Since the first report on a discovery of helical graphitic carbon microtubules [1], carbon nanotubes (CNTs) have received a great deal of worldwide attention from government, academia, industry and the investment community. Because of its high strength-to-weight ratio and superior electrical and thermal conductivities, CNTs and CNT-reinforced materials are considered to be one of the most promising novel materials suitable for a wide range of applications in nanotechnology from nanoelectronics and bioengineering to biotechnology. 

Defects in nanotubes can, however, significantly degrade the material properties [2-3] and affect the performance and reliability of nanotube-based devices. Thus, the defects must be understood and controlled before carbon nanotubes can be fully commercialized. 

Native Defects in Single-Walled Nanotubes (SWNTs) - Perfect carbon nanotubes are tubular carbon molecules in which a carbon atom is bonded to three carbon atoms to form a hexagonal network. Similar to graphite, the bonding between two carbon atoms, known as sp² bond, occurs when the 2s- and 2p-shells of each carbon atom are combined with the 2s- and 2p-shells of its neighboring atoms. 

Three types of native defects, which are formed during the CNT synthesis process, have been identified; (i) isolated point defects or vacancies, (ii) topological defects and (iii) sp²-sp³ hybridization defects.

Vacancies in the nanotube lattice, where carbon atoms are missing from their original positions, are formed as a result of incomplete bonding during the synthesis process. In diamond-like materials, the formation of vacancies and vacancy clusters depends upon the processing temperatures and material cooling rate. 

Thus far, vacancy clusters have been observed only in electron- and ion-irradiated carbon nanotubes [4]. From the calculations using the spin-polarized density functional theory, vacancies can change the electronic structure of the SWNTs from metallic to semiconducting nanotubes or vice-versa [5].

In contrast to a perfect hexagonal carbon lattice, topological defects are created as a result of pentagon and heptagon carbon lattice formation along the CNT side wall. A topological defect consisting of a pair of 2 pentagons and 2 heptagons is known as the Stone-Wales defect. Stone-Wales defects (SWDs) have a unique characteristic in adsorption of hydrogen and other foreign atoms and hence CNTs containing SWDs has a potential usage for energy storage. Hybridization defects are defects that are generated as a result of an interaction between carbon atoms and adsorbing hydrogen [6], which is readily available as a byproduct from hydrocarbon decomposition during the CNT synthesis process.

Structural Defects - During the nanotube synthesis process, individual carbon tubes can grow into several branches to form nanotube-nanotube junctions or several tubes in a bundle that split off and follow different directions [7]. Several types of such structural defects have been characterized based upon their appearance as a two-dimensional Y junction branch, T junction branch, H junction branch and three-dimensional multiple junctions or tree-like defects.

Although the defect formation mechanism is not well understood, it has been demonstrated that nanotubes with some types of junction branches can be synthesized in the laboratory [8]. Structural defects such as the T junction branch, which form the transition from metallic to semiconducting SWNT, could play a significant role in future nanoelectronics.

Extrinsic Defects - Extrinsic defects are impurities and contaminants originating from the gas source or those from the metallic catalysts used in the synthesis process. Impurities such as nitrogen, boron and potassium are intentionally doped into the carbon nanotubes to alter their electronic properties. 

Although the catalyst-free synthesis process developed by IBM (NYSE:IBM) eliminates metallic contaminations such as iron, nickel and cobalt [7], oxygen and hydrogen still remain as impurities present in the synthesized CNTs. Some research suggests that O₂ physisorbs on the nanotube wall and then migrates to the vacancy sites [9] while adsorbing hydrogen interacts with carbon atoms to form hybridization defects [6].

In nanoelectronics, doping with nitrogen or boron alters electronic properties of SWCTs and provides a means for tuning the field emission of nanotube emitters [10]. Nitrogen atoms, which favor the formation of pentagons and heptagons, causes a high-degree of distortion to the carbon lattice, resulting in wound C-N nanotubes.

It has been reported that N-type field effect transistors (CNTFETs) can be made by doping with an electropositive element such as potassium [10]. Potassium doping changes the carriers in the nanotube from holes to electrons, typical values on the order of ~100-1000 electrons/mm. 

REFERENCES

[1] S. Iijima, Nature, 354 (1991) 56-58.
[2] Chenyu Wei, Kyeongjae Cho, and Deepak Srivastava, Phys. Rev. B 67, 115407 (2003).
[3] Jianwei Che, Tahir Cagin, and W. A. Goddard, III, Thermal Conductivity of Carbon Nanotube, presented at The Seventh Foresight Conference on Molecular Nanotechnology, Santa Clara, CA. October 14-17, 1999.
[4] A. V. Krasheninnikov and K. Nordlund, Irradiation effects in carbon nanotubes, August 15, 2003, submitted for publication.
[5] Yuchen Ma, P. O. Lehtinen, A. S. Foster and R. M. Nieminen, Presented at International Conference on the Science and Application of Nanotubes, San Luis Potosí, S.L.P., México, July 19-24, 2004.
[6] K. Tada, S. Furuya and K. Watanabe, Phys. Rev. B 63, 155405 (2001).
[7] V. Derycke, R. Martel, M. Radosavljevi , F. M. Ross, and Ph. Avouris, Nano Letters 2, 1043(2002).
[8] Ping‘an Hu, Yunqi Liu, Xianbiao Wang, Biao Wangand Daoben Zhu, Presented at the 8th International Conference on ElectronicMaterials (IUMRS-ICEM 2002, Xi’an, China, 10–14 June 2002.
[9] S.M.Lee et al, Phys. Rev. Lett. 82, 217(1999).
[10] Nitrogen Doped Carbon Nanotube, Laboratory of Properties and Microstructures (LEPM), Swiss Federal Institute of Technology, Lausanne, Switzerland.
[11] M. Bockrath, J. Hone, A. Zettl, P.L. McEuen, A.G. Rinzler, R.E. Smalley, Phys. Rev. B 2000, 61, R10606.