Supramolecular Assembly of Oriented Spherulitic Crystals of Conjugated Polymers Surrounding Carbon Nanotube Fibers.

The directed assembly of conjugated polymers into macroscopic organization with controlled orientation and placement is pivotal in improving device performance. Here, the supramolecular assembly of oriented spherulitic crystals of poly(3-butylthiophene) surrounding a single carbon nanotube fiber under controlled solvent evaporation of solution-cast films is reported. Oriented lamellar structures nucleate on the surface of the nanotube fiber in the form of a transcrystalline interphase. The factors influencing the formation of transcrystals are investigated in terms of chemical structure, crystallization temperature, and time. Dynamic process measurements exhibit the linear growth of transcrystals with time. Microstructural analysis of transcrystals reveals individual lamellar organization and crystal polymorphism. The form II modification occurs at low temperatures, while both form I and form II modifications coexist at high temperatures. A possible model is presented to interpret transcrystallization and polymorphism.

Optoelectronic Switching of a Carbon Nanotube Chiral Junction Imaged with Nanometer Spatial Resolution.

Chiral junctions of carbon nanotubes have the potential of serving as optically or electrically controllable switches. To investigate optoelectronic tuning of a chiral junction, we stamp carbon nanotubes onto a transparent gold surface and locate a tube with a semiconducting-metallic junction. We image topography, laser absorption at 532 nm, and measure I-V curves of the junction with nanometer spatial resolution. The bandgaps on both sides of the junction depend on the applied tip field (Stark effect), so the semiconducting-metallic nature of the junction can be tuned by varying the electric field from the STM tip. Although absolute field values can only be estimated because of the unknown tip geometry, the bandgap shifts are larger than expected from the tip field alone, so optical rectification of the laser and carrier generation by the laser must also affect the bandgap switching of the chiral junction.

Electron-Impact Ionization Cross Sections of Molecules Containing Heavy Elements (Z > 10).

The binary-encounter-Bethe (BEB) theory has been successful for computing electron-impact ionization cross sections of many molecules. For molecules that contain heavy atoms (defined here as atoms with valence principal quantum number n > 2), there are two alternative BEB procedures in the literature. The first involves a kinetic-energy correction for molecular orbitals that are dominated by atomic orbitals with n > 2. The second alternative is to use effective core potentials (ECPs), which were developed for other purposes but yield valence pseudo-orbitals with reduced kinetic energies. In the present study, the results of these two approaches are compared with experimental cross sections for several molecules containing heavy elements. Although both procedures perform well, the ECP results agree somewhat better with experimental measurements. Cross sections are presented for C2Cl6, C2HCl5, C2Cl4, both isomers of C2H2Cl4, CCl4, TiCl4, CBr4, CHBr3, CH2Br2, P2, P4, As2, As4, GaCl, CS2, H2S, CH3I, Al(CH3)3, Ga(CH3)3, hexamethyldisiloxane, and Zn(C2H5)2. Incorrect BEB calculations have been reported in the literature for several of these molecules. As an ancillary result, the dipole polarizability of Zn(C2H5)2 is predicted to be 12.1 Å(3).