Uniform low-dimensional nanostructures—nanoparticles, nanosheets, and nanowires—are important materials in materials science and nanotechnology. Nanowires offer large specific surface areas useful for sensing platforms, anisotropic emissions, and one-dimensional (1D) transport. To realize anisotropic transport at a macroscopic scale, nanowires need to be aligned while maximizing their in-plane density. A vertical array of uniform ultrathin nanowires is ideal, providing both large surface area and highly aligned unidirectional 1D pathways. Various fabrication methods for inorganic nanowire arrays exist, such as top-down lithography and bottom-up epitaxial growth. For example, silicon nanowires have been fabricated with diameters of ~10–100 nm and integrated into field-effect transistors. Vertically aligned carbon nanotube (VA-CNT) arrays, grown by chemical vapor deposition, have also been developed. However, standing organic nanowires have been largely unexplored. Bottom-up crystalline growth methods have limitations in structural purity and aspect ratio control, while lithography methods result in lower aspect ratios. Template methods using anodic aluminum oxide (AAO) have also been explored, but the nanowires often collapse upon template removal. Therefore, a technique to fabricate standing organic nanowires with ultrahigh aspect ratios from a variety of organic materials is highly desirable.

The existing literature extensively covers inorganic nanowire fabrication techniques such as top-down lithography and bottom-up epitaxial growth, showcasing examples like silicon and vertically aligned carbon nanotubes (VA-CNTs). However, the fabrication of standing organic nanowires with high aspect ratios and controlled properties remains a significant challenge. While bottom-up approaches using organic molecules have shown promise, they often suffer from low structural purity and difficulties in controlling the aspect ratio and diameter of the nanowires. Top-down methods, on the other hand, struggle to achieve the desired ultrahigh aspect ratios. Template methods, although successful in producing thin organic nanowires, often face the problem of nanowire collapse during the removal of the template. This research addresses the gap in the literature by introducing a novel method for the fabrication of free-standing, high-aspect-ratio organic nanowires using a dry process.

The researchers developed a novel technique called single particle-triggered linear polymerization (STLIP) to fabricate uniform nanowires with controlled length, diameter, and number density. This method utilizes high-energy charged particles, which, upon passing through a condensed organic layer, deposit their kinetic energy within a nanometer-scale region. This energy transfer initiates polymerization/cross-linking reactions, forming 1D gels, ultimately resulting in the formation of organic nanowires. A key innovation is the use of a dry process—sublimation—to isolate the nanowires. The unreacted organic molecules are removed by heating in a vacuum, leaving behind the polymerized nanowires standing vertically on the substrate. This all-dry process eliminates the issues of nanowire collapse and aggregation often associated with wet processes. The study used Buckminster fullerene (C60) as a model material due to its sublimability and ability to undergo chain polymerization reactions. Further experiments explored the feasibility of using different molecules, including PC61BM and C70, and other sublimable aromatic molecules with triple carbon-carbon or aryl-halogen bonds. The researchers investigated the effect of irradiation fluence, finding that sufficient fluence is crucial for maintaining the vertical alignment of nanowires. Furthermore, heterojunction structures were created using bilayer films of different organic molecules and also by electropolymerization of π-conjugated monomers around the existing nanowires to form coaxial heterojunctions. Various characterization techniques were employed, including scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, and DC conductivity measurements, to analyze the morphology, structure, and electrical properties of the fabricated nanowires.

The STLIP method successfully produced free-standing organic nanowires with ultrahigh aspect ratios (over 300) and controlled number density. The all-dry sublimation process proved crucial in preventing nanowire collapse, which is a common problem in wet processing methods. The method's versatility was demonstrated by successfully fabricating nanowires from various sublimable organic molecules, including C60, PC61BM, C70, and several other aromatic compounds. The researchers established a correlation between the reaction efficiency, molecular structure, and nanowire diameter. Nanowire networks with controlled three-dimensional structures were also fabricated by tilting the substrate during irradiation. The study successfully demonstrated the formation of heterojunction structures in the nanowires, both by using bilayer films of different molecules (creating planar heterojunctions) and by electropolymerization (creating coaxial heterojunctions). The electrical conductivity of the nanowire plexus was found to be proportional to the number density of nanowires and the area of top electrodes, suggesting high uniformity in the electrical conduction. Furthermore, the p-n heterojunction nanowires showed rectifying behavior, demonstrating their potential as ultrasmall rectifier diodes. Finally, the vertically aligned nanowires showed high water repellency.

The results address the long-standing challenge of creating high-aspect-ratio, free-standing organic nanowires. The STLIP method offers a unique approach by combining high-energy particle irradiation with a dry isolation process to produce highly uniform nanostructures with precisely controlled parameters. The versatility of the method, shown by the successful use of various organic molecules, expands its applicability to a wide range of applications. The ability to fabricate heterojunctions provides a pathway to create functional nanodevices with tailored electronic and optical properties. The findings have significant implications for various fields, including nanoelectronics, nanophotonics, sensing, and biomedicine, opening up possibilities for developing novel organic-based devices with enhanced functionalities.

The study successfully demonstrated a novel, versatile method for fabricating high-aspect-ratio, free-standing organic nanowires using single particle-triggered linear polymerization (STLIP) and a dry sublimation process. The method enables precise control over nanowire length, diameter, and number density, as well as the ability to create both planar and coaxial heterojunction structures. The successful fabrication of nanowires from diverse organic molecules and their demonstrated functionalities highlight the potential of this technique for developing advanced nanomaterials and devices in various fields. Future research could focus on exploring the potential of these nanowires in specific applications such as organic electronics, sensing, and biomedicine, as well as further optimizing the synthesis process to achieve even higher aspect ratios and improved control over nanowire properties.

While the study demonstrates the successful fabrication of high-aspect-ratio organic nanowires, further investigations are needed to fully explore the limitations of the STLIP method. The current research primarily focused on the fabrication and characterization of the nanowires. Detailed studies of the long-term stability of the nanowires, their mechanical properties, and their performance in actual device applications need further exploration. Additionally, the scalability of the method and its cost-effectiveness for large-scale production remain to be investigated.