ABSTRACT
Separation of single-walled carbon nanotubes (SWNT) by diameter is an important prerequisite for controlled experimental studies and efficient application of these systems. By comparing experimental data with molecular dynamics (MD) simulations, we demonstrate that water filling has a significant, tube-diameter dependent effect on the effective mass density of individual single-walled carbon nanotubes suspended in aqueous surfactant suspensions. We present a model for the effective density of the nanotube-surfactant complex in aqueous solution that permits a comprehensive description of its density across the entire, experimentally relevant range of SWNT diameters. Parameters for this model can be obtained from molecular dynamics simulations and/or experiment and help explain the subtle interplay of surfactant coverage and endohedral water in the separation of a particular diameter species of SWNT by gradient centrifugation.
ABSTRACT
We measure electron tunneling in transistors made from C(140), a molecule with a mass-spring-mass geometry chosen as a model system to study electron-vibration coupling. We observe vibration-assisted tunneling at an energy corresponding to the stretching mode of C(140). Molecular modeling provides explanations for why this mode couples more strongly to electron tunneling than to the other internal modes of the molecule. We make comparisons between the observed tunneling rates and those expected from the Franck-Condon model.
Subject(s)
Carbon/chemistry , Electrochemistry/methods , Electrons , Models, Chemical , Nanostructures/chemistry , Nanostructures/ultrastructure , Transistors, Electronic , Carbon/analysis , Computer Simulation , Electric Conductivity , Electron Transport , Materials Testing , Nanostructures/analysis , VibrationABSTRACT
Polymeric forms of C60 are now well known, but numerous attempts to obtain C70 in a polymeric state have yielded only dimers. Polymeric C70 has now been synthesized by treatment of hexagonally packed C70 single crystals under moderate hydrostatic pressure (2 gigapascals) at elevated temperature (300 degrees C), which confirms predictions from our modeling of polymeric structures of C70. Single-crystal x-ray diffraction shows that the molecules are bridged into polymeric zigzag chains that extend along the c axis of the parent structure. Solid-state nuclear magnetic resonance and Raman data provide evidence for covalent chemical bonding between the C70 cages.
