Single‐Crystal Organic Nanowires of Copper–Tetracyanoquinodimethane: Synthesis, Patterning, Characterization, and Device Applications

Single-crystal, one-dimensional semiconductor nanostructures are very important building blocks for nanoscale optical and electronic devices. Currently, much attention is focused on inorganic materials, such as carbon nanotubes and inorganic oxide nanowires, whose properties endow them with considerable potential. Fewer studies have been performed on organic semiconductor nanowires, despite their widespread development for inexpensive organic thin-film optoelectronic devices. Organic semiconducting materials comprised of smaller molecules normally have conjugated structures. As a consequence of their diversity, tailorability, multifunctionality, and ease in forming very highly ordered crystals, it is highly desirable to synthesize single-crystal organic semiconducting nanowires from small molecular compounds, which could serve as low-cost, high-performance building blocks for the next-generation lightweight displays and for applications in information storage. Copper–tetracyanoquinodimethane (Cu–TCNQ) is a widely studied organic semiconductor because of its capability for reversible, bistable switching of conductivity from a high to a low state through induction by an electric field or optical excitation. Bulk and thin-film forms of Cu–TCNQ have been widely studied, and thus far various methods for the fabrication of Cu–TCNQ nanostructures have been reported, such as vacuum vapor deposition, solution processes in organic solvents, as well as chemical and electrochemical reduction of TCNQ in the presence of metallic Cu or Cu ions. However, the use of these techniques to fabricate microand nanoscale Cu–TCNQ structures have resulted in significant drawbacks with respect to the yield, size, phase, material multiplicity, production of high-density arrays, and ability to achieve patterned structures. Herein we report the synthesis and patterning of single-crystal semiconductor Cu–TCNQ nanowires by a low-temperature vapor–solid route. The advantage of this synthetic route lies in its simplicity and mild reaction conditions, as well as the high crystallinity of the nanowires grown in the arrays. To demonstrate the potential of the technique we report the design, fabrication, and characteristics of cross-point nonvolatile memory devices based on a network of organic singlecrystal nanowires on a flexible poly(ethylene terephthalate) (PET) flexible substrate. Cu–TCNQ organic nanowires were grown in a controllable manner from reactions of a Cu thin film and TCNQ vapor at low temperatures (Figure 1). The diameters of the nanowires range from 20 to 100 nm, while the lengths could be varied between 1 mm and 20 mm by adjusting the growth time and thickness of the Cu film. There are several important features of these Cu–TCNQ nanowires. First, the nanowires initially grow directly from the patterned Cu film on the substrate surface and make good mechanical connections to the untreated Cu layer, an essential starting point for good electrical connections to device electrodes (Figure 1a). As the nanowires grow longer, they curve to form a network of nanowires (Figure 1b). Second, Cu–TCNQ nanowires can be synthesized at temperatures as low as 120 8C by using a simple chemical vapor deposition (CVD) method. Therefore, Cu–TCNQ nanowires can be grown on some temperaturesensitive substrates, such as glass and flexible plastics, which is promising for large-area, low-cost flexible electronic applications. Third, as a result of the vapor–solid reaction growth, the nanowires grow only in areas patterned with Cu thin films and readily yield patterned nanowires. The direction of the nanowires can be influenced by carefully exposing regions of the Cu thin film. Various techniques, such as electron-beam lithography and photolithography, can be used to create patterned Cu films for subsequent growth of Cu–TCNQ nanowires. SEM studies reveal massive growth of fine, long, [*] Dr. K. Xiao, Dr. Z. Pan, Dr. A. A. Puretzky, Dr. I. N. Ivanov, Dr. D. B. Geohegan Center for Nanophase Materials Sciences Oak Ridge National Laboratory 1 Bethel Valley Road, Oak Ridge, TN 37831-6030 (USA) Fax: (+1)865-574-4143 E-mail: xiaok@ornl.gov geohegandb@ornl.gov