| dc.description.abstract | Carbon nanotubes possess various unique and interesting properties. They have very high thermal and electrical conductivities, high stiffness, mechanical strength, and optical properties. Due to these properties, CNTs are widely used materials in a variety of fields. It is used for biotechnological and biomedical applications, as chemical and biosensor, in energy storage and field emission transistor. Experimentally synthesized CNTs are generally found in bundle form due to the strong vander Waals (vdW) at-traction between the individual tubes. To use CNTs in real life applications, we often require specific nanotubes with particular characteristics. The nanotube bundle is a mixture of various chirality, diameters and electronic properties (metallic and semiconducting). Only thermal energy is not sufficient to disperse nanotubes from the bundle geometry overcoming the strong vdW attraction between nanotubes. The hydrophobic and insoluble nature of CNTs in the aqueous medium makes the dispersion of CNTs even more difficult. So, it is a big challenge to get single pristine nanotube from the bundle geometry.
Many experimental and theoretical studies have addressed the problem of nanotube dispersion from the bundle geometry. Ultrasonic dispersing method is a widely used technique for this purpose where ultrasonic sound is applied to agitate particles in a system. Other methods include using different organic and inorganic solutions, various surfactant molecules, different polymers as dispersing agents. In this study we extend our e orts to develop some better methods and improved dispersing agents. In this thesis, we address the problem of CNT dispersion. To address this issue, we rst give a quantitative estimation of the effective interaction between nanotubes. Next, we introduce different polymers (ssDNA and dendrimers) as external agents and show that they help to overcome the strong adhesive interaction between CNTs and make nanotube dispersion possible from the bundle geometry. For all of the works presented in this thesis, we have used fully atomistic MD simulation and DFT level calculations. We study ssDNA-CNT complex using all-atom MD simulation and calculate various structural quantities to show the stability of ssDNA-CNT complex in aqueous medium. The adsorption of ssDNA bases on CNT surface is driven by - interaction between nucleic bases and CNT. Using the potential of mean forces (PMF) calculation, we study the binding strength of the polynucleotide ssDNA for poly A, T, G, and C with CNT of chirality (6,5). From the PMF calculation, we show the binding sequence to be A > T > C > G. Except for poly G, our result is in good agreement with earlier reported single molecule force spectroscopy results where the sequence of binding interaction was reported to be A > G > T > C. To explore how the interaction between two CNTs mod-i ed in presence of ssDNA between them, we perform PMF calculation between the two ssDNA-wrapped CNTs. The PMF shows the sequence of interaction strength between two ssDNA-wrapped CNTs for different nucleic bases to be T > A > C > G. Thus, from PMF calculations we show the poly T to have the highest dispersion efficiency, which is consistent with earlier reported experimental study. Our PMF calculation shows that poly C and poly G reduce the attraction between two CNTs drastically, whereas poly A and poly T make the interaction fully repulsive in nature. We also present microscopic pictures of the various binding conformations for ssDNA adsorbed on CNT surface.
We also study the dendrimer-CNT complex for both the PAMAM and PETIM dendrimers of different generations at various protonation states and present microscopic pictures of the complex. We calculate PMF between two dendrimer wrapped CNTs and show that protonated and higher generations (G3, G4, and so forth) non-protonated PAMAM dendrimers can be used as e ective agents to disperse CNTs from bundle geometry. We also study the chirality dependence of PMF respectively.
Finally, we study the interaction of mannose dendrimer with CNTs and show that the wrapping of mannose dendrimer can drive a metal to semiconducting transition in a metallic CNT. We attribute the carbon-carbon bond length assymetry in CNT due to the wrapping of mannose dendrimer as the reason for this band gap opening which leads to metal-semiconductor transition in CNT. Thus, the wrapping of mannose dendrimer on CNT can change its electronic properties and can be used in the band gap engineering of CNT in future nanotechnology.
Thus, the works carried out here in this dissertation will help to address the problem of nanotube dispersion from the bundle geometry which will in turn help to use CNT for various applications in diverse fields. | en_US |
