Vibrational Properties of Disordered Carbon Nanotube

Abstract

Carbon nanotube (CNT) is considered as an ideal candidate for next generation nanoelectronics owing to unusual thermal, electrical, optical and other physical properties. It provides the opportunity to understand one dimensional (ID) physics. Due to strong electron phonon coupling in carbon-based nanomaterials, lattice vibrations have a significant effect on the electron transport properties in CNT. From a realistic point of view, defects such as atomic vacancies, adatoms, isotopes and impurities are very common during the synthesis of CNTs. Even small concentration of vacancy changes the phonon properties of CNT significantly. Many studies on the vibrational properties of CNT have been performed. However, these works are limited on pristine CNT only. Here I have performed an in-depth theoretical analysis of the effect of vacancy defects and curvature on phonon properties of (10,0) zigzag scmiconducting carbon nanotube (ZCNT) and (10,10) armchair metallic carbon nanotube (ACNT). In the first part of this work, a simple model to calculate the vibrational eigcnfrcqucncies and cigcnvectors for I D disordered systems is developed using the forced vibrational method, which is based on the mechanical resonance to extract the pure vibrational eigcnmodcs and suitable to treat very large and complex disorder physical system. This model is then used to study (10,0) ZCNT and (10,10) ACNT with different concentrations of atomic vacancies. This dissertation reports some unique and interesting lindings based on computational simulations on (10,0) and (10,10) CNTs both in low and high-frequency region. The correction of force constant parameters due to the curvature effect of CNTs lead to the Raman active E29 mode phonon peaks are at 1576 cn11 and 1581 cm' for (10,0) and (10,10) CNTs, respectively. A softening and shifting of the E2g mode towards the low-frequency region are observed with the increasing of vacancies and curvature of CNTs. For vacancy concentrations of 10% or higher, the E2g peak has been reduced into a shoulder or it has been completely disappeared. The other high symmetry point's peaks in the h igh- frequency region are also broadened and softened for both CNTs with the increase of defect density. Vacancy induces some new peaks at low-frequency region of phonon density of states. Due to scattering by atomic vacancies, the phonon wave function becomes localized in the real space. To investigate phonon localization effect with vacancy type defects and curvature of MI CNTs, the mode pattern and localization length for K point in-plane TO mode at Raman D-band frequency is calculated. Strong localization is observed with variation of vacancy concentration and curvature of CNTs. With increasing defect densities localization effect shows stronger confinement. The localization effect is stronger in (100) CNT than (10,10) CNT due to curvature effect. These findings show the significant impact of vacancy defects on the phonon properties that strongly affect the electron transport properties of CNT-based nanodevices. These findings will also open a route towards better understanding the thermal conductivity, specific heat capacity, electron phonon interaction, resistivity and superconductivity in disordered CNT.