Nanotubos

highlights
Chirality in nanotubes
DOI: 10.1002/smll.200500238

Controlling Nanotube Chirality and Crystallinity by Doping
Mauricio Terrones*
Keywords:
· carbon nanotubes · chirality · doping · nitrogen

urrently, researchers are demonstrating that carbon nanotubes can be used in the fabrication of numerous devices, such as field-emission sources, sensors, actuators, batteries, polymerfillers, to name but a few.[1] It is clear that some of these applications will result in commercial products very shortly. Since the industrial applications of nanotubes are developing rapidly, it is important to be able to produce carbon nanotubes (single- and multi-walled) in copious amounts, and furthermore, control their crystallinity and chirality (that is, the way in which the hexagonal ringsare oriented along the nanotube axis). Carbon nanotubes (both single- and multi-walled) can be produced by a wide variety of techniques, which include: 1) arc discharge between graphite electrodes, 2) chemical vapor deposition (CVD) using hydrocarbons and metal catalysts, 3) laser vaporization of graphite targets, 4) electrolysis of graphite electrodes in molten ionic salts, 5) solar vaporizationof carbon targets, and 6) ball-milling methods on carbon powders together with heat treatments.[1] Out of these techniques, CVD is the most efficient and it is capable of producing bulk amounts of multi-walled carbon nanotubes (that is, on the grams-per-hour scale). In addition, this method is economically viable. Unfortunately, this process is carried out at relatively low temperatures (650–9008C), and the resulting nanotubes usually exhibit low crystallinity, and may contain a large amount of defects within the hexagonal network. Therefore, CVD routes that are able to produce highly crystalline material at low temperatures need to be developed. In this context, the work by Windle and co-workers[2] reports an astonishing result, which describes the production of highly crystalline thickmulti-walled carbon nanotubes (MWNTs) with three-dimensional (3D) order (the concen-

C

tric cylinders adopt the crystal structure of perfect graphite). The authors produce these nanotubes using a CVD process involving the thermolysis of toluene/1,4-diazine (C7H8/ C4H4N2) solutions containing 2 wt % ferrocene (FeCp2) under an Ar atmosphere at 760 8C. Interestingly, these tubes contain 3 %nitrogen, but very differently to previous Ndoped nanotubes, they exhibit an extremely high crystallinity which is very similar to that of 3D graphite (Figure 1). These results break new ground in the nanotube field for two main reasons:[2] 1) MWNTs with high 3D order can now be produced at low temperature via CVD processes, and 2) N atoms under specific circumstances appear to con-

[*] Prof. M.Terrones Advanced Materials Department, IPICYT Camino a la Presa San JosØ 2055 Col. Lomas 4a. sección, San Luis Potosí 78216 (MØxico) Fax: (+ 52) 444-834-2039 E-mail: mterrones@ipicyt.edu.mx

Figure 1. TEM image of a highly crystalline MWNT doped with N, exhibiting 3D order among the concentric tubes. These tubes were produced by Windle and co-workers using CVD methods.[2] Note that the tubeexhibits some compartmentalization within the inner cylinders, which is caused by an increased concentration of N within the internal cylinders (scale bar = 100 nm).
small 2005, 1, No. 11, 1032 – 1034

1032

 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim

trol the nanotube chirality. This appears to be the first report on the production of highly crystalline MWNTs using nitrogen asa promoter. Previous reports have demonstrated that N-doped MWNTs exhibit increased disorder within layers, leading to a breakdown in the crystallinity of the nanotubes. These tubes tend to be metallic and always exhibit bamboo-like morphologies. Their compartmentalized shapes can be explained in terms of pyridine-type nitrogen atoms (N atoms bonded to two carbon atoms, which are responsible...