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Nanomaterials based on functional organic molecules have attracted increasing attention in the past few years. Organic nanostructures offer some unique advantages such as relative ease of chemical doping, high reactivity and good processability, which make them complementary to their inorganic counterparts. While significant progress has been made in inorganic nanowire synthesis, much less has been done with organic nanowires. For example, ordered vertical arrays of inorganic nanowires have been fabricated and shown to be beneficial for many photonic and electronic devices. However, vertical growth of organic nanowires has been rarely reported. Site-selective growth of nanowires in predefined patterns has been proved to simplify the device fabrication. Therefore, site-selective, orientation-controlled synthesis of organic nanowire arrays should be very attractive since it could allow direct growth of these nanostructures onto prefabricated patterns for device integration.

With the support from PRF, we were among the first to achieve the direct growth and patterning of vertically aligned nanowires of small organic compounds on solid substrates. 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. Vertical arrays of DAAQ nanowires on solid substrates were prepared by a physical vapor transport method. 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. During the vapor transport growth, we noticed that the DAAQ tends to preferably nucleate and grow faster on high surface energy sites such as dust particles and scratches on substrates. When silica beads were deposited on a smooth silicon wafer as artificial “dust”, it could be seen that the DAAQ grew mainly on the spheres, which provided preferred nucleation centers for the DAAQ vapor. The preferred nucleation and growth of DAAQ nanowires on high surface energy sites made it possible to grow ordered arrays of nanowires on prepatterned substrates, too. For example, geometrical patterns on Si wafer made by scouring the surface to create alternating ridges and grooves could be used as a physical pattern to selectively grow DAAQ nanowires on ridge areas. Also, microcontact printing of self-assembled monolayers was exploited to alternatively pattern a solid substrate with different surface energy domains, which can also direct the growth of nanowires.

The structure of as-grown nanowires was thoroughly studied by both electron diffraction and X-ray powder diffraction method. In short, precession electron diffraction was applied to reduce the dynamic scattering, which enabled us to identify the space group of the crystal. The phase was then resolved using the powder charge flipping algorithm with both X-ray powder diffraction and precession electron diffraction patterns as the input data. The structural model of the DAAQ crystal was then built by replacing the peaks in the reconstructed maps with the known DAAQ molecule. Based on a series of tilted selected area diffraction patterns, the nanowires determined to be single crystalline with a monoclinic lattice. Transmission electron microscope (TEM) studies also revealed the smooth surface and uniform width of a typical nanowire product.

The π*→ π type transition in the DAAQ molecules has a predominant intramolecular charge transfer character from amino groups to the carbonyl ones, which leads to strong fluorescence emission in visible region. 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, the photoluminescence (PL) intensity rapidly decreased, reaching over 90% quenching after only 30 s. 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. 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 quenching efficiencies was observed when the nanowires were exposed to cycles of acid and base vapors.

Furthermore, the resulting photoluminescence emission could be self-guided along the single crystalline DAAQ nanowires. Compared to horizontal nanowires deposited on a solid substrate, our vertical nanowires show much less optical loss. The higher optical loss of the horizontal wires on glass is due to the energy leakage through the underlying substrates. However, such loss is minimized in the vertical nanowires. DAAQ films prepared by solution casting or vapor deposition are usually amorphous to polycrystalline, which are less favorable materials for waveguiding applications due to high scattering loss. Vertically aligned, single crystalline nanowires thus offer the best combination of materials and geometry in supporting the low loss waveguiding modes, which can potentially be used as optical interconnects.

In summary, vertical organic nanowire arrays of DAAQ dye molecules were prepared by a facile physical vapor transport method. DAAQ grows much faster on the substrates with higher surface energies. Based on this, patterned growth of the nanowire arrays was achieved by both microcontact printing and a physical scouring method. The structure of the nanowires was analyzed by both electron and X-ray diffraction method. The DAAQ nanowires can serve as nanosized optical waveguides, and the optical loss of the vertical wires is much lower compared to that of the horizontally placed wires on glass substrates. The as-prepared nanowire arrays were also integrated to sensing devices for the detection of trace amounts of acidic vapors with very high sensitivity.