Physical properties of active materials built up from small molecules are dictated by their molecular packing in the solid state. Here we demonstrate for the first time the growth of n-channel single-crystal field-effect transistors and organic thin-film transistors by sublimation of 2,6-dichloro-naphthalene diimide in air. Under these conditions, a new polymorph with two-dimensional brick-wall packing mode (b-phase) is obtained that is distinguished from the previously reported herringbone packing motif obtained from solution (a-phase). We are able to fabricate single-crystal field-effect transistors with electron mobilities in air of up to 8.6 cm 2 V À 1 s À 1 (a-phase) and up to 3.5 cm 2 V À 1 s À 1 (b-phase) on n-octadecyltriethoxysilane-modified substrates. On silicon dioxide, thin-film devices based on b-phase can be manufactured in air giving rise to electron mobilities of 0.37 cm 2 V À 1 s À 1 . The simple crystal and thin-film growth procedures by sublimation under ambient conditions avoid elaborate substrate modifications and costly vacuum equipment-based fabrication steps.

Ribbon-shaped single crystal transistors based on naphthalene diimide Cl2-NDI exhibit excellent n-channel performance with the mobility as high as 8.6 cm(2) V(-1) s(-1) in air. The combination of ambient stability and high mobility n-channel transport closes the gap between p- and n-channel SCFETs and opens the door for the manufacture of high performance complementary organic circuits.

Abstract-This paper demonstrates how to use multiple channels to improve communication performance in Wireless Sensor Networks (WSNs). We first investigate multi-channel realities in WSNs through intensive empirical experiments with Micaz motes. Our study shows that current multi-channel protocols are not suitable for WSNs, because of the small number of available channels and unavoidable time errors found in real networks. With these observations, we propose a novel tree-based multichannel scheme for data collection applications, which allocates channels to disjoint trees and exploits parallel transmissions among trees. In order to minimize interference within trees, we define a new channel assignment problem which is proven NPcomplete. Then we propose a greedy channel allocation algorithm which outperforms other schemes in dense networks with a small number of channels. We implement our protocol, called TMCP, in a real testbed. Through both simulation and real experiments, we show that TMCP can significantly improve network throughput and reduce packet losses. More importantly, evaluation results show that TMCP better accommodates multi-channel realities found in WSNs than other multi-channel protocols.

Understanding relationships between microstructure and electrical transport is an important goal for the materials science of organic semiconductors. Combining high-resolution surface potential mapping by scanning Kelvin probe microscopy (SKPM) with systematic field effect transport measurements, we show that step edges can trap electrons on the surfaces of single crystal organic semiconductors. n-type organic semiconductor crystals exhibiting positive step edge surface potentials display threshold voltages that increase and carrier mobilities that decrease with increasing step density, characteristic of trapping, whereas crystals that do not have positive step edge surface potentials do not have strongly step density dependent transport. A device model and microelectrostatics calculations suggest that trapping can be intrinsic to step edges for crystals of molecules with polar substituents. The results provide a unique example of a specific microstructure–charge trapping relationship and highlight the utility of surface potential imaging in combination with transport measurements as a productive strategy for uncovering microscopic structure–property relationships in organic semiconductors.

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