Nano-architecture

Conjugated organic materials such as conducting polymers offer the tunable electrical properties of semiconductors with the flexibility of plastics. These organic materials lead to applications such as transparent conductors, serve as hole transport layers, and are the active component in state-of-the-art electronic devices including solar cells, electrochromics, thin film transistors, and light emitting diodes. Among the plastics that conduct electricity, poly(3,4-ethylenedioxythiophene) (PEDOT) has attracted a great deal of attention due to its high electrical conductivity, outstanding chemical stability, and electrochromic behavior. In order to engineer the use of PEDOT in an electronic device, standard fabrication protocols are required to deposit a thin film. The quality of the coating is directly related to the efficiency of the device. Therefore, deposition of a high quality film of PEDOT that possesses a thin, continuous, and homogenous morphology is desired in order to make efficient use of the intrinsic electrical properties of the polymer. Coatings can be cast by solution processing using methods such as dip-coating, drop-casting, and spin-coating or alternatively by electrochemical techniques. Typically, these approaches have served as the most common routes for the deposition of thin films of PEDOT; however, they suffer from limitations ranging from a lack of scalability and versatility to poor material processability. Chemical vapor deposition is an alternative method for making thin films of PEDOT that overcomes these problems.

Thin films of PEDOT are readily produced from the vapor phase to create transparent, homogeneous coatings over large areas on virtually any substrate. PEDOT forms by step-growth-polymerization when vaporized molecules of the EDOT monomer and the oxidant make contact with each other. This process oxidizes the monomer into radical cations and can be carried out using a variety of oxidizing agents such as iron(III) salts. For example, an oxidant such as iron(III) p-toluenesulfonate leads to ordered polymer chains, so that when the rate of vapor phase polymerization is controlled, PEDOT of a sufficiently long conjugation length deposits as a thin film exhibiting a conductivity as high as 4500 S/cm. The simultaneous vaporization of the EDOT monomer with oxidant molecules enables conformal coatings for even coverage over complex topographies. Alternatively, selectively pre-coating the oxidant onto a substrate followed by exposure to monomer vapors leads to micro-patterning of a PEDOT thin film. Polymerization occurs concomitantly with the deposition of a film inside a reaction chamber; therefore, controlling synthetic parameters such as temperature, concentration, and pressure, leads to high quality thin films of PEDOT. The thickness of a film is controlled by the feed rate of the reactant vapors; thus, by systematically varying the concentration of EDOT molecules inside the reaction chamber, film thicknesses from 20 – 300 nm can be produced.

Traditionally, vapor phase deposition has been employed for growing smooth textured films. More recently, PEDOT nano-structured films comprised of nanofibers, nano-bowls, and nano-snowflakes have been produced. These findings have led us to explore the versatility of polymerization from the vapor phase in order to create nano-architectured PEDOT. Our previous studies have focused on the synthesis of nanostructures of the conducting polymers polyaniline, polythiophene, polypyrrole and their derivatives. We observed that the main mechanism leading to nanofiber formation stems from classical nucleation theory where homogeneous nucleation predominates. We are now investigating the mechanism that leads to the growth of highly crystalline PEDOT during vapor phase polymerization. We have developed a protocol for the synthesis of various anisotropic PEDOT nano-architectures deposited from the vapor phase that can produce either vertically or horizontally oriented PEDOT nanofibers. Different nanostructures can be synthesized by modifying traditional chemical vapor deposition techniques, using a simple glass reaction chamber, and by controlling the exposure time to reactant vapors.

This issue's cover image shows the nanoscale morphology of a thick film of PEDOT produced on a glass substrate using our modified vapor phase deposition technique. The topography of this nanostructured PEDOT film shows a surface coating of vertically oriented one-dimensional nanostructures of low aspect ratio. Anisotropic stubby PEDOT nano-architectures have lengths that range between 500 nm and 2 μm with diameters from 300 to 500 nm. Our synthetic protocol is optimized for the formation of an intensely blue colored nanostructured material possessing a long conjugation length and exhibiting a sheet resistance as low as 4 Ω/square. The PEDOT nanostructures shown in the image belong to an intermediate morphology that was obtained when the polymerization was not allowed to reach completion; instead, the polymerization was quenched by removing the substrate from the reaction chamber and inserting it into a cold vessel filled with dry ice. The deposited mass of PEDOT was peeled off from the glass substrate and directly attached to a piece of double-sided conductive copper tape for collecting the image using a Nova NanoSEM 230 (FEI, USA) scanning electron microscope (SEM).