Properties of Carbon Nanotubes under External Factors

Carbon nanotubes are quasi-one dimensional nanostructures with many unique properties. These materials are considered as promising building blocks in nanoelectronics, spintronics, and nano-electromechanics. Potential applications of carbon nanotubes have strongly relied on their extraordinary ability of property changes upon external factors, such as mechanical alterations or applied external fields. The success in application of carbon nanotubes has significantly depended on predictions from theoretical calculations and modeling. In particular, first principal simulation methods have proven to be useful techniques in predicting the fundamental nature of changes in carbon nanotube properties and how these properties can be altered under external factors. There is a considerable amount of research on simulations of carbon nanotubes under one type of external factors, such as mechanical defects, mechanical deformations, applied external electric fields, or applied external magnetic fields. However, the development and design of current and new carbon nanotube based nanodevices has led to the search of additional ways to modify and tailor their properties. To achieve this flexibility, researchers have begun to study the influence of more than one external factor simultaneously. This is a relatively new area which opens up more possibilities for new devices. Thus, the knowledge of physical property changes in carbon nanotubes under several external factors is crucial for fundamental science and technological prospects. Our primary focus in this chapter is first principal simulations of electronic and magnetic properties of single wall carbon nanotubes under two external factors. The discussion involves understanding the combined effect of radial deformation and mechanical defects, such as a Stone-Wales defect, nitrogen impurity, and mono-vacancy in the carbon network. Furthermore, we will discuss the nanotube property changes due to the application of radial deformation and external electric field. To provide the basis for further arguments, we start with a review of important results of carbon nanotubes under one external factor such as mechanical deformations, mechanical defects, or external fields. After we provide the description of the calculation method and model, we arrive at the major issue of radially deformed carbon nanotubes with defects or external electric field. We start with the analysis of the structural changes in defective carbon nanotubes under two types of radial deformations. One is deformation achieved by squeezing of the nanotube between two hard 3