Molecular-Level Control in the Deposition of Ultrathin Films of Highly Conductive, In-Situ Polymerized P-Doped Conjugate
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ABSTRACT A novel thin film processing technique has been developed for the fabrication of ultrathin films of conducting polymers with angstrom-level control over thickness and multilayer architecture. Molecular self-assembly of in-situ polymerized conjugated polymers consists of a layer-by-layer process in which a substrate is alternately dipped into a solution of a p-doped conducting polymer (e.g. polypyrrole, polyaniline) and a solution of a polyanion. In-situ oxidative polymerization produces the more highly conductive, underivatized form of the conjugated polymer, which is deposited in a single layer of precisely controlled thickness (30 to 60 A). The thickness of each layer can be fine-tuned by adjusting the dipping time and the solution chemistry. The surface chemistry of the substrate (e.g. hydrophobic, charged, etc.) also strongly influences the deposition, thereby making it possible to selectively deposit conducting polypyrrole onto well defined regions of the substrates. Typical multilayer films exhibit conductivities in the range of 2050 S/cm, but samples with conductivities as high as 300 S/cm have been realized. There is no limit to the number of layers that can be built up nor to the complexity of the multilayer architecture of the film; achieved simply by alternating the sequence of dips into solutions of various polycations and polyanions. This new self-assembly process opens up vast possibilities in applications which require large area, ultrathin films of conducting polymers and, more importantly, in applications that can take advantage of the unique interactions achievable in the complex, supermolecular architectures of multilayer films.
INTRODUCTION Conducting polymers continue to look promising as the active elements of novel electronic and chemical devices such as flexible light-emitting diodes [1], chemical sensors [2] and photovoltaic devices [3]. As a result of this, the thin film processing techniques for these materials have become increasingly important to the successful fabrication and optimization of useful allorganic thin film devices. Techniques such as spin-coating, electrochemical deposition, and Langmuir-Blodgett thin film transfer [4] have all been utilized with varying degrees of success to manipulate conjugated polymers into thin films. To date, however, no technologically useful technique exists to provide the level of molecular and supermolecular control needed to fabricate the ever more complex multilayer heterostructures required in many of these devices. In this paper, we describe a new thin film processing technique that affords a more precise, angstrom-level control over film thickness and enables the fabrication of ultrathin, multilayer films with unique and complex supermolecular architectures [5]. The basis for this new process is the electrostatic attraction developed between the ionized functional groups of a polyanion and the delocalized positive charges along the backbone of a ptype doped conducting polymer. Through a process involving the alternate dipping of a
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