond to form primary C atoms; free radical fragments, lost when the third bond breaks, condense on the free radical defects to form the asperities. The extent of primary and secondary C atoms, and thus their functionalization on air exposure, may be controlled by the extent of treatment, offering a method for the controlled surface functionalization of CNTs by low-energy Ar+ treatment.
Introduction Carbon nanotubes (CNTs) have attracted interest since their discovery in 19911 because they exhibit fascinating electronic, thermal, and mechanical properties, by virtue of their unique molecular structure, geometrical confinement, and quantized electronic states.2 However, some structural and chemical modifications are necessary in order to incorporate them into mesoscale structures having predetermined orientations and locations. As well, the bonding of CNTs to other materials is carried out by chemical (e.g., the surface oxidation of the CNTs3) and physical (e.g., by energetic particle treatment) modifications: the treatment of CNTs by electrons4-13 and ions14-41 * Corresponding author: e-mail
[email protected]; tel (514) 340-4711, ext 4858; fax (514) 340-3218. (1) Iijima, S. Nature (London) 1991, 354, 56 (2) Carbon Nanotubes, Synthesis, Structures, Properties, and Applications; Dresselhaus, M. S., Dresselhaus, G., Avouris, Ph., Eds.; Springer: Berlin, 2001. (3) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; RodriguezMacias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (4) Terrones, M.; Terrones, H.; Banhart, F.; Charlier, J.-C.; Ajayan, P. M. Science 2000, 288, 1226. (5) Terrones, M.; Ajayan, P. M.; Banhart, F.; Blase, X.; Carroll, D. L.; Charlier, J. C.; Czerw, R.; Foley, B.; Grobert, N.; Kamalakaran, R.; Kohler-Redlich, P.; Ru¨hle, M.; Seeger, T.; Terrones, H. Appl. Phys. A 2002, 74, 355. (6) Terrones, M.; Banhart, F.; Grobert, N.; Charlier, J.-C.; Terrones, H.; Ajayan, P. Phys. Rev. Lett. 2002, 89, 075505. (7) Banhart, F. Rep. Prog. Phys. 1999, 62, 1181. (8) Smith, B. W.; Luzzi, D. E. J. Appl. Phys. 2001, 90, 3509. (9) Kiang, C.-H.; Goddard, W. A.; Beyers, R.; Bethune, D. S. J. Phys. Chem. 1996, 100, 3749. (10) Ajayan, P. M.; Ravikumar, V.; Charlier, J.-C. Phys. Rev. Lett. 1998, 81, 1437. (11) Salvetat, J. P.; Bonard, J. M.; Thomson, N. H.; Kulik, A. J.; Forro´, L.; Benoit, W.; Zuppiroli, L. Appl. Phys. A 1999, 69, 255. (12) Luzzi, D. E.; Smith, B. W. Carbon 2000, 38, 1751. (13) Li, J. X.; Banhart, F. Nano Lett. 2004, 4, 1143. (14) Mickelson, W.; Aloni, S.; Han, W. Q.; Cumings, J.; Zettl, A. Science 2003, 300, 467. (15) Zhu, Y.; Yi, T.; Zheng, B.; Cao, L. Appl. Surf. Sci. 1999, 137, 83.
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10.1021/la0514922 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/04/2005
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may cause the creation of defects,29 a reduction of CNT dimensions,15 and CNT coalescence4,5 and welding,6 as well as tunneling barrier formation.16 Further, ion beam treatment may cause the formation ofCNTsfromhighlyorientedpyrolyticgraphite(HOPG),35-37 the creation of links between nanotubes,22 the modulation of chemical and electrical properties,38 and the hardening of CNT thin films.39 It is also found that the sputtering behavior of CNTs is different than that of bulk materials,40,41 due to their spatial and localized electronic structures, similar to the case of metal nanoparticles.42 These studies suggested that the defects produced in CNTs by 50-3000 eV Ar+ treatment are metastable vacancies: free radicals that facilitate the welding of contacting CNTs to create molecular junctions.30 Surface chemical modifications (or functionalizations) of CNTs by energetic ions may be performed by hydrogen plasma treatment,43 reactive gases such as CF4,44 or treatment with 10-45 keV reactive molecular ions: as an example, it was recently shown34,38 that the treatment of CNTs by a focused Ga+ beam (FIB) could not only cause localized electrical property changes but also be used for the cutting, thinning, and welding of CNTs. This opens the possibility of using ion beams to locally modify CNTs so as to site-selectively alter the CNT structure and chemistry and create interconnected networks for device application.38 However, the controlled modification of CNTs by energetic particle beams has not yet been studied in great detail, particularly as to the physics and chemistry provoked by the beam. In this work, we present the results of our surface chemical and physical modifications of MWCNTs through 2 keV Ar+ treatment and their characterization by X-ray photoelectron (XPS) and Raman spectroscopies and field emission scanning (FESEM) and high-resolution transmission (HRTEM) electron microscopies. This work is part of our program on the search for methods for the mild, controlled functionalization of CNTs. Experimental Section Multiwalled carbon nanotubes (95% purity, diameter 20-30 nm, length 1-5 µm) were obtained from Nano-Lab, Brighton, MA. They were dispersed in 18 MΩ deionized water and sonicated for a few minutes before being deposited onto 15 mm square pieces of Au-coated Si wafer (10 nm Ti, followed by 200 nm Au, both by electron beam evaporation) by dropping and drying. The samples were left under mechanical vacuum, at room temperature, for at least 24 h. Such CNT samples did not wet the Au well, forming films with cracks, through which the Au was visible. Ar+ beam treatment was carried out in the sample preparation chamber of our VG Escalab 3 Mark II instrument, with the VG EX03 ion gun; 2 keV ions were used, with the angle between the beam and sample surface set at 33°. The beam current density was ∼ 1 × 1013 ions/cm2‚s, at an Ar gas pressure of 5 × 10-6 Torr, and could be changed by changing the Ar gas pressure, the beam focus, the beam energy, and/or the filament current. A special sample holder was used, containing a hole under where the beam strikes the sample, to avoid ion backscatter. (39) Foerster, C. E.; Garcia, I. T. S.; Zawislak, F. C.; Serbena, F. C.; Lepienski, C. M.; Schreiner, W. H.; Abbate, M. Thin Solid Films, 2002, 411, 256. (40) Krasheninnikov, A. V.; Nordlund, K. Nucl. Instrum. Methods B 2004, 216, 355. (41) Pomoell, J. A. V.; Krasheninnikov, A. V.; Nordlund, K.; Keinonen, J. J. Appl. Phys. 2004, 96, 2864. (42) Yang, D.-Q.; Piyakis, K. N.; Sacher, E. Surf. Sci. 2003, 536, 67. (43) Khare, B. N.; Meyyappan, M.; Kralj, J.; Wilhite, P.; Sisay, M.; Imanaka, H.; Koehne, J.; Baushchilcher, C. W. Appl. Phys. Lett. 2002, 81, 5237. (44) Ni, B.; Andrews, R.; Jacques, D.; Qian, D.; Wijesundara, M. B. J.; Choi, Y.; Hanley, L.; Sinnott, S. B. J. Phys. Chem. B 2001, 105, 12719.
Yang et al. XPS was carried out in the analysis chamber of the VG Escalab 3 Mark II, using nonmonochromated Mg KR X-rays (1253.6 eV) and a base pressure of < 10-10 Torr. High-resolution spectra were obtained at a perpendicular takeoff angle, with a pass energy of 20 eV and 0.05 eV steps. The instrument resolution was ∼0.7 eV. After Shirley background removal, the component peaks were separated by the VG Avantage V1.62 program (Thermo VG Scientific). Peak positions, shapes, and widths were those previously used for studying both HOPG and MWCNTs, and the program was instructed to iterate fits until a peak/fit convergence of
