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Vertical arrangement of nanowire materials has been found to be advantageous for applications such as solar cells, batteries, electrochemical sensors, field emitters, light emitting diodes, and transistors and has led to the discoveries of nanolasers and nanopiezotronics. So far, the predominating body of work has focused on inorganic materials. Organic nanowires can bring new opportunities since they are composed of molecular building blocks with relatively weaker interactions, thus allowing highly flexible structural tunability. Organic nanostructures have emerged to play increasingly important roles in miniaturized optoelectronic devices.

With the support from PRF, we were among the first to achieve the direct growth of vertically aligned nanowires of small organic compounds on solid surfaces. Our first example was 1,5-diaminoanthraquinone (DAAQ), which belongs to a class of quinoid compounds that have been used in the dye industry for over a century. In our recent work, vertical arrays of DAAQ nanowires on solid substrates were prepared by a physical vapor transport method. In a typical synthesis, a thin coating of DAAQ was made on the inner wall of a round bottle flask by spreading an ethanol solution. This ensured uniform heating over the entire sample area in the subsequent sublimation steps. Then the flask was heated to a predetermined temperature of 150-200 °C to vaporize the powders. The vapor of DAAQ can be transported downstream by air, nitrogen, or diffusion under vacuum to deposit on a solid substrate placed downstream. The length and diameter of the nanowires were controlled by the growth conditions. For example, the diameter can be tuned from 80 to over 500 nm by the evaporation temperature, while the length can be tuned from 500 nm to over 10 μm by the deposition time.

Transmission electron microscope (TEM) studies revealed the smooth surface and uniform width of a typical nanowire product. Based on the electron diffraction patterns, as-grown nanowires determined to be single crystalline with a monoclinic lattice. The XRD pattern of the nanowires matched the powder pattern except for the relative peak intensities, suggesting that DAAQ did not undergo phase transition or chemical reaction during the vapor transport. Scanning electron microscopy observation of DAAQ nanostructures in early stage of formation on solid substrates revealed that DAAQ first deposited as well-separated nanoparticles of 100 nm in diameter with a density comparable to that of the final nanowire arrays. This indicates that the nanowires were grown on these seed nanoparticles. Since vertical nanowires were obtained on many different substrates, it implies that the vapor may have condensed to form similarly oriented seeds on those surfaces as well.

The intramolecular charge transfer between the neighboring amine and carbonyl groups is responsible for the color and fluorescence of DAAQ. Since the charge transfer can be interrupted if the N atoms are protonated, we carried out acid sensing experiments based on the changes in color and fluorescence on exposure to 5 ppm of HCl vapor in air. Upon exposure of the nanowires to HCl for different lengths of time, apparent bleaching of the red color of the sample was observed. On exposure to HCl, the photoluminescence (PL) intensity rapidly decreased, reaching over 90% quenching after only 30 s. Fluorescence quenching could be seen by eye when the samples were illuminated with blue light. In contrast, DAAQ powders composed of micron sized particles did not show an apparent response even after 30 min of exposure, demonstrating higher sensing capability of as-grown nanowires of same compound. Both the color and PL could be recovered in air to 95% of their original intensity in 2 h. However, they could be rapidly reset by basic vapors (e.g., NH3) within 2 s. This also provides a mechanism for detecting basic vapors. The basic vapor deprotonated the amine groups and helped to restore the intramolecular charge transfer, leading to the recovery of color and fluorescence. A good consistency in the bleaching and quenching efficiencies was observed when the nanowires were exposed to cycles of acid and base vapors.

In summary, vertical organic nanowire arrays of DAAQ dye molecules were prepared by a facile physical vapor transport method. The crystal structure of the nanowires was determined by TEM and electron diffraction. These fluorescent nanowires were sensitive to acidic and basic vapors. The ease of oriented vertical growth should make it possible to directly integrate these nanowires into photonic sensing devices.