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44009-G10
Electronic Structure Studies of Adsorption on Defective and Deformed Carbon Nanotubes

Lilia M. Woods, University of South Florida

The research in the second year of this ACS-PRF Grant was directed towards understanding how different external stimuli affect the electronic structure properties of carbon nanotubes (CNT).  First, we study theoretically how CNT electronic structure properties change when mechanical alterations and/or external electric fields are present. Second, we develop a model of how the adsorption properties of defective and deformed carbon nanotubes are modified as compared to those of perfect nanotubes. For this grant we performed first principle calculations based on density functional theory (DFT) implemented in the VASP code.         

In our studies we show additional ways to make fundamental changes in the CNT structure and related properties in order to achieve greater experimental and theoretical versatility of these nanostructured materials. We investigated how CNT electronic structure properties can be modified when the tube is radially deformed, a mechanical defect is created and an external electric field is applied.         

Radially Deformed CNT under Transverse External Electric Fields

We consider semiconducting and metallic single wall CNT with various degrees of radial deformation placed under an external transverse to the CNT cross-section electric field – TOC. The calculations are analyzed in terms of energy band gap changes, electronic structure changes and charge density plots in terms of degree of radial deformation, strength and direction of the electric field and type of CNT. If nanotubes are deformed, significant changes in the CNT properties can be achieved simply by changing the direction of the applied electric field. For example, the energy band gap for a slightly deformed semiconducting CNT can be reduced by a factor of two simply by switching E_y to E_x – TOC. At the same time, for deformed metallic CNT smaller fields along y can change the electronic structure significantly, while fields along x axis affect very little or not at all their band structure [1, 2].               

Electronic Structure Modulations of Radially Deformed and Defective CNT

Further, we calculate the electronic structure of single wall CNT under two types of external mechanical stimuli – radial deformation and an isolated defect. The particular way of altering the tube corresponds to first squashing it radially and then creating a defect on its highest curvature region. Two deformations are investigated – squashing between smaller than the tube’s cross-section hard walls and squashing between larger than the tube’s cross section hard walls. Three types of defects are studied – a Stone-Wall (SW) created by rotating on C-C bond, a N impurity by substituting one C atom by a N atom, and a mono-vacancy (MV) by removing one C atom.

Our calculations show that in general it is easier to create a defect in the carbon network for nanotubes with larger degree of applied radial strain. In addition, if the defect involves larger disturbance in the CNT structure, such as creating a vacancy, more energy is needed to make this defect, as compared to the cases when this involves less disturbance, such as creating a C-C rotated bond – Nugget.

We also find that various defects may induce different magnetic properties in the deformed CNT. More specifically, for the tube with smaller h and a N impurity, the calculations show that the total DOS for spin “up” and “down” are different, and the extra N e is relatively localized. For larger h, however, practically no spin-polarized effects are found and the extra N e is not localized any more. At the same time, for the CNT with large h and a mono-vacancy spin-polarization effects were found [3].           

Adsorption on Defective and Deformed CNT

The final stage of this grant involves electronic structure studies of adsorption on the defective and deformed nanotubes. Currently calculations are underway of benzene adsorption on mechanically altered nanotubes. We are investigating how the type of radial squashing, degree of radial deformation, and type of defect present will influence the adsorption process. We are analyzing the adsorption energies, equilibrium distances and configurations, and electronic structure for the various cases.        Comparing with the previous studies of aromatic molecule adsorption on perfect CNT [5] and the electronic structure of perfect and mechanically altered nanotube [2,3], we will be able to complete the theoretical modeling proposed in this grant. A regular research article with the final results will be submitted to the journal of Physical Review B.

Summary

1 research article – Journal of Physical Chemistry C, under review

1 research article – Carbon, submitted

1 research article – Physical Review B, under preparation

1 Conference Proceeding paper - published

References:
1. Y. V. Shtogun amd L.M. Woods, NSTI-Nanotech 2008, Vol. 1, p. 108;
2.  Y.V. Shtogun and L.M. Woods, J. of Phys. Chem. C, under review;

3. Y.V. Shtogun, L.M. Woods, and T.L. Reinecke, Carbon, submitted;

4. Y.V. Shtogun and L.M. Woods, Phys. Rev. B, to be submitted;

5. Y. V. Shtogun, L.M. Woods, and G.I Dovbeshko, Journal of Physical Chemistry C 111, 18174 (2007).

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