Energetic comparison of double-walled carbon nanotube systems

Shalayna Lee Lair, University of Texas at El Paso

Abstract

Controlled fabrication of specific size and type nanotubes would lay the foundation for production of many novel carbon nanotube (CNT) devices. It is well known that changes in the structure of CNTs result in dramatic variations in their material properties and because many devices take advantage of this structure-property paradigm, successful fabrication of these devices depends directly on the ability to control the structure of CNTs. While many researchers have concentrated on varying production parameters, manufacturing methods or focusing on purification methods to control structure, it may be more productive to understand the basic energetics of nucleation and growth of CNTs. The objective of this research is to systematically study and classify electronic energy trends in double-walled carbon nanotubes (DWCNTs) through ab initio simulations. Understanding if there is a preferred structural motif for DWCNTs and also clarifying which nucleation and growth paths are favored by nanotubes will elucidate if controlled fabrication can be achieved. ^ Previous work by the author has shown how the energetics of single-walled carbon nanotubes (SWCNTs) behave as the length and circumference of the tube changes. The SWCNT work will be used in combination with computational simulations of DWCNTs to obtain a detailed description of the electronic energies for these nanotubes. A variety of configurations will be examined in order to help understand the growth behavior of DWCNTs, and different combinations of type, length and circumference will be studied to establish criteria for high stability systems. In order to confirm the validity of the results a variety of different models and quantum mechanical basis sets will be used to evaluate the energies and comparisons of accuracy and computational time will be made. Finally, this project not only encompasses understanding the electronic energies of DWCNTs but the theoretical methodology developed in this research will be used to examine questions related to tube growth and nucleation of multi-wall carbon nanotubes (MWCNTs). ^ The major findings and contributions of this work are: (1) As the length of DWCNTs increases the atomic energy is constant for capped tubes, and decreases for uncapped tubes. (2) As the circumference of both tubes increases the stability increases for capped tubes and decreases for uncapped tubes. (3) Nucleation appears to be most favorable from two nested end caps, although there is evidence for other possible nucleation pathways. (4) Growth of uncapped or single capped tubes occurs because of an increase in stability; there is a threshold atomic energy occurring at approximately -37 a.u./atom. When this threshold is reached the second end cap would form. Once all ends are capped there is energetically little drive for growth. (5) An IWS of 3.3 Å to 3.5 Å is preferred for uncapped tubes. (6) The favored structural pairing for capped tubes is the combination of an armchair inner tube and armchair outer tube (A//A), for uncapped tubes the combination of an armchair inner tube and zigzag outer tube (Z//A) is energetically more stable. (7) A computational confirmation of the mechanisms of carbon nanotube growth proposed in 1995 by Guo, et al. [24]. ^

Subject Area

Engineering, Materials Science

Recommended Citation

Lair, Shalayna Lee, "Energetic comparison of double-walled carbon nanotube systems" (2007). ETD Collection for University of Texas, El Paso. AAI3291005.
http://digitalcommons.utep.edu/dissertations/AAI3291005

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