Directed Integration of Tetracyanoquinodimethane‐Cu Organic Nanowires into Prefabricated Device Architectures

1D nanostructures (such as nanotubes and nanowires) of semiconductor materials have attracted a lot of interest in recent years due to their potential applications in optical and electronic nanodevices such as chemical and biological sensors. However, the bottleneck to making functional nanodevices has been the difficulties involved in connecting these nanostructures to microscale electrodes. Many methods for fabricating nanowire-based devices have been explored, but most of these techniques require complex post-processing. For example, the “pick-and-place” approach has been successful for making individual devices, such as field-effect transistors and chemical sensors; however, it cannot be used for the reproducible, large-scale fabrication of dense, low-cost device arrays. Processes directly integrating growth into device fabrication, such as the precise and rational growth of nanowires within prefabricated device architectures, are highly desirable since they will markedly simplify the overall device assembly while avoiding the deterioration of the nanowires during processing. The lateral bridging growth of nanostructures was first demonstrated for GaAs nanowhiskers grown by metal–organic vapor phase epitaxy (MOVPE) from vertical ridges on a GaAs substrate. Recently, Williams and co-workers and Yang and co-workers have successfully grown Si nanowires across a trench on a Si substrate. The lateral growth of ZnO nanowires across a Si trench via a vapor–solid (VS) method has also been reported. All these examples demonstrate that nanowire growth and device fabrication can be achieved simultaneously and suggest that lateral bridging growth techniques may be a viable technology for the reliable integration of nanowires into dense arrays for sensors and other devices. The charge-transfer organic compound tetracyanoquinodimethane-Cu (TCNQ-Cu) has attracted much attention because of its intriguing structural, electronic, and optical properties, and its potential applications in electrical and optical memory devices, sensors, and magnetic devices. Bulk and thin-film TCNQ-Cu devices have been widely studied in the past. While the properties of TCNQ-Cu organic nanowires offer many advantages over their bulk and thin-film counterparts, their integration into high-density memory and sensor arrays is limited by the sensitivity of this organic nanomaterial during normal nanofabrication processing steps. TCNQ-Cu nanowires are sensitive to electron-beam irradiation, ruling out electron-beam lithography and focused ionbeam deposition as processing techniques for their integration into devices. Such materials are also too soft to be processed by methods such as sputtering, etching, or milling. Therefore, direct nanowire growth between prefabricated electrodes appears to be the most promising method for the fabrication of single-crystal organic nanowire devices. In this paper, we report a facile, low-temperature technique for the synthesis of single-crystal TCNQ-Cu organic nanowires between prefabricated electrodes by reactive chemical vapor deposition, demonstrating the self-assembled integration of organic nanowires into devices. Compared to current synthesis processes for inorganic nanowires and carbon nanotubes, this method avoids the high temperatures that often damage pre-existing electrodes and other microelectronic components. The lateral bridging growth method used here is demonstrated to be a reproducible method for the directed assembly and integration of TCNQ-Cu nanobridges into prespecified locations on different substrates, eliminating the need for subsequent assembly processes. We are able to effectively synthesize high-aspect-ratio and dendrite-free single-crystal TCNQ-Cu semiconducting nanowires with controlled dimensions. Electrical measurements show that the nanowires are in good contact with the prefabricated electrodes and possess electrical-switching properties. The TCNQ-Cu nanowires are grown by the reaction of TCNQ vapor with Cu via a selective VS reaction mechanism according to Equation 1.