Elucidating Nucleation and Growth Behavior of Single-Walled Carbon Nanotubes obtained via Catalyzed Synthesis

The catalytic growth of single-walled carbon nanotubes (SWCNTs) is studied using reactive molecular dynamics (RMD) simulations and density functional theory (DFT) calculations. Computational calculations are performed in order to achieve a better understanding of the catalytic reaction mechanism at the initial stages of synthesis, where most of the structural characteristics are defined. Different process variables such as catalyst chemical composition and size, temperature, pressure, and the nature of catalyst support, can be optimized with the purpose of tuning the structure and physical properties of SWCNTs. Controlling the structure of SWCNTs during synthesis and avoiding additional purification and/or separation processes are critical for the direct use of SWCNTs in electronic devices. RMD simulations demonstrate that small catalyst particles favor the growth of lengthy nanotubes over catalyst encapsulation as a result of an increase of the curvature energies of the carbon capsule. Furthermore, simulations performed over deposited catalyst particles demonstrate that the catalyst-support adhesion must be controlled in order to grow nanotubes with high structural quality and avoid catalyst poisoning. Results herein reported suggest that growth conditions must be optimum to minimize the nucleation of topological defects in nanotubes. RMD trajectories prove the vital role played by the catalyst surface in healing defects via adsorption and diffusion. These results significantly impact the field of chirality control since the presence of defects introduce misorientation of hexagons, shifts the overall chiral angle, and therefore, modifies the physical properties of the nanotube. DFT calculations are employed to evaluate the interaction between SWCNTs and the ST-cut quartz substrate. The outstanding performance of CNT-based FET relies on the alignment of the horizontally grown nanotubes on silica substrates, as well as on the selective growth of semiconducting nanotubes. It is demonstrated that finite-length zigzag nanotubes are adsorbed stronger than armchair tubes on the quartz support. This suggests that the nanotube electronic band structure is a key factor on the preferential adsorption of zigzag tubes. DFT calculations suggest that patterns of unsaturated silicon atoms of silica surfaces define the crystallographic directions of preferential alignment. These patterns might be chemically altered in order to favor other directions of alignment

Typical and atypical presentation pantothenate kinase deficiency : reports of two cases and a review of current literature

Los síndromes de neurodegeneración asociada al hierro son una causa secundaria importante de extrapiramidalismo (1). De aparición entre los 6 y 40 años, se asocian además a cambios comportamentales y demencia (2). La base fisiopatológica son los depósitos de hierro a nivel ganglio basal. De estas enfermedades, la más frecuente es el déficit de pantotenato quinasa (PKAN por su siglas en inglés), constituyendo más del 50% de los casos de esta enfermedad. Se han descrito 2 formas de presentación típica o temprana, y atípica o tardía2. Se reportan a continuación dos casos: uno de presentación típica y otro de presentación atípica, diagnosticados en el hospital San Ignacio de Bogotá, Colombia.Caso clínico124-127Neurodegenerations associated with iron deposites are an important secondary cause extrapiramidalism. Their onsets are between 6 and 40 years, and are associated with behavioral changes and dementia. The pathophysiological bases are iron deposits at the basal ganglia. Of these diseases, the most frequent is Pantothenate kinase associated neurodegeneration (PKAN by its acronym), which constitutes over 50% of cases of the disease. Two forms of presentation have been described: typical (early onset) and atypical (late onset). We report two cases in the following: one typical and one atypical presentation, both diagnosed in San Ignacio Hospital in Bogotá, Colombia

Elucidating Nucleation and Growth Behavior of Single-Walled Carbon Nanotubes obtained via Catalyzed Synthesis

The catalytic growth of single-walled carbon nanotubes (SWCNTs) is studied using reactive molecular dynamics (RMD) simulations and density functional theory (DFT) calculations. Computational calculations are performed in order to achieve a better understanding of the catalytic reaction mechanism at the initial stages of synthesis, where most of the structural characteristics are defined. Different process variables such as catalyst chemical composition and size, temperature, pressure, and the nature of catalyst support, can be optimized with the purpose of tuning the structure and physical properties of SWCNTs. Controlling the structure of SWCNTs during synthesis and avoiding additional purification and/or separation processes are critical for the direct use of SWCNTs in electronic devices. RMD simulations demonstrate that small catalyst particles favor the growth of lengthy nanotubes over catalyst encapsulation as a result of an increase of the curvature energies of the carbon capsule. Furthermore, simulations performed over deposited catalyst particles demonstrate that the catalyst-support adhesion must be controlled in order to grow nanotubes with high structural quality and avoid catalyst poisoning. Results herein reported suggest that growth conditions must be optimum to minimize the nucleation of topological defects in nanotubes. RMD trajectories prove the vital role played by the catalyst surface in healing defects via adsorption and diffusion. These results significantly impact the field of chirality control since the presence of defects introduce misorientation of hexagons, shifts the overall chiral angle, and therefore, modifies the physical properties of the nanotube. DFT calculations are employed to evaluate the interaction between SWCNTs and the ST-cut quartz substrate. The outstanding performance of CNT-based FET relies on the alignment of the horizontally grown nanotubes on silica substrates, as well as on the selective growth of semiconducting nanotubes. It is demonstrated that finite-length zigzag nanotubes are adsorbed stronger than armchair tubes on the quartz support. This suggests that the nanotube electronic band structure is a key factor on the preferential adsorption of zigzag tubes. DFT calculations suggest that patterns of unsaturated silicon atoms of silica surfaces define the crystallographic directions of preferential alignment. These patterns might be chemically altered in order to favor other directions of alignment