Computational predictions of properties and applications of unique carbon nanotube configurations

Carbon nanotubes are thin cylindrical molecular tubes with nanometer length scale cross-sections. Their unique shape and nanoscale dimensions give them extraordinarily interesting mechanical, chemical and electrical properties that can be exploited to create unique useful devices. In this work, we examine different configurations of carbon nanotubes and predict their properties and applications through accurate atomic-level classical and quantum mechanical computer simulation techniques.; Firstly, we examined in detail the configuration changes in single wall carbon nanotubes as the diameter was increased. We found that for diameters over 4 nanometers, van der Waal attraction forces cause single wall carbon nanotubes to energetically prefer a flattened cross-section over a round one. This flattened nanotube structure can be dynamically transitioned into a round nanotube structure by application of a static electric charge greater than .4 electrons/atom on the nanotube walls. Furthermore, we found that the introduction of a substrate reduced the diameter requirement for stable flattened nanotubes to only 2.6 nanometers. However, we also calculated that these results applied only to perfectly straight nanotubes and that the introduction of small bends along the nanotube axis created a structure which prefers the round configuration state. We also studied in detail the electronic structure of the flattened nanotube configuration and found significantly different electronic behavior from the round nanotube configuration. Based on these results, we proposed novel applications such as nanometer-scale fluid pumps and electrical switches. We also offer a possible explanation for some experimental result anomalies previously attributed to measurement error.; Secondly, we conducted a detailed analysis of the binding energy of atomic hydrogen as a function of increasing hydrogen coverage. The possibility of making use of the highly controllable curvature of the flattened SWNT structure has provided impetus to understanding the general principles of hydrogen chemisorption on single wall carbon nanotubes. We found that hydrogen preferred to bind to the nanotube in such a way as to (1) create high curvature "kink" areas where the H atoms are adsorbed to allow for the larger sp 3 bonding angles; (2) create large low curvature regions for remaining non-hydrogen bonded carbon atoms due to increased pi-bonding among the sp2 bonded atoms. Based on these results, we deduced the maximum spontaneous hydrogen coverage to be between 50% and 75% with an energy preference of approximately 25meV against increasing hydrogen coverage despite calculating that binding energies were stable up to 100% coverage consistent with other studies.