Author Chua, Hui Li
Title Ab initio study of low dimensional carbon nanostructures.

Academic exercise (B.Sc. Hons.)--National Institute of Education, Nanyang Technological University

Year 2005
Supervisor Xu, Shuyan
Call no. QD181.C1 Chu

Nano-size carbon-based structures are widely regarded as the most promising materials for nanotechnology applications. Recently, various types of low dimensional carbon nanostructures have attracted enormous research interests because of their boundless, technologically important applications, including electron field emission displays, molecular electronic device components, novel composite materials with unique, enhanced mechanical properties, hydrogen storage devices, bio-imaging markers and smart drug delivery.

In this work, a novel and unique class of carbon crystals is described and their characteristics are investigated. Self organized, single crystal (001) carbon nanotips with the dimensions of the sharp tips ranging around 10 - 20 nm have been discovered in our Plasma Sources and Application Centre (PSAC). These nano-size graphite layered, sharp-tip crystals are fundamentally different from all carbon nanostructures reported to date. Scanning Electron Microscopy (SEM) reveals high density, vertically-aligned sharp-tip carbon crystal structures oriented along the conventional z-direction. High resolution Transmission Electron Microscopy (HRTEM) shows that these tips are made up of continuous horizontal planes (up to several thousand layers) perpendicular to the growth direction (001). Sharp tip carbon nanostructures are ideal for electron field emission displays (FEDs).

State-of-the-art First Principles calculations based on the local density approximation to the density functional theory (DFT) are performed. A sophisticated package, DMol3, calculates variational self-consistent solutions to the DFT equations, is employed to study the geometrical structures and stabilities of the low dimensional, single crystal carbon nanotips. Based on the SEM, HRTEM and XRD measurements of the carbon nanotips, we proposed atomistic models of such structures.

These atomic structures were geometry optimized and total energy calculations confirmed the existence of such structures. Results showed that hydrogen termination at the periphery edges stabilizes the geometry of the entire molecular network. Molecular orbital analysis reveals that the carbon nanotips are very narrow band gap (of the order of 0.01 to 0.05 eV) semiconductors. Doping of Group 111 and V impurities, in particular, boron and nitrogen impurities into the atomic structure was found to change the structures' electronic properties, which is useful for making molecular devices.