Organic thin-film transistors (OTFTs) have potential for use in many low-cost, large-area electronic applications such as smart cards, radio-frequency (RF) identification tags, and flatpanel displays. To date, OTFTs with the best overall reported performance have been based on the organic semiconductor pentacene, with carrier field-effect mobility > 2 cm 2 V ±1 s ±1 .[1]Molecular ordering is believed to play a large role in the performance of devices based on organic active layer materials, [2,3] especially for small molecule materials such as pentacene. In particular, the amount of p-orbital overlap is expected to have a strong influence on field-effect mobility. Interestingly, investigation of the solid-state order of pentacene reveals ªherringboneº packing with a combination of edge-to-face and face-to-face molecular interactions. This packing arrangement yields limited p-orbital overlap and may limit the field-effect mobility possible in pentacene. One approach to improve pentacene-based device performance is to modify the pentacene molecule to form molecular crystals with increased p-orbital overlap. This paper reports on OTFTs fabricated with pentacene functionalized to improve p-orbital overlap. Previous work on functionalized pentacene focused on using the material as a soluble precursor allowing the fabrication of solution-cast unsubstituted pentacene OTFTs.[4±6] The functional groups were removed from the molecule before completion of device fabrication and did not result in molecular ordering with improved electronic transport potential. This study reports on the use of molecular modifications to engineer improved organic semiconductor carrier-transport properties. The approach reported here is to add bulky functional groups at the 6,13-positions of the pentacene molecule to discourage edge-to-face molecular interactions. The desired result is modified molecular ordering with primarily face-toface interactions and improved p-orbital overlap. In addition to providing modified molecular ordering, the functional groups may also improve the solubility of the material in organic solvents, allowing solution-cast devices, and may also enhance the oxidative stability of the material. Figure 1 shows (left) the molecular structure of the pentacene derivatives investigated in this work and (right) the five functional groups used. The substituent is separated from the pentacene molecule by a rigid alkyne spacer used to hold the bulky groups away from the aromatic core to allow the closest possible approach between the aromatic rings. X-ray crystallography data on triisopropylsilyl (TIPS) pentacene reveals that the material stacks in a two-dimensional columnar array with significantly increased p-orbital overlap and reduced interplanar spacing compared to unsubstituted pentacene. [7,8] OTFTs were fabricated using the five functionalized pentacene derivatives as the active layer. A heavily doped silicon wafer was used as a convenient substrate and gate electrode. Thermally grown silicon dioxide was used as the gate ...

We show that the thermal and electrical properties of single wall carbon nanotube (CNT)-polymer composites are significantly enhanced by magnetic alignment during processing. The electrical transport properties of the composites are mainly governed by the hopping conduction with localization lengths comparable to bundle diameters. The bundling of nanotubes during the composite processing is an important factor for electrical, and in particular, for thermal transport properties. Better CNT isolation will be needed to reach the theoretical thermal conductivity limit for CNT composites.

We report EHT calculations of the band electronic structure of substituted pentacene derivatives and the polymorphs of the parent compound. The results show that there are wide disparities among the bandwidths and electronic dimensionalities of these compounds. The parent pentacene polymorphs are 2-dimensional in their band electronic structure with moderate dispersions; the bandwidths in the 14.1 Å d-spacing polymorph are noticeably larger than for the 14.5 Å d-spacing polymorph, reported by Campbell. Whereas the parent pentacene polymorphs adopt the well-known herringbone packing, the new, substituted pentacenes are noticeably different in their solid state structures and this is reflected in the band electronic structures. TMS adopts a highly 1-dimensional structure that leads to a large bandwidth along the stacking direction; TIPS also adopts a stacked structure, but because the molecules are laterally interleaved in the fashion of bricks in a wall, this compound is strongly 2-dimensional.