referidas en estas orientaciones
1. FASE DEL RECORDAR
[2]rotaxane 20 and polymer functionalized with azide groups.
1.2 Single Walled Carbon Nanotubes (SWNTs)
Carbon nanotubes (CNTs) are an allotropic form of carbon, like diamond, graphite, or the fullerenes. Diamond is composed of four-coordinate sp3 carbon
atoms that form a three-dimensional network. In contrast, graphite has three- coordinate sp2 carbon atoms forming two-dimensional sheets constituted by
hexagonal rings. Graphite forms a three-dimensional structure due to the stacking of the two-dimensional sheets through van der Waals interactions. Fullerene C60, discovered by Kroto et al.23 in 1985, is a spherical cage due to the
addition of pentagonal rings, which break the planarity of the hexagonal sheets of the graphitic structure. The ability to obtain C60 in gram scale, together the
promising results in photovoltaic applications of fullerene derivatives, produced a great interest in the research community. In 1991, Ijima24 observed the
formation of nanotubules of graphite during an arc-discharge experiment, similar to the synthesis of fullerenes. These nanotubes are formed by rolled graphitic
23. H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl and R. E. Smalley, Nature, 1985, 318, 162-163. 24. S. Iijima, Nature, 1991, 354, 56-58.
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sheets, which have a concentric cylinders structure. Multi-walled carbon nanotubes (MWNTs), have a centric nanotube with ca. 1 nm diameter covered by graphitic cylindrical layers separated by ~ 3.4 Å. Single walled carbon nanotubes (SWNTs) were simultaneously discovered by Ijima25 and Bethune,26
in 1993. In 2004, A. K. Geim and K. S. Novoselov isolated and characterized a single layer of graphite, graphene,27, 28 which raised great interest in the scientific
community due to its outstanding electronic properties.
Figure 6. Chemical models of: a) Fullerene C60, b) SWNT and c) Graphene sheet.
Conceptually SWNTs are the result of rolling up a graphene sheet. Depending on the angle with which the graphene sheet is rolled, there is a great variety of SWNTs with different diameters, electronic behavior and chiralities. The crystal lattice of a SWNT is defined by an n and m chiral index. Depending on the value of the chiral indices (n, m), the SWNTs are classified in three groups: zig-zag nanotubes (m = 0), armchair nanotubes (n = m) and chiral nanotubes (n ≠ m ≠ 0). SWNTs have a typical diameter of 1 – 2 nm and length of several micrometers. The large aspect ratio (typically ca. 300 – 1000) makes SWNTs a quasi-1D material, which exhibit high flexibility29 and low mass density.30
25. S. Iijima and T. Ichihashi, Nature, 1993, 363, 603-605.
26. D. S. Bethune, C. H. Klang, M. S. de Vries, G. Gorman, R. Savoy, J. Vázquez and R. Beyers, Nature, 605- 607.
27. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666.
28. A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183-191.
29. C. A. Cooper, R. J. Young and M. Halsall, Composites Part A, 2001, 32, 401-411. 30. G. Guanghua, Ç. Tahir and A. G. William, III, Nanotechnology, 1998, 9, 184.
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Figure 7. Left: schematic of a two-dimensional graphene sheet illustrating lattice vectors a1 and a2, and the
roll-up vector Ch = na1 + ma2. The limiting cases of (n,0) zigzag and (n,n) armchair tubes are indicated with
red arrows. Right: models of zigzag, armchair and chiral nanotubes
Because of the strength of the C–C double bond, SWNTs have great mechanical properties showing a Young’s modulus up to 1.25 TPa measured in SWNT ropes.31,32 This value is one order of magnitude higher than steel’s.
Moreover, the electronic properties of SWNTs are also remarkable. Depending only on their diameter and/or chilarity, SWNTs can be either metallic or semiconductors without the necessity of any doping.33,34,35 Finally, the
combination of the high aspect ratio, sharp geometry, high chemical stability and mechanical strength, make SWNTs suitable to experiment the field emission of electrons. This phenomenon consists on the emission of electrons through a solid surface of the material, when a high electric field (ca. 107 V cm-1) with a negative
electrical potential is applied.36
SWNTs are considered one of the most promising building blocks for future nanoelectronic technology due to the extraordinary properties showed above. Currently, they have already found application in many different fields. To illustrate it, here were enumerate some of their applications: i) as electrodes in electrochemical devices such as supercapacitors37,38; ii) as field emission
31. A. Krishnan, E. Dujardin, T. W. Ebbesen, P. N. Yianilos and M. M. J. Treacy, Phys. Rev. B, 1998, 58, 14013-14019.
32. J.-P. Salvetat, G. A. D. Briggs, J.-M. Bonard, R. R. Bacsa, A. J. Kulik, T. Stöckli, N. A. Burnham and L. Forró, Phys. Rev. Lett., 1999, 82, 944-947.
33. J. W. Mintmire, B. I. Dunlap and C. T. White, Phys. Rev. Lett., 1992, 68, 631-634. 34. M. Ouyang, J.-L. Huang, C. L. Cheung and C. M. Lieber, Science, 2001, 292, 702. 35. M. Ouyang, J.-L. Huang and C. M. Lieber, Acc. Chem. Res., 2002, 35, 1018-1025. 36. Y. Saito and S. Uemura, Carbon, 2000, 38, 169-182.
37. K. H. An, W. S. Kim, Y. S. Park, J. M. Moon, D. J. Bae, S. C. Lim, Y. S. Lee and Y. H. Lee, Adv. Funct.
Mater., 2001, 11, 387-392.
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electron sources39 for flat panel displays;40,41 iii) as components in nanometer
size electronic devices such as nanotube field effect transistors (NT-FETs);42 iv)
as active materials in chemical sensor applications;43 v) as components in
polymer composites;44,45, vi) as drug delivery46 systems and medical
nanorobots.47
Figure 8. Left: TV screen based on carbon nanotubes OLED.48 Middle: Photograph of a collection of SWNT
transistors and circuits on a thin sheet of plastic (PI).49 Right: CNT sheets and yarns used as lightweight data
cables and electromagnetic shielding material.50
1.3 Chemical Functionalization of SWNTs
Carbon nanotubes present outstanding physical properties, but their synthesis leads to complex mixtures of carbon nanotubes with different diameters, chiralities and lengths, which make them less suitable for real applications. Currently, many research efforts are focused not only in the synthesis51,52,53 of
39. W. A. de Heer, A. Châtelain and D. Ugarte, Science, 1995, 270, 1179.
40. N. S. Lee, D. S. Chung, I. T. Han, J. H. Kang, Y. S. Choi, H. Y. Kim, S. H. Park, Y. W. Jin, W. K. Yi, M. J. Yun, J. E. Jung, C. J. Lee, J. H. You, S. H. Jo, C. G. Lee and J. M. Kim, Diamond Relat. Mater., 2001, 10, 265-270.
41. M. A. McCarthy, B. Liu, E. P. Donoghue, I. Kravchenko, D. Y. Kim, F. So and A. G. Rinzler, Science, 2011, 332, 570.
42. S. J. Tans, A. R. M. Verschueren and C. Dekker, Nature, 1998, 393, 49-52.
43. J. Kong, N. R. Franklin, C. Zhou, M. G. Chapline, S. Peng, K. Cho and H. Dai, Science, 2000, 287, 622- 625.
44. P. M. Ajayan, O. Stephan, C. Colliex and D. Trauth, Science, 1994, 265, 1212. 45. O. Breuer and U. Sundararaj, Polym. Compos., 2004, 25, 630-645.
46. Z. Liu, X. Sun, N. Nakayama-Ratchford and H. Dai, ACS Nano, 2007, 1, 50-56.
47. A. M. Popov, Y. E. Lozovik, S. Fiorito and L. Yahia, Int J Nanomedicine, 2007, 2, 361-372. 48. Patent US7473930,Use of patterned CNT arrays for display purposes.
49. Q. Cao, H.-s. Kim, N. Pimparkar, J. P. Kulkarni, C. Wang, M. Shim, K. Roy, M. A. Alam and J. A. Rogers,
Nature, 2008, 454, 495-500.
50. M. F. L. De Volder, S. H. Tawfick, R. H. Baughman and A. J. Hart, Science, 2013, 339, 535-539. 51. H. Dai, Acc. Chem. Res., 2002, 35, 1035-1044.
52. S. M. Bachilo, L. Balzano, J. E. Herrera, F. Pompeo, D. E. Resasco and R. B. Weisman, J. Am. Chem. Soc., 2003, 125, 11186-11187.
53. A. R. Harutyunyan, G. Chen, T. M. Paronyan, E. M. Pigos, O. A. Kuznetsov, K. Hewaparakrama, S. M. Kim, D. Zakharov, E. A. Stach and G. U. Sumanasekera, Science, 2009, 326, 116.
37 carbon nanotubes and their purification though physical methods,54,55,56 but also
in the chemical functionalization of SWNTs covalent57 or non-covalently58 to
purify59 the complex mixture of them or to modulate their interesting properties.
Besides the functionalization of the outer surface of the SWNTs (exohedral functionalization), the endohedral functionalization of SWNTs is an alternative that is also being extensively studied.60,61
1.3.1 Covalent Functionalization
The covalent modification of SWNTs involves the rupture or saturation of a C–C double bond of the scaffold of the SWNTs to form at least a new strong covalent bond between the nanotube and the molecule with which we are functionalizing it. As consequence, the products obtained have high stability, but the native properties of the pristine material change. There are two approximations to functionalize SWNTs covalently: i) amidation or esterification of carbon nanotubes previously oxidized; ii) addition reactions to the sidewalls of the SWNTs. The covalent functionalization of carbon nanotubes is a very extensive field, we have chosen to illustrate it with a few selected examples.
Amidation or esterification strategies must be carried out with pre-oxidized SWNTs. Many methods of oxidation of carbon nanotubes have been described using mixtures of H2SO4: HNO3 or H2SO4: H2O2 with different proportions.62,63
These oxidizing treatments endow SWNTs with oxygenated functional groups, a significant part of which are carboxyilic acids, which can be activated through chlorides or carbodiimide intermediates to promote the coupling with amines or alcohols. To solubilize SWNTs, Haddon and coworkers functionalized shortened SWNTs through amidation with octadecylamine to obtain the
54. R. Krupke, F. Hennrich, H. v. Löhneysen and M. M. Kappes, Science, 2003, 301, 344. 55. M. S. Arnold, S. I. Stupp and M. C. Hersam, Nano Lett., 2005, 5, 713-718.
56. H. Liu, T. Tanaka, Y. Urabe and H. Kataura, Nano Lett., 2013, 13, 1996-2003. 57. S. Banerjee, T. Hemraj-Benny and S. S. Wong, Adv. Mater., 2005, 17, 17-29. 58. Y.-L. Zhao and J. F. Stoddart, Acc. Chem. Res., 2009, 42, 1161-1171. 59. X. Tu, S. Manohar, A. Jagota and M. Zheng, Nature, 2009, 460, 250-253. 60. A. Hirsch, Angew. Chem. Int. Ed., 2002, 41, 1853-1859.
61. A. de Juan and E. M. Pérez, Nanoscale, 2013, 5, 7141-7148.
62. Z. Chen, K. Kobashi, U. Rauwald, R. Booker, H. Fan, W.-F. Hwang and J. M. Tour, J. Am. Chem. Soc., 2006, 128, 10568-10571.
63. K. Flavin, I. Kopf, E. Del Canto, C. Navio, C. Bittencourt and S. Giordani, J. Mater. Chem., 2011, 21, 17881-17887.
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