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44009-G10
Electronic Structure Studies of Adsorption on Defective and Deformed Carbon Nanotubes
The mechanical, electronic, and adsorption properties of carbon nanotubes (CNT) play crucial role in their integration into nano-technological devices. Applications such as gas storage, sensors, biocompatible agents, functionalized elements and more have been envisioned and/or demonstrated. The ability of CNTs to interact with chemical species combined with their high surface area is a key component in many of these applications.
Aromatic compounds interacting with CNTs are of particular interest. This is related to the great potential that CNTs have for biomedical research. For example, functionalization with aromatic compounds, DNA encapsulation, DNA conformal transformation, etc…has been demonstrated. These achievements show the versatility and promise of aromatic/CNT complexes. Therefore, a fundamental understanding of CNT interaction with aromatic molecules, which can appear as building blocks in large organic and biological systems, is necessary.
To achieve this goal, which is the goal of this grant, we have divided the work in several parts. These are related to the different factors determining the aromatics adsorption on CNTs. All of our studies are based on first principle calculations. These are related to: i) different aromatic compounds adsorbed on perfect CNTs; ii) investigating the combined role of defects and radial deformations on single walled CNTs; iii) aromatics adsorption on defective and deformed CNTs will be done; and iv) investigating the role of electric fields on the electronic structure of defective and deformed single wall nanotubes and aromatics adsorption.
For the first part, we have done first principle calculations within density functional theory (DFT) applying the local density approximation (LDA) for the exchange-correlation functional. The state-of –the-art VASP (Vienna Ab initio Simulation Package) code was used. We have determined that the adsorption process for aromatics molecules is governed by several important factors – the existence of a &pi ring in the molecule, the type of electron shell valence electronic structure (open or closed), charge transfer due to different functional groups in the molecules. We have examined two types of aromatics – simple benzene derived molecules (carbon ring) and DNA bases (nitrogeneous ring). We have determined that the most important one is the existence of the aromatic &pi ring, responsible for the stacking above the &pi carbon rings from the nanotubes themselves. For all structures, the adsorption is physisorption. In addition, we determined that additional higly localized peaks associated mainly with the open shell molecular orbitals can appear in the CNT electronic structure around the Fermi level. The role of each functional group is secondary resulting in a very small charge transfer in the system. Our model establishes that aromatic compounds can be easily immobilized on the walls of perfect CNTS and used for functionalizing perfect CNTs also. The transport properties of the perfect CNTs are not changed significantly upon aromatics compounds adsorption. The results from these calculations were presented in two papers – one published Physical Review B paper (cited) and one paper accepted for publication in Journal of Physical Chemistry C.
For the second part, we are in the process of finalizing results from first principle calculations also based on the DFT-LDA approach using the VASP code. We study the effect of different degrees of radial deformation, defects, and the combined influence of radial deformations with defects on the electronic structure of metallic and semiconducting single walled CNTs. The bulk of the work consists of performing many calculations for the different cases. Although we have access to the USF computer cluster, where VASP is installed and supported, time restrictions apply for users. Currently, we are analyzing the results from the calculations and writing a manuscript to present a model for the influence of the different mechanical alterations in relation to the electronic and transport properties of single walled CNTs.
For the third part, the bulk of the work will be done in the second year of the grant, since it is dependent on the finished calculations and results from the second part.
For the fourth part, we have done tight binding model calculations to study the electronic structure changes in single walled CNTs when different degrees of radial deformations and radially directed external electric fields are present. All calculations are done and we are in the process of finalizing a manuscript. We find that when CNTs are radially deformed and an external electric field is applied, additional ways of modulating the CNT electronic structures can be achieved. For example, it is possible to trigger a semiconductor to metal transition for a nanotube under radial deformation. However, at that specific deformation an external electric field can produce another transition back to semiconductor. In fact, we find that for different degrees of deformations, the electric field is capable of causing several such transitions as its strength is varied.
