Browsing by Author "Singh, Dilip Kumar"

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    Spectroscopic Studies of Diameter Dependent Properties, Defects and Defect Engineering in Single-Walled and Multi-Walled Carbon Nanotubes
    (2010) Singh, Dilip Kumar
    Carbon nanotubes (CNTs) are unique one-dimensional nanostructures with remarkable electronic, optical, mechanical and thermal properties. Since its discovery in 1991, CNTs have caught widespread attention of researchers worldwide for studies of fundamental properties as well as device applications. Carbon nanotubes were first thought of as perfect seamless cylindrical graphene sheets- a defect-free structure. However, with time and as more studies have been undertaken, it has emerged that nanotubes are not necessarily that perfect i.e. it may contains various kinds of topological defects. The issue is further complicated by the fact that the quality of a nanotube sample depends very much on the growth method such as commonly used chemical vapor deposition (CVD). As-grown nanotubes containing various forms of carbon and other impurities can be purified by various methods. However, often defects are created on the tube walls during growth as well as in the process of purification. Several exotic properties of single walled carbon nanotubes (SWCNTs) predicted by theory have been verified experimentally to a reasonable extent. However, presence and role of defects in the nanotube properties are yet to be understood properly. Detrimental effects of defects in CNTs have been recognized from studies of nanoelectronic devices fabricated from CNTs that often have very short lifetime and high failure rates. Defective nanotubes are known to be highly electron emitting and lifetime of these devices are too short for commercial applications. Defect induced enhanced conductivity and defect-enhanced dispersion of carbon nanotube in DNA solutions have been reported recently for multiwalled carbon nanotubes (MWCNTs). Thus, defects are expected to play a significant role in determining various functionalities of carbon nanotube based devices. A controlled spectroscopy, positron annihilation lifetime spectroscopy etc. Use of the sophisticated techniques like ultrahigh vacuum scanning tunneling microscopy (UHV-STM), high resolution transmission microscopy (HRTEM) for monitoring structural defects in CNTs are often time consuming and can not be used for the bulk characterization. In the literature, UHV-STM and aberration corrected low voltage HRTEM have been used recently to obtain evidence of defects and defect dynamics in nanotubes. However, nondestructive techniques like Raman spectroscopy, PL spectroscopy and ESR spectroscopy are not explored in the literature to study the presence and influence of structural defects in the structural and optical properties of carbon nanotubes. introduction of defects and nondestructive evaluation of defects in nanotubes are of immense significance for their potential applications in various devices. Understanding the influence of specific type of defects in the nanotube structure on the electronic and optical properties is of particular interest. During the growth of carbon nanotubes, the number and nature of defects is decided by various factors like temperature of growth, carbon source feeding rate, carrier gas flow rate, nature of carbon source being used etc. The identification of various kinds of defects and their methods of characterization is not established for carbon nanotubes using traditional methods like Raman spectroscopy, electron spin resonance (ESR), photoluminescen...................