Templates for integrated nanofiber growth

    Publikation: AfhandlingPh.d.-afhandling


    Para-hexaphenylene (p6P) molecules have the ability to self-assemble into organic nanofibers. These nanofibers hold unique optoelectronic properties, which make them interesting candidates as elements in electronic and optoelectronic devices. Typically these nanofibers are grown on specific single-crystalline substrates, such as muscovite mica, on which long, mutually parallel nanofibers are self-assembled upon vapor deposition of the organic material under high vacuum conditions. However, the lack of ability to further process these substrates and for integration of such fragile nanostructures with the necessary interface circuitry such as metal electrodes for electrical connection continues to be a significant hindrance toward their large-scale implementation. In-situ growth constitutes a very promising strategy for integrating functional nanostructures into device platforms due to the possibility of parallel, high-volume integration. Besides such single-crystalline templates, the nanofibers can also be grown on non-crystalline gold surfaces, on which the orientation of the nanofibers can be manipulated by structuring the gold surface prior to p6P deposition.
    In this work, a novel method for in-situ growth of p6P nanofibers on nano-structured gold surfaces is presented. The substrates are prepared by conventional nanofabrication techniques such as e-beam lithography and metal deposition, which increase their potential as device platforms. Some of the results presented here demonstrate, that both the growth direction and the nanofiber length and position can be controlled by placement of nano-structured lines on the substrate. These lines can be used to guide the surface diffusion and thereby steer the self-assembly process of the organic molecules leading to morphologically well-defined molecular nanofibers with preferred growth directions. It is shown that the preferred growth direction of the nanofibers is perpendicular to these structures whereas their length scales are limited by the size and placement of the structures. This work therefore demonstrates a new technique, which can be useful within future organic nanofiber based applications. We also demonstrated how gold gratings fabricated on an insulating substrate can enable electrical contact to in-situ grown p6P nanofibers.
    In a further development of this method, in-situ directed growth of such organic nanostructures were performed between pre-fabricated contacts, which are source–drain gold electrodes on a transistor platform (containing bottom- or top-gate) on silicon dioxide. The substrates were patterned by a combination of optical lithography and electron beam lithography. The dimensions of the gold electrodes strongly influence the morphology of the resulting structures leading to notably different electrical properties. The transistor design influences its electrical characteristics, and the top-gate configuration shows to have the stronger gate effect. In addition, platforms for light-emitting devices were fabricated, and the nanofibers did emit light when an AC voltage was applied to the gate. Platforms for 4-point measurements were fabricated to eliminate contact resistances and determine the nanofibers intrinsic resistance and resistivity.
    The large-scale fabrication of such small device platforms was demonstrated by using nano-imprint lithography (NIL).
    The ability to achieve in-situ growth of p6P nanostructures on device platforms opens a wide range of possible applications including fabrication of organic LEDs and other optoelectronic devices.
    StatusUdgivet - 2011


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