Synthesis and Directed Growth of Single-Crystal TCNQ-Cu Organic Nanowires

K. Xiao, J. Tao, and Z. Liu (CNMS Postdocs); I. N. Ivanov, A.A. Puretzky, Z. Pan, and D.B. Geohegan (CNMS Staff); and S. J. Pennycook (ORNL)

Achievement

Few synthesis experiments have been reported for nanowires of organic semiconductors, despite the proposed use of organic thin-film materials in energy-related optoelectronic devices such as solid state lighting and photovoltaic cells. Although nanostructures of conductive polymers such as polythiophene and polyaniline have been synthesized, there have been no demonstrations of the well-controlled synthesis of organic semiconductor nanostructures in single crystalline form, which could be especially valuable for understanding photocarrier generation-recombination and carrier-transport behavior. To address this need, a CNMS team recently demonstrated low-temperature (120-130°C) reactive chemical vapor deposition (CVD) synthesis of semiconducting TCNQ-Cu organic nanowires, and their direct integration onto prefabricated electrode structures for transport measurements. Transmission electron microscopy and selected area electron diffraction were used to ascertain the single-crystal structure and growth direction. Current-voltage (I-V) characteristics indicate that the semiconducting TCNQ-Cu nanowires make excellent contact with the electrodes. The I-V characteristics also exhibit reversible hysteretic bistable switching behavior. This has been explained by others as due to an electric field-induced reversible redox reaction that builds conduction channels into the TCNQ-Cu material. Single-crystal TCNQ-Cu nanowires also were controllably synthesized on substrates such as Si, glass, copper foils and plastics, and a non-volatile memory device utilizing the switching behavior was demonstrated from TCNQ-Cu nanowires that were grown directly on a plastic substrate.

Sigificance

High quality single-crystal semiconductor nanostructures are critical for understanding carrier behavior in organic materials. However, most nanowires synthesized to date are of inorganic materials. TCNQ-Cu (tetracyanoquinodimethane-Cu) is an organic charge-transfer complex with intriguing structural, electronic, and optical properties. Moreover, the simple CVD method used to synthesize single-crystal TCNQ-Cu nanowires is directly applicable to many other organic semiconductors. In particular, the low growth temperature is compatible with the direct synthesis and integration of semiconducting organic materials into prefabricated electrode structures on low-cost plastic substrates. The CVD approach and use of pre-patterned metal electrodes are scalable without the need for post-processing, thereby facilitating the integration of organic materials into large-scale systems such as photovoltaic cells.

Publications

Xiao, K., I. I. Ivanov, A. A. Puretzky, Z. Liu, and D. B. Geohegan, “Directed integration of tetracyanoquinodimethane-Cu organic nanowires into prefabricated device architectures,” Adv. Mater. 18, 2184 (2006).

This research was conducted in the Functional Nanomaterials Theme at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy.